brandl - 2006 - energy foundations and other thermo-active ground structures

Energy foundations and other thermo-active ground structures

H. BRANDL*

Energy foundations and other thermo-active groundstructures, energy wells, and pavement heating representan innovative technology that contributes to environmen-tal protection and provides substantial long-term costsavings and minimised maintenance. The paper focuseson earth-contact concrete elements that are already re-quired for structural reasons, but which simultaneouslywork as heat exchangers. Absorber pipes filled with aheat carrier fluid are installed within conventional struc-tural elements (piles, barrettes, diaphragm walls, base-ment slabs or walls, tunnel linings), forming the primarycircuit of a geothermal energy system. The naturalground temperature is used as a heat source in winterand for cooling in summer. Hence no additional elementshave to be installed below surface. The primary circuit isthen connected via a heat pump to a secondary circuitwithin the building. ‘Free cooling’ may even run withouta heat pump. The paper describes heat transfer in theground, and between absorber fluid and concrete/soil.Temperature-induced changes of soil properties or offoundation behaviour are also discussed, and recommen-dations for design and operation are given. Pilot researchprojects and case histories bridge the gap between theoryand practice, and special applications reveal the widefield of geothermal geotechnics.

KEYWORDS: basements; case history; design; diaphragm and insitu walls; environmental engineering; footings/foundations; piles;soil/structure interaction; temperature effects; time dependence

Les fondations energetiques et autres structures de solthermoactives, les puits d’energie et le chauffage destrottoirs, representent une technologie innovante qui con-tribue a la protection de l’environnement et permet deseconomies de couts substantielles a long terme tout enreduisant la maintenance. Cet expose etudie les elementsde beton au contact du sol qui sont necessaires pour desraisons structurales mais qui servent en meme tempsd’echangeurs de chaleur. Des tuyaux absorbeurs remplisd’un liquide conducteur de chaleur sont installes dans deselements structuraux conventionnels (piles, barrettes, ri-deaux souterrains, dalles ou murs de sous-sol, doubluresde tunnels) formant le circuit primaire du systeme d’ener-gie geothermique. La temperature naturelle du sol estutilisee comme source de chaleur en hiver et source derefroidissement en ete. Il n’est donc pas necessaire d’in-staller d’autres elements sous la surface. Le circuit pri-maire est alors connecte par l’intermediaire d’une pompea chaleur a un second circuit a l’interieur du batiment. Ce« refroidissement gratuit » peut meme fonctionner sanspompe a chaleur. Cet expose decrit le transfert de chaleurdans le sol et entre le liquide absorbeur et le sol/beton.Nous analysons egalement les changements dus a la tem-perature des proprietes du sol ou des comportements desfondations et nous faisons des recommandations quant ala conception et a l’exploitation. Des projets de recherchepilotes et des histoires de cas comblent le vide entre latheorie et la pratique et des applications speciales revelentl’etendue du domaine de la geotechnique geothermique.

INTRODUCTIONSubsurface geothermal resources represent a great poten-

tial of directly usable energy, especially in connection with(deep) foundations and heat pumps. Environmental andeconomical aspects were the incentive for the invention ofthe heat pump by the Austrian mining engineer Peter Rittervon Rittinger in 1855. Already two years later he couldprove an annual saving of 293 000 m3 firewood if the heatpumps were installed in all Austrian saltworks. Geothermalenergy can also be obtained by means of flat collectors,trench collectors, or borehole heat exchangers. These sys-tems have been widely used for many years in Austria(Fig. 1).

Since the beginning of the 1980s, geothermal energy hasalso been increasingly obtained from foundation elements inAustria and Switzerland: at first from base slabs, then frompiles (1984) and diaphragm walls (1996). This innovationmakes use of the high thermal storage capacity of concrete.Moreover, these concrete members are already required forstructural reasons, and need not be installed as additional

elements like conventional thermal energy utilisation sys-tems. Fig. 2 shows the increasing use of energy piles inAustria since the year 1984.

With combined geothermal cooling/heating systems heatenergy is fed into and withdrawn from the ground viaenergy foundations or other thermo-active ground structures.This innovative method is significantly more cost-effective

Brandl, H. (2006). Geotechnique 56, No. 2, 81–122

81

Manuscript received 26 April 2005; revised manuscript accepted 27October 2005.Discussion on this paper closes on 1 August 2006, for furtherdetails see p. ii.* Institute for Soil Mechanics and Geotechnical Engineering,Vienna University of Technology, Austria.

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Fig. 1. Scheme for heating/cooling a small one-family housewith energy foundations. Also indicated are additional thermo-active ground-source systems

than conventional systems, and it is environmentally friendlybecause it uses clean, renewable energy. In Austria nearly300 buildings are fitted with energy piles or energy dia-phragm walls. The number of foundation slabs, retainingwalls, basement walls, garage walls etc. used for geothermalheating or cooling is of similar magnitude. Furthermore, thistechnology and fitting techniques have been exported toseveral countries, even to tropical regions.Conventional ground heat exchangers consist of one or

more U-shaped loops of plastic tubes (absorber pipes)inserted into a borehole. The heat transfer from the sur-rounding soil/rock to the heat carrier fluid (or vice versa)takes place via the absorber pipes and the groundwater ormaterial that fills the borehole. The heat transfer of such aclosed system is consequently not as efficient as for an opensystem that consists of a single plastic tube, through whichthe fluid is transported from/to the bottom of the borehole.Energy foundations represent a closed system too, but

owing to the good thermal properties of concrete the heattransfer is significantly higher than in boreholes. Further-more, thin pipes are used, installed as loops, whereby thebending radius of the absorber pipes in the bottom zone islarger than in the case of conventional heat extraction/storage boreholes. Thus the flow resistance of the heatcarrier fluid is smaller, and consequently the operation costsare lower.Energy foundations contain closed coils of plastic piping

through which a heat carrier fluid is pumped that exchangesenergy from a building with the ground. The same principleis used for other thermo-active elements such as energywalls (retaining walls, basement walls), energy wells andenergy tunnels. The essential difference from conventionalearth-collector systems or ground heat exchanger boreholesis that the earth-contact concrete elements that serve as heatexchangers are already required for structural reasons andneed not be constructed separately. Furthermore, concretehas a higher thermal conductivity than soil.

Energy wells are wells temporarily used for groundwaterlowering and/or groundwater recharging but simultaneouslyadapted for energy extraction/storage purposes. The energysystems therefore have a double function, and they workmost efficiently if the thermo-active elements are in contactwith mobile groundwater in the case of heating or coolingonly. However, for seasonal operation (i.e. heating in winterand cooling in summer) rather steady groundwater condi-tions with a low hydraulic gradient are favourable for

seasonal energy extraction and feeding (recharging, storage).Nevertheless, a sufficient seasonal performance factor of thesystem is available even without closed groundwater and,moreover, the use of renewable clean energy favours envir-onmental protection.

Thermo-active foundations, tunnels etc. use earth-contactstructural elements with closed circuits. In contrast, opensystems use water from an aquifer, which is pressed througha heat exchanger or heat pump. These are simpler but hardlyused in Austria because of operational problems such asclogging or bio-fouling in the wells and heat exchangers.Clogging may occur by precipitation of dissolved mineralscaused by temperature changes and precipitation of iron-manganese hydroxides. It increases with temperature varia-tions in the aquifer and with air entering the wells orpipework. The latter can be avoided by operating the systemwith a slight overpressure. Furthermore, such wells needsubmersible pumps that can be lifted for maintenance.

PRINCIPLES OF GEOTHERMAL UTILISATION OFFOUNDATIONS (ENERGY FOUNDATIONS)

Energy foundations may comprise base slabs, piles, barr-ettes, slurry trench systems (single elements or continuousdiaphragm walls) and concrete or grouted stone columns(‘energy columns’). Combinations with near-surface earthcollectors or retaining structures are also possible. Energyfoundations can be used for heating and/or cooling buildingsof all sizes, as well as for road pavements, bridge decks etc.

Concrete has a good thermal conductivity and thermalstorage capacity, which makes it an ideal medium as anenergy absorber (heat exchanger). To use these propertiesfor energy foundations, high-density polyethylene plasticpipes of 20 or 25 mm diameter, with 2.0 or 2.3 mm wallthickness respectively, have to be installed within the con-crete. They are placed to form several individual closed coilsor loops, which circulate a heat carrier fluid (heat transfermedium) of either water, water with antifreeze (mainlyglycol), or a saline solution.

The plastic piping can be fixed to the reinforcement cagesof the energy foundation in a plant or on the site (Figs 3 to5). The latter is more common, whereby the piping isdelivered to site on reels, and a special working area isneeded. At the start (fluid inflow) and return end (outflow)of the pipework in each pile, diaphragm wall panel or otherindividual foundation element a locking valve and a man-

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Fig. 2. Number of energy piles installed in Austria

82 BRANDL

ometer are fixed. This allows the pipe circuit to be pres-surised to (normally) 8 bar for integrity check and to resistthe head of the wet concrete without collapsing. Thispressure is typically maintained until after the foundationconcrete is a few days old, and is again applied before theentire primary circuit is definitely closed.

Figure 6 gives a partial view of the laying system ofabsorber pipes on the subconcrete of a piled raft foundation.

Connected to this primary circuit is the secondary circuitwithin the building where the thermal energy is distributed.

Until recently prefabricated driven piles of reinforced con-crete with integrated heat exchanger (absorber pipes) repre-sented the majority of energy piles. But the percentage of(large-diameter) bored piles has been steadily increasingsince the year 2000. Pile excavation may be supported bycasing or fluid. Continuous flight auger piling and plungingof the heat exchanger with the reinforcement cage into wetpile concrete may affect the final integrity of the plasticabsorber pipes. The risk of absorber pipe damage can belowered by a stiff reinforcement cage—that is, by weldingthe helical reinforcement to the vertical reinforcement bars.Merely connecting with wires would allow excessive defor-mations of the reinforcement cage during lifting and insert-ing into the pile concrete. Consequently, the rotary boredtechnique or excavation by grab should be preferred forenergy piles, and even in this case a relatively stiff reinfor-cement cage is recommended.

An increasingly used alternative to driven piles of prefab-ricated reinforced concrete is ductile cast iron piles withintegrated heat exchangers. Contrary to grey cast iron withlamellar graphite, ductile cast iron contains spherical gra-phite. It therefore exhibits higher stresses at failure and aductile behaviour. The piles consist of 5 m long standardelements that can easily be assembled to longer sectionsduring the driving procedure. The tubes are filled underpressure with concrete. Shaft grouting to increase friction isalso possible. Since 1985 more than 1 million metres of

Fig. 3. Absorber pipes fitted to the reinforcement cage of alarge-diameter bored energy pile. The connecting ends areprotected by a tube at the pile head

Fig. 4. Reinforcement with attached absorber pipes insertedwithin casing of an auger pile; ready for casting concrete

Fig. 5. Absorber pipes attached to the reinforcement cage of adiaphragm wall (see Fig. 53)

Fig. 6. Installation of absorber pipes (filled with heat-carrierfluid) on the subconcrete of a piled raft foundation

ENERGY FOUNDATIONS AND OTHER THERMO-ACTIVE GROUND STRUCTURES 83

ductile cast iron piles have been installed in Austria; atpresent about 130 000 m are driven every year, with anincreasing proportion of energy piles. The heat exchangersare inserted into the fresh concrete, and have to be securedagainst uplift until the concrete has sufficiently hardened.The standard diameter of such driven piles is d ¼ 170 mm,but this can be increased significantly by shaft grouting.Nevertheless, the geothermal effectiveness of such thin en-ergy piles is smaller than that of driven precast concretepiles or large-diameter bored piles, despite the high thermalconductivity of cast iron. The small diameter enables theinstallation of only one pipe loop and no coiled piping(similar to Fig. 6). Moreover, the contact area with theground is relatively small. In soft soils, buckling of the pilesalso has to be considered.Other alternatives for energy piles are

(a) driven steel tube piles filled with concrete and insertedheat exchangers (absorber pipes) (Fig. 7)

(b) vibrated concrete columns installed after the vibroflota-tion technique and then fitted with absorber pipes.Partially grouted vibro stone columns can also be usedas ‘energy columns’ but have a lower geothermalefficiency.

The primary circuit contains closed pipework in earth-contact concrete elements (piles, barrettes, diaphragm walls,columns, base slabs, etc.) through which a heat carrier fluidis pumped that exchanges energy from the building with theground. The heat carrier fluid is a heat transfer medium ofeither water, water with antifreeze (glycol) or a salinesolution. Glycol–water mixtures have proved most suitable,containing also additives to prevent corrosion in the headerblock, of valves, of the heat pump, etc. Once cast, thepipings within the underground-contact concrete elementsare individually joined to a header and manifold block. Theyare joined by connecting pipes which, in the case of energyfoundations, are normally laid within the blinding beneaththe base slab. The secondary circuit is a closed fluid-basedbuilding heating or cooling network (secondary pipework)embedded in the floors and walls of the structure or inbridge decks, road structures, platforms etc. (Fig. 8).Commonly, primary and secondary circuits are connected

via a heat pump that increases the temperature level, typi-cally from 10–158C to a level between 258C and 358C(Fig. 9).All that is required for this process is a low application of

electrical energy for raising the originally non-usable heat

resources to a higher, usable temperature. The principle of aheat pump is similar to that of a reverse refrigerator (Fig.10). In the case of the heat pump, however, both the heatabsorption in the evaporator and the heat emission in thecondenser occur at a higher temperature, whereby the heat-ing and not the cooling effect is utilised.

The coefficient of performance, COP, of a heat pump is adevice parameter and is defined by

COP ¼ energy output after heat pump kW½ �energy input for operation kW½ � (1)

A value of COP ¼ 4 means that from one portion ofelectrical energy and three portions of environmental energy(from the ground) four portions of usable energy are derived(Fig. 9).

The efficiency of a heat pump is strongly influenced bythe difference between extracted and actually used tempera-ture. A high user temperature (inflow temperature to theheating system of the secondary circuit) and a low extractiontemperature (due to too low a return-flow temperature) inthe heat exchanger (primary circuit) reduce its efficiency.For economic reasons a value of COP > 4 should beachieved. Therefore the usable temperature in the secondarycircuit should not exceed 35–458C, and the extraction tem-perature in the absorber pipes should not fall below 0–58C.Consequently, this technology tends to be limited to low-temperature heating (and cooling).

Commonly, electric heat pumps are used, less often heatpumps with internal combustion and occasionally absorptionheat pumps. For environmental reasons only refrigerantswithout ozone reduction potential and with a minimumgreenhouse potential are allowed for heat pumps. Thereforehalogenated fluorinated hydrocarbons should not be used.

The seasonal performance factor (SPF) of a thermo-activesystem with a heat pump is the ratio of the usable energyoutput of the system to the energy input required to obtainit. Therefore SPF includes not only the heat pump but alsothe other energy-consuming elements (e.g. circulationpumps). At present, values of SPF ¼ 3.8–4.3 are achievedwith standard electric heat pumps; special devices withdirect vaporisation increase SPF by 10–15%.

SPF ¼ usable energy output of the energy system kWh½ �energy input of the energy system kWh½ �

(2)

In Austria about 200 000 heat pumps are running at present,and their number increases by more than 5000 per year.Their main purpose was at first provision of warm water, but

Fig. 7. Installation of absorber pipes in a driven steel tube pilefilled with concrete

Fig. 8. Absorber pipes entering the thermo-active concrete slabin the first floor of a building

84 BRANDL

since the year 2000 heating (and cooling) of buildings hasdominated. It is estimated that these heat pumps save morethan 250 000 t of fuel oil per year.

Experience has shown that these geothermal cooling/heating systems from energy foundations and other thermo-active ground structures may save up to two-thirds of con-ventional heating costs. Moreover, they represent an effectivecontribution to environmental protection by providing cleanand self-renewable energy.

If only heating or only cooling is performed, high-permeability ground and groundwater with a high hydraulicgradient are of advantage. However, the most economicaland environmentally friendly is a seasonal operation with anenergy balance throughout the year, hence heating in winter(i.e. heat extraction from the ground) and cooling in summer(i.e. heat sinking/recharging into the ground). In this caselow-permeability ground and groundwater with only low

hydraulic gradients are favourable. Dry soil makes deeperpiles and a larger area of the heat exchanger necessary.Depending on soil properties and the installation depth ofthe absorbers, 1 kW heating needs roughly between 20 m2

(saturated soil) and 50 m2 (dry sand) of the surface ofconcrete structures in contact with soil or groundwater.

There is no limitation to the depth of piles or diaphragmwalls as far as the installation of energy absorber systems isconcerned. The energy potential increases with depth: hencedeeper foundations are advantageous. The economicallyminimum length of piles, barrettes or diaphragm wall panelsis about 6 m.

The production of electric current from energy founda-tions and other thermo-active ground structures is theoreti-cally possible but not effective. This is similar to biomassesas base materials: they exhibit a high efficiency for heating(85%) but an extremely low efficiency for producing electriccurrent (25%). This would be an inexpedient application ofenvironmental technologies, whereas combined heat andpower plants reduce CO2 production significantly. The com-bustion of wood is CO2-neutral and replaces fossil resources.In Vienna the largest biomass-operated power station in theworld is operating. Waste incineration for heating purposesalso saves fossil fuels, especially in connection with districtheating. Furthermore, combinations of geothermal and solarheating and cooling have proved suitable to contributeeffectively to environmental protection. The economicallyoptimal solution depends on numerous local conditions (e.g.twin circuits and/or photovoltaically operated heat pumps).

HEAT TRANSFER IN THE GROUNDGeneral

Soil is a multiple phase system with a complex heattransfer mechanism involving

(a) conduction(b) radiation(c) convection

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Fig. 9. Scheme of a geothermal energy plant with energy piles and an energy flux for COP4 of the heat pump. COP coefficient of performance defining the heat pump efficiency

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Fig. 10. Scheme of a compression heat pump with temperaturesand pressures for the refrigerant medium R290 (as example).Heat exchange occurs from primary circuit to refrigerantmedium in the evaporator and from refrigerant medium tosecondary circuit in the condenser


(d) vaporisation and condensation processes(e) ion exchange( f ) freezing–thawing processes.

Heat transfer in unfrozen soil occurs mainly by conductionand secondly by convection. Heat convection is also possibleif there is a phase change of water (latent heat duringvaporisation and condensation). In soils, radiation usuallycontributes only negligibly to heat transfer; its effect in sandis less than 1% of the overall heat transfer (Rees et al.,2000). Freezing and thawing, however, can also transfersignificant heat, but such processes should be avoided forthermo-active ground structures.

Heat convection occurs between thermo-dynamic systemsthat move relative to each other (i.e. by means of circulationflows). In the soil the solid phase is static: hence onlyconvection with water and convection with gas has to bedistinguished. Heat transfer by fluid convection can bedescribed by

_qql,conv ¼ cwrwvw T � T 9ð Þ (3)

where cw is the specific heat capacity of the soil water, rwis the density of the soil water, vw is the vector of watervelocity (vw ¼ ki), and T 9 is a reference temperature.A similar equation is obtained for vapour (pore gas)

convection

_qqv,conv ¼ cvrvvv T � T 9ð Þ (4)

where cv is the specific heat capacity of soil vapour.The latent heat transfer that occurs as a result of a phase

change of water (vaporisation) depends mainly on the quan-tity of vapour transfer occurring in the soil pores. It in-creases with decreasing water content, and can be expressedby

_qqlat ¼ L0rwvv (5)

where L0 is the latent vaporisation heat at temperature T9.Thus the total heat transfer in the soil, _qqtot, may be

defined as (Rees et al., 2000)

_qqtot ¼ _qqcond þ _qql,conv þ _qqv,conv þ _qqlat (6)

where _qqcond is the conductive heat flux, _qql,conv is the heatflux generated by liquid convection, _qqv,conv is the heat fluxgenerated by vapour convection, and _qqlat is the heat flux dueto latent heat.

Heat conduction is a process whereby energy is passedfrom one region of a medium to another by moleculartransfer. According to Fourier’s law, the heat flux for a heatvolume Q through an arbitrary area during time t, that is,the heat flux density _qqcond , can be written

_qqcond ¼Q

At¼

_QQ

A¼ �º

@T

@n(7)

where º is the thermal conductivity and @T/@n is the tem-perature gradient in the actual flow direction n

@T

@n¼ @T

@xex þ

@T

@ ye y þ

@T

@zez ¼ gradT (8)

If the grain sizes of the soil particles and the pore sizes arenegligibly small in relation to the considered soil volume,the complex heat transfer process may be reduced to onlyconduction, which dominates in the case of thermo-activefoundations, retaining walls, and tunnels.Equation (7) can be written in rectangular coordinates as

_qq ¼ �º@T

@xex þ

@T

@ ye y þ

@T

@zez

� �¼ �ºgradT (9)

If the thermal conductivity and the temperature gradient areconstant over the area and in its normal direction, respec-tively, equation (7) can be modified for an energy pile withradius R and length l to

_QQ ¼ 2RºldT

dr[W] (10)

This is indicated in Fig. 11 for an energy pile utilised forcooling (¼ heat transfer into the ground) or heating (¼ heatextraction from the ground), describing a steady state wherethe temperature does not change within a period of time t. Atemperature change is caused by an alternation of the heatflux density within this period, thus leading to a change ofthe internal energy

�rc@T

@ t¼ @ _qq

@xþ @ _qq

@ yþ @ _qq

@z(11)

Differentiating equation (7) with respect to spatial coordi-nates and combining it with equation (11) yields

@T

@ t¼ a

@2T

@x2þ @2T

@ y2þ @2T

@z2

!¼ a div grad Tð Þ ¼ a˜T

(12)

with the thermal diffusivity a (m2/s) given by

a ¼ º

rc(13)

where º (W/mK) is the thermal conductivity, c (J/kgK) isthe specific heat capacity (thermal capacity), and r (kg/m3)is the density of the solid medium. If an inner heat source(internal heat generation) exists in a considered soil volume,the basic heat conduction equation becomes

@T

@ t¼ a˜T þ

_QQi

rc(14)

Equation (12) in rectangular coordinates can be transformedinto cylindrical coordinates with radius r, azimuth � andaxis z

@T

@ t¼ a

@2T

@ r2þ 1

r

@T

@ rþ 1

r2@2T

@j2þ @2T

@z2

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Fig. 11. Temperature curves in the soil around an energy pileutilised for heating or cooling. Schematic; constant heat fluxassumed

86 BRANDL

In spherical coordinates with radius r, angle of declination �and angle of ascension ł, equation (12) becomes

@T

@ t¼ a

@2T

@ r2þ 2

r

@T

@ rþ 1

r2@2T

@ł2þ cosł

r2 sinł

@T

@łþ 1

r2 sinł

@2T

@j2

!

(16)

The full mathematical description of ground heat transferas a physical problem requires the assumption of initial andboundary conditions. The initial state considers the tempera-ture distribution at time t ¼ 0. Regarding the boundaryconditions, three different assumptions can be made (Adam& Markiewicz, 2002; Hofinger, 2002)

(a) Dirichlet’s boundary condition. Surface temperature TSis constant or depends only on time t.

(b) Neumann’s boundary condition. Heat flux density _qq atthe surface is constant or depends only on time t.

(c) Cauchy’s or mixed Neumann’s boundary condition. Onthe surface of the considered body a heat exchangeoccurs with the liquid or gaseous surroundings of thebody exhibiting a temperature TU. According to New-ton’s law of cooling the heat flux is proportional to thetemperature difference between the surroundings andthe surface temperature, Tinterface. The factor of propor-tionality is defined as the heat transfer coefficient Æ(W/m2 K)

�º@T

@n

� �interface

¼ Æ Tinterface � TUð Þ (17)

Because of mathematical difficulties, analytical solutions ofthese equations are possible only for simple cases. One-dimensional problems can easily be solved, since equation(12) depends only on one coordinate then. Two and three-dimensional problems can be solved in some cases bycombining one-dimensional solutions. Examples are two orthree-dimensional edges/corners of bodies or other bodiesgained by Boole’s mathematical operations of basic forms.

Analytical and numerical calculationsAbsorbers for geothermal heat extraction or feeding/

storage exhibit different forms depending on the shape ofthe structures used as thermo-active elements. Energy pilesor vertical bored heat exchangers can be modelled bycylinders, whereas complicated foundations or energy tun-nels require sophisticated geometric considerations. In thefollowing, three basic cases are discussed, which can be usedto simulate most of the commonly designed absorber ele-ments (Table 1).

(a) Case 1: Semi-infinite body. Simulates plane interfacesbetween soil and atmosphere or foundations (slabs;basement walls, retaining walls, pile walls, diaphragmwalls; horizontal earth collectors and trench collectors).

(b) Case 2: Infinite body with cylindrical gap. Simulateslong vertical ground heat exchangers, and hence heatextraction/storage boreholes, energy piles and energywells; also, the lower part of pile or diaphragm wallsembedded on either side.

(c) Case 3: Infinite body with spherical gap. Simulatesspherical thermo-active ground openings (undergroundstructures), and approximates the toe zone of heatextraction/storage boreholes and energy piles.

Analytical solutions are available only for case 1 (semi-infinite body) considering simple boundary conditions. Morecomplicated boundary conditions require semi-analyticalsolutions that reduce the differential equation to a semi-analytically solvable function, the so-called Gaussian errorfunction (Adam & Markiewicz, 2002; Hofinger, 2002). Thecomplement of the Gaussian error function is defined asfollows to be solved numerically

erfc �ð Þ ¼ 1� 2ffiffiffi�

pð�0

e�ø2

dø (18)

where � is the integration limit and ø is the integrationvariable.

This error function is also the basis for the solution ofcase 3 (infinite body with spherical gap). Independently ofboundary conditions, case 2 (infinite body with cylindricalgap) can only be found numerically using a differentialequation solver.

Semi-infinite body (case 1)The problem was solved for different boundary conditions

and heat supply or loss at the surface (Adam & Markiewicz,2002; Hofinger, 2002).

First, a harmonic temperature oscillation on the surfacewas assumed (Cauchy’s or mixed Neumann’s boundary con-dition) with heat transfer between soil and air. Thus thedaily or seasonal temperature fluctuations between groundand atmosphere can be treated, whereby a time lag has to beconsidered. This depends on the considered depth in the halfspace, and may even cause an anti-cyclic behaviour in theseasonal histogram of the mean daily air temperature(Tm,out). The surface of a semi-infinite body is assumed toexchange heat with the air, which performs a sinusoidaltemperature oscillation F(t): see Fig. 28 for an example.

Table 1. Basic cases for heat conduction in soil

Case Sketch Differential equation

1: Semi-infinite body@2

@x2Ł x, tð Þ ¼

1

a

@

@ tŁ x, tð Þ

2: Infinite body withcylindrical gap

@2

@ r2Ł r, tð Þ þ

1

r

@

@ rŁ r, tð Þ ¼

1

a

@

@ tŁ r, tð Þ

3: Infinite body withspherical gap

@2

@ r2Ł r, tð Þ þ

2

r

@

@ rŁ r, tð Þ ¼

1

a

@

@ tŁ r, tð Þ


Furthermore, assuming the transient period of the initialvibrations to be over, a steady state therefore exists. Underthese conditions the soil temperature fluctuates according tothe mean yearly air temperature Tm,out if radiation effectsand geothermal temperature gradient are neglected. However,the amplitudes decrease depth with owing to the thermalinertia of the soil. A function that fulfils the differentialequation (12) with the corresponding boundary conditions is

T z, tð Þ ¼ Tm,out þ ˜Tout�e�z=d cos ø t � �ð Þ � z

d

� �(19)

where

d ¼ffiffiffiffiffi2a

ø

r¼

ffiffiffiffiffiffiffia P

�

s(20)

� ¼ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 2k þ 2k2

p (21)

� ¼ arctank

1þ k(22)

k ¼ º

Æ d¼ º

Æ

ffiffiffiffiffiffi�

aP

r(23)

Tm,out is the mean yearly air temperature (8C), Tm,out is themean daily air temperature (8C), ˜Tout is the temperatureamplitude (8C), ø ¼ 2�=P (1/s), P is the period duration oftemperature oscillation (s), d is the damping depth, and Æ isthe heat transfer coefficient from soil to air (W/(m2K)).On the surface (z ¼ 0) the solution reduces to

T 0, tð Þ ¼ Tm,out þ ˜Tout� cos øt � �ð Þ (24)

It is evident that the amplitude of the surface temperaturedecreases by a factor � , 1 in relation to the air tempera-ture and, moreover, undergoes a time lag of �. Fig. 12illustrates the time–depth curves for a full period at differenttimes t.The damping depth d depends on the number of periods

and temperature conductivity. In soils with high a values(moist sand) the temperature wave penetrates relativelydeeply. Comparing the response of the soil body to yearlyand daily periods yields the following ratio

Pyearly

Pdaily

¼ffiffiffiffiffiffiffiffi365

p¼ 19:1 (25)

Equation (25) reveals that the yearly temperature wavepenetrates 19 times deeper into the ground than the dailywave. Furthermore, the heat flux in the ground reaches itsmaximum earlier than the temperature.

The second solution of the basic case 1 is achieved byassuming a sudden temperature fall on the surface of thesemi-infinite halfspace (Dirichlet’s boundary condition). Thisallows the investigation of the heat transfer between a planethermo-active element and the ground in contact.

Assuming a temperature fall from T0 to TS at time t ¼ 0,the solution of equation (12) is obtained as follows

T z, tð Þ ¼ TSerfcz

2ffiffiffiffiffiat

p� �

(26)

T z, tð Þ is the temperature function with T0 as reference(initial) temperature

T z, tð Þ ¼ T (z, t)� T0 (27)

Hence

T z, 0ð Þ ¼ 0 (28)

Comparative calculations of the basic casesThe basic cases 1 to 3 were simulated by numerical

models and the results compared with the (semi-) analyticalsolutions. As an example for time–temperature curves ofdifferent heat exchanger shapes the following assumptionsare made.

(a) The soil, initially at constant temperature T0, issuddenly heated up to TS ¼ T0 + 258C on its surface(i.e. the interface between soil and heat exchanger).

(b) The soil parameters are as given in Table 2.(c) The radius of the cylindrical underground space (case

2) and spherical underground space (case 3), R ¼30 cm.

(d) The thermal conditions and properties are as given inFig. 11 and Table 2.

(e) The time period considered is half a year, that is, 182.5days.

Figure 13 shows the results plotted for two-week intervals.In order to keep the surface temperature constant, a certainheat supply must be provided. This is given by

_qq 0, tð Þ ¼ �º@

@zT z, tð Þ

��z¼0, t>0

¼ ºTSffiffiffiffiffiffiffi�at

p (29)

If _qq is integrated over the considered time period of 182.5

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-��(�� 5�1*�6

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7��:��+��

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��

�2� �2� �2� ��2� ��2� ��2�

��1�

��1�

��1��1�

��1��1�

��1�

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Fig. 12. Temperature distribution within soil at different timescaused by a harmonic oscillation of outdoor temperature

Table 2. Example parameters for calculating the basic cases ofTable 1

Property Value

Thermal conductivity, º: W/mK 2.5Density, r: kg/m3 2700Specific heat capacity, c: J/kgK 800Temperature conductivity, a: m2/s 1.1574 3 10�6

Radius of cylindrical and sphericalgap: m

0.3

Sudden temperature rise, ŁS: K 25Observed period, t Half a year ¼ 182.5 days

88 BRANDL

days, the total heat supply to be provided is obtained. Thisis 72.37 kWh/m2 for case 1.

The heat supply needed for case 2 results from equation(15), whereby differentiation with respect to r on the surfaceand integration over time were performed numerically. Thisyielded 287.04 kWh/m2.

The solution for the spherical underground space withradius R ¼ 30 cm (case 3) can be found from equation (16)as follows

T r, tð Þ ¼ TS

R

rerfc

r � R

2ffiffiffiffiffiat

p for R < r < 1 (30)

The heat flux density is given by

_qq 0, tð Þ ¼ �º@

@ rT r, tð Þ

��r¼R, t>0

¼ ºTS

Rþ TSffiffiffiffiffiffiffi

�atp

� �: (31)

If R ! 1 the solution for the semi-infinite body accordingto equation (29) is obtained (case 1). Independently of thetime a basic heat flux ºTS/R must occur to keep the tem-perature of the spherical underground space constant (at t !1). Integrating the heat flux over half a year yields therequired heat supply of 984.81 kWh/m2. Table 3 summarisesthe results of the comparative investigations.

A comparison of these three basic cases illustrates thatthere are significant differences of the heat penetration velo-city caused by temperature changes at the surface. Higher-dimensional heat conduction leads to a more efficient heattransport. This means that spherical underground spaces canextract more heat energy per unit area. Spherically shapedabsorber elements or heat-exchanging structures are therefore

most effective, whereas plane elements such as base slabs orearth collectors are less productive.

THERMAL SOIL PROPERTIESIn most regions of Europe the seasonal ground tempera-

tures remain relatively constant below a depth of 10–15 m.Values between 108C and 158C predominate to a depth ofabout 50 m (Fig. 14). Such temperatures permit economicalheating and cooling by thermo-active ground structures, andthey represent ideal conditions for heat pumps. Substantialtemperature fluctuations during the year would reduce theefficiency of both heat pumps and absorber systems. In thetropics the constant ground temperature at a depth of morethan 10–15 m below the surface varies between 208C and258C (locally even 288C), which still allows cooling ofbuildings.

The thermal conductivity º, heat capacity c and density rof the soil are temperature-dependent parameters, and arecoupled in the basic equation of heat conduction (equation(13)) with the thermal diffusivity a. This parameter (cm2/h)describes the depth and velocity of penetration of a tempera-ture wave into the ground. If the ground temperature is

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��%� �

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��%� �

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��%� �

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Fig. 13. Comparison of the basic cases of heat conduction at equal boundary conditions and thermal and material properties.Temperatures in soil due to a temperature change from T 0 to 258C on the surface for half a year: (a) case 1, semi-infinitebody; (b) case 2, infinite body with cylindrical gap; (c) case 3, infinite body with spherical gap

Table 3. Results of comparative calculations of basic cases 1 to3 (to Tables 1 and 2)

Basic case Specific heat exchange: kWh/m2

1 72.3672 287.0403 984.807


measured at at least two depths z1 and z2, the thermaldiffusivity can be calculated either from the amplitude de-crease or from the time lag.The proportionality factor that relates the rate at which

heat is transferred by conduction to the temperature gradientis known as the thermal conductivity º (SI units W/mK). ºis strongly influenced by the water content and density ofthe soil, and hence also by the mineralogical componentsand by the chemical properties of the pore water. Freezingincreases the thermal conductivity significantly, becauseºwater ¼ 0.57 W/mK changes to ºice ¼ 2.18 W/mK. Conse-quently, the thermal conductivity of the soil can only beexpressed approximately (Rees et al., 2000).The heat capacity c (J/kgK) defines the amount of energy

stored in a material per unit mass per unit change intemperature. It is required when non-steady solutions are tobe determined. The heat capacity does not depend on themicrostructure. In most cases, therefore, it is satisfactory tocalculate the heat capacity of soils from the specific heatcapacities of the different constituents according to theirvolume ratios

c ¼ csxs þ cwxw þ caxa (32)

where x is the specific volume: hence xs ¼ 1 � n for thesolid phase, xw ¼ nS for the pore water, and xa ¼ n(1 � S)for the pore air, where S is the degree of saturation and n isthe soil porosity.The heat capacity of a soil with more than three constitu-

ents (e.g. contaminated ground) can be calculated by simplyadding more terms into equation (32) (Rees et al., 2000).The heat capacities of solid and liquid constituents can beconsidered constant, and xa is negligible. Thus equation (32)reduces approximately to

c ¼ 2:0 xs,min þ 2:5 xs,org þ 4:2 xw (W s=kgK) (33)

where xs,min refers to the mineral components and xs,org tothe organic components of the solid fraction.As the mineral and organic solid components have some-

what similar thermal capacities, only the water contentremains as a relevant variable, at least in the short term. Inthe long term consolidation or shrinkage processes of soils(under external loads or self-weight, or due to excessive heatextraction) may play a role because the volume ratioschange. The overall thermal capacity increases with thewater content and decreases in the case of freezing. The

specific heat capacity of ice is only ci ¼ 1884 J/kgK com-pared with cw ¼ 4186 J/kgK for water.

The volumetric capacity CV is derived from the specificheat capacity and the bulk density of the soil, and representsthe weighted arithmetic average of the particular soil compo-nents. Hence

CV ¼X

ricixi (34)

or

CV ¼ rs cs þ cww

100

� �(35)

where cs is the specific heat capacity of mineral components(most minerals have cs ¼ 1000 W s/kgK at a temperature of108C); cw is the specific heat capacity of water (cw ¼4186 W s/kgK); and w is the water content related to thedry mass in percent.

The most important thermal soil parameter is the thermalconductivity º. For the preliminary design of complexenergy foundations, or for the detailed design of standardgeothermal systems, it can be taken with sufficient accuracyfrom diagrams considering water content, saturation densityand texture of the soil (Fig. 15).

For large complex projects º should be determined fromlaboratory or/and field tests, whereby a soil body has to beexposed to a temperature gradient. However, this may causea significant moisture transfer in unsaturated soils, whichshould be taken into account when interpreting the measuredresults. In the field, the thermal response test has provedsuitable, as it can be performed directly with vertical groundheat exchangers—that is, with heat extraction/storage bore-holes, energy foundations or energy wells. In the laboratoryboth the steady state method and the transient method areused, but the latter is preferred, especially the hot wiremethod. In this method an electrical wire is implanted in theexperimental soil sample. A steady current is supplied to theelectric wire, and the temperature rise and fall of the heatingwire is measured by a thermocouple and recorded over ashort heating and cooling interval. Comparative tests haveshown that the thermal conductivity during heating is com-monly slightly higher than that during cooling (Abu-Hamdehet al., 2001).

The specific heat capacity can be determined in thelaboratory by mixing water and soil of different tempera-tures. If the total thermal energy of both componentsremains constant, and the specific heat capacity of onecomponent is known (e.g. cw of water), then the specificheat capacity of the soil (cs) can be achieved. Commonly,test temperatures of 08C for soil and 208C for water areused, and the steady-state temperature of the mixture ismeasured. Moist soil requires a correction of the cs valueaccording to its water content. Considering the temperaturedependence of the specific heat capacity c(T) the energybalance of the water–soil mix can be written as (Leu et al.,1999)

cs Tsð ÞTsms þ cw Twð ÞTwmw ¼ cs Tminð Þms þ cw Tminð Þmw½ �Tmin

(36)

where Ts is the temperature of the cooled soil (8C); Tw is thetemperature of the water before mixing (8C); Tmin

is the minimum temperature of the mixture (8C); ms, mw arethe mass of the soil and the water (g); and cs, cw are thespecific heat capacities of the soil and the water (J/gK).

5��

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-��

<��

�/" �/"

�4 ��

��( ��=��/�� =��/��

)��'�+

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0�%��

Fig. 14. Heat transport and geothermal situation for deepfoundations (schematic). Ground temperature at depth z >10–15 m tends to be constant (Europe, T 10–158C; tropics, T

20–258C, locally 288C)

90 BRANDL

TEMPERATURE-INDUCED CHANGES OF SOILPROPERTIES

The range of temperature changes in the ground causedby geothermal energy utilisation is rather small. Neverthe-less, they should be checked, and environmental assessmentsare frequently required by national laws. Moreover, thethermal properties of soils vary considerably in a freezingsoil, where the transfer coefficients across the freezing frontchange greatly. Consequently, excessive heat extraction fromenergy foundations causing soil freezing should be avoided.

Stress–strain propertiesUnlike the linear stress–strain relationship of ideal elastic

materials, the thermal expansion coefficient of soil cannot bedetermined on the basis of Hooke’s law, but only by meansof tests whereby the water content has to be kept constant toavoid shrinkage effects. Pure minerals do not undergo sig-nificant strains until very high temperatures (> 3008C).Consequently, the thermal expansion coefficient of verymoist ground corresponds to that of the soil water.

Thermal processes in the ground induce water migrationtowards the colder regions. In fine-grained sensitive soils thismay gradually cause shrinkage in the warm zone and expan-sion in the cold one. A thermal expansion of pore water

increases the pore water pressure, and consequently de-creases the effective stress of the soil. Furthermore, anincreasing temperature reduces the internal viscosity, andhence the shear resistance. Oedometric tests on silt andsands have shown larger settlements at higher temperatures;however, with increasing load the modulus went up more(Ennigkeit, 2002). These tests also revealed an acceleratedconsolidation at higher temperatures caused by lower viscos-ity of the pore water and thus a higher hydraulic conduct-ivity.

Organic constituents increase the temperature sensitivityof soils, especially clays. Nevertheless, experience has shownthat properly operated energy systems do not affect the load-bearing capacity of thermo-active piles embedded in suchstrata. Commonly, the interactions are rather negligible inpractice, but they need to be considered if the above-groundpart of a building is extremely sensitive to differentialsettlements.

Hydraulic propertiesLowering the groundwater temperature causes an increase

in viscosity, and hence a decrease of hydraulic conductivity.This leads to lower flow velocities and to smaller flowgradients of the groundwater. However, comprehensive

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Fig. 15. Thermal conductivity against dry density and water content for frozen and unfrozensoils (Jessberger & Jagow-Klaff, 1996): (a) coarse-grained soil, frozen; (b) coarse-grained soil,unfrozen; (c) fine-grained soil, frozen; (d) fine-grained soil, unfrozen


parametric studies have shown that in practice these effectsare negligible in relation to geothermal energy systems.

Physico-chemical propertiesToo intensive cooling of the groundwater (due to exces-

sive energy extraction for heating buildings) increases thepH value and reduces calcium solubility, which favours theclogging of pores. On the other hand, the solubility ofgaseous substances such as CO2 increases, and hence alsothe hardness of the groundwater. Hitherto investigations haveshown that temperature changes smaller than ˜T ¼ 58Chave a negligible effect within a temperature range of0–208C.

Biological propertiesTemperature is one of the most important environmental

factors for the microorganisms in groundwater. Many ofthem can exist only within a very limited temperature range.In particular, the activity of bacteria-consuming microorgan-isms drops significantly at temperatures below 108C. There-fore, for instance, the mortality rate of pathogens is reducedby about half if the temperature drops from 78C to 28C.

HEAT TRANSFER BETWEEN ABSORBER FLUID ANDCONCRETE/SOILGeneralAssuming that the walls of the absorber pipes of a ground

heat exchanger have the same temperature as the surround-ing concrete or soil respectively reduces the complex ther-mal problem (Fig. 16) to the heat transfer from pipe wall toabsorber fluid (heat carrier fluid). This is essentially influ-enced by the flow behaviour of the fluid, that is, laminar orturbulent.Pipe flow is described by two zones (Fig. 17): the

transient inflow zone, where flow velocity and temperatureprofile change with pipe length; and the steady-state condi-tion with a constant hydrodynamic and thermal profile. Theheat transfer does not then change (at constant thermalconductivity). In absorber pipes in thermo-active founda-tions, retaining walls, tunnels, pipe wells and roads thesteady-state phase dominates. Furthermore, this state isreached after only a short distance. Hence the followingtheoretical considerations are limited to the steady-state flowand heat transfer problem (Adam & Markiewicz, 2002).

Laminar flow in a pipe is based on flow paths withdifferent velocities u and interface friction �, which isproportional to the velocity gradient du/dx perpendicular tothe flow direction. The coefficient of proportionality is theviscosity �, which increases with temperature. Fig. 18 showsan example of a typical absorber fluid for energy founda-tions (an antifreeze–water mixture). For this purpose New-ton’s friction law can be applied

� ¼ �du

dx(37)

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)��'��=��$��$��'��%�� :�:�:��

)��'�� :��::�:�:�:�:>5

)��'��=�� :��:�

��

)��%�� :�:��: )��'��=��:�:�:��

)��$��$��'��%�� :�:�:�:�:�:�:�

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Fig. 16. Heat transport from soil to heat carrier fluid within theabsorber pipe of an energy pile (GW, ground water)

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Fig. 17. Flow velocity and temperature distribution in absorberpipes filled with heat carrier fluid

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Fig. 18. Kinematic viscosity against temperature for differentmixtures of water and Antifrogen L

92 BRANDL

The mean velocity of laminar flow is umean ¼ 0.5umax, andfor turbulent flow umean ¼ (0.80–0.85)umax. The transitionfrom laminar to turbulent flow condition is described by theReynolds number, Re

Re ¼ ud

�with � ¼ �

r(38)

where u is the mean velocity (m/s), d is the pipe diameter(m), � is the kinematic viscosity (m2/s), � is the dynamicviscosity (kg/m s), and r is the density (kg/m3). Below thecritical Reynolds number Re ¼ 2300 laminar flow occurs;above Re . 104 full turbulence exists. Between these bound-ary values transient conditions occur. Turbulence increasesthe diffusive transfer of energy, impulse and mass. Thiseffect increases with flow velocity.

Heat transfer by convectionHeat transfer through contact is based on Fourier’s law of

molecular heat transport, which can be written for one-dimensional problems as

_qq ¼ �º@T

@x

� �(39)

where _qq is the heat flux density (W/m2), º is the thermalconductivity of the flowing medium (W/(mK)), T is thetemperature, and x is the local coordinate.

Heat transfer between masses not moving relative to eachother occurs by conduction. Heat transfer by convection isbased on differential movements. The latter occur betweenpipe wall and absorber fluid, whereby molecular heat transfertakes place at the interface

_qqW ¼ �º@T

@ r

� �wall

(40)

where _qqW is the heat flux density at the pipe wall (W/m2).The heat transfer between the pipe wall and the fluid can

be described by the heat transfer coefficient Æ

Æ ¼ _qqWTwall � Tfluid

¼ �º @T=@ rð Þwall˜T

(41)

and by the Nusselt number Nu, which is defined as follows

Nu ¼ Æd

º¼ � @T=@ rð Þwall

˜T=D(42)

Under turbulent conditions the heat transfer velocity dependsnot only on the self-velocity of the energy carrier (heatcarrier fluid) but also on that of the turbulent fluctuations,which is connected to the average flow velocity of theabsorber fluid. Consequently, the heat transfer depends alsoon the flow velocity, and the heat transfer coefficient Æ is afunction of material properties, geometric dimensions, lengthof heat transfer occurrence, and flow velocity of absorberfluid.

Calculation of the temperature gradient (@T/@r)wall at thepipe wall is possible only as long as equation (39) is valid ateach point of the absorber fluid. But this applies only tolaminar flow without friction, or to motionless media. Forturbulence, equation (39) is valid only for the pipe wall(according to equation (40)) but not for the interior of theflowing medium. To date there is no exact theory for thisthermal problem; it can be solved only by equations basedon experimental data (VDI, 1997).

Heat transfer by forced convectionThe absorber pipes of a heat exchanger are part of a

closed circuit (primary circuit in Fig. 9) where the flow iscreated by a pump. Therefore this is called forced convec-tion. Commonly, the calculation is based on steady-stateconditions, whereby the flow velocities should not be toolow.

The flow velocity u(r) within a circuit is different at eachpoint of the cross-section. Consequently, the period t forwhich individual fluid particles remain within a certainabsorber pipe section differs. According to equation (12)one-dimensional conditions can be described by

rc@T

@ t¼ º

@2T

@ r2(43)

where r is the density, c is the specific heat capacity and ºis the thermal conductivity of the flowing medium. Thedifferent time periods of the fluid absorber staying inparticular sections are t ¼ x/u(r) with the radial distance raccording to Fig. 19. This leads to

rc u rð Þ @T

@x¼ º

1

r

@

@ rr@T

@ r

� �(44)

Furthermore, another dimensionless coefficient is used forparametric studies, the Prandtl number Pr, which has amaterial-dependent value and is defined as

Pr ¼ �

a¼ �rc

º(45)

For Pr ! 0 the velocity profile along a flow path x, wherethe heat is transferred, is equivalent to the profile of a pistonflow. In the case of Pr ! 1 the velocity profile correspondsto the Hagen–Poiseuille flow (Fig. 19). Common absorberfluids exhibit values of about Pr ¼ 7 for clean water closeto the freezing point, and Pr ¼ 70 for a viscous fluid, suchas water–glycol mixture as anti-freeze medium

Commonly, the heat transfer from concrete or soil to theabsorber fluid occurs at a widely constant pipe wall tempera-ture (Tw) along the entire pipe length, if laminar flowconditions prevail. For a mean fluid temperature Tm themolecular heat transfer can be then be described by equation(47) and Fig. 20 (left)

@T

@x¼ TW � T

TW � Tm

dTm

dx(46)

rc u rð Þ TW � T

TW � Tm

� �dTm

dx¼ º

1

r

@

@ rr@T

@ r

� �(47)

In the case of constant heat flux density _qqw ¼ Æ(Tw � Tm)the heat transfer coefficient Æ is constant

Æ ¼ qW

TW � Tm

¼ º

R

@

@ z=Rð ÞTW � T

TW � Tm

� �� wall

(48)

In this case the temperature difference Tw � Tm is alsoconstant, leading to

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��

Fig. 19. Flow velocity distribution in pipes (Hagen–Poiseuille’sparabola)


@T

@x¼ dTW

dx¼ dTm

dx(49)

This finally gives a molecular heat transfer according toequation (50) and Fig. 20 (right):

rc u rð Þ dTm

dx¼ º

1

r

@

@ rr@T

@ r

� �(50)

Turbulent flow conditions in the absorber pipe include alaminar zone close to the pipe wall where the local flowvelocity is finally zero (Fig. 21). Equation (37) is then validonly along the pipe wall but not within the core of the flux.The shear stress �W along the wall is

�W ¼ ��u9

�l¼ �

8ru2

c (51)

where uc is the flow velocity in the core of the fluid, u9 isthe flow velocity along the laminar (viscous) edge zone, � isthe coefficient of flow pressure loss within the pipe system,and �l is the thickness of the laminar (viscous) edge zone.Energy and impulse transfer within this core is achieved byso-called turbulence balls continually entering and leavingthe laminar edge zone, and thus undergoing an averagevelocity change from ucore to u9 and vice versa. These

oscillating balls have a mass flux density of _mmr, and theshear stress �9 along �l is

�9 ¼ _mmr uc � u9ð Þ (52)

In a similar way the heat flux density _qq transferred by theseturbulence balls can be expressed by

_qq9 ¼ _mmrc T 9� Tcð Þ (53)

The temperature profile in a cross-section is similar to theflow velocity profile (Fig. 21). Thus the heat flux density _qqwat the pipe wall becomes

_qqW ¼ �ºT 9

�l¼ ÆTc or Æ ¼ _qqW

TW � Tc

(54)

where Tc is the temperature in the core of the fluid, T 9 is thetemperature at the laminar (viscous) edge zone, Tw is thetemperature on the wall, and �l is the thickness ofthe laminar (viscous) edge zone

Equations (51)–(54) lead to Prandtl’s basic equation forthe relationship between heat transfer and flow resistance

Nu

Re Pr¼ �

8

1

1þ Pr� 1ð Þ u9=ucð Þ (55)

The velocity ratio u9/uc is then substituted by 12.7ffiffiffiffiffiffiffiffi�=8

p,

which represents a suitable approach. Further sophisticationsare based on experimental data, leading finally to a formulathat also considers the length of the pipe system (Oertel,2001)

Num,T ¼ �=8ð Þ Re� 1000ð ÞPr1þ 12:7

ffiffiffiffiffiffiffiffiffiffiffi�=8ð Þ

p ffiffiffiffiffiffiffiPr2

3p

� 1� � f (56)

with

f ¼ 1þ

ffiffiffiffiffiffiffiffiffiffiffiffid

L

� �23

s(57)

for 0.5 , Pr , 104 and 2300 , Re ,106, and for 0 , D/L, 1.

Figure 22 considers laminar and turbulent flow conditionsdepending on the dimensionless Reynolds, Nusselt andPrandtl numbers. When determining these parameters apossible temperature dependence of the material propertieshas to be taken into account, whereby in practice only thedynamic viscosity is influenced by temperature changes in arelevant way. Thus the Nusselt number becomes

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Fig. 20. Heat conditions at the wall of an absorber pipe for: (a) constant temperature; (b) constant heat flux density

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Fig. 21. Turbulent flow conditions with a laminar flow areaalong the wall of an absorber pipe

94 BRANDL

Nu ¼ Num�m�w

� �0:14

(58)

where �m is the dynamic viscosity at a caloric mean tem-perature Tm, and �w is the dynamic viscosity at a walltemperature of TW.

Summarising remarksThe material properties should be related to a mean tem-

perature of Tm ¼ (Tinflow + Treturn flow)/2 considering the in-flow and return flow of the absorber fluid into/from theprimary circuit.

The heat transfer coefficient Æ depends on the pipediameter d, the pipe length L, the flow velocity u, theviscosity �, the density r, and the specific heat capacity c orthermal conductivity º respectively. For laminar flow it canbe determined theoretically; turbulence, however, requiresexperimental data. The heat transfer coefficient of turbulentflow is always higher than that of laminar flow: Ælaminar <f (ˇu) whereas Æturbulent ¼ f (u3=4), if equal boundary condi-tions are assumed. The flow velocity is a criterion not onlyfor the contact period but also for the intensity of a turbulentmixing.

The Nusselt number Nu is a valuable criterion to describethe heat transfer intensity from the absorber fluid to aparticular section of the absorber pipe, but it does notdescribe the overall heat extraction (or storage) Q of theentire absorber system. For that variable the time period foran absorber fluid circulates within the heat exchanger hasalso to be considered.

Some guidelines for geothermal energy utilisation recom-mend the creation of turbulent flow in the absorber pipes.However, this should not be generalised. In the case oflonger heat extraction (or storage) the critical point is notthe heat transfer but the quantity of heat energy economic-ally extracted from or stored in the surrounding soil. High-performance pumps, required to create turbulent conditions,would therefore reduce the seasonal performance factor(SPF) of the overall geothermal system.

Figure 23 illustrates the performance balance of soil andabsorber (heat exchanger) per metre run of an energy pileunder steady-state conditions. The heat flux density _qq of thesoil is compared with that of the absorber, _qqw, whereby thegeometric conditions (circumference of pile Cpile, circumfer-ence of absorber pipe Cpipe) and the number of absorberpipes have to be considered

_qqCpile $ n _qqwCpipe ¼ nÆ Tw � Tmð ÞCpipe (59)

where n is the number of absorber pipes filled with heatcarrier fluid.

Equation (59) does not include the heat transfer throughthe concrete cover and the pipe wall. This can only besimulated numerically. Therefore numerous parametric stud-ies were conducted comprising the parameters discussedbelow.

Numerical simulationsIn order to investigate the influence of individual para-

meters and their interaction, comprehensive comparativestudies were conducted (Markiewicz, 2004). They referred tothe following parameters: concentration of absorber fluid(water–glycol), Conc.; cooperation temperature, Tm; pumpperformance, P; efficiency of pump, �p; inner pipe diameter,d; roughness of inner pipe wall, �; length of pipe for aparticular pump performance, LP; specific heat flux resis-tances, Ri; pipe length for heat transfer, L; and temperatureof pipe wall, TW.

Figure 24 illustrates the scheme for a one-family house asan example. The ground heat exchanger is idealised as one

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absorber structure. For thermal-flow-relevant investigations itis sufficient to consider only the primary circuit of thegeothermal energy system. The secondary circuit is opti-mised by the building’s HVAC expert (for heating, ventila-tion, air-conditioning) considering the interaction with theprimary circuit.The calculations are based on a clear limit between

laminar and turbulent flow state for Re ¼ 2300. Hence astep function is assumed theoretically, whereas a transientzone occurs in practice. Prior investigations revealed thatthere is no qualitative difference between Nu and the heattransfer coefficient Æ per metre run of the absorber pipe.Thus Nu is the most suitable for comparing heat transferprocesses in parametric studies. The results of these numer-ical simulations can be summarised as follows.Heat transfer from the absorber wall to the absorber fluid

increases with Nu. It depends widely on the flow conditions:zones of laminar or turbulent flow have do be considered ina different way. In principle, heat transfer under turbulentconditions is significantly higher than in laminar zones.Hence the parameters of an earth-source geothermal systemought to be chosen such that turbulence occurs in theabsorber pipes. However, economical aspects have to beconsidered too: increasing the pump performance to achieveturbulence causes higher costs for operation. Consequently,an economical optimisation requires an overall view, balan-cing ground-extracted and equipment-supplied energy.Heat transfer under laminar flow conditions depends on

how long the fluid stays in the absorber pipes, on the pipediameter, and on the density, thermal conductivity andspecific heat capacity of the absorber fluid. Thus heattransfer between absorber pipe wall and fluid increases with

(a) decreasing operating temperature(b) decreasing concentration of the water–glycol mixture(c) decreasing pipe length(d) increasing pipe diameter(e) increasing pump performance and flow velocity( f ) increasing temperature of the pipe wall.

Heat transfer under turbulent flow conditions depends onRe and Pr, and on the diameter and length of the absorberpipes, but Re number has the greatest influence. Heattransfer between absorber pipe wall and carrier fluid in-creases or decreases qualitatively similarly to laminar condi-tions with one exception: decreasing the operatingtemperatures of the pipe fluid reduces heat transfer.When laminar and turbulent flow conditions are compared

directly, the following conclusions can be drawn.

(a) Heat transfer under turbulent conditions dependssignificantly more on the input parameters. Underlaminar conditions Nu tends rather constant for allparameters.

(b) Increasing the concentration of the water–glycol mix-ture increases the laminar zone, and hence reduces theturbulent zone and therefore heat transfer. If pure wateris used as absorber fluid, turbulence occurs in nearly allcases, and Nu number is about 15 times higher than fora water–glycol mixture. Nevertheless, in most cases ananti-freeze is unavoidable, because temperatures below08C are possible during operation. It should beconsidered that—when using a heat pump—the fluidtemperature at the vaporiser is still about 28C lowerthan the inflow temperature. This involves the danger offreezing/icing in the heat pump.

(c) Under laminar conditions the flow velocity is indepen-dent of the pipe diameter. It depends only on pumpperformance, pump efficiency and pipe length, and onthe flow parameters (kinematic viscosity and density).

Furthermore, it increases with operating temperature,whereas under turbulent conditions the flow velocitytends to be independent of the temperature.

(d) Under laminar conditions the pipe diameter does notinfluence the residence period of the heat carrier fluidwithin the ground heat exchanger (absorber pipes),whereas under turbulence this duration increases withpipe diameter.

(e) In small-diameter pipes and at low operating tempera-tures laminar flow occurs in practically all cases.

( f ) If small-diameter pipes are used, Nu can hardly beincreased by increasing the pump performance. Henceinstalling pumps with higher capacity is then of no use.However, for large-diameter pipes turbulent flowconditions can be achieved rapidly by increasing thepump performance. This also increases the heat transferfrom absorber pipe wall to absorber fluid.

(g) With decreasing pipe diameter the total flow resistancethat has to be overcome by the pump increases, as pipewall friction related to the diameter increases. Fordiameters of d ¼ 2–4 cm the pressure remains nearlyconstant, but for smaller diameters it increases over-proportionally. Furthermore, the pressure increases withreduced operating temperature because the viscosity ofthe fluid increases.

DESIGN ASPECTS OF THERMO-ACTIVE SYSTEMSAND INSTALLATION SCHEMES

Early ecological energy planning for building can in manycases prevent costly refurbishment and renovation in thefuture. High-quality energy design involves not only heatingand cooling (rooms, water) but also lighting, and it requiresa multi-objective optimisation.

An optimised energetic-thermal design should also con-sider the seasonal heat loss from (un-)insulated slab-on-grade floors or basement walls. Far more energy and costsare expended in running an inefficiently laid out buildingthan in constructing an efficient one. A proper design shouldconsider the efficiency of the overall building process,including the sustainability of all elements.

Proper design of energy foundations for a complex thermo-active system requires detailed ground investigation and—atleast for large projects—a numerical simulation of the entiresystem, including the secondary energy system within thebuilding. Accordingly, the following parameters should beconsidered.

(a) Hydrogeological ground properties, especially(i) depth and seasonal fluctuation of the groundwater

table(ii) flow direction and velocity of groundwater.

(b) Geotechnical soil properties, especially(i) soil layering(ii) water content(iii) density and void ratio(iv) permeability (saturated and unsaturated hydraulic

conductivity)(v) swelling–shrinking behaviour (if intensive heat

extraction is required)(vi) freezing–thawing behaviour (if intensive heat

extraction is required)(vii) shear parameters and stress–strain behaviour (for

foundation design).(c) Geothermal soil properties, especially

(i) thermal conductivity and heat capacity at specifictemperature levels

(ii) in situ ground temperature(iii) thermal gradient.

96 BRANDL

(d) Mineralogical composition and geochemical soil proper-ties (only in exceptional cases).

(e) Structural details of the building(i) type and size of energy foundations (length, width,

thickness, diameter)(ii) depth of foundation below original ground surface

and basement floor(iii) position, arrangement and spacing of foundation

elements(iv) pile pattern(v) method of installation of foundation, construction

sequence (with regard to absorber pipeworkinstallation)

(vi) details of reinforcement, concrete properties ofenergy foundations.

( f ) Installation details of geothermal heating/cooling sys-tem(i) available space for connecting lines(ii) position of header block (distributor/collector)(iii) position of heat pump and technical service centre.

(g) Building physics(i) insulation thickness of roof, walls floors(ii) size and quality of windows(iii) location and design of staircase (closed/open)(iv) design of heat bridges(v) temperature conditions in primary and secondary

energy circuits.(h) Climate conditions, energy concept, and optional

concept(i) monthly heating/cooling demand and peak de-

mands within building(ii) temperature conditions in primary and secondary

energy circuits(iii) type of heating/cooling system in building(iv) type/mixture and velocity of circulating fluid (heat

carrier fluid) within energy system(v) heating/cooling intervals, operation plan.

Most parameters are widely interacting. For instance, theinstallation of pipe coils within energy piles can mean thatthe most cost-efficient piling technique for the particularground conditions may not be possible, and advice must besought from an experienced piling contractor at the begin-ning of the design process.

In practice it is frequently sufficient to supplement thestandard site investigation with empirically establishedgeothermal values as shown in Fig. 15. However, detailedgeothermal investigation is recommended for large, complexprojects.

For general feasibility studies and pre-design of energyfoundations the following assumptions can be made regard-ing the energy volume that can be extracted from thermo-active energy foundations

(a) pile foundations with piles D ¼ 0.3–0.5 m: 40–60 Wper metre run

(b) pile foundations with piles D > 0.6 m: 35 W per m2

earth-contact area(c) diaphragm walls, pile walls (fully embedding the soil):

30 W per m2 earth-contact area(d) base slabs: 10–30 W per m2 earth-contact area.

The heat that can be extracted from or fed into/stored inthe ground depends on the maximum possible heat fluxdensity in the absorber pipe system. There, the heat transportoccurs by forced convection of the fluid (usually an anti-freeze–water mixture). In order to optimise the absorberpipe system the following parameters have to be considered

(a) diameter and length of pipes

(b) properties of pipe wall (roughness)(c) heat conductivity, specific heat capacity, density and

viscosity of fluid circulating in absorber pipes(d) flow velocity and flow conditions (laminar-turbulent)

within absorber pipes.

Figure 25 gives a schematic overview of the heat transportwithin a thermo-active system consisting of energy piles. Itillustrates that the heat flux _QQprim transported by heat carrierfluid in the primary circuit is given by the specific heatcapacity cprim, the mass flow _mmprim and the temperaturedifference ˜Tprim:

Complex ground properties and pile groups require nu-merical modelling of the geothermal heating/cooling system.Fig. 26 shows for example the temperature distribution with-in a section of a piled raft foundation during the winter andsummer maxima.

Figure 27 shows the daily mean temperatures in Viennafor the year 2001. Such data are needed to design aheating–cooling system whereby it is assumed that heatingtypically starts at external temperatures lower than 128C.This provides the heating period for the unsteady numericalmodels. Fig. 28 illustrates that the seasonal course of the airtemperature can be simulated by a sinusoidal curve accord-ing to the following equation

TGS ¼ Tm,out þ ˜Tout cos2�

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where TGS is the ground surface temperature, t is time, Tm,out

is the average yearly temperature, ˜Tout is the temperatureamplitude, P is the duration period, and �t is the phasedisplacement.

In the end, the monthly heating and cooling demands haveto be compared with the available output, as indicated inFig. 29 (and Fig. 60). Moreover, the seasonal course of theabsorber fluid temperature (heat carrier fluid temperature)should be predicted.

Usually, a numerical simulation of the geothermal systemis recommended for buildings with a heating and coolingdemand of more than 50 kW. This rough value decreases toabout 20 kW for buildings where rooms have to be cooledthroughout the year. Geometric simplification may lead tosignificant errors in heat calculation. Therefore three-dimen-sional analyses should be conducted. The simulation shouldcomprise the expected inflow and outflow temperatures atthe energy foundations and the temperature distribution inthe ground. Numerical models and computer programsshould be reliably calibrated—that is, on the basis of long-term measurements and experience from other sites, and onphysical plausibility—otherwise wrong results may begained, even from well-known suppliers. Experience hasshown that the results are very sensitive to even smallchanges in the finite element mesh. Consequently, the im-portance of numerical simulations lies rather in parametricstudies (to investigate the influence of specific parameters)than in gaining ‘exact’ quantitative results.

Calculation of the temperature distribution in the grounddue to energy foundations is increasingly being demandedby local authorities for environmental risk assessment. Thisrefers mainly to possible influences on adjacent groundproperties and on the groundwater by the long-term opera-tion of thermo-active deep foundations.

Proper geothermal energy utilisation requires an interdisci-plinary design, especially in the case of houses. The geo-technical engineer, architect, building equipment (sanitation)designer and installer, heating engineer and specialisedplumber should cooperate as early as possible to create themost economical energy system. However, the tender for


construction should clearly specify the individual perfor-mances on the site. It has proved suitable to entrust thegeothermally experienced plumber with all details of theprimary and secondary circuits, beginning with the mountingof the absorber pipe systems in the foundation elements.Figure 30 shows an installation scheme for heating with

energy foundations (piles or diaphragm wall elements) and aheat pump comprising the primary energy circuit (in theground) and the secondary energy circuit (in the building).Reversible heat pumps can achieve heating and cooling. Fig.31 illustrates an installation scheme for ‘free cooling’. Inthis case the necessary energy input is limited to theelectricity required to operate a circulation pump, wherebyabout 1 kWh of electricity is needed to obtain up to 50 kWhof cooling energy. The same fluid, which cools as it passesthrough the absorber loops in the foundation, is pumpedthrough the cooling system of the building. Coolness can betransferred into the building and distributed there by aceiling, floor, or wall cooling system.

PILOT RESEARCH PROJECT A: HEATING ANDCOOLING OF A REHABILITATION CENTREProjectFigure 32 shows the ground plan of a large rehabilitation

centre, with areas A to J, and Fig. 33 depicts area F, wheremost of the energy piles were installed. The building has a

volume of 90 000 m3, a usable area of 21 500 m2, plus aswimming hall, a fitness centre, etc. It comprises sevenfloors, two of them beneath the ground surface, and it had tobe constructed on a slide-prone, unstable slope. Extensiveground investigations during the design stage revealed thatthe operating costs of this centre could be minimised byextracting/sinking geothermal energy for heating/cooling andby using groundwater from the subsoil for non-drinkingpurposes.

The foundation work was performed in the years 1994–1995. The bored piles were excavated with a casing, andtheir structural integrity was checked by dynamic testing(TNO). In all 175 bored piles (D ¼ 1.2 m) were installed,having three functions:

(a) foundation of the statically critical area F(b) retaining structures for the slide-prone slope(c) retaining walls for the 14 m deep excavation pit.

Owing to the sloped surface of the building area and thedeep-seated collector galleries, the piles had to be installedat rather different levels (Fig. 34). The pile depth variedaccording to static requirements and local soil character-istics: 9–11 m for the foundation piles and 9–18 m (meanvalue 14 m) for the retaining piles. Most of the retainingstructures had to be tied back with prestressed anchors (Fig.34).

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98 BRANDL

Of the piles, 143 are fitted with heat exchangers andhence act as energy piles, as indicated in Fig. 33. The pilesof the retaining structures/walls also form a part of theheating/cooling system, as shown in Fig. 34. The relevanttechnical data are as follows

(a) 2050 m run of energy piles(b) HDPE absorber pipes: do ¼ 25 mm, di ¼ 20 mm(c) 260–290 m absorber pipes per pile(d) absorber fluid: 70% water, 30% antifreeze L(e) required absorber fluid passage: 77.4 m3/h( f ) condensation capacity of heat pump: 270 kW(g) minimum absorber fluid inflow temperature: +18C(h) minimum absorber fluid return flow temperature: +48C.

Some 40 000 m of polyethylene piping were fitted in thereinforcement cages of the piles. Throughout the construc-tion period the absorber units were pressurised at 8 bar sothat any defects could be detected immediately. The pipesleading from the piles to the distributors were placedbeneath the raft of the building and behind the pile walls.

Ground propertiesThe subsoil consists of weathered talus (mainly loam)

down to 2–5 m below original ground level. It varies fromsilty sand to clayey silt, and is underlain by tertiary sedi-ments without a clear interface. The piles are fully em-bedded in these sediments, which are weathered (oxidised)

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and locally severely jointed to a depth of 10–15 m. Theyconsist predominantly of clayey to sandy silt, with a naturalwater content near the plasticity limit. The main soil charac-teristics are (rounded)

(a) natural water content wn ¼ 25–40%(b) plasticity index Ip ¼ (0) 10–35%(c) density rd ¼ 1.3–1.7 g/cm3

(d) moduli (oedometric) E ¼ 5–35 MN/m2

( f ) friction angle � ¼ 25–358

(g) residual angle �r > 108(h) compressive strength qu ¼ 20–500 kN/m2.

The density and strength-deformation properties of the soilimprove with depth. The permeability coefficient k of un-disturbed silty soil samples varied between 10�8 and10�10 m/s. The overall hydraulic conductivity was muchhigher owing to joints and sandy interlayers, generally aboutk ¼ 5 3 10�7 m/s and locally k ¼ 10�4 m/s.

The mineral composition of the fines (silt–clay) varied asfollows: mica group, 20–30%; montmorillonite, 10–30%;quartz, 10–20%; calcite, 10–20%; chlorite, 5–10%; kaoli-nite, 5–10%.

Groundwater was found 4–5 m below the original surface,though not as a continuous aquifer but locally in sandyinterlayers and joints. Accordingly, predominantly localwater ingress occurred during piling and excavation of the14 m deep pit. The maximum was about 20 l/s during theconstruction stage. This groundwater situation required adrainage blanket beneath the (piled) raft foundation. Long-term monitoring disclosed a steady state of about q ¼ 7 l/sas a mean value over the year. This is that portion ofgroundwater from the slope that is collected in the drainagesystems along the retaining structures and beneath thefoundation raft. It is used to a high percentage for therehabilitation centre’s non-drinking water purposes.

MeasurementsThe (long-term) stability of the retaining structures has

been monitored since the initial construction period byinclinometers in the piles and by pressure cells on theanchor heads. The pipes of the absorber system have beenmonitored with pressure gauges. One load-bearing pile ofthe piled raft foundation was fitted with the following meas-uring devices

(a) pressure cells on pile toe and head(b) fissuremeters at three levels(c) thermo-elements at five levels.

The aim of the measuring pile (Fig. 33) was to investigatethe effects of temperature changes within an energy pile onits bearing capacity, and especially on its shaft resistanceduring hydration and subsequent energy extraction. More-over, the influence of natural temperature fluctuation in theground should be monitored.

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Fig. 27. Mean daily outdoor temperatures in Vienna, 2001. Iftemperature drops below 128C, room heating is usuallynecessary

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100 BRANDL

The head of the measuring pile is situated in an easilyaccessible place, which facilitates long-term monitoring. Thepile length is 9 m and the calculated total load Q ¼2000 kN. In fact, up to now the maximum theoretical lifeload has not yet been imposed, so that the measured headloads are clearly smaller. Moreover, part of the load istransferred directly from the piled raft into the subsoil.

The first testing phase was conducted during the winter of1996/1997 and provided initial data that could be used tooptimise the absorber system. The rehabilitation centre wasopened in late 1997, and the energy piles have been in fulloperation for the floor heating since then.

Figure 35 shows the load changes on the head and toe ofthe measuring pile against time. The rough structure of thebuilding was completed in the autumn of 1996, providingabout 90% of the dead load. The initial results illustrate thatresidual stresses were imposed on the pile before any load-ing, caused by heat development in the fresh pile concretedue to hydration: thermal contraction after peak temperaturecaused a temporary reduction of the base pressure in the piletoe. To a certain extent this continued for a short period and,simultaneously, shaft friction was increasingly mobilised.Over a long-term period the point load in the base of thepile, Qb, remained constant, independently of the increase in

the total load and temperature variations within and aroundthe energy pile. This proved to be the first evidence that aproper operating of energy piles has no relevant influence onthe shaft resistance, as could then be confirmed on severalother sites and by numerical modelling.

In Fig. 36 the temperature fluctuations during the firstyears are plotted against pile depths. The first measurementwas performed during the hydration phase of the pile con-crete and showed a temperature of up to 608C. The secondmeasurement (27 September 1995) provided typical summerresults and the third one (12 February 1996) winter results,whereby the measuring pile was already loaded by fourfloors. At that time, only the concrete structure was underconstruction, and no temperature insulation had been in-stalled. This explains the value of 08C on the pile head.Measurement No. 4 was taken during a test phase ofoperating the energy piles, after the structure was finished atthat time. The measurement on 9 February 1998 was takenin the first winter period during excessive heat extraction fortesting purposes. The minimum pile temperature was closeto the allowable limit of +28C.

Since autumn 1997 the energy piles have been under fulloperation. Long-term monitoring has revealed typical seaso-nal temperature fluctuations with relatively large amplitudes

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Fig. 30. Heating of buildings with energy foundations and heat pump: installation scheme with primary andsecondary circuit. With reverse heat pumps this energy system can also be used for cooling


of maximum and minimum pile temperatures in summer andwinter, as intended for this pilot project. Consequently, thepile temperatures in the subsequent winter were somewhathigher than in the quasi-steady state before. Moreover, Fig.37 shows that daily outdoor mean values are needed for areliable interpretation. Single outdoor temperature measure-ments parallel to pile temperature measurements are notsufficient (indicated in the diagram from April 2004 toDecember 2005).

In Fig. 38 some temperature curves are selected asexamples of heating and cooling periods in relevant years.Immediately after commencing, continuous operation of thegeothermal system started (with excessive heating); the year2003 had a very high cooling demand, and there was aquasi-steady state in the year 2004–2005.

Figure 39 shows the total strains, comprising load-inducedand temperature-induced parts. The load influence dominatesin the upper zone of the pile and diminishes toward the piletoe. Consequently, the shaft resistance in the lower part ofthe pile is quite small. The skin friction distribution along

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Fig. 31. ‘Free cooling’ of buildings with energy foundations and circulation pump: installation schemewith primary and secondary circuit

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Fig. 32. Ground plan of the rehabilitation centre at BadSchallerbach, Austria, with foundation areas A to J and partof the retaining walls, fitted with energy piles. Area F rests on apiled raft foundation; the other areas are founded only on rafts

102 BRANDL

the pile shaft was back-calculated from fissuremeter meas-urement data by approximately assuming Hooke’s law ofelasticity.

Between the retaining structure and building an accessi-ble gap has remained. It avoids horizontal forces from thecreeping slope onto the building and enables long-termmonitoring of the tied-back retaining wall (including ther-mal effects). Thus the influence of excessive heat extraction

from the ground could be clearly observed along sections1–1 of the piled retaining structure (Fig. 33). Operationaltemperatures between �28C and �38C (temporarily even�58C) caused the formation of ice lenses in the groundand thus a frost heave, H, of the surface behind the piles.The maximum of H ¼ 15 cm was observed near thecollection shaft, and it decreased with distance from theheat extraction sources, similar to Fig. 11. After stopping

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this improper test run of the energy system, the tempera-tures increased again, also favoured by warmer weather.Figs 40 and 41 show the temperature profiles along thedepth of two piles and in the subsoil behind the pile wall.A comparison of the temperature profiles along the pileembedment illustrates the significant influence of thegroundwater flow, which is much stronger around pile No.29 than No. 69. Also clearly visible are the influence of airtemperature above the embedment of the piles and the

temperature decrease in the soil behind the retaining struc-ture. The deep temperatures were caused partly by the lowair temperature in the winter and partly by the low opera-tion temperature in the energy piles.

Hydration of fresh pile concreteMeasurements within the research programme for deep

energy foundations have also shown that the hydration of

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pile or diaphragm wall concrete causes a heat development,which reaches its maximum within 15–24 h after casting theconcrete. In large-diameter cast in situ piles, temperatures upto 708C develop. The subsequent decrease of concrete tem-perature can last from several days up to one month, untilthe ground temperature is reached. Temperature increase anddecrease depend on the thermal and hydrological propertiesof the surrounding ground, and on the diameter of the pile.Furthermore, they are strongly influenced by the pile con-crete with regard to its composition, sort and grindingfineness of cement, and additives (e.g. plasticiser, retarders).

Hydration of the cast in situ deep foundation may cause atemporary decrease of the base pressure due to thermalcontraction of the young concrete. Increasing external loadsthen lead to an increase of the base pressure once more.Shrinkage due to the hydration of in situ cast concrete pilesmay cause thermal strain-induced cracking. Therefore ten-sion piles should have sufficient reinforcement.

Figure 42 illustrates the alteration of the shaft resistancein the initial phase of a large-diameter bored pile. Whencasting the concrete, hydrostatic stress conditions exist along

the pile depth, and the shaft resistance is practically negli-gible (Fig. 42(a)). The following hydration of the youngconcrete causes shrinkage, and hence a contraction of thepile. In homogeneous ground this leads to a distribution ofskin friction according to Fig. 42(b), if the pile weight isneglected. Superposing the effects of shrinkage and pileweight leads to a reduction of negative skin friction alongthe lower part of the pile and to an increase in positive skinfriction along the upper part (Fig. 42(c)). Fig. 42(c) gradu-ally develops into Fig. 42(d) when external loads are trans-ferred into the pile.

In Fig. 42 the vertical displacement of the pile head andthe movement of the pile base are also sketched, on anenlarged scale. When comparing Figs 42(a), (b) and (c),those measuring results become clear, which show a tempor-ary decrease of the base pressure shortly after casting thepile, and then a gradual increase due to external loads (e.g.Fig. 35).

The time–base pressure curve of a cast in situ concretepile that exhibits strong thermally induced contraction isshown schematically in Fig. 43. This scheme depends notonly on physical and thermal concrete and ground propertiesand on external loads, but also on the slenderness of thepiles.

PILOT RESEARCH PROJECT B: FIRST THERMO-ACTIVE TRAFFIC TUNNEL (‘ENERGY TUNNEL’)Overview

To upgrade the railway line between Vienna and westernEurope for four-track operation, a tunnel through the north-ern Vienna woods, a hilly landscape consisting of flysch andmolasse formations, is being built. The core section is the12.8 km long Lainzer Tunnel, which partly also serves aslarge energy-absorbing tube.

A tunnel activates a significantly larger quantity of usablegeothermal heat than deep foundations. The energy can beused for heating and/or cooling of railway stations, adminis-tration and residential buildings, and for keeping platforms,bridges, passages etc. free from ice in winter. Consequently,shallow tunnels (especially in cut and cover) make a widerapplication possible than deep-seated tunnels, because theheat transfer between source and user is easier. However,deep-seated road or railway tunnels may be used simulta-neously for heat transport if hot groundwater occurs. Butthis represents another technology, completely different fromthermo-active tunnel linings based on energy exchange bymeans of earth-contact structures.

The Lainzer Tunnel has been constructed in severalsections and by different methods

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(a) cut and cover, consisting of large-diameter bored piles,reinforced concrete base slab and roof

(b) New Austrian Tunnelling Method (NATM), with aprimary support of reinforced shotcrete, rockbolts andanchors, and a secondary lining of reinforced concrete.

To optimise the energy design of the tunnel, and forresearch purposes, the following geothermal projects werecarried out.

(a) Energy plant: LT24, Hadersdorf–Weidlingau. The sec-tion LT24 was selected as a test plant, to investigateboth the technical and the economic aspects. In thissection the tunnel was constructed using the cut andcover method, allowing the application of alreadyproven absorber techniques combined with structuralengineering.

(b) Energy well: Hetzendorferstraße. Another test plant wasconstructed in order to investigate the performance ofenergy wells that were simultaneously used for loweringthe groundwater level.

(c) Energy membrane: LT22, Bierhauslberg. The sectionLT22 is located in an NATM section, where the provensystem of attaching absorber pipes to the reinforcement

cages of piles, diaphragm walls or base slabs could notbe used. Therefore a completely new technology had tobe developed.

Cut and cover tunnelIn section LT24 the primary side wall lining of the tunnel

consists of bored piles, whereby each third pile is used as anenergy pile (Figs 44 and 45). Thus the energy plant LT24:Hadersdorf–Weidlingau comprises 59 bored piles with adiameter of 1.2 m and an average pile length of about17.1 m. The intermittent pile wall exhibits jet-grouting col-umns between the piles. Pile excavation (by grab) wassupported by casings using rotating equipment. The energypiles are equipped with absorber pipes connected to collec-tor/distributors, which are located at a central point of thetunnel. The pipes leading from the piles to the collector/distributors are placed alongside the cover of the tunnel. Theconnecting pipes lead into a collector/distributor room thatis easily accessible on top of the cut and cover tunnel. Fig.46 shows the header block with the collector/distributor forall collecting pipes. The manometers allow a detailed water-tightness check of all absorber pipes. A manifold with adiameter of 150 mm connects the collector/distributors withheat pumps in an adjacent school in order to heat thebuilding. Table 4 gives the relevant technical data.

Preliminary calculations yielded an extractable thermalpower of about 150 kW in the long term. In one heatingperiod an energy amount of 214 MWh can be gained.Furthermore, the benefits of this new energy concept are thatit is both environmentally friendly and economical: thereduction of natural gas of 34 000 m3 per year leads to adecrease of annual CO2 emissions of 30 t. Furthermore,annual savings in operation costs of A10 000 will beachieved, compared with the old natural gas heating systemof the school building.

The plant was constructed as a demonstration project inthe context of a major research initiative by the Austriangovernment. Because of this scientific background, the plantis intensively instrumented with measurement devices. Sixenergy piles are fitted with 18 temperature gauges at differ-ent levels; additionally, one pile is fitted with combinedstrain–temperature gauges at five levels for measuringstrains and temperature. The aim of this measuring system is

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Fig. 44. Schematic cross-section of energy plant LT24, Hadersdorf–Weidlingau. One sidewall of cut-and-cover tunnel used as energy wall

106 BRANDL

to investigate the effects of temperature changes within anenergy pile on its bearing capacity and the temperaturefluctuation in the energy piles during operation. Moreover,heat carrier fluid passage, total extracted heat, and tempera-tures in the manifold are monitored. The groundwater tem-

perature surrounding the energy plant is also registered (atdifferent distances). Temperature differences between energypiles (thermo-active and bearing function) and standard piles(bearing function only) have been checked by heat picturephotographs. The differences can be registered even after theplacement of the secondary lining, that is, the reinforcedconcrete cover (Fig. 47).

The hydration heat caused by concreting the secondarytunnel lining could be clearly observed in the energy piles.Furthermore, the base slab of the tunnel had a significanttemperature separation function (Fig. 48). Fig. 48 also showsthat—even without energy operation—temperature differ-ences exist between the pile head and toe, and between theinner and outer side of the pile skin.

The operation of the energy plant started in February2004, and during the first testing phase initial data wereobtained that could be used to optimise the absorber system.About 40 MWh of heating energy could be extracted fromthe energy piles during the first six weeks of operation.Since autumn 2004 the energy system has run permanentlyfor a school near the tunnel. The external air temperature(outdoor temperature) is used as criterion for regulating theenergy system. Down to �58C the school building can befully heated with ground source energy. At lower tempera-tures the existing gas boiler furnace is added.

Figure 49 shows for example the specific strain in meas-urement pile S-7-20 at different times plotted for the innerand outer side and the central axis. The zero reading wasbefore soil excavation in front of the pile wall. Therefore thecurves include both mechanical and thermal effects, as canbe clearly seen from the seasonal differences. The tempera-

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Fig. 45. Detail of Fig. 44: longitudinal section through energytunnel wall. The primary lining consists of bored piles with jet-grouted columns in between and is connected to the watertightsecondary lining by dowels. Every third pile is equipped withabsorber pipes that are situated behind reinforcement bars andthus protected from damage through dowel installation for theinner lining (reinforced concrete panels). Also shown is thelocation of measurement instruments in pile S-07-20

Fig. 46. Header block to Fig. 44 where the connecting lines andmanifolds are collected (see also Fig. 9)

Table 4. Technical data of the energy plant LT24: Hadersdorf–Weidlingau

Annual heating output: MWh 214Energy piles 59 pilesMean length: m 17.1Heating capacity: kW 150Required antifreeze passage: m3/h 51.6Absorber pipes HDPE,

do ¼ 25 mm,di ¼ 20 mm

Absorber circuits 80 unitsAbsorber pipes in piles, total length: m 9709Connecting lines, total length: m 13 754

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ture-induced deformations are significantly smaller thanthose caused by earth pressure, and the natural fluctuation ofthe tunnel temperature has a greater influence than thetemperature changes due to energy extraction/storage in theenergy piles. Energy operation even creates a more uniformtemperature in the piles, that is, smaller temperature differ-ences between pile head/toe and inner/outer side. Thisthermal balancing-out reduces the temperature-induced mo-ments in the energy piles. Of course, the energy operationcauses a stronger cooling or heating of the piles, but thisoccurs uniformly, and hence causes a volumetric deformation

without constraints, and therefore no additional load on thestructure.

NATM sectionThe utilisation of tunnels excavated according to the New

Austrian Tunnelling Method or sprayed concrete method forheat extraction/storage requires special absorber elements.The construction sequences make the installation of contin-uous absorber pipes in the longitudinal direction rathercomplicated.

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In principle, three structural elements can be used forthermal energy extraction/storage (i.e. heating/cooling;Fig. 50)

(a) energy anchors or nails that thermally activate thesurrounding soil or rock

(b) energy geosynthetics that make use of the groundaround the tunnel circumference—mainly non-wovengeotextiles and geocomposites (Fig. 51), but also somegeomembranes

(c) thermo-active secondary lining (inner reinforced con-crete).

PILOT RESEARCH PROJECT C: COOLING ANDHEATING OF METRO STATIONS

Until now the rooms of stations on the Vienna Metro haveusually been actively air-conditioned (monitoring rooms,transformer rooms, switchrooms, storage rooms, shops etc.).District heating or electric current serve for heating, andcooling is achieved by electric refrigeration systems. Gas isexcluded for safety reasons. The extension of the Metro lineU2 offered the chance to supply four stations with geother-mal energy from earth-contact structures for both heatingand cooling purposes. These projects represent the first full-scale application of the thermo-active technology in metroengineering worldwide. Hence comprehensive measurementand monitoring systems were installed. They serve foroptimisation and quality assurance (similar to the observa-tional method), for maintenance control and for furtherresearch.

The design was based on previous experience withgeothermal energy systems. The stations were constructed bythe cut and cover method. Consequently, absorber pipes aresituated in diaphragm walls, in bottom slabs and betweenthe primary and secondary lining of the station tunnels.Feasibility studies, including numerical analyses of tempera-

ture flow in the ground, revealed that geothermal energyextraction/storage would have only a very limited influenceon the soil close to the absorber elements (i.e. within a fewmetres). Unfavourable thermal effects within the surroundingsoil and groundwater will not occur. Therefore the projectpassed all legal proceedings regarding environmental impact,compatibility and risk assessment without problems, and itwas greatly supported by public opinion.

The first station was U2/2-Taborstraße, where significantwaste heat is created, causing very high room temperaturesall the year round. Therefore a geothermal cooling systeminvolving 1865 m2 of energy diaphragm wall and 1640 m2

of energy base slab was designed. If the surplus energycannot be used for heating, it is transferred into the soil viathe absorber system, thus avoiding noisy or unsightly out-door cooling towers. Thermal simulations revealed that theheat carrier fluid will have temperatures between 108C and288C. A total fluid volume (water–glycol mixture) of about10 m3 circulates in the absorber system, providing a maxi-mum cooling capacity of 81 kW.

In the design the following energy demand of the Metrostation was assessed

(a) maximal heating 95 kW(b) maximal cooling 67 kW(c) average yearly heating energy 175 MWh(d) average yearly cooling energy 437 MWh.

Figure 52 shows the cooling capacity–time histogram andthe seasonal fluid temperature in the absorber pipes. It showsthat sufficient energy (heat) can be transferred into theground to achieve yearly cooling of the entire Metro station.

Figure 53 gives a section of the Metro station, and alsoindicates the ground profile and the instrumentation in ameasuring panel of the diaphragm wall. The right wallserves for heating, and hence for energy extraction from theground. The standard measuring programme for all Metrostations with geothermal heating/cooling systems comprisestemperature sensors installed in the diaphragm walls, in thetunnel tubes, and 5.0 m below the bottom slabs. Thesedevices serve not only for quality control and safety assess-ment but also for regulating, controlling and optimising theoperation of the geothermal system.

In addition to this standard equipment, several measuringdevices were installed to investigate the thermal influence on

Fig. 50. Scheme of energy tunnel excavated with the NewAustrian Tunnelling Method

Fig. 51. Energy geotextile (geocomposite) installed in an energytunnel (testing plant LT22 – Bierhauselberg)


the load–deformation behaviour of the underground struc-ture, and especially of diaphragm wall panels. The measur-ing panel in Fig. 53, for instance, was equipped with

(a) 21 strain gauges to register tension and compressionstrains and temperature along the front and rear zone ofthe diaphragm wall on the supporting points and in themiddle between them

(b) chain extensometers consisting of seven segments tomeasure the longitudinal deformations between theindividual support points and below the bottom slab,down to the toe of the diaphragm wall

(c) temperature sensors to measure the temperature oneither side of the diaphragm wall

(d) temperature sensors to monitor inflow and return-flowtemperatures of both absorber circuits.

Already it can be concluded that geothermal absorber

systems for cooling and heating Metro stations and linesrepresent an innovation that is not only environmentallyfriendly but also highly economical.

FURTHER CASE HISTORIESCooling and heating of a paper-processing plant

This case history refers to the piled raft foundation of apaper-processing plant where 400 employees are printingdaily 50 000 m2 of paper and other materials. Such plantsrequire special temperature and humidity conditions withintheir production halls. The overall structural volume of thisplant is about 130 000 m2. The subsoil consists of weak silt,underlain by sandy gravel.

The deep foundation comprised groups of two to sixenergy piles in a different pattern according to structuralrequirements; pile spacing within the groups is typically1.4 m. The external walls are founded on single piles at

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variable spacing of 1.4–4.0 m. In all 570 driven reinforcedconcrete piles (0.4 m 3 0.4 m) were installed, thus thermallyactivating a heat storage volume of about 100 000 m3 in theground. The plant was opened in autumn 1995, and theenergy system has been running without problems since thattime.

Cooling and heating are based on multiple energy systemscomprising not only the energy piles but also cooling withnear-surface earth collectors and—to cover peaks—an icestorage refrigeration machine. For heating the building, atfirst the waste heat from compressors and from the printingmachine cooling system is used, then the energy piles andthe heat pump are activated, and, finally, only peaks arecovered by fossil fuel (oil) heating.

Figure 54 shows the temperature fluctuation in the subsoil

before the energy system was installed and after its opera-tion was started. It shows clearly that the increase due tocooling exhibits its maximum in the centre of the buildingbut decreases with depth.

Figure 55 shows the fluctuation of used and generatedenergy during the first heating period in winter 1995/1996.The diagram clearly shows that the portion of ground-extracted energy using the absorber piles prevails. In all,95 000 absorber pipes (d ¼ 20 mm) provide 520 kW of cool-ing and heating. They have been positioned in the top 14 mof the 24 m long piles.

Cooling of rooms, structural elements and machines inthis industrial plant also requires a relatively high amount ofenergy. In comparison to Fig. 55, Fig. 56 shows the coolingenergy generated within the same period. The portion of

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Fig. 54. Soil temperature against time at three levels (5.0, 10.0, 14.5 m) below energy pileheads (piled raft foundation); location of temperature sensors within ground area of thebuilding and among the piles as parameter: (a) in pile groups, 1 m from centre piles; (b)between pile groups, 3.5 m from centre piles; (c) 0.5 m next to pile at building’s edge; (d)between two piles, 1.5 m to building’s edge


heat pumps/absorber piles predominates, followed by coolingwith near-surface earth collectors if the ground temperatureis not too low. Hence the energy needed from refrigerationmachines could be reduced to a negligible minimum.The geothermal energy system has been running now for

10 years without any problem. The pile temperature fluctu-ates with a steady envelope corresponding to the maximumand minimum values already registered during the first twoyears of operation. Hence no relevant change of the ground-water properties has occurred.

Heating and cooling of an arts centreThe second case history refers to an example with energy

piles and diaphragm walls, both installed for the foundationof the Arts Centre in Bregenz, Austria. The building has astructural volume of 28 000 m3 and an area of 33 500 m2. Itsconstruction required an excavation depth of up to 11 m.The foundation elements comprised a perimeter diaphragmwall of 0.5 m, 0.9 m, and 1.2 m thickness around the excava-tion pit, and diaphragm wall panels (barrettes) and piles of1.2 m diameter. The wall depth reached up to 28 m, and thepile depth varied between 17 m and 25 m.The subsoil consisted of 3.5 m of near-surface gravel

covering loose sands and weak clays to a depth of about21 m below ground. These young sediments were underlainby a moraine and finally by rock. Hence the foundationswere designed as end-bearing elements. The groundwaterlevel was about 1 m below surface.The building consists of a cast concrete structure, incor-

porating piping loops for heating in the winter and coolingin the summer to provide a comfortable room climate.Moreover, the sensitive art collection makes an optimal

hygrothermal behaviour of the building necessary. Theallowable variations in daily and long-term temperature andhumidity are very small, < �28C and �3% respectively.

The thermal energy for conditioning is extracted from theconcrete absorber system integrated in the diaphragm walls,which serve as retaining structure for the excavation pit andas perimeter foundation of the building (Fig. 57). In total24 000 m of DN 25/2.3 mm PE pipes were installed within4 500 m3 of diaphragm wall concrete, forming 249 units,each approximately 100 m in length, which are connected toflow and return circuits placed around the building next tothe diaphragm wall. The absorber units were mounted inloops within the reinforcing cages of the diaphragm wallpanels and were then lowered into the slurry trenches beforethe concrete was cast. Throughout the construction phase theabsorber units were pressurised at 8 bar and monitored, sothat any pipe failure could be detected immediately.

A total water volume of 26 m3 circulates in the absorbersystem and provides a maximum cooling capacity of120 kW, owing to the heat exchange between the absorberunits in the diaphragm walls and the ground (i.e. primarycircuit). The secondary circuit within the building comprisessome 30 000 m of 16/2 mm piping positioned in the concretefloors and walls. Moreover, the solar energy entering throughthe windows into the concrete of the structure is transferredinto the diaphragm walls and stored there (and in thesurrounding soil).

The benefits of this energy concept for the Bregenz ArtsCentre are both environmental and economical. The savingin investment costs was A1.32 million, and the annual sav-ings in energy and operation costs are A22 700 comparedwith a conventional air-conditioning system. Furthermore,excessive energy can be temporarily stored in the ground forshort periods.

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112 BRANDL

Heating and cooling of a multipurpose hall and of a spahotel

A multipurpose hall with a capacity of 8 000 persons wasdesigned for exhibitions and fairs and as a sports hall,especially as an ice rink. The latter required intensive cool-ing and temporary heating. The complex energy manage-ment could be solved with energy piles, because piles werealready needed for a deep foundation of the structure restingon weak clays. The deep foundation comprises 320 cast insitu concrete piles (bored piles, d ¼ 0.5 m) 18 m long. Thepiles contain in total about 65 km absorber pipes (HDPE; d¼ 25 mm). This cooling/heating system provides an annualsaving of 85 000 m3 of natural gas, which is equivalent to anenvironmental relief of 73 t of CO2.

In the same region a 43 m high spa hotel with geothermalheating and cooling was built. The core of the spa centrecomprises four floors with 6500 m2 of spa and fitness zones,and a 2000 m2 bath and sauna area. The energy foundationconsists of 357 auger piles, 30 m long, and includes69 000 m of plastic pipes. Groundwater temperature is con-stant at 128C. Primary and secondary energy circuits areconnected by a 400 kW heat pump. In winter 1.6 GWh areextracted from the ground, corresponding to the energydemand of about 160 modern one-family houses. About thesame heat volume is then sunk back into the ground whencooling the building in summer.

Heating and cooling of Keble College in OxfordThe first energy pile project in the United Kingdom

started in 2001, using Austrian geothermal piling technology.It is a new building for Keble College at Oxford University,a six-storey structure including a basement up to 7 m belowexisting ground level, providing a new lecture theatre, teach-ing rooms and study bedrooms. The soil profile can beroughly summarised as follows: made ground (3 m); firmalluvial clay (1 m); medium dense Thames river deposits(3.5 m); very stiff to hard Oxford clay (below). There is aperched groundwater table in the upper part of the riverdeposits. According to Figs 58 and 59 foundation piles (d ¼0.75 m and 0.45 m) and secant piles (d ¼ 0.6 m) of theretaining wall for the excavation pit were designed as energypiles fitted with absorber pipes.

Because of an alternative bid, the retaining wall wasexecuted as a hard/soft secant pile wall, eventually compris-ing three pile types (Suckling & Smith, 2002): 115 primarypiles (D ¼ 0.60 m, l ¼ 9 m); 53 secondary hard piles (D ¼0.75 m, l ¼ 15 m) in areas of higher ground retention; and55 secondary hard piles (D ¼ 0.60 m, l ¼ 12 m) in areas oflower ground retention. Moreover, 61 bearing piles (D ¼0.45 m, l ¼ 12 m) were installed within the basement to takethe structural loads.

Thus the original pile design was somewhat modified bythe contractor, but only with regard to the required structuralor geotechnical applied loads. The amount of pipeworkrequired within the piles was designed to fit within thestructural requirements. Hence no pile diameter or length wasincreased to suit the geothermal requirements above thatrequired for the applied loads. Installation details are givenby Suckling & Smith (2002). Piping comprises 6 500 m,whereby 5 200 m of absorber pipes were installed in the piles.

The heating load of the building is 85 kW and the coolingload 65 kW. Thus the monthly costs for heating and coolingcan be fully covered by this geothermal system (Fig. 60).

Heating and cooling of office centres in ViennaTable 5 gives an overview of some office centres in

Vienna that use energy foundations for heating and cooling.

For comparison, relevant data of the Lainzer Railway Tunnel(pilot research project B) and of the Vienna Metro (pilotresearch project C) are added. All thermo-active energysystems have been running without problems since theirinception.

SPECIAL APPLICATIONSOverview

Subsurface geothermal resources can be widely used forheating and/or cooling all sorts of building and traffic areaby utilising structural elements for energy extraction orstorage

(a) piles, barrettes and diaphragm walls as foundationelements of bridges

(b) shallow foundations(c) retaining walls(d) embankments(e) tunnel linings (especially near the portals).

Special applications are

(a) heating/cooling of multipurpose buildings(b) heating/cooling of bridge decks(c) heating/cooling of road pavements and parking places(d) heating of airport runways(e) ‘energy tunnels’ for heating/cooling of buildings near

tunnel portals( f ) ‘energy wells’ for heating/cooling of buildings near

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groundwater extraction wells (e.g. for temporary orpermanent groundwater lowering)

The Austrian railway authorities plan to use geothermalenergy from thermo-active foundations, retaining walls andtunnels for heating (and cooling) railway stations, platforms,pedestrian ramps and staircases along the new high-speedrailway lines.Heating of turf in sport stadiums is another possibility for

the use of geothermal energy. This is increasingly beingperformed in countries with long, cold winters, but is ratherexpensive because of its low efficiency.

Heating and cooling of bridge decksIn countries with cold winters and hot summers the

heating and cooling of bridge decks (Fig. 61) providesnumerous environmental, technical, and economical advan-tages

(a) keeping the pavement free from ice and snow, thussignificantly reducing traffic hazards for road users

(b) substituting a clean, renewable energy for gritting andthe use of de-icing salt

(c) reducing the temperature-induced rutting of asphaltpavements caused by dense, heavy traffic

(d) reducing temperature constraints in bridge decks, whichincreases the service life of the superstructure andpavement

(e) reducing maintenance costs( f ) reducing environmental impacts.

Heating and cooling of road pavementsGeothermal technology in road engineering refers mainly

to the heating of pavements during the winter months, andcomprises the following goals

(a) road surface free from ice, and hence increased trafficsafety

(b) reduced winter road clearance(c) increased environmental protection, because salt or grit

for icy roads is not necessary(d) increased lifetime of the road pavement/surface(e) increased traffic comfort (no fitting of snow chains)( f ) minimisation of freeze–thaw damage to the road

structure, especially in the case of frost-susceptiblesub-bases

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Table 5. Examples of some office centres in Vienna using energy foundations for heating and cooling. Tunnel and metro projects withgeothermal energy systems for comparison

Building Year ofconstruction

Energy foundation Absorber pipes:m

Thermal power: kW Annual output: MWh

Generali Office Tower 1995 6000 m2 diaphragm walls 30 000 Cooling: 400Heating: 600

Cooling: 220Heating: 630

Strabag Office Centre 2001/2002 242 piles and 6000 m2 offoundation raft

78 000 Cooling: 2026Heating: 1680


Uniqa Tower 2002 7800 m2 diaphragm walls andfoundation raft



Columbus Centre 2002 12 400 m2 diaphragm walls and300 piles



Lainzer Tunnel 2002 60 piles (cut and cover) 18 000 Heating: 150 Heating: 214

Foursections of

U2/1 2003/2004 1425 m2 diaphragm walls and 14piles



ViennaMetro U2

U2/2 2650 m2 diaphragm walls, 820 m2

of foundation raft and 6 piles27 000 Cooling: 67

Heating: 95Cooling: 437Heating: 175

U2/3 7125 m2 diaphragm walls and3744 m2 of foundation raft



U2/4 2348 m2 of foundation raft 12 000 Cooling: 37Heating: 62


114 BRANDL

(g) cost savings for the road authorities/owners.

The use of snow chains increases the costs of winter roadclearance and road maintenance, for the following reasons

(a) mechanical abrasion of the road surface(b) no self-regeneration of bituminous pavements in tunnels

and galleries(c) dust development due to abrasion(d) rapid pollution of tunnel and gallery walls due to road

surface abrasion.

It is estimated that savings of about 50% of maintenancecosts would be possible along numerous road sections in theAlpine regions if no snow chains are required. This could beachieved by geothermal heating of the pavement. Further-more, winter-long safe access to the skiing centres would behighly appreciated by tourists.

In order to keep a road surface free from ice, its tempera-ture should be higher than +28C. The critical range of airtemperature lies between 08 and �108C. Lower temperaturesallow the heating system to be operated intermittently oreven turned off, because commonly there is no snow fallthen.

At present a long-term research project is running inAustria in order to determine the optimal position of absor-ber pipes from the thermal, energetic and structural pointsof view (Fig. 62). In part, these aspects exhibit contraryoptima, and thus certain compromises are required.

The scheme of thermo-active road pavements can be usedalso for airfields and basically even for turf heating of sportsstadiums (e.g. soccer matches during winter).

Energy tunnelsUntil recently geothermal heating from tunnels was used

only in connection with hot water, mostly without heatpumps. But the heat potential along a tunnel can also beutilised by using the tunnel support and lining as energyabsorbers. These may be anchors, rock/soil nails, geosyn-thetics or secondary concrete lining. Anchors or nails reach-

ing deeply into the surrounding ground can activate arelatively large mass for geothermal utilisation.

Energy tunnels may be excavated as closed systems, forexample by the NATM (Figs 50 and 51), or by the cut andcover method (Fig. 44).

Near the portals of transportation tunnels with geothermalequipment the following groups may take the availableenergy

(a) the owner or operator of the tunnel(b) private users (large residential blocks in particular, but

also one-family houses)(c) commercial and industrial users(d) public users (municipal and federal).

An example from a railway tunnel in Vienna underlinesthese advantages: about 1200 private flats could be suppliedwith geothermal energy, but also large public buildings.

Energy tunnels are an exciting challenge to geotechnicalengineering, whereby the optimisation of energy extractionor feed/storage, transfer and distribution requires multidisci-plinary cooperation. Ground investigation and geotechnicaldesign should incorporate geothermal aspects at an earlystage. The main advantages of this innovative technology areas follows.

(a) Commonly, tunnels are situated at a depth where theseasonal ground temperature is widely constant.

(b) Tunnels exhibit large interfaces between structure andground, and thus favouring the extraction and/or feed(and hence storage) of geothermal energy.

(c) Very deep-seated mountain tunnels can make use ofgreat geothermal gradients.

(d) In long tunnels, significant inner heat is available,mainly thanks to the waste heat of transportation. Inmetro tunnels, for instance, temperatures of more than+208C are possible even during the winter months.

(e) Utilising clean and self-renewable energy from tunnelsis environmentally friendly and economical. Thereforeenergy tunnels have high public acceptance andpolitical support, which makes the approval procedureseasier.

Until now only cut and cover tunnels and open face tunnels(excavated using the NATM) have been equipped withthermo-active geosynthetics or anchors/nails, but bored tun-nels with segmental lining can also be used as energytunnels, as they exhibit earth-contact structural elements.Furthermore, optimised energy tunnels may use not only theground temperature from their large underground contactarea but also inner heat sources from traffic, lighting etc.

Energy wellsThere are three different groups of geothermal well that

serve for environmentally friendly heating/cooling at lowcost

(a) exploiting hot water from the ground by boreholesreaching to a depth up to �2000 m

(b) conventional ground heat exchanger boreholes drilledup to about 300 m deep

(c) wells that are required for temporary groundwaterlowering, but which can serve simultaneously as heatextraction/storage systems.

Drilling deep geothermal boreholes for (a) and (b) requiresspecialised equipment, considerable skill and experience.Such systems serve only for heat exchanging. Energy wellsof group (c), however, represent a two-purpose system, andhence a technology involving geotechnical and geothermalengineering. Many construction sites require wells for

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groundwater lowering. Sometimes discharge wells arecoupled with recharge wells in order to minimise groundsettlement. These temporary measures can also be used forheating and cooling adjacent buildings. This may be per-formed temporarily during the construction period but alsopermanently after ceasing groundwater lowering. Experiencehas revealed that the public acceptance of metros, railwaysand roads by neighbouring people increases if they areprovided with cheap, renewable energy from such energywells or other geothermal systems.Figure 63 shows the scheme of a large-scale test with

energy wells for heating and cooling. Simultaneously, thesewells were used for long-term groundwater lowering along anew railway line under construction. Hence the tests couldrun from 2001 to 2003.The following investigations were carried out for research

purposes and to optimise an adjacent energy tunnel in cutand cover

(a) in situ determination of thermal soil parameters(thermal conductivity, specific heat capacity)

(b) maximum amount of extractable heat and energy influx,and storage capacity

(c) long-term behaviour and temperature conditions(d) influence of groundwater flow.

The 45 m deep test wells were installed in 50 m deep bore-holes of 600 mm diameter. The subsoil consisted of man-made fills (down to 4.7 m) underlain by heterogeneoustertiary sediments: silty clay, sand, sandy silt, gravel andwide-grained silty-sandy gravel, locally with sandstone andboulders. Below 23.0 m stiff cohesive layers with interlayersof sand and clay dominate.The hydraulic conductivity was extremely scattered: lo-

cally from k ¼ 10�3 to 10�10 m/s with an overall value ofabout k ¼ 10�6 m/s. Owing to the layered ground profile thehorizontal permeability exceeded the vertical one: hence kh¼ (10–50)kv.Seepage water occurred between 9.5 and 13.4 m depth,

the closed groundwater table lay at 20–23 m depth (seasonalamplitude), and at 36 m depth artesian groundwater wasfound.Both boreholes were fitted with U-pipe heat exchangers

consisting of HDPE pipes of 25 mm outer diameter andlocated within the filter tube of the wells. In order to achieve

an optimal heat exchange between absorber and surroundingsoil the spacing is typically filled with a cement–bentonitesuspension for common energy wells. However, in this caseonly sand and gravel were used, because the wells shouldalso be used again—at least temporarily—for groundwaterextraction. Moreover, the influence of different contact med-ia should be investigated. In well GB 2/97 (Fig. 63) sandwas filled between tube and filter; in well GB 4/97 the heattransfer into the absorber pipes should occur mainly bycontact with the groundwater.

Thermal energy was taken from the heat source GB 2/97(discharge well) and transported to a heat pump, where theenergy was raised to a higher temperature level, and thentransferred back to the ground by the heat sink GB 4/97(recharge well) (Fig. 63).

Figure 64 shows the temperature fluctuation in the absor-ber system after the start of operation on 28 March 2001. Inthe heat source (GB 2/97) the temperature difference of theheat carrier fluid between absorber inflow and outflow wasabout ˜T ¼ 1.58C, whereby the temperature level was verylow and fluctuated at about T ¼ �58C. This was caused bya high-performance circulating pump, which had too muchpower in relation to the relatively short absorber pipes in thewell. But the over-capacity of the pump had been chosendeliberately to investigate the effects of an over-design.

RECOMMENDATIONS FOR PRACTICEGeneral

High-permeability soil and groundwater flow are espe-cially suitable if only heating or cooling is performed. How-ever, energy balance is the ideal form of seasonal heating/cooling in winter/summer. In this case low-permeability soiland a low hydraulic gradient of the groundwater are favour-able. Moreover, the smaller the temperature difference be-tween ground source energy and used energy, the higher isthe seasonal performance factor (SPF), and hence the effi-ciency of the thermo-active system. Consequently, detailedsoil investigation is essential for optimising an absorbersystem for thermal energy extraction/storage.

Usually, temperature fluctuations caused by energy foun-dations have no relevant effect on the surrounding soil,assuming its temperature remains higher than +28 C. How-ever, cooling below 08C, as a result of improper operation,

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116 BRANDL

should be avoided, as it may cause freeze–thaw impacts inthe soil, and hence affect the load-bearing behaviour of thepiles, barrettes or diaphragm walls. Moreover, thermal con-ductivity increases and thermal storage capacity decreaseswith freezing, especially in soil with high water content.Freezing is avoided if the absorber system is operated onlywith water without antifreeze additives, but experience hasshown that this reduces the efficiency of the energy systemsignificantly. Therefore it has proved suitable to use awater–glycol mixture and to limit the freezing temperaturein the core of piles or diaphragm walls to about �18C.

Commonly, a temperature difference of about ˜T ¼ 28Cbetween inflow and return-flow temperature of the absorberfluid is sufficient for economical operation of the energysystem. Operational fluctuation of the groundwater tempera-ture should be kept as low as possible (˜T < 58C). Lower-ing the groundwater temperature causes an increase inviscosity, and hence a decrease of the hydraulic conductivity.For ˜T < 58C this influence is practically negligible.

Too intensive cooling of the groundwater increases the pHvalue, reduces calcium solubility, and raises the solubility ofgaseous substances such as CO2. Too intensive heatingresults in a relatively large reduction in oxygen solubility,which may make the groundwater unfit for drinking. Further-more, temperature is one of the most important environmen-tal factors for the microorganisms in water. Many of themcan exist only within a very limited temperature range. Inparticular, the activity of bacteria-consuming microorganismsdrops significantly at temperatures below 108C.

If there is sufficient heat supply from the ground, inter-mittent operation of the heating/cooling system is possible.This means, for instance, one to two days of operation andturn-off, alternately. In the case of a piled raft foundation,the raft should be properly isolated in order to minimise heatloss in winter and cooling reduction in summer.

Geothermal utilisation of concrete retaining walls is alsopossible: gravity walls, cantilever walls, pile walls, dia-phragm walls, etc. In this case, a proper design of theabsorber system has to take into consideration the naturaltemperature fluctuations along the free-standing front face of‘energy walls’, which differ widely from those in the fullyembedded zone beneath surface.

Pile walls may consist of secant piles, contiguous orintermittent piles. The spacing of the piles has a stronginfluence on the efficiency of the energy system. A generallyvalid rule about which pattern provides the highest efficiencycannot be given, because this depends on several interactingfactors

(a) the surface area and volume of the concrete elements(b) the heat conductivity and thermal storage capacity of

the piles and ground(c) the hydrogeological conditions.

Installation of absorber pipes in energy foundations or wallsThe installation of reinforcement cages fitted with absor-

ber pipes into bored piles, barrettes or diaphragm wallsrequires the following measures.

(a) Protection from mechanical damage, especially in thecase of cutting by machine and non-deburred reinforce-ment bars.

(b) Protection from thermal damaging (during reinforce-ment welding).

(c) Exact positioning of the reinforcement cages (orienta-tion of the connecting box).

(d) The construction of stiff reinforcement cages for deepfoundations (e.g. welding of helical reinforcement tovertical rebars of deep piles, barrettes etc.).

(e) Lifting long reinforcement cages at both ends toprevent damage to the pressurised absorber pipe loops.

( f ) The use of full tremie pipes to place concrete in pilebores; also for dry rotary-bored piles, where commonlyself-compacting concrete is placed via a short tremiepipe from the ground surface.

(g) Upon completion of the pipework fixing on thereinforcement cage, a visual check on the final locationof the pipes is imperative to ensure that the floor of thewet pile concrete through the reinforcement cage wouldnot be impaired. The pipe ends near the bottom of thecage should be placed at different levels to help this.

(h) Very long reinforcement cages fitted with absorberpipes have to be installed in sections, which should becoupled by screwing, not welding. The pipes areextended/coupled by electrically welded sleeves. Weld-ing of the reinforcement sections is unavoidable only ifa lightning protection element is attached. In such casesthe absorber pipes have to be protected during weldingby welding mats.

(i) Careful insertion and withdrawal of the tremie pipes.( j) Protection from torsion and heave of the reinforcement

cage during concreting and steel pipe withdrawal.(k) Sufficient distance of the absorber pipes from the

reinforcement on the head and toe of the piles,barrettes, or diaphragm wall panels.

(l) Special precautions have to be taken for energy piles or

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diaphragm walls used for cut and cover tunnels orretaining walls if they are covered by a secondarylining. In order to avoid damage of the absorber pipeswhen installing the connecting nails or dowels, thepipes should be protected by twin bars.

For energy rafts (base slabs) the following constructionsequence has proved suitable.

(a) First lean concrete layer (blinding) on ground.(b) Place a wire mesh to facilitate fixing of the absorber

pipes.(c) Install the absorber pipes, which must be clearly and

durably marked.(d) Second lean concrete layer to protect the absorber

pipes.(e) Place the reinforcement of the raft and casting concrete.

The absorber pipes in the foundation elements should bekept under pressure during all construction stages. Thisenables prompt localisation of possible defects and repair intime. The collectors and distributors of the absorber pipesshould be fitted with optical flowmeters for long-term mon-itoring of the fluid circulation in the geothermal system.

Interaction of primary and secondary energy systemsThe temperature felt by persons in a room consists of the

air temperature and the radiation temperature (i.e. the tem-perature of walls and floors), whereby the ratio between airand surface temperature is essential. Compensating too lowa wall or floor temperature by higher air temperature is feltas uncomfortable (e.g. in temporarily uninhabited coldhouses that have to be warmed up). Surface temperatures of20–258C are optimal, corresponding to an air temperaturebetween 168C and 218C. Low-temperature heating systems,such as wall and floor heating with a large surface radiation,fulfil this prerequisite, whereby the feed temperature of wallheating should not exceed 408C and the floor temperature amaximum of 288C. This heating system can be coupled inan ideal way with energy foundations, retaining walls andother thermo-active ground structures.Thermo-active foundations, retaining walls, etc. should be

combined with thermo-active building systems (‘tabs’). Thetemperature difference between rooms and structure shouldnot exceed 2–38C in order to create a comfortable roomclimate. This is effectively achieved by optimal glazing,basic ventilation and minimal thermal resistance (e.g. nothick floor covers, double floor system). Low and locallydifferent inflow temperatures are essential: consequently,corner areas should have their own feed circuits.The energy demand during day/night and seasonally does

not always coincide with the available renewable energy.Therefore energy must be stored temporarily, either in theground or within the structure of the building. The installa-tion of plastic pipes in the concrete elements is easy andeconomical during the construction of new buildings. How-ever, thermo-active installations in existing buildings arequite complicated. Innovative technologies are thin floorelements of light weight with a high heat storage capacityusing phase-change materials.In many cases the restoration of old buildings or their

heightening by one or more floors requires an underpinningof the existing foundation. This can be used to install energyfoundations and couple them with the thermo-active heating(and cooling) system of the improved building.The optimal low-energy building is an ‘energy-active’

house using the entire structural envelope and foundationand even garden walls for heat absorption and emission, andhence as a low-temperature thermal storage system. Such

buildings are superior to ‘passive’ houses as they allow morenatural air circulation in the rooms and avoid too intensiveinsulation.

Proper operation of geothermal energy systemTemperature changes of the soil directly bordering on

thermo-active foundations have negligible influence on bear-ing-deformation behaviour if the energy system is properlyoperated. Granular soil is affected less than clay or silt;furthermore, the temperature sensibility of soil increaseswith its organic contents.

Proper heat extraction for heating a building reduces theground temperature locally from 10–158C to about 5–108C,whereas excessive heat extraction with freezing-stable absor-ber fluids may create sub-zero temperatures. This causes theformation of ice lenses in the soil whereby clayey to sandysilt is especially affected, depending on its mineralogicalcomposition. Soil freezing hardly has any influence on deepenergy foundations, but may cause severe heave of baseslabs, or lateral deformations (and cracks) in retaining walls.During the subsequent thawing period the ground losesstrength, and settlements of the building are possible. Ex-perience and long-term monitoring have shown that thetemperature along the skin of energy piles or diaphragmwalls should not fall below +28C.

Intensive heat input into the ground for cooling a buildingis less critical for the foundation but possibly so for thequality of groundwater and microorganisms. Temperatureincrease causes a slight decrease of effective stress, andhence of shear strength; furthermore, shrinkage may alsolead to settlements. Numerous papers on general temperatureeffects between 28C and 658C on soils show rather divergentresults (e.g. Ennigkeit, 2002). This can be explained mainlyby the different behaviour of clay minerals, the influence ofwater content/saturation, density etc. Even the speed ofwarming up has an influence, but less than the freezingspeed in the case of excessive heat extraction. However,these investigations refer only to general laboratory tests,and cannot be applied directly to thermo-active groundstructure operation. Here the temperature differences arerather small, and have less physical-mechanical influencethan heat extraction.

Seasonal operation of the energy system—that is, heatextraction and heat input (recharging) by alternate heatingand cooling—causes a superposition of the above-mentionedeffects, and hence cumulative settlements occur until asteady state is reached. Cumulative heave would occur onlyin the case of freezing of frost-susceptible soil below baseslabs. When energy piles, barrettes or diaphragm walls areexposed to geothermal temperature changes, their head andtoe react in a different way: Heating of the building—that is,cooling of the deep foundation—causes its shrinkage, andhence head settlement and toe heave. Cooling of the build-ing leads to reverse movements.

Numerical simulations of such temperature-induced defor-mations have yielded temporary settlements of up to 10% ofthe static settlements for low- and high-rise buildings. En-nigkeit (2002) predicted somewhat higher values. However,in situ monitoring has clearly shown smaller values, for bothseasonally maximal and cumulatively residual values; more-over, differential settlements have been negligible from astatical point of view. This refers also to temperature-induced differential settlements among central piles, edgepiles and corner piles of pile groups, and to temperature-induced changes of the skin friction of thermo-active deepfoundations.

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Promotion of geothermal energy utilisationEarly ecological energy planning for buildings can in

many cases prevent costly refurbishment and renovation inthe future. High-quality energy design involves not onlyheating and cooling (rooms, water) but also lighting.

Building biology (including building ecology) is gainingincreasing importance in the fight against global warming,depletion of the ozone layer and the exploitation of materialresources. Building biology has become (or should become)a multidisciplinary science combing architecture, civil andgeotechnical engineering, physics and chemistry, installationengineering, medicine and related sciences. It considers notonly interactions of buildings and human health, but alsoenergy concepts, the life cycle of building materials, sustain-ability etc.

Geothermal geotechnics offers a promising alternative toconventional heating/cooling systems, providing solutions tothe challenges of today’s energy policies.

The targets for renewable energy and for energy buildingscan in general be reached only by political measures.

(a) High taxes on fossil fuels are the most importantprerequisite for energy saving and promotion ofrenewable energy sources.

(b) In order to promote the installation of thermo-activesystems or other heating/cooling systems based onrenewable energy, the economic incentives for privateinvestors, house owners, companies, and also for publicadministrators to invest in renewable energy systemsshould be improved in many countries. Strong supportby European Union policy is necessary.

(c) Legislation.(d) Public grants.

Since January 2004 each person who wants to build a familyhouse in Austria has received financial support by the localgovernment only if they present an energy performancecertificate with a low energy number. This number describesthe energy consumption (provided by heating energy minusheating losses) and is expressed in kWh/m2 and year.Promotion by public funds is granted only if this energynumber is smaller than 50 kWh/m2 for each floor. At valuesless than 40, 30, 25, 20 and 15 kWh/m2 the grant increasesstep by step.

However, if a building is heated/cooled by means of clean,renewable energy, for example by geothermal systems, theallowable limit value for energy consumption may be in-creased. The target of multidisciplinary innovation shouldapproach heat-and-light systems combined with ground-sourced or solar residual heating/cooling.

Thermo-active structures (including energy foundations)are therefore very helpful in reaching this low energy num-ber. Their installation is widely supported by politicians andmedia. Consequently, about 500 buildings with energy foun-dations or retaining/basement walls already exist in Austria.

This philosophy is fully supported by Directive 2002/91/EC of the European Parliament and of the Council on theenergy performance of buildings, which will come into forcein the European Community on 4 January 2006 at the latest.Thus an energy performance certificate has to be presentedif a building with more than 500 m2 is sold or rented.

CONCLUSIONSWorld supplies of fossil fuels are rapidly being depleted.

Consequently, multidisciplinary efforts are needed to developinnovative building practices using renewable energy—including new energy storage technologies. Near-surfacegeothermal and deep geothermal energy, solar photovoltaicand solar thermal energy, and wind energy are promising

alternatives (like conventional hydropower energy). Optimumeconomical efficiency and environmental protection aregained in most cases by an ‘energy mix’ from differentsources. Local climate and ground properties, technologicallevel, the specific use of a building, seasonal fluctuations,environmental conditions and actual energy prices are themain influencing parameters.

Integral planning and balancing of buildings means con-sidering the technical, economic, aesthetic and ecologicalaspects. An integral design, therefore, is always a sustainabledesign too, and it requires multi-objective optimisation.Balancing refers to materials, energy, emissions, waste water,waste/rubbish and its disposal or recycling, costs (invest-ment, maintenance, demolition), and life cycle.

Both thermo-active ground structures (energy foundations,retaining walls, tunnels, etc.) and energy wells are innova-tions that promise sustainable and clean energy consumption.A significant advantage of such systems is that they areinstalled within elements that are already needed for statical/structural or geotechnical reasons. Hence no additional struc-tural or hydraulic measures are required. Foundations, walls(below and above ground) and tunnel linings can be useddirectly for the installation of absorber pipes for heatexchange. Moreover, concrete has a higher thermal conduc-tivity than soil. Wells for groundwater lowering may besimultaneously used for heat extraction/storage, thus becom-ing energy wells. This innovation is a key improvement overthe conventional geothermal methods such as (deep) bore-hole heat exchangers or near-surface earth collector systems.At a certain depth, ground temperature remains widelyconstant throughout the year (e.g. 10–158C below 10–15 min most European regions), and a heat exchanger allows it tobe used as a heat source in winter and for cooling insummer.

Energy systems based on earth-contact structural elementshave a double function, and they work most efficiently if thethermo-active elements are in contact with groundwater.Nevertheless, a sufficient seasonal performance factor of thesystem is achievable even without groundwater, especiallyfor seasonal operation—that is, heating in winter and coolingin summer. Energy balance is the ideal form of heating andcooling. Moreover, the smaller the temperature differencebetween ground source energy and used energy, the higher isthe seasonal performance factor, and hence the efficiency ofthe thermo-active system. Usually, a temperature differenceof only ˜T ¼ 28C between absorber fluid inflow and returnflow from the primary circuit is sufficient for economicaloperation of the energy system. Consequently, such geother-mal systems represent low-temperature systems. Experiencehas shown that the electricity required for operating theentire system commonly varies between 20% and 30% ofthe total energy output. If no heat pump is necessary (e.g.for free cooling) this value drops to 1–3% for merelyoperating a circulation pump.

Cost–benefit analyses depending on the climatic condi-tions in the region concerned have shown that the investmentpayback period for such heating/cooling systems usuallyvaries between 2 and 10 years, depending on the particularground properties, foundations systems, building character-istics and energy prices. In special cases even the initial costoutlay may be lower than for conventional heating/coolingsystems. Nearly 20 years of experience with energy founda-tions has shown that heating or cooling costs can be reducedby up to two thirds over the design life of a building.Balanced seasonal operation of heating and cooling mayeven save up to 75% in electricity compared with conven-tional air-conditioning systems.

The shaft resistance and base pressure of energy piles,barrettes or diaphragm walls, and the bearing capacity of the


soil around their toe, are not affected by the heat absorptionprocess in a statically relevant magnitude. Hence tempera-ture-induced settlement or heave of buildings is negligible inrelation to displacements caused by static loads (this hasbeen shown by detailed site measurements since 1994), butexposure of the surface of deep foundations to temperaturesbelow freezing should be avoided. Fine-grained soils with ahigh content of active clay minerals (e.g. montmorillonite)are especially critical in this context.The temperature fluctuations around an energy foundation

decrease significantly with distance from the heat exchanger(roughly an exponential decay). Thermal properties andefficiency of energy foundations can be influenced not onlyby their design (always coupled with statical/structural andgeotechnical requirements) but also by the concrete charac-teristics.Proper geothermal energy utilisation requires an interdisci-

plinary design, especially in the case of houses. The geo-technical engineer, structural engineer, architect, buildingservices designer and installer, heating engineer and specia-lised plumber should cooperate as early as possible to createthe most economical energy system. In the first phase ofoperation precise adjustment is recommended to optimisethe performance of the engineering system. Furthermore,some operation rules have to be considered (as mentioned inthis paper).The benefits of energy foundations and other thermo-

active earth-contact structures may be summarised as fol-lows.

(a) They are environmentally friendly (non-polluting, sus-tainable energy).

(b) They offer a reduction of fossil energy demand, andhence of CO2 emissions.

(c) They promote compliance with international environ-ment obligations, such as the Kyoto and Torontotargets.

(d) They are economical, at least in the long term.(e) Although thermo-active earth-contact structures com-

monly require investment costs that are similar to orslightly higher than those of conventional systems, theyhave lower running costs and hence lower life cyclecosts.

( f ) They are low maintenance, long lifetime systems.(g) Geothermal energy systems run fully automated.(h) Thanks to the low temperature and pressure in the heat

carrier circuits, geothermal heating/cooling systems canbe operated without risk.

(i) The closed primary heat carrier circuit embedded inconcrete prevents damage of pipework or groundwaterpollution.

( j) They offer increased personal comfort in buildings(indoor rooms). The temperature personally felt there—that is, the ambience experienced—consists of airtemperature and radiation temperature, which are influ-enced by wall and floor temperatures. Comfort isenhanced by low-temperature heating of walls andfloors exhibiting a large heat-radiating surface.

(k) Optimal hygrothermal behaviour of buildings is possi-ble (e.g. for museums and arts centres).

(l) There is no storage of fossil fuel, no stove or chimney,and no visible radiators are needed.

(m) Geothermal cooling may replace conventional air-conditioning, which is often felt to be loud andunhygienic.

(n) Geothermal energy may be easily combined with otherenergy systems.

(o) Unlike hydroelectricity, geothermal energy is notvulnerable to droughts.

(p) The cost of geothermal energy is not prone tounpredictable price fluctuations.

(q) The consequent reduction of energy imports means areduced dependence on external economical or politicalsituations.

(r) Geothermal energy has a positive public image, and inseveral regions is supported by government grants.

ACKNOWLEDGEMENTSThe author would like to thank his assistants, Associate

Professor Dr D. Adam and Dr R. Markiewicz at the Institutefor Soil Mechanics and Geotechnical Engineering, ViennaUniversity of Technology, for their valuable contributions tothis paper. In the framework of ‘Universities meet researchinterested companies’ the long-established cooperation withthe engineering company Naegelebau/enercret is also ac-knowledged. Moreover, the Vienna Authorities (WienerLinien), the Austrian Federal Ministry for Transport, Innova-tion and Technology, and the Austrian Railway Authorities(HLAG) have strongly supported the research project ‘Ther-mo-active ground structures’. Finally, the author is gratefulto the iC consultants, Austria, for discussions on metrosubjects of this paper.

NOTATIONA areaa thermal diffusivity

COP coefficient of performanceC circumference

CV volumetric heat capacityc specific heat capacity

cv specific heat capacity of soil vapourcw specific heat capacity of soil waterD pile diameter; thickness of diaphragm walld damping depth; pipe diameter

ex, e y, ez unit vectorsi hydraulic gradientk hydraulic conductivity; permeability coefficientL pipe lengthl length

m mass_mmr mass flux density

Nu Nusselt numbern flow direction; soil porosity; quantity of pipesP pump performanceP period duration of temperature oscillationPr Prandtl numberQ heat volume; total load_QQ heat flux_qq heat flux density

_qqcond conductive heat flux density_qql,conv heat flux density generated by liquid convection

_qqlat heat flux density due to latent heat_qqtot total heat transfer in soilqu compressive strength

_qqv,conv heat flux generated by vapour convectionR, r radiusRe Reynolds number

S degree of saturationSPF seasonal performance factor

T temperatureT9 reference temperatureTm caloric mean temperature

Tm,out mean daily air temperatureTm,out mean yearly air temperature

TS surface temperatureTU surrounding temperaureT0 reference (initial) temperature

˜Tout temperature amplitudet timeu flow velocity of absorber fluid

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vv vector of vapour velocityvw vector of water velocityw water contentx specific volumez axisÆ heat transfer coefficient� roughness of inner pipe wall�l thickness of laminar edge zone�t phase displacement� integration limit� dynamic viscosity; efficiencyº thermal conductivity� kinematic viscosity� coefficient of flow pressure loss within pipe systemr densityrv density of soil vapourrw density of soil water� shear stress� azimuth; angle of declination; friction angleł angle of ascensionø integration variable

Subscriptsa air; pore airc core (of fluid)

GS ground surfaceGW ground water

h horizontali ice; wateri inner

m meanmin minimum

o outerp pump

prim primary circuits soil; solid

s,min mineral component of solid fractions,org organic component of solid fractionsec secondary circuitv verticalW wallw water; pore water

REFERENCESAbu-Hamdeh, N., Khadair, A. & Reeder, R. (2001). A comparison

of two methods used to evaluate thermal conductivity for somesoils. Int. J. Heat Mass Transfer 44, No. 14, 1073–1078.

Adam, D. & Markiewicz, R. (2002). Nutzung der geothermischenEnergie mittels erdberuhrter Bauwerke. Osterr. Ing. Archit. Z.147, No. 4, 5 and 6.

Ennigkeit, A. (2002). Engerieanlage mit Saisonalem Thermospei-cher, Mitteilungen des Institutes und der Versuchsanstalt furGeotechnik der Technischen Universitat Darmstadt, No. 60.

Hofinger, J. (2002). Nutzung geothermischer Energie und Umwelt-warme auf niedrigen Temperaturniveau mittels erdberuhrterBauteile. Master’s thesis, Institute for Soil Mechanics and Geo-technical Engineering, Technical University of Vienna.

Jessberger, H. L. & Jagow-Klaff, R. (1996). Frost im Baugrund.Grundbau-Taschenbuch. Teil 1, 5th edn. Berlin: Ernst & SohnVerlag.

Leu, W., Keller, B., Matter, A., Scharli, U. & Rybach, L. (1999).Geothermische Eigenschaften Schweizer Molassebecken. Bern:Schlussbericht Bundesamt fur Energie.

Markiewicz, R. (2004). Numerische und experimentelle Untersu-chungen zur Nutzung von geothermischer Energie mittels erd-beruhrter Bauteile und Neuentwicklungen fur den Tunnelbau.Doctoral thesis, Institute for Soil Mechanics and GeotechnicalEngineering, Technical University of Vienna.

Oertel, H. Jr (Ed.) (2001). Prandtl: Fuhrer durch die Stromung-slehre. Braunschweig/Wiesbaden: Friedr. Vieweg & Sohn Ver-lagsgesellschaft mbH.

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(2000). Ground heat transfer effects on the thermal performanceof earth-contact structures. Renewable and Sustainable EnergyRev. 4, No. 3, 213–265.

Suckling, T. P. & Smith, P. E. H. (2002). Environmentally friendlygeothermal piles at Keble College, Oxford, UK. Proc. 9th Int.Conf. Exhib. on Piling and Deep Foundation, Nice, 445–452.

Verein Deutscher Ingenieure (1997). VDI-Warmeatlas. Berlin/Heidelberg: Springer-Verlag.

FURTHER READINGAdam, D., Hofinger, J. & Ostermann, N. (2001). Utilization of

geothermal energy from railway tunnels. Proc. 15th Int. Conf.Soil Mech. Geotech. Engng, Istanbul 3, 2029–2034.

Andersland, O. B. & Anderson, D. M. (1978). Geotechnical en-gineering for cold regions. New York: McGraw-Hill.

Brandl, H. (1980). The influence of mineral composition on frostsusceptibility of soils. Proc. 2nd Int. Symp. on Ground Freezing,Trondheim, 815–823.

Brandl, H. (1998a). Energy piles for heating and cooling of build-ings. Proc. 7th Int. Conf. Exhib. Piling and Deep Foundations,Vienna, 3.4.1–3.4.6.

Brandl, H. (1998b). Energy piles and diaphragm walls for heattransfer from and into ground. Proc. 3rd Int. Geotech. Seminar,Deep Foundations and Auger Piles III, Ghent, 37–60.

Brandl, H. & Markiewicz, R. (2001). Geothermische Nutzung vonBauwerksfundierungen (‘Energiefundierungen’). Osterr. Ing. Ar-chit. Z 146, No. 5–6, 216–222.

Brandl, H., Adam, D. & Kopf, F. (1999). Geothermische Energie-nutzung mittels Pfahlen, Schlitzwanden und Stutzbauwerken.Pfahl-Symposium 1999, Braunschweig, 329–356.

Brandl, H., Adam, D. & Markiewicz, R. (2004). Nutzung vonVerkehrstunneln als Absorberbauwerke fur die Gewinnunggeothermischer Energie, Research Report. Vienna: Federal Min-istry for Transport, Innovation and Technology.

Brauner, M. (2002). Oberflachennahe Nutzung der GeothermischenEnergie: Analyse ihrer Umweltauswirkungen und der rechtlichenRahmenbedingen im internationalen Vergleich. Master’s thesis,Institute for Soil Mechanics and Geotechnical Engineering,Technical University of Vienna.

Enercretnagele (2004). Thermoaktive Fundamente. Nagele-Reports,Rothis, Austria.

Ennigkeit, A. & Katzenbach, R. (2001). The double use of piles asfoundation and heat exchanging elements. Proc. 15th Int. Conf.Soil Mech. Geotech. Engng, Istanbul 2, 893–896.

Farouki, O. T. (1986). Thermal properties of soils, Series on Rockand Soil Mechanics, Vol. 11. Clausthal-Zellerfeld: Trans TechPublications.

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Hellstrom, G. (1994). Fluid-to-ground thermal resistance in ductground heat storage. Proc. Calorstock ’94, Espoo, pp. 373–380.

Holdsworth, B. (2003). Cool thinking. European Foundations,Spring, 10–11.

von der Hude, N. & Kapp, Chr. (1998). Einsatz von Energiepfahlenam Beispiel des Main Tower in Frankfurt am Main. Reports ofthe Institute for Geotechnics, Technical University of Darmstadt,Vol. 39, pp. 15–28.

von der Hude, N. & Volkner, R. (2003). Erdwarmenutzung undAusfuhrung auf der U-Bahnbaustelle U2/1, ‘Schottenring’ inWien. Osterreichische Vereinigung fur Beton- und Bautechnik54, 15–11.

Hueckel, T. & Pellegrini, R. (1991). Thermo-plastic modelling ofundrained failure of saturated clay due to heating. Soils Found.31, No. 1, 1–16.

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Katzenbach, R., Arslan, U. & Ruckert, A. (1998). Das Prinzip dessaisonalen Thermospeichers, Report No. 39 of the Institute forGeotechnics. Technical University of Darmstadt.

Kuntiwattanakul, P., Towhata, I., Ohishi, K. & Seko, I. (1995).Temperature effects on untrained shear characteristics of clay.Soils Found. 35, No. 1, 1–16.

Laloui, L., Moreni, M., Fromentin, A., Pahud, D. & Vulliet, L.(1999). In-situ thermo-mechanical load test on a heat exchangerpile. Proc. 4th Int. Conf. on Deep Foundation Practice, Singa-pore, 273–279.

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Lund, P. (1985). Effect of ground water flow on the performance oflong-term pipe heat storage in the ground, Report TKK-F-A586(1985). Helsinki University of Technology Finland.

Markiewicz, R. & Adam, D. (2003). Utilisation of geothermalenergy using earthcoupled structures: theoretical and experimen-tal investigations, case histories. Proc. 13th Eur. Conf. SoilMech. Geotech. Engng, Prague 2, 3–10.

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Mullner Energieberatung & Haustechnik (1990). Anlagenbeschrei-bungs Ph. Maser-Maselli Strickwarenfabrik, Dornbirn. NageleReports, Rothis, Austria.

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Preg, R. (1997). Energy savings with concrete: Heating and coolingwith energy from subsoil. Nagele-Reports, Rothis, Austria.

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Suckling, T. & Cannon, R. (2004). Energy piles for Pallant House,Chichester, UK. Ground Engng 37, No. 7, 27–29.

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VOTE OF THANKSProfessor J. B. BURLAND, Imperial College of Science,Technology and Medicine, London

Over the last four years I have had the great pleasure oflistening to the following four major lectures presented byProfessor Brandl: the Special Lecture to the Hamburg Con-ference; the Young Geotechnical Engineers’ Conference inSantorini; the First International Conference on GeotechnicalEducation in Sinaia, Romania; and GeoEng 2000 in Mel-bourne, Australia.Tonight’s lecture is the fifth that I have attended. I know

that Professor Brandl has given a number of other majorlectures as well, and I am full of admiration for the way hehas been able to find the time to do this on top of a mostdemanding professional and teaching workload. Each ofthese lectures has been on very different topics, rangingfrom the practical through to the ethical and even philoso-phical, all of them thought-provoking and all of themchallenging.I am sure you will agree that this evening’s Rankine

Lecture has also been thought-provoking and challenging,and has covered a remarkable range of topics.

Much of Professor Brandl’s work has been to do with theconstruction and stabilisation of transport infrastructure inalpine country. Many of the projects are extraordinarilydaring and audacious, in conditions where the ground prop-erties vary enormously and precise calculations are not help-ful. This work has bred in Professor Brandl a deep relianceon understanding the basic mechanisms of behaviour,coupled with an insistence on observation and measurement.His work is also characterised by a willingness to useinnovative solutions for dealing with very challenging pro-blems.

These alpine projects have also given rise to a highlydeveloped sense of the need for compatibility of deforma-tions between ground and structure—a sense of when rigid-ity is beneficial and when flexibility and robustness are whatis required—particularly in ground-retaining structures. Inthis evening’s lecture we have seen displayed all thesecharacteristics of Professor Brandl’s work.

There are three topics that are exercising society as awhole right now, and will do so for the foreseeable future.They are: conservation and enhancement of the environment;risk assessment; and sustainable development. All three ofthese involve many disciplines. Professor Brandl has touchedon all three of these topics, and has demonstrated thatgeotechnical engineering has major roles to play in all ofthem.

The idea of energy foundations is exciting, and the energysavings look attractive and viable. I believe that the conceptsand case histories outlined by Professor Brandl will stimu-late study and discussion and hopefully further successfulapplications worldwide.

With his background in alpine construction it is naturalthat aesthetics should be a key issue, and Professor Brandlhas shown us that it is possible to make high retainingstructures using novel and varied techniques that are veryattractive. He has certainly offered us a challenge in thisrespect.

Professor Brandl touched on landfill engineering, which isof course a significant aspect of environmental geotechnicsbut by no means the dominant part of it. He has given usmuch detailed valuable advice and a number of alternativestrategies. What I found particularly interesting in this partof his lecture is his perception of risk and in particular theimportance of involving laypeople, those directly affected, indialogue.

In many senses, the assessments of technical and commer-cial risk are the easy bit. It is dealing with society that isthe difficult bit. The communication of risk in terms that arereadily understood, together with informed presentation ofthe options available, is something that the engineeringprofession is just beginning to learn. I believe that we shouldnot be reluctant to enter into dialogue with the lay public(the stakeholders) because it is as much about understandingand making opportunities as it is about preventing conflictand mitigating risk. The key is to do it at the right time—and to know when to call on the artist or psychologist!

I know that you will all agree with me that we havelistened to a challenging and stimulating 41st RankineLecture. The published version will, sadly, be more con-densed, but I believe that it will be a seminal paper. It iswith the utmost pleasure that I propose this vote of thanksto Professor Brandl.

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brandl - 2006 - energy foundations and other thermo-active ground structures

Documents

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