the cooling and heating potential of an earth tube system in buildings

The cooling and heating potential of an earth tube system in buildings

Kwang Ho Lee *, Richard K. Strand

University of Illinois at Urbana-Champaign, Champaign, IL, United States

Received 3 September 2006; received in revised form 3 April 2007; accepted 13 April 2007

www.elsevier.com/locate/enbuild

Energy and Buildings 40 (2008) 486–494

Abstract

A new module was developed for and implemented in the EnergyPlus program for the simulation of earth tubes. The model was validated

against and showed good agreement with both theoretical and experimental data. Using the new module, a parametric analysis was carried out to

investigate the effect of pipe radius, pipe length, air flow rate and pipe depth on the overall performance of the earth tube under various conditions

during cooling season. Pipe length and pipe depth turned out to affect the overall cooling rate of the earth tube, while pipe radius and air flow rate

mainly affect earth tube inlet temperature. The cooling and heating potential of earth tubes in four different locations were also investigated.

Whether or not an earth tube is beneficial turned out to be heavily dependent on the climate of the location.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Earth tube; EnergyPlus; Parametric analysis; Cooling and heating potential

1. Introduction

The utilization of geothermal energy to reduce heating and

cooling needs in buildings has received increasing attention

during the last several years. An earth tube is a long,

underground metal or plastic pipe through which air is drawn.

As air travels through the pipe, it gives up or receives some of its

heat to/from the surrounding soil and enters the room as

conditioned air during the cooling and heating period.

Due to the significance of earth tube system, numerous

research studies have been performed by Krarti et al. (1995) [1],

Puri (1986) [5] and Labs at al. (1989) [2]. Krarti (1995)

developed the model to determine the annual mean soil surface

temperature and amplitude of soil surface temperature, which is

essential to the calculation of soil surface temperature. Labs [2]

developed the model to determine the soil temperature at

various depths and elapsed time using the annual mean soil

surface temperature and the amplitude of soil surface

temperature. Recently, a sophisticated model describing the

complex mechanisms of simultaneous heat and mass transfer

occurring around the earth tube has been developed and

integrated into TRNSYS by Mihalakakou et al. (1995) [4].

* Corresponding author at: 711 W. Main Street #6, Urbana, IL 61801, United

States. Tel.: +1 217 419 6067.

E-mail address: [email protected] (K.H. Lee).

0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2007.04.003

Nevertheless, those research studies have focused either on

heat transfer to/from the surrounding soil or on the prediction of

soil temperature separately. To date, a detailed algorithm

calculating the soil temperature variation around the earth tube

directly from weather data files has not been encoded within an

existing simulation tools. Since an accurate ground temperature

prediction is also an essential factor for the simulation of an

earth tube, both the heat transfer occurring around the earth

tube and the soil temperature should be modeled together in

order to successfully simulate an earth tube. In order to

compute the soil temperature surrounding the earth tube and the

subsequent heat transfer rate, three important parameters

should be determined: annual mean ground surface tempera-

ture, amplitude of the annual soil surface temperature variation

and the phase constant of the soil surface. The existing

algorithms require users to input these three values, but finding

out these three values is another big challenge for users and it is

done by experiments in most cases. In this circumstance, the

new model is developed to calculate those three parameters

directly from the weather data file in this study.

The objective of this paper is to discuss the development and

implementation of a new module handling both the heat

transfer and soil temperature algorithms into the EnergyPlus

program for the simulation of earth tubes. The model is

validated against the data from other studies. Also, using the

new model, the effects of four parameters on the overall

performance of the earth tube is quantitatively assessed through

the parametric analysis. In addition, the cooling and heating

mailto:[email protected]

http://dx.doi.org/10.1016/j.enbuild.2007.04.003

Nomenclature

a constant (103 Pa/8C)

As amplitude of the soil surface temperature varia-

tion (8C)

b constant (609 Pa)

Ca specific heat of air (J/kg 8C)

f fraction of evaporation rate

hc convective heat transfer coefficient at the inner

pipe surface (W/m2 8C)

hs convective heat transfer coefficient at the soil

surface (W/m2 8C)

kair thermal conductivity of the air (W/m 8C)

kp pipe thermal conductivity (W/m 8C)

ks soil thermal conductivity (W/m 8C)

L pipe length (m)

ma mass flow rate of ambient air through pipe

(kg/s)

ra relative humidity

r1 inner pipe radius (m)

r2 pipe thickness (m)

r3 distance between the pipe outer surface and

undisturbed soil (m)

Rc thermal resistance due to convection heat transfer

between the air in the pipe and the pipe inner

surface (8C/W)

Rp thermal resistance due to conduction heat tra-

nsfer between the pipe inner and outer surface

(8C/W)

Rs thermal resistance due to conduction heat transfer

between the pipe outer surface and undisturbed

soil (8C/W)

Rt total thermal resistance between pipe air and soil

(8C/W)

DR radiation constant (63 W/m2)

Sm average solar radiation (W/m2)

Sv amplitude of the solar radiation (W/m2)

t time elapsed from beginning of calendar year

(days)

t0 phase constant of the soil surface (s; days)

t0a phase constant of the air (s; days)

Ta air temperature above the ground surface (8C)

Ta(y) air temperature of the pipe at the distance y from

the pipe inlet (8C)

Tam ambient air temperature (8C)

Tm average soil surface temperature (8C)

Tma average air temperature (8C)

Tsur ground surface temperature (8C)

Tva amplitude of the air temperature (8C)

Tz,t ground temperature at time t and depth z (8C)

Ut overall heat transfer coefficient of the whole earth

tube system (W/8C)

Va average pipe air velocity (m/s)

w annual angular frequency (=1.992 � 10�7 rad/s)

z depth of the radial center of pipe below soil

surface (m)

Greek letters

as soil thermal diffusivity (m2/s; m2/days)

b soil absorption coefficient (=1– soil albedo)

e hemispherical emittance of the ground surface

wI phase angle between the insolation and the air

temperature (rad)

ws phase angle difference between the air and soil

surface temperature (rad)

K.H. Lee, R.K. Strand / Energy and Buildings 40 (2008) 486–494 487

potential of the earth tube is investigated in four representative

locations in the U.S.

2. Earth tube model description

The simulation program in which the earth tube module was

implemented is EnergyPlus, since EneryPlus is considered to be

the next-generation building performance simulation program

combining the best features of DOE-2 and BLAST. As a result,

it has the great flexibilities and capabilities for the compre-

hensive building simulation. The integrated solution manager

in EnergyPlus consists of three managers: the surface heat

balance manager, the air heat balance manager and the building

systems simulation manager. The earth tube module presented

in this paper was implemented at the air heat balance manager

level.

Due to the complex mechanisms occurring around the earth

tube, several simplifying assumptions were made:

� C
onvection flow inside the pipe is hydrodynamically and
thermally developed.

� S
oil temperature in the pipe vicinity can be calculated using
the soil model discussed below beyond a particular distance

from the center of the pipe (thickness of the annulus).

� T
he temperature profile in the pipe vicinity is not affected by
the presence of the pipe. As a result, the pipe surface

temperature is uniform in the axial direction.

� T
he soil surrounding the pipe is homogeneous and has a
constant thermal conductivity.

� P
ipe has a uniform cross sectional area in the axial direction.
2.1. Soil temperature calculation

Prior to the calculation of the soil temperature around an

earth tube, the ground surface temperature directly above earth

tube should be predicted. According to Kusuda and Achenbach

(1965) [8], the ground surface temperature satisfies the

following expression:

Tsur ¼ Tð0; tÞ ¼ Tm þ As ReðeiwtÞ (1)

where T(x,t) is the soil temperature profile as a function of depth

x and time t. Tm and As are the annual mean value and the

amplitude of the ground surface temperature variation, respec-

tively, which should be calculated by considering the convec-

tive heat transfer between the air and ground, the solar radiation

K.H. Lee, R.K. Strand / Energy and Buildings 40 (2008) 486–494488

absorption by the ground, the long-wavelength radiation

emitted from the soil, and the latent heat loss due to the

moisture evaporation at the ground surface.

The model is described in depth by Krarti et al. [1], and the

derivation process will not be discussed in this paper.

According to Krarti et al. [1], the annual mean ground surface

temperature, Tm, can be estimated as follows:

Tm ¼1

he

½hrTma � eDRþ bSm � 0:0168hs fbð1� raÞ� (2)

where he ¼ hsð1þ 0:0168a f Þ; hr ¼ hsð1þ 0:0168ara f ÞMore detailed description of the algorithm is provided in

reference [7].

The amplitude of the soil surface temperature variation (8C),

As, the phase constant of the soil surface (s), t0, and phase angle

difference between the air and soil surface temperature (rad),

ws, can also be determined as follows [1]:

As ¼�� hrTva � bSv eifI

he þ dks

�� (3)

t0 ¼ t0a þfs

w(4)

fs ¼ �Arg

�hrTva � bSv eifI

he þ dks

�(5)

In Eqs. (3) and (5) the symbols jj jj and Arg are used to signify

the modulus and the argument of a complex number, respec-

tively.

The phase constant of the air (s), t0a, is the time elapsed from

the beginning of the year at which the air temperature reaches

the minimum value in the year.

The value of d is evaluated as follows:

d ¼ 1þ i

D(6)

where the dampening depth (m), D, is calculated from the

following equation:

D ¼ffiffiffiffiffiffiffi2as

w

r(7)

Assuming homogeneous soil of constant thermal diffusivity,

the temperature at any depth z and time t can be finally

estimated by the following expression (Labs et al. 1989).

Tz;t ¼ Tm � As exp

�� z

�p

365as

�1=2�

cos

�2p

365

�t � t0 �

z

2

�365

pas

�1=2�(8)

2.2. Heat transfer and earth tube inlet air temperature

calculation

In order to calculate the heat transfer between the earth tube

and the surrounding soil, the overall heat transfer coefficient

should be determined using the following three thermal

resistance values:

Rc ¼1

2pr1Lhc

(9)

Rp ¼1

2pLkp

lnr1 þ r2

r1

(10)

Rs ¼1

2pLks

lnr1 þ r2 þ r3

r1 þ r2

(11)

where Rc is thermal resistance due to convection heat transfer

between the air in the pipe and the pipe inner surface (8C/W),

Rp the thermal resistance due to conduction heat transfer

between the pipe inner and outer surface (8C/W) and

Rs is the thermal resistance due to conduction heat

transfer between the pipe outer surface and the undisturbed

soil (8C/W). The distance between the pipe outer surface and

undisturbed soil, r3, is assumed to be equal to the radius of

the pipe.

The convective heat transfer coefficient at the inner pipe

surface in Eq. (9) (W/m2 8C), hc, is a function of Nusselt

number, Nu, and thermal conductivity of air (W/m 8C), which

can be expressed by the following expression:

hc ¼Nu kair

2r1

(12)

Using the three thermal resistance values, Rc, Rp and Rs,

overall heat transfer coefficient of earth tube can be estimated

as follows:

Ut ¼1

Rt

(13)

Rt ¼ Rc þ Rp þ Rs (14)

Now, the heat transfer between the soil and the air inside the

pipe is equal to the amount of heat loss or gain as air flows along

the pipe [3]:

Ut dy½TaðyÞ � Tz;t� ¼ �maCa½dTaðyÞ� (15)

By solving for air temperature inside the pipe, Ta(y), the

following earth tube inlet air temperature (defined as the air

leaving the earth tube and entering the space in this paper) can

be finally obtained.

� I
n case Tam > Tz,t
TaðLÞ ¼ Tz;t þ eA (16)

� I
n case Tam = Tz,t
TaðLÞ ¼ Tz;t (17)

� I
n case Tam < Tz,t
TaðLÞ ¼ Tz;t � eA (18)

where

A ¼ maCa lnjTam � Tz;tj � UtL

maCa

(19)

Fig. 1. E+ earth tube model validation (ambient air temperature 25.56 8C).


3. Model validation

The model developed in the previous section was validated

against both theoretical model and experimental data. The

theoretical model for the comparison was developed by Al-

Ajmi et al. [3], which was validated against relevant

experimental and theoretical studies. The experimental study

was carried out by Dhaliwal et al. [6] at North Carolina under

the configuration of a pipe diameter of 30 cm, a pipe length of

24.7 m, and a pipe depth of 1.7 m. The detailed input

parameters for the comparison to both experimental and

theoretical studies are described in Table 1. The results for both

the experimental and theoretical studies using two different

ambient air temperature conditions are presented in Tables 2

and 3, Figs. 1 and 2. As can be seen, the results show good

agreement and have the values between the two studies under

consideration. The difference in the earth tube inlet temperature

between the experimental data and the modeling results can be

seen to be a result of an obvious data error in the experimental

results (see Fig. 1). However, the discrepancies between the

EnergyPlus model and the Al-Ajmi model are insignificant,

indicating that the model can properly predict the performance

Table 1

Input parameters for comparative validation

Pipe diameter (cm) 30

Pipe length (m) 24.7

Air velocity (m/s) 1.5

Soil temperature (8C) 18.89

Pipe depth (m) 2.13

Soil thermal conductivity (W/m8C) 1.16

Soil thermal diffusivity (m2/s) 0.00232

Table 2

E+ earth tube model validation (ambient air temperature: 25.56 8C)

Axial distance

from the pipe

inlet (m)

Experimental

data of Dhaliwal

et al. (8C)

Theoretical data

of Al-Ajmi

et al. (8C)

E+ earth

tube model

(8C)

3.35 25.00 24.94 25.04

6.4 24.40 24.43 24.60

9.45 25.00 23.97 24.19

12.5 24.40 23.54 23.82

15.55 23.80 23.15 23.46

24.7 23.80 22.16 22.55

Table 3

E+ earth tube model validation (ambient air temperature: 20.55 8C)

Axial distance

from the pipe

inlet (m)

Experimental

data of Dhaliwal

et al. (8C)

Theoretical

data of Al-Ajmi

et al. (8C)

E+ earth

tube model

(8C)

3.35 20.55 20.40 20.42

6.4 20.00 20.27 20.31

9.45 20.00 20.16 20.21

12.5 20.00 20.05 20.11

15.55 20.00 19.95 20.03

24.7 20.00 19.71 19.80

of an earth tube system and the heating/cooling load reduction

due to the earth tube.

4. Description of simulation conditions and example

building

Using the newly developed earth tube model, the cooling

and heating potential of an earth tube system was predicted in

four different locations which represent four typical climatic

conditions in order to investigate the influence of soil

temperature and soil condition. Also parametric studies were

carried out to determine the effect of four important variables

influencing the earth tube inlet air temperature: pipe radius,

pipe length, air flow rate and pipe depth under the ground

surface. Simulations were performed on five different values of

each parameter while the other parameters were maintained at

the same values. 21 August is chosen as summer design day for

Key West and Spokane, and 21 July was chosen for Peoria and

Phoenix. The maximum dry bulb temperatures were set at

30.6 8C, 30.4 8C, 35.7 8C and 28.3 8C for Key West, Peoria,

Phoenix and Spokane, respectively. 21 December was chosen

as winter design day for Phoenix and Spokane, and 21 January

was chosen for Key West and Peoria. The maximum dry bulb

Fig. 2. E+ earth tube model validation (ambient air temperature 20.55 8C).

Table 4

Description of simulation conditions

Conditions

Location

Spokane, WA: mild and dry

Peoria, IL: mild and wet

Phoenix, AZ: hot and dry

Key West, FL: hot and wet

Run period

Summer design day

7/21: Peoria and Phoenix

8/21: Key West and Spokane

Winter design day

12/21: Phoenix and Spokane

1/21: Key West and Peoria

Variables

Pipe radius: 0.05 m, 0.075 m, 0.1 m, 0.15 m, 0.2 m

Pipe length: 10 m, 30 m, 50 m, 7 0m, 90 m

Air velocity: 2 m/s, 5 m/s, 8 m/s, 11 m/s, 14 m/s

Pipe depth: 1 m, 2.5 m, 4 m, 5.5 m, 7 m


temperatures were set at 14.3 8C, �17.7 8C, 4.5 8C and

�12.3 8C for Key West, Peoria, Phoenix and Spokane,

respectively. Table 4 shows the details of the parametric studies.

The ‘‘base case’’ values for each variable were set at:

0.075 m for pipe radius, 30 m for pipe length, 5 m/s for air

velocity, and 2.5 m for pipe depth. These values were selected

for the prediction of cooling and heating potential of the

system. In addition, when changing only one variable at every

simulation process for parametric studies, the other variables

were kept at those values. Table 5 describes the inputted soil

conditions and parameters for each location. The annual

average ground surface temperature, Tm, and amplitude of the

soil surface temperature variation, As, were calculated by a

utility program that is provided with EnergyPlus.

A three-zone residential building was chosen for this study.

The building is not actually existing building, but the

constructions and thermal conditions are thoroughly based

on the real buildings. It consists of a living space, an attached

garage and attic above living space and garage having floor

areas of approximately 140 m2, 37 m2, and 177 m2, respec-

tively. In this study, the living space will be analyzed, which is

located on the northern side of the building with the ceiling

height of 3.05 m. The internal heat gains for lights and

equipment were set at 5.4 W/m2 and two people were placed in

the living space during the simulation Infiltration was set at 0.25

ACH and the earth tube was set to run constantly at the same

volumetric flow rate of 285 m3/h during the whole running

period.

Table 5

Soil related parameters

Soil condition Tm (average) (8C) As (amplitude) (8C)

Key West Heavy and moist 24.3 5.0

Peoria Heavy and moist 10.0 18.8

Phoenix Heavy and dry 25.0 9.4

Spokane Heavy and dry 9.9 18.5

5. Parametric analysis

The effects of four important parameters significantly

affecting the performance of an earth tube system were

investigated through the parametric analysis. Since the findings

discussed later in this paper showed that earth tubes have more

potential for cooling than for heating, the parametric analysis

was carried out only for cooling.

5.1. Influence of pipe length

Fig. 3 presents the effect of pipe length on the earth tube inlet

air temperature at the highest ambient air temperature. As the

pipe length increases, the inlet air temperature decreases due to

the fact that the longer pipe provides a longer path over which

heat transfer between the pipe and the surrounding soil can take

place given the same overall heat transfer coefficient of earth

tube.

However, the temperature range and decrease rate in terms

of pipe length were different among each location due to the

different soil conditions, ambient air temperature, and soil

temperature distribution. In addition, Peoria and Spokane are

appropriate locations to employ earth tubes since the earth tube

inlet air temperature is lower than the specified indoor

temperature of 23 8C. In both cases of Peoria and Spokane

after a certain point around 50–70 m increasing the length does

not result in much better performance and the improvements

begin to level off, indicating that these values can be the optimal

design values in these special cases which can be determined

using the developed earth tube model.

5.2. Influence of pipe depth

Fig. 4 shows the influence of pipe depth under the ground

surface on the earth tube inlet air temperature. As the pipe depth

increases, the inlet air temperature decreases, indicating that the

earth tube should be placed as deeply as possible. However, the

trenching cost and other economic factors should be considered

when installing earth tubes.

Similarly the temperature range and decrease rate with

regard to pipe depth were different at each location due to

Fig. 3. Influence of pipe length on inlet temperature.

Fig. 6. Influence of pipe radius on inlet temperature.Fig. 4. Influence of pipe depth on inlet temperature.


different soil and weather conditions. In addition like the case

of pipe depth, Peoria and Spokane are the better places for the

utilization of earth tubes than Key West and Phoenix which

have higher soil temperatures and the earth tube inlet

temperature. Moreover, after a certain point (around 4–

5.5 m) the improvements begin to level off, which can be

considered to be the optimal design values without factoring in

economic factors such as trenching and drilling costs. Based on

these results, pipe depth appears to have as large of an influence

on earth tube performance as pipe length.

6. Influence of air velocity inside pipe

Fig. 5 presents the effect of air velocity inside the pipe on the

earth tube inlet air temperature. As the air flow rate increases

the inlet air temperature increases in all locations, since the air

spends more less in the tube and thus in contact with the lower

soil temperature. This can be seen in the earth tube modeling

equations since according to Eq. (19) a higher air flow rate

causes a higher mass flow rate and higher air temperature.

Likewise, the range and rate of increase of the inlet air

temperatures as a function of air velocity were different at each

location due to different soil conditions. Like the cases of other

parameters described above, Peoria and Spokane turned out to

be more appropriate locations for earth tubes, and there is a air

Fig. 5. Influence of air velocity on inlet temperature.

velocity limit above after which the inlet temperatures become

relatively constant despite additional increases in velocity.

However, when considering the air flow rate during the

design process, simply reducing the flow rate does not

necessarily improve the earth tube performance since the

cooling heat transfer rate due to the earth tubes depends on both

air flow rate and temperature difference, not on each factor

alone (q = maCaDT). Thus, both the air flow rate and the

temperature difference should be considered simultaneously.

6.1. Influence of pipe radius

Fig. 6 illustrates the effect of pipe radius on the earth tube

inlet air temperature. As the pipe radius increases, the earth tube

inlet air temperature also increases due to the fact that higher

pipe radius results in a lower convective heat transfer

coefficient on the pipe inner surface and a lower overall heat

transfer coefficient of earth tube system.

Similarly, the temperature range and the increase in the inlet

temperature in terms of pipe radius were different at each

location. Peoria and Spokane can be considered to be better

places for earth tubes due to a lower inlet air temperature than

the specified indoor temperature of 23 8C. Compared to the

other three parameters discussed above, pipe radius did not

affect the results as much as the other parameters. Moreover,

simply reducing the pipe radius under same air flow rate will

increase the air velocity inside the pipe, resulting in an increase

in the earth tube inlet air temperature. Thus pipe radius and air

flow rate should be considered together and optimized using

simulated results.

The trends of the results in terms of the influence of design

parameters on the performance discussed above were similar to

those of other studies (Mihalakakou et al., 1995).

7. Cooling and heating potential

The simulations were carried out for the investigation of the

cooling and heating potential of an earth tube by the

comparison of the results with and without earth tubes.

Figs. 7–10 illustrate the cooling and heating rate variation under

design day conditions with and without an earth tube in four

Fig. 7. Cooling/heating rate with and without earth tube (Key West).

Fig. 8. Cooling/heating rate with and without earth tube (Peoria).

Fig. 10. Cooling/heating rate with and without earth tube (Spokane).

Table 6


different locations. Table 6 describes the required cooling load

requirement of the building to maintain the indoor air

temperature of 23 8C during the whole summer design day

period with and without the earth tube. Similarly Table 7

describes the required heating load requirement of the building

to maintain the indoor air temperature of 20 8C during the

winter design day with and without the earth tube. As can be

Fig. 9. Cooling/heating rate with and without earth tube (Phoenix).

seen in Figs. 7–10, the cooling rate varies more than the heating

rate in all four locations since winter design days have constant

ambient air temperatures. The increases in the cooling rate

around hours 12–20 are due to the increase in the ambient air

temperature during that time period. The cooling rate around 2–

8 h reaches zero in Peoria and Spokane due to the fact that

ambient air temperature is low enough to maintain the

comfortable indoor temperature without air-conditioning and

earth tubes.

In case of cooling performance, whether or not the earth tube

is beneficial clearly depends on climatic conditions and

locations as can be seen in Table 6. The amount of reduced

total cooling load requirement due to an earth tube system were

6.6 kWh and 8.8 kWh in case of Peoria and Spokane,

respectively, an energy saving of 31% and 52%, respectively.

Therefore, large amounts of cooling energy can be saved by

employing earth tube systems. However, cooling load require-

ments with an earth tube system is even 41.8 kWh and

26.9 kWh higher than those without an earth tube in Key West

and Phoenix, respectively. This is due to the fact that the earth

tube inlet air temperatures were higher than specified indoor

temperature in the two locations, indicating that the hot weather

Total cooling load requirement of the building with and without an earth tube

Locations Cooling load requirement

without earth tube (kWh)

Cooling load requirement

with earth tube (kWh)

Key West 88.3 129.7

Peoria 20.8 14.3

Phoenix 79.7 106.6

Spokane 16.9 8.1

Table 7

Heating load requirement of the building with and without an earth tube

Locations Heating load requirement

without earth tube (kWh)

Heating load requirement

with earth tube (kWh)

Key West 20.9 21.2

Peoria 170.7 196.1

Phoenix 64.7 65.7

Spokane 142.7 161.3

Table 8

Total cooling load requirement under different earth tube air flow rates (design

day simulation)

Conditions Peoria Spokane

Cooling load requirement without

earth tube (kWh)

20.9 16.9

Cooling load requirement with

earth tube (0.75 ACH) (kWh)

14.3 8.1



12.8 6.2



14.2 7.0

Table 10

Heating load reduction by preheating the fresh air using an earth tube (design

day simulation)


with 0.35 ACH infiltration

rate (kWh)


with 0.35 ACH earth tube

air flow rate (kWh)

Key West 22.3 14.5

Peoria 184.1 159.3

Phoenix 69.7 52.0

Spokane 153.3 132.3


of the two locations also increased the soil temperature

significantly and that the weather and soil conditions should be

considered in order to determine whether or not earth tubes

should be used in particular locations.

Furthermore, additional simulations were carried out to

investigate the cooling load reduction variation under different

earth tube air flow rate conditions: 0.75, 1.5 and 3.0 ACH.

Since Peoria and Spokane showed the cooling load reduction

due to an earth tube, simulations were carried out only for

these two locations. Tables 8 and 9 show the total cooling load

requirements under different earth tube air flow rates

simulated during the design day and the whole cooling season

respectively. As can be seen in the tables, large portions of the

cooling loads are reduced due to earth tubes. In case of the

whole cooling season simulation, more than 50% of cooling

loads can be saved in both locations. In both the design day and

the whole cooling season simulations, the earth tube air flow

rate of 1.5 ACH turned out to cause the largest total cooling

load reduction, even larger than 3.0 ACH. This is due to the

fact that the earth tube inlet air temperature becomes higher as

the air velocity inside the pipe also gets higher due to the

higher earth tube air flow rate under the same pipe radius

condition, which can reduce the cooling provided by the earth

tube to the zone.

Therefore, it should be noted that the higher earth tube air

flow rate does not necessarily mean the highest cooling load

reduction due to the earth tube, indicating that the optimal air

flow rate under different conditions should be determined in

advance using an earth tube model in a program such as

EnergyPlus. Although the earth tube system did not appear to

have the capacity to replace the conventional air-conditioning

Table 9

Total cooling load requirement under different earth tube air flow rates (June–

August simulation)

Conditions Peoria Spokane

Cooling load requirement without

earth tube (kWh)

1099.8 1147.8



427.1 497.2



336.5 394.4



389.5 444.5

system completely, it can significantly reduce the cooling load

requirement to maintain the specified indoor conditions when

the weather and soil conditions are properly considered for each

location.

On the other hand, in case of heating performance, the

heating load requirements in all the four locations with an earth

tube were higher than those without an earth tube as can be seen

in Table 7. This is due to the fact that the earth tube inlet air

temperatures are not higher than the specified indoor

temperature within the comfort zone, indicating that an earth

tube inlet temperature high enough to provide heating cannot be

obtained. Therefore, an earth tube system has better capability

for cooling than for heating buildings given the same location.

Although an earth tube did not turned out to reduce the

heating load, additional simulations were carried out to find out

the contribution of earth tubes to the preheating of the fresh air.

According to ASHRAE Standard 62-1999, at least 0.35 ACH of

the air should be ventilated for living spaces. Therefore, two

cases were simulated: the case with the infiltration rate of 0.35

ACH without the assistance of an earth tube, and the case with

the earth tube air flow rate of 0.35 ACH. Tables 10 and 11 show

the heating load requirements of the two cases simulated during

the design day and the whole heating season, respectively. As

can be seen in the tables, a considerable amount of the heating

load can be reduced through the preheating of the fresh air using

earth tubes, though not as much as the cooling loads. In the case

of Phoenix simulated for the whole heating season, 45% of the

heating loads can be saved by preheating the fresh air. The

reason why the heating loads in Key West simulated for the

whole heating season are zero is that most ambient air

temperatures were higher than the specified indoor air

temperature of 20 8C even in the winter season and that the

indoor temperature did not fall below the 20 8C at any point

during the simulation.

Table 11

Heating load reduction by preheating the fresh air using an earth tube (Decem-

ber–February simulation)


with 0.35 ACH infiltration

rate (kWh)


with 0.35 ACH earth tube

air flow rate (kWh)

Key West 0 0

Peoria 9513.6 8542.6

Phoenix 1788.0 990.1

Spokane 8283.9 7564.3


8. Conclusion

In this paper, the algorithm for the simulation of earth tubes

is described, and validated against the data of other studies.

Parametric studies were carried out to investigate the effect of

each parameter on earth tube. The cooling and heating potential

of an earth tube was also investigated using the newly

developed model, and the following conclusions can be made

based on the data presented above:

1. A
s a result of the validation against the data from other
theoretical and experimental studies, the earth tube model

showed a good agreement with the work performed by

others. Thus it can be suitably used to predict the thermal

performance of earth tube systems.

2. A
deeply placed and longer earth tube with a lower air
velocity and smaller radius should result in lower earth tube

inlet temperatures. However, the trenching cost and other

factors should also be considered when installing earth tubes.

3. P
ipe length, air velocity inside pipe and pipe depth turned out
to have more influence on earth tube performance than pipe

radius. However, pipe radius and air flow rate as well as

cooling heat transfer rate should be considered simulta-

neously.

4. W
hen properly designed, an earth tube were shown to save
more than 50% of the total cooling load in the cases

presented in this paper, depending on the weather and soil

conditions. Although the earth tube alone cannot replace

conventional air-conditioning system in these case studies, it

can significantly reduce the cooling load in buildings.

5. E
arth tubes turned out to have a larger capacity for cooling
than for heating given the same location. Even among the

case of cooling, whether or not earth tubes are beneficial is

dependent on the local climate, indicating that weather and

soil conditions should be properly considered and investi-

gated using the new model.

The availability of an earth tube model in a program such as

EnergyPlus is an important step forward when attempting to

determine whether or not earth tubes should be used for a

particular building and to determine the most optimal

combination with regard to depth, length, radius, and air

velocity. Based on this study, future work that should be done

includes the economic assessment and the environmental

effects of the earth tube since those factors affect the feasibility

of the system.

Acknowledgment

The authors of this paper wish to thank the U.S. Department

of Energy’s Lawrence Berkeley National Laboratory for

funding the work which led to this paper under Grant 6712530.

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