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Performance simulation of a solar-assistedmicro-tri-generation system: hotel case study

Ana I. Palmero-Marrero* and Armando C. Oliveira

Faculty of Engineering, University of Porto (New Energy Tec. Unit), Rua Dr RobertoFrias, 4200-465 Porto, Portugal

*Corresponding author:

[email protected]

AbstractIn this work, a micro-tri-generation system integrated with a solar system is studied. A basic micro-cogeneration technology [micro-CHP (combined heat and power) system] integrating solar collectors,storage tank, micro-turbine and a thermodynamic cycle based on the organic Rankine cycle (ORC) iscombined with an absorption chiller. The heat rejected at the condenser of the micro-CHP system isused for water heating (WH), and the absorption chiller is used for space cooling. Hot water from thesolar storage tank is the heat source for the cooling system (absorption chiller) and the micro-CHPsystem. A heat exchanger is used to transfer heat from the hot water circuit to the power cycle (whichuses an organic refrigerant). The micro-CHP system under analysis uses a micro-turbine and an electricgenerator with a power output of 5 kW. The turbine inlet temperature is 808C and the working fluid iscyclohexane. The absorption chiller, which is a single-effect water-fired chiller, operates with a lithiumbromide and water mixture, and water inlet temperature is between 80 and 1008C. The performance fordifferent solar collector areas and tank capacities was evaluated through a numerical model. A hotelbuilding was used as a case study and the analysis was extended throughout the cooling season, forclimatic conditions of different European cities: Athens (Greece), Lisbon (Portugal), Madrid (Spain),Paris (France) and London (UK). The monthly average solar fraction was evaluated for different cases:the micro-CHP system, the cooling system and the micro-tri-generation system with the usefulcondenser energy used for WH. The solar fraction of the micro-CHP system was low, compared withthat of the cooling system, because the efficiency of the micro-CHP system is lower than 7%. However,when the tri-generation system is considered, the monthly average solar fraction is much higher, due tothe utilization of the condenser heat. The solar system, cooling system and its components weremodelled with the TRNSYS simulation program. The micro-CHP system was modelled with EESsoftware.

Keywords: micro-tri-generation; solar energy; absorption cooling; solar fraction

Received 11 June 2011; revised 7 July 2011; accepted 21 July 2011

1 INTRODUCTION

A tri-generation system, also referred to as CCHP (combinedcooling, heating and power), is a type of plant where the pro-duction of power, heating and cooling all come from the samesource, resulting in advantageous energy savings and environ-mental benefits over conventional generation. A slight differ-ence between CCHP and CHP (combined heat and power) isthat thermal or electrical/mechanical energy is further utilizedto provide space or process cooling capacity in a CCHP appli-cation [1]. In tri-generation plants, the waste heat from theplant prime mover is used to provide the energy needed forheating, for instance, domestic hot water [2]. Limited studieshave been made in micro-tri-generation (micro-scale CCHP)

systems for different applications [36]. There are someobstacles to the diffusion of these systems, such as a relativelyhigh initial cost and complex optimum matching of differentparts of the system, which limit their development [7].

From a sustainability perspective, the direct use of solarenergy as a primary energy source is attractive because of itsuniversal availability, low environmental impact and low or noon-going fuel cost. The fact that peak cooling demand insummer is associated with high solar energy availability offersan opportunity to further exploit solar energy for cooling [8].Solar-powered cooling is one of the technologies which allowsobtaining an important energy saving compared withtraditional air-conditioning plants [9]. Until now, there are afew studies on the utilization of solar energy as the primary

International Journal of Low-Carbon Technologies 2011, 6, 309317# The Author 2011. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]:10.1093/ijlct/ctr028 Advance Access Publication 20 September 2011 309

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energy source in tri-generation systems [10, 11], particularlywhen heating and cooling are used simultaneously. Therefore,this work aims to contribute to the performance assessment ofthis type of systems, with a hotel building used as a case study.

2 DESCRIPTION AND MODELLING OF THEMICRO-TRI-GENERATION SYSTEM

Figure 1 shows the integrated micro-tri-generation system.Hot water from the storage stank is the heat source for the

cooling sub-system (absorption chiller) and the micro-CHPsub-system. A heat exchanger is used to transfer heat from thehot water circuit to the power cycle (which uses an organicrefrigerant). System components are listed in Table 1.

The solar sub-system, cooling sub-system and itscomponents were modelled with the TRNSYS simulationprogram [12]. The micro-CHP sub-system was modelled withthe EES software [13]. The heat exchanger which connects thesolar and micro-CHP sub-systems was modelled with TRNSYS.

2.1 Solar sub-systemFor the solar system, certified flat-plate collectors with selectivecoating and tempered glass cover were considered. Theefficiency parameters are: F(ta)n 0.757, where (ta)n is thecollector transmittanceabsorptance product for normal

incidence, and FUL 4.0 W/m2/K, where F is the collector heatremoval factor and UL the collector heat loss coefficient, with awater flow rate of 0.02 kg/s/m2. The storage tank model didnot include thermal stratification. A storage volume equal to50 Acol (in litres) was considered, which is a typical value insolar thermal systems [14]. A control system activates the cir-culation pump, so that water is circulated in the collectors onlywhen the outlet temperature is higher than the storage temp-erature. The inclination of the solar collectors was consideredto be equal to the latitude.

2.2 Cooling sub-systemThe cooling system has an absorption chiller, which is a single-effect water-fired chiller, operating with a lithium bromide andwater mixture. It is a Yazaki chiller (model WFC-SC20). Datafor this model were used in the TRNSYS component (type

Figure 1. Schematic diagram of the micro-tri-generation system.

Table 1. Components of the micro-tri-generation system.

Solar sub-system Cooling sub-system Micro-CHP sub-system

Flat-plate collector Auxiliary boiler Sensible heat exchanger

Tank Absorption chiller Auxiliary boiler

Pumps Cooling tower Micro-turbine with a generator

Regenerator

Condenser

Pump

A.I. Palmero-Marrero and A.C. Oliveira

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107). The chiller characteristics were obtained from the catalo-gue (Table 2). It has a nominal cooling capacity of 73 kW. Theexternal auxiliary heater guarantees that the water inlet temp-erature is at 888C, which leads to a cooling coefficient of per-formance (COP) close to the maximum (0.7). Cooling waterfor the absorber is supposed to be available at a temperature of308C, which means that the use of a cooling tower was notexplicitly modelled.

The waterair heat exchanger is a counter-flow heat exchan-ger with a global heat transfer coefficient equal to 50 W/8C. Itis used for building space cooling and can be located insidethe building, as in the case of fan-coils.

2.3 Micro-CHP sub-systemThe micro-CHP sub-system under analysis uses a micro-turbine and an electric generator with a power output of 5 kW.This was chosen as the smallest turbine recently available inthe market [15]. A larger turbine was not considered, as thecycle would lead to a too high output heat, which is onecharacteristic of small organic Rankine cycles (ORCs)verylarge heat to power ratios. The turbo-generator has an overallefficiency around 70% [15]. In this system, the working fluid

(cyclohexane) expands in the turbine, passes in a regeneratorand condenses in the condenser. The heat is supplied throughthe tank connected with the solar collectors. In this case, aboiler is considered for supplying auxiliary energy (gas asenergy backup) when necessary. The heat rejected in the con-denser can be used for water heating (WH), or space heatingin the heating season. Previous studies have demonstrated thatcyclohexane presents a higher efficiency as the working fluidfor this type of micro-CHP system [16].

The inlet temperature in the micro-turbine was taken at808C. At the condenser outlet, the fluid is at 458C (saturatedliquid). The efficiency of electricity production was calculatedthrough Equation (1). This efficiency represents the ratiobetween electrical power obtained from the micro-CHP sub-system ( _W elect _Wpump) and thermal power input in theboiler and solar system, through the heat exchanger ( _Qinput).

h _Welect _Wpump

_Qinput1

Regeneration was considered if the temperature at the turbineoutlet (point 5) was higher than that at the pump outlet(point 2). A regenerator efficiency of 80% was considered:

1reg h5 h6h6 hP6;T2 2

2.4 Building characteristicsA hotel building was considered as a case study, using a modeldeveloped with TRNSYS, with three floors and 500 m2 in eachfloor. Each floor is considered mono-zone with identicalcharacteristics. Each floor has four external walls, a largewindow area on the south facade and smaller windows in theeast, west and north facades. All windows have double glazing(with clear glass) with a U-value equal to 3.21 W/m2K and ag-value equal to 0.72 (15% frame). The total window andfacade areas of the building are 180 and 851 m2. The ratio ofthe total window area to floor area is 12%. Per floor, thewindow area is 35 m2 for the south facade, 5 m2 for the northfacade and 10 m2 for the west and east facades. The facade areais 281 m2 for the south and north facades and 144 m2 for theeast and west facades. The window area included the frame.The characteristics and properties of building materials areshown in Table 3.

Besides the definition of geometry and materials, otherinput data considered were: infiltration rate of 0.6 air changesper hour, ventilation of 1 air change per hour, internal gainscorresponding to 150 persons and artificial lighting of 5 W/m2.The percentages of occupation were 100% from 11 p.m. to 6a.m., 30% from 6 a.m. to 10 a.m., 20% from 10 a.m. to 7 p.m.and 60% from 7 p.m. to 11 p.m., every day. The metabolic rate(heat production depending on the activity level) was assumedas 1.2 met (1 met 58.2 W/m2). This corresponds to seated

Table 2. Characteristics of the absorption chiller (commercial Yazakimodel) [19].

Model Yazaki WFC-SC20

Type (refrigerant/absorbent): H2O/LiBr, simple effect

Cooling capacity 73 kW

COP 0.7

Chilled water

Temperature

Inlet 12.78COutlet 7.08C

Evaporator pressure loss 66.0 kPa

Maximum operating pressure 588 kPa

Flow rate 3.06 l/s

Water retention volume 47 l

Cooling water

Heat rejection 177 kW

Temperature

Inlet 29.58COutlet 33.78C

Absorber/condenser pressure loss 45.3 kPa

Coil fouling factor 0.086 m2h K/kW


Flow rate 10.2 l/s


Heat medium

Heat input 104 kW

Temperature

Inlet 888COutlet 82.88CRange 75958C

Generator pressure loss 42 kPa


Flow rate 4.8 l/s


Performance simulation of a solar-assisted micro-tri-generation system

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and light activity at home, office etc. [17]. The scheduleassumed for the use of artificial lighting was from 8 p.m. to 1a.m. (100%), from 1 a.m. to 5 p.m. (45%) and from 5 p.m. to8 p.m. (80%), every day. Simulation results were obtainedusing the climatic conditions of Athens (Greece), Lisbon(Portugal), Madrid (Spain), Paris (France) and London (UK).Table 4 shows the latitude, longitude and elevation of thedifferent cities.

For circulation in the fan-coils, an onoff control wasassumed, with on when the indoor air temperature was equalto or higher than 248C, and off when the temperature waslower than 248C.

The climatic data were obtained through METEONORM,provided by TRNSYS and distributed under licence fromMeteotest [18]. METEONORM is a meteorological referencedatabase, incorporating a catalogue of meteorological data andcalculation procedures for solar applications and system designat any desired location in the world. It is based on over 23years of experience in the development of meteorologicaldatabases for energy applications.

The study was extended throughout the cooling season(from May to October).

3 SIMULATION RESULTS

3.1 Micro-cogeneration (micro-CHP sub-system)Figure 2 shows the temperature versus entropy diagram forcyclohexane. The different points corresponding to Figure 1are indicated in the diagram.

The turbine inlet fluid is saturated vapour, and the outletfluid, in the condenser and in the pump, is in saturated liquidcondition. The inlet temperature in the micro-turbine (T4) was

808C (saturated vapour), and the condenser outgoing fluid wasat 458C (saturated liquid, T1). Considering these values, thesimulation in EES gave the following temperature values:53.468C for the inlet temperature in the heat exchanger (T3);59.88C for the outlet temperature in the micro-turbine (T5);and 48.048C for the inlet temperature in the condenser.

Table 5 shows the thermal performance values for the ORC:the turbine inlet and outlet pressures (P4 and P5), the heat

Table 3. Thermophysical properties of building components.

Building

components

Material Thickness (m) Thermal conductivity

(kJ/h m K)

Thermal capacity

(kJ/kg K)

Density

(kg/m3)

U-value

(W/m2K)

Outwall 0.320 0.367

Inside Plaster 0.02 4.68 0.837 1900

Brick 0.20 1.368 0.936 1224

EPS (insulat.) 0.08 0.144 1.21 15

Back Cover 0.02 4.68 0.837 1900

Interior Wall 0.353 1.824

Inside Plywood (wood) 0.03 0.468 2.75 650

Weak concrete 0.10 7.2 1.080 2400

Stone 0.23 3.067 0.965 1320

Back Plaster 0.02 4.68 0.837 1900

Ground 0.370 0.332


Concrete 0.25 4.68 0.837 1900

Back EPS (insulat.) 0.10 0.144 1.21 15

Roof 0.370 0.332


Concrete 0.25 4.68 0.837 1900

Back EPS (insulat.) 0.10 0.144 1.21 15

Table 4. Weather data locations.

Cities Latitude (8N) Longitude (8W) Elevation (m)

Athens (Greece) 37.97 223.72 107

Lisbon (Portugal) 38.7 9.15 77

Madrid (Spain) 40.45 3.55 582

Paris (France) 48.82 22.33 75

London (UK) 51.6 0.12 77

Figure 2. Temperature versus entropy diagram for cyclohexane.





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required to obtain 5 kW of electricity (the heat from the solarsub-system and auxiliary boiler, _Qinput), the condenser heat( _Qcond), the flow rate ( _m), the pump power ( _Wp) and theelectrical efficiency (h).

The heat from the solar system and auxiliary boiler ( _Qinput)is the sum from the input heats for saturated liquid (frompoint 3 to point 30, _Qliq:sat 10:06 kW) and for liquidvapour

transition (from point 30 to point 4, _Qliq;vap 65:2 kW).When the solar sub-system is connected to the CHP sub-system, the heat obtained from the solar system (collectors andtank) through the heat exchanger is lower than 10.06 kW, ascan be seen in the calculation results. Thus, the fluid throughthe heat exchanger is only saturated liquid and a sensible heatexchanger must be chosen.

Table 5. Thermal performance of the cyclohexane ORC.

Fluid P4 (kPa) P5 (kPa) _Qinput kW _Qcond kW _m kg=s _Wp kW h (%)Cyclohexane 99.07 29.98 75.26 70.27 0.1829 0.01 6.63

Figure 3. Monthly average indoor air temperature with and without chiller, and outdoor ambient temperature during the cooling season (May to October) for:

(a) Athens, (b) Lisbon, (c) Madrid, (d) Paris and (e) London. Solar collector area of 250 m2.





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3.2 Micro-tri-generation systemThe solar sub-system, cooling sub-system, building and theheat exchanger of the micro-CHP sub-system were modelledwith TRNSYS. Initially, flat-plate collectors with Acol 250 m2and a storage tank with storage volume equal to 50 Acol(12 500 l) were considered.

Figure 3 shows the simulated average indoor air temperaturewith and without cooling system, and the outdoor ambienttemperature for all cities, during the different months(monthly average values were calculated and represented). Theindoor air temperature is the average temperature of the threefloors (all floors have similar temperatures).

Note that the average indoor air temperature is keptbetween 20 and 258C during the cooling season, when thecooling system is used. If the chiller is not used, comfortconditions are almost never reached in the building for Julyand August in all cities.

Different monthly solar fractions were calculated to assessthe system thermal performance. For the micro-CHPsub-system, the monthly average solar fraction (fCHP) dependson the total heat transferred between fluids in the heatexchanger (QHX) and Qinput (75.26 kW):

fCHP QHXQinput

QHXQaux;boiler QHX 3

For the cooling sub-system, monthly average solar fraction(fcooling) represents the percentage of energy input in thechiller that is due to solar energy and depends on the energydelivered to the chiller by the hot water (Qhw), auxiliary energy(Qaux, chill) and useful solar energy (Qsol usef ):

fcooling Qsol usefQhw

1 Qaux;chillQhw

4

The total solar fraction, when the micro-CHP and coolingsub-systems are considered, can be calculated through:

ftotal QHX Qsol usefQinput Qhw

QHX Qhw Qaux;chillQinput Qhw 5

The condenser heat ( _Qcond) of the micro-CHP sub-system canbe used for WH because the condenser outlet fluid is at 458C( _Qcond 70:27 kW; Table 4). If the system is working at fulltime in the cooling season (May to October), the useful con-denser energy obtained at 458C is 310 MWh. The energyconsumed for room hot water with a total water consumptionof 100 l of hot water per person and per day (with 150 personsin the hotel per day) is 101 MWh in the cooling season. Theremaining heat may be used to heat an existing swimmingpool, with a total estimated load of 209 MWh, during theperiod analysed. Therefore, it was considered that all condenserenergy is used for WH (QWH). When operating as atri-generation system (powerheatingcooling), the monthly

average solar fraction (fTRIG) is defined by:

fTRIG QHX Qsol usef QcondQinput Qhw QWH 6

Table 6 shows the energy performance values and Table 7shows the monthly average solar fraction results, for the fullsystem with 250 m2 of collector area.

Auxiliary energy for the chiller is needed only a few hoursper year, because the outlet temperature of the tank is higherthan 808C most of the time. Therefore, the solar fraction in thecooling system (fcooling) is near 100% when the cooling systemis used. Note that fcooling for London in May and October, andfor Paris in October, is zero because the indoor air temperaturewithout the chiller is lower than 208C and, therefore, thecooling system is not necessary.

The solar fraction of the micro-CHP sub-system is very low,because the electrical efficiency is lower than 7% (Table 5).When the tri-generation system is considered, the monthlyaverage solar fraction (fTRIG) varies between 55 and 58% in thecooling season, for all cities.

Figure 4 shows the monthly solar fraction for the totalsystem (CHP cooling) and the tri-generation system fordifferent solar collector areas (50, 250, 450 m2), for all cities.

Note that the monthly solar fraction varies considerablywhen the solar collector area increases from 50 to 250 m2.However, the solar fractions are similar for 250 and 450 m2

collector area. This variation is similar for all cities. Therefore,it is not interesting to use higher collector areas. For thissystem, a solar collector area equal to 250 m2 seems to beadequate for the requirements.

4 CONCLUSIONS

A micro-tri-generation system integrating a solar sub-system, amicro-CHP sub-system (with organic Rankine cycle) and acooling sub-system (absorption chiller) was evaluated. Theenergy performance and different monthly solar fractions wereobtained for a case studyhotel building.

The solar fraction of the micro-CHP sub-system was foundto be very low, compared with that of the cooling system,because the electrical efficiency of the micro-CHP system islower than 7%.

When the tri-generation system is considered (useful power,cooling and heating), the monthly average solar fraction(fTRIG) varies between 55 and 58% in the cooling season for allcities, when a collector area of 250 m2 is used. This value ishigher than ftotal (with CHP and cooling systems) due to theutilization of the condenser heat for WH. Also, the variationof the solar fractions with the solar collector area was studied.Solar fractions are similar for 250 and 450 m2, but higher thanthe ones obtained with 50 m2. A solar collector area of 250 m2

seems adequate for a system providing 5 kW of electricity, plusspace cooling for a building with 1500 m2 floor area and a WH





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Table 7. Monthly solar fraction for the CHP sub-system, cooling sub-system, total system (CHP cooling) and tri-generation system (Acol 250 m2).Month Athens Lisbon Madrid Paris London

fCHP(%)

fcooling(%)

ftotal(%)

fTRIG(%)

fCHP(%)

fcooling(%)

ftotal(%)

fTRIG(%)

fCHP(%)

fcooling(%)

ftotal(%)

fTRIG(%)

fCHP(%)

fcooling(%)

ftotal(%)

fTRIG(%)

fCHP(%)

fcooling(%)

ftotal(%)

fTRIG(%)

May 10 99 19 56 7 100 19 57 4 100 18 57 0 100 16 57 0 0 16 57

June 17 97 21 56 13 100 22 57 13 99 21 57 4 100 17 56 2 99 15 56

July 21 96 23 56 17 99 23 57 20 99 25 58 8 97 16 55 5 95 14 55

August 22 97 25 57 18 99 24 58 20 99 25 58 9 96 16 55 5 97 15 55

September 18 98 23 57 16 98 22 57 15 99 22 57 3 97 15 55 1 100 15 56

October 10 99 19 56 11 99 19 56 5 100 17 56 0 0 15 56 0 0 16 57

Season 16 97 22 57 14 99 22 57 13 99 22 57 4 97 16 56 2 97 15 56

Table 6. Energy performance results (Acol 250 m2).Month Athens Lisbon Madrid Paris London

QHW (chiller)

(MWh)

Qaux, chill(MWh)

QHX (CHP)

(MWh)

QHW (chiller)

(MWh)

Qaux, chill(MWh)

QHX (CHP)

(MWh)

QHW (chiller)

(MWh)

Qaux, chill(MWh)

QHX (CHP)

(MWh)

QHW (chiller)

(MWh)

Qaux, chill(MWh)

QHX (CHP)

(MWh)

QHW (chiller)

(MWh)

Qaux, chill(MWh)

QHX(CHP)

(MWh)

May 5.3 0.0 6.2 3.8 0.0 7.8 2.2 0.0 8.4 0.1 0.0 9.1 0.0 0.0 9.0

June 9.1 0.3 4.6 7.2 0.0 6.3 6.9 0.1 6.2 2.4 0.0 7.5 1.2 0.0 7.2

July 11.8 0.5 4.2 9.4 0.1 5.9 11.2 0.1 5.6 4.6 0.2 5.1 2.8 0.1 5.9

August 12.5 0.4 4.7 10.4 0.1 5.8 11.3 0.1 5.5 5.1 0.2 4.7 3.0 0.1 5.8

September 10.0 0.2 5.0 8.6 0.1 5.3 7.9 0.0 5.8 1.7 0.1 6.6 0.8 0.0 7.8

October 5.6 0.0 6.0 6.1 0.1 5.9 2.5 0.0 7.6 0.0 0.0 8.2 0.0 0.0 8.9

Season 54.2 1.4 30.8 45.5 0.4 37.1 41.9 0.3 39.2 13.9 0.4 41.2 7.8 0.2 44.6

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load of 301 kWh (enough for room hot water plus a swimmingpool), although performance depends on climatic conditions.

ACKNOWLEDGEMENTS

The authors wish to thank FCT (Fundacao para a Ciencia e aTecnologia) of the Ministerio da Ciencia, Tecnologia e EnsinoSuperior of Portugal, for partially funding the work done.

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Figure 4. Monthly solar fractions for different solar collector areas for: (a) Athens, (b) Lisbon, (c) Madrid, (d) Paris and (e) London.





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