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14 Exergetic performance evaluation of heat pump systems having


INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2008; 32:1279–1296 Published online 28 April 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/er.1414

Exergetic performance evaluation of heat pump systems having various heat sources
Ayd?n Dikici1 and Abdullah Akbulut2,*,y
2 1 Department of Mechanical Education, University of Firat, 23279 Elazig, Turkey Department of Mechanical Engineering, University of Dumlupinar, 43100 Kutahya, Turkey.

SUMMARY In this study heat pump systems having di?erent heat sources were investigated experimentally. Solar-assisted heat pump (SAHP), ground source heat pump (GSHP) and air source heat pump (ASHP) systems for domestic heating were tested. Additionally, their combination systems, such as solar-assisted-ground source heat pump (SAGSHP), solar-assisted-air source heat pump (SAASHP) and ground–air source heat pump (GSASHP) were tested. All the heat pump systems were designed and constructed in a test room with 60 m2 ?oor area in Firat University, Elazig (38:418N; 39:148E), Turkey. In evaluating the e?ciency of heat pump systems, the most commonly used measure is the energy or the ?rst law e?ciency, which is modi?ed to a coe?cient of performance for heat pump systems. However, for indicating the possibilities for thermodynamic improvement, inadequate energy analysis and exergy analysis are needed. This study presents an exergetic evaluation of SAHP, GSHP and ASHP and their combination systems. The exergy losses in each of the components of the heat pump systems are determined for average values of experimentally measured parameters. Exergy e?ciency in each of the components of the heat pump systems is also determined to assess their performances. The coe?cient of performance (COP) of the SAHP, GSHP and ASHP were obtained as 2.95, 2.44 and 2.33, whereas the exergy losses of the refrigerant subsystems were found to be 1.342, 1.705 and 1.942 kW, respectively. The COP of SAGSHP, SAASHP and GSASHP as multiple source heat pump systems were also determined to be 3.36, 2.90 and 2.14, whereas the exergy losses of the refrigerant subsystems were approximately 2.13, 2.996 and 3.113 kW, respectively. In addition, multiple source heat pump systems were compared with single source heat pump systems on the basis of the COP. Exergetic performance coe?cient (EPC) is introduced and is applied to the heat pump systems having various heat sources. The results imply that the functional forms of the EPC and ?rst law e?ciency are di?erent. Results show that Exloss;total becomes a minimum value when EPC has a maximum value. Copyright # 2008 John Wiley & Sons, Ltd.
KEY WORDS:

solar-assisted heat pump; ground source heat pump; air source heat pump; multiple source heat pump; energy analysis; exergy analysis; exergy e?ciency

1. INTRODUCTION Renewable energy sources have been the subjects of many theoretical and experimental investiga-

tions; their important roles in many aspects of engineering have recently given a new impetus for more detailed studies of these sources. All over the world, unfortunately, the fossil fuels are being

*Correspondence to: Abdullah Akbulut, Department of Mechanical Engineering, University of Dumlupinar, 43100 Kutahya, Turkey. y E-mail: aakbulut1@gmail.com Received 23 October 2007 Revised 7 February 2008 Accepted 10 February 2008

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depleted and as its inevitable result, the energy demand is increasing rapidly. As the renewable energy sources provide a solution to the energy demand, the necessity of using them are becoming more and more urgent. Recently, in many countries the heat pumps that are working with renewable energy sources have become a popular choice for both heating and cooling. Commonly the solar, soil, ambient air and geothermal energies are used as heat sources. The performance of heat pump systems has been improved considerably. The performance of heat transfer equipments used in a heat pump system such as condenser, evaporator and compressor can be improved by research and by engineering, the second law of thermodynamics and exergy concept must be used in their design. As it is known, exergy is the maximum work that can be produced from the given energy under ambient conditions. Owing to the exergy being the optimal use of energy, the exergy analysis is a useful method to establish for the design of operation of all energy resources and many industrial processes. As far as some recent studies conducted on exergy analysis of heat pump systems are concerned, Ding et al. [1] studied an improved air source heat pump (ASHP) and presented a new sub-cooling system employing a scroll compressor with supplementary injections that can e?ectively solve the problems. The prototype of an ASHP was validated and relevant dynamic performance was tested in this study. Ozgener and Hepbasli [2] investigated the performance analysis of a solarassisted ground source heat pump (SAGSHP) greenhouse system and researched the performance characteristics with a 50 m vertical 1 1/4 in. nominal diameter u-bend group heat exchanger. Kaygusuz and Ayhan [3] studied a solar-assisted heat pump (SAHP) system experimentally and analyzed the data using the exergy concept method. Gang Pei et al. [4] presented the performance of multi-functional domestic heat pump systems and discussed the working principles and the basic features of these systems. Gang Pei et al. [4] also reported that a multi-functional domestic heat pump system that can provide much better energy performance and higher equipment utilization causes less thermal pollution than the
Copyright # 2008 John Wiley & Sons, Ltd.

heat pump water heater and the conventional air conditioner. Koroneos et al. [5] studied exergy analysis of renewable energy sources and also presented the exergy analysis of solar energy, wind power and geothermal energy. Cervantes and Torres Reyes [6] studied exergy analysis and optimization of an SAHP system and discussed the optimum evaporation temperature as a function of a parameter that involves ambient conditions and the overall heat transfer coe?cient. Badescu [7] studied ?rst and second law analysis of an SAHP system and found that most of the exergy losses occur during compression and condensation. He reported that the photovoltaic array can provide all the energy required to drive the heat pump compressor. Hepbasli and Akdemir [8] studied energy and exergy analysis of a GSHP system. They also presented the exergy transports between the component and the consumptions in each of the components of the GSHP system. In this paper, theoretical and experimental studies of SAHP, ground source heat pump (GSHP), ASHP and multiple source heat pump systems (SAGSHP, solar-assisted-air source heat pump (SAASHP) and ground–air source heat pump (GSASHP)) are presented. The present study di?ers from the previously conducted studies on energy and exergy analysis of heat pump systems as follows: (i) it consists of single source and multiple source heat pump systems; (ii) it includes an investigation of some thermodynamic parameters such as exergy losses and exergy e?ciency and (iii) it calculates the exergetic coe?cient of performance (COP) values for the heat pump systems and the whole systems. Exergy losses and exergetic e?ciencies for each component of the heat pump systems are identi?ed.

2. SYSTEM DESCRIPTION AND MEASUREMENT PROCEDURE 2.1. Experimental setup Schematic diagrams of heat pump systems are shown in Figures 1 and 2. The experimental setup mainly consists of three separate circuits for gaining the heat from the energy sources: (i) the
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

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16 VIII 14 III 13 IX FLOW METER M 8 I DRYER 3 2 LIQUID TANK 3

7

V

OBSERVING GLASS

HOT AIR OUTLET II

2

(a)

15

CIRCULATION PUMP

OBSERVING GLASS

LIQUID TANK

3 V 5 11 IV 6 CIRCULATION PUMP 12 VII I 1 2

DRAYER

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HOT AIR OUTLET II

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(b)
OUTSIDE INSIDE OBSERVING GLASS VI 9 V 3 DRAYER LIQUID TANK 3 II

1

2

HOT AIR OUTLET 2

10

I WALL

(c)

Figure 1. Experimental setup of single heat pump systems: (a) SAHP; (b) GSHP; and (c) ASHP.

ground heat exchanger cycle or water–antifreeze solution cycle, (ii) solar collector and (iii) air source evaporator cycle. The speci?cation and characteristics of the main components of the SAHP, GSHP and ASHP systems are given in Table I, where the numbers in parentheses correspond to the elements given. Conversion of the heating cycle from one source
Copyright # 2008 John Wiley & Sons, Ltd.

heat pump system to another source heat pump system is obtained by using a four-way valve. To avoid freezing the water under the working condition during the winter season a 30% propylene glycol solution was prepared. The refrigerant cycle was built on the closed-loop copper tubing and R-22 was preferred as the refrigerant ?uid. In the experiments, six kinds of heat pumps were used
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16

14 VIII 14 7 13 III Vs 8 13 LIQUID TANK DRAYER 3 HOT AIR OUTLET 2 II

IX 1 6 IV 15 FLOW METER M CIRCULATION PUMPS FLOW M METER 11 3 5 Vg

OBSERVING GLASS 2 I

12

VII

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OUTSIDE 14 16 VIII 9 13 IX Va 7 Vs 3 8 DRAYER OBSERVING GLASS III LIQUID TANK 3 II INSIDE

HOT AIR OUTLET 2

VI

10

1

I

2

15 FLOW METER M

(b)
9 V 3

WALL CIRCULATION PUMP

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LIQUID TANK 3 HOT AIR OUTLET 2 II

OUTSIDE AIR VI 10 11

1 6 IV 5

I

2

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V

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12

3 VII

(c)

FLOW METER

M

Figure 2. Experimental setup of multiple systems: (a) SAGSHP; (b) SAASHP; and (c) GSASHP.

Copyright # 2008 John Wiley & Sons, Ltd.

Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

Table I. The main components speci?cation and characteristics of the SAGSASHP system. Component Compressor (I) Technical speci?cation

Main circuit

Copyright # 2008 John Wiley & Sons, Ltd.

Refrigerant circuit

Condenser and fan

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Sources of heat pumps

Solar source evaporator (III) Capillary tube (V) Ground source evaporator (IV) Dryer Fan of air condenser (II) Ground heat exchanger (VII) Circulating pumps for GHE and SHE Solar collector (IX) Solar heat exchanger (VIII) Air source evaporator (VI) Air source evaporator (VI)

Type: Hermetic; speed: 2900 rpm; the rated power of electric motor driving: 1.5 HP; refrigerant: R-22; connection: Suction line 5/8 in, discharge line 3/8 in Heat transfer surface: 08 * 78 * 41 mm; connection: 3/4–1/2 in; maximum ?ow rate: 3:6 in3 h?1 Copper capillary tube: 2 m; Inside diameter: 1.5 mm Heat transfer surface: 208*78*41 mm; connection: 3/4–1/2 in; maximum ?ow rate: 3:6 in3 h?1 Capacity: 11,1–12,8 kW; connection: 3/8 in Heat transfer surface: 5:5 m2 ; type: aluminum ?n, 0.012 m thickness; connection copper pipe: 3/8 in; power of fan: 50 W; Diameter: 0.3 m; capacity: 1000 m3 h?1 Horizontal heat exchanger; pipe distance: 0.37 m; pipe diameter: 0.017 m; piping depth: 1 m; material: polyethylene, PX-B cross link Three speed; speed step: 1250, 1750, 2250 rpm; power: 40, 62, 83 W 1:85 m2 ; total surface area of six collector: 11:1 m2 ; standard ?at-plate Capacity of tank: 180 l; thickness of tank: 0.002 m; isolation: glass wool; thickness 0.005 m; type: spiral copper pipe of 13.5 m length and 3/8 in diameter Heat transfer surface: 5:5 m2 ; power of fan: 50 W; type of evaporator: 0.12 mm thickness, aluminum ?nned and 3/8 in diameter copper pipe Heat transfer surface: 5:5 m2 ; power of fan: 50 W; type of evaporator: 0.12 mm thickness, aluminum ?nned and 3/8 in diameter copper pipe

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as mentioned above (SAHP, GSHP, ASHP, SAGSHP, SAASHP and GSASHP). In the winter, the water–antifreeze mixture in the pipes of the ground heat exchanger extracts the heat from the soil and carries it indoors. The heat transfer from the soil to the heat pump is maintained by the water–antifreeze mixture circulated through the ground heat exchanger for the GSHP. The heat transfer from the solar radiation to the heat pump is obtained from closed-loop solar collector-heat exchanger circuit for the SAHP. The heat transfer from the outdoor temperature by air source evaporator to the heat pump is very high in summer or extremely low in winter. The capacity of the ASHP decreases sharply when the outdoor temperature deviates worse than that of mild working conditions. The ?uid transfers its heat to the refrigerant ?uid in the evaporator for each heat pump system. The refrigerant, which ?ows through closed loop in the heat pump, evaporates by absorbing the heat from the water–antifreeze solution circulated through the evaporators for each system and then enters the compressor. The refrigerant is compressed by the compressor and enters the condenser, which condenses it. After the refrigerant leaves the condenser, the liquid tube provides safety to reduce the risk of liquid droplets entering the compressor. Then a condenser’s fan blows the warmed air into the room. 2.2. Experimental procedure During the experiment, the following experimental data were regularly recorded with a time interval of 30 min: (a) Measurement of solar radiation by a Kipp– Zonen Solar meter. (b) Measurement of temperature of the inlet and outlet temperatures of R-22 in the condenser, compressor and evaporator by T-Type thermocouples. (c) Measurement of the temperature of water– antifreeze solution while entering and leaving the ground heat exchanger and solar collector. (d) Measurement of the outdoor and indoor air temperatures.
Copyright # 2008 John Wiley & Sons, Ltd.

(e) Measurement of inlet and outlet air temperatures of the condenser. (f) Measurement of mass ?ow rates of the water–antifreeze solution, which ?ows through the ground heat exchanger and solar heat exchanger by rotometer. (g) Measurement of mass ?ow rates of refrigeration by a ?ow meter. (h) Measurement of volumetric air ?ow rates of the condenser by wind rose. (i) Measurement of the inlet and outlet pressures of the compressor and evaporators by using Bourdon-type manometers. (j) Measurement of the electrical power input to the compressor and circulating pump by a wattmeter. The experiments were conducted on the six heat pump systems under steady-state conditions in the heating mode. The average values of some measurements for each source of heat pump system with an interval of 30 min were measured. Brie?y, the temperatures, pressure drops, ?ow rates and voltages were measured during the experiments. 2.3. Uncertainty analysis Errors and uncertainties in the experiments can arise from selection, condition and calibration of the instruments, environment, observation, reading and test planning [8, 9]. A more precise method of estimating uncertainty in experimental results has been presented by Holman [9]. The method is based on a careful speci?cation of uncertainties in the various primary experimental measurements. These measurements are then used to calculate some desired result of the experiments. The total uncertainties of the measurements are estimated to be 3.43% for the mass ?ow rates, ?1:65% for the water temperatures, ?2:95% for pressures, ?3:00% for compressor electric current, ?1:40% for the air temperature, ?2:75% for power inputs to the compressor, ?1:75% for the solar radiation and ?3:35% for the refrigerant temperatures. Uncertainty in reading values for the table is assumed to be ?0:35: The total uncertainties of calculated parameters are estimated to be ?3:13% for the
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exergy losses, ?1:57% for the exergy e?ciencies, ?5:29% for the COP values and ?3:32% for the exergetic COP values. Uncertainty analysis calculation procedure is given in Appendix A.

sor, Sc is the speed of compressor (rpm) and Vrsv is the speci?c volume of the refrigerant inlet to the compressor ?m3 kg?1 ?: The general energy balance is ’ ’ E in ? E out ?3? ’ where E in is the rate of net energy transfer by heat, ’ work and mass, E out is the rate of net energy transfer out by heat, work and mass. The general energy balance can also be written as follows: X X ’ ’ Q? min hin ? W ? mout hout ?4? ’ ’ ’ ’ where Q is the rate of net heat input, W is the rate of net work output and h is the enthalpy of per unit mass. The exergy associated with mass ?ow can be divided into four components, namely chemical exergy Exche ; physical exergy Exphy ; kinetic exergy Exkin and potential exergy Expot [4] Ex ? Exche ? Exphy ? Exkin ? Expot ’ ’ ’ Exin ? Exout ? Exloss ?5? ?6?

3. ANALYSIS Since the exergy is the optimal use of energy, exergy analysis is a useful method to establish for the design of operation of all energy resources and many industrial processes. This method derived from laws of thermodynamics can be used to identify the main sources of irreversibility and to minimize the generation of entropy in a given process where the transfer of energy and matter takes place [6, 10, 11]. Basically, the terms of exergy, available energy and availability are similar. The concepts of exergy destruction, exergy consumption, irreversibility and lost work are also essentially similar [2, 12, 13]. Exergy is the measurement of maximum useful work that can be produced by the system interacting with an environment having constant pressure ?P0 ? and temperature ?T0 ? [2]. The exergy transfer can be associated with mass ?ow rate, work interaction and/or with heat interaction [14]. 3.1. Mass, energy and exergy balance equations and performance analysis For a general steady-state, steady-?ow process, namely mass, energy and exergy balance equations are given to ?nd the heat input, the rate of exergy losses, the e?ciencies of the energy and exergy [2, 15–17]. The mass balance equation can be expressed in the rate form as follows: X X min ? mout ?1? ’ ’ where min is the inlet mass ?ow rate and mout is the ’ ’ outlet mass ?ow rate. The mass ?ow rate of the refrigerant R-22 can be expressed as follows: Z Vc Sc mr ? c ?2? ’ Vsrv where mr is the mass ?ow rate of refrigerant ’ ?kg s?1 ?; Vc is the stroke volume of the compressor, Zc is the volumetric e?ciency of the compresCopyright # 2008 John Wiley & Sons, Ltd.

’ ’ where Exin ? Exout is the rate of exergy transfer by ’ heat, work and mass and Exloss is the rate of exergy loss. Brie?y, in the GSHP, SAHP and ASHP systems, the exergy transfer in the terms of mass ?ow and heat interaction can be de?ned as follows: ’ ’ ’ ’ ’ Exheat ? Exwork ? Exmass;in ? Exmass;out ? Exloss ?7? The rate of the general exergy balance can also be written as  X X X T0 ’ ’ 1? Qi ? W ? min cin ? mout cout ’ ’ Ti ’ ? E loss c is the ?ow exergy and can be expressed as c ? ?h ? h0 ? ? T0 ?s ? s0 ? ?9? ’ where Qi is the heat transfer rate, Ti is the ’ temperature at which the heat transfer occurs, W is the work rate, h is the enthalpy, s is the entropy and the subscript zero indicates properties at the dead state of P0 and T0 : The rate form of the entropy balance can be expressed as ’ ’ ’ Sin ? S out ? S gen ? 0 ?10?
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

?8?

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’ ’ where S in ? Sout is the rate of net entropy ’ transferred by heat and mass and S gen is the rate of entropy generation, where the rates of entropy ’ transfer by heat transferred at a rate of Qi and ’ ’ mass ?owing at a rate of m are S heat ? Qi =Ti and ’ ’ S mass ? m ? S: The rate form of the general ’ entropy equation can also be written as X X XQ ’ ’ ?11? Sgen ? mout S out ? mi S i ? ’ ’ ’ ’ T ’ The irreversibility rate I directly from the following equation is ’ I ? T0 Sgen ?12? The exergetic analysis allows one to evaluate for each component of the system the exergy destroyed or exergy loss and to determine which components have the most e?ects on the overall system ine?ciency. Exergetic or exergy e?ciency (second low e?ciency) can be written as follows [3]: ’ Exloss ZII ? 1 ? ?13? ’ Exin The coe?cient of performance of the overall heating system ?COPsys ? can be de?ned as ’ Qc or COPsys ? Wsys COPsys ? Qc Wcomp ? Wfancoils ? Wpumps ?14?

The balance equations of mass, energy and exergy for the SAHP, GSHP and ASHP components are obtained by using Equations (1)–(12) and are summarized in Table II. The exergy equations and exergy loss of each component were calculated using thermodynamic properties of water and R-22.

4. RESULTS AND DISCUSSION The following assumptions are used during the energy and exergy analyses: (a) All processes are in steady state and in steady ?ow with negligible potential and kinetic energy e?ects and no chemical reactions. (b) Heat transfer to the system and work transfer from the system are positive. (c) Air is an ideal gas with a constant speci?c heat. In the present study, the experimental results in heating season from January to March in 2003 and 2004 are given to determine the performance coe?cient, energy, exergy and exergetic e?ciency analysis of the heat pump systems. Table III presents the mass ?ow rate, temperature, pressure, speci?c enthalpy and entropy for the R-22, water, water–antifreeze solution and brine according to their state numbers speci?ed in Figure 1. In this study, the restricted dead states were taken to be the environment at which the temperature and the atmospheric pressure are 258C and 101.325 kPa, respectively. The thermodynamic properties data of R-22 and water can be obtained from Reference [17]. The solar collector, ground heat exchanger and the air source evaporator are used as heat sources to heat the R-22 in the heat pump cycle. In other words the heating performance of the heat pump systems is a function of the water temperature produced by the solar collector and the ground heat exchanger used in SAHP and GSHP, respectively. Moreover air temperature produced by the air source evaporator is the key parameter to determine the performance of the ASHP system. These experiments and theoretical analyses were carried out to determine the highest heating
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

’ where Qc is the heat transfer rate of the condenser, Wsys is total work loaded system, Wcomp ; Wfancoils and Wpumps are the rates of work inputs to the compressor, fan coils and circulating pumps. The collector’s instantaneous e?ciency is computed as follows: Qu ?15? Zc ? Ac I A new exergetic criterion called exergetic performance coe?cient (EPC) allows evaluating the in?uence of irreversibility in the system on the performance and can be formulated as follows [18]. ’ Qcond EPC ? ?16? Exloss;total Total exergy loss rate of the system can be calculated with the sum of each component’s respective exergy loss rate.
Copyright # 2008 John Wiley & Sons, Ltd.

Table II. The balance equation for the SAHP, GSHP and ASHP system components. Mass analysis using Equation (1) m1 ? m2 ? mr ’ ’ ’ m2 ? m3 ? mr ’ ’ ’ m3 ? m4 ? mr ’ ’ ’ m14 ? m13 ? mw ’ ’ ’ m12 ? m11 ? mw;a ’ ’ ’ m9 ? m10 ? mr ’ ’ ’ ’ ’ Qeva ? Qfan ? mr ?h10 ? h9 ? ’ ’ Qeva ? mr ?h6 ? h5 ? ’ h3 ? h4 ’ Qeva ? mr ?h4 ? h7 ? ’ ’ Qcond ? mr ?h2 ? h3 ? ’ ’ W comp ? mr ?h2 ? h1 ? ’ ’ ’ Exloss ? mr ??h1 ? h2 ? ? T0 ?s1 ? s2 ?? ? W ’ Energy analysis using Equations (3) and (4) Exloss using Equations (1)–(12)

Component

Copyright # 2008 John Wiley & Sons, Ltd.

Compressor

Condenser

Capillary tube

Evaporator A-SAHP

  ’ loss ? mr ??h2 ? h3 ? ? T0 ?s2 ? s3 ?? ? Qc 1 ? T0 ’ Ex ’ Tout;r ’ Exloss ? mr ??h3 ? h4 ? ? T0 ?s3 ? s4 ?? ’ ’ Exloss ? mr ??h7 ? h4 ? ? T0 ?s7 ? s4 ?? ? mw ??h14 ? h13 ? ? T0 ?s14 ? s13 ?? ’ ’

B-GSHP

C-ASHP

Ground heat exchanger m15 ? m16 ? mbrine ’ ’ ’ m14 ? m13 ? mw ’ ’ ’ ’ Qu ? mw Cpw ?T14 ? T13 ? ’ ’ Qrad ? Ac ? G

m12 ? m11 ? mw;a ’ ’ ’

’ Qghe ? mw;a Cpw;a ?T12 ? T11 ? ’

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Solar collector

Solar heat exchanger

’ Exloss ? mr ??h5 ? h6 ? ? T0 ?s5 ? s6 ?? ? mw;a ??h11 ? h12 ? ? T0 ?s11 ? s12 ?? ’ ’   T0 ’ ’ Exloss ? mr ??h9 ? h10 ? ? T0 ?s9 ? s10 ?? ? Qfan 1 ? ’ Tout;air   T0 ’ ? mr ??h9 ? h10 ? ? T0 ?s9 ? s10 ?? ? Qfan 1 ? ’ Tin;r   T0 ’ ’ Exloss ? mw;a ??h11 ? h12 ? ? T0 ?s11 ? s12 ?? ? Qghe 1 ? ’ Tsoil   T0 ’ ’ Exloss ? mbrine ??h15 ? h16 ? ? T0 ?s15 ? s16 ?? ? Qrad 1 ? ’ Tp   T0 ’ ’ ?Tp ? Tw ? Exloss ? mw ??h13 ? h14 ? ? T0 ?s13 ? s14 ?? ? Qu 1 ? ’ Tw

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Table III. Thermodynamic properties used in (a) single systems and (b) multiple systems.
Temperature, T ?8C? Pressure, P (kPa) Speci?c enthalpy, h ?kJ kg?1 ? Speci?c entropy, s ?kJ kg?1 K?1 ?

State number Fluid S 25 25 12 103 103 19.6 19.6 2 } } 2 12 } } } } 42 44.8 19 47 } } } } 25 25 19.6 102 102 27.4 2.6 } ?1.1 } } 2.6 41.8 25 2.8 110 110 27.5 } ?1.3 ?0.5 ?1.3 3.1 } } 25 100 100 166 1115 1115 1090 480 490 } 490 482 480 166 } } } } 25 25 20.3 97.1 97.1 22 3.1 ?1.1 } ?1.1 2.7 3.1 30.2 100 150 150 200 } } } } 100 100 200 1180 1180 1115 450 } 480 } } 450 200 3.4 } } 200 } } } } } 100 100 220 1130 1130 1116 } 460 550 460 170 } } } 175.9 187.8 81.1 196.9 274 104 270 315.1 315.1 135 253 128.6 } 128.6 252.2 126 276.6 14.45 } } } } 274 104 269 318.4 318.4 154 253 } 132 } } 126 284 } } } } } 274 104 257.5 325.3 325.3 164.8 } 250 44 126 258.7 } } 3.1 116.9 116.9 21.5 21.5 ?0.8 ?0.8 3.1 } } } } 1.3 0.14 127.2 127.2 26.3 26.3 ?1.1 } } } } ?1.1 0.14 } 400 1104 1104 900 900 420 } } 420 400 } } } 550 1102 1102 950 950 480 480 550 } } } } 150 500 1102 1102 1080 1080 480 } } } } 480 500 } 260.4 320.2 320.2 128 128 126 } } 126 260.4 } } } 248.4 320.8 320.8 131.4 131.4 129 129 258.4 } } } } 5.571 258.4 335.4 335.4 155.2 155.2 154.4 } } } } 145.4 258.4 } 25 25 100 100 100 104 104 104 0.36 0.984 1.074 1.074 0.462 0.462 0.474 } } 0.474 0.984 } } } } 0.599 0.635 0.282 0.67 1.16 0.36 1.06 1.13 1.13 0.56 0.94 0.484 } 0.484 0.938 0.4743 1.119 25 25 100 100 100 274 274 274 1.16 1.16 0.36 0.996 1.075 1.075 0.525 0.525 0.485 0.485 1.113 } } } } 0.019 0.051 } } } } 1.16 0.36 0.997 1.0637 1.0637 0.543 0.94 } 0.499 } } 0.4743 1.126 G A S G A S G A S G R-22 } } 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.35 0.35 0.1 0.1 0.033 0.033 } } 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 Water

Item number

Mass ?ow rate, m ’ ?kg s?1 ?

A 1.16 0.36 1.01 1.15 1.15 0.615 0.615 0.634 } } } } 0.634 1.113 } } } } } } 1.16 0.36 0.975 1.0857 1.0857 0.589 } 0.93 0.03 0.4743 1.055 } }

Copyright # 2008 John Wiley & Sons, Ltd. A. DIKICI AND A. AKBULUT R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 Water, antifreeze Water, antifreeze Water Water Brine Brine R-22 Water R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22 R-22

(a) 0 (Dead state) 0 (Dead state) 1 2 2 3 3 4 5 6 7 8 9 10 11

I (inlet) I (outlet) II (inlet) II (outlet) V (inlet) V (outlet) IV (inlet) IV (outlet) III (inlet) III (outlet) VI (inlet) VI (outlet) VII (inlet)

12

VII (outlet)

13 14 15 16

VIII (inlet) VIII (outlet) IX (inlet) IX (outlet)

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(b) 0 (Dead state) 0 (Dead state) 1 2 2 3 4 4 4 5 6 7 8

0

I (inlet) I (outlet) II (inlet) II (outlet) Vs (outlet) Vg (outlet) Va (outlet) IV (inlet) IV (outlet) III (inlet) III (outlet)

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performance and lowest exergy loss in each of the heat pump systems. It is obvious from Table IV that the heating load of the condenser for SAHP, GSHP and ASHP systems are determined to be 4.805, 4.735 and 4.505 kW, respectively. The heating load of the evaporator is 3.36 kW for the SAHP system. The heating load of the evaporator for the GSHP and ASHP systems are obtained to be 3.235 and 2.6 kW, respectively. The compressor power depends on inlet and outlet pressures. If the temperature di?erence between the condensing and evaporating temperatures decreases, it will reduce the power of the compressor. The highest and the lowest compressor powers occur in ASHP and SAHP systems. It is clear from Table IV that the compressor powers of SAHP, GSHP and ASHP are obtained to be 1.495, 1.81 and 1.925 kW, respectively. Table IV represents the COPsys values of the heat pump systems. The highest COPsys occurs in SAHP, which is 2.95. The lowest COPsys is approximately 2.22 in ASHP. For GSHP, the COP value is estimated to be 2.44. These results show that the increasing the temperature di?erence between inlet and outlet of the condenser and evaporator increases the heating load and consequently increases the COPsys : Additionally, the increasing mass ?ow rate of the compressor increases the required electric power and consequently decreases COPsys : By comparison, Ozgener et al. [2] reported that the COPsys values for the whole system are obtained to be 2.00 on a cloudy day and 3.13 at the end of a sunny day for the SAGSHP system. It may be concluded that the COPsys values obtained from the present study are fairly close to those reported by Ozgener et al. [2]. The variation of COPsys ; heating load of the condenser and evaporator, power input are compared at di?erent inlet and outlet temperatures in each of the multiple source heat pump systems. Each multiple source heat pump system has two evaporators. The compressor outlet temperature increases with the decrease in outlet evaporator temperature. The compressor input power increases with respect to increase in temperature di?erence between inlet and outlet at the compressor, and
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

0.499 1.053 0.03

Speci?c entropy, s ?kJ kg?1 K?1 ?

A

0.499 1.0904 }

131.8 258 8.573

19.893 }

Speci?c enthalpy, h ?kJ kg?1 ?

A

175.97 181.89 82.67 206.1 13 14 15 16 VIII (inlet) VIII (outlet) IX (inlet) IX (outlet) 0.1 0.1 0.033 0.033 30.9 32.8 15.3 38 41 43.4 19.4 49.2 } } } } 200 210 200 240 250 180 260 165 } } } } 125.9 137.65 65.8 159.4

132.8 259.3 }

} } } } S: SAHP system; G: GSHP system; A: ASHP system; SG: SAGSHP system; SA: SAASHP system; GA: GSASHP system.

G

} } 8.543

550 170 210

Pressure, P (kPa)

480 120 }

Table III. Continued.

} } 180

?0.5 2.1 2

Temperature, T ?8C?

?1.2 2.68 }

Mass ?ow rate, m ’ ?kg s?1 ?

0.025 0.025 0.3–0.28

} } 2

S

State number

9 10 11

Copyright # 2008 John Wiley & Sons, Ltd.

12

VII (outlet)

VI (inlet) VI (outlet) VII (inlet)

Item number

R-22 R-22 Water, antifreeze Water, antifreeze Water Water Brine Brine

Fluid

0.3–0.28

4.1

}

G

4.7

A

210

S

}

G

180

A

17.4

S

0.4368 0.4752 0.230 0.545

0.0625

} } 0.03

S

0.599 0.617 0.285 0.693

G

}

} } } }

1290
Capillary tube (kW) COPsys

A. DIKICI AND A. AKBULUT

increases the mass ?ow rate of the refrigerant. Besides, the compressor input power is approximately 1.127 kW when the compressor inlet temperature is 20:38C and compressor outlet temperature is 97:18C in the SAGSHP system. The variation of compressor power input is obtained to be 1.235 when the compressor inlet and outlet temperatures were 19.6 and 1028C; respectively in the SAASHP system. As for the GSASHP system, the power input is found to be 1.695 when the compressor input and output temperatures were, 2.8 and 1108C; respectively. The heating load increases with the decrease in the condenser outlet temperature. For the SAGSHP system, the condenser heating load is obtained to be 1.329 kW when the condenser outlet temperature is 228C: The variations of condenser heating load for SAASHP and GSASHP systems are found to be 2.139 and 1.985 kW, when the condenser outlet temperatures are 27.4 and 27:58 C; respectively. It is obvious from the variations given in Table III that the COPsys is 3.36 for the SAGSHP system. For the SAASHP system and the GSASHP system, the COPsys are obtained to be 2.90 and 2.14, respectively. Compared with the measured di?erent state temperatures, the variation of COPsys increased with the increase in the condenser heating load and the decrease in the compressor power input. It is obvious that the heating load of the evaporator, in relation to the inlet and outlet temperatures of R-22, depends on the type of heat source and the inlet and outlet ?uid temperature to the evaporator. For the SAGSHP and SAASHP heating load occurring in the solar source evaporator, the evaporator heating load increased with the increase in inlet water temperature to the solar source evaporator. The evaporator heating load is obtained to be 3.765 kW when the water inlet temperature to the evaporator was 32:88C for the SAGSHP system. The variation in solar source evaporator heating load for the SAASHP system is found as 3.95 kW when the water inlet temperature was 43:48C: For the SAASHP system, the inlet and outlet temperatures of the R-22 were approximately 2.6 and 41:88C: The ground source evaporator outlet temperatures were 2.7 and 3:18C: The variations in ground
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

Solar collector (kW)

Solar collector (kW) Solar heat exchanger (kW) Ground heat exchanger (kW) Air evaporator (kW) Ground evaporator (kW) Solar evaporator (kW) Evaporator (kW) Condenser (kW) Compressor (kW) Heat pump systems

3.45 } } 3.80 4.05 }

Table IV. (a) Energy analysis results of the components and (b) exergy loss analysis results of the components.

Solar heat exchanger (kW)

Ground heat exchanger (kW)

} 2.988 } 3.648 } 3.4944

Air evaporator (kW)

Ground evaporator (kW)

} } } 3.09 } 3.3175

} } } } 3.162 3.155

Solar evaporator (kW)

} } } 3.765 3.95 }

Evaporator (kW)

Condenser (kW)

?4.805 ?4.735 ?4.505 ?4.502 ?4.11 ?4.01

Compressor (kW)

?1.495 ?1.81 ?1.925 ?1.127 ?1.235 ?1.695

Heat pump systems

Copyright # 2008 John Wiley & Sons, Ltd.

(b) SAHP GSHP ASHP SAGSHP SAASHP GSASHP

(a) SAHP GSHP ASHP SAGSHP SAASHP GSASHP

0.8 0.75 0.616 0.598 0.637 0.824

0.242 0.395 0.521 0.203 0.22 0.303

0.43 0.56 0.650 } } }

3.36 2.88 2.6 } } }

} } } 1.033 0.896 }

} } } 0.296 } 1.011

} } } } 1.243 0.975

} 0.148 } 0.072 } 0.113

0.105 } } 0.136 0.103 }

1.17 } } 0.78 1.01 }

2.16 } } 1.52 1.97 }

0.14 0.12 0.16 0.46 0.477 0.481

2.95 2.44 2.22 3.36 2.90 2.14

EXERGETIC PERFORMANCE EVALUATION

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source evaporator heating load for SAGSHP and GSASHP are obtained to be 3.09 and 3.317, respectively. For the SAASHP system, the heating load of air source evaporator is obtained to be 3.162 kW when the temperature di?erence of R-22 between inlet and outlet at the evaporator was 3:18C: The variation of the heating load of air source evaporator for the GSASHP system is found as 3.155 kW when the temperature di?erence in R-22 was approximately 2:68C: Taking into consideration the temperature measured di?erent state, the heating load of the evaporator is better for the solar source evaporator than for the ground source evaporator and air source evaporator. For the multiple source heat pump system, the highest heating load of condenser is obtained to be 4.502 kW in the SAGSHP system and the lowest heating load of condenser is approximately 4.01 kW in the GSASHP system. Table IV illustrates the values for exergy loss as a function of thermodynamic parameters. Exergy loss values are evaluated for each component in the SAHP, GSHP, ASHP, SAGSHP, SAASHP and GSASHP systems. The causes of exergy loss in each source of the heat pump systems include the compressor, condenser, evaporator, capillary tube, ground heat exchanger, solar heat exchanger and solar collector and solar source evaporator, ground source evaporator and air source evaporator for multiple-source heat pump systems. Table IV presents the exergy losses occurring in each of the components of the systems. As can be seen from the results given in Table IV, the exergy losses in the solar collectors have the greatest value. The exergy losses in the solar collector used in the SAHP, SAASHP and SAGSHP systems are obtained to be 2.16, 1.52 and 1.97 kW, respectively. The exergy losses in the solar heat exchanger and the ground heat exchanger have the lowest value. The exergy losses in the ground heat exchanger used in the GSHP, SAGSHP and GSASHP systems are approximately 0.148, 0.072 and 0.113 kW, respectively. The exergy losses in the solar heat exchanger used in the SAHP, SAGSHP and SAASHP systems are obtained to be 0.105, 0.136 and 0.103 kW, respectively. The
Copyright # 2008 John Wiley & Sons, Ltd.

highest exergy loss in the evaporator is approximately 0.650 kW in the ASHP system. For the multiple source heat pump system, the lowest exergy loss in the evaporator is obtained to be 1.329 kW in SAGSHP. The highest exergy loss in the condenser occurs in the ASHP system. The magnitude of the loss was over 0.516 kW. The lowest exergy loss in the condenser is obtained to be 0.203 kW in SAGSHP. Table V presents the values for exergy rate and the second law e?ciencies as a function of thermodynamic parameters. The reasons of exergy rate in each heat pump include the compressor, condenser, evaporator, ground heat exchanger, solar heat exchanger, solar source evaporator, ground source evaporator and air source evaporator. It is obvious from Table V that the exergy rate and the second law e?ciency in each component are calculated as a function of thermodynamic parameters. Using Equation (13), the second law e?ciency is determined for each component given in Table V. By comparison, in a study performed by Reyes et al. [19], the exergy losses of the component are found as follows: the exergy loss of the compressor is 0.104 kW; the exergy loss of the condenser is 0.663 kW; the exergy loss of the collector-evaporator is 3.518 kW. It may be concluded that the exergy loss values obtained in the present study are fairly close to those reported by Reyes et al. [19]. The largest exergy loss occurs in the condenser (0.89 kW) followed by the evaporator (0.18 kW), the expansion valve (0.04 kW) as con?rmed by Pridasawas et al. [20]. The energy balance of the single source heat pump systems are given in Table VI. The energy balance can only determine the overall system e?ciency. The overall system energy e?ciency for SAHP, GSHP and ASHP are calculated to be 67.53, 96.1 and 56.2%, respectively. Table VI also presents the energy e?ciency values for the multiple heat pump systems. It is obtained to be 42.1% for the SAGSHP system. For the SAASHP system and GSASHP system, the overall system energy e?ciency values are obtained to be 47.4 and 39.9%, respectively. The solar radiation data measured during the experiments, are given in Appendix B.
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

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Table V. (a) Exergy analysis results of the systems and (b) second law analysis results of the systems. Condenser (kW) Inlet 1.78 1.58 1.6 1.251 1.827 1.836 } } } 1.395 1.396 } } } } 0.362 0.5 } } } } 1.402 } 1.408 } } } 1.106 } 0.397 } } } } 1.391 1.369 1.547 1.19 1.09 1.008 1.594 1.521 1.4 1.389 0.928 } } } 0.971 0.829 0.278 } } } } } } } 0.148 0.394 Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Inlet } 0.648 } 0.639 } 0.497 Evaporator (kW) Solar evaporator (kW) Ground evaporator (kW) Air evaporator (kW) Ground heat exchanger (kW) Outlet } 0.796 } 0.711 } 0.610 Solar heat exchanger (kW) Inlet 0.106 } } 0.137 0.138 } Outlet 0.211 } } 0.273 0.241 }

Copyright # 2008 John Wiley & Sons, Ltd. A. DIKICI AND A. AKBULUT

Compressor (kW)

Inlet

Outlet

(a) SAHP GSHP ASHP SAGSHP SAASHP GSASHP

0.98 0.831 0.993 0.645 1.163 0.965

1.78 1.58 1.6 1.251 1.827 1.836

Compressor (%) 84.2 75.6 67.4 82.8 86.3 82.5 } } } 25.9 35.8 } 69.2 59.7 29.9 } } }

Condenser (%)

Evaporator (%)

Solar evaporator (%)

Ground evaporator (%) } } } 78.8 } 28.2

Air evaporator (%) } } } } 10.6 28.8

Ground heat exchanger (%) } 77.2 } 88.7 } 77.3

Solar heat exchanger (%) 50.2 } } 50.1 57.2 }

(b) SAHP GSHP ASHP SAGSHP SAASHP GSASHP

55.1 41.9 31.5 53.7 53.2 51.3

Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

EXERGETIC PERFORMANCE EVALUATION

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Table VI. (a) Energy balance of single source heat pump systems and (b) exergy balance of multiple sources heat pump systems. Energy received (kW) S (a) Solar radiation 5.49 Ground heat exchanger Air source evaporator Refrigeration cycle 4.985 Overall 7.115 G 2.880 2.6 4.820 4.625 4.928 4.625 4.805 4.805 A Energy delivered (kW) S 3.45 2.588 4.735 4.735 2.6 4.505 2.6 G A Energy losses (kW) S 2.14 0.1 0.085 0.082 0.12 96.4 2.31 0.19 2.025 67.53 Energy losses (kW) SG GA SA G A Energy e?ciency (%) S 62.8 96.27 98.23 96.1 97.4 56.2 G A

Energy received (kW) SG (b) Solar radiation Ground heat exchanger Air source evaporator Refrigeration cycle Overall 5.71 3.648 GA SA

Energy delivered (kW) SG 3.8 3.09 4.502 4.502 GA 3.317 3.155 4.01 4.01 SA 4.050 3.2 4.11 4.11

Energy e?ciency (%) SG GA 94.9 60.2 47.04 48.2 39.9 SA 72.20

5.61 3.494 3.155 3.2 8.192 6.652 8.527 10.695 8.524 10.287

1.91 1.560 66.4 0.558 0.177 84.7 3.69 2.64 6.193 4.417 54.9 6.177 42.1

5. CONCLUSIONS This work deals with six di?erent kinds of heat pump systems that use solar, ground and air sources as input. An experimental system was installed and designed for investigating the heating performance, energy and exergy analysis of these systems. Energy and exergy analyses were derived using mass, energy and exergy balance equations. Exergy analysis is a useful tool for ?nding the losses in the system. The exergy rate and the second law analysis results of each component in each of the heat pump systems were also given. A new exergetic criterion called the EPC is applied to the heat pump systems having various heat sources. Some concluding remarks can be drawn from the results: 1. Experimental results show that the SAHP, GSHP and ASHP systems can be used for domestic heating in the east regions of Turkey. The multiple source heat pump systems can be suggested as a best solution for domestic heating. 2. The heating COP of the SAHP, GSHP and ASHP systems are obtained to be 2.95, 2.44 and 2.22, respectively. The heating COP of the SAGSHP, SAASHP and GSASHP systems are
Copyright # 2008 John Wiley & Sons, Ltd.

approximately 3.36, 2.90 and 2.14, respectively. The heating coe?cients of performance values for multiple source heat pump systems were higher than for the single source heat pump systems. 3. For the single source heat pump systems, the largest exergy loss in the refrigeration subsystem occurs in the compressor followed by the evaporator and the condenser. The largest exergy loss in the multiple source heat pump systems are associated with the evaporator, followed by the compressor, capillary tube and condenser. 4. Exergetic performance coe?cient values (EPC) of the SAHP, GSHP and ASHP systems are obtained to be 1.23, 2.39 and 2.31, respectively. EPC values for the SAGSHP, SAASHP and GSASHP systems are found to be 1.04, 0.74 and 1.08, respectively. EPC gives information about the loss rate availability. The results imply that the functional forms of EPC and ?rst law e?ciency are di?erent. The maximum value of the EPC signi?es the importance of getting a heat gain from the heat pump system by causing lesser dissipation in the environment. 5. The performance of a heat pump system is in?uenced by three primary factors: (i) the heat pump machine; (ii) the circulating pump and
Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

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A. DIKICI AND A. AKBULUT

(iii) the capacity of evaporator and ground heat exchanger coupling unit, solar heat exchanger. 6. The results can focus an engineer’s attention on components where the greatest potential is destroyed. For future studies, the new criterion called EPC can be used in the analysis of di?erent heat pump systems having various heat sources fed by using di?erent components and refrigerants. APPENDIX A The total uncertainty in the measurement of the mass ?ow rate wm may be calculated as follows [9]: ’ wm ? ?w2 ? w2 ? w2 ?1=2 ’ r0 sl td where wr0 is the uncertainty in the rotameter reading (%), wsl is the uncertainty associated with the system leakages (%), wtd is the uncertainty associated with the temperature di?erences (%) wm ? ?2:22 ? 2:52 ? 0852 ?1=2 ? 3:43% ’ The uncertainties arising in calculating a result ?wR ? due to several independent variables is given in Reference [9] as " 2  2  2 #1=2 @R @R @R wR ? w1 ? w2 ? ? ? ? ? wn @x1 @x2 @xn
900 800 700 Solar Radiation (W/m2) 600 500 400 300 200 100 10

The result R is a given function in terms of the independent variables. Let wR be the uncertainty in the result and w1 ; w2 ; . . . ; wn are the uncertainties in the independent variables. The COPsys is a function of several variables, each subject to an uncertainty COPsys ? f ?m; h; W? ’ COPsys ? Qcond ? 3:08 Wsys

@COPsys 1 ? Wsys @Qcond @COPsys Qcond ?? 2 @Wsys Wsys "  ! #1=2 1 2 2 ?Qcond 2 wp ? wsys ? wcond 2 W Wsys " #1=2 2   1 3:844 2 2 2 wp ? ?0:005? ? ?0:001? 1:159 1:246 ? 5:29%

APPENDIX B Measured solar radiation data are given in Figure B1.

January February March

11

12

13 Time (h)

14

15

16

Figure B1. Data of measured solar radiation.
Copyright # 2008 John Wiley & Sons, Ltd. Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er

EXERGETIC PERFORMANCE EVALUATION

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NOMENCLATURE A c COP ’ E ’ Ex h ’ I I m ’ P ’ Q s ’ S T V ’ W Greek letters Z c Subscripts 0 1 to 16 17 18 19 c comp cond eva f fan gen ghe in out p r rsv s ? dead state ? location numbers as shown in Figure 1 ? outdoor air ? indoor air ? outlet air of condenser ? collector ? compressor ? condenser ? evaporator ? fluid ? condenser fan unit ? generation ? ground heat exchanger ? inlet ? outlet ? plate ? refrigerant ? refrigerant speci?c volume ? solar ? efficiency ? flow exergy (kW) ? area ?m2 ? ? specific heat ?kJ kg?1 K?1 ? ? coefficient of performance of heat pump (-) ? energy rate (kW) ? exergy rate (kW) ? specific enthalpy ?kJ kg?1 ? ? rate of irreversibility (kW) ? rate of instantaneous radiation ?kW m?2 ? ? mass ?ow rate ?kg s?1 ? ? pressure (kPa) ? heat transfer rate (kW) ? entropy ?kJ kg?1 K?1 ? ? entropy rate ?kW K?1 ? ? temperature ?8C? ? volume ?m3 ? ? work rate (kW)

sys u w w,a Superscripts che kin phy pot

? system ? useful ? water ? water–antifreeze solution

? chemical ? kinetic ? physical ? potential

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Int. J. Energy Res. 2008; 32:1279–1296 DOI: 10.1002/er



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