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Energy 32 2007 1361 1374 Experimental performance analysis and optimization of a direct expansion solar assisted heat pump water heater Y W Lia b R Z Wanga b J Y Wua b Y X Xua b aInstitute of Refrigeration and Cryogenics Shanghai Jiaotong University 200240 Shanghai China bEngineering Research Center of Solar Power Water heater Coeffi cient of performance Solar collector effi ciency Exergy 1 Introduction Solar energy is renewable and free which can be a heat source of heat pump like the air source In order to improve the heat pump COP the idea of combining the heat pump with solar energy application system has been proposed and developed by many researchers around the world In a so called direct expansion solar assisted heat pump DX SAHP the collector and evaporator are combined into one unit collector evaporator where the refrigerant circulating in heat pump system gets evaporated by absorbing the incident solar energy and or ambient air energy The DX SAHP offers several advantages over the conventional SAHP such as superior thermodynamic performance lower system cost and longer life time of collector evaporator It is estimated that in China solar water heater have been marketed for about 10 billion RMB Yuan each year It is also reported that China has 5 million solar water heaters installed in families in 2000 and it is still being developed more and more rapidly In virtue of its advantages over conventional solar water heaters the DX SAHPWH is expected to have a giant potential market in China The DX SAHP concept was fi rst considered by Sporn and Ambrose in West Virginia 1 Following their work many theoretical and experimental studies have been reported in the past 27 years 2 26 A review paper in this fi eld has indicated that the COP values of the DX SAHP systems range from 2 to 9 and the collector evaporator effi ciencies vary between 40 and 75 under different climatic conditions experimentally 25 With differentmatchingbetweenthesystemcomponents especially the collector evaporator and the compressor the COP and Zcollvalues are very different each other So the proper matching between each component has animportantimpactontheperformanceofthe DX SAHPWH ARTICLE IN PRESS 0360 5442 see front matter r 2006 Elsevier Ltd All rights reserved doi 10 1016 j energy 2006 11 003 Corresponding author Institute of Refrigeration and Cryogenics Shanghai Jiaotong University 200240 Shanghai PR China Tel 862162933838 fax 862162932601 E mail address rzwang sjtu edn cu R Z Wang Except for the collector evaporator the rest of the DX SAHPWH system employs ordinary materials and components currently available in the refrigerating and air conditioning industry The study of the performance enhancement for collector evaporator is signifi cant for the development of the DX SAHPWH Inthisstudy thefocusaimsatdevelopinga DX SAHPWHwithhigherperformancesuitablefor Chinese potential market At present two prototypes DX SAHPWH A and B were built in sequence in Engineering Research Center of Solar Power i 10 where E is the exergy rate and its subscript rad stands for solar radiation and subscript Qw for heat transfer rate released from condenser to water P i Irr istands for the total exergy loss rate of main components of the DX SAHPWH system The subscript i represents the ith component of the DX SAHPWH system The exergy loss rate calculation equations of each component are given as follows The exergy loss rate in the collector evaporator can be concluded as Irr eva Erad E4 in f E1 out f Erad H4 H1 T0 S4 S1 11 where Eradis the exergy rate of the incoming solar radiation Erad AcITZcoll 1 T0 Tp 14 ITis the total solar radiation intensity Tpis the temperature of the collector evaporator panel surface ARTICLE IN PRESS Y W Li et al Energy 32 2007 1361 13741365 The exergy loss rate in the compressor can be concluded as Irr comp Wcomp E1 in f E2 out f Wcomp H1 H2 T0 S1 S2 12 The exergy loss rate in the condenser can be concluded as Irr cond E2 in f E3 out f EQw H2 H3 T0 S2 S3 Qw1 T0 Tw 13 The exergy loss rate in the expansion valve can be concluded as Irr v E3 in f E4 out f H3 H4 T0 S3 S4 T0 S4 S3 14 As for the second law evaluating indicator exergy effi ciency does not appear to have been standardized In this paper the exergy effi ciency of the DX SAHPWH system is defi ned as Zex E Qw Wcomp Erad Qw1 T0 Tw Wcomp Erad 15 The exergy loss rate in each component of the system can be examined more clearly by using the concept exergy loss coeffi cient w which is defi ned as wi Irr i Wcomp Erad 16 The correlation of the Zexand the wexcan be concluded as Zex 1 wex 1 X i wi 17 4 Experimental results and discussions At present a series of seasonal sunny day experi ments were done in April 2005 under the meteorological conditions at Shanghai to study the performance of DX SAHPWH A system during the spring period The experimental data are summarized in Table 2 It is indicated that it takes 90 98min for heating 150L water from about 14 20 to 501C and the total electric consump tion for compressor was 0 98 1 06kWh The average COP and Zcollwas 5 21 6 61 and 88 105 respectively So it was proven that the Zcollcan be more than 1 0 when the evaporatingtemperatureislowerthantheambient temperature This characteristic is helpful to improve the performance of the solar collector evaporator A set of experimental data under typical working condition on April 22 2005 are selected to analyze the time dependent performance of the DX SAHPWH A system From data shown in Fig 4 it can be concluded that the average value of solar radiation ITand ambient tempera ture T0are 812W m2and 24 41C during the experimental period respectively And there was no large variation about the instantaneous ambient temperature but the instantaneous solar radiation varied unpredictably because of the cloud 4 1 COP and Zcollof the DX SAHPWH system It can be concluded from the experimental results on April 22 2005 that the COP and the Zcollof the DX SAHPWH A system is 5 21 and 88 respectively It was found that after about 30minutes the running DX SAHPWH A system comes into a so called trans steady state working condition period stage for about an hour until the end of measurements The system running parameters such as Tcond i Tcond o Teva o Teva i Tw Tpand so on are all increasing almost linearly during this period Tostudytime varyingperformanceofthe DX SAHPWH system the entire trans steady state work ing condition period is divided into seven intervals in this paper They are a former 1 8 period b former 1 4 period c former 1 2 period d whole period e later 1 2 period f later 1 4 period g later 1 8 period The time integrated average COP and Zcollof the DX SAHPWH A system at a g seven intervals were calculated using Eqs 7 and 8 and plotted at seven mid points of the corresponding intervals respectively as shown in Fig 5 From Fig 5 it is noted that Zcollvalues of the DX SAHPWH A system almost reach about 90 they are much more than that of the conventional solar collector ARTICLE IN PRESS Table 2 Experimental data of DX SAHPWH A taken in spring No Date mm dd yy Local time hh mm Dt min T0 1C IT W m2 Tw 1 1C Tw 2 1C Tp 1C Peva in MPa abs Peva out MPa abs Pcond MPa abs Thermodynamic states P j Wcomp tj Kwh COPZcoll T1 1C T2 1C T3 1C T4 1C 104 04 0510 30 12 049420 695513 450 531 91 0390 8841 68033 076 840 826 10 9870 026 6170 349174 0 204 05 059 58 11 329622 185814 350 630 00 9280 8021 58929 272 940 625 71 0070 026 3670 339774 5 304 15 0511 39 13 179825 166317 449 325 10 8880 7671 68323 475 045 021 71 0670 025 2670 2910575 8 404 20 059 58 11 289025 781220 350 429 60 9800 8441 74628 679 646 826 01 0070 025 2670 298874 8 504 22 0510 21 11 519024 481220 550 531 40 9630 8301 73830 480 048 927 31 0170 025 2170 298874 8 Y W Li et al Energy 32 2007 1361 13741366 and its variation range is not great But the COP values generally decrease mainly because of the gradual increase of Twand Tcond It is worth noticing that Twin Fig 5 is transient measured value As to experimental operation once the measured value of Twreached 551C the compressor is switched off and the circulating water pump is switched on to mix the stratifi ed hot water in tank According to experimental results the actual equilibrium water tempera ture Tw 2was about 4 81C lower than the measured one As above mentioned in order to measure the pressure drop in the collector evaporator there were two low pressure transducers at inlet and outlet of the collector evaporator respectively A high pressure transducer was locatedatoutletofthecondensertomeasurethe condensing pressure These three pressure values were plotted in Fig 6 It is noticed from Fig 6 that both Pcond and Pevaincrease with the increase of Tw and the increasing rate of Pcondis higher The compressor exhaust temperature is higher and higher e g the discharge temperature is 90 41C when the actual Tw 2reaches 50 51C So the excessive terminal setting value of Twwill do harm to the stability and reliability of the DX SAHPWH system ARTICLE IN PRESS 10 26 10 36 10 46 10 56 11 06 11 16 11 26 11 36 11 46 500 600 700 800 900 1000 20 22 24 26 28 30 11 56 April 22 2005 T0 C IT IT W m2 Local time hh mm 10 16 T0 Fig 4 Variation of ITand T0with time for the DX SAHPWH A system 30405060708090 0 2 4 6 8 10 12 20 25 30 35 40 45 50 55 April 22 2005 Tw C coll COP 10 coll TSky is the sky temperature TSky 0 0552T1 5 0 23 ULcis collector evaporator heat loss coeffi cient ULc hw 4 sT3 0 hw is the wind heat transfer coeffi cient hw 5 7 3 8Vw 20 21 Vwis the wind velocity at ambient environment s is the Stefan Bolzmann constant T is the temperature the subscript p standsforcollectorplate eva for evaporating refrigerant and 0 for the ambience F0is the collector effi ciency factor The exergy gain by the collector evaporator is given by the heat absorbed from the incoming solar radiation times ZCarnot 14 The exergetic effi ciency of the collector evaporator can be defi ned as Zex coll F0 S ULc Teva T0 1 T0 Teva IT 1 T0 Tp 19 Taking the derivative of Zex collwith respect to Teva we fi nd the maximum exergetic effi ciency can be obtained at Teva optimum T1 2 0 S ULc T0 1 2 20 The second term in the right hand side of Eq 20 will be referred to as sol air temperature Tsa S ULc T0 21 Combining Eqs 20 and 21 we obtain Teva optimum T0Tsa 1 2 22 As for the experimental data on April 22 2005 the average ambient temperature and the average evaporating temperature is 24 4 and 19 81C respectively The average optimum evaporating temperature is calculated to be 39 11C As shown in Fig 8 the difference between the Teva optimumand the Tevadecreases with increasing Tw This is accordant to the fact that the exergy loss coeffi cient of the collector evaporator is dropping gradually in Fig 7 4 4 The design optimization of collector evaporator As calculated above the optimum evaporating tempera ture is much higher than the actual evaporating temperature So the higher evaporating temperature should be required But the actual evaporating temperature depends upon the design confi guration and the operating conditions However the twin goals of effi ciency and reliability of this system have a confl icting object Teva The compressor exhaust temperature may be unacceptable with too high value of Teva As Chaturvedi et al s point of view Tevamay be higher or lower than T0 depending upon the design system matching and the weather condition 4 Chatur vedi et al further shows that an SAHP using a bare collector and a variable frequency compressor has an optimum performance based on maximization of exergy effi ciency of the collector evaporator provided the eva porator temperature is maintained in a temperature range of5 101Caboveambient 6 Amethodologyfor determination of an optimal evaporation temperature proposed by Torres Reyes et al was also based on maximization of exergy effi ciency of the collector evapora tor 14 Ito 20 21 and Hawlader 22 also designed an SAHP with a bare collector operatingat Teva4T0 respectively But Huang 18 consider that an SAHP operating at TevaoT0has an advantage of having lower compressor exhaust temperature and dual heat source from both solar radiation and ambient air with higher Zcoll In this experimental study Tevais designed as about 51C below ambient TevaoT0 according to the limit of the compressor suction pressure As shown in Fig 6 the pressure loss between the inlet and the outletis large because of the complicated refrigerant fl ow path in the collector evaporator So the fl ow path in the collector evaporator would be designed properly so as to reduce the large pressure loss of the refrigerant fl ow The procedure of design calculation for the collector evaporator is described as follows Provided the average ambient temperature is 251C the average solar radiation intensity is 800W m2and the ARTICLE IN PRESS 0102030405060708090 0 5 10 15 20 25 30 35 40 45 April 22 2005 Teva optimum T0 Teva Ti C Time minutes Fig 8 Variation of Teva optimum Tevaand T0with time for the DX SAHPWH A system Y W Li et al Energy 32 2007 1361 13741369 average wind velocity is 3 1m s in spring at Shanghai and a and e were both about 0 9 for the collector evaporator used in this experiment Then the S and ULccan be calculated using Eq 18 as 650W m2and 22 9W m2K respectively Combing Eqs 7 and 18 gives Zcoll F0 IT S ULc Teva T0 23 Assumed the Teva T0 and Zcollis 51C and 90 respectively then F0can be calculated as about 0 94 The collector effi ciency factor can be expressed as 20 F0 F 1 F D W 24 whereFisthe fi n effi ciency F tanh Ub Ub Ub W D 2 ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi ULc lP dP p W is the pitch of the tubes D is the outer diameter of the tube lPis the conductivity of the collector evaporator plate the material of the collector evaporator is recommended as aluminum which can be easily shaped so lP 236W mK dPis the thickness of the collector evaporator plate 1 60mm is suggested as same as that in the DX SAHPWH A system Then D and W can be confi rmed as 12 0 and 130 0mm respectively Lastly Accan be determined by Eq 18 once Qevais requested In theory the schematic of the optimum designed collector evaporator is shown in Fig 9 However under the limitation of the available products in the market the actual collector evaporator utilized in present study is made of 8 solar collector slab cores as shown in detail in Fig 10 The total collector area is 2 08m2 The collector evaporator plate is a 0 18mm thick copper sheet with its surface black painted by solar selective coating A copper tube having inner and outer diameters of 11 0 and 12 0mm respectively was soldered to the backside of the copper sheet with a pitch between tubes of 140 0mm The collector evaporator was installed on the roof facing south with an inclination angle of 31 221 Based on the experiment and analysis on the DX SAHP A a small type DX SAHPWH B system with 400W input power is developed to optimize the system perfor mance and economic cost In this study Zcompof 0 60 was used in the calculations for the DX SAHPWH B system The specifi cation of each main component of the DX SAHPWH system B is listed in Table 3 4 5 Experimental comparison between the DX SAHPWH A and B system To examine the performance of the DX SAHPWH B system during autumn period and compare it with that of the DX SAHPWH A system series of sunny day experimentsweredoneinOctober2005underthe meteorological conditions at Shanghai The experimental data are summarized in Table 4 With the contrast of the data in Tables 2 and 4 it can be noted that the small type DX SAHPWH B system need much longer time than the DX SAHPWH A system at similar ambient conditions and hot water request However both DX SAHPWH A and B need electric energy consumption no more than 1kWh At this similar running cost the lower capital cost and smaller collector evaporator area which is advantageous for integrating it with buildings roof or walls makes the benefi ts of the small type DX SAHPWH B system over the DX SAHPWH A system Aset of experimental data under typical working condition on October 15 2005 are selected to analyze the time dependent performance of the DX SAHPWH B systemsoas tocompareit withthedataforthe DX SAHPWH A system on April 22 2005 for further analysis ARTICLE IN PRESS W D p Fig 9 Schematic diagram of the design optimization for collector evaporator Y W Li et al Energy 32 2007 1361 13741370 From data shown in Fig 11 it can be concluded that the average value of the solar radiation IT and ambient temperature T0is 795W m2and 24 71C during the experimental period respectively As similarly to Fig 4 the ambient temperature was almost unchanged However the instantaneous solar radiation varied with a sinusoid rule without the effect of cloud It can be concluded from the experimental results on October 15 2005 that the COP and the Zcollof the DX SAHPWH B system is 4 97 and 86 respectively As shown in Fig 12 the variable characteristics of Zcoll COP and Twfor the DX SAHPWH B system was similar to that for the DX SAHPWH A system The main difference is that after about only 10minutes ARTICLE IN PRESS Copper sheet R22 R22 Copper tube Fig 10 Schematic diagram of collector evaporator in the DX SAHPWH B system Table 3 Specifi cation of the main components of the DX SAHPWH B system NameTypeRemarks CompressorRotaryRated input power 0 40kW displacement volume 7 40cm3 rev Domestic water tankPressure resistance and heat insulation 150L water immersed 50m copper coil f9 90 0 75mm as condenser Solar collector evaporatorSlab cores made of copper sheets and tubes 8 slab cores 0 18mm thick copper sheet soldered on f12 0 0 50mm copper tube connected in series total collector evaporator area 2 08m2 Thermostatic expansion valveTEX 2 type manufactured by Danfoss Denmark External balance type Table 4 Experimental data for the DX SAHPWH B system taken in autumn No Date mm dd yy Time hh mm Dt min T0 1C IT W m2 Tw 1 1C Tw 2 1C Tp 1