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System optimization and experimental research on air source heat pump water heater J Zhang R Z Wang J Y Wu Institute of Refrigeration and Cryogenics Shanghai Jiao Tong University 1954 Huashan Road Shanghai 200030 PR China Received 30 May 2005 accepted 16 July 2006 Available online 27 October 2006 Abstract This paper deals with the system optimization of air source heat pump water heater ASHPWH including calculating and testing The ASHPWH system consists of a heat pump a water tank and connecting pipes Air energy is absorbed at the evaporator and pumped to storage tank via a Rankine cycle The coil pipe condenser releases condensing heat of the refrigerant to the water side An ASHPWH using a rotary compressor heated the water from initial temperature to the set temperature 55 C The capillary tube length the fi lling quantity of refrigerant the condenser coil tube length and system matching are discussed accordingly From the testing results it could be seen that the system performance COP could be improved obviously 2006 Elsevier Ltd All rights reserved Keywords Heat pump Water heater Condensing coil tube Filling quantity System matching Optimization 1 Introduction The prevailing products in the domestic market of water heater are gas water heater GWH electric water heater EWH and solar water heater SWH while heat pump water heater HPWH as the forth kind of water heater appears in the market recently Compared with the three formers HPWH has several advantages such as energy saving low running fare and safety in using which all bring it a promising prospect in domestic water heating Air source heat pump water heater ASHPWH based on the principle of Rankine cycle could absorb heat from air at lower temperature and through the work of heat engine the absorbed heat and the consumed work is trans ferred into water tank the higher temperature heat source From the environment the system gets energy that is possibly 3 4 times of the electric power it will consume Totally it contributes 4 5 times of the electric energy So ASHPWH is preferred by users due to its virtues such as high effi ciency and energy saving Since the 1950s researches have been performed on HPWHs including structure thermodynamics working fl uids operation controlling numerical simulation and economical analysis Condenser design underwent two stages bayonet style with annular fl ow and U style pipe Mei et al tested perfor mance of 8 condensers in a water tank When considering COP as the function of mean water temperature they found that the performance of U style pipe system is com monly better than that of bayonet condenser system Sys tem COP and the rate of heat production increase with the loop number increases 1 Hiller led a group studying dual tank water heating sys tem from 1991 Elementary research shows that more than 60 kinds of dual tanks have potential study values Diff er ent pipe connections and control strategies can achieve goals such as optimizing control of hot water supply and power controlling Continuous tests suggest that the effi ciencies of 38 kinds of dual tank water heating systems are higher than HPWH of other structures Of course 1359 4311 see front matter 2006 Elsevier Ltd All rights reserved doi 10 1016 j applthermaleng 2006 07 031 Corresponding author Tel 86 21 62933838 fax 86 21 62932601 E mail address rzwang R Z Wang Applied Thermal Engineering 27 2007 1029 1035 the heat loss of dual tank water heating system is more than single tank system with the same volume 2 Huang and Lin also studied the dual tank HPWH The water tank volume was 100 L Results showed that heating water form 42 C to 52 C need 10 20 min and the all year COP reached 2 0 3 0 Compared with electrical water the energy saving fraction was 50 70 and the hot water dis charge effi ciency was 0 912 3 Hasegawa et al proposed a two stage compression and cascade heating heat pump system for hot water supply Using R12 it could heat water from 10 C directly to 60 C The inlet and outlet water temperatures of evapora tor are 12 C and 7 C and the system COP is 3 73 4 Ji et al combined HPWH and conventional air conditioning and realized a multi functional domestic heat pump MDHP This equipment could implement multi functions in moderate climate areas and operate long time with high effi ciency When refrigeration and heating run simulta neously the average of COP and EER could reach 3 5 5 6 R12 R22 were the most common used work fl uids in HPWH As the proposal of ozonosphere protection R22 became the only conventional fl uid still been used In devel oping countries such as China the dead line using R22 is 2040 Until now it is still widely used So there still have some meaning to do research on the R22 system perfor mance improvement which is also a means of saving energy Sloane et al using ribbed roil pipe in the middle of water tank in ambient temperature of 24 C and water temperature 27 C COP is 2 4 Mei et al also use R22 as refrigerant the result is that when water temperature is 27 C an ambient temperature are 20 C and 27 C the COP can be respectively 4 0 and 4 5 7 From the litera ture using conventional working fl uids it could be seen that when the environmental temperature is moderate and condensing temperature is not high R22 could get fi ne thermodynamic performance and effi ciency However when the system runs in the high temperature area for example above 50 C the discharge temperature and pres sure of compressor are both very high especially in cold winter The working condition of compressor is worse than ordinary air conditioning heat pump which seriously aff ect system safety and reliability So it is urgent to fi nd new fl uid of better performance Much related research has been conducted to enable the ASHPWH to run eff ectively Morrison et al 8 demon strated a method for annual load cycle rating of ASH PWH Kim et al 9 proposed a dynamic model for a water heater driven by a heat pump system Ding et al 10 and Yao et al 11 have done much research on defrosting to improve the ASHPWH system working in the winter Fan et al used a 7500 W HPWH to study its energy saving character Considering the power consumed by fan and water pump the system COP was 3 3 If only consider the compressor the COP became 4 18 12 However as far as ASHPWH is concerned manufactur ers have not agreed on the parameters and the matching of heat pumps and water tanks mainly due to the diff erent working conditions including areas living habits and all year round running Heat pump water heater system is consisted of out door heat pump water tank and connect ing pipes etc Some manufacturers use air conditioning heat pump out door machine directly and complete the system just by adding a water tank Obviously the working conditions of a ASHPWH vary from those of air condi tioner Temperature at the hot side of a ASHPWH rises gradually but its cooler side is changing according to the climate year round Thus it is necessary to standardize the products of ASHPWH In order to enhance the system performance COP reduce the product cost and optimize the running condi tion system components should be investigated fi rst Besides compressor condenser evaporator and thermal valve or capillary the refrigerant fi lling quantity matching between water tank and heat pump unit are also important for the system This paper deals with the system optimiza tion of the air source heat pump water heater ASHPWH including calculating and testing The capillary tube length the fi lling quantity of refrigerant the condenser coil tube length and system matching are discussed accordingly From the testing results it could be seen that the system performance COP could be improved obviously after sys tem optimization We hope that it could provide some valuable suggestions for future development of ASHPWH 2 ASHPWH experiment system The testing system of ASHPWH is shown in Fig 1 It is composed of a temperature and humidity controlled room heat pump water tank control system and test system A data logger Keithley 2700 and a PC are used to record temperatures of water in the water tank Also tempera tures at the inlet and outlet water pipes the ambient tem perature the saturated evaporating temperature and the transient electric input power are stored in the PC automat ically as fi les During the ASHPWH system running the working fl uid absorbs heat from air evaporates in the evaporator then is compressed into the high pressure and temperature vapor which is then condensed into liquid and release heat in the coil condenser to heat up the water in the water tank The liquid goes through capillary tube or thermal expansion valve turning into gas liquid mixture with low temperature and pressure The low temperature liquid is vaporized in the evaporator after absorbing heat from the air In the experiment a controller sets the start end temperature points and the running mode When the water temperature goes up to the end point the system stops automatically If it gets down to some temperature the system will restart to compensate for the water heating We discuss the parame ters just in one heating process in which we preset the tem perature and mix the water in water tank to unify its initial and fi nal temperatures The COP could be calculated based on the heat gained and the electric power consumed 1030J Zhang et al Applied Thermal Engineering 27 2007 1029 1035 3 Refrigerant fi lling quantity of ASHPWH After the heat pump system is leak checked and vacuu mized refrigerant should be fi lled into it In this experimen tal system R22 is used as the working fl uid Obviously the fi lling quantity is related to the evaporator condenser and compressor If the refrigerant fi lling quantity is too much the load of compressor will be aggravated and the odd refrigerant will take up part area of the condenser which will descend the heating effi ciency On the other hand if refrigerant is not enough the suction and discharge pres sure will be partially low the heat fl ow will be too weak to satisfy its rated capability Neither of the two conditions can make heat pump reach the ideal working status Addi tionally in winter and summer the ambient temperature is quite diff erent 10 C to 35 C in Shanghai for example and so is the refrigerant fl ow rate The working condition in summer may need more refrigerant liquid than that in winter All these are the factors aff ecting the refrigerant fi llings Our goal is to fi nd which may be most energy saving In the experiment a 750 W heat pump is chosen the constant room temperature is kept at 25 C 150 L water tank and 60 m condensing coil pipe B9 9 0 75 mm are used in testing The start and end temperature of water in the tank is 15 and 55 C respectively In the test we chose digital freon ration fl ow meter from whose LCD screen the refrigerant quantity can be read directly In order to keep the compressor within rated power the max imum current should not be more than 3 8 A In Fig 2 we could see the COP curves versus the refrigerant fi lling quantity and the maximum COP is reached at the fi lling refrigerant for 1 5 kg Theoretically speaking in the heat pump refrigerant fl ow rate could be calculated based on the heat load and performance parameters of ASHPWH According to 13 total fi lling quantity is equal to the sum up of those in each component mT ma mP me mc 1 where mT is total fi lling quantity ma mP meand mcare accumulator fi lling quantity fl uid pipe fi lling quantity Fig 1 Sketch of the experimental system to test the performance of ASHPWH 1 Shell body 2 heat exchanger 3 fan 4 store 5 compressor 6 fi lter 7 valve 8 thermal expansion valve 9 copper pipe 10 outer shell 11 thermal insulation 12 inner tank 13 coil pipe condenser 14 temperature and humidity controlled room 15 inlet water pipe 16 cycle water pump 17 water mixing valve 18 three way valve 19 outlet water pipe 20 controller 21 ammeter 22 computer 23 data scrutiny 24 data logger 25 hot water tank 26 cycle water pipe A H temperature sensors I J water meters Fig 2 Filling quantities and COP J Zhang et al Applied Thermal Engineering 27 2007 1029 10351031 evaporator fi lling quantity and condenser fi lling quantity respectively Table 1 is the calculated results in which the winter summer and spring autumn working set ambient tempera ture are 5 C 30 C and 25 C It can be seen that the best fi lling quantities are quite dif ferent with the climatic changes However for the sake of safety we choose the one of spring autumn to ensure the compressor not over loading too much in summer 4 Condenser coil pipe length of ASHPWH The length of condenser coil pipe should match up to the compressor type system load and evaporator area If the pipe is short compressor suction discharge tempera ture may be a little higher than rated On the other hand if the pipe is too long there will be some length unused Thus calculating a suitable condenser coil length is important For the chosen a 750 W heat pump with 150 L water tank with R22 as the refrigerant If the set temperature of hot water is 55 C the calculation process is as follows 1 System running parameters The heating load of ASHPWH is 3375 W take the spring autumn for example condensing temperature is 60 C evaporating temperature is 15 C and the super cooling is 5 C the copper pipeused is B9 9 0 75 mm 2 Heat exchange area Due to the refrigerant phase change in condenser heat exchange area is divided into gas liquid and two phase sections Thus each section is calculated separately The physical model temperature section division of the water tanker is shown in Fig 3 Ignoring the thickness and thermal resistance of copper pipe the average temperature in superheated section is the same as the hot water temperature at the heating end hot water temperature of Ts 55 C Set refrigerant inlet outlet temperature and top bottom water tank temperature Tr in 80 C Tr out 50 C Tw top 60 C Tw bottom 50 C Through calculation we get the pipe length of each section which add up to the total length LTotal 47 64 m shown in Table 2 Similarly in the set of 200 L water tank and an 1125 W heat pump the calculated pipe length is 69 9 m These data are close to those in testing which are shown in Fig 4 Table 1 Theoretical calculation results of heat pump working fl uid fi lling quantities SeasonWinterSummerSpring autumn Ambient temperature C 53025 Filling quantity kg 1 2682 4761 577 Fig 3 Physical model of water tank temperature sections Table 2 Pipe length of each section SectionSuper heatTwo phaseSuper cooling Pipe length m 10 07135 2722 301 Total length m 47 64 Fig 4 Experimental results of the COP versus condenser pipe length a 150 L 1 Hp and b 200 L 1 5 Hp 1032J Zhang et al Applied Thermal Engineering 27 2007 1029 1035 5 System matching Through the test it is discovered that diff erent capillar ies make the performance of heat pump distinct in various environments When the ambient temperature is high a stubby capillary performances better when it is in low tem perature a slender one is better For example at 35 C capability of system with shorter capillary is 21 higher than that of long capillary while at 15 C the latter is 3 higher than the former Minitype family use ASHPWH using double capillaries is a simple eff ective and cheap choice In one aspect turn ing on off of electromagnetic valve decides the working capillary which adapt to the changing working condition at the same time the total cost will not be much aff ected by the low price of capillaries However the double capillaries cannot satisfy higher requirement because certain diameter and length deter mine a certain pressure diff erence of the capillary So its capability is almost constant although the water tempera ture changes a lot which needs an increase of refrigerant fl ow rate Based on this we considered to use thermal expansion valve which makes use of the superheat at evap orator outlet to adjust the refrigerant fl owing instead of double capillary in ASHPWH system 5 1 Relation between refrigerant fi lling quantity and thermal expansion valve TXV opening degree A 750 W heat pump at air source of 25 C is tested with 150 L water tank and 60 m condensing coil pipe The initial and fi nal temperatures of water are 15 C and 55 C respectively Fig 5 shows that under some fi lling quantity system COPs are diff erent with the opening degrees among which there exists a best opening degree COP That is to say in the same fi lling condition the opening degree of the TXV impacts the system COPs a lot which should be optimized in experimental research We discovered that there are some coupling of refriger ant fi lling quantity and thermal expansion valve opening degree Theoretical correlation has not been found but we can obtain several qualitative conclusions as follows 1 Under some fi lling quantity if the TXV is open too much the pressure diff erence will be small but it may cause refrigerant accumulated in the rail part of condenser coil pipe If the TXV is not open enough the outlet temperature of condenser will be high whichmeansthattheheattransferis insuffi cient 2 At some opening degree of TXV if the fi lling quan tity is too much the refrigerant will accumulate at the bottom of condenser and will not be in function fully And the compressor may run below the rated load if the fi lling quantity is not enough 5 2 System stability In the running process of heat pump it is found that the inlet temperature of condenser will rise slightly but the outlet will appear a few fl uctuations with the water temper ature rising This instability suggests the instability of suc tion of TXV and fl uctuation of transient compressor output power which indicates the instability of system running To solve the problem the system was fi rst adjusted to a suitable fi lling quantity thereby we could see the fl uctua tion swing diminished and COP increased considerably Then the TXV opening degree is changed to make the COP of whole system to reach the maximal v