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A new heating system based on coupled air source absorption heat pump for cold regions Energy saving analysis Wei Wu Wenxing Shi Baolong Wang Xianting Li Department of Building Science School of Architecture Tsinghua University Beijing 100084 China a r t i c l ei n f o Article history Received 6 June 2013 Accepted 18 August 2013 Keywords Absorption heat pump Air source Double stage coupled Heating Cold regions a b s t r a c t Energy consumption for heating and domestic hot water is very high The heating system based on an air source absorption heat pump ASAHP had been assessed to have great energy saving potential However the single stage ASAHP exhibits poor performance when the outdoor air temperature is very low A double stage coupled ASAHP is proposed to improve the energy saving potential of single stage ASAHP in cold regions The heating capacity and primary energy effi ciency PEE of the proposed system oper ated in both coupled mode and single stage mode are simulated under various working conditions The building load and primary energy consumption of different heating systems applied in cold regions are analyzed comparatively to investigate the energy saving potential of the coupled ASAHP Results show that the coupled ASAHP exhibits stable PEE and provides high heating capacity in very cold condi tions The energy saving rate of the coupled ASAHP in all the typical cities is above 20 In addition the energy saving potential of the single stage ASAHP in severely cold areas can be improved obviously by coupled ASAHP with an improvement of 7 73 in Harbin 2013 Elsevier Ltd All rights reserved 1 Introduction 1 1 Energy consumption and problems of traditional heat supply systems Energy consumption for heating and domestic hot water is very high In urban areas of north China heating accounted for 23 of the total building energy consumption in 2008 and this was dou bled from 72 million ton of standard coal equivalent tce in 1996 to 153 million tce in 2008 1 Regarding domestic hot water the energy consumption in urban areas is about 28 1 million tce accounting for up to 23 4 of the total building energy consump tion in 2008 1 Traditionally boiler is the most commonly used heating and domestic hot water system in cold regions 2 In China the coal boiler is still widely applied due to the coal dominated energy structure 3 However the coal boiler is of low energy effi ciency as well as high air pollution which is regarded as one of the main sources of CO2 SO2 NOXand particulate matters such as PM2 5 and PM10 4 6 1 2 Air source absorption heat pump ASAHP and its limitations A heat supply system combining a conventional heating system with an air source absorption heat pump has been assessed to have great potential in primary energy saving and emission reduction 7 However similar to the air source electrical heat pump ASEHP 8 9 the ASAHP exhibits poor performance or might not work when the outdoor air temperature is very low 10 When the ASAHP cannot meet the heating demand the boiler has to undertake the residual heating load Consequently the energy saving will be reduced in colder regions Therefore it is of great signifi cance to improve the performance of ASAHP in cold and severely cold regions just as is the case with ASEHP 1 3 Research objectives on alternative heating systems Regarding the ASAHP there are few researches reported of its use for heating purposes let alone its applicability to colder climates In this work a double stage coupled ASAHP is proposed for heating purposes in cold regions The coupled ASAHP can switch to a normal single stage ASAHP when the ambient temper ature rises to obtain higher effi ciencies In order to investigate the energy saving potential of this novel heating system the perfor mance of the coupled ASAHP is simulated under various outdoor air temperatures Then the energy saving potential of the coupled ASAHP applied in typical cold cities is analyzed taking the conven tional coal boiler as a baseline heating system In addition the en ergy saving is compared with that of the single stage ASAHP system to ascertain the improvement contributed by the coupled ASAHP 0196 8904 see front matter 2013 Elsevier Ltd All rights reserved http dx doi org 10 1016 j enconman 2013 08 036 Corresponding author Tel 86 10 62785860 fax 86 10 62773461 E mail address xtingli X Li Energy Conversion and Management 76 2013 811 817 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage 2 Methodology 2 1 Description of double stage coupled ASAHP The double stage coupled ASAHP is a hybrid system in which the condenser of the low temperature stage and the evaporator of the high temperature stage are connected through a middle water loop as illustrated in Fig 1 The ASAHP is located in the low temperature stage and a water source absorption heat pump WSAHP is placed in the high temperature stage In the operational mode of the coupled ASAHP pump 1 valve 1 and valve 5 are open while valves 2 3 and 4 are closed The heat production in the condenser of the low temperature stage C1 be comes the heat source of the evaporator of the high temperature stage E2 In this way the condensation temperature of the low temperature stage is low and the evaporation temperature of the high temperature stage is high Both the ASAHP in the low temper ature stage and the WSAHP in the high temperature stage can operate effi ciently even when the air temperature is very low The returned hot water is heated sequentially in the condenser C2 and absorber A2 of the high temperature stage and the ab sorber A1 of the low temperature stage When the air tempera ture increases theheating performanceofthesingle stage ASAHP improves and can meet the building heating demand Then the double stage coupled ASAHP can be switched to a single stage ASAHP by opening valves 2 3 and 4 while at the same time closing pump 1 valves 1 and 5 In this mode the returned hot water is heated sequentially in the condenser C1 and absorber A1 of the low temperature stage In a coupled ASAHP heating system both the heating safety in lower ambient temperatures and energy effi ciency in higher ambi ent temperatures can be guaranteed by the mode switching 2 2 Modeling and design of coupled ASAHP In order to investigate the performance of the proposed heating system and to compare it with a conventional boiler system math ematical models of the coupled ASAHP are developed Based on these models the heating capacity and energy effi ciency of both the coupled ASAHP and the single stage ASAHP under various air temperatures can be simulated 2 2 1 Modeling of absorption heat pump To simplify the model of the absorption heat pump AHP some reasonable assumptions should be made 11 12 1 the system is in steady fl ow and heat balance 2 the refrigerant leaving the evaporator and condenser is sat urated vapor and liquid respectively 3 the solutions leaving the generator and absorber are both saturated 4 the fl ow resistance pressure losses and heat losses in pipes and components are all ignored 5 the throttling in the expansion valves are isenthalpic pro cesses and 6 the electricity consumption of water pumps is not included Based on these simplifi cations the mathematical models of the ASAHP system can be built based on the mass and energy balance of each component 13 14 validated in previous work 7 X mout X min 1 X moutxout X minxin 2 Q X mouthout X minhin 3 Q UA LMTD 4 where UA is the product of the heat transfer coeffi cient and the heat transfer area of each heat exchanger and LMTD is the logarithmic mean temperature difference NH3 LiNO3 is used as the working fl uid for both stages of the coupled ASAHP owing to its advantages of low freezing point and lack of need of a rectifi er 15 16 The thermodynamic proper ties of the fl uids are obtained from 17 18 Coeffi cient of perfor mance COP for heating is defi ned as the useful heat loads of the absorber and condenser divided by the required heat load of the generator For the proposed system operated in coupled ASAHP mode COPcoupled ASAHP Qc2 Qa2 Qa1 Qg1 Qg2 5 For the proposed system operated in single stage ASAHP mode COPASAHP Qc1 Qa1 Qg1 6 where Qa1 Qc1and Qg1are the heat loads of the absorber condenser and generator in the low temperature stage and Qa2 Qc2and Qg2are the heat loads of the absorber condenser and generator in the high temperature stage Nomenclature G air volume fl ow rate m3 s h specifi c enthalpy kJ kg ppressure Pa Qheating load kW qunit refrigeration capacity kW kg V fl uid volume fl ow rate m3 s Wwork consumption kW Abbreviations AHPabsorption heat pump ASAHPair source absorption heat pump COP coeffi cient of performance EHPelectrical heat pump ESRenergy saving rate LMTDlogarithmic mean temperature difference PEprimary energy PEE primary energy effi ciency WSAHPwater source absorption heat pump Subscripts aabsorber ccondenser ggenerator Greeks g effi ciency qdensity kg m3 e refrigeration coeffi cient 812W Wu et al Energy Conversion and Management 76 2013 811 817 2 2 2 Primary energy effi ciency of the coupled system The power consumption of solution pump is calculated as fol lows 18 Wp Vp pout pin gp 7 where Vp is the volume fl ow rate of the pump poutand pinare the outlet and inlet pressure of the pump andgp is the pump effi ciency The power consumption of the fan is calculated using a simpli fi ed method 7 Wfan Dpfan G gfan n Dpcoil Dpout gfan Qsupply qcpDt 1 1 COPhigh 1 1 COPlow 8 whereDpfan is the evaporator resistance G is the air volume fl ow rate of the fan gfan is the fan effi ciency n is the number of rows of the evaporator coil Dpcoilis the coil resistance of each row Dpout is the excess pressure of the fan outlet Qsupplyis the heat rate sup plied to users qis the density of air cp is the specifi c heat of air Dt is the temperature rop of air through the evaporator and COPhigh and COPloware the COP of the high temperature stage and low tem perature stage respectively The specifi cs of these parameters can be found in Ref 7 Because different kinds of energy i e electricity and coal are involved in the analysis the primary energy effi ciency of the ASAHP is defi ned for the performance evaluation 19 PEE Qsupply Qg gboiler Wp Wfan gpower 9 where Qsupplyis the heat rate supplied to users Qgis the heat rate consumed by the generator of the ASAHP Wpis the electricity power consumed by the solution pump Wfanis the electricity con sumption of the fan andgboilerandgpower are the boiler effi ciency and power generating effi ciency respectively The PEE of the cou pled ASAHP and single stage ASAHP can be calculated accordingly 2 2 3 Equipment design of the coupled ASAHP The main parameters involved in the equipment design are listed in Table 1 Several parameter settings are remarked based on the general situation in China The temperature of the middle water loop has great infl uence on the performance of the double stage coupled ASAHP If the set temperature is high the evaporation temperature of the high tem perature stage is high and consequently the performance is good However the condensation temperature of the low temperature stage is also high in this situation which leads to poor effi ciency Therefore there is an optimal set temperature at which the PEE of the coupled ASAHP attains its highest value In the design pro cess a number of PEE values of the coupled ASAHP are calculated with different temperatures of the middle water loop and the tem perature corresponding to the PEE peak is chosen as the design temperature in this work Based on the above mathematical models of ASAHP and the optimization principal of middle water loop the UA values and water air fl ow rates of each heat exchanger the pressure lift and fl ow rates of solution pump fan are calculated The design results are listed in Table 2 After the equipment design is completed the performance sim ulation of the proposed coupled ASAHP under different working conditions can be conducted in Matlab by programming Consider ing that the fl ow rates of water and air are kept unchanged the UA G1 A1 C1 E1 SHX1 P1 G2 A2 C2 E2 P 144 SHX2 P2 High Temperature Stage Low Temperature Stage Supplied Water Heat Source Loop Hot Water Loop Middle Water Loop G Generator A Absorber C Condenser E Evaporator SHX Solution Heat Exchanger P Precooler Valve2 Valve3 Valve5 Valve1 Return Water Driving Source Pump1 Valve4 Fig 1 Schematic diagram of the coupled ASAHP W Wu et al Energy Conversion and Management 76 2013 811 817813 values of all the exchangers are regarded as constant for simplifi ca tion in the simulation 12 2 3 Energy saving rate of heating system In order to investigate the potential of the proposed coupled ASAHP systems applied in cold regions the primary energy con sumptions in typical cities in Northern China are calculated based on the PEE simulation Then the corresponding energy saving rates ESRs are calculated using a conventional coal boiler as the baseline heating system The ESR of the single stage ASAHP is also analyzed for comparison in order to obtain the improve ment contributed by the coupled ASAHP The ESR of the proposed system is defi ned as ESR PEboiler PEproposed PEboiler 100 10 where PEproposedis the primary energy consumption of the proposed heating system kW h and PEboileris the primary energy consump tion of the conventional boiler heating system Three typical cities in northern China Shenyang Changchun and Harbin are chosen to investigate the application potential of the proposed heating system A typical hotel building is chosen for simulation and the building layout is illustrated in Fig 2 The building characteristics given in Table 3 are designed according to the Chinese design standard for energy effi ciency of public buildings 20 The weather characteristics and the heating periods of different cities are also listed in Table 3 21 Based on the building information weather characteristics and heating confi gu rations the hourly heating loads of the hotel building are calcu lated using a dynamic energy simulation tool DeST 22 The design heating loads and accumulated heating loads of different cities can be found in Table 3 3 Results Based on the design parameters provided in Tables 1 and 2 operation performances of the double stage coupled ASAHP under different working conditions are simulated 3 1 Performance under design condition The performance of the coupled ASAHP under the design condi tion is listed in Table 4 Additionally if switching to the single stage ASAHP its performance is simulated as well Although the temperature lift between the condensation and evaporation temperature in the entire coupled ASAHP is as high as 67 C the temperature lift in each stage is only 29 C and 44 C Therefore the COP in each stage is relatively high with a value of 1 63 in the high temperature stage and 1 52 in the low temperature stage The COP of the coupled ASAHP is slightly lower than that of the single stage ASAHP However the heating capacity of the coupled ASAHP is more than three times greater The PEE of the coupled ASAHP is 87 29 whereas that of the conventional coal boiler is only 70 00 3 2 Performance under off design conditions During a heating season in a cold region the outdoor air temperature varies over a wide range which causes the heating capacity and heating COP to change greatly Table 4 shows that the lowest air temperature could approach as low as 30 C in the typical cities and therefore the ambient temperature range is taken as 30 to 20 C for the performance simulation of the coupled ASAHP Fig 3 shows the COP of the low temperature stage ASAHP and the high temperature stage WSAHP in the coupled ASAHP system It can be seen that the COP of both stages is relatively stable It is maintained in the range 1 58 1 69 in the high temperature stage and within the range of 1 47 1 58 in the low temperature stage Table 1 Main design parameters of coupled ASAHP ParametersValueRemarks Heating capacity30 kW Design outdoor air temperature 10 C Supplied hot water temperature 45 CTypical temperature for low temperature heating Heat source temperature 130 CTypical temperature for district heating in China 1 Effi ciency of coal boiler 70 Coal boiler heating is widely used in China and the typical effi ciency is 70 1 Effi ciency of coal generation 33 Electricity of coal generation is the most widely used in China and the effi ciency is about 33 1 Effi ciency of solution pump 80 Effi ciency of fan80 Table 2 Design parameters of each component of the coupled ASAHP ComponentUA W K Water air fl ow rate kg s Heat exchanger Generator1718 40 5833 Generator2505 40 5450 Condenser12489 90 8573 Condenser2849 20 7160 Evaporator1873 01 2533 Evaporator22489 90 8573 Absorber1728 90 7160 Absorber2513 00 7160 SHX1463 1 SHX2162 1 Precooler149 9 Precooler253 4 LiftFlow rate kg s Pump and fan Solution pump 175 m0 0560 Solution pump 2105 m0 0261 Fan90 Pa1 2533 Fig 2 Building layout of a typical hotel 814W Wu et al Energy Conversion and Management 76 2013 811 817 Consequently the COP of the coupled ASAHP consisting of these two stages can be very stable as shown in Fig 4 Fig 4 indicates that the COP of the coupled ASAHP can still reach 1 25 at an ambi ent temperature as low as 30 C whereas the single stage ASAHP cannot work if the ambient temperature is below 20 C The PEE of the coupled ASAHP and single stage ASAHP at differ ent outdoor air temperatures is illustrated in Fig 5 The PEE of the coupled ASAHP is higher than that of the single stage ASAHP when the ambient temperature is below 15 C When the ambient tem perature is 30 C the coupled ASAHP still has a PEE of 85 50 which is 15 50 higher than the conventional boiler 70 00 As the ambient temperature gets higher the PEE of the single stage Table 3 Building characteristics and heating loads in typical cities ShenyangChangchunHarbin Building characteristics Building information 5 fl oor 21 m high Total heating area 8700 m2 Window wall ratio 0