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Heating Cold regions ASAHP is capacity and show that the coupled ASAHP exhibits stable PEE and provides high heating capacity in very cold condi of traditional domestic heating in 2008 Traditionally boiler is the most commonly used heating and sources of CO 2 SO 2 NO X and 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 lity to colder is proposed ASAHP ambient temper ature rises to obtain higher efficiencies In order to investigate energy saving potential of this novel heating system the 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 Corresponding author Tel 86 10 62785860 fax 86 10 62773461 Energy Conversion and Management 76 2013 811 817 Contents lists available lsev E mail address xtingli X Li 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 efficiency as well as high air pollution which is regarded as one of the main use for heating purposes let alone its applicabi climates In this work a double stage coupled ASAHP for heating purposes in cold regions The coupled switch to a normal single stage ASAHP when the 0196 8904 see front matter C211 2013 Elsevier Ltd All rights reserved http dx doi org 10 1016 j enconman 2013 08 036 can the perfor 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 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 1 Introduction 1 1 Energy consumption and problems systems Energy consumption for heating and high In urban areas of north China the total building energy consumption 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 C211 2013 Elsevier Ltd All rights reserved heat supply hot water is very accounted for 23 of and this was dou 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 significance to improve the performance of ASAHP in cold and Air source Double stage coupled 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 A new heating system based on coupled air 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 article info Article history Received 6 June 2013 Accepted 18 August 2013 Keywords Absorption heat pump abstract Energy consumption for heating source absorption heat pump the single stage ASAHP exhibits double stage coupled ASAHP in cold regions The heating ated in both coupled mode Energy Conversion journal homepage www e and domestic hot water is very high The heating system based on an air had been assessed to have great energy saving potential However poor performance when the outdoor air temperature is very low A proposed to improve the energy saving potential of single stage ASAHP and primary energy efficiency PEE of the proposed system oper single stage mode are simulated under various working conditions source absorption heat pump China at ScienceDirect and Management 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 efficiently 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 Nomenclature G air volume flow rate m 3 s h specific enthalpy kJ kg p pressure Pa Q heating load kW q unit refrigeration capacity kW kg V fluid volume flow rate m 3 s W work consumption kW Abbreviations AHP absorption heat pump ASAHP air source absorption heat pump COP coefficient of performance EHP electrical heat pump ESR energy saving rate 812 W Wu et al Energy Conversion and ture increases the heating performance of the single 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 efficiency 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 efficiency 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 flow 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 flow 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 simplifications 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 m out X m in 1 X m out x out X m in x in 2 Q X m out h out X m in h in 3 Q UA C1LMTD 4 LMTD logarithmic mean temperature difference PE primary energy PEE primary energy efficiency WSAHP water source absorption heat pump Subscripts a absorber c condenser g generator Greeks g efficiency q density kg m 3 e refrigeration coefficient Management 76 2013 811 817 where UA is the product of the heat transfer coefficient and the heat transfer area of each heat exchanger and LMTD is the logarithmic mean temperature difference NH 3 LiNO 3 is used as the working fluid for both stages of the coupled ASAHP owing to its advantages of low freezing point and lack of need of a rectifier 15 16 The thermodynamic proper ties of the fluids are obtained from 17 18 Coefficient of perfor mance COP for heating is defined 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 COP coupled ASAHP Q c2 Q a2 Q a1 Q g1 Q g2 5 For the proposed system operated in single stage ASAHP mode COP ASAHP Q c1 Q a1 Q g1 6 where Q a1 Q c1 and Q g1 are the heat loads of the absorber condenser and generator in the low temperature stage and Q a2 Q c2 and Q g2 are the heat loads of the absorber condenser and generator in the high temperature stage 4 and G1 A1 SHX1 G2 A2 SHX2 Valve3 Valve5 Driving Source Valve W Wu et al Energy Conversion 2 2 2 Primary energy efficiency of the coupled system The power consumption of solution pump is calculated as fol lows 18 W p V p p out C0p in g p 7 where V p is the volume flow rate of the pump p out and p in are the outlet and inlet pressure of the pump and g p is the pump efficiency The power consumption of the fan is calculated using a simpli fied method 7 W fan Dp fan C1 G g fan nC1Dp coil Dp out g fan C1 Q supply qc p Dt 1C0 1 COP high C18C19 C1 1C0 1 COP low C18C19 8 where Dp fan is the evaporator resistance G is the air volume flow rate of the fan g fan is the fan efficiency n is the number of rows of the evaporator coil Dp coil is the coil resistance of each row Dp out is the excess pressure of the fan outlet Q supply is the heat rate sup plied to users q is the density of air c p is the specific heat of air Dt is the temperature rop of air through the evaporator and COP high and COP low are the COP of the high temperature stage and low tem perature stage respectively The specifics 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 efficiency of the ASAHP is defined for the performance evaluation 19 PEE Q supply Q g g boiler W p W fan g power 9 Fig 1 Schematic diagram C1 E1 P1 C2 E2 P 144 P2 High Temperature Stage Low Temperature Stage Supplied Heat Source Loop Hot Water Loop Middle Water Loop G Generator A Absorber C Condenser E Evaporator SHX Solution Heat Exchanger P Precooler Valve2 Valve1 Return Water Pump1 Management 76 2013 811 817 813 where Q supply is the heat rate supplied to users Q g is the heat rate consumed by the generator of the ASAHP W p is the electricity power consumed by the solution pump W fan is the electricity con sumption of the fan and g boiler and g power are the boiler efficiency and power generating efficiency 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 influence 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 efficiency 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 flow rates of each heat exchanger the pressure lift and flow 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 flow rates of water and air are kept unchanged the UA Water of the coupled ASAHP Table 1 Main design parameters of coupled ASAHP Parameters Value Remarks Heating capacity 30 kW Design outdoor air temperature C010 C176C Supplied hot water temperature 45 C176C Typical temperature for low temperature heating Heat source temperature 130 C176C Typical temperature for district heating in China 1 Efficiency of coal boiler 70 Coal boiler heating is widely used in China and the typical efficiency is 70 1 Efficiency of coal generation 33 Electricity of coal generation is the most widely used in China and the efficiency is about 33 1 Efficiency of solution pump 80 Efficiency of fan 80 Table 2 Design parameters of each component of the coupled ASAHP 814 W Wu et al Energy Conversion and values of all the exchangers are regarded as constant for simplifica 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 defined as ESR PE boiler C0PE proposed PE boiler C2100 10 where PE proposed is the primary energy consumption of the proposed heating system kW h and PE boiler is 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 Component UA W K Water air flow rate kg s Heat exchanger Generator1 718 4 0 5833 Generator2 505 4 0 5450 Condenser1 2489 9 0 8573 Condenser2 849 2 0 7160 Evaporator1 873 0 1 2533 Evaporator2 2489 9 0 8573 Absorber1 728 9 0 7160 Absorber2 513 0 0 7160 SHX1 463 1 SHX2 162 1 Precooler1 49 9 Precooler2 53 4 Lift Flow rate kg s Pump and fan Solution pump 1 75 m 0 0560 Solution pump 2 105 m 0 0261 Fan 90 Pa 1 2533 building characteristics given in Table 3 are designed according to the Chinese design standard for energy efficiency 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 configu 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 C176C the temperature lift in each stage is only 29 C176C and Fig 2 Building layout of a typical hotel Management 76 2013 811 817 44 C176C 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 C030 C176Cin the typical cities and therefore the ambient temperature range is taken as C030 to 20 C176C 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 and Table 3 Building characteristics and heating loads in typical cities W Wu et al Energy Conversion 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 C030 C176C whereas the single stage ASAHP cannot work if the ambient temperature is below C020 C176C Shenyang Changchun Harbin Building characteristics Building information 5 floor 21 m high Total heating area 8700 m 2 Window wall ratio 0 5 Heating configuration 24 h heating Temperature 20 C176C Fresh air 30 m 3 h p Heat transfer coefficient of roof W m 2 K 0 35 0 30 0 30 Heat transfer coefficient of floor W m 2 K 0 40 0 39 0 39 Heat transfer coefficient of outer wall W m 2 K 0 40 0 38 0 38 Heat transfer coefficient of window W m 2 K 2 50 1 80 1 80 Shading coefficient of window 0 75 0 50 0 50 Heating loads Average air temperature C176C 8 56 5 47 4 12 Minimum air temperature C176C C023 40 C028 10 C028 68 Heating period November 1 April 1 October 15 April 15 October 15 April 15 Design heating load kW 703 704 741 Accumulated heating load kW h 750 258 1 007 812 1 166 646 Table 4 Performance of the coupled ASAHP under design conditions High temperature stage Low temperature stage Generation Temperature 115 C176C 120 C176C Heat load 11 42 kW 12 22 kW Absorption Temperature 47 C176C50C176C Heat load 11 24 kW 11 37 kW Condensation Temperature 45 C176C22C176C Heat load 7 40 kW 7 18 kW Evaporation Temperature 16 C176C C020 C176C Heat load 7 18 kW 6 30 kW SHX Heat load 3 41 kW 7 97 kW Precooler Heat load 0 49 kW 0 60 kW Solution pump Power 0 04 kW 0 05 kW Fan Power 0 12 kW Stage Heating capacity 18 63 kW 11 37 kW COP 1 63 1 52 PEE 113 47 103 37 Single stage ASAHP Heating capacity 9 82 kW COP 1 39 PEE 93 04 Coupled ASAHP Heating capacity 30 00 kW COP 1 27 PEE 87 29 1 65 1 70 1 75 1 80 COPL COPH Management 76 2013 811 817 815 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 C015 C176C When the ambient tem perature is C030 C176C 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 1 20 1 25 1 30 1 35 1 40 1 45 1 50 1 55 1 60 30 25 20 15 10 5 0 5 10 15 20 COP Outdoor Air Temperature Fig 3 COP of low and high temperature stage of the coupled ASAHP 1 00 1 05 1 10 1 15 1 20 1 25 1 30 1 35 1 40 1 45 1 50 1 55 1 60 30 25 20 15 10 5 0 5 10 15 20 COP Outdoor Air Temperature Coupled ASAHP ASAHP Fig 4 COP of the coupled ASAHP and si