Performance Analysis And Comparison Study Of Different Compression-assisted GAX Cycles For Heat Pump Applications

The GAX cycle can improve the energy efficiency of ammonia water absorption heat pump.However,when the generator driving heat source temperature is not high enough and the evaporator heat-source temperature is too low,the GAX cycle efficiency decreases,and adding compressor in the absorption heat pump cycle may solve the above limitations.In this thesis,the operation characteristics of a compressor added on the high-pressure side and low-pressure side of GAX ammonia-water absorption heat pump system are compared,and the operation characteristics of the compression-absorption ammonia water absorption heat pump system are studied.Based on that,the optimal coupling scheme of the compression-absorption heat pump is identified.The dissertation starts with the establishment of the physical and mathematical model of the system.Four kinds of compressor combined with GAX ammonia water absorption heat pump were simulated.In HGAX 1a and HGAX 1b systems,the compressor is used between the evaporator and absorber.While,in HGAX 2a and HGAX 2b,the compressor is implemented between the desorber and condenser.The compressor has different purposes in each of the models described above.The compressor is used to increase the absorber pressure in HGAX 1a and to decrease the evaporator pressure in HGAX lb.While,it is used to decrease the desorber pressure in HGAX 2a and to increase the condenser pressure in HGAX 2b.First,the thesis simulates and compares HGAX 1a and HGAX 2a by controlling the desorber and absorber pressures,makes a detailed thermodynamic comparison,optimizes the operating parameters of the system,and analyses the influence of the designed operating parameters such as the compression ratio,evaporator,condenser,absorber and desorber temperatures on the models'performance and circulation ratio.The results show that,compared to HGAX 2a,the compressor in HGAX 1a can improve the absorption characteristics by increasing the absorption pressure,which proved to be more advantageous to the system coefficient of performance,especially when the evaporator temperature is lower than 2.2?.However,under evaporator temperatures higher than 2.2?,reducing the generator pressure in HGAX 2a tends to be more beneficial to the system coefficient of performance.For the same heating capacity,compression ratio,and an evaporator temperature of-10?,HGAX 1a achieved 6.98%higher COPh,p compared to HGAX 2a.However,under other certain simulated conditions,HGAX 2a can yield 11.71%higher COPh,p compared to HGAX la.The results also pointed out that the circulation ratio is the prevailing factor in estimating the performance of HGAX 1a at low evaporator temperatures,and HGAX 1a always acquiring a lower circulation ratio compared to HGAX 2a under the same design temperatures and compression ratios.In addition to that,optimum values of the compression ratio were found that maximizes the COPh,p for each cycle.Secondly,in this dissertation,the HGAX lb and HGAX 2b models are simulated and compared with the conventional GAX cycle.The results reveal that the use of the compressor in HGAX 1b can significantly reduce the evaporator temperature compared to the GAX cycle.At 40? of condenser and absorber temperatures,and 185? of desorber temperature,HGAX lb achieved a reduction in evaporator temperature from 0? to-50? using a compression ratio of 10 compared to a drop from 0 to-15.8? in the GAX cycle(without a compressor).On the other hand,the usage of the compressor in HGAX 2b expanded the operational range of the condenser temperature to more than 1.5 times of that in the GAX cycle.Similarly,at 0?,40?,and 185? of evaporator,absorber,and desorber temperatures,respectively,the results show that HGAX 2b accomplished an increase in condenser temperature from 40? to 100? using a compression ratio of 4.08 compared to an increase from 40? to 62.1? in the GAX cycle(without a compressor).Overall,the results of this work indicates that the hybridization of GAX ammonia water absorption heat pump cycle enhanced its thermal performance and allows the cycle to operate at severe working conditions.