Irreversible Losses And Performance Study Of A Transcritical CO₂ Two-stage Compression/Ejector Refrigeration System For Shipboard Cold Chambers

The adverse effects of global warming and climate change require critical measures for shipboard refrigeration technology because of its impact on greenhouse gas emissions.In this paper,two systems are built based on the shipboard cold chamber transcritical CO₂ base refrigeration cycle using two methods,namely two-stage compression intercooling and introduction of an ejector,respectively.Namely,the transcritical CO₂two-stage compression refrigeration system（CTRC）and the transcritical CO₂ two-stage compression/ejector refrigeration system（TERC）,where the ejector is located on the low-pressure side,i.e.,the low-pressure ejector,and the main flow is part of the high-pressure compressor inlet gas,and the secondary flow is the saturated vapor at the evaporator outlet.The main work done in this paper is as follows.Firstly,the theoretical study of TERC and CTRC systems using entropy analysis was conducted in this paper to investigate the entropy production and power capability loss of each component in TERC and CTRC systems,as well as the effects of gas cooler outlet temperature,high-pressure side discharge pressure,evaporating temperature and intermediate pressure on the entropy production of the components.The results show that in the TERC system,the entropy production of the low-pressure ejector is the largest,accounting for 26.94%of the whole system,followed by 26.06%of the low-pressure compressor.The entropy production of the low-pressure ejector decreases significantly with increasing evaporating temperature and increases significantly with increasing intermediate pressure.In the TERC and CTRC systems,the entropy production of the high-pressure expansion valve increases significantly with the increase of the gas cooler outlet temperature,while the entropy production of the high-pressure expansion valve decreases with the increase of the evaporating temperature,intermediate pressure and high-pressure side discharge pressure.Secondly,this work adopts a combination of conventional and advanced exergy analysis to evaluate the exergy destruction characteristics of TERC system.According to the advanced exergy analysis,75.88%of the exergy destruction are endogenous,which indicates that the system exergy destruction is mainly caused by the irreversibility of the components themselves,and the interactions between the components are not very close.Among all components,the low-pressure compressor has the largest endogenous avoidable exergy destruction,indicating that it has the greatest potential for optimization.While the rest of the improvement order is the low-pressure ejector,high-pressure compressor,intercooler,evaporator and gas cooler,the low and high-pressure expansion valves have the lowest improvement priority.It is noteworthy that there is a significant difference between the conclusions obtained from the conventional and the advanced exergy analysis.In addition,the efficiency of the low-pressure ejector and compressors as well as the effect of the high-pressure side discharge pressure and intermediate pressure on the system’s exergetic performance was considered.When the efficiency of the low-pressure ejector,high-pressure and low-pressure compressor was increased from0.5 to 0.9,the system’s exergetic efficiency increased by 9.52%,26.76%and 46.15%,respectively.From the system’s exergetic performance perspective,there exists an optimum intermediate pressure of 4 MPa and an optimum high-pressure side discharge pressure range of 8.5～9.0 MPa.Again,the system performance of CTRC and TERC was compared and analyzed through theoretical studies,and the effects of intermediate pressure and evaporating temperature on the low-pressure ejector performance were also investigated.The results show that for the TERC system,the maximum COP varies from 0.35～0.57 when the intermediate pressure increases from 3 MPa to 5 MPa under the condition that the discharge pressure on the high-pressure side is 9 MPa.In addition,the COP of the TERC and CTRC systems increases significantly with the decrease of the intermediate pressure and the increase of the evaporating temperature.With the increase of intermediate pressure,the low-pressure ejector entrainment ratio and lift pressure ratio increase significantly;at standard operating conditions,the increases are 60%and 24.8%,respectively.However,the low-pressure ejector entrainment ratio does not change significantly with the increase of evaporation temperature,while the lift pressure ratio decreases significantly.In addition,the introduction of the low-pressure ejector can effectively reduce the power consumption of the low-pressure compressor,but the effect diminishes with the increase of the intermediate pressure.Finally,experimental studies were conducted on TERC and CTRC systems by building an experimental setup.The effects of gas cooler outlet temperature,evaporator flow rate,operation time,high-pressure side discharge pressure and low-pressure stage electronic expansion valve opening on the system performance were investigated.The results show that the COP of the CTRC system generally shows a decreasing trend with the increase of the opening of the low-pressure stage electronic expansion valve at different gas cooler outlet temperatures.Under the experimental conditions of the TERC system,the temperature inside the cooler was maintained at about 0℃,the flow rate on the low-pressure side was less,and the entrainment ratio and pressure recovering ratio of the low-pressure ejector were 0.09 and 1.09,respectively,while the difference between the mainstream inlet pressure of the low-pressure ejector and the evaporator pressure was maintained at about 1.05 MPa.At the gas cooler outlet temperature of 10～20℃,the evaporator inlet flow rate of the CTRC system is more than three times that of the TERC system,resulting in a lower cooling effect of the TERC system than the CTRC system.By adjusting the solenoid valve of the low-pressure ejector to improve the evaporator flow,it was found that with the increase of the evaporator flow,the effect of the low-pressure ejector was enhanced and the cooling effect of the evaporator was gradually improved,and finally the performance of the TERC system was slightly better than that of the CTRC system under a larger evaporator flow.