| With the successive signing of the Montreal Protocol and the Kigali Amendment,it has been a trend to shift the refrigerants toward the direction of both zero ODP?ozone depletion potential?and low GWP?global warming potential?.As a natural refrigerant,carbon dioxide?CO2?has been favored by people for its excellent environmental performance.In addition,the air-source transcritical heat pump water heater using CO2as working fluid has a series of advantages,such as wider application range,better low-temperature performance and higher water-outlet temperature,which fully meets the requirements of heat pump in the“coal to electricity”policy that is currently being vigorously promoted in China.Therefore,the transcritical CO2 heat pump become a hot spot in the field of heat pump at home and abroad in recent years and then has broad development prospects.Based on the air-source transcritical CO2 heat pump water heater,the influence of gas-cooler outlet status,compressor speed,water-inlet flowrate and water-inlet temperature on the system performance were experimentally studied in turn.Then,a new standard named UTC?unit temperature-lift cost?was proposed.Combined with the UTC and other factors,the three parameters that affecting the hot water outlet temperature of the system?compressor speed,water-inlet flowrate and water-inlet temperature?were comprehensively compared and evaluated.From the above two aspects of work,the following main conclusions were obtained:?1?Upon increasing the compressor discharge pressure,the gas-cooler outlet temperature of CO2(tgc,o)rises temporarily at first,then decreases,and finally remains almost unchanged.Moreover,the tgc,o also affects the compressor suction temperature caused by the effect of internal heat exchanger.?2?As the compressor discharge pressure increases,the system reaches the maximum COP(COPopt)firstly,with further increase of discharge pressure,the maximum heating capacity(Qmax)is achieved.The operating range between COPoptpt and Qmax of the system is a suitable operating situation,and the system can be operated within the operating range by adjusting the compressor discharge pressure.?3?Rise of hot water temperature can always lead to the decrease of COPopt that the system can achieve,and the increase of the corresponding optimal discharge pressure.As the compressor speed increases,COPopt is decreased while the corresponding heating capacity is increased,and the rise of water-outlet temperature at the optimal status is gained.When the water-inlet flowrate decreases,COPopt is decreased and the water-outlet temperature at the optimal status shows an increase,while the corresponding heating capacity decreases slightly or even rises a little.When it comes to the water-inlet temperature,as the water-inlet temperature increases,both COPopt and the corresponding heating capacity are declined,while the water-outlet temperature at the optimal status increases.?4?In the ranges of the experiment,the average UTCs corresponding to the variations of compressor speed,water-inlet flowrate and water-inlet temperature are0.054°C-1,0.042°C-1 and 0.099°C-1,respectively.In general,for these three methods of increasing the hot water temperature,the water-inlet flowrate has the lowest UTC and then the best lifting effects of water-outlet temperature.Besides,the water-inlet flowrate has more advantages when the demand for water-outlet temperature rise is large.The compressor speed is suitable to meet the needs of increasing water-outlet temperature and heating capacity simultaneously.However,no competitive advantages can be found for the water-inlet temperature to rise the hot water temperature compared to the other two ways. |
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