Translated Abstract
The energy consumption for domestic hot water requirements accounts for almost 1/4 of all residential energy requirement in the city at present. Most residential water heaters(WHs) are equipped with conventional heaters generating heat by consuming fossil fuels or high-grade electricity. Compared to those WHs, air-source heat pump water heater(ASHPWH) could supply much more heat just with the same amount of electric input. In recent years, ASHPWH are successfully applied in south China. However, the outdoor temperature in north China is very low in winter. Thus the compressor discharge temperature and the pressure ratio of ASHPWH system are very high, and the coefficient of performance(COP) decreases sharply. These shortcomings limit its application in north China. In order to improve its heating performance, two kinds of ASHPWH using auto-cascade system were proposed because it can enlarge the temperature range of system operation in cold regions. The main differences between two systems are as follows: after the liquid flowing out from vapor-liquid separator is throttled, the resulted two-phase fluids directly flows into compressor inlet for the first system However, for the second system, the liquid flowing out from the separator is firstly throttled, and then enter a heat regenerator to provide refrigeration for the vapor flowing out from the separator Finally, two streams flowing out from the heat regenerator would enter compressor inlet. Two kinds of non-azeotropic refrigerant mixtures of R123/R22 and R600a/R22 were selected as the system’s refrigerants. Their thermodynamic properties were computed and analyzed. Experimental apparatus for the auto-cascade air-source heat pump water heater was built up based on theoretical analysis. After several exploratory experiments, the control strategies of electronic expansion valves for above systems were established.At first stage, a series of experiments on the thermodynamic performance of the two kinds of ASHPWH system at -5℃ condition were conducted. The experimental results showed that the first ASHPWH system can provide 55℃ hot water with the compressor discharge temperature below 120℃ by using 123/R22 and R600a/R22. But for the second system, the discharge temperature of compressor would go beyond 120℃ with hot water temperature of 55℃. When these two experimental systems adopted R600a/R22 as their mixture refrigerant, a lower discharge temperature and a higher COP would be obtained. In addition, at the ambient temperature -15℃ only the first system adopting R600a/R22 as its mixture refrigerant could provide 55℃ hot water with its compressor discharge temperature less than 100℃.This research revealed that at low outdoor temperatures the auto-cascade system would effectively increase the heating performance and enlarge the application scope of ASHPWH. A number of further improvement measures were proposed for future researches.
Translated Keyword
[Air-source heat pumpWater heaterAuto-cascadeNon-azeotropicmixture refrigerant]
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