Comparison analysis of ideal performance coefficients for different heat pumps driving air conditioning modes

The liquid dehumidification and heat pump hybrid (LDHP) system, the temperature-humidity independent control (THIC) system and traditional heat pump (HP) system have been being studied by researchers. In the LDHP system, the supply air was cooled by heat pump firstly and then was sent to the dehumidifier for dehumidification, finally the air after dehumidification was sent to the indoor environment. The heat and mass transfer in dehumidifier and regenerator would release/absorb moisture latent heat; therefore, heat absorption/extraction through a heat pump could be used to achieve isothermal dehumidification/regeneration in an ideal process. In the THIC system, the liquid dehumidification system was adopted to handle the fresh air and part of the return air, which was utilized to deal with the entire latent heat load indoors. Meanwhile, the rest of return air was cooled by the heat pump so as to remove indoor sensible heat load. In the ideal HP system, the outdoor air was preheated by indoor exhaust air in the total heat exchanger, and then mixed with return air, Then the mixed supply air was cooled and condensate by the heat pump. Under ideal conditions, the process from reheating the air under dew point to reach the air supply state was able to absorb heat from outdoor by the ideal heat engine in which the air under dew point was heated and output work yielded at the same time. In this paper, models of ideal reversible process were constructed to analyze the theoretical maximum of the coefficient of performance (COP) for the above-mentioned system under different conditions. Meanwhile, the influences of different supply air temperature, enthalpy humidity ratio, indoor and outdoor environmental air conditions on the COP of those system were also discussed. Results indicated that increasing the supply air temperature difference or reducing the enthalpy humidity ratio would decrease the COP of three mentioned systems in ideal condition, and simultaneously the COP was more significantly affected by the supply air temperature difference. When the enthalpy humidity ratio was 5 000, the supply air temperature difference increased from 2 to 10℃, COP of LDHP system and HP system fell by 26% and 18% respectively, which was far below the 63% of THIC system. When the enthalpy humidity ratio ranging from 4 000 to 20 000, the COP of LDHP, THIC and HP system increased by 28%, 99% and 33%, respectively. At the same time, the influences of indoor and outdoor environmental air parameters on the COP of those systems were also involved. The COP of the three systems all increased with increasing the indoor air design temperature or relative humidity, which had nearly doubled in the whole simulation range. Meanwhile, during the process of the outdoor environmental air temperature and relative humidity varying from 28℃, 60% to 38℃, 85%, the COP of LDHP system, HP system, THIC system decreased by 78.3%, 59.7% and 52.1% respectively, and the COP was more obviously affected by the air temperature. Furthermore, the COP of the LDHP system was the largest, followed by the HP system, and the THIC system was the lowest under the simulation conditions. The research results in this paper provided a theoretical guidance for the selection and optimization of the three air-conditioning methods. © 2018, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.