Abstract
The effect of the discharge pressure on the performance of the trans-critical CO2 heat pump with a low gas-cooler outlet temperature is experimentally investigated on a test rig of water source heat-pump water heater. The optimal discharge pressure of the trans-critical CO2 heat pump is investigated under different external operation conditions. When the tap-water temperature is low, the characteristic of the S-shape isotherm at the supercritical region has little effect on the occurrence of the optimal discharge pressure; while the mass flow rate of CO2, the suction pressure and the gas-cooler outlet temperature play a significant role in determining the emergence of the optimal discharge pressure. At the optimal discharge pressure, the COP reaches the peak; however, the corresponding heating capacity is still lower than its maximum, which is reached as the discharge pressure is slightly above the optimal discharge pressure. Reducing the tap-water flowrate or increasing the water-source temperature can increase the optimal discharge pressure. The COP is positively dependent on both the tap-water flowrate and the water-source temperature. In addition, the tap-water flowrate has a negligible influence on the maximum heating capacity while increasing the water-source temperature can greatly enhance the heating capacity.
Highlights
Owing to numerous incomparable advantages like excellent environmental compatibility and heat transfer characteristic, carbon dioxide (CO2 ) has been widely acknowledged as the most competitive alternative refrigerant
In a trans-critical system, the heat rejection process occurs above the critical point while the heat absorbing process takes place below the critical point
In a sub-critical system, both the absorbing and rejection of heat process take place below the critical point. It is the unique characteristic of the trans-critical CO2 system that makes it different from the traditional sub-critical system [2,3]
Summary
Owing to numerous incomparable advantages like excellent environmental compatibility and heat transfer characteristic, carbon dioxide (CO2 ) has been widely acknowledged as the most competitive alternative refrigerant. Since Lorentzen and Pettersen put forward the application of the trans-critical. CO2 system in the mobile air-conditioning [1], replacing CFC and HCFC refrigerants with CO2 has attracted great attention in many industrial fields. Conventional air-conditioning and refrigerating systems with CO2 as the working medium operate in trans-critical mode. In a trans-critical system, the heat rejection process occurs above the critical point while the heat absorbing process takes place below the critical point. In a sub-critical system, both the absorbing and rejection of heat process take place below the critical point. It is the unique characteristic of the trans-critical CO2 system that makes it different from the traditional sub-critical system [2,3]. The simplified sketch and the corresponding pressure-enthalpy diagram of a trans-critical CO2 system are shown in Figures 1 and 2 respectively
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