Abstract
Carbon dioxide heat pump is considered a potential low-carbon combined cooling and heating solution. However, there is a strong need for simple methods to improve the energy efficiency of the carbon dioxide heat pump due to the larger throttling losses that result in poor performance. Given this, a compact water-cooled subcooler-assisted carbon dioxide heat pump for combined cooling and heating is designed in this paper. An experimental test platform is built, where experimental investigations are performed to reveal the working performance. Furthermore, a matching control strategy is developed and verified as well. The experiments demonstrated that the adoption of the water-cooled subcooler has significantly improved the cooling performance. The cooling capacity (Q̇c) and cooling coefficient of performance (COPc) are increased by up to 20.2% and 31.5%, respectively. Moreover, the optimal discharge pressure is lowered by 0.5 MPa. However, the heating performance is somewhat degraded, for which the control strategy with the synergistic variation of subcooling degree and compressor rotation speed is developed. The deteriorated heating performance is thus mitigated. The developed control strategy has been verified in practice, and the carbon dioxide heat pump performance could be maintained at the highest possible level. Q̇c and COPc are further improved by a maximum of 40.7% and 37.7%, respectively. The heating and overall coefficient of performance could reach 2.55 and 5.10 at the return water and outlet water temperatures of 40 °C and 75 °C, respectively.
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