Optimum high pressure for transcritical CO2 heat pumps considering isentropic efficiency and gliding heat extraction.

Paper published in poster form during the conference. The coefficient of performance COP of a transcritical CO2 heat pump system is greatly affected by the compressor discharge pressure. An internal heat exchanger is also important to improve the system performance. Simulations of the ideal transcritical heat pump cycles show maxima in the COP at optimal high pressures depending on evaporation temperature and gas cooler outlet temperature. The real compressor behavior has been incorporated by the isentropic efficiency as a function of the pressure ratio. Then slightly lower optimal high pressures and lower COP are obtained. The correlations for the optimal high pressure are given for the ideal and real heat pump. The low temperature waste heat of a large computing center is considered as heat source of a high capacity CO2 heat pump system. A gliding heat extraction has been chosen using the pinch point method to design the heat exchangers for a district heating system. Different modes of operation (room heating, domestic water heating, absorption cooling) are discussed.

本文以墙报形式在学术会议上发表。跨临界二氧化碳（transcritical CO₂）热泵系统的性能系数（coefficient of performance, COP）受压缩机排气压力的影响显著，内部换热器（internal heat exchanger）对于提升系统性能同样至关重要。理想跨临界热泵循环的仿真结果表明，性能系数COP会在最优高压侧压力下取得峰值，且该最优压力随蒸发温度与气体冷却器出口温度的变化而改变。研究将以压比（pressure ratio）为自变量的等熵效率（isentropic efficiency）纳入模型以表征压缩机的实际工作特性，此时得到的最优高压侧压力与性能系数COP均略低于理想循环的结果。本文分别给出了理想与实际热泵系统的最优高压侧压力关联式。本研究将大型计算中心的低温余热作为大容量CO₂热泵系统的热源，采用夹点法（pinch point method）通过变温取热的方式设计区域供热系统（district heating system）的换热器。此外还讨论了三种运行模式：房间供暖、生活热水制备以及吸收式制冷（absorption cooling）。