Abstract Nowadays heat pumps are crucial elements of air conditioning systems and still gaining popularity, but they suffer from limitations in very cold climates such as low coefficient of performance (COP) and air leaking. Recently-developed trans-critical CO2 heat pumps are proper replacements for their (H)CFC counterparts, especially in environmental concerns, but they have not answered the temperature limitations yet. The temporal solution is to bridge the temperature gap between low temperature ambient air (lower than −15 °C) and normal operating temperature of heat pump (about 0 °C) by intermediate systems such as supplementary heaters or geothermal sources. The main idea of this study is to define a highly-efficient heat pump instead of intermediate systems. Consequently, instead of a heat pump with auxiliary heater, there is a cascade of heat pumps which operates properly in very cold climates. In this regard, two sub-critical CO2 heat pumps were studied. One uses expansion valves and another utilizes expanders. COPs more than 3.7 were obtained in −40 °C. Using the first and second laws of thermodynamics, analyses on the performance characteristics of the cycles were carried out. Also, a parametric study was conducted to optimize the performance of each cycle under various operating conditions. The results show that these cycles perform properly in both energy and exergy points of view and expander increases COP and reduces lost exergy. Inter-stage pressure and performance of internal heat exchanger were studied and optimal values were extracted. Lastly, a thermo-economic comparison between the purposed cycles and conventional fossil fuel heating systems were done. The findings show that this sub-critical CO2 heat pump can make a conventional heat pump to remain functional at low temperatures without any significant fall in COP.

中文翻译：

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适用于低温气候的高效亚临界二氧化碳辅助热泵（能量和火用分析）

摘要 如今，热泵​​是空调系统的关键元件，并且仍然越来越受欢迎，但它们在非常寒冷的气候下受到限制，例如性能系数 (COP) 低和空气泄漏。最近开发的跨临界 CO2 热泵是其 (H)CFC 对应物的合适替代品，尤其是在环境问题上，但它们尚未解决温度限制问题。临时解决方案是通过辅助加热器或地热源等中间系统弥合低温环境空气（低于-15°C）和热泵正常运行温度（约0°C）之间的温度差距。本研究的主要思想是定义一个高效的热泵而不是中间系统。因此，代替带有辅助加热器的热泵，有一个级联的热泵，可以在非常寒冷的气候下正常运行。在这方面，研究了两个亚临界 CO2 热泵。一种使用膨胀阀，另一种使用膨胀器。在-40°C 下获得超过 3.7 的 COP。使用热力学第一定律和第二定律，对循环的性能特征进行了分析。此外，还进行了参数研究以优化每个循环在各种操作条件下的性能。结果表明，这些循环在能量和火用方面均表现良好，并且膨胀机增加了 COP 并减少了火用损失。研究了内部换热器的级间压力和性能，并提取了最佳值。最后，对特定循环和传统化石燃料供暖系统进行了热经济比较。研究结果表明，这种亚临界 CO2 热泵可以使传统热泵在低温下保持功能，而不会显着降低 COP。