High efficiency sub-critical carbon dioxide supplementary heat pump for low temperature climates (energy and exergy analysis)

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.