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Progress in Ground-source Heat Pumps Using Natural Refrigerants



Wei Wu, Harrison Skye, Lingnan Lin


Hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) refrigerants with high Global Warming Potential (GWP) will be phased down significantly under the Montreal Protocol. Natural refrigerants have a very low GWP and are inexpensive, and therefore are attractive candidates for replacing HFCs and HCFCs currently used in ground-source heat pumps (GSHPs). This paper presents a comprehensive literature survey on the progress in GSHPs using natural refrigerants, including CO2, NH3, water, and hydrocarbons. First, we compared the refrigerants' thermodynamic properties, as well as their performance in brine-to-air and brine-to-water basic-cycle vapor- compression GSHPs. The remainder of the paper discusses recent research for each natural refrigerant applied to GSHPs. Studies of the use of CO2 were the most common because CO2 is non-flammable, is non-toxic, has high volumetric capacity, and has transport properties favorable for high heat transfer and low pressure drop. The CO2 studies reviewed covered basic and advanced vapor-compression cycles, direct-expansion systems, CO2 as a secondary fluid, multi-source hybrid GSHPs, and hybrid GSHPs for lower ground thermal imbalance. NH3 was the second most studied, though concerns about its toxicity can inhibit broad adoption. Vapor- compression NH3 GSHPs were mainly used in heating, absorption-type NH3 GSHPs were applied to eliminate ground thermal imbalance and reduce the number of boreholes, and the hybrid compression-absorption NH3 GSHPs widened the operating temperature range and enhanced the efficiency. A few studies evaluated water as a refrigerant for absorption-type GSHPs, including applications to solar cooling, ground thermal imbalance, and district heating. Our literature survey found propane was the only hydrocarbon considered for GSHPs; propane studies included analyses on refrigerant charge, system performance, comparison with air-source heat pumps, and propane as a secondary fluid.
International Journal of Refrigeration


Ground-source heat pump, Natural refrigerant, Carbon dioxide, Ammonia, Water, Hydrocarbon


Wu, W. , Skye, H. and Lin, L. (2018), Progress in Ground-source Heat Pumps Using Natural Refrigerants, International Journal of Refrigeration, [online], (Accessed June 20, 2024)


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Created July 31, 2018, Updated October 12, 2021