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Laboratory Tests of a Prototype Carbon Dioxide Ground-Source Air Conditioner



Harrison M. Skye, Wei Wu


Environmental concerns are driving regulations to reduce the use of hydrofluorocarbons (HFCs) with high global warming potential (GWP) as refrigerants in heat pumps. CO2 is an attractive alternative refrigerant because is it 'environmentally friendly' in terms of 'direct' emissions, with GWP = 1, and no ozone depletion potential (ODP). However, CO2 heat pumps generally have a lower efficiency than HFC-based systems, and therefore have higher 'indirect' emissions, related to generating the electricity that powers them. The indirect emissions dwarf the direct emissions for most heating, air-conditioning and refrigeration applications, so it is critical for the equipment to operate with high efficiency. CO2 air-source heat pumps (ASHPs) provide cooling with particularly low efficiency at high ambient temperatures where the CO2 operates in a transcritical cycle. Using CO2 in a ground-source heat pump (GSHP) offers the potential to overcome the low efficiency since a GSHP operates with lower heat-rejection temperature (for cooling), enabling the system to operate some of the time in a more-efficient subcritical cycle. This report details the laboratory tests of a prototype residential liquid-to-air ground-source air conditioner (GSAC) using CO2 as the refrigerant. The tests were performed in an environmental chamber and followed the ISO 13256-1 standard for rating GSHPs. The CO2 GSAC operated either in a subcritical or a transcritical cycle, depending on the entering liquid temperature (ELT). The test results included the coefficient of performance (COP), capacity, sensible heat ratio (SHR), and pressures. The system incorporated a liquid-line/suction-line heat exchanger (LLSL-HX), which was estimated to cause a COP penalty of (0 to 2) % for ELTs ranging (10 to 25) °C, and benefit of (0 to 5) % for ELTs ranging (30 to 39) °C. The CO2 system was compared to a 'low-cost', commercially-available R410A-based GSHP. With ELTs ranging (10 to 39) °C the CO2 system cooling COP ranged (7.3 to 2.4), whereas the R410A system values ranged (6.1 to 3.2). At the 'standard' rating condition (ELT 25 °C), the CO2 GSAC cooling COP was 4.14 and the R410A GSHP COP was 4.57. At 'part-load' conditions (ELT 20 °C) both systems had a COP of ≈4.92. Further effort is needed to increase the CO2 system efficiency at ELTs greater than 20 °C, since it underperformed the R410A system in that temperature range.
Technical Note (NIST TN) - 2068r1
Report Number


Air conditioner, carbon dioxide, CO2, ground-source heat pump, subcritical and transcritical cycles


Skye, H. and Wu, W. (2021), Laboratory Tests of a Prototype Carbon Dioxide Ground-Source Air Conditioner, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online],, (Accessed June 16, 2024)


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Created April 12, 2021, Updated November 29, 2022