Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Modeling and Experiments for a CO2 Ground Source Heat Pump with Subcritical and Transcritical Operation



Wei Wu, Harrison M. Skye, John Dyreby


Ground-source heat pumps have high energy efficiency and CO2 is an environmentally-friendly refrigerant with no ozone depletion potential (ODP) and a low global warming potential (GWP = 1), yet CO2 is not regularly applied to GSHPs. We developed a prototype residential CO2 liquid- to-air GSHP to investigate the potential for CO2 GSHPs. Further, we developed a detailed model of the system that simulates both cooling and heating operation; the model is the primary focus of this report. The model simulates both subcritical and transcritical operation since the system regularly operates near and above the critical temperature of CO2 (30.98 °C) during normal heating and cooling operation. The detailed model considered both the thermodynamic and transport processes in the cycle, and air-side heat transfer and condensation. We performed cooling tests for the prototype CO2 GSHP that included those from the International Standards Organization (ISO) 13256-1 standard for liquid-to-air heat pumps, as well as extended tests at additional entering liquid temperatures (ELTs). The model predicted the measurements within: COP 2.2 % to 7.6 %, total capacity 2.5 % to 4.7 %, and sensible capacity 1.0 % to 3.0 %. We compared the cooling measurements with those of a commercially- available R410A GSHP and found the CO2 GSHP has a higher cooling COP and total capacity than the R410A GSHP with ELTs below 20 °C. The R410A GSHP had a lower sensible capacity and correspondingly a better dehumidification capability. In the future, the model can be used to investigate methods to improve the CO2 GSHP performance to meet or exceed that of the R410A system; possible studies include replacing the electronic expansion valve (EEV) with an ejector, optimizing the charge, and optimizing the heat exchanger geometry and circuiting.
Energy Conversion and Management
September 2021


CO2, carbon dioxide, ground-source heat pump, subcritical cycle, transcritical cycle, model validation


Wu, W. , Skye, H. and Dyreby, J. (2021), Modeling and Experiments for a CO2 Ground Source Heat Pump with Subcritical and Transcritical Operation, Energy Conversion and Management, [online], (Accessed May 30, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created September 1, 2021, Updated July 6, 2022