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.
, Skye, H.
and Dyreby, J.
Modeling and Experiments for a CO2 Ground Source Heat Pump with Subcritical and Transcritical Operation, Energy Conversion and Management, [online], https://doi.org/10.1016/j.enconman.2021.114420
(Accessed December 6, 2023)