Modeling and experiments for a CO₂ ground-source heat pump with subcritical and transcritical operation

CO₂-based ground-source heat pumps (GSHPs) have the potential to be very environmentally friendly, since GSHPs operate with high energy efficiency, and CO₂ has no ozone depletion potential (ODP) and a low global warming potential (GWP). We developed a prototype CO₂ liquid-to-air GSHP to investigate its performance potential in residential applications. 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 heating and cooling operation. The model considered both the refrigerant-side thermodynamic and transport processes in the cycle, as well as the air-side heat transfer and moisture removal. 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 0.5% to 6.7% for COP, 1.0% to 3.6% for total capacity, and 3.3% to 4.9% for sensible capacity. We compared the measured cooling performance to published performance data for a commercially-available R410A GSHP and found that for ELTs below 20 °C, the CO₂ GSHP has a higher cooling COP and total capacity than the R410A GSHP. At the ‘standard’ cooling rating condition (ELT 25 °C), the CO₂ GSHP COP was 4.14 and the R410A GSHP COP was 4.43. At ‘part-load’ conditions (ELT 20 °C) the CO₂ GSHP COP was 4.92 and the R410A GSHP COP was 4.99. 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 over a wider range of ELTs; possible studies include replacing the electronic expansion valve (EEV) with an ejector, optimizing the charge, and optimizing the heat exchanger geometry and circuiting.