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 degrees 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 degrees C, the CO2 GSHP has a higher cooling COP and total capacity than the R410A GSHP. At the 'standard' cooling rating condition (ELT 25 degrees C), the CO2 GSHP COP was 4.14 and the R410A GSHP COP was 4.43. At 'part-load' conditions (ELT 20 degrees 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.

Automated summary

The study developed a prototype CO2 liquid-to-air GSHP and a detailed model to simulate its performance. Testing and comparison showed that the CO2 GSHP has better cooling performance than the R410A GSHP under low temperature conditions.