61 Theoretical and Experimental Investigation of Ground Source Co₂ Heat Pump and Air Conditioning Systems

Increased concern about the environmental impact of refrigeration systems is leading toward improved energy efficiencies of such systems with ozone-friendly refrigerant with the least possible global warming potential. Carbon dioxide is one of the most promising refrigerants attaching great interest in industrial and scientific fields. However, the coefficient of performance of air-source CO₂ heat pumps (ASHP) decreases with decreasing outdoor temperature while that of air conditioning systems decreases with increasing outdoor temperature. Ground energy is a clean, renewable source with a higher temperature than the air and the advantage of a more stable temperature range. This paper describes a combination of this ideal refrigerant and renewable energy sources in a ground source CO₂ heat pump (GSHP) water heater and air conditioning system. Theoretical analyses of the GSHP and ASHP show that the COP of the ground source refrigeration system is much higher than that of the air-source refrigeration system since the reservoir temperature at the condenser is much lower in the ground source refrigeration system than in the air-source refrigeration system. The COPh of the GSHP is also higher than that of the ASHP, especially in cold climates. The heat pump system, when used not only for space heating but also for water heating, has an optimal high side pressure which maximizes the COPh. The water inlet temperature into the gas cooler has an important influence on the heater performance with the heating COP increasing as the outlet temperature decreases.