Laboratory Tests of a Prototype Carbon Dioxide Ground-Source Air Conditioner
Harrison M. Skye, Wei Wu
CO2 is an attractive refrigerant option for ground-source heat pumps (GSHPs) because is it environmentally friendly with low global warming potential (GWP = 1) and no ozone depletion potential (ODP). The cycle efficiency is also important for a refrigerants total environmental impact since the CO2 emissions from the power source (i.e., indirect emissions) far outweigh the direct GWP impact (i.e., the GWP from the release of the fluid into the atmosphere) of the working fluid in nearly all heating, ventilation, air-conditioning (HVAC) equipment. For HVAC applications, depending on the sink/source temperatures, CO2 operates in a subcritical or transcritical cycle, where the subcritical cycle has higher efficiency. The lower temperature lift associated with GSHPs, compared to air-source heat pumps (ASHPs), enables a CO2 based system to operate more often in a subcritical cycle. A prototype residential liquid-to-air CO2 ground-source air conditioner (GSAC) was developed and tested in a laboratory according to the ISO 13256-1 standard for rating GSHPs. (A GSAC, rather than a GSHP, was developed to reduce the design and test complexity.) A range of entering liquid temperatures (ELTs) were tested so the system operated in both a subcritical and transcritical cycle. Performance was reported in terms of coefficient of performance (COP), capacity, sensible heat ratio (SHR), and pressures. The CO2 system was compared to an entry-level, commercially-available R410A-based GSHP; with ELTs ranging (10 to 39) °C the CO2 system cooling COP ranged 2.3 to 7.3, whereas the R410A values ranged 3.3 to 6.0. The R410A system had higher efficiency at ELTs greater than 20 °C, so further effort is needed to increase the CO2 system efficiency there.