Development of high-efficiency carbon dioxide commercial heat pump water heater

Abstract

Although heat pump water heaters are widely accepted in both Japan and Europe, where energy costs are high and government incentives for their use exist, acceptance of such products in the United States has been limited. This trend is slowly changing with the introduction of heat pump water heaters into the residential market, but acceptance remains low in the commercial sector. The objective of the presented work is the development of a high-efficiency R744 heat pump water heater of approximately 35-kW (10-ton) heating capacity for commercial applications with effective utilization of the cooling capability for air conditioning and/or refrigeration. This unit will be targeted at commercial use where some cooling load is typically needed year round, such as restaurants, hotels, nursing homes, and hospitals. The improvement process concentrated on the heat exchangers of the system. Further optimization potential was identified by investigating the gas cooler as well as the expansion device of the heat pump water heater by using a two-phase ejector. In addition, a comparison to a commercially available baseline R134a unit of the same capacity and footprint was made where significant package size reduction potential of the R744 heat pump water heater was discovered as well as performance improvement, especially at high water temperature lifts.