CO2 Heat Pump Water Heaters (HPWHs) were first commercialized in Japan in 2001. This now mature technology was incentivized by utilities and marketed under the brand name "EcoCute.” CO2 HPWHs are offered by Panasonic, Daikin, DENSO, Sanden, Itomic, Mitsubishi, Sanyo, and Hitachi. The Japanese government has subsidized this technology since 2002 with an initial goal of installing 5.2 million units by 2010.
Japanese companies redesigned their CO2 heat pump water heaters for the European market to comply with European Union (EU) safety standards. Safety standards are stringent as CO2 pressures can be in the range of hundreds of pounds per square inch. Stiebel, enEX, ICS, Thermea, Kylma, CTC, JCA and Viessmann have added CO2 heat pumps to their product offerings. Sales of HPWHs in the EU doubled to 48,000 units in 2011. These HPWHs are also sold in Australia by Sanden which has provided the systems for the Technology Innovation Projects (TIPs) funded by BPA.
CO2 (or R744) HPWHs achieve a COP greater than 3.0 compared to 2-2.5 for standard refrigerant integrated heat pump water heaters (BPA , 2014). R744 has a global warming potential of 1 compared to about 2,000 for typically used refrigerants (like R410). It also has a broader range of operating temperatures, allowing CO2 HPWHs to maintain capacity at low temperatures with a COP of around 2.0 at 17 F. Split system CO2 HPWHs do not add to the space heating load like integrated HPWHs located in the interior space. However, these systems are much more expensive than standard refrigerant HPWHs.
The CO2 HPWH technology is emerging in the U.S. market for residential applications, however a commercial/industrial version described in ET #293 is currently available. Code officials in Portland, Oregon have approved CO2 HPWH for residential space and water heating applications. Sanden is currently in the process of obtaining a UL listing of its split system which will be available in Fall, 2015. Design modification issues for the U.S. market include operation with a 240 V, 60 Hz power supply, meeting pressure vessel standards, meeting UL requirements and incorporating advanced freeze protection features. Oak Ridge National Laboratory (ORNL) is partnering with General Electric to develop a CO2 heat pump water heater meeting Energy Star standards with an installed cost to enable widespread market acceptance.
The Bonneville Power Administration through its Technology Innovation Program is funding research on CO2 heat pumps conducted by the WSU Energy Program. This includes recently completed laboratory testing of CO2 heat pumps (Larson, 2013). Research on the performance of Japanese-designed CO2-based HPWH units in four Northwest homes is underway (BPA , 2014). Participants in this research include the Northwest Energy Efficiency Alliance (NEEA), Avista, Tacoma Power, Ravalli Electric Co-op and the Energy Trust of Oregon. Related research is being conducted on demand response and combined space heat and water heating (BPA, 2013) and (BPA, 2014). Because of its high cost, this technology might be most cost-effective for combined space heating and water heating applications.