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Experiment: Heat Pump Water Heater Performance, Demand Response Capability, and Space Conditioning Impact

As buildings become better insulated and air sealed due to tougher code requirements, building scientists and the codes community itself are turning their attention to other pieces of the residential energy consumption pie. One appliance that contributes 17.7% to the residential energy use pie is water heating (U.S. Energy Information Administration, Residential Energy Consumption Survey). Efficient water heating options are an important way to achieve significant energy savings in the residential sector.

Heat pump water heaters (HPWH) offer an efficient option for the many homes with electric water heaters. They have a theoretical energy savings of up to 63% in comparison to an electric resistance water heater1. Previous research has demonstrated the laboratory performance of HPWHs and has shown that impressive savings are possible, based on standardized testing protocols.

Heat pump water heaters use the refrigerant compression and expansion cycle (much like a refrigerator operating in reverse) to heat water. Heat is extracted from the air and transferred to water in an enclosed tank. A low-pressure liquid refrigerant is vaporized in the heat pump's evaporator and passed into the compressor. As the pressure of the refrigerant increases, so does its temperature. The heated refrigerant runs through a condenser coil within the storage tank, transferring heat to the water stored there. As the refrigerant delivers its heat to the water, it cools and condenses, and then passes through an expansion valve where the pressure is reduced and the cycle starts over.

Significant barriers must be overcome before this technology will reach widespread adoption nationwide. Experiments in the PNNL Lab Homes are investigating two of these barriers: their impact on space conditioning (indoor heating and cooling) and their use in utility demand response programs (for managing energy loads). Because heat pump water heaters draw heat from the surrounding environment, they usually heat water more efficiently when located inside a home. However, because they exhaust cool air, if they are located in the home (i.e., in conditioned space), they will impact the home’s heating load in winter. Modeling studies performed indicate that the installation location of heat pump water heater can significantly impact their performance and the resultant whole-house energy savings (Ecotope. 2012. Heat Pump Water Heaters: Tier 2 - Ducted Interior Installations. Presented to the Pacific Northwest Regional Technical Forum February 14, 2012, by Adam Hadley and Jeff Harris, Portland, Oregon). Ducting cold air exhaust to discharge outside the home is one solution PNNL is investigating with support from the Northwest Energy Efficiency Alliance.

For demand-response (DR) programs, many utilities currently employ electric resistance water heaters to reduce peak load by turning off the water heater during times of peak demand and storing energy as hot water in the tank.

Title Publication Date Citation
Impact of Ducting on Heat Pump Water Heater Space Conditioning Energy Use and Comfort July 2014 Widder SH, JM Petersen, GB Parker, and MC Baechler. 2014. Impact of Ducting on Heat Pump Water Heater Space Conditioning Energy Use and Comfort. PNNL-23526, Pacific Northwest National Laboratory, Richland, WA.
Abstract | Full Publication
Formal Report
Demand Response Performance of GE Hybrid Heat Pump Water Heater July 2013 Widder SH, JM Petersen, GB Parker, and MC Baechler. 2013. Demand Response Performance of GE Hybrid Heat Pump Water Heater . PNNL-22642, Pacific Northwest National Laboratory, Richland, WA.
Abstract | Full Publication
Formal Report
Evaluation of Ducted GE Hybrid Heat Pump Water Heater in the PNNL Lab Homes April 2013 Widder SH. 2013. "Evaluation of Ducted GE Hybrid Heat Pump Water Heater in the PNNL Lab Homes." Presented by Sarah Widder at DOE Building America Program Review, Richland, WA on April 24, 2013. PNNL-SA-95180.
Abstract | Full Publication
Presentation

1 Based on the DOE test procedure (10 CFR 430.32(d)) and comparison of an ERWH (Energy Factor, EF = 0.90) versus a HPWH (EF = 2.4)

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