Development of a Low Cost Heat Pump Water Heater - Second Prototype
- Oak Ridge National Laboratory (Retired)
- ORNL
Since the 1980s various attempts have been made to apply the efficiency of heat pumps to water heating. The products generated in the 80s and 90s were not successful, due in part to a lack of reliability and difficulties with installation and servicing. At the turn of the century, EnvironMaster International (EMI) produced a heat pump water heater (HPWH) based on a design developed by Arthur D. Little (ADL), with subsequent developmental assistance from Oak Ridge National Laboratory (ORNL) and ADL. This design was a drop-in replacement for conventional electric water heaters. In field and durability testing conducted by ORNL, it proved to be reliable and saved on average more than 50% of the energy used by the best conventional electric water heater. However, the retail price set by EMI was very high, and it failed in the market. ORNL was tasked to examine commercially available HPWH product technology and manufacturing processes for cost saving opportunities. Several cost saving opportunities were found. To verify the feasibility of these cost saving measures, ORNL completed a conceptual design for an HPWH based on an immersed condenser coil that could be directly inserted into a standard water tank through a sleeve affixed to one of the standard penetrations at the top of the tank. After some experimentation, a prototype unit was built with a double-wall coil inserted into the tank. When tested it achieved an energy factor (EF) of 2.12 to 2.2 using DOE-specified test procedures. A.O. Smith contacted ORNL in May 2006 expressing their interest in the ORNL design. The prototype unit was shipped to A.O. Smith to be tested in their laboratory. After they completed their test, ORNL analyzed the raw test data provided by A.O. Smith and calculated the EF to be approximately 1.92. The electric resistance heating elements of a conventional electric water heater are typically retained in a heat pump water heater to provide auxiliary heating capacity in periods of high demand. A.O. Smith informed us that when they applied electric resistance backup heating, using the criterion that resistance heat would be applied whenever the upper thermostat saw water temperatures below the heater s nominal setpoint of 135oF, they found that the EF dropped to approximately 1.5. This is an extremely conservative criterion for backup resistance heating. In a field test of the previously mentioned EMI heat pump water heater, residential consumers found satisfactory performance when the criterion for use of electric resistance backup heating was the upper temperature dropping below the set point minus 27 degrees. Applying this less conservative criterion to the raw data from the original A.O. Smith EF tests indicates that electric resistance heating would never have come on during the test, and thus the EF would have remained in the vicinity of 1.9. A.O. Smith expressed concern about having an EF below 2, as that value triggers certain tax advantages and would assist in their marketing of the product. We believe that insertion of additional length of tubing plus a less conservative set point for electric resistance backup heating would remedy this concern. However, as of this writing, A.O. Smith has not decided to proceed with a commercial product.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Building Technologies Research and Integration Center (BTRIC)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 984746
- Report Number(s):
- ORNL/TM-2007/155; BT0302000; CEBT002; TRN: US201016%%1878
- Country of Publication:
- United States
- Language:
- English
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