Air source heat pumps are easy to install, and in particular for those units capable of achieving higher temperatures, are able to be directly retrofitted into the higher-temperature hot water heating systems typically found in UK homes. However their performance is impeded by colder source air temperatures resulting in higher temperature lifts and lower efficiencies. Phase change materials (PCMs) are attractive for use in thermal energy store applications due to their high-energy storage density over a small temperature range, therefore allowing the air-source heat pump to operate during winter warmer afternoon ambient air conditions. This energy is then stored for evening use, with the added benefit of delaying the air-source heat pump start until after the peak electricity demand time of early evening. A finite-volume numerical simulation model has been used to examine the thermal performance of PCMs augmented hot water thermal energy store for a heat pump energy storage application. These simulation results will be used to improve the system operation in winter and identify limitations when used for heat pump on-off control application.
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