Heat pumps use mechanical energy to transfer thermal energy from a source at a lower temperature to a sink at a higher temperature. The bigger advantage of electrically driven heat pump heating systems, compared to electric resistance heating or expensive fuels, is that the heat pump's coefficient of performance (COP; ratio of heating performance to electrical energy) is greater than unity for heating; so it yields 9 to 15 MJ of heat for each kilowatt hour of energy supplied to the compressor, which saves on purchase of energy.
The original system concept, proposed by Charters et al. (1980), was a system with direct evaporation of the working fluid of the heat pump in the solar collector. The condenser of the heat pump was actually a heat exchanger wrapped around the storage tank. In this way, the initial system cost and the parasitic energy requirements of the system are minimized. A possible disadvantage of this system is that the condenser heat transfer is limited by the free convection from the tank wall, which can be minimized by using a large heat transfer area in the tank. A more important disadvantage of this system is that the heat pump refrigeration circuit must be evacuated and charged on site, which requires special equipment and expertise.
This disadvantage is removed by using compact solar heat pump systems. These incorporate an evaporator mounted outside the water storage tank with natural convection air circulation. This system needs to be installed outdoors, and if installed adjacent to the ventilation duct outlet of a building, it can also work as a waste heat recovery unit. The advantages of this system are that it has no parasitic energy requirement and, because the system is packaged, all its components are assembled in the factory and thus the system is pre-charged. The installation of this system is as simple as a conventional electric water heater because the unit requires no high-power electrical connection (Morrison, 2001).
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