The vapour compression cycle is used in the majority of heat pumps, and the use of electric motor drives dominates such units. The principal components in the basic vapour compression cycle circuit are shown in the figures. These comprise an evaporator and condenser, the compressor and its drive and an expansion valve. Not to be omitted is the working fluid, or refrigerant, which circulates around the system.
The fluid is evaporated at low temperature and pressure in the evaporator, using heat from the heat source. The vapour is then compressed, raising its temperature before proceeding to the condenser, which gives out useful heat by condensing the working fluid at this higher temperature. The flow of the fluid through the expansion valve reduces the pressure to that of the level in the evaporator.
The net heat output at the condenser is effctively the sum of that taken in at the evaporator, and the heat equivalent of the work of compression put in by the drive. This leads to an energy "bonus" which sets the heat pump apart from other heating methods - the heat delivered is greater than that which could result from fully effective use of the primary energy alone. In the case of a gas engine drive heat may be recovered from the water jacket and exhaust to supplement that delivered at the condenser.
Source: Brochure "Heat Pumps - an option for an energy efficient and clean society" of the Commission of the European Communities.
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