Thermodynamics

ator coils is highly undesirable since it seriously disrupts heat transfer. Thecoils can be defrosted, however, by reversing the heat pump cycle (runningit as an air conditioner). This results in a reduction in the efficiency of thesystem. Water-source systems usually use well water from depths of up to80 m in the temperature range of 5 to 18°C, and they do not have a frostingproblem. They typically have higher COPs but are more complex andrequire easy access to a large body of water such as underground water.Ground-source systems are also rather involved since they require long tubingplaced deep in the ground where the soil temperature is relatively constant.The COP of heat pumps usually ranges between 1.5 and 4, dependingon the particular system used and the temperature of the source. A new classof recently developed heat pumps that use variable-speed electric motordrives are at least twice as energy efficient as their predecessors.Both the capacity and the efficiency of a heat pump fall significantly atlow temperatures. Therefore, most air-source heat pumps require a supplementaryheating system such as electric resistance heaters or an oil or gasfurnace. Since water and soil temperatures do not fluctuate much, supplementaryheating may not be required for water-source or ground-source systems.However, the heat pump system must be large enough to meet themaximum heating load.Heat pumps and air conditioners have the same mechanical components.Therefore, it is not economical to have two separate systems to meet theheating and cooling requirements of a building. One system can be used asa heat pump in winter and an air conditioner in summer. This is accomplishedby adding a reversing valve to the cycle, as shown in Fig. 11–9. AsChapter 11 | 619HEAT PUMP OPERATION—HEATING MODEOutdoor coilReversing valveIndoor coilFanFanExpansionvalveCompressorHigh-pressure liquidLow-pressure liquid–vaporLow-pressure vaporHigh-pressure vaporHEAT PUMP OPERATION—COOLING MODEOutdoor coilReversing valveFanIndoor coilCompressorFanExpansionvalveFIGURE 11–9A heat pump can be used to heat ahouse in winter and to cool it insummer.