The flow direction of refrigerant through a heat pump is controlled byvalves. When the refrigerant flow is reversed, the heat exchangers switchfunction. This flow-reversal capability allows heat pumps either to heat orcool room air.
Now, if under certain conditions a heat pump puts out more thermal energythan it consumes in electrical energy, has the law of energy conservation beenchallenged? No, not even remotely: the additional input of thermal energy intothe circulating refrigerant via the evaporator accounts for the difference inthe energy equation.
Unfortunately there is one real problem. The heating capacity of a heatpump decreases as the outdoor temperature falls. The drop in capacity is causedby the lessening amount of refrigerant mass moved through the compressor at onetime. The heating capacity is proportional to this mass flow rate: the less themass of refrigerant being compressed, the less the thermal load it can transferthrough the heat-pump cycle. The volume flow rate of refrigerant vapor throughthe single-speed rotary compressor used in heat pumps is approximatelyconstant. But cold refrigerant vapor entering a compressor is at lower pressurethan warmer vapor. Therefore, the mass of cold refrigerant and thus the thermal energy itcarries is less than if the refrigerant vapor were warmer before compression.
Here, then, lies a genuine drawback of heat pumps: in extremely coldclimates where the most heat is needed heat pumps are least able tosupply enough heat.
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