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Refrigerators and heat pumps
Intermediate
Use work to move heat from cold to hot.
COP = Q_c/WThey use work to move heat from cold to hot. COP (fridge) = Q_c/W; for a heat pump COP = Q_h/W. Ideal values: T_c/(T_h − T_c) and T_h/(T_h − T_c).
A heat pump with COP 4 gives 4 kWh heat for 1 kWh electricity.
COP can exceed 1 because heat is moved, not created.
In more detail
A refrigerator or heat pump uses work to move heat from a cold reservoir to a hot one. Its performance is measured by a coefficient of performance, the heat moved divided by the work input.
Real-world uses
- Home refrigerators keep food cold by pumping heat out.
- Heat pumps provide heating by extracting heat from outside air or ground.
- A heat pump uses electricity to move heat from cold outdoor air into a house, which is more efficient than direct electric heating.
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Frequently asked questions
Why is the coefficient above 1?
Because heat pumps move more heat than the work input, drawing extra from the environment.
Is a heat pump more efficient than a heater?
Often, because it moves heat rather than generating it directly.
What is a coefficient of performance?
The heat moved divided by the work input, which for a heat pump can exceed one.
Related topics in Heat & Thermodynamics
Heat engines · Carnot engine · Carnot cycle · Otto cycle · Diesel cycle · Entropy