Difference between a Heat Engine, Refrigerator and Heat Pump

Difference between a Heat Engine, Refrigerator and Heat Pump

Heat Engine

In heat engine the heat supplied to the engine is converted into useful work. If Q₂ is the heat supplied to the engine and Q₁ is the heat rejected from the engine, the net work done by the engine is given by:

The performance of a heat engine is expressed by its efficiency. We know that the efficiency or coefficient of performance of an engine,

T₁ >T_a

Refrigerator

Refrigerator is a reversed heat engine which either cool or maintain the temperature of a body (T₁) lower than the atmospheric temperature (Ta). This is done by extracting the Heat from a cold body and delivering it to a hot body (Q₂). In doing so, work W_R is required to be done on the system. According to First law of thermodynamics,

W_R = Q₂ – Q₁

The performance of a refrigerator is expressed by the ratio of amount of heat taken from the cold body (Q₁) to the amount of work required to be done on the system (W_R). This ratio is called coefficient of performance. Mathematically, coefficient of performance of a refrigerator,

T₁ < T_a

Heat Pump

A refrigerator used for cooling in summer can be used as a heat pump for heating in winter. In the similar way, as discussed for refrigerator, we have

W_p =- Q₂ – Q₁

The performance of a heat pump is expressed by the ratio of the amount of the heat delivered to the hot body (Q2) to the amount of work required to be done on the system (Wp). This ratio is called coefficient of performance or energy performance ratio (E.P.R.) of a heat pump. Mathematically, coefficient of performance or energy performance ratio of a heat pump,

From above we see that the C.O.P. may be less than one or greater than one depending on the type of refrigeration system used. But the C.O.P. of a heat pump is always greater than one.