Heat Engines, Heat Pumps & Refrigerators

Applied Science: BTEC Level 3 National Extended Diploma, topic 4.9.7

Second Law and Engines

It is impossible for engines to work only using the first law of thermodynamics. The second law must also be obeyed.

Source and sink

Source and sink

In order to obey the second law of thermodynamics, all engines need a source and a sink of heat.
Because the sink cannot be at absolute zero, no engine can be 100% efficient.

Efficiency

Efficiency

Efficiency $= \frac{W}{Q_H} = \frac{Q_H - Q_C}{Q_H}$

Maximising efficiency

Maximising efficiency

Maximum efficiency $= \frac{T_H - T_C}{T_H}$
To maximise efficiency, (T_H - T_C) must be large, but T_C is usually limited by the temperature of the environment.
T_H is limited by the material characteristics of the engine.
In modern systems, the use of Q_H and W is maximised, whilst practical uses for Q_C are sought.

Reversed heat engines

Any engine that runs from cold to hot is a reversed heat engine.

Fridges

Fridges

A reverse heat engine does work to move heat energy from a hot body to a cold body.
It is the same as a normal heat engine but the heat energy transferred does not do work, it needs work.

Coefficient of performance

Coefficient of performance

The coefficient of performance of a refrigerator is like its efficiency.
$COP= \frac{{Q_c}}{W}$
Where $Q_c$ is the heat from the cold reservoir.

Heat pumps

Heat pumps

Heat pumps are another type of reversed heat engine.
A reverse heat engine does work to move heat energy from a cold body to a hot body.

Coefficient of performance

Coefficient of performance

The coefficient of performance of a heat pump is like its efficiency.
$COP= \frac{{Q_h}}{W}$
Where $Q_h$ is the heat from the warm reservoir.

Test yourself on Heat Engines, Heat Pumps & Refrigerators

Which of these is an example of a reversed heat engine?
Which is the correct equation for the coefficient of performance of a refrigerator?

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