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Heat Engines and Refrigerators and Heat Pumps

Let us see how various thermodynamic processes can be applied to a device. One of the thermodynamic devices is heat engine.

A heat engine is a thermodynamic device which continuously converts heat supplied to it into mechanical work.

There are three types of heat engines, which are,
  • The Steam Engine
  • The internal Combustion Engine
  • The Steam or Gas Turbine
All these different types of heat engines have the following characteristics, and also define the specific conditions under which they can operate.
  • There must be a source of a heat or hot reservoir of infinite thermal capacity, maintained at a constant high temperature; so that any amount of heat that can be withdrawn from it or given to it does not affect its temperature.
  • There must also be a sink or cold reservoir of infinite thermal capacity, maintained at a lower constant temperature; so that any amount of heat withdrawn from it or given to it does not affect its temperature.
  • There must be some substance through which heat is absorbed from the source or rejected to the sink. This substance is called as working substance. For example, a mixture of fuel vapour and air in a gasoline or diesel engine or steam in a steam engine are the working substances.
  • The difference between the quantities of heat absorbed by the working substance from the source and rejected to the sink is converted into work.
The efficiency (η) of heat engine can be defined as the ratio between its output of work to the heat supplied to heat engine.

W is the work done on the environment in a cycle and Q1 is the heat absorbed from the source.

At the end of each cycle, the working substance regains its original state so that its internal energy is not affected. Hence we can write,
W = Q1 - Q2

Where Q2 is the quantity of heat rejected to sink.

Usually the efficiency is expressed in percentage.

Refrigerators and Heat Pumps

A refrigerator is the reverse of a heat engine. Here the working substance extracts heat Q2 from the cold reservoir at temperature T2, and some external work W is done on it and heat Q1 is released to the hot reservoir at temperature T1.

A heat pump is the same as a refrigerator. If the purpose is to cool a portion of space, like the inside of a chamber, and the higher temperature reservoir is surrounding, we call the device a refrigerator.

In a refrigerator the working substance usually, in gaseous form goes through the following steps:
  • Sudden expansion of the gas from high to low pressure which cools it and converts it into a vapour-liquid mixture.
  • Absorption by the cold fluid of heat from the region to be cooled converting it into vapour.
  • Heating up of the vapour due to external work done on the system.
  • Release of heat by the vapour to the surroundings, bringing it to the initial state and completing the cycle.
The coefficient of performance (α) of a refrigerator is given by

Where Q2 is the heat extracted from the cold reservoir and W is the work done on the system.

If the heat is pumped into a portion of space, the device is called a heat pump and α for heat pump is defined as

By definition η can never exceed 1. But α can be greater than 1.

By energy conservation, the heat released to the hot reservoir is
Q1 = W + Q2
In a heat engine, heat cannot be fully converted to work. Similarly a refrigerator cannot work without some external work done on the system, i.e., the coefficient of performance in a refrigerator cannot be infinite.

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