As previously defined, a heat engine is any mechanism operating cyclically with the primary aim of partially converting heat into work. The most efficient cycle is the Carnot heat engine which can operate between two constant heat reservoirs. The Carnot efficiency represents the upper limit.
A typical heat engine (shown below) produces high pressure steam in the boiler, expands it in the turbine to obtain work. The low-pressure steam exiting the turbine is sent to a condenser where heat is dissipated to the surroundings. Before the resultant liquid is returned to the boiler, its pressure is increased using a pump. A small amount of work obtained from the turbine is used in the pump.
The boiler-turbine-condenser-pump
assembly can be represented as a circle in a schematic representation of a heat
engine as shown below
A heat engine's performance is measured in terms of its efficiency, η, which is defined as:
Heat pumps are considered reversed heat engines, thus., they are used to transfer heat from a low temperature reservoir to a high temperature reservoir with the help of an external work. This can be schematically represented as follows:
For a heat pump, its performance is
measured using its coefficient of performance (COP) which is as follows:
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