Waste heat

Waste heat refers to heat produced by machines and technical processes for which no useful application is found, and is regarded as a waste by-product. A fraction of input energy is always converted to heat by friction between machine parts and other dissipative processes such as liquid friction (see: viscosity). Heat production in bearings can be minimised by the use of rolling-element bearings.

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Conversion of energy

Whereas mechanical drives can be designed to run smoothly, with little dissipation of energy to heat, machines for conversion of energy contained in fuels to mechanical work or electric energy necessarily produce large quantities of by-product heat (see: Second law of thermodynamics).

The electrical efficiency of thermal power plants, defined as the ratio between the primary product and input energy, ranges from 30 to 70%. It is often difficult to find useful application for large quantities of low quality heat, so the heat is qualified as waste heat and is rejected to the environment. Economically most convenient is the rejection of such heat to water from a sea, lake or river. If sufficient cooling water is not available, the plant has to be equipped with a cooling tower to reject the waste heat into the atmosphere.

Cogeneration

Waste of the by-product heat is avoided if a cogeneration system is used. Limitations to the use of by-product heat arise due to difficulties in heat transport and heat storage.

See also

• Heat recovery steam generator