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Waste heat

Waste heat is by necessity produced both by machines that do work and in other processes that use energy, for example in a refrigerator warming the room air or a combustion engine releasing heat into the environment.

Waste heat is the by-product heat of machines and technical processes for which no useful application is found.

A fraction of input energy is always converted to heat by friction between machine parts and other processes such as liquid friction (see: viscosity).

Mechanical drives can be designed to run smoothly, with little loss of energy to heat. Machines for conversion of energy contained in fuels to mechanical work or electric energy produce large amounts of waste heat (see: Second law of thermodynamics).

This waste heat can be re-used if a cogeneration system is used. It is hard to use waste heat because it is difficult to transport the heat to another place, and it is difficult to store the heat for use at another time.

In electrification of waste heat, organic matter can be used as a working medium to produce electricity on much lower temperatures compared to the water steam cycle.