Waste heat is the thermal energy which is commonly discarded after being created as a byproduct of industrial processes and working machinery of all kinds, including domestic appliances and cars. The recovery of waste heat can be carried out in a number of different ways, but techniques are limited by the need to be cost-effective in order to be commercially viable. One successful example is the waste heat recovery boiler, which can be situated above heat-generating equipment to generate steam that can drive electricity-producing generators.
There are many ways in which waste heat can be lost, including flue gases, heat loss from pipes, boiler exhaust and warm air vented from heated rooms. Only if the heat can be used in a way which causes benefit can it cease to be defined as waste. Possible uses include heating of water, production of steam to drive turbines, and heating of public spaces. Pipes filled with fluid, known as heat pipe exchangers, are able to transfer heat and are used in situations where hot exhaust from kilns, furnaces and boilers can be recovered and used to warm air or water. What are known as waste heat recovery boilers often transform the heat of combustion, in engines or incinerators for example, to form steam or to heat water or other fluids.