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.