Waste heat recovery is the collection of heat created as an undesired by-product of the operation of a piece of equipment or machinery to fill a desired purpose elsewhere.
Waste heat recouping methods range from the simple to the complex. A common simple example is household water drain heat recovery. In this method, the heat going down a sink or shower drain is recovered by a copper pipe coiling around the drain pipe. The coil is then used to heat water as it passes through pipes on the way to a hot water heater.
On the more complex side, heat recovered from liquid cooling systems in data centers can be used for parts of the facilities where warmer temperatures are desired. Other sources of waste heat that can be recovered for practical uses include car exhaust, industrial exhaust, thermoelectric generation and turbines. Depending on the application, the heat itself may be the desired product or may be subjected to another process to provide clean electricity.
According to the United States Department of Energy, up to 50 percent of the energy from all fuels burned in the U.S. ends up in the atmosphere as waste heat. Research indicates that recovery of the energy waste from industrial facilities could fulfill up to 20 percent of total domestic electricity demand and simultaneously effect a 20 percent reduction in greenhouse gas emissions.
See a video about GE's waste heat recovery system: