Any conservation system whereby "some space heating or water heating is done by actively capturing byproduct heat that would otherwise be ejected into the environment." Sources of water-heat recovery in commercial buildings include refrigeration/air-conditioner compressors, manufacturing or other processes, data processing centers, lighting fixtures, ventilation exhaust air, and the occupants themselves.
This concept is being tried in the realm of transportation. "Honda engineers are trying to make the old internal combustion engine more efficient by recapturing energy that is usually lost as heat through the exhaust, writes Michael Graham Richard at TreeHugger.com. "There's lots of room for improvement: According to Amory Lovins from the Rocky Mountain Institute, only about 1% of the energy contained in a gallon of gasoline moves the driver of a car (if that's not shocking, we don't know what is). The rest is partly used to move the vehicle's weight, but it is mostly just lost as heat." If that heat can be turned into electricity, it could then be used to power the car via the electric motor of a hybrid.
A more domestic application of waste heat recovery has sprung from the fact that 80-90% of all hot water energy goes down the drain. A device called the "GFX" is "reuses the heat from hot water going down your drain, and uses it to preheat water going into your hot water tank. Its operation is based upon a natural phenomenon whereby surface tension and gravity create failing water films."