From my book, Appendix A., p. 179-180: Waste Heat
Strictly speaking, this is: "heat energy produced in an energy conversion or transfer process that is lost during conversion or transfer and is not available for useful purposes" (as defined by Pacific Northwest National Laboratory). For example, a typical automobile engine is 20% efficient, meaning that 80% of the heat from the burning gasoline goes out of the exhaust system, and 20% converts to vehicle propulsion. . . .
However, the 80% of the waste heat from an automobile engine is not all necessarily wasted in the literal sense. In wintertime, you move a lever to open a baffle, directing a stream of fresh air across the hot engine block into the passenger compartment. In other words, you use waste heat to keep yourself warm. At a typical electric power plant, 66% of the heat is wasted, producing billowing clouds of steam from the cooling towers. This leftover heat is not hot enough to generate electricity with conventional turbines, but it can be used for space heating and other purposes. See: Cogeneration.
