Cogeneration is the production of two useful forms of energy in a single energy conversion process. For example, a gas turbine may produce both rotational energy for an electric generator and heat for a building.

During the energy conversion process, an energy converter converts some form of energy to a form having a more suitable use. A light bulb and a gasoline engine are two familiar converters. People invest in electric energy to operate a light bulb because light is useful; likewise, people invest in gasoline for energy to run the automobile internal combustion engine because automobiles are useful. The laws of nature require that there be no loss of energy in the conversion. If 100 joules of energy are converted, then 100 joules remain after the conversion. However, the laws of nature neither require the converted energy to be in the form we desire, nor do they require that...