Combined heat and power


Combined heat and power - Definition

Cogeneration (also combined heat and power or CHP) is the use of a power station to simultaneously generate both heat and electricity. CHP allows a more total use of energy than conventional generation, potentially reaching an efficiency of 70-90%, compared with approximately 50% for conventional plants. This means that less fuel needs to be consumed to produce the same amount of energy.

Thermal power plants (including those that use uranium or burn coal, petroleum, or natural gas) do not convert all of their available energy into electricity. Inevitably, a large amount of heat is released as a by-product. Conventional power stations emit this heat into the environment through cooling towers, as flue gas, or by other means.

It is more energy-efficient to use this waste heat in places where it would otherwise need to be generated by other means. (Often, these other means involve drawing upon electric power, while it would be more efficient to supply the heat directly, without converting it into electricity first.) Heat is widely used, not only for heating of residential buildings, but also for high-temperature industrial processes and other applications.

Cogeneration systems are generally economic on a large scale, for example to provide heating water and power for an industrial site or an entire town. Common CHP plant types are:

Gas turbine CHP plants using the waste heat in the flue gas of gas turbines
Combined cycle power plants adapted for CHP
Steam turbine CHP plants using the waste heat in the steam after the steam turbine.

Small cogeneration units for hospitals, swimming pools or groups of dwellings are also economic if standardized, mass-produced CHP plants are used. Examples are the internal combustion (IC) engines (gas or diesel engines) used for car manufacture. They use the waste heat in the flue gas and cooling water of gas or diesel engines and replace the traditional gas- or oil-fired boiler (furnace) used in central heating systems.

Generating high-temperature heat (e.g. from industrial processes) usually results in some wasted low-temperature heat, which is simply dumped into the environment. A CHP system can also be used to recover some of this waste heat and to use it to generate electric power. For small systems, the waste heat can be recovered by a Stirling engine.

A problem with cogeneration is that heat transmission over long distances requires thick, heavily insulated pipes, whereas electricity can be transmitted along a comparatively simple wire.

See also