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Cogeneration
(Also called combined heat and power, CHP) is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat. It is one of the most common forms of energy recycling.
Conventional power plants emit the heat created as a by-product of the electricity generation into the natural environment through cooling towers, flue gas, or by other means. By contrast, CHP captures the by product for domestic or industrial heating purposes, either very close to the plant, or as hot water for district heating with temperatures ranging from approximately 80˚ to 100˚ C. This is also called Combined Heat and Power District Heating or CHPDH. Small CHP plants are an example of decentralized energy.
By-product heat at moderate temperatures (212˚ – 356˚ F/ 100˚ – 180˚ C) can also be used in absorption chillers for cooling. A plant producing electricity, heat and cold is sometimes called Trigeneration or more generally: a polygeneration plant.
Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.
Thermal power plants (including those that use fissile elements or burn coal, petroleum, or natural gas), and heat engines in general, do not convert all of their thermal heat energy into electricity. In mostheat engines, a bit more than half is lost as excess heat. By capturing that excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 80%, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy.
Some Tri-Cycle plants have utilized a combined cycle in which several thermodynamic cycles produced electricity, and then a heating system was used as a condenser of the power plant’s bottoming cycle. A more modern system might use a gas turbine powered by natural gas, whose exhaust powers a steam plant, whose condensate provides heat. Tri-cycle plants can have thermal efficiencies above 80%.
CHP is most efficient when the heat can be used on site or very close to it. Overall efficiency is reduced when the heat must be transported over longer distances. This requires heavily insulated pipes, which are expensive and inefficient; whereas electricity can be transmitted along a comparatively simple wire, over much longer distances for the same energy loss.
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