Cogeneration

From Academic Kids

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 the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of energy. Cogeneration plants are commonly found in universities, hospitals, hotels, prisons, oil refineries, paper mills, wastewater treatment plants, enhanced oil recovery wells and industrial plants with large heating needs.

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. A conventional power station emits this heat into the environment through cooling tower, 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.

Topping cycle plants produce electricity first, then the exhuast is used for heating. Bottoming cycle plants, which are rare, produce heat for an industrial process first, then electricity is produced using a waste heat recovery boiler. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing.

Large cogeneration systems provide heating water and power for an industrial site or an entire town. Common CHP plant types are:

Smaller cogeneration units usually use a reciprocating engine. 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.

"Micro cogeneration" is on the scale of one household or small business[1] (http://www.cogen.org/about/workinggroup_microcogeneration.htm). Instead of burning fuel to merely heat the house or hot water, some of the energy is converted to electricity in addition to heat. The potential market for micro cogeneration is obviously vast.

Generating high-temperature heat (e.g. from industrial processes) usually results in some wasted low-temperature heat, which is simply dumped into the environment. Using a bottoming cycle plant, some of this waste heat can be recovered 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. Therefore, the consumer of the heat must be located in or adjacent to the cogeneration plant.

Large or small, most cogeneration projects only produce, more or less, the amount of energy the facility requires. However, thermally enhanced oil recovery (TEOR) plants often produce a substantial amount of excess electricity. After generating electricity, these plants pump leftover steam into heavy oil wells so that the oil will flow easier, increasing production. TEOR cogeneration plants in Kern County, California produce so much electricity that it cannot all be used locally and is transmitted to Los Angeles.

See also

fr:Cogénération

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