Combined heat and power   

Combined heat and power or CHP (also Cogeneration) 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 and can improve energy efficiency up to 50%.

[edit] Efficiency of an Ideal Heat engine

Conventional Colal, Oil and Gas power plants convert the chemical energy in the fuel into heat and then convert the heat into electricity. Thermodynamics tell us that the maximum possible efficiency for a system converting heat into work is:

where

W is the work done by the system (energy exiting the system as work),

Q_H is the heat put into the system (heat energy entering the system),

T_C is the absolute temperature of the cold reservoir, and

T_H is the absolute temperature of the hot reservoir.

Power stations use cooling towers or rivers or the sea as their cold reservoir which means T_C is the outside air or water temperature - 0C to 35C depending on the location and the season. This corresponds to 273K to 308K. The materials used to make power stations mean the temperature of the combustion gases is limited to about 500C, 800K which gives us a maximum efficiency of about 60% of the heat energy converted into work (electricity) and the rest rejected to the cold reservoir. This is for an ideal heat engine. Losses in real engines mean efficiencies for large conventional power stations are about 55%.

Note that the above applies to heat engines. Fuel cells, which convert chemical energy directly into electrcity, without a heat stage, are not bound by this theoretical limit, although the efficiency of current practical fuel cells is not much better.

Combined Heat and Power engines convert slightly less of the heat energy into electricity and use some of the heat directly 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 130 °C. This is also called Combined Heat and Power District Heating or CHPDH. In this way CHP plants use up to 89% of the heat.

Small CHP plants are an example of decentralized energy or distributed generation.

[edit] Design Considerations

The important design consideration in Combined Heat and Power systems is the characteristics of the heat load. How hot does it need to be? How cold is the water leaving it? Domestic Low Pressure Hot Water heating systems operate at 40C to 80C. to heat houses to about 20C. Underfloor heating systems and swimming pool heating systems can operate at lower temperatures. District heating systems must operate at higher temperatures to ensure water at 40C to 80C is available at the far end of the system. Frost protection and de-icing can get useful heat from water as cold as 15C. The higher the water flow temperature required the less heat there is for electricity generation. The lower the temperature of the water coming back the less heat that needs to be rejected to the environment.

Heating loads which need higher water temperatures such as De-salination plants or Adsorption chillers or many industrial processes will have less scope for electricity generation.

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[edit] Interwiki links

• Wikipedia:Combined heat and power