How does a cogeneration system work?

Topping Cycle
In the topping cycle, natural gas is burned in a gas reciprocating engine or gas turbine that drives an electric generator to produce electric power. 

Waste heat, obtained from the engine's jacket water and/or the exhaust gases, is transferred via heat exchangers or waste-heat boilers to replace heat normally supplied from conventionally fired gas equipment.

Bottoming Cycle
With the bottoming cycle, high-temperature exhaust heat from a high-temperature process furnace is converted to steam in waste-heat boilers to run a steam turbine driving an electric generator. Electric power production is dependent upon the amount of waste heat available.

Combined Cycle
Practical for 5,000 kW or greater installations, this process uses a gas turbine topping cycle process and uses steam produced in a waste-heat recovery boiler to power an auxiliary steam turbine-driven electric generator similar to the bottoming cycle.

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