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How does combined heat and power (CHP) cogeneration work?

There are two types of CHP installations, topping and bottoming operations.

In topping operations, a fuel source such as natural gas, diesel, gasified coal or crude oil is used to power a prime mover such as a gas turbine or reciprocating engine. Attached to this prime mover is an electrical generator that produces electricity. Waste heat from the prime mover is captured and used to produce steam for use in a nearby industrial or commercial operation.

The steam produced in the second phase of CHP topping operations can be used for processing, or local heating or cooling needs. For cooling loads, an absorption chiller is used.

In bottoming operations, excess heat from an existing industrial operation is recovered and used to generate electricity. The resulting electricity can either be used within the industrial facility or sold to the local utility. When excess electricity is sold in this way, it is absorbed by the distribution grid.

Bottoming cycle operations could use the heat from a large furnace in the glass or metals industries, for example.

Both combined cycle and CHP cogeneration plants are classified by the type of prime mover they use to drive the electrical generator. These are the most common components used in cogeneration systems:

  • steam turbine
    This is a common cogeneration component. Fuels of various types are burned in a boiler to produce high-pressure steam that passes through a steam turbine to produce power. Steam can be extracted part way through the turbine for industrial use.

  • gas turbine
    A gas turbine uses large amounts of intake air which is first compressed and then rapidly expanded by adding a fuel which is combusted. The rapidly expanding air rotates the turbine blades. Like a jet engine, the gas turbine can burn light oil or natural gas as a fuel. The gas turbine drives an electrical generator which produces electricity. Its exhaust gas goes to a heat recovery boiler, which produces both steam and heat.

  • microturbine
    A microturbine is a small, compact system that combines a gas turbine, compressor and generator on one shaft. This low-cost, flexible unit can run on any liquid fuel, including natural gas, flare gas, landfill gas and coal extraction.

  • reciprocating engine
    This cogeneration system uses an internal combustion engine similar to that used in a car or locomotive. It harnesses heat from two sources the exhaust gas and the engine cooling system to produce hot water.

  • heat recovery steam generator
    In this component, steam is generated from the hot exhaust gases from a gas turbine, or the hot exhaust and engine coolant from a reciprocating engine. This steam drives a steam turbine to generate additional electricity or is used for heat or cooling in an industrial operation.

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