Cogeneration

Overview

Cogeneration orcombined heat and power (CHP) is the use of a heat engine to simultaneously generate both electricity and useful heat.

Thermal power plants and heat engines in general, do not convert all of the available primary energy into electricity. In most heat engines, more than 50% of the primary energy is wasted as excess heat. By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 70%, compared with at most 40% for the conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy. As well, less pollution is produced for a given economic benefit.

Cogeneration plants are commonly found in central heating and/or cooling systems for hospitals, hotels and industrial plants with large heating needs. Cogeneration plants are fired by biomass or industrial and municipal waste.

Applications

Cogeneration is ideal for distributed energy or on-site generation.

Cogeneration plants are commonly found in central heating and/or cooling systems for hospitals, hotels and industrial plants with large heating needs.

Fuels and Technologies

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Efficiency

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, and over much longer distances for the same energy loss.

Technology

There are two categories of cogeneration plants:

1. Topping cycle - produces electricity first, and then the exhausted steam is used for heating. The hot water from condensed steam is well-suited for space and water heating.
2. Bottoming cycle - produces high heats for an industrial process, and then a waste heat recovery boiler feeds an electrical plant. Bottoming cycle plants are only used when the industrial process requires very high temperatures, such as furnaces for glass and metal manufacturing hence rare.

Common CHP plant types are:

• Gas turbine CHP plants using the waste heat in the flue gas of gas turbines
• Combined cycle power plants adapted for CHP
• Steam turbine CHP plants that use the heating system as the steam condenser for the steam turbine.

Further Information

• Guide: Cogeneration & Trigeneration - How to produce energy efficiently (GIZ, 2016)
  
• wikipedia.org: Cogeneration
• cogen.org: what is CHP
  
• localpower.org