At a large scale, combined heat and power (CHP sometimes referred to as cogeneration) is generally a process in which the heat that is created as a by-product of power generation is captured and used rather than being wasted. Typically, a gas-powered turbine or reciprocating engine is used to produce electricity, and the heat recovered is used for local water or space heating, or to support an industrial process.
Microgeneration (or micro-generation) is the local production of electricity and/or heat on a very small scale. Typically, microgeneration has an output of under 45kW for heat and under 50kW for electricity.
Micro-CHP or micro combined heat and power is the small-scale generation of heat and electricity from a single energy source.
Micro-CHP is becoming more common in domestic buildings, where it can be installed as direct replacement for gas-fired boilers as micro-CHP units (sometimes referred to as CHP boilers) are similar in size and shape to conventional domestic boilers and can be floor standing or wall hung.
The main output of domestic micro-CHP units is generally heat, with some electricity generation, in the ratio of around 6:1. Typically, domestic units can generate up to 1 kW of electricity. This can be used to power the dwelling, or sold back to the grid.
Currently, domestic scale micro CHP units tend to generate electricity using a stirling engine. The boiler element of the unit heats water for central heating and to provide a hot water supply, and the ‘waste’ heat from the boiler heats an enclosed working gas such as helium in the stirling engine. The gas expands and contracts as it is heated and cools, pushing a magnetic piston up and down within copper coil windings, generating an alternating current.
Domestic micro CHP units tend to be powered by natural gas or by liquefied petroleum gas (LPG) and because of their efficiency they are considered to be a low-carbon technology. In the future, they may use bio-fuels, and so might be considered to be a ‘renewable’ technology. They may also develop to adopt fuel cell CHP technology which may be more efficient than gas turbines or stirling engines.
Consumers who generate their own electricity from a renewable or low-carbon source can qualify for a payment known as the feed-in tariff (FIT) for each unit of electricity generated. Consumers can also qualify for an ‘export tariff’ by selling surplus electricity back to their supplier. Micro-CHP is an allowable technology, qualifying for the feed in tariff.
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