CHP systems, also known as cogeneration systems, provide both heat and power from a single source of energy; working to the principles of a Stirling engine. Unlike central station generation, CHP is a type of distributed generation which is located at or near the point of consumption.

By using the heat created, which is normally a by-product that would be lost, CHP systems typically achieve efficiencies greater than 80%. It therefore makes sense to consider CHP units when there is a need for both power and heat at a site. The technology can also be deployed quickly, cost-effectively, and with few geographic limitations.

The term CHP covers a suite of technologies that can use a variety of fuels, e.g. biomass or gas, to generate electricity or power at the point of use. In the case of a gas CHP system, the input fuel is natural gas.

In a gas CHP system, the combustion turbine or reciprocating engine CHP systems burn natural gas as a fuel to turn generators that subsequently produce electricity. Heat recovery devices are then used to capture the heat from the turbine or engine. As with biomass CHP, this heat is converted into useful thermal energy, usually in the form of steam or hot water.

Gas CHP has become a well-established and proven technology in the UK. They can operate with up to 90% efficiency and a commercial CHP unit turns approximately 60% of its input energy into heat and 30% into electricity. Unlike biomass CHP that constantly needs to run at full capacity, gas CHP units can modulate their output between 50% and 100% to match the base heating load of a building. As a result, end-users typically see savings in the region of 20% to 30%.

Gas CHP units are designed to run long hours, ideally 16 hours a day and very often 24 hours a day. To do this the unit needs to be sized correctly to be able to supply the base heat and electrical loads. The CHP unit will operate automatically and can modulate but ideally needs to run at full load for as long as possible to maximise efficiency and minimise wear.

If the electrical site load drops below the CHP output, then there are two possible outcomes depending on the localised conditions. The CHP power could be exported under a pre-agreed import/export agreement (however sometimes this may not be financially beneficial) or the other consideration is to modulate the CHP electrical output, to match the site load requirements so all the site power comes from the CHP. On some occasions the site heat load falls below the CHP heat output and when this happens the heat will need to be rejected. This part of the operation may not be eligible for the financial incentives.

Gas CHP systems need to have access to a reliable gas supply; if a mains gas connection is not available, one would need to be installed. Space for the unit is needed in or next to the existing boiler plant room. The CHP unit can be integrated to sit alongside existing boilers but can be installed in a new dedicated plant room if required. For integration alongside existing boilers, an available installation space of at least 8m squared with a minimum ceiling height of 2m and entrance of at least 0.8m wide is required.

In jurisdictions that have significantly decarbonised grid derived electricity through the mass deployment of renewable generation, electricity derived from fossil fuelled CHP’s such as gas, could lead to increased carbon emissions.