Combined heat and power systems burn fuel such as natural gas, LPG (liquid petroleum gas), biomass, biogas or biomethane to generate both heat and electricity.

Requires: Natural gas, LPG (liquid petroleum gas), Biomass, or Biogas & Biomethane fuel source.

Micro-combined heat and power systems burn fuel to generate both electricity and heat. The heat given off by this process can be used for space and water heating. When considering heat generation, condensing boilers are more efficient, however as electricity is also generated micro-CHP systems are more efficient overall, as they utilise a larger fraction of the energy from burning fuel to produce a used product. (1)

A wide variety of micro-combined heat and power systems are available. The most common systems are boilers that burn natural gas or LPG (liquid petroleum gas). As micro-combined heat and power boilers are much more efficient than traditional boilers, even micro-combined heat and power systems fuelled in this way are classed as low carbon heating (1). Maintenance demands for fossil fuel micro-combined heat and power boilers are very similar to those of traditional boilers (2). Additionally, micro-combined heat and power systems are also available which use biomass or biogas as fuel (3).

In a micro-combined heat and power system, the prime mover generates mechanical power, like an engine. An electrical generator converts some of this energy into electricity. Finally, heat recovery units capture the heat generated by the prime mover and the electrical generator and use it for space or water heating.

There are three technologies suitable at micro-combined heat and power scale: Internal combustion engines, Stirling engines, and fuel cells. (4)

Internal combustion engine generation is similar to that used in vehicle engines but uses natural gas or compression-ignition diesel for fuel. Heat produced is as hot water rather than steam. One-two units of heat are produced for each unit of electricity generated. This ratio increases for larger engines. As a result, internal combustion engine micro-combined heat and power systems are best suited to sites with low electrical and high heating and hot water demand. Some smaller engines can only operate at a fixed output. (5)

Stirling engines have two pistons, each piston surrounded by a fixed quantity of gas such as helium or air. One piston moves to compress the gas. The compressed gas heats up, expands, and pushes the piston back. This moves the other piston to compress the gas in that chamber, repeating the cycle. They are more reliable than internal combustion engines but generally have much lower efficiencies. This low efficiency means Stirling engines are not recommended as replacements for high efficiency condensing boilers. (4)

Fuel cells generate electricity and heat from electrochemical reactions. This is reminiscent of batteries however fuel cells are unable to store the energy they produce. Hydrogen rich fuels are used for these reactions. Fuel cells generate cleanly and quietly, and are more electrically efficient than internal combustion engines, and much more so than Stirling engines. Fuel cells are however more expensive than the other two technologies. If fuelled by pure hydrogen, fuel cells emit no carbon dioxide (CO₂) (4).

(1) The Renewable Energy Hub UK, Micro Combined Heat and Power, mCHP Systems. Available from: https://www.renewableenergyhub.co.uk/main/micro-combined-heat-and-power-micro-chp-information/ [Accessed 26^th January 2021].

(2) The Renewable Energy Hub UK, Micro Combined Heat and Power, mCHP Systems, Is My Property Suitable for An mCHP Boiler [sic]. Available from: https://www.renewableenergyhub.co.uk/main/micro-combined-heat-and-power-micro-chp-information/property-suitability-for-microchp/ [Accessed 26^th January 2021].

(3) Department for Business, Energy & Industrial Strategy, Combined Heat and Power – Additional Guidance for Renewable CHP: A Detailed Guide for Developers – Part 6. London: UK Government; 2020. Available from: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/924597/chp-additional-guidance-for-renewable-chp-6.pdf [Accessed 26^th January 2021].

(4) The Renewable Energy Hub UK, Micro Combined Heat and Power, mCHP Systems, CHP and Its Types Types of Micro CHP Technology [sic]. Available from: https://www.renewableenergyhub.co.uk/main/micro-combined-heat-and-power-micro-chp-information/the-different-types-of-chp-and-microchp-technologies/ [Accessed 26^th January 2021].

(5) The Renewable Energy Hub UK, Micro Combined Heat and Power, mCHP Systems, Internal Combustion Engine CHP Generators. Available from: https://www.renewableenergyhub.co.uk/main/micro-combined-heat-and-power-micro-chp-information/internal-combustion-engine-chp-generators/ [Accessed 26^th January 2021].