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Combined heat and power station

Combined heat and power (CHP) plants

Most biomass power plants use steam produced by combustion to drive turbines which generate electricity. However, electricity generation alone generally has a low conversion efficiency (30% to 45%), with a significant proportion of energy lost as 'low-grade' heat.


CHP plants, on the other hand, are much more efficient, generating electricity whilst also capturing the usable heat produced in this process. They can be up to 90% efficient and offer one of, if not the most cost-effective way to meet carbon reduction targets.

CHP plants work well where there is a constant heat load.  CHP is also suitable:

  • When there is a requirement for space heating or process heat close to the generator
  • To provide low temperature (up to 90 degrees C) hot water heating for local district schemes
  • For applications that require (low grade) process heat, especially those that can supply their own fuel (i.e. sawmills and wood process industry which use heat for timber drying and steaming)
  • At sites such as hospitals, leisure centres, greenhouses, and retirement complexes which have a year round heat demand
  • For the provision of steam to industrial applications
  • Where there is a requirement for environmentally responsible disposal of waste (i.e. sewage sludge, clinical waste or agricultural residue) and where transport costs for disposal are high
  • For powering absorption refrigerators to provide cooling in summer, giving tri-generation.

Types of CHP Installations

Industrial CHP

Ranging in scale from a few MWe to the size of a conventional power station, these plants provide high value heat - at the temperatures and pressures often required by industry - along with electricity. In some cases surplus heat can also be used to meet the heat requirements of the surrounding local community. Likewise, electricity that is surplus to the needs of the site can be fed into the local network.

Examples of industrial CHP in Scotland include UPM Caledonian Paper in Irvine, Balcas' pellet plant in Invergordon, Land Energy in Girvan and the Macallan Distillery at Aberlour.

CHP with District Heating

Connected to a district heating network, CHP can provide heat and power in areas of concentrated demand, such as in city centres, towns, villages, industrial zones and other built environments with a dense 'heat load'.

Small scale and packaged CHP

Proven technology is now available from 30kW electricity and 75kW of heat upwards. These units are suitable for timber drying, small rural businesses, care homes, hotels, supermarkets, small health centres, leisure centres, larger residential properties and country estates. Where relevant, these units can also feed power into the electricity network and can contribute excess heat to a district heating network.

As of July 2015, the following companies have highlighted that they offer small scale CHP solutions in Scotland:  HWEnergy, and Arbor Heat and Power.

Micro CHP

Micro-CHP is a specific form of CHP designed for individual households and micro businesses. Micro CHP typically has an electrical output of <2kWe. As a replacement for a standard domestic gas boiler, it generates both electricity and heat for space heating and hot water.

Gas micro-turbines are being developed for CHP applications making use of advances in automotive turbocharger technology. They can be driven by hot flue gases or indirectly heated air. They can run on highly efficient foil bearings and comprise very few moving parts offering the potential for reliable long term operation.

A range of CHP case studies are available on the Association for Decentralised Energy Website, click here to access.

Funding for CHP

Information on financing CHP projects can be found on the Association for Decentralised Energy website here, and on the website here.

Please note, CHP finance can be more complicated than heat only woodfuel installations and the funding mechanisms will depend upon the technology being used. Please speak to your potential installer for further information.

Combined Heat and Power Quality Assurance (CHPQA)

This is a voluntary scheme used to define, assess and monitor CHP schemes for their energy efficiency and environmental performance. Each CHP plant is evaluated on its fuel use, power generation and heat supply and provided with a Quality Index (QI) and efficiency rating.  Various benefits are linked to whether a CHP is "Good Quality" or not, such as the ROC and ECAs.  For the RHI, biomass CHP must be CHPQA accredited but is not required to meet any specific quality standards. For further information on the CHPQA scheme go to the Association for Decentralised Energy website or click  here.

Types of CHP Technology

A CHP plant consists essentially of an electrical generator combined with equipment for recovering and using the heat produced by that generator. The generator may be a prime mover such as a gas turbine or a reciprocating engine. Alternatively, it may consist of a steam turbine generating power from high-pressure steam produced in a boiler. In some cases, a CHP scheme may be a combination of prime mover(s), boiler(s) and steam turbine(s). For further information on the different types of CHP technology, click  here.

Woodfuel for CHP installations

Please note that some CHP installations, especially the small scale units, require wood chip of a different specification and moisture content to conventional woodfuel heat only boilers.  Make sure that you can source the correct fuel for your system locally at the outset of planning any CHP installation.

Key Tips

  • Most CHP installations should be sized based upon the heat output, not the electricity output;
  • Where possible ensure that the electricity produced is used on site; transporting electricity to the grid is not necessarily easy or profitable;
  • Each individual CHP system is different; only your potential installer can provide detailed information on the make-up of your system.