The principle behind combined heat and power (or “cogeneration” or simply CHP) is to recover the waste heat generated by the combustion of a fuel (see note 1) in an electricity generation system. This heat is often rejected to the environment, thereby wasting a significant portion of the energy available in the fuel that can otherwise be used for space heating and cooling, water heating, and industrial process heat and cooling loads in the vicinity of the plant. This cogeneration of electricity and heat greatly increases the overall efficiency of the system, anywhere from 25 - 55% to 60 - 90%, depending on the equipment used and the application.

Combined heat and power systems can be implemented at nearly any scale, as long as a suitable thermal load is present. For example, large scale CHP for community energy systems and large industrial complexes can use gas turbines (Figure 1), steam turbines, and reciprocating engines with electrical generating capacities of up to 500 MW. Independent energy supplies, such as for hospitals, universities, or small communities, may have capacities in the range of 10 MW. Small-scale CHP systems typically use reciprocating engines to provide heat for single buildings with smaller loads. CHP energy systems with electrical capacities of less than 1 kW are also commercially available for remote off-grid operation, such as on sailboats. When there is a substantial cooling load in the vicinity of the power plant, it can also make sense to integrate a cooling system into the CHP project (see note 2). Cooling loads may include industrial process cooling, such as in food processing, or space cooling and dehumidification for buildings.

The electricity generated can be used for loads close to the CHP system, or located elsewhere by feeding the electric grid. Since heat is not as easily transported as electricity over long distances, the heat generated is normally used for loads within the same building, or located nearby by supplying a local district heating network. This often means that electricity is produced closer to the load than centralized power production. This decentralized or “distributed” energy approach allows for the installation of geographically dispersed generating plants, reducing losses in the transmission of electricity, and providing space & process heating and/or cooling for single or multiple buildings (Figure 2).

A CHP installation comprises four subsystems: the power plant, the heat recovery and distribution system, an optional system for satisfying heating (see note 3) and/or cooling (see note 4) loads and a control system. A wide range of equipment (see note 5) can be used in the power plant, with the sole restriction being that the power equipment rejects heat at a temperature high enough to be useful for the thermal loads at hand. In a CHP system, heat may be recovered and distributed as steam (often required in thermal loads that need high temperature heat, such as industrial processes) or as hot water (conveyed from the plant to low temperature thermal loads in pipes for domestic hot water, or for space heating).

Worldwide, CHP systems with a combined electrical capacity of around 240 GW are presently in operation. This very significant contribution to the world energy supply is even more impressive when one considers that CHP plants generate significantly more heat than power. Considering that most of the world’s electricity is generated by rotating machinery that is driven by the combustion of fuels, CHP systems have enormous potential for growth. This future growth may move away from large industrial systems towards a multitude of small CHP projects, especially if a decentralized energy approach is more widely adopted and the availability of commercial products targeted at this market.