The model that is widely adopted today for the fulfillment of the energy needs of buildings is the supply of electrical energy from the grid and the combustion of fuel (e.g. oil, natural gas, biomass) in boilers operating inside the buildings for the production of thermal energy.

CHP (Cogeneration of Heat and Power), on the other hand, refers to systems which simultaneously produce electrical and thermal/cooling energy from the same source and within the same process.

Why can the widespread application of CHP contribute to an important reduction in primary energy consumption?

CHP refers to technologies which are installed in close proximity to the consumer and can contribute significantly to the decentralization of the energy systems. The application of decentralized energy systems leads to a reduction in the electrical energy transmission and distribution losses. Furthermore, during the operation of a CHP system the produced thermal energy is utilized and not wasted like in the case of central power plants

The available CHP technologies include the following:

• Combined cycle gas turbine

• Back-pressure steam turbine

• Condensing steam turbine

• Open cycle gas turbine

• Reciprocating internal combustion engine

• Stirling engine

• Fuel cell

• Steam engine

• Organic Rankine cycle 

While the energy costs are increasing continuously, CHP systems are a promising technology that can offer a viable solution, especially for big buildings. There are many examples of CHP systems running in hospitals or commercial buildings. Micro-CHP systems for houses do the job of a traditional boiler while occupying a similar amount of space. The noise levels they produce are also comparable with that of conventional boilers. The difference of micro-CHP systems is that they also produce electrical energy. Their price, however, is still much higher in comparison with boilers. CHP and micro-CHP systems can also be combined with absorption/adsorption refrigerators to produce cooling. The combination of the two technologies is referred to as trigeneration (generation of electrical energy, heating and cooling).

Among the available CHP systems, those employing reciprocating Otto engines are the most widespread ones in the residential and tertiary sector. Less widespread are CHP units employing a Stirling engine or an Organic Rankine cycle. In countries like Germany and the UK, the installation of micro-CHP units is promoted with the provision of subsidies as part of an effort to promote environmentally friendly solutions that can contribute to the reduction of CO₂ emissions.

Industrial facilities consume significant amounts of fuel and electrical energy for their processes. The application of CHP technologies in industrial facilities can lead to an important reduction in their energy bills and can contribute to their energy autonomy. CHP technologies which are applied in the industrial sector have many differences regarding their cost and their energy efficiency. The selection of the appropriate technology should take into account factors like the electrical energy demand, the thermal energy demand, the price and availability of the fuels and the time profile according to which the CHP system will operate.