There are several types of CHP plants, which can be powered by most types of fuel, including natural gas, coal, oil and biomass. In the most common type of CHP plant, electricity is generated by heated steam that passes a turbine. The remaining steam then heats cold district heating water and, rather than being wasted, the heat is transferred to homes and businesses via a network of underground pipes: a district heating network.
Water at high temperature (approximately 100 degrees Celsius) is transported to a city via a primary line. In secondary networks, the heat is further brought to the premises for building heating, under floor heating and hot water. The temperature in the secondary networks is lower (around 70 degrees Celsius, 40 degrees Celsius return).
The most modern, efficient CHP plants use natural gas as fuel
CHP plants have very high efficiency levels. When generation is based on natural gas, the steam turbine can be combined with a gas turbine – which further improves efficiency. Regardless of type of fuel used, approximately 90 per cent of the energy is utilised. This has clear advantages over other power plants, which use only the electricity generated and thus utilise only around 35-50 per cent of the energy content.
Contribution to a better climate
Highly efficient district heating – CHP plants combined with large-scale heat storage, and "power to district heating" concepts or heat pumps using excess power from renewable energies – offers for example substantial potential to increase renewables-based heating particularly in urban areas. It also provides for considerable savings of fossil primary energy sources like oil and gas.
Another important feature of district heating is that it not only allows the use of waste heat from traditional electricity generation, but can also use bio-fuels, waste and large heat pumps for generation in an environmentally sustainable way.
How district heating works
Water is transported in well-insulated underground pipes to a district heating centre in the building that is to be heated. A heat exchanger conveys the heat (but not the water) to the building's own heating system of radiators and hot tap water. The cooled-down district heating water is returned to the district heating plant to be reheated and pumped back into the district heating system.