District heating – how it works

Heat for a district heating network is often generated in a Combined Heat and Power (CHP) plant.

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 produced 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 the 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

When generation is based on natural gas, the steam turbine can be combined with a gas turbine – which further improves efficiency. Another common type of CHP plant is smaller and located near landfills, where harmful methane gases are combusted and converted into heat instead of being diffused into the atmosphere and contributing to the greenhouse effect.

CHP plants have very high efficiency levels. Regard­less of type of fuel used, approximately 90 per cent of the energy is utilised. This has clear advantages over condensing 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

Another important feature of district heating, is that it not only allows the use of waste heat, but also bio-fuels, waste fuels or large heat pumps available in an environmentally sustainable way.

By using residual heat instead of fossil fuels, district heating has a relatively low impact on the environment and low CO2 emissions in particular.

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 district heating water is returned to the district heating plant to be reheated and pumped back into the district heating system.

Last updated: 2013-10-02 08:34