District heating networks (DHNs) consist of a central plant where hot water or other heat source is generated and then piped into buildings to provide them with space heating and domestic hot water. Usually, the network of well-insulated pipes is led underground and each building has a heat exchange unit that allows individual heating control via heat meters. District cooling works in a similar way but uses waste heat to drive a refrigeration process. Commonly, district heating networks are managed by an Energy Services Company (ESCO).
Where possible, ‘waste heat’ can be used to power the network. As this heat is a by-product of another process such as electricity generation, it is considered as ‘emission free’, since all emissions have already been attributed to the primary product, and this leads to a better environmental performance.
Usually, the central boiler plant is combined with a heat and power unit (CHP). This process allows the production of electricity and useful heat from a single plant, which is more efficient than generating electricity and heat separately.
The implementation of these networks is particularly favourable in areas where a sizeable amount of waste heat is available at a relatively reduced distance and there is a large and steady demand of heat. This often implies the coexistence of domestic and non-domestics users within the district heating network.
District heating networks can operate with fossil fuel or renewable energy-based fuels such as natural gas, biomass, geothermal energy or energy from residual municipal waste plants. A major advantage of district heating networks from an environmental perspective is precisely the possibility of changing fuel and heat sources, which is not usually possible in smaller networks that remain locked mainly to one source. This flexibility of the system allows switching from carbon-intensive heat generation to low- or zero-carbon sources without affecting the consumer.
It should be noted that although heat networks with a CHP can be a more efficient way of using fossil fuels, they are not truly sustainable.