District energy systems consist of a network of underground insulated pipes that pump hot or cold water to multiple buildings in a district, neighbourhood or city. Some systems just connect a few buildings, while others connect thousands of buildings and homes across a city.

Modern district energy systems combine district heating, district cooling with combined heat and power, thermal storage, heat pumps and/or decentralised energy. They are increasingly climate resilient and low carbon, allowing the:

• Recovery and distribution of surplus and low-grade heat and cold to end-users (e.g., previously unused waste heat from industry or power stations, waste water and use of natural water reserves such as lakes, rivers);
• Storage of large amounts of energy – such as surplus wind power or surplus heat in the summer – at the lowest cost compared to other energy storage options;
• Integration and balancing of a large share of variable renewable power – for example, through conversion to heat and stored for use seasonally or during peak thermal demand.

It is time for a redefinition of District Energy. It is no longer exclusively about heat or surplus energy, the traditional drivers of district energy. It's about local production matched to local use – and not only at a building level, but also at the neighbourhood and city level. It's about sharing energy between buildings. And it's about resource efficient neighbourhoods and resilient cities. District energy is an approach to applying technologies to co-ordinate the production and supply of heat, cool, domestic hot water and power to optimise energy efficiency and local resource use.