Abstract

1.1 What is District Energy?

District energy systems distribute steam, hot water and/or chilled water from a central plant to individual buildings through a network of pipes. District energy systems provide space heating, air conditioning, domestic hot water and/or industrial process energy, and often also cogenerate electricity. With district energy, boilers and chillers in individual buildings are unnecessary.

District energy systems can use a diversity of energy resources, ranging from fossil fuels to renewable energy to waste heat. They are sometimes called "community energy systems" because, by linking a community's energy users together, district energy systems maximize efficiency and provide opportunities to connect generators of waste energy (e.g., electric power plants or industrial facilities) with consumers who can use that energy. The heat recovered through district energy can be used for heating or can be converted to cooling using absorption chillers or steam turbine drive chillers.

Storage of chilled water or ice is an integral part of many district cooling systems. Storage allows cooling energy to be generated at night for use during the hottest part of the day, thereby helping manage the demand for electricity and reducing the need to build power plants.

1.2 How Many Systems Are There?

There are an estimated 5,800 district energy systems in the U.S., providing 1.1 quads of energy and serving over 8% of commercial floorspace. Most district energy output is in institutional systems, which serve groups of buildings owned by one entity, such as a college, university, hospital or military base. (Figure 1) However, the greatest current growth in district energy is occurring in utility systems serving downtown areas, particularly district cooling systems.

Phase-out of ozone-depleting refrigerants has motivated building owners to opt for cost-effective alternatives such as district cooling. At the same time, electric and gas utilities are increasingly implementing district cooling systems as competition increases for retail energy customers. Once district cooling systems are established, they grow substantially. (Figure 2)

Figure 1. District energy output by system type1

Figure 2. Growth of selected utility district cooling systems, showing first year of operation

| District Energy in U.S. Climate Change Strategy | Environment |