District energy systems distribute thermal energy (steam, hot water, and/or chilled water) through a network of underground pipes to multiple buildings in an area, such as a downtown district, college or hospital campus, airport, or military base. By aggregating the heating and air conditioning supply for multiple buildings, district energy systems optimize thermal energy efficiency. They also achieve economies of scale that allow for the use of low-carbon, cost-effective thermal energy sources—such as the “waste” heat from power plants or industrial processes, combined heat and power, geothermal energy, deep lake water, or municipal solid waste and other types of biomass—that may not be feasible for individual buildings. More information is available here.