District Energy

The dominant model for home heating and cooling consists of highly centralized and expensive generating or distribution facilities powered by natural gas, hydroelectric, coal, or nuclear energy sources that send energy, gas, or fuel to many thousands of homes. This model is becoming increasingly strained because of its inherent inefficiencies, social inequities, and overall environmental unsustainability.

Now many are advocating a return to an older model: decentralized, or district, energy production that distributes space heating, domestic hot water, and cooling within a complex of buildings or over a range of city blocks. District energy (DE) has the potential to address issues related to network vulnerability in terms of disrepair, accidents, catastrophic weather, and even acts of terrorism. However, the particular value of this approach in terms of sustainability is that it allows communities to transition from less sustainable (or more expensive) fuel sources to “greener” alternatives.

Strictly speaking, DE does not refer to the production of the energy itself but, rather, to a thermal network and the way it is managed on a community level. Instead of piping gas into a housing unit from a central location to be combusted in a furnace, or electricity to be consumed by an air conditioner, a DE system uses a medium (usually water or steam, but also synthetic fluids) to carry and transport the heat (or coolant) from the point of origin to the user through a network of underground pipes. Heating and cooling needs are met in individual buildings, and the medium is then returned to the DE plant to be once again heated or cooled as needed.

Far from being a new technology, DE was first made commercially available in Lockport, New York, in 1877, and by the late 19th century it was in wide use in North America and Europe. Much of Manhattan is still heated by Con Edison's steam plants, with new structures added to the system all the time. However, following World War II, Europe continued to expand its DE services, whereas North American cities turned instead to coal, oil, and natural gas.

Common DE markets include municipalities, or clusters of buildings within a municipality. Campus settings such as hospitals, universities, and industrial parks are ideal potential customers. DE systems work well within an industrial ecology model in which waste products from one industrial process may be used by another. In the case of DE, this is referred to as cogeneration, or combined heat and power, wherein the waste heat from one industrial process is used to heat the medium in a DE system to generate electricity, rather than being discharged into the environment. This flexible model extends to taking advantage of other natural and built assets. DE cooling systems can pump already-chilled water from deep lakes to cool buildings or draw warm water from abandoned and flooded mines to heat nearby towns. Alternately, they can be teamed with other green energy solutions, such as thermal solar energy collectors.