A typical district energy customer is the owner/manager of large office or condominium buildings or facilities such as hospitals, universities and other municipal buildings.

District energy systems exist in most major North American and European cities where cooling and heating is essential, some of which have been in operation for over 100 years. In many of these locations, district energy is a very appealing alternative to on-site chiller/heater plant installation and operation, due to its year-round reliability, predictable long-term operating and maintenance costs, reduced space utilization and major capital expenditure avoidance.

The process

In the case of Thermal Chicago's downtown Chicago district cooling system, cold or "chilled" water is made in chillers. The chillers use a refrigerant to cool water to low temperatures, near freezing. A refrigerant will typically be a liquid under pressure and flash to a gas when heated and allowed to expand. This is much like heated water that flashes to steam, but a refrigerant will flash to a gas at a very low temperature and still carry heat away with it. The heated gas is then cooled via a separate system and compressed back into a liquid state and the cycle begins again. The majority of the cooling action that chills the water is caused when the refrigerant changes from a liquid to a gas.

Thermal Chicago has 5 plants in the downtown Chicago area that contain many chillers. The chillers use refrigerants to chill distribution system water. They can also freeze some distribution water for "thermal storage" and then melt it for cooling when needed. The distribution system water is then sent via a system of underground pipes to heat exchangers at customer facilities. Thermal Chicago's distribution system water does not mix with building air-conditioning water. Instead the thermal energy is transferred via these heat exchangers, which allow the transfer of cooling energy from the Thermal distribution water to the building's cooling water system for use in the distribution of cool air throughout the building. The distribution system water then returns to the plants for re-chilling through the same continuous process.

Operational framework

District energy provision is largely unregulated in the United States, although each multi-customer system usually has an agreement with the city in which it operates that provides permission to lay pipes under the streets (generally in the form of a use agreement or concession). The plans for laying these pipes need to be drawn up and provided to the city engineers for approval.

In the case of Thermal Chicago, its operations are conducted subject to the terms of a long-term Use Agreement with the City of Chicago.