District energy (DE) may be defined as the heating and/or cooling of two or more structures from a central heat source (Fig. 1). Heat may be provided in the form of either steam or hot water and may be utilized to meet process, space, or hot water requirements. Chilled water or an ice slurry may be distributed to meet the needs for space cooling. The thermal energy is distributed through a network of insulated pipes consisting of delivery and return mains. Heat can be provided through the use of conventional boilers that burn conventional fuels such as oil, natural gas, or coal, or from cogeneration plants that produce both electricity and heat. Cooling can be provided through the use of absorption chillers or centrifical chillers driven by electricity, steam, or a binary turbine. District energy systems may also utilize renewable resources such as geothermal, biomass, or waste heat resources such as industrial waste heat. Fossil fuel peaking or back-up is often an integral part of district heating systems and thermal storage in the form of chilled water. Brine or ice is often incorporated into the design and generation of most modern district cooling systems. The delivery of thermal energy from a central source is not a new idea. During Roman times, warm water was circulated through open trenches to provide heating for buildings and baths in Pompeii. In the ruins are the remains of the old heating plants, for example, the one used in conjunction with the so called Thermer (bath). In many places it is still possible to see how the heated water was circulated from home to home through a network of trenches that went through the cellars of the buildings. Hundreds of years later, but several hundred years before our time, in Chaudes Aigues Cantal in France, geothermal water was distributed as early as the 14th Century through wooden pipes. That system is still used today. The French were already at that time utilizing a large main with laterals to provide heat to individual houses.

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