Figure 2. Simplified arrangement of a crude oil refining system. SOURCE: Chevron Research and Technology Company, a division of Chevron USA Inc.

Thermal energy delivered to a building from an outside source is known as district heating and cooling, which can range in size from small systems serving two or three buildings to networks serving entire cities. District heating and cooling is widely used in developed countries throughout the world and offers numerous advantages over individual building apparatus, including greater safety and reliability, reduced emissions, and greater fuel flexibility, particularly in using alternative fuels such as biomass or waste.

The earliest examples of district heating were Roman hypocausts, a type of hot-air furnace often adapted to warm several buildings in close proximity, such as the three temples at Carnutum (Vienna). The hypocaust and other Roman technologies were reintroduced during the Renaissance, serving primarily...