heat. Environmental considerations and the problem of delivery prevent
the direct combustion of coal in individual buildings and the remaining
options are not likely to be located within the inner city.
 The options are limited for individual building measures, but the
 high density urban area is uniquely suited for the application of district
 heating.

 DISTRICT HEATING AND ITS POTENTIAL BENEFITS

 District heating is a thermal distribution system. It consists of
 a network of underground pipes, carrying hot water or steam, whereby
 energy is collected from one or more central sources and delivered
 throughout a community to provide space conditioning, water heating, and
 process heat requirements. It is a proven, well developed technology.
 District heating has been used in the United States since the early
 1900s, but urban applications were primarily steam systems with limited
 transmission distance, and the concept never developed to its full
 potential. In contrast, modern district heating systems that primarily
 use hot water are in extensive use in Europe and Russia, and large-scale
 expansions are being planned. With district heating, conservation of
 energy and scarce fuels results from the use of innovative and more
 efficient heat sources. Virtually any energy technology available to
 the nation and local communities can be incorporated into the district
 heating and cooling system. These thermal energy sources can include
 abundant domestic fuels (coal, nuclear, and urban waste), natural sources
 (solar, geothermal, and groundwater), or manufactured sources (industrial
 waste heat and synfuels). In fact, for the urban areas, the only feasible
 way to make extensive use coal, solid waste, and other renewable heat
 sources is with a district heating network.
 Cogeneration, the simultaneous production of electricity and useful
thermal energy, can be a valuable heat source for district heating. As
shown in Fig. 1, an electric-only generating plant has an efficiency of
about 33%; the other 67% of input fuel energy is lost to the atmosphere
or to bodies of water. With cogeneration, electric output is reduced to
25% but one gains 55% of the input fuel energy as hot water suitable for