district heating applications and the advantages of district heating
systems.


 URBAN CHARACTERISTICS AFFECTING CONSERVATION OPTIONS

 The conservation options to be considered include improving the
thermal envelope (or structure) of buildings, use of renewable natural
resources such as solar and geothermal energy, use of more plentiful
domestic fuels, and recovery of energy from refuse and waste heat. The
characteristics of high density urban areas limit the feasibility of
many of these options.
 It is estimated that 45% of commercial and 65% of residential
floorspace that will exist in the year 2000 already existed in 1980.
This affects the practicality of improving the thermal integrity of
individual buildings because such conservation programs will require
extensive retrofit measures. Retrofit conservation measures are more
expensive than when implemented with a new building design, and the
types of conservation measures that can be used are more limited. There
are low-cost quick-payback measures that should be used before considering
any other option, including more efficient management of building operation,
decreased lighting levels, and improvements in air handling systems and
controls. An analysis of engineering audits at Minnesota state-owned
facilities indicated improvements to the thermal envelope had very long
payback periods, an average of 23 years, and would provide only about
eighteen percent of the energy saving possible by implementing all
measures considered.
 High density urban areas are very unlikely candidates for use of
renewable energy sources. The availability of land and roof top area of
highrise buildings is too limited to supply much of the building's
thermal demand with individual-building solar collectors. It is also
unlikely that geothermal energy, groundwater or ground coupled heat
pumps, or wind energy could be used as individual building systems to any
large extent.
 Excluding the scarce fuel sources (natural gas and oil), the remaining
options include coal, nuclear power, urban waste, and industrial waste