Hot Water Is Storable
Another benefit of hot water is that, similar to chilled-water
storage, the energy can be stored to reduce peak demand and
tap energy sources that may be out of synch with the related
load. For example, in some climates in the spring and fall there
are cold mornings and warm afternoons. If the district heating
and district cooling systems are integrated, hot water can be
used to store afternoon heat recovered from heat recovery
chillers for use the following morning. By reducing peak
demand, storage systems can also help to increase the load
factor for renewable thermal energy systems including
Examples of Transitions to Hot Water
Most North American district heating systems are steam
systems, and these systems serve their users and the environment
well. However, as these systems expand and renew themselves
in a carbon-constrained policy environment, we will see a trend
of incremental moves to hot water for new service areas and
in some cases comprehensive moves to hot water distribution.
Replacement of entire distribution systems is costly, but some
district energy systems have concluded it is both cost-effective
and a key to carbon reduction. For example:
• District Energy St. Paul accomplished this nearly 30 years ago,
replacing an old steam system with a Swedish design for hot
water. Now, in addition to its biomass CHP plant, District
Energy St. Paul is installing a solar energy system to feed the
district heating system.
• In conjunction with implementation of CHP, in 2005 the
University of Rochester replaced a major portion of its steam
distribution system with a hot water system with a supply
temperature under 200 F. This project was the cover story in
the 3rd quarter 2007 issue of District Energy.
• Stanford University is now planning to replace its steam system
with hot water as part of a long-term plan to integrate the
heating system with the district cooling system, recovering
rejected chiller heat for a low-temperature district heating
system. Operating temperatures are still under study.
The Question of Cost
Converting to hot water production and distribution may
seem like a daunting capital challenge. However, there are ways
to reduce the capital investment in hot water infrastructure, and
there can be significant operation and maintenance savings.
The University of Rochester spent $10.1 million on its new
hot water system, including 16,950 feet of paired supply and
return piping and installation of 118 heat exchangers in buildings. In Rochester we found that significant savings were realized
by running distribution pipes through existing building spaces or
tunnels whenever feasible.
As discussed above, obtaining low return temperatures from
the buildings is critical to effectively tapping many low-carbon
heat sources. Hot water heating and reheat coils have far more
heat transfer capability than many engineers realize. With tight
flow control and consistent temperature control, 180/160 F coils
don’t have to be replaced to get back 110 F or lower.
Operating savings will be realized by reducing distribution
losses. Thermal losses in properly designed hot water systems are
generally in the range of 5 to 7 percent. Maintenance costs can
be reduced with new piping infrastructure and the elimination of
steam traps and condensate piping.
Effective energy policies to reduce GHG emissions will
encourage increased use of recovered waste heat and renewable
energy. Low-temperature district hot water will increasingly
become the preferred district heating approach because it
facilitates the ability to tap many of these low-carbon or
Mark Spurr is legislative director of IDEA. He also is
president of FVB Energy Inc., a U.S. consulting firm
specializing in district energy and CHP business development, engineering and marketing. FVB has offices
in Minneapolis, Minn.; Seattle, Wash.; Manama,
Bahrain; Edmonton, Toronto and Vancouver, Canada;
and in Stockholm, Västerås and other cities in
Sweden. Spurr represents the United States on the
executive committee of the International Energy Agency Implementing
Agreement on District Heating and Cooling, including Implementation
of CHP. He may be reached at email@example.com.
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