District Energy - 2010 4th Quarter

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 biomass boilers.

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 no-carbon sources.

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 mspurr@fvbenergy.com.