Sidebar: In the Pipeline: District Energy and Green Building

The nation’s largest hot-water district heating system operates in St. Paul, Minnesota, serving 80% of the downtown buildings, including the State Capitol Complex, most downtown office buildings, and 300 single-family homes. An older steam system was converted to hot water between 1983 and 1990. In 2003, District Energy St. Paul added a wood-chip powered CHP plant to its energy mix (which includes coal). The wood-chip plant generates 25 megawatts of electrical power (MW_e) and distributes 65 MW_th of hot water through 18 miles (30 km) of pipe. The new plant reduced District Energy’s reliance on coal by 80%, and reliability exceeds 99.99%.

In addition to providing heat, District Energy St. Paul has been providing district cooling since 1993. Two insulated tanks store 6.5 million gallons (25 million l) of water that is chilled at night using off-peak electricity; this chilled water is distributed in the daytime to district cooling customers that currently include 60% of buildings in downtown St. Paul. According to the city, the hot water district heating system is twice as efficient as the previous steam heating system that it replaced—heating twice the square footage of building with the same amount of fuel. In one month alone, November 2005, customers saved $1 million in energy, compared to what they would have spent on heating with natural gas. The system also significantly reduces air pollution emissions; it eliminated 150 smokestacks, 50 cooling towers, and 300 chimneys in the city. Overall, the new wood-fired CHP plant reduced particulate emissions by 50% and greenhouse gas emissions by 280,000 tons per year. CFCs and HCFCs have also been eliminated by the district cooling system. The wood-fired CHP plant is powered with urban wood waste. The city stores wood chips at their Pigs Eye Wood Recycling Center from storm damage in the region as well as from tree cutting and pruning in the city. Annual wood chip use is about 300,000 tons—approximately half of the wood waste generated by the Twin-Cities area. By comparison, an October 2006 storm in identically sized Buffalo, New York, generated at least 10 million tons of wood waste, according to The New York Times, which would be enough to power the St. Paul plant for 30 years.

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