Cool, Hot and Green
District Energy St. Paul Inc. relies predominantly on biomass power to heat and cool the majority of downtown St. Paul, Minn. The goal is to replace all fossil fuels in the future.
While district energy systems have been widely used in densely populated urban areas for more than a century, St. Paul's system is unique as it's the largest biomass-powered system in North America. In 1978, St. Paul was selected by the U.S. DOE and the Minnesota Energy Agency to be the subject of a study to determine the feasibility of a modern hot-water district-heating system in a major northern U.S. city. The favorable results from the study led to the building of a hot-water system that was completed in 1983. The system primarily used coal-fired traveling grate boilers to heat water, although fuel oil and natural gas could also be used. The system cost $45.8 million in 1982 dollars. It was completed one year ahead of schedule and was $1.3 million under budget. It replaced an old steam-heat system that was highly inefficient with more than 50 percent distribution losses, compared with the hot-water system which has distribution losses of only 5 percent. Both heating and cooling systems run year-round to supply hot water for heating, domestic hot water, industrial process heat and chilled water for air conditioning.
Anders Rydaker, who served as a consultant in developing the original district heating project, returned to the city in 1993 from his native Sweden to become the president of the independent nonprofit organization. "When I got to St. Paul the second time, it was my vision to move to renewable energy," he says. "It's been my priority for a long time, way before it was the thing to talk about in the United States." It took another 10 years before the CHP plant began turning clean urban wood waste into electrical energy delivered to the city's power grid as well as heat for the hot-water heating system and evaporative coolers. Biomass now provides nearly 70 percent of the system's energy demand, he says, approaching the goal of 75 percent. The boilers from the 1983 system are still used for peak-load conditions when the city experiences its famous winter cold snaps and temperatures drop as low as minus 30 degrees Fahrenheit. Rydaker says that a CHP system large enough to meet the remaining peak load would not be economical, although his next goal is to replace the remaining portion of fossil-fuel energy used. The 1983 system had been cofiring coal with wood chips in the 1990s, but the feedstock system was dismantled to make room for the biomass CHP plant. Studies begin this year to examine the best way to modify the 1983 boiler system. The primary challenge in meeting peak demand with biomass will be developing new feedstock sources.
As vice president of operations and engineering, it is Michael Burns' job to manage the biomass acquisition program which at times scrambles to find enough clean wood. When the system started, the Minneapolis-St. Paul metropolitan area had a fragmented infrastructure of businesses handling tree waste. "We had to build the infrastructure to handle 250,000 tons per year," he says. That meant adding chipping and screening equipment, and finding collection and processing points. Many of the original businesses still supply District Energy with already processed wood, which is run through screens to make sure the 4-inch maximum diameter required for the CHP plant's vibrating grate boiler system isn't exceeded. The main wood collection site is located in an industrial area along the Mississippi River just a few miles downstream from downtown St. Paul. The biomass project took over management of a tree waste site developed by the city when Dutch elm disease devastated its urban forest.
Developing the infrastructure has been only one part of the challenge, however. At times, there isn't enough clean wood waste in the entire 13 county metropolitan area to fuel the plant, Burns says. That forces him to turn to the northern Minnesota forestry industry for supplies. "We try to avoid it whenever possible because of the transportation costs, and because they have higher harvesting costs," he says.
In order to fuel the remaining energy load, District Energy will begin testing various feedstocks such as corn stover and other agricultural wastes. Among the options to be studied will be pelletized biomass, which could be handled in the 1983 boiler system much like stoker coal. "Now that we have the CHP optimized and the market has developed, we are looking at the opportunities to incorporate biomass in those boilers without major modifications," he explains. Rydaker adds that retrofitting costs will determine how quickly the conversion to 100 percent biomass will proceed.
Several related businesses actually make up District Energy, which was called District Heating Development Co. when it was launched in 1983. When cooling capacity was added in 1993, District Cooling St. Paul Inc. was organized as a separate entity. Both are independent, nonprofit organizations with individual boards, although most board members serve both organizations. The seven-member boards of directors include three elected by customers, three appointed by the city of St. Paul and one elected by the six. Ever-Green Energy LLC is a for-profit affiliate of District Energy which develops renewable energy projects for others. One biomass project being developed in St. Paul is evaluating whether a paper recycling plant could utilize agricultural waste biomass for its thermal process energy. Ever-Green owns and operates the CHP plant along with Duke Generating Services LLC, a subsidiary of Duke Energy Corp. Biomass procurement is handled by an affiliate of Ever-Green called Environmental Wood Supply.
District Energy Benefits
District energy systems like St. Paul's are not uncommon. The International District Energy Association reports that in 2005 about 6 percent of commercial buildings in the United States were heated with district heating. District energy systems are typically located in the central business districts of larger cities, on university or college campuses, on hospital or research campuses, military bases and airports. The number of customer buildings served ranges from as few as three or four in the early stages of developing a new system to as many as 1,800 customer buildings as in the system served by Con Edison Steam Business Unit in Manhattan, N.Y., the largest district steam system in the world.
IDEA lists several benefits found in district energy systems:
› The systems tap into economies of scale and the operational benefits of having a large, diverse portfolio of customers
› The systems can utilize industrial-grade equipment not economically feasible for individual buildings
› The diversity of energy options and fuel flexibility creates a market advantage and establishes the district energy system as an asset for community energy planning
› The availability of district energy service reduces the capital cost of developing an office building by cutting the boiler and chiller plant capital cost from the project
› District energy systems can utilize local fuel resources to keep energy dollars recirculating in the local economy, and as a renewable energy source may qualify for a production
tax credit under a renewable energy portfolio standard
District Energy St. Paul receives calls weekly from operators of some of the older steam heat systems in the country asking about retrofitting a hot water system as was done in St. Paul. "Hot water is not very common in the United States," Rydaker says. "It's so much better. With hot water you can recover waste heat in whatever form." In buildings still using older steam-heat systems, the retrofit to hot water can be quite costly. However, buildings constructed after 1975 that utilize hot water in heat exchanger systems have an easy conversion. Some building complexes have both, such as the Minnesota State Capitol on the north end of downtown St. Paul which tapped into District Energy when it was developed.
Looking down from Kellogg Boulevard as the trucks deliver wood chips to the CHP plant.
A major benefit for district energy customers is the system's reliability-after 25 years District Energy St. Paul reports 99.99 percent reliability for heat and District Cooling St. Paul reports 100 percent reliability. Stable energy costs for customers have been another benefit, although Rydaker says that in the 1990s when natural gas was at $2 per million British thermal units the District Energy rates were higher. Today that is reversed with District Energy rates below natural gas. Rising energy costs for fossil fuels have boosted the cost for wood chips however and after a decade of flat rates, District Energy's customers now are seeing higher rates on their monthly bills. Regardless, customer acceptance of the system is high-St. Paul's district heating system serves 85 percent of the downtown area's square footage, while the district cooling system serves 65 percent. More than 180 buildings in the downtown district and 300 single-family homes in adjacent areas are served by District Energy. Offices and homes across the Mississippi River from the plant are also supplied via a pipeline encased in a bridge, which is another example of the problem solving and innovations that have made District Energy St. Paul an industry leader.
Susanne Retka Schill is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 738-4962.