Making Sparks with Steam for Big Success
Middlebury College in Middlebury, Vt., has an enrollment of 2,100. Its 36 buildings comprise 1.6 million sq. ft. on a 350-acre campus. Each building is connected to a steam plant at the center of campus that supplies heating, domestic hot water, laundry and steam for food service. There are four boilers.
Middlebury College in Middlebury, Vt., has an enrollment of 2,100. Its 36 buildings comprise 1.6 million sq. ft. on a 350-acre campus.
Each building is connected to a steam plant at the center of campus that supplies heating, domestic hot water, laundry and steam for food service.
There are four boilers. Two were installed in the 1960s, one in 1985, and the newest, in 1999 to meet the requirements of Bicentennial Hall, which opened in 2000. The two oldest boilers operate at 125 psig, and the newer ones at 250 psig. Because surplus steam is available in the warmer months, the college installed a 150-ton absorption chiller in 1976 to serve the Johnson Art and Art History Building. Due to the need of complete climate control for art materials, the chiller is in a state of year-round readiness.
The school installed a second, 500-ton absorption chiller in 1991 to serve the Fine Arts Building. Two more 500-ton single-stage units were installed in 2000 in Bicentennial Hall; they typically operate between May and October. In all three above mentioned buildings, the chillers provide 44°F water to air handlers that serve VAV boxes.
What is unique about the system is that it uses backpressure steam-turbine/generator sets in line with the boilers to reduce steam pressure and provide electricity for the school. "The college recognized the opportunity to extract useful energy from the steam in the process of reducing it to our application levels of 22 psig or lower," says Michael Moser, the school's central heating plant manager. "We use these systems rather than pressure-reducing valves and receive a significant amount of electric energy year-round."
The three turbine/generator sets were designed to match the campus' specific steam supply conditions. A 250-kW unit receives steam at 125 psig and reduces it to 22 psig. The other two units, rated at 600 and 850 kW, receive steam at 250 psig and reduce it to 22 psig. The outlet steam from all three units is diverted to a common campus steam supply. The first turbine/generator set was installed in 1980, and its output is 480 volts AC, which is stepped up to the campus distribution voltage of 12.5 kV.
Moser estimates that the generators provide around 10% of the school's electrical requirements depending on the time of year. The school's power supplier delivers energy at about 8.5 cents per kWh, and the college uses 20 million kWh per year. The energy the school generates by itself costs about 2 cents per kWh, which at this rate, is valued at $170,000.
Essentially, by using absorption chillers during the cooling season, the potential output of the turbine/generators at times when steam output is otherwise low, increases. By using the combination of turbine/generators and absorption cooling, the school has saved over $850,000 over the last ten years, nearly double the amount that could be achieved using the turbine/generators alone.
These savings will grow even faster with the newer chiller and turbine/generator installations. There are plans to extend steam lines and chilled water service to new campus buildings, including a dormitory and dining hall. Moser speculates that with the inevitable increase in steam use by the new buildings, the on-campus generation of electricity will increase proportionally.