Middlebury College maintains its 200-year-old sense of history while taking advantage of new technologies for building comfort and energy use.
MIDDLEBURY, Vt. - "The college wanted to get the fullest possible use of its steam plant. Installing back-pressure turbo-generators along with absorption chillers was a logical step."

That was how Michael Moser, central heating plant manager at Middlebury College, described the college's decision to use a steam plant.

Middlebury College is located in Middlebury, Vt., about 30 miles south of Burlington. Timothy Dwight, then president of Yale College, founded it in 1800. The school's original purpose was to train young men from Vermont and neighboring states for the ministry and other scholarly professions of the early 19th century.

In 1883, it became one of the first formerly all-male schools in New England to admit women. Today, it has an international reputation for its programs in the sciences, arts and humanities.

Trane single-stage absorption chillers are supplied by gas-fire boilers, also used for campus heating.

Pioneering programs

One of the college's best-known programs is its Bread Loaf School, a summer academic program offered since 1920 at Green Mountain, a location away from the central campus. Among the long list of mentors at the Bread Loaf School was New England poet Robert Frost, who participated in the program for 42 years.

The college has an enrollment of 2,100 students, most of whom live on campus, and offers undergraduate and graduate degrees. The picturesque main campus in Middlebury features academic buildings, a chapel and residence halls, all constructed of gray stone. The college has a close, historic relationship with the Middlebury village community.

Turbosteam back-pressure turbo-generators reduce steam pressure for campus heating and cooling while also supplying up to 20 percent of campus electric requirements.

Campus steam system

The 36 buildings on the 350-acre campus comprise 1.6 million square feet. All receive heating service from a campus steam system. The system supplies heating, domestic hot water, laundry and steam for food service. Steam is produced by boilers at the service building in the central part of the campus. The boiler fuel is No. 6 oil.

The college's four steam boilers consist of a 1963 Babcock & Wilcox, a 1968 Cleaver-Brooks, a 1985 Zurn and a 1999 Babcock & Wilcox. The two earlier boilers operate at 125 pounds per square inch, gauge (psig), and the two newer ones at 250 psig. The newest boiler was installed to meet the requirements of Bicentennial Hall, the science center that opened in 2000.

All of the major building cooling on campus is done by Trane absorption equipment. Because of the availability of surplus steam in the warm months, in 1976 the college installed its first Trane single-stage absorption chiller, serving the Johnson Art and Art History Building.

The 150-ton machine is located in a basement mechanical room of the building. Because of the need for complete climate control for art materials, this chiller is kept ready year-round.

A second 500-ton Trane absorption machine was installed to serve the Fine Arts Building when it was completed in 1991. In 2000, two more 500-ton single-stage Trane absorption chillers were installed in Bicentennial Hall, which was dedicated as the college celebrated its 200th anniversary. The operating season for these three machines is typically from early May through October.

These machines all supply 44?F chilled water to air handlers that serve VAV boxes in the three buildings. According to George McPhail, who manages the HVAC system on campus, the college has been pleased with the comfort levels achieved with the absorbers and chilled water system. McPhail anticipates extension of building cooling systems in additional campus buildings.

Electric generation

The unusual aspect of the Middlebury College system is its use of back-pressure steam turbine-generator sets in line with the boilers to reduce the steam pressure and to provide electric energy for campus use.

"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," Moser said. "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 specific steam supply conditions on the campus. One unit, rated at 250 kilowatts, receives steam at 125 psig and reduces it to 22 psig. The other two units, rated at 600 kW and 850 kW, receive steam at 250 psig and also reduce it to 22 psig.

The largest and most recent unit was designed and installed in 2000 by Turbosteam Corp. of Turners Falls, Mass. The outlet steam from all of the units goes to a common campus steam supply.

According to Moser, the first turbine-generator set was installed in 1980. "Over the years, these units have been really trouble-free," he said.

The generator output is at 480 VAC, which is stepped up to the campus distribution voltage of 12.5 kilovolts. He estimates that the generators provide approximately 10 percent of the campus electric requirement, depending on the time of year.

Power is supplied is by Central Vermont Public Service and the delivered energy cost is about 8.5 cents per kilowatt-hour. The college uses 20 million kwh per year. The units are located next to the boilers in the service building.

Growing opportunity

Sean Casten, chief executive officer at Turbosteam said that Middlebury College has been a pioneer in a growing trend of colleges and other institutions using this type of equipment.

"Middlebury College understood that the pressure reduction process was an opportunity to increase their energy security and reduce their annual operating costs," Casten said.

Casten noted that rather than generate all of their energy at a distant power plant and have it pushed down the line to the college, they can meet some of their requirements right here on campus.

"And the incremental cost is very minor - less than 2 cents per kwh versus the 8.5 cents per kwh cost from the utility," he said.