"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," said Michael Moser, central heating plant manager at Middlebury College.

Broad Campus Steam System

Currently 36 buildings comprise 1.6 million square feet on the 350-acre campus. 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 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 held in readiness year round.

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

The chillers supply 44 degree F (7 degree C) 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. He anticipates extension of building cooling systems in additional campus buildings.

Electricity Generation

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

According to Moser, "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. 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. One unit, rated at 250 kW, 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 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.

Moser said the first turbine-generator set was installed in 1980. "Over the years, these units have been really trouble free," he said. The units are located next to the boilers in the Service Building.

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

The school's power supplier is Central Vermont Public Service; the delivered energy cost is about 8.5 cents/kWh. The college uses approximately 20 million kWh/per year.

According to Sean Casten, CEO at Turbosteam, "Middlebury College understood that the pressure-reduction process was an opportunity to increase their energy security and reduce their annual operating costs."

Casten pointed out that rather than generate all of their energy at a distant power plant and have it pushed down the line to the college, the school can meet some of its requirements on campus. "And the incremental cost is very minor - less than 2.0 cents per kWh versus the 8.5 cents per kWh cost from the utility." Casten noted that at this retail rate, the turbine-generators on campus are providing electric energy valued at $170,000.

Casten said the Middlebury system is a very good example of the benefits of backpressure turbine-generators. "The college has demonstrated that submegawatt sizes of back-pressure turbine-generator sets make economic sense. At Middlebury College we have seen a payback for the systems of less than two years. There are many other institutions out there that have discovered that they can do the same thing."

Synergies from Absorption Plant

In effect, the use of absorption chillers during the cooling season increases the potential output of the turbine-generators at times when the output of steam would otherwise be quite low.

According to Casten, the savings achieved for the college over the past 10 years with the turbine-generators, combined with absorption cooling, has totaled $852,000, nearly double the amount achievable with the turbine-generators alone. These savings will grow even faster with the newer chiller and turbine-generator installations.

Emil Krieger from Trane Albany works with the college on a regular basis. He pointed out that the electric generation system has no impact on the absorption chillers, except to make them an even more attractive comfort solution.

"Single-stage absorbers like these need low-pressure steam. The college simply makes fuller use of the steam before it goes to the chillers. We'd recommend other steam users take a look at this option."

Middlebury College is growing, and plans are being made to extend steam lines and chilled-water service to new campus buildings, including a dormitory and dining hall. Moser noted that with the greater level of steam use for new campus building, on-campus electric generation would increase proportionately.

Sidebar: Pioneering Academic Programs

One of Middlebury College's best-known programs is its Bread Loaf School, a summer academic program offered since 1920 at a Green Mountain location away from the central campus.

New England poet Robert Frost is on the long list of mentors at the Bread Loaf School. Frost participated in the program for 42 years.

The college has an enrollment of 2,100 students and offers undergraduate and graduate degrees. The main campus, situated on rolling hills in the Village of Middlebury, features academic buildings, a chapel, and residence halls all constructed of gray stone and fitting into a harmonious overall appearance.

The college has a close, historic relationship with the Middlebury village community.

Publication date: 12/01/2003