ENERGY PLANT UPGRADE NEEDEDFor decades, Clemson has operated a central plant facility to supply steam and chilled water for heating and cooling. In the late 1990s, the University Facilities group at Clemson recognized a need to upgrade its central plant with a more efficient and reliable steam and chilled water supply. At the same time, there was strong interest in installing an electric generation capability to carry at least a portion of the campus electric requirement.
The Clemson campus is also the home for the South Carolina Institute for Energy Studies (SCIES), which in partnership with the university planned to create an Energy Systems Laboratory (ESL) adjacent to the existing Central Energy Facility. Lawrence Golan, director of SCIES, commented, “Our goal was to make the campus a real-world energy laboratory. As the plans developed for the central plant improvements, we saw the opportunity to integrate this facility with the university’s mission for research and teaching.”
Clemson solicited proposals from engineers to design an integrated energy plant with combustion turbines for electric generation and a heat recovery boiler on the turbine exhaust to recover heat energy in the form of steam. The most urgent need was a new chiller plant to replace three older existing centrifugal chillers with equipment that could operate at much higher efficiency.
PLANT INCLUDES LEARNING MISSIONThe university encouraged submittal of designs which would allow the equipment to be closely observed by students, researchers, and industrial partners from the Energy Systems Laboratory. The engineer selected to design the central plant facility was I.C. Thomasson of Tampa, FL. Using the university’s criteria, the integrated facility design included demolition of three of the four existing coal-fired boilers, leaving in place Boiler Number Four, rated at 75,000 lbs/hr at 125 psig. A new natural gas-fired boiler was specified to produce an additional 150,000 lbs/hr, for a total system capacity of 225,000 lbs/hr.
Electric generation was to be provided by two natural gas-fired combustion turbines manufactured by Solar, rated at 4.2 and 4.8 MW each. The combined capacity of 9.0 MW represents about 40% of the university’s summer peak electrical demand. One of the combustion turbines, the Solar Mercury 50, is an advanced recuperative cycle machine that was installed as a part of an energy technologies research project at the university.
The design uses a heat recovery boiler on the combustion turbine exhaust as a supplementary source of system steam. Steam from the heat recovery boiler reduces the need to purchase natural gas or coal and improves overall system efficiency.
The chilled water aspect of the design involved replacing three 30-year-old single-stage centrifugal chillers which had a nominal capacity of 6,000 tons with three new centrifugal chillers and a double-effect absorption chiller, providing a combined capacity of 6,400 tons. According to Jeffrey Hinson, the actual goal of the replacement was far greater than the slight increase from 6,000 to 6,400 tons. “Those old machines had already been dated because of plugged tubes, and because of changes in the inlet temperature of the condenser cooling water.”
He pointed out that the plant was designed to use water from nearby Lake Harwell for condenser cooling. “Because of the operation of the Oconee nuclear power plant upstream, we have seen an increase in the summer water temperatures from 65 degrees F (18 degrees C) to 80 degrees F (27 degrees C). Since our old chillers were designed for 65 degrees F water, they suffered a major derate. We were unable to come close to meeting campus cooling loads. In fact, even though the nominal increase in capacity was only 7%, the actual increased cooling capability was more like 30%.”
MINIMAL CONDENSER WATER FLOW REQUIREDA key specification for the replacement chillers was that they be able to operate using the existing 20-inch buried pipeline carrying condenser cooling water from the lake to the central plant, a distance of over 1,000 feet. Hinson noted, “The expense of replacing this pipeline would have been phenomenal. Among other things, it runs under our football stadium.”
He said, “Replacement just wasn’t an option. We needed the new chillers to be able to continue to use the existing pipeline.” According to Hinson, the engineer determined that the effective flow limit for the existing pipeline was 10,000 gpm, which meant that the electric chillers would need to be able to stay on-line with flow rates as low as 1.5 gpm/ton.
The chillers ultimately selected for the project were three Trane Duplex CenTraVac™ chillers, Model CVHD, rated at 1,800 tons each, and a Trane Horizon™ absorption chiller rated at 1,000 tons. This combination of machines met the university’s requirement to be able to use the existing pipeline and to operate efficiently at condenser water inlet temperatures of 80 degrees F, or even occasionally above that.
The purpose of the absorption chiller was to take advantage of surplus steam, particularly during times of year that the combustion turbines were operating and there was low demand for heating steam. Additionally, the university wanted increased flexibility in energy choices, and the ability to help minimize electric load during peak demand hours.
The electric supplier is Duke Power, and peak demand is established monthly year-round, based on usage coincidental to Duke’s system peak. In order to reduce demand charges, the university commonly operates its own generation during the afternoon hours, the typical time of the peak. Hinson said, “We try to anticipate when that peak will occur, and have our generation on-line at that time. Usually we guess right.”
PLANT INSTALLED ON TIGHT SCHEDULEThe mechanical contractor chosen was Stanley Jones Corp. of South Fulton, TN, and the electrical contractor selected was Walker Electric of Nashville. It took five months from the start of the installation in winter to have the plant up and running. While work was in progress, the university relied on a 2,000-ton remote plant to carry the entire campus load.
Hinson indicated that the installation went without major problems. The old boilers and chillers needed to be completely removed from the central plant building before the new boiler and chillers could be moved in. An air jack was used to move the heavy equipment into place.
Hinson said that the chilled water system startup was uneventful. “That went really well, too. We did a few tweaks on the communication system, but nothing serious. The entire new chiller plant was able to carry the full campus load almost immediately. The units have performed as we expected, and we are providing much colder water to the entire campus,” he remarked. As designed, the plant delivers chilled water at 40 degrees F (4 degrees C).
Most of the chilled water is used for cooling academic buildings, laboratories, and administrative facilities. It is used in a wide variety of air handler and fan coils. Chilled water is also sold to the independent authority that operates residence halls. According to Hinson, this customer immediately noticed the difference in chilled water temperature, and found that much more effective cooling and dehumidification was now possible.
Today, with the new central plant in operation, plans are being finalized to integrate operations with the Energy Systems Laboratory. Research on the advanced cycle combustion turbine is a project being done for the U.S. Department of Energy (DOE). Students and faculty are learning to use the central plant as a learning center.
According to ESL program manager Dave Stubblefield, in coming months, students and researchers will be able to visit the World Wide Web to observe operations at the center, looking at energy use, operating temperatures and pressures, and comfort levels throughout the campus. Stubblefield said, “With our close affiliation with the central energy plant, we feel we have a unique learning and research opportunity. This will only expand in coming years.”
For more information, visit the Trane website at www.trane.com.
Publication date: 11/11/2002