Two-stage absorption chillers reduce college's energy costs

COLLEGE STATION, TEX. - Established in 1876 as the first public college in Texas, Texas A&M University, located here, is a world leader in teaching, research, and public service with an enrollment of more than 43,000. Texas A&M's central plant provides heating, ventilating, and air-conditioning to the university's 700-acre main campus.

As part of an energy reduction project, Texas A&M replaced five 25-year-old absorption chillers that had outlived their useful life and required heavy maintenance. In 1995, the university installed four new 1500-ton Carrier 16JT double-effect absorption chillers, cutting its energy costs by a projected $1 million annually.

The University's facility planning division and the power plant chose the two-stage design in conjunction with Energy Engineering Associates (EEA), an Austin, Tex., based engineering firm, which conducted the studies for the energy reduction.

The chiller produces cost-effective cooling in many ways. First, only the controls and refrigeration/solution pumps require electricity. In addition, with its two-stage, double-effect design, each chiller incorporates both a high- and low-stage generator for two stages of solution reconcentration. The low-stage generator acts as the condenser for the high-stage generator, increasing unit efficiency by more than 70 percent over a single-stage unit. High-efficiency operation with a steam rate of 10 lb per hr per ton of cooling - as compared to the 18 lb per hr ton steam rates of the university's old absorption chillers - yields significantly lower operating costs.

The university's absorption chillers operate on steam from the central plant's gas-fired boilers and from a heat recovery boiler on a gas turbine, which drives a generator that produces 15 megawatts of electricity.

The plant is essentially a cogeneration plant, producing both electricity and thermal utilities.

The new units also use a lithium bromide and water absorption refrigeration cycle instead of chlorine-based refrigerants.

According to Emil Linnstaedter, system mechanical engineer, facilities planning division, the entire project cost approximately $8 million. The U.S. Dept. of Energy gave the university two matching grants totaling $1.5 million as a result of the chillers' high energy efficiencies.

Two new absorption units have been operating since last summer and two since last fall. While the university has not yet conducted final performance testing to determine actual efficiencies and energy savings, university officials believe the system is on target to meet projections. In addition, they report that faculty and staff are enjoying increased comfort in the main-campus buildings.

According to Ray Neff, systems manager, utility division, the new equipment is more reliable than the old chillers. During the summer months, when outdoor air temperatures exceed 100 F, the old units would often break down when they were overloaded, and he would receive complaints from students and faculty. Now, with the new chillers running continuously, there hasn't been a single complaint.