In 2000, The University of Texas at Austin (UT) was paying about $2 per million British thermal units of natural gas, its primary fuel. By September 2006, the cost had risen to more than $14; over the next two to three years, it averaged about $8. In less than a decade, UT's annual gas budget had gone from about $9 million to approximately $36 million.

At the same time, the UT campus was experiencing significant growth. Over the span of 15 years, the area served by UT's combined-heat-and-power district energy plant had grown from approximately 13 million sq ft to nearly 20 million sq ft (Figure 1).

Looking to meet its growing cooling needs using less power, UT recently embarked on a district-cooling-optimization project that has seen gas-use quantities return to 1970s levels.

Central Utility Plant
With a peak load of 61,000 kw and total annual output of 350 million kwh, UT's central utility plant provides 100 percent of the power, steam, chilled water, deionized laboratory water, and compressed air for the 350-acre, 200-plus-building campus. The peak steam load is 220,000 lb per hour, with total annual output of 750 million lb and about 95 percent of condensate being returned. The cooling system consists of four central chilling stations with 45,000 tons of capacity tied into 6 miles of chilled-water-distribution-system piping. The four central chilling stations serve 135 buildings—more than 17 million sq ft—with a peak load of 35,000 tons and growing. Annual chilled-water production in 2008 was 143 million ton-hr.

The central utility plant is fueled with natural gas, with No. 2 diesel the emergency backup fuel. Natural gas is fired in a combustion-gas turbine capable of generating up to 45,000 kw. Heat in exhaust gas from the combustion turbine is captured in a heat-recovery steam generator that produces up to 288,000 lb of 425 psig/710°F steam per hour. A natural-gas-fired boiler tied into a high-pressure-steam (HPS) line is left at low fire to ensure continuous delivery of steam to the campus in the event the combustion-gas turbine trips offline. Steam from the HPS line drives a 27,000-kw, single-stage steam-turbine generator. Medium-pressure steam (MPS) is "extracted" from the steam-turbine generator at 155 psig and approximately 565°F. This MPS is distributed for various uses, including domestic water heating. It is used directly (process steam in laboratories) or converted to high-temperature hot water and used indirectly to heat buildings.

Compressed air is drawn from the final stage of the compressor in the combustion-gas turbine or air compressors located in the central utility plant and four chilling stations. Deionized water is provided from the central utility plant's boiler makeup-water system, which uses reverse-osmosis trains and demineralizing beds. This water is of sufficient quality for use in many of the laboratories across campus and the production of high-quality water in various special-use laboratories.

The central utility plant's electrical distribution grid is tied to the City of Austin through four 50-mva transformers. The university maintains a standby agreement with the city that virtually ensures nearly 100-percent reliability in campus electrical service.

The four chilling stations house 11 electric centrifugal chillers ranging in size from 3,000 tons to 5,000 tons. A 4-million-gal., 36,000-ton-hr thermal-energy-storage tank is being constructed to provide capacity and backup to ensure the campus is never without cooling. Approximately 33 percent of the central utility plant's output is consumed by the chilling stations. At their peak load of 35,000 tons, the chillers use 20,000 kw, and their associated auxiliaries (chilled-water pumps, condenser-water pumps, and cooling-tower fans) use more than 8,000 kw.

Despite differences in equipment, systems, pressures, temperatures, and distribution methods, the central utility system at UT is similar to the systems on hundreds of other university and industrial campuses throughout the United States.

Challenges
In the four chilling stations, both new and old technologies were used to optimize efficiency and operation. The challenge was integrating the four chilling stations and optimizing their combined operation. Over time:

• Attempts to save energy had resulted in frequent "hot calls" from building occupants, for which chilled-water operation was blamed.
• Cooling demand had increased as buildings in older areas of the campus were replaced or renovated. Management was concerned about the capacity of the existing infrastructure.
• Aging building mechanical systems had begun returning chilled water well below design, resulting in low-delta-T syndrome.