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With a peak electrical load for chilled-water production of nearly 30,000 kw and annual consumption of nearly 110 million kwh, a significant opportunity for cost savings existed. Figure 2 shows total campus plant efficiency from 2000 through 2010. The first large dip, in 2009, occurred when steam chillers were eliminated from operation, while the second dip, in 2010, was the result of the addition of a new chilling station, Station 6.

New Chilling Station
The $39.25 million Station 6 was funded to make way for a new computer-science building. Station 2 was to be demolished. Plans called for the replacement of the 7,800 tons of steam-driven centrifugal chillers in Station 2 with new electricity-driven centrifugal chillers and the installation of a 3,000-ton inlet air coil on the combustion-gas turbine to boost summer capacity from 36 Mw to 45 Mw. Based on an extensive 30-year life-cycle-cost analysis, a 15,000-ton all-variable-speed system was selected.

Station 6 incorporates:

• 15,000 tons of cooling capacity.
• A primary-only all-variable-speed system.
• Three 5,000-ton variable-speed electric chillers with 39°F chilled-water design.
• Three variable-speed condenser-water pumps (15,000 gpm, 110 ft head, 500 hp).
• Three variable-speed chilled-water pumps (10,000 gpm, 250 ft head, 800 hp).
• Three variable-speed counterflow cooling-tower cells (15,000 gpm each, 250-hp fans, 85°F to 95°F, 78°F wet bulb).
• A programmable-logic control system.

Figure 3 shows the efficiency difference between a 5,000-ton constant-speed chiller and a 5,000-ton variable-speed chiller with entering condenser-water temperature (ECWT) ranging from 55°F to 85°F and load ranging from 50 percent to 100 percent. Note that the only point at which the constant-speed chiller is more efficient than the variable-speed chiller is at design (100 percent capacity and 85°F ECWT).

To operate the four chilling stations at the lowest kilowatts-per-ton level possible, relational controls based on the Equal Marginal Performance Principle1 were selected. Relational-control algorithms use mathematical relationships between subsystems.2 At UT, the algorithms took full advantage of the existing supervisory-control-and-data-acquisition system integrating all of the systems and equipment in the chilling stations. Through that integration, UT has been able to optimize system operation without compromising safety or reliability. Figure 4 shows the total efficiency of Station 6 based on outdoor wet-bulb temperature and given plant capacities. The typical annual wire-to-water performance range is 0.33 to 0.78 kw per ton.

Today, UT's on-site utility system self-generates 100 percent of needed electricity, steam, and chilled water using the same amount of power the university consumed in 1977, despite the addition of 8 million sq ft. The efficiency improvements have added value to the campus, enabled self-funded improvements, and provided significant emissions reductions (Figure 5).

References
1) Hartman, T. (2005). Designing efficient systems with the equal marginal performance principle. ASHRAE Journal, 47, 64-70.
2) Erpelding, B. (2009, Fourth Quarter). How high can you go? New approaches for cooling system efficiency. District Energy, pp. 13-16. Available at http://bit.ly/8wbxP5

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A Bit of History

From its founding in 1883 until 1928, The University of Texas at Austin (UT) used power from the City of Austin and coal-fired boilers in each of its buildings for heating. In 1928, a central heating plant was constructed.

At UT, as on most large university and industrial campuses throughout the United States, demand for mechanical cooling of buildings did not emerge until the 1940s, when owners took one of two considerably different paths:

• Construct a central cooling plant at or near the central heating plant and use the steam-production equipment that provided heating during winter to drive cooling during summer. Many of the first central cooling plants contained steam-turbine-driven centrifugal chillers or steam absorption chillers.
• Install a chiller in each building, using the existing steam supply as the prime mover. Because steam typically was delivered to buildings at a pressure lower than that at central heating plants, low-pressure steam absorption chillers frequently were chosen for building cooling. This type of system required chilled-water pumps, condenser-water pumps, and cooling towers at each building.

In the ensuing years, the demand for comfort cooling exploded, and those who had chosen the central-cooling-plant approach found themselves rapidly adding capacity to their original central plants and building additional plants in other locations on campus, a highly capital-intensive proposition.

Around 1980, chillers purchased during the 1940s and '50s were reaching the end of their useful life, and chiller-maintenance needs rapidly increased, as did chiller outages, and the replacement of chillers became a high priority for owners. Finally, the wisdom of the central-plant concept was realized. When a chiller in a central plant fails, it still is possible on all but the hottest design day to provide cooling; when a chiller in a building fails, that building is without cooling until the chiller can be repaired or replaced. At the same time, another variable—energy costs—was entering the equation. The energy required to cool 15 buildings each with a load of 500 tons using 15 individual 500-ton chillers and associated equipment was far greater than the energy required to serve those same 15 buildings with a central cooling plant that might contain three 3,000-ton chillers. The central cooling plant, as with the central heating plant, also provided a much higher degree of reliability and reduced maintenance.

During the 1980s, those who had chosen the one-building, one-chiller approach began to run chilled-water piping between buildings and replace failing chillers with larger chillers, creating a "distributed chiller plant." While that did not provide all of the advantages of a central plant, it was a significant step forward in terms of reduced energy, reduced maintenance, and higher reliability.

A minority of one-building, one-chiller owners decided to "bite the bullet" and construct central plants serving all or part of their campuses, quite probably arguing that the reduced operation and maintenance costs, as well as the higher reliability, of a central plant would offset the higher capital investment required to "loop" together building chillers.

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