©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 7, July 2013.
Blake Ellis, P.E., Member ASHRAE
About the Author
Blake Ellis, P.E., is a principal at Burns & McDonnell where he leads the OnSite Energy & Power group in the Kansas City, Mo. He is the program chair for ASHRAE Technical Committee 6.2, District Energy. Raymond Mosier, P.E., is an associate mechanical engineer at Burns & McDonnell in Kansas City, Mo.
Everything is big in Texas, including the Texas Medical Center in Houston (the largest medical center in North America) and the plant that provides its energy. Thermal Energy Corporation (TECO) was operating an 80,000 ton (281 360 kW) chilled water system in 2007 that served the Texas Medical Center. Planning efforts indicated cooling load demand would double to 160,000 tons (562 720 kW) over the next decade. An area just east of the existing central plant was identified as available for expansion. This area was less than half the size of the existing plant, but it was the only land available for expansion.
Analysis of several options suggested a two-step approach to meet the increased cooling load demand. The first step was to add 8.8 million gallons (33.3 million L) of thermal energy storage (TES), which became operational in January 2010. The second step was the addition of the new East Chilled Water Building (ECHB), which became operational in May 2011 with 32,000 tons (112 544 kW) of chilled water production, bringing the system total to 120,000 tons (422 040 kW) of production. The ECHB has provisions for an ultimate production capacity of 80,000 tons (281 360 kW).
Modern chilled water plant design philosophy uses variable speed drives (VSDs) for the major pieces of equipment such as chillers, cooling tower fans and pumps to optimize energy efficiency. However, at the time this project was being designed, VSDs had never been applied to the 8,000 ton (28 136 kW) chillers required for this project.
Several chiller options were analyzed including constant speed 5 kV and 15 kV motors and VSDs serving 5 kV motors. After several iterations, the configuration selected was to use VSDs with a 15 kV input to reduce the electric cable costs and a 5 kV output to drive the 7,000 hp (5220 kW) motors on each chiller. VSDs were also used on the 24,000 gpm (1514 L/s) chilled water pumps and the cooling tower fans. In addition to the VSDs, the efficiency of the new chillers was 0.094 kW/ton (0.027 kW/kW) lower than the existing chillers.
The result of the all variable speed chilled water plant design was an average reduction in energy use of 3.4 MW during the first year of operation. Figure 1 shows the energy savings in kW for every hour from August 2011 through July 2012. During this period, a total of 303 million ton-hours of chilled water was produced with the new chillers saving a total of 26.1 GWh of electricity. This is an average decrease of 12.7% in annual energy use.
The project site is extremely compact. (Figure 2) Consequently, a project goal was established to maximize the potential chilled water production capacity on the space-constrained site. This resulted in the cooling towers being located on the roof of the plant to provide the most compact arrangement.
Initial desires were to locate the condenser water basins at grade; however, given the condenser water pump energy requirement, locating the basins as high as possible was desired to reduce the required net static head on the pumps. The final location saved nearly 3 MW of condenser water pumping energy versus condenser water basins located at grade. The savings are nearly equal to the energy saved by the variable speed chiller plant concept.
Maximizing the amount of chilled water production per square foot of the project site was a project goal because TECO did not have additional property on which to expand. Capacity of the chilled water plant was constrained by the footprint of the cooling towers on the roof. Several plant arrangements were studied, but single inlet counter flow cooling towers with a 15°F (8.3°C) range for the condenser water temperature provided the highest heat rejection per square foot of tower footprint while using only slightly more energy (2%) than the most efficient cooling towers.
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