Page 2 of 2

Nova Southeastern University. A new central energy plant at the main campus of Nova Southeastern University (NSU) in South Florida includes ice-based TES. With more than 28,000 students, NSU is the sixth-largest independent not-for-profit university in the United States. The new plant is expected to serve the entire 300-acre Fort Lauderdale-Davie campus. The first phase, which went online in November 2009, included two 2,300-ton compound chillers and 30 ice coils in field-erected concrete tanks rated for 17,000 ton-hr of capacity. When completed, the plant will have a total capacity of 11,500 tons of chilled water with 68,000 ton-hr of TES, making it one of the largest TES systems in the United States.8

When in ice-building mode, the design-efficiency penalty for each chiller is approximately 13 percent (2,300 tons derated by 13 percent equals 2,000 tons). Based on a peak-to-off-peak differential of 1.6 (calculated from actual electric energy billing), the economics appear viable. However, unless the local utility's off-peak power-generation emissions are 13-percent less than their on-peak levels, the process is not truly sustainable. According to the local utility, Florida Power & Light, only 8 percent of its current generating capacity is fueled by oil and coal; the remaining 92 percent is from natural-gas, nuclear, and renewable sources, resulting in a CO2 emissions rate that is 59 percent of the U.S. industry average.9 While laudable, this means that the best case for peak-to-off-peak emissions reduction is limited to 8 percent, which by itself is not sufficient to offset the efficiency penalty.

Fortunately, the project's anticipated overall efficiency improvements are more than what is needed to offset the 5-percent increase in source CO2 emissions. The improvements are a result of the reduction in cooling-tower-fan horsepower (two installed two-cell, 3,000-gpm-per-cell cooling towers were designed for the capacity requirements of all of the chiller/TES-system operation modes), the ability to optimize performance by using any or all of the chillers in any mode (chiller, charge, or discharge/off), and the means to vary the chilled-water/brine-solution temperature from 18˚F to 38˚F, depending on mode and load.

Because we are more concerned with sustainability than economics, first cost is not addressed in this analysis. However, one would hope that good design practices would include some degree of equipment diversity that would help offset the expected increase in construction costs. And because simple payback periods based solely on avoided energy costs would be long, this type of project could be a good candidate for a return-on-investment analysis using a capital-recovery-factor-based cost-to-benefit ratio.10

Conclusion
A relatively simple analysis of only two installations cannot definitively address the overarching question of whether ice-based TES represents a sustainable solution. However, it can—and does—confirm that these systems usually have the potential to reduce energy costs and, on a case-by-case basis (depending primarily on plant design and the local utility's infrastructure), may be truly green.

Going forward, the impact of renewable energy on the sustainability of these types of solutions probably should be addressed in some detail. If, in either of the two examples given, the electric energy required for chiller-plant operation had been provided—or augmented—by on-site generation using a renewable-energy source, such as solar or wind, project sustainability could have been demonstrated easily. In that case, the emphasis would have been on optimizing the availability of the energy source—either sunlight or wind—rather than on shifting load from peak to off-peak hours of operation. In that instance, the model theoretically could make ice during the day and melt it at night.

References
1) Ice thermal storage. (n.d.) Retrieved from http://www.baltimoreaircoil.com/english/products/ice/tsum/index.html
2) Thermal energy storage benefits. (n.d.) Retrieved from http://www.calmac.com/benefits/
3) Wiens, J. (2008, June). Thermal energy storage. Buildings.
4) California Energy Commission. (1996). Source energy and environmental impacts of thermal energy storage. Sacramento, CA.: California Energy Commission.
5) Henze, G., Krarti, M., & Brandemuehl, M. (2003, February). Guidelines for improved performance of ice storage systems. Energy and Buildings, pp. 111-127.
6) Xu, P., Huang, J., Jin, R., & Yang, G. (2007, January). Measured energy performance of a US-China demonstration energy-efficient office building. ASHRAE Transactions, 113, 56-64.
7) U.S. Environmental Protection Agency. (2009). U.S. annual non-baseload CO2 output emission rate: Year 2005 data. Washington, DC: U.S. Environmental Protection Agency.
8) Hill York. (2009). Building a green-er NSU. Fort Lauderdale, FL: Hill York.
9) FPL Group Inc. (2009). Sustainability report 2009. Juno Beach, FL: FPL Group Co.
10) Clark, L. (2009, January). Engineering economics goes green. Engineered Systems, pp. 82-84.
11) Struble, L., & Godfrey, J. How sustainable is concrete? (n.d.) Retrieved from www.cptechcenter.org/publications/sustainable/strublesustainable.pdf

Did you find this article useful? Send comments and suggestions to Associate Editor Megan White at megan.white@penton.com.

Larry Clark, LEED AP, is director of corporate business development for air-conditioning contractor Hill York. Previously, he served as president, vice president of sales and marketing, and regional sales manager for MEPCO and regional sales manager for Vapor Power.

Construction Consumes Energy

It should be noted that this article does not attempt to address the embodied energy associated with the construction of ice-storage tanks. When fully built-out, the NSU plant will have four concrete tanks each measuring approximately 106 ft by 65 ft by 42 ft. That represents a significant amount of concrete using cement that requires nearly 5 mj per kilogram to produce (approximately 2,150 Btu per pound), which does not include the energy required to manufacture the rebar that also must be incorporated.11