Aug. 1, 2010
The May 2010 article The Benefits of Ice-Based Thermal Energy Storage by Larry Clark, LEED AP, ( has generated interesting discussion

The May 2010 article “The Benefits of Ice-Based Thermal Energy Storage” by Larry Clark, LEED AP, ( has generated interesting discussion on

“p valenta” writes:

“Interesting article; I like the discussion. Think of a Hummer vs. a Prius. If the odometer of each vehicle starts at zero, and both cars travel 100 miles, you could say both cars went 100 miles. But the mileage does not indicate how efficiently the cars traveled. The same is true of the utility meter. A utility meter tells how many kilowatt-hours were used, but not how many source British thermal units were used to make and transport the electricity.

“I would like to clarify this statement, ‘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).’ This is a capacity reduction. I think the efficiency number is in the correct range for efficiency reduction for water-cooled equipment. For air-cooled, in many locations, the energy-efficiency ratio (EER) is the same for ice making as peak day cooling because of ambient relief.

“In this example, you can see why the distinction between capacity reduction and efficiency reduction is important. A water-cooled centrifugal (chiller) might be 1,000 tons nominal at 0.6 kw per ton (600 kw). Ice-making capacity at night for this chiller is in the range of 782 tons at 0.68 kw per ton (532 kw). It draws less overall kilowatt demand at night, even though it has reduced efficiency because its capacity is derated for lower operating temperatures. To further optimize the cooling system, the chiller can be right-sized, the delta-T enlarged to reduce flow, and medium-temperature air distributed, saving fan horsepower. Additionally, in a partial storage system, the chiller can be upstream of the ice storage, operating at warmer discharge temperatures, thus, improving its EER over a conventional cooling system's. In other words, the design of the system is critical to efficient operation. One cannot look, in my mind, at only the chiller and storage operation.

“A presentation by Florida Power & Light Co. during the American Society of Heating, Refrigerating and Air-Conditioning Engineers' 2008 Annual Meeting stated: Typically, cooling load is shifted from a 10,000-Btu-per-kilowatt-hour-heat-rate power generator to a combined-cycle generating plant that uses only 7,000 Btuh per kilowatt-hour. That reduces fossil-fuel usage and emissions a great deal.

“Because source energy is seen as an important measurement of overall efficiency and performance, California Code of Regulations, Title 24, applies hourly time-dependant values to each hour of consumption to tie power-plant heat rates into the energy code. In this way, a life-cycle-cost analysis of a building takes into account not only the kilowatt-hours on the meter, but the source energy consumed to make the electricity, based on location and hour of use.

“Storage of all types will be necessary to transform our fossil-fuel use to more and more dependable and renewable energy.”

In response, Clark wrote:

“Interesting analogy with the odometers, but I'm not clear on the basis of the conclusions. For example, you state that, ‘A utility meter tells how many kilowatt-hours were used, but not how many source British thermal units were used to make and transport the electricity.’ However, the source-site ratio for electric energy (which actually is a measure of the efficiency of that energy and is dependent on the fuel being burned and the equipment being used to burn it) has been established on a national level by the U.S. Environmental Protection Agency and the U.S. Department of Energy through their Energy Star program. For electric energy purchased off the grid, that factor presently is 3.34. So, the value in source British thermal units of a kilowatt-hour consumed is:

“1 kwh (site) × 3.413 × 103 Btu per kwh × 3.34 (source-site ratio) = 1.14 × 104 Btu

“The comments regarding the distinction between capacity reduction and efficiency reduction are interesting. As stated in the article, the chillers are compound (low stage and high stage), with 3,000-gpm-per-cell cooling towers — in other words, water-cooled centrifugal chillers, not air-cooled screw chillers. In ice-building mode, the input power to a chiller at rated capacity of 2,000 tons is approximately 1,700 kw, resulting in an efficiency of 0.85 kw per ton (1,700 kw divided by 2,000 tons). The same chiller in ‘day mode’ has input power of 1,458 kw at its full-load capacity of 2,300 tons, resulting in an efficiency of 0.634 kw per ton (1,458 kw divided by 2,300 tons). As you suggest, this does not address the efficiencies from the cooling towers seeing lower ambient air temperatures that should be achieved during ice building, which is presumed to be primarily at night. However, that is addressed early in the article.

“I am glad you agree with the need to explore ways to store energy and reduce our fossil-fuel consumption and dependence. It's not just conservation and stewardship; it's a vital national-defense imperative.”

“EnergyGod” writes:

“My firm makes ice by cooling 100 percent water to 29°F under pressure (water freezes at a lower temperature under pressure). We then release the sub-cooled water to an atmospheric (sealed) tank, where it forms a slush — the ice floats to the top, and the 32°F water goes to the bottom, where it is recycled.

“Because we are operating at a much higher temperature, cool night temperatures yield a compressor loading virtually equal to ‘normal’ daytime chiller performance, so no additional energy is required.

“Because we have made 32°F chilled water, a 28°F (32°F to 60°F), rather than a normal 16°F (44°F to 60°F), differential flow is pumped, requiring a 40-percent smaller pump and piping with 40-percent lower water flow and horsepower.

“Because the air-handling units (AHUs) see colder water, they make colder air (44°F vs. 55°F), which means they can be smaller and need less airflow.

“Because the air is colder, it also is drier, allowing the system to support space conditions of 80°F and 35-percent relative humidity (RH) (which has nearly the same enthalpy as 74°F and 50-percent RH). The warmer room allows even lower supply air (44°F to 80°F, or a 36°F differential, vs. 55°F to 74°F, or a 19°F differential), requiring 47 percent less air.

“The warmer room will see a 15°F delta-T to the outside at 95°F ambient vs. the 21°F delta-T with a 74°F room, cutting transmission heat gain by nearly 30 percent and requiring less air to meet the cooling need while reducing the total amount of cooling capacity needed to satisfy the spaces. This yields additional fan and chiller energy savings and additional reductions in environmental releases.

“Ice-based thermal-energy storage can yield lower energy usage and much greener overall site operation. You just have to think about how you use it and what type of system you use at each stage of the operation.”

About the Author

Scott Arnold | Executive Editor

Described by a colleague as "a cyborg ... requir(ing) virtually no sleep, no time off, and bland nourishment that can be consumed while at his desk" who was sent "back from the future not to terminate anyone, but with the prime directive 'to edit dry technical copy' in order to save the world at a later date," Scott Arnold joined the editorial staff of HPAC Engineering in 1999. Prior to that, he worked as an editor for daily newspapers and a specialty-publications company. He has a bachelor's degree in journalism from Kent State University.