CHICAGO - Citing nearly five years of reliable, trouble-free operation, a federal project officer has identified this system as his agency's most successful thermal storage project to date. The system stores cool energy in the form of ice during the night when the price of electricity is low. The ice is used the next day to provide air conditioning and to act as emergency back-up cooling for computers at the Harold Washington Building on Madison St. in Chicago.

The most unique feature of this system is that existing water storage tanks were converted to ice thermal storage. Significant cost savings were achieved by altering four 8,400 gallon tanks measuring 6.5 feet in diameter and 36 feet long.

Midwesco Mechanical fitted the existing steel tanks with internal distribution piping and then loaded the tanks with Ice Balls manufactured by Cryogel of San Diego, CA (

By converting to ice, the system is capable of 1,960 ton hours of storage capacity as compared to about 400 ton hours as would be possible from chilled water storage. Ice is produced over a period of 8 hours during off-peak night hours. Chillers operating at a capacity of 250 tons, cool Wintrex heat transfer fluid supplied by Houghton Chemical of Boston to an average temperature of 22.2 degrees F. That cold fluid is then circulated through the storage tanks and Ice Balls to make ice and store energy until it is needed during the afternoon when air conditioning loads peak.

Bill Davis, P.E., Midwesco project manager, noted, "Thermal storage allowed us to eliminate two 25-ton chillers and reduce maintenance costs by $4,000 per year while maintaining the cooling redundancy required by the government".

"Retrofifting existing tanks and upgrading the storage capacity of chilled water systems is an increasingly popular approach to thermal storage because of the obvious cost savings on tanks," according to Cryogel sales manager Bruce McDavid. McDavid points out that, "In the case of chilled water thermal storage, a relatively simple retrofit to ice allows for storage capacities to be increased by as much as four to five times."

Ice Ball is a trade name of Cryogel, San Diego, CA. For more information, visit

Publication date: 08/13/2001