Thermal Storage Retrofit Takes a Byte Out of Computer Cooling Demands
The 10-story, 790,000-sq-ft building was built with electricity driving its hvac system. An electric heat pump system used the interior heat from the building to heat its skin, or the exterior 12 ft of the building, and electric boilers augmented the heat pumping cycle when outside temperatures dropped below 20 degrees F.
Tim Corbett, U.S. Department of Health and Human Services project manager, says the system had become inefficient from both ventilation and financial perspectives. The building's 2,000 full-time employees logged frequent temperature complaints. Additionally, the system had originally been designed to capture heat from the building's light fixtures. And while the system had been upgraded in 1980 to provide more heat, it was still exceedingly expensive to run; in order to keep the building warm on cold Chicago winter nights, the lights had to be left on.
Appeasing '5,000 Bosses'When it was time to put a new system in place, an Energy Savings Performance Contract (ESPC) was initiated, with FH Paschen and Midwesco Mechanical and Energy working in a joint venture on the contract. Bill Davis, P.E., Midwesco project manager, says the project presented plenty of challenges. All of the work was done in phases and the bulk of it was done at night when the building was unoccupied. Communication was one of the biggest keys to the job, he adds.
"We had meetings throughout the project where we'd bring in people and conduct partnering sessions with the local and federal government people and all the engineers," he says, adding that a quarterly newsletter about the project was also distributed to tenants. "Dealing with a building that was occupied, we had what seemed like five thousand different bosses, so we used a coordinator who met with the occupants daily to make sure it was all going smoothly," Davis says.
Freezing CostsThe idea to use a thermal storage system was not a stretch at all, says Corbett, since the building's four 8,400-gal water tanks (that are 6.5 ft in diameter and 36 ft long) could be converted to accommodate ice. Midwesco fitted the existing steel tanks with internal distribution piping and then loaded the tanks with Ice Balls manufactured by Cryogel (San Diego).
The system now in place 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 backup cooling for the building's computers. By converting to ice, the system is capable of 1,960 ton-hrs of storage capacity as compared to about 400 ton-hrs that would be possible from chilled water storage.
Ice is produced over a period of eight hrs during off-peak night hours. Chillers operating at a capacity of 250 tons cool Wintrex heat transfer fluid supplied by Houghton Chemical (Boston) to an average temperature of 22.2 degrees. That frigid 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.
Davis says that thermal storage allowed the elimination of four 20-ton chillers that had served the building's computer room facility. "It also allowed us to reduce operating costs by $8,000 per year, while maintaining the cooling redundancy required by the government." Additionally, since the lighting system was upgraded and is not relied upon for heating, there have been tremendous savings in that area.
"Retrofitting 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. He 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." From his perspective, Corbett identifies the building's system as his agency's most successful thermal storage project to date. ES