4) Thermal Energy Storage

June 1, 2008

Explore thermal energy storage as a new HVAC possibility

What it is:
Thermal energy storage (TES) systems shift the creation of cooling to off-peak hours. "TES systems operate chillers in the middle of the night, when electric rates are cheap, and generate either chilled water that gets stored in a large tank, or actually make ice," explains Erik Ring, director of MEP services at Irvine, CA-based LPA Inc. Then, that stored thermal energy is used for cooling during daytime hours, cooling your building without operating chillers.

How it works:
As David Callan, senior vice president, director of sustainable design and high-performance building technology, Syska Hennessy Group, Chicago, explains, with TES, you use electricity to run your chiller in the evening, and the system turns out very cold water (water mixed with glycol or antifreeze). You either store that chilled water or make ice. "During the day, your thermal ice-storage system goes into a melt cycle, where you circulate water through the building, and that water passes through the ice tanks. Then, the heat from the building goes into the ice tanks and melts the ice," says Callan.


  • TES may not help you save energy, but it can help the environment. "For peak demand, coal is usually the source because it's quick: turn the switch and it's going," says Richard Williams, VP, architecture and sustainability, at HOK, Toronto. "If you load shift, you help reduce dependency on coal."
  • TES systems make a difference in terms of dollar savings. "Most commercial customers are charged not only for the amount of energy they use, but also for the peak amount of energy they demand. By taking this large energy load out of the peak period of the day and shifting it to the nighttime period, there can be substantial utility bill savings," says Ring. "In some places, the local utility provides really attractive rates to customers who put this type of system in." As Greg Allen, sustainable design strategist at HOK, Toronto, points out, the rate differential often pays for the technology investment.


  • Maintenance is more difficult than with traditional HVAC systems. "The ice systems tend to be trickier because you're operating equipment at lower temperatures," says Ring.
  • Space constraints can be an issue. Do you have enough room to store the tanks? "A chiller is a piece of machinery; it sits in the basement or on a roof, and it's only as big as it needs to be. Now, imagine that you've got these huge tanks to store all this ice or chilled water. They could be stored underground, they can be stored in the basement, but you're not going to put it up on the roof because it's very heavy. These systems take up square footage," says Rob Bolin, senior vice president, Syska Hennessy Group, Chicago.
  • TES systems really aren't green. As Callan points out, the efficiency of a chiller depends on how cold you make the water. "Making ice at night actually uses more kilowatt-hours than you would need during the day because it's a less efficient process," he explains. "It's not an energy-saving measure or a carbon-footprint-saving measure. It's an energy-cost-saving measure."

Where it works:
You can use these systems just about anywhere; climate doesn't matter. What does matter: the demand charges in your area. "Anywhere where the demand is higher than $20 per kilowatt-hour, off-peak should be evaluated. When the demand charges are less, it can still be evaluated, but it won't pay back quickly," says Callan. Obviously, if your utility offers incentives for reducing peak load, that's a plus.

Something else that matters: the size of your facility. "Larger buildings and campuses that have significant on-peak demand charges are the best candidates for thermal energy storage systems. There has been a push to apply the same concepts to smaller and smaller buildings, but, at this point, TES is still most common in larger facilities," says Ring. Large office buildings, convention centers, and airports are also possibilities. And, if your building is big enough, the rule of thumb mentioned by Callan may not apply—you could still see some economic benefits from TES.

Bolin also emphasizes that TES works well in owner-occupied buildings. "They're going to occupy it and they're going to pay the utility rates, so there's incentive for them to build additional square feet to house these things, or pay the additional cost to bury the tanks, because they're going to pay the bills. Since owner-occupiers are in the building for the long haul, the additional capital costs can be amortized over longer periods."

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