Is Your Building a Candidate for Thermal Energy Storage?

Do you know your cost for on-peak cooling? Chances are it’s at least double your off-peak rates. Using technologies that leverage this difference can ease the burden on your pocketbook.

Thermal energy storage (TES) is a method by which energy is produced and stored at one time period for use during a different time period. The most common application in the U.S. is for cooling and entails making chilled water or ice during off-peak times (at night) and storing it for use during peak demand (the next day). By shifting energy use, a thermal storage system can lower operation and maintenance costs and reduce equipment size.

Building type, space available for storage, and existing equipment status will affect implementation of TES. Thermal storage for off-peak cooling is a proven, simple, and practical solution to rising energy costs, but can it work for you? Let these factors help you decide if the time is right for thermal storage.

Where To Begin
If your application is for a new construction project, TES can be considered from the very beginning. If you’re evaluating thermal storage for a retrofit, other triggering events will make the project more feasible and provide a more attractive payback period.

Start by studying your electric rates. “Utility bills can be confusing. Be aware of the peak demand charge,” advises Mark M. MacCracken, president and CEO of CALMAC Manufacturing, a provider of thermal energy systems. “Every rate I’ve seen for commercial buildings turns out to be 50-60% less at night, and that may not be evident at first glance. You have to study it – or have a consultant study it and give it back to you in English.”

The electricity you buy during the hottest part of the day can cost nearly three times as much as the power you could buy in the middle of the night, says John S. Andrepont, president and founder of thermal storage consulting firm The Cool Solutions Company.

“Usually, you will get at least an annual savings of $100-150 for every kilowatt of peak demand reduction,” Andrepont explains. Because TES can save thousands of kilowatts, there is potential to save hundreds of thousands of dollars.

Some local electric utilities pay one-time cash incentives to customers that install and operate TES equipment – usually a fixed amount of several hundred dollars per kilowatt of load reduction during a defined on-peak period of several hours per day, Andrepont says.

“In the most attractive cases, TES can be economically justified even as a pure retrofit,” adds Andrepont. “But TES economics are always better during times when a TES investment offsets an otherwise necessary investment in conventional chiller plant capacity – whether that’s new construction, retrofit expansion, or retirement of existing chillers.”

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New Construction
A TES system takes the place of conventional chiller capacity, so payback is almost immediate. The Chrysler Tech Center in Auburn Hills, MI, had a 16,000-ton peak load, so a conventional 17,700-ton chiller plant was specified to meet that and maintain a little extra capacity for emergencies.

With chilled water TES implementation, conventional chillers only need to account for average load plus spare (about 11,400 tons) since storage will account for the difference. Thus, initial investment is reduced. “Chrysler had capital savings of $3.6 million,” Andrepont says. “They also save about $1 million annually in energy.”

“Cost is basically a wash for new construction,” MacCracken agrees. “But for retrofits, some other event should be helping to drive the decision.”

Expansion and Replacement
Are you planning to expand your current facility? Consider investing in TES to account for growth in cooling load instead of buying another chiller. “If you are about to start spending money on your cooling system, see if redesigning with a storage system could save you a lot of money,” MacCracken advises.

Another prime opportunity to consider TES is when you’re facing the retirement of existing chillers. The usual life expectancy of a chiller is considered 20-25 years, but the economic life expectancy can be much less, Andrepont says. Perhaps your chillers are becoming less reliable. Maybe maintenance costs are increasing. Also consider if your chillers are working too often or too hard, indicating poor energy performance.

“If, for whatever reason, your chillers need replacing, that’s a good opportunity to downsize those chillers and put some storage tanks in,” MacCracken says. In most applications, the chiller system can be downsized by 30-50% when storage is entered into the equation because conventional chillers will only need to account for average load as opposed to peak.

When you’re facing a necessary upgrade in chiller capacity, it’s easy to see how thermal storage is comparable to the initial investment of adding on conventional equipment. When you factor in the operational savings from off-peak rates, thermal storage systems look even more favorable.

“On average, the goal is to shift 30-40% of the HVAC peak load, but it’s not uncommon to shift up to 50%,” Andrepont says. “The investment you avoid in conventional equipment offsets the cost of storage so you’re looking at an immediate payback, or a very rapid one when you factor in operational savings.”

Once you’ve crunched the numbers of your current situation, determine which type of TES is right for you.

Chilled Water vs. Ice
Generally speaking, chilled water systems are best used in large applications. They can serve industrial facilities, large airports, and multi-building networks like hospitals or campuses. These sites generally have space for the large tank that is usually customized for the application.

Ice storage systems are factory-manufactured, so small, modular equipment has traditionally dominated smaller, individual building applications. There are exceptions, however, with some large ice and a few small chilled water systems being appropriate and successful choices in certain instances, Andrepont says.

Applications for each type range from elementary schools to corporate campuses. A typical school might use a system as small as 1,000-2,000 ton-hours, which would require a 100,000- to 200,000-gallon chilled water storage tank. At an installation cost of $2-3 per gallon for this size ($500,000 for a 250,000-gallon tank), this is at the unfavorable side of the economy of scale, Andrepont says. But even without factoring in if an upgrade is necessary, a system like this would save about $60,000 per year, he adds, and generate a 5- to 10-year payback.

Rockefeller Center in New York City installed 41 CALMAC ice tanks that together produce 8,600 ton-hours of storage capacity and address inefficient staging on its four-chiller system. They had anticipated $500,000 in annual savings and at least a five-year payback, but the retrofit saves in excess of $1 million per year and shaved payback down to two or three years, MacCracken says.

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Testing for Results
Once you’ve implemented TES at your facility, use ASHRAE Standard 150, “Method of Testing the Performance of Cool Storage Systems,” to analyze its success. “The Standard 150 test is the functional performance test at the end of the project that evaluates if the project as a whole achieved the owner’s requirements,” explains Lucas B. Hyman, president of Goss Engineering, Inc., in Engineered Systems.

To meet requirements, the following tests are performed: discharge, charge, cool storage capacity, and cool storage system efficiency. Some of the key data documented includes tank capacity, heat gain, and load profile achievement. All figures and calculations are gathered in a report to verify the performance of your project.

Win-Win-Win
If a motivating factor is present, consider thermal storage right now, Andrepont advises. “As electric rates begin to more closely match the true cost of generating power, which varies so much from day to night, the time is right. Thermal storage is becoming more attractive both to customers and utilities,” he says.

Thermal storage users clearly benefit by paying a fraction of their typical electricity bill, but there are far-reaching benefits for other parties.

“The utility also wins because you’re shifting loads to nighttime when they have idled equipment as opposed to when they’re completely strained during the hot part of the day,” MacCracken explains. “They utilize their equipment more effectively because the peak-load plants that have to be turned on during the day are usually dirtier and less energy-efficient. This also saves fuel and reduces emissions, so the environment wins too.”

As more projects come online and the industry continues to grow, as Andrepont forecasts, assess your situation and determine if TES can work for you.

“If bags of ice were on sale at the corner store last night for 50% off, would you start making ice for a party when people walk through the door? It’s kind of laughable, but that’s what we do with air conditioning systems,” MacCracken says. “It makes no sense to have your equipment idle at night when it could be taking advantage of off-peak rates.”

Chris Curtland ([email protected]) is assistant editor of BUILDINGS.