Making ice at night to cool buildings
July 4, 2009
Calmac IceBank tanks at One Bryant Park, one of the nation's greenest high-rise buildings.
Photo: © Gunther Intelmann for Cook+Fox ArchitectsWhat surprised me most in researching thermal energy storage for the EBN feature article this month is that it's not incorporated into virtually all commercial buildings. In a nutshell, the idea is to use electricity at night to make ice and then use that ice during the daytime as the cooling source for the building. Thermal energy storage (TES) can also involve chilled water (instead of ice) or electric heat stored in bricks or other thermal mass, but I focused on ice with this article. A number of very well-known green buildings rely on ice-based TES cooling. One of the newest such buildings is the 2.1-million square-foot (195,000 m2) Bank of America building in New York City at One Bryant Park. I visited the sub-basement (three floors down) to see the 44 eight-foot-diameter, insulated CALMAC tanks in the building that collectively provide about a quarter of the building's cooling. Each of these tanks holds about 1,600 gallons of water that is alternately frozen and thawed by circulating a glycol solution through about three miles of plastic tubing. It's high-tech, but the result is surprisingly simple. Benefits of ice-based TES include the following:
- Saving money by using less expensive off-peak electricity for cooling. Most utility companies offer less expensive off-peak electricity rates for commercial and industrial customers.
- Saving money by reducing electric demand charges. Demand charges are based on the peak electricity consumption of a building. By shifting the operation of energy-intensive chillers or compression-cycle air conditioners from daytime (when electricity consumption in commercial buildings is highest) to nighttime, peak demand can be significantly reduced.
- First-cost savings can often be achieved by downsizing chillers, pumps, ducts, and other components. In some cases, floor-to-floor height can also be reduced, because smaller ducts are used, resulting in dramatic savings.
- Even though there is an efficiency loss with any heat-exchange process, a lot of the losses inherent in ice-based TES can be offset by higher efficiency that results from operating the chiller or A/C system continuously at night (eliminating the on-off cycling) and by operating the equipment with cooler nighttime air temperatures.
- Reduced pollution emissions? This depends on where the building is located and how the local utility company generates power during on-peak and off-peak periods. If the baseload generation is hydropower and nuclear and peaking plants are natural gas or oil, minimizing peak electricity use can significantly reduce emissions — but with baseload coal plants and peaking hydropower or cleaner-burning natural-gas plants, the opposite can be true.
- Using off-peak electricity for cooling will allow us to benefit from wind power and other renewable electricity sources. When the wind is blowing isn't always when we need power. That's fine if wind energy is only providing a few percent of our electricity, but if that fraction grows to 20% or more, it could be a problem. Shifting cooling loads to nighttime hours is an important way to help us benefit from wind power.