In some areas of the nation, up to 70% of energy demand at peak hours comes from air conditioning for residential and commercial units. The vast majority of those units — almost three-quarters — are refrigerant-based air conditioners. Lessening the burden on the nation's energy grid during those peak times is one of the Department of Energy's main concerns. It's also the principle on which one company bases its products.

Harvesting ice from lakes and rivers to use for food preservation and air conditioning is the oldest form of energy storage. The ice was collected in the winter and stored in insulated warehouses to be sold throughout the year. A technology from Ice Eenrgy, Windsor, Colo., is on a daily cycle, using standard air-conditioning units to freeze water in a tank during off-peak hours. Then, during the peak hours of the day, a small pump circulates the refrigerant through copper coils in the ice tank, replacing the traditional condensing unit. The system is contained in either a packaged rooftop unit or inside an air handler associated with a split-style system. The 550-gallon tank can deliver approximately 6 hours of cooling. The newest product is a 6-foot cube that weighs in at about 6,000 pounds when full. A typical fast-food restaurant would use two or three units.

According to Randy Zwetzig, Ice Energy's vice president of business development, since the majority of the electricity expenditure is during off-peak hours, the system may reduce the facility's energy use by 97%. A utility's peak period is between noon and 6 p.m., so most charge more for energy use during that time. “We're shifting energy use,” says Zwetzig. “Normally the air conditioner is running during those peak hours of the day, during the hottest part of the day, so it's less efficient. We replace that 10,000W air conditioner that's having to do a lot of work to cool the building with a small pump or two small pumps that consume 300W.”

Dry climates — where there is a larger differential between the daytime high and the nighttime low — is where systems like these are most efficient. Title 24 2005 Building Energy Efficiency Standards in California have actually put a limit on the amount of energy that can be used during peak hours. The technology doesn't burden the already-threatened water supply in those climates either. A closed system doesn't allow the water to evaporate as it melts: The temperature of the room — between 32°F and 40°F degrees — causes the evaporation to condense and literally rain back into the tank.