First cost savings can pay for equipment

TES systems can reduce first costs in a number of ways. For starters, they allow chillers or packaged air conditioners to be downsized. Consider this example: If a building's cooling load is 100 tons, during times of peak afternoon cooling requirements in the summer it will take as much as 100 tons of cooling to maintain comfort. (We still use "ton" as a measure of cooling-it is the rate of cooling provided by one ton, 2,000 pounds, of ice as it melts over a 24-hour day; in modern terminology, one ton is equivalent to 12,000 Btu/hour of cooling capacity.) Without energy storage, that building would need a 100-ton chiller to meet that peak demand. With energy storage, however, ice is available to meet that peak load so the chiller's work can be spread throughout the day and night, meaning that a much smaller chiller can provide the building's cooling needs. Cooling tonnage can often be reduced by 35 to 50 percent, according to Mark MacCracken, president of CALMAC Manufacturing, yielding cost savings that more than offset the cost of adding an ice-storage system.

With a full-storage system, enough ice is created to meet the building's entire daytime load, avoiding higher electric rates. By installing a partial-storage TES system (a more common approach), the ice supplements the chiller's output during the day, which provides less of a reduction in electricity costs but allows the chiller-as well as the ice storage system-to be downsized.

Data: Ice Energy

 

Along with downsizing chillers with TES, further first-cost savings are often realized by decreasing the size of ducts, pumps, fans, cooling towers, and power supply infrastructure. The smaller ducting is possible in part because ice-based TES systems typically deliver cooler air. In rare situations, these reduced duct sizes can even reduce floor-to-floor height. The Cool Storage Technology Guide, published in 2000 by the Electric Power Research Institute (EPRI), suggests that "floor-to- floor height can be reduced by 4 inches when the supply air temperature is reduced from 55 degrees to 45 degrees Fahrenheit," as is typical with ice-based TES systems. This height reduction, in turn, reduces structural and envelope costs, according to EPRI, and can cut total construction cost by 3 percent.

Even without reductions in floor-to- floor height, it is not uncommon for the first-cost savings in chillers and other equipment to entirely pay for the TES system, according to MacCracken, so that the operating cost savings are achieved at very little-or even zero-additional upfront cost. In retrofit applications, the cost of TES is higher, but the payback of that additional investment will be relatively short-three to seven years, claims MacCracken.

Minimizing air pollution

In some places, shifting cooling loads to off-peak electricity results in less pollution. If baseload power generation is primarily hydro or nuclear and peaking plants are fossil-fuel powered, reducing peak demand reduces pollution. In a study of the environmental impacts of thermal energy storage, the California Energy Commission (CEC), found that shifting cooling loads from peak to off-peak hours in the Pacific Gas & Electric territory reduces pollution emissions by 47 percent.

Where peaking power is provided with relatively new gas turbines or hydropower from reservoirs (where the flow can be readily controlled) and coal-fired power plants provide baseload power, shifting loads to off-peak hours might not reduce emissions. "Emission characteristics will be utility dependent," says Reindl.

Another factor in the environmental benefits of TES is the conversion of pollution emissions into smog. Nitrous oxide (NOx) emissions from a given fossil-fuel-fired power plant will result in less smog at night than during the day because smog production is a photoreactive process-catalyzed by sunlight. A 2005 report conducted for Ice Energy by E3 Ventures of Boulder, Colorado, showed that NOx emissions in the Sacramento Municipal Utility District (SMUD) are 56 percent lower during off-peak hours than during peak hours, due both to the shift in generation source and the absence of sunlight.