Is ice-based thermal-energy storage (TES) really green? Although the success of ice-based TES in reducing energy costs has been substantiated, there are differing opinions within the HVAC community regarding whether TES is a green technology. As one would expect, manufacturers of related equipment generally claim that ice-based TES reduces emissions, while outside authorities have disagreed on record.1,2,3 Because TES often can offer heat-recovery opportunities—such as for domestic water heating—its environmental friendliness is a legitimate concern. So, is ice-based TES economically and environmentally justifiable? How can we determine if the benefits gained by utilizing ice-based or any other type of TES are worth the resources used?

Electric Chillers and TES
Electric chillers have been utilized for ice-based TES since the 1980s. This method can decrease costs by shifting electric energy consumption from higher-priced peak rates to less-costly off-peak (predominantly nighttime) rates. Chilled water is used to produce ice during off-peak periods, which is melted to absorb heat in conditioned spaces during peak periods, thus reducing peak chiller loads. This method also may reduce first costs in new construction by allowing equipment downsizing.

But is ice-based TES really a green/sustainable solution? On one hand, it consumes more energy than conventional chilled-water cooling. And because the sub-freezing temperatures required for ice production are lower than those generally used for space cooling (typically 38˚F to 45˚F), the chillers may be operating at a lower mechanical efficiency. Because frequent cycling reduces overall efficiency, higher energy consumption can be offset at least partially if the chillers are operating continuously at a constant load. It often is presumed that cooling towers experience lower air temperatures at night and that decreased pumping horsepower—as well as more efficient condenser-water cooling—help balance chiller-efficiency losses because of lower flows resulting from a wider temperature range. With a higher delta-T across the cooling coil, fewer rows and, therefore, a lower pressure drop and decreased fan-motor horsepower would be expected. Finally, off-peak-produced power is arguably "cleaner."

The California Energy Commission concluded that a reduction in source fuel typically results in a reduction of the greenhouse-gas emissions produced by a power plant.4 Data from one utility, Southern California Edison, shows that carbon-dioxide (CO2) emissions are 40-percent lower for power generated during off-peak periods (Table 1). However, this may not always be the case. For example, if a base-load power plant is coal-fired and the corresponding intermediate and/or peak-load plant is gas-fired, the off-peak power may be "dirtier" than the peak-load plant's output. In any case, chillers with lower efficiencies at sub-freezing evaporator temperatures will require larger on- to off-peak differentials—in energy and demand rates—if the project is going to be economically viable.5 If a TES project does not make good economic sense, its sustainability—or lack thereof—is moot.

Determining TES Sustainability
Several often conflicting factors that influence the sustainability of a project, such as higher energy consumption, differing chiller efficiencies, and potentially cleaner off-peak power generation, must be examined to determine whether an ice-based TES system is green. Reviewing a couple of actual ice-based TES installations can help give a better understanding of TES-system sustainability.

Beijing demonstration building. Completed in late 2003 and occupied in 2004, a joint U.S./China demonstration building in Beijing was the first Chinese office building to achieve Leadership in Energy and Environmental Design (LEED) Gold certification from the U.S. Green Building Council.6 The nine-story, 140,000-sq-ft building has an ice-based TES system in its basement. Primary cooling and ice-making is provided by two high-efficiency screw chillers, each rated for 100 tons with a coefficient of performance of 4.4. The TES system can produce 50 tons of ice per night. Using data from summer 2005, the additional off-peak energy consumption for the system averaged just over 9,000 kwh per month, resulting in whole-building off-peak energy consumption of approximately 21,000 kwh per month. With 7,700 tons of stored cooling per month, the peak cooling load was reduced by 7,000 tons per month, resulting in a reduction of peak electric-energy consumption of 6,100 kwh per month for a monthly net savings of approximately $609. Although there was some modest economic benefit, the real question is whether this particular TES system is green.

If we assume a source-site ratio of 3.34 (the U.S. average for the same period),7 then the additional 9,000 kwh per month needed for the TES system required consumption of approximately 30,000 kwh per month for generation. If there were no difference in the fuels fired during peak and off-peak periods, then the TES system would have produced the equivalent of an additional 22 metric tons of CO2 per month. However, if we assume an emissions reduction similar to that shown in Table 1, the additional off-peak energy consumption of 9,000 kwh per month actually was lower than the peak energy consumption of 6,100 kw per month when a 40-percent emissions penalty was considered. When peak and off-peak carbon footprints were calculated, peak energy consumption was greater than 10,000 kwh per month. In that case, the TES system would be demonstrably sustainable, both environmentally and—at a peak-to-off-peak differential of greater than 4—economically. In this example, the break-even point is a peak-to-off-peak emissions reduction of approximately 32 percent.