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Thermal Energy Storage: Putting Green Solutions on Site

Fortnightly Magazine - August 1996

used3 (in particular, whether the savings from reduced power plant "unit commitment" are included, as under the incremental energy method described above).

In many TES installations in California, 40 to 80 percent of the annual kilowatt-hours of electricity used for air conditioning can be shifted from day to night. For such installations, the incremental energy method showed significant source energy savings. Using that method, the savings for each kilowatt-hour of shifted energy use ranged between 36 to 43 percent for SCE, and between 20 to 30 percent for PG&E. The savings indicated by the marginal plant method were lower, but still significant (em from 12 to 24 percent for SCE and 8 to 10 percent for PG&E. Thus, even if some TES systems use more kilowatt-hours altogether than conventional air conditioning, they can still yield a net savings in "source energy."

In fact, if TES could achieve a

20-percent market penetration by 20054, it would save enough source energy from load-shifting only (ignoring kilowatt-hour impacts) to supply the energy needs of all the electric cars running on California highways in 2005, as projected by the CEC.

At the customer site, TES systems can improve energy efficiency to a significant degree

compared with conventional air-conditioning systems. Although early TES systems squandered more kilowatt-hours than conventional systems, recent systems consume as much as 12-percent fewer kilowatt-hours than conventional cooling systems. These efficiencies present attractive alternatives to the 20- to 50-percent energy penalties inflicted by conventional utility storage technologies such as pumped storage for hydroelectric generation.

When energy savings at the customer site are combined with those at the power plant source, again assuming a 20-percent market penetration, TES could save enough energy to supply over a third of the new air-conditioning load that the CEC projects for California by 2005.

ENVIRONMENTAL IMPACTS

At the power plant source, TES can reduce air emissions significantly. Indeed, in California, where natural gas serves as the fuel of choice for the marginal power plant, the extent of reductions in power plant emissions (on a percentage basis) is comparable to energy savings from TES.5 Assuming a 20-percent market penetration by 2005, TES could save 260,000 tons of CO2 annually statewide. Just as important, it could save about 1.6 tons of nitrogen oxide (NOx) per day in the South Coast Air Quality Management District (SCAQMD), which encompasses the greater Los Angeles area. These NOx savings are equivalent to the emissions from almost 100,000 cars.

TES can also help reduce combustion air emissions at the customer's building site. SCAQMD, in fact, explicitly identifies thermal storage as an option for reducing site emissions.6

In addition, TES can help in the transition to air-conditioning refrigerants without chloroflorocarbons (CFCs). When existing chillers are converted to a non-CFC refrigerant, for example, their effective cooling capacity may be reduced. Some key facility managers envision TES making up the difference. In addition, partial-storage TES systems often can require half as much chiller capacity, which means half as much refrigerant.

COMPETITIVE ADVANTAGES

Several other aspects of TES work to make energy suppliers and building owners more competitive.

For