Keep It Cool with Thermal Energy Storage Tomorrow's Energy Here Comes Summer
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U.S. Department of Energy Keep It Cool with Thermal Energy Storage Tomorrow's Energy Here comes summer. Temperatures are rising, but energy costs aren’t, Today thanks to an innovative way of storing nighttime off-peak energy for daytime peak use—cool thermal energy storage. for Cities and Counties In most states, demand for electrical imbalance between daytime need and power peaks during summer. Air- nighttime abundance. Although “cool conditioning is the main reason, in thermal energy” sounds like a contra- some areas accounting for as much as diction, the phrase “thermal energy 50% of power drawn during the hot storage” is widely used to describe midday hours when electricity is storage of both heating and cooling most expensive. But during the night, energy. Heating TES usually involves utilities have electricity to spare, and using inexpensive, off-peak power to this “off-peak” electricity is much add heat to a storage medium for cheaper. Now there’s a way to air- later use. condition during the day using elec- Patrons at the Pasadena Central tricity produced at night. In contrast, cool TES uses off-peak Library can enjoy a good book and power to provide cooling capacity cool air despite stifling summer Cool thermal energy storage (TES) by extracting heat from a storage temperatures. The library uses a cool has become one of the primary medium, such as ice, chilled water, or storage system to keep energy costs solutions to the electrical power “phase-change materials.” Typically, down during daytime peak use. a cool storage system uses refrigera- tion equipment at night to create a reservoir of cold material. During the day, the reservoir is tapped to pro- vide cooling capacity. There are many advantages to using a cool TES system. Lower nighttime temperatures allow refrigeration equipment to operate more efficiently than during the day, reducing energy consumption. Less chiller capacity is required, which means lower capital equipment costs. And by using off-peak electricity to store energy for use during peak hours, daytime peaks of power consumption are reduced, forestalling the need to build expensive new power plants. James Caccavo / PIX 1302 “We’re certainly putting our money “Cool storage is a Cool Storage Using Ice where our mouth is,” says Moudood Aslam, conservation specialist at the dynamic way to gain Ice is an efficient cool storage city-owned utility, Pasadena Water medium. Cool storage systems using and Power. “The city’s resource plan summer peak capacity ice can store and release 144 British anticipated a shortfall of electricity by shifting the on-peak thermal units (Btu) per pound around 1996 or 1997. To prepare for (334,000 joules per kilogram) during that, we can either build new power load to off-peak hours.” melting and freezing, whereas chilled plants or shift power loads and con- water systems can store only about serve energy. We encourage cus- —Moudood Aslam 18 Btu per pound (41,780 joules per tomers to both shift and conserve. Conservation Specialist kilogram)—about one-eighth the Cool storage is a dynamic way to gain Pasadena Water and Power capacity per pound of an ice storage summer peak capacity by shifting the system. on-peak load to off-peak hours.” Pasadena, California The utility goes even farther to pro- The city of Pasadena, California, is mote cool storage, paying an owner installing ice storage systems in two as much as $5,000 for a building’s of its buildings, the Pasadena Central cool storage feasibility study. The Library and the Pasadena Civic Cen- utility also pays a rebate on cool stor- ter. It’s Pasadena’s way of taking the age installations; the rebate amount is lead in the Commercial Cool Storage based on the number of kilowatts Incentive Program the city started in shifted from on-peak to off-peak 1992 to promote cool storage systems hours, up to a maximum of $250,000. in the community. Pasadena also markets cool storage to building owners through direct-mail brochures and personal contacts. Homes and Small Commercial Buildings: A Coming Market Utilities such as Pasadena Water and Ice is now well established as a cool which is worth approximately $6,200 to Power let customers decide which storage medium for larger buildings. But SMUD in avoided power costs (present cool storage medium is most advan- it can also store cooling power in a small value) over 20 years. This means that space, and that’s opening new opportu- just 1000 residential ice storage systems tageous to them. A potential benefit nities in homes and small commercial would save SMUD roughly $6.2 million. of ice is that it’s generally colder than buildings. As of mid-1994, 518 residen- chilled water or phase-change materi- tial units in Sacramento, California, had als, unless the chilled water is treated installed ice storage systems and col- with an additive. A cooler storage lected cash rebates on the installations medium produces cooler air, so less under the Residential Thermal Energy air needs to be moved to cool a build- Storage Program of the public power ing. Because fans that move the air utility, the Sacramento Municipal Utility can be smaller, they cost less and use District (SMUD). Because these residen- 30% to 40% less energy than a con- tial systems also heat domestic water ventional system does, according to and provide space heating in winter, they Electric Power Research Institute. can provide a total annual energy sav- ings of about 5000 kilowatt-hours (in a Also, duct size can be 20% to 40% residence of 1500 to 1700 square feet smaller and air handlers 30% to [140 to 158 square meters]), compared 50% smaller, requiring less initial to an all-electric residence with electric equipment cost and less cost for the resistance space heating, water heating, building space needed to house and central air-conditioning. This mechanical equipment. amounts to an annual savings of about $800 to the home owner. Lincoln Electric Goes for the Cold This Sacramento home (above) benefits The benefit? The average residential from a cool storage system. These systems Utilities have their own reasons for ice storage system shifts 2 to 3 kilo- are integrated units that provide three using ice and chilled water storage watts of summer peak load and about services: hot water, space heating, and air- systems. Feeding the cold air from conditioning. The systems can be built-in or 3100 kilowatt-hours per year off-peak, located in a side yard. these systems into a turbine can boost Phenix Energy Systems / PIX 1300, 1301 2 Lincoln also uses ice storage down- How Cool Storage Works in Lincoln town to cool 330,000 square feet (30,660 square meters) in the city and Ice makers Electricity input county administration building and Pump the Lancaster County Jail. The utility Ice storage tank also will use the system in a new • Makes ice all night and on weekends 120,000-square-foot (11,150-square- • Uses ice in the late afternoon meter) records-storage and office Chilled when heat pumps are shut off building. water to reduce electric demand Electricity Pump input Lincoln took an innovative approach to building and operating its down- Well water Chilled water for town cool storage system. In 1989, the cooling building county and utility formed the District 47˚ F (8˚ C) Energy Corporation to govern the city and county heating and cooling Heat pumps Lancaster system. This nonprofit corporation Water • Extract cooling from city and county discharge well water provided the financing to build the 70˚ F (21˚ C) buildings and jail ice storage system as part of a district Electricity input 47˚ F (8˚ C) energy system. Lincoln Electric con- tracts with the corporation to handle Well water management and maintenance. Existing chillers • Provide supplemental cooling Water discharge 70˚ F (21˚ C) Chilled-Water Cool Storage During the summer, heat pumps remove buildings during the afternoon, the peak heat from the buildings and transfer it to demand period. Chillers are available to One advantage of using water as a underground water wells. During the provide supplemental cooling on the cool storage medium is that con- cooler late evening and early morning hottest days and may also be used for structing chilled-water storage tanks hours, when electricity is less expensive, cooling buildings in off-peak periods is economically attractive in larger the heat pumps act as high-efficiency while the heat pumps are making ice. buildings. ice makers. This ice is stored to cool the Chicago’s McCormick Place energy production. Combustion tur- One of the world’s largest cool stor- bine generators activated by utilities age systems is located at Chicago’s to provide extra power during peak McCormick Place exhibition center. demand are, ironically, least efficient Chicago’s Metropolitan Pier & in hot weather. As a turbine’s air inlet Exposition Authority chose chilled- temperature increases, air density water storage for a new annex to Savings on energy costs decreases, and generating capacity McCormick Place. The 2.2-million- falls. Lowering that air inlet tempera- for Chicago’s annex to square-foot (204,400-square-meter) ture increases efficiency tremen- annex is slated for 1997 completion, McCormick Place is dously, producing more power. but by 1994, McCormick Place was already using off-peak power expected to approach Lincoln Electric System, the city- to store cooling capacity in the new owned utility of Lincoln, Nebraska, 8.5-million-gallon (32-million-liter) $1 million by 1997. uses an ice storage system to cool chilled-water tank and apply it by inlet air on a 57-megawatt turbine at day to cool the center’s existing two its Rokeby Station.