Thermal Energy Storage
Content
Energy Efficiency
Thermal Energy Storage
Description: Thermal energy storage (TES) systems cool a storage medium and then use that cold medium to cool air at a later point in time. Using thermal storage can reduce the size and initial cost of cooling systems, lower energy costs, and reduce maintenance costs. If electricity costs more during the day than at night, thermal storage systems can reduce utility bills further. Systems can be sized to eliminate compressor energy use during periods when electricity is most expensive, but most systems are designed to simply augment mechanical cooling in order to limit peak demand. The latter type of system can be considerably smaller than the former. Two forms of TES systems are currently used. The first system uses a material that changes phase, most commonly water and ice. The second type just changes the temperature of a material, most commonly water. Applications: TES may be economical if one or more of the following conditions exist: ♦ High utility demand costs ♦ Utility time-of-use rates (some utilities charge more for energy use during peak periods of day and less during off-peak periods) ♦ High daily load variations ♦ Short duration loads ♦ Infrequent or cyclical loads ♦ Capacity of cooling equipment has trouble handling peak loads ♦ Rebates are available for load shifting to avoid peak demand Effective applications of thermal energy storage include: ♦ Electrical power use management by shifting the cooling load to off-peak hours and reducing peak load ♦ Reducing required capacity of building and process cooling systems, or helping existing cooling equipment to handle an increased load
Thermal Energy Storage
Factsheet
Water storage systems are often used in new large cooling system applications in conjunction with cogeneration and/or district energy systems. Water-ice storage is the most common cooling storage in smaller applications. Because latent heat storage (phase change between water and ice) has a smaller volume, it is often chosen for retrofit applications with limited space. In general, the buildings that offer the highest potential are offices, retail, and medical facilities. The Dallas Veterans Affairs Medical Center installed a 24,628 ton-hours chilled water TES that resulted in a reduction in demand of 2,934 kW and a reduction in annual electricity cost of $223,650. The local utility provided $500,000 of the total cost of $2.2 million required for design and installation. Savings resulting from installation of the thermal storage technology will allow the VA to recoup its investment within 7 years.
Performance/Costs: Thermal energy storage systems are installed for two major reasons: lower initial project costs and lower operating costs. Initial cost may be lower because distribution temperatures are lower and equipment and pipe sizes can be reduced. Operating costs may be lower due to smaller compressors and pumps as well as reduced time-of-day or peak demand utility costs. The economics of thermal storage is very site- and system-specific. A feasibility study is generally required to determine the optimum design for a specific application. Several examples exist of effective TES systems that were installed for less cost than conventional alternatives and that also provided significant energy and energy cost reductions. TES projects often profit from unexpected benefits that are secondary to the primary reason for an action. For example, a well designed TES air conditioning application may experience reduced chiller energy consumption, lower pump horsepower, smaller pipes, high reliability, better system balancing and control, and lower maintenance costs.
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Thermal Energy Storage
Factsheet
Determining electrical cost savings from shifting chiller operation from daytime to night-time (or off-peak operation) can be complicated, depending upon the local utility rate structure. Your utility may have time-of-day peak rates as well as differing peak demand rates. Without thermal storage, a chiller will typically operate during times of peak electrical demand, and thus be included in the peak monthly demand charge. Your greatest savings will occur when you shift chiller operation to off-peak times. Table 1 shows the benefit of using thermal storage to shift chiller use to a time when the rest of the facility is unoccupied. The chiller load (in terms of ton-hours delivered) is the same, but demand charges for the chiller would be eliminated. The table shows the effect of two different utility demand rates ($6 and $12 per kW). Availability: Thermal energy storage equipment and installations are available from a number of suppliers. For a list of equipment manufacturers, contact the Thermal Storage Applications Research Center (TSARC) or go to the Energy User News Buyers Guide website (see “For Additional Information” below). For Additional Information: Heating, Ventilating, Air Conditioning, and Refrigerating Center and Thermal Storage Applications Research Center (TSARC) These centers, located at the University of Wisconsin, perform research to develop technologies that increase the efficiency of systems, and offer technical assistance and education to industry. http://www.engr.wisc.edu/centers/tsarc/tsarc.html Energy User News Buyers Guide This list of manufacturers can be searched by technology type, e.g. search for Thermal Energy Storage. http://www.energyusernews.com/buyers.htm Thermal storage and deregulation From a paper published in the April 1998 ASHRAE Journal http://www.pwi-energy.com/main/whitepapers/tsdereg.htm Benefits and savings of chiller replacement & various thermal storage projects U.S. Army case studies http://www.cecer.army.mil/ul/gascool/storage.htm Thermal Energy Storage at a Federal Facility Describes the benefits of a project in which the Dallas Veterans Administration Medical Center and Texas Utilities Electric Company join in an unprecedented partnership to lower energy costs. http://www.eren.doe.gov/femp/financing/fed_facility_439.html Ice Storage Retrofit for Rooftop Air Conditioning Federal Energy Management Program (FEMP) New Technology Demonstration Program evaluates ice storage retrofit for rooftop air conditioning. http://www.eren.doe.gov/femp/prodtech/icesum.html © 2003 Washington State University Cooperative
Extension Energy Program. This publication contains material written and produced for public distribution. You may reprint this written material, provided you do not use it to endorse a commercial product. Please reference by title and credit Washington State University Cooperative Extension Energy Program. WSUCEEP00-127
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Thermal Energy Storage
Description: Thermal energy storage (TES) systems cool a storage medium and then use that cold medium to cool air at a later point in time. Using thermal storage can reduce the size and initial cost of cooling systems, lower energy costs, and reduce maintenance costs. If electricity costs more during the day than at night, thermal storage systems can reduce utility bills further. Systems can be sized to eliminate compressor energy use during periods when electricity is most expensive, but most systems are designed to simply augment mechanical cooling in order to limit peak demand. The latter type of system can be considerably smaller than the former. Two forms of TES systems are currently used. The first system uses a material that changes phase, most commonly water and ice. The second type just changes the temperature of a material, most commonly water. Applications: TES may be economical if one or more of the following conditions exist: ♦ High utility demand costs ♦ Utility time-of-use rates (some utilities charge more for energy use during peak periods of day and less during off-peak periods) ♦ High daily load variations ♦ Short duration loads ♦ Infrequent or cyclical loads ♦ Capacity of cooling equipment has trouble handling peak loads ♦ Rebates are available for load shifting to avoid peak demand Effective applications of thermal energy storage include: ♦ Electrical power use management by shifting the cooling load to off-peak hours and reducing peak load ♦ Reducing required capacity of building and process cooling systems, or helping existing cooling equipment to handle an increased load
Thermal Energy Storage
Factsheet
Water storage systems are often used in new large cooling system applications in conjunction with cogeneration and/or district energy systems. Water-ice storage is the most common cooling storage in smaller applications. Because latent heat storage (phase change between water and ice) has a smaller volume, it is often chosen for retrofit applications with limited space. In general, the buildings that offer the highest potential are offices, retail, and medical facilities. The Dallas Veterans Affairs Medical Center installed a 24,628 ton-hours chilled water TES that resulted in a reduction in demand of 2,934 kW and a reduction in annual electricity cost of $223,650. The local utility provided $500,000 of the total cost of $2.2 million required for design and installation. Savings resulting from installation of the thermal storage technology will allow the VA to recoup its investment within 7 years.
Performance/Costs: Thermal energy storage systems are installed for two major reasons: lower initial project costs and lower operating costs. Initial cost may be lower because distribution temperatures are lower and equipment and pipe sizes can be reduced. Operating costs may be lower due to smaller compressors and pumps as well as reduced time-of-day or peak demand utility costs. The economics of thermal storage is very site- and system-specific. A feasibility study is generally required to determine the optimum design for a specific application. Several examples exist of effective TES systems that were installed for less cost than conventional alternatives and that also provided significant energy and energy cost reductions. TES projects often profit from unexpected benefits that are secondary to the primary reason for an action. For example, a well designed TES air conditioning application may experience reduced chiller energy consumption, lower pump horsepower, smaller pipes, high reliability, better system balancing and control, and lower maintenance costs.
2
Thermal Energy Storage
Factsheet
Determining electrical cost savings from shifting chiller operation from daytime to night-time (or off-peak operation) can be complicated, depending upon the local utility rate structure. Your utility may have time-of-day peak rates as well as differing peak demand rates. Without thermal storage, a chiller will typically operate during times of peak electrical demand, and thus be included in the peak monthly demand charge. Your greatest savings will occur when you shift chiller operation to off-peak times. Table 1 shows the benefit of using thermal storage to shift chiller use to a time when the rest of the facility is unoccupied. The chiller load (in terms of ton-hours delivered) is the same, but demand charges for the chiller would be eliminated. The table shows the effect of two different utility demand rates ($6 and $12 per kW). Availability: Thermal energy storage equipment and installations are available from a number of suppliers. For a list of equipment manufacturers, contact the Thermal Storage Applications Research Center (TSARC) or go to the Energy User News Buyers Guide website (see “For Additional Information” below). For Additional Information: Heating, Ventilating, Air Conditioning, and Refrigerating Center and Thermal Storage Applications Research Center (TSARC) These centers, located at the University of Wisconsin, perform research to develop technologies that increase the efficiency of systems, and offer technical assistance and education to industry. http://www.engr.wisc.edu/centers/tsarc/tsarc.html Energy User News Buyers Guide This list of manufacturers can be searched by technology type, e.g. search for Thermal Energy Storage. http://www.energyusernews.com/buyers.htm Thermal storage and deregulation From a paper published in the April 1998 ASHRAE Journal http://www.pwi-energy.com/main/whitepapers/tsdereg.htm Benefits and savings of chiller replacement & various thermal storage projects U.S. Army case studies http://www.cecer.army.mil/ul/gascool/storage.htm Thermal Energy Storage at a Federal Facility Describes the benefits of a project in which the Dallas Veterans Administration Medical Center and Texas Utilities Electric Company join in an unprecedented partnership to lower energy costs. http://www.eren.doe.gov/femp/financing/fed_facility_439.html Ice Storage Retrofit for Rooftop Air Conditioning Federal Energy Management Program (FEMP) New Technology Demonstration Program evaluates ice storage retrofit for rooftop air conditioning. http://www.eren.doe.gov/femp/prodtech/icesum.html © 2003 Washington State University Cooperative
Extension Energy Program. This publication contains material written and produced for public distribution. You may reprint this written material, provided you do not use it to endorse a commercial product. Please reference by title and credit Washington State University Cooperative Extension Energy Program. WSUCEEP00-127
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