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Energy storage technologies run the gamut from large-scale pumped hydropower systems and compressed air energy storage systems to relatively small supercapacitors. Along with supercapacitors, a number of exotic technologies have been commercialized in recent years, including flywheels and superconducting magnetic energy storage systems. But among DE technologies, the lead-acid battery remains the most widely used technology. Case studies of each of these technologies are presented below.
Information about DOE-sponsored storage projects can be found on the Energy Storage Program Projects page.
Some of the following documents are available as Adobe Acrobat PDFs. Download Acrobat Reader.
Batteries
The most common energy storage system for distributed power applications are deep-cycle lead-acid batteries. Combined with intermittent energy sources like wind or solar power, batteries provide a reliable supply of power for off-grid applications. Batteries can also be incorporated into grid-connected systems to serve as a backup power source. Finally, battery energy storage systems can be used for grid support or for peak shaving and backup power in industrial settings.
Case Study: 102-kWh Battery Bank Supplements Off-Grid PV System
Channel Islands National Park, Santa Rosa Island, California (PDF 184 KB)
Two off-grid photovoltaic systems have been providing power to the Channel Islands' employee housing site since May 1998. Each 6.4-kW system includes a battery storage bank consisting of eight 6-V batteries connected in series, providing 51 kWh of electrical storage, for a total of 102 kWh of storage. That storage capacity equals eight hours of PV power production at peak capacity. Once the batteries are fully charged and all loads are provided for, excess electricity is shunted off.
Superconducting Magnetic Energy Storage (SMES)
SMES systems are used to address power quality problems and short-term power losses, such as those that may occur while switching between a power grid and a backup power supply. They have also been used for electricity-grid support.
Case Study: SMES System Maintains Power Quality at Auto Parts Plant
Georg Fischer Mössner GmbH factory, Gleisdorf, Austria
A portable SMES system, mounted in a 48-foot trailer, has been providing power quality protection to the Georg Fischer Mössner auto parts factory in Austria since June 1999. In 1998, power sags cost the factory about $150,000. The SMES system, operated by the Steweag-Steg electric utility, has the ability to ride through sags lasting up to 0.8 seconds and up to a rated power of 1.4 MW — a total energy storage of 0.3 kWh. At lower electrical loads, it can bridge longer power dips. When it detects a voltage disturbance, the system immediately injects real and reactive power into the factory substation to stabilize the power entering the Mössner facility. In the system's first three years of operation, the Mössner facility rode through 79 voltage sags lasting up to 1.7 seconds and dropping the power voltage to as low as 80% of normal levels.
Case Study: Six SMES Systems Provide Grid Support
Wisconsin Public Service Northern Loop, Wausau to Rhinelander, Wisconsin
Wisconsin Public Service Company's 200-mile, 115-kV Northern Loop transmission line faces voltage transients due to a large number of inductive loads, both from the area's paper mills and from home owners' air conditioning systems. With a permanent transmission grid upgrade pending, the utility installed six SMES systems at substations along the loop in mid-2000. The systems allowed the Northern Loop to maintain power quality during a system fault in September 2000 and after a lightning strike in November 2000. For more information, see the American Superconductor Web site. American Superconductor has purchased the SMES systems.
Flywheels
Flywheels can discharge their power either slowly or quickly, allowing them to serve as backup power systems for low-power applications or as short-term power quality support for high-power applications. Flywheel technology is being installed in the field, but it is still in its infancy.
Case Study: Flywheel System Supplies Backup Power to Cable and Internet Systems
WinDBreak Cable Company, Lyman, Nebraska
In early 2002, a 6-kWh demonstration flywheel storage system was installed at WinDBreak Cable Company's cable and Internet service facilities in Lyman, Nebraska. The 36-V system provides backup power to the facility, allowing uninterrupted cable and Internet service. According to the company, the system is able to operate in the extreme cold and heat that may be experienced at the site.
Supercapacitors
Supercapacitors are electrochemical storage devices that work like large versions of common electrical capacitors. They are characterized by relatively low storage capabilities and high charging and discharge rates. In power systems, they are most likely to be used as bridging power sources in uninterruptible power supplies, much like flywheels. Although no case studies are available, an example is available from Maxwell Technologies. The TC2700 is a 2700-farad capacitor that provides 8400 Joules of energy storage, or about 0.002 kWh.
Compressed Air Energy Storage (CAES)
CAES is really a hybrid storage/power production system. The system stores compressed air that is fed into a natural-gas-fired combustion turbine, allowing the turbine to operate at high efficiency. At present, the only existing CAES systems are combined with large central-station power plants. However, the technology could potentially be applied to distributed energy by using a small air compression station with a gas cylinder that feeds a single combustion turbine or a modified microturbine. The case study presented here is of the only CAES facility in the United States at present.
Case Study: 110-MW Compressed Air Energy Storage system
McIntosh Unit 1, McIntosh, Alabama
The 110-MW McIntosh CAES system was declared commercial on May 31, 1991. The system uses an underground cavern to store compressed air. The cavern was formed by "solution mining" a salt deposit -- pumping water into and out of it to dissolve the salt and form the cavern. The cavern is 220 feet in diameter and 1000 feet tall, for a total volume of 10 million cubic feet. At full charge, the cavern is pressurized to 1100 psi, and it is discharged down to 650 psi. During discharge, 340 pounds of air flow out of the cavern each second. The cavern can discharge for 26 hours.
The compressed air feeds a 100-MW gas-fired combustion turbine. Compared to conventional combustion turbines, the CAES-fed system can start up in 15 minutes rather than 30 minutes, uses only 30% to 40% of the natural gas, and operates efficiently down to low loads (about 25% of full load).
Pumped Hydropower Storage
Pumped hydropower is a large-scale energy storage system that has traditionally been used to store off-peak power generation from nuclear and large coal facilities that are not easily cycled. That off-peak generation is then used to meet peak load needs or to provide emergency power injection to the grid when a plant goes offline unexpectedly. Although all existing facilities are large in scale, this technology could potentially be used on a smaller scale for distributed energy applications, although the economies of scale would work against it.
Case Study: 1080-MW Pumped Hydropower Storage Facility
Northfield Mountain, Northfield, Massachusetts
Northfield Mountain went into service in 1972 at a cost of $685 million and was at that time the largest pumped hydropower storage facility in the United States. The facility draws water from the Connecticut river and pumps it up an 1100-foot shaft to an artificial reservoir, 800 feet in elevation above the river. The 300-acre reservoir is capable of storing 5.6 billion gallons of water and provides 10 hours of continuous full-power discharge.
The underground powerhouse includes four large reversible turbines, each capable of pumping about 20,000 gallons of water per second and generating 270,000 kW of electricity. Seven hundred feet below the surface, the cavern is longer than a football field and higher than a ten-story building.
A significant advantage of the system is its ability to inject a large amount of power into the power grid in a short period of time. It can go from zero power production to full power within a few minutes.
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