Electric Storage Technologies

A number of energy storage technologies have been developed or are under development for electric powe r applications, including:

• Pumped hydropower

• Compressed air energy storage (CAES)

• Superconducting magnetic energy storage (SMES)

• Supercapacitors

Thermal energy storage technologies, such as molten salt, are not addressed in this appendix.

Pumped Hydro: Pumped hydro has been in use since 1929, making it the oldest of the central station energy storag e technologies. In fact, until 1970 it was the only commercially available storage option for generation applications . Conventional pumped hydro facilities consist of two large reservoirs, one is located at base level and the other i s situated at a different elevation. Water is pumped to the upper reservoir where it can be stored as potential energy . Upon demand, water is released back into the lower reservoir, passing through hydraulic turbines which generat e electrical power as high as 1,000 MW. The barriers to increased use of this storage technology in the U.S. include high construction costs and long lead times as well as the geographic, geologic and environmental constraints associate d with reservoir design. Currently, efforts aimed at increasing the use of pumped hydro storage are focused on th e development of underground facilities [20].

Compressed Air Energy Storage (CAES): CAES plants use off-peak energy to compress and store air in an air-tight underground storage cavern. Upon demand, stored air is released from the cavern, heated and expanded through a combustion turbine to create electrical energy. In 1991, the first U.S. CAES facility was built in Mcintosh, Alabama , by the Alabama Electric Cooperative and EPRI, and has a capacity rating of 110 MW. Currently, manufacturers ca n create CAES machinery for facilities ranging from 5 to 350 MW. EPRI has estimated that more than 85% of the U.S. has geological characteristics that will accommodate an underground CAES reservoir [21]. Studies have conclude d that CAES is competitive with combustion turbines and combined-cycle units, even without attributing some of th e unique benefits of energy storage [22].

Batteries: In recent years, much of the focus in the development of electric energy storage technology has bee n centered on battery storage devices. There are currently a wide variety of batteries available commercially and man y more in the design phase. In a chemical battery, charging causes reactions in electrochemical compounds to stor e energy from a generator in a chemical form. Upon demand, reverse chemical reactions cause electricity to flow ou t of the battery and back to the grid. The first commercially available battery was the flooded lead-acid battery whic h was used for fixed, centralized applications. The valve-regulated lead-acid (VRLA) battery is the latest commercially available option. The VRLA battery is low-maintenance, spill- and leak-proof, and relatively compact. Zinc/bromine is a newer battery storage technology that has not yet reached the commercial market. Other lithium-based batterie s are under development. Batteries are manufactured in a wide variety of capacities ranging from less than 100 watt s to modular configurations of several megawatts. As a result, batteries can be used for various utility applications in the areas of generation, T&D, and customer service.

Flywheels: Flywheels are currently being used for a number of non-utility related applications. Recently, however , researchers have begun to explore utility energy storage applications. A flywheel storage device consists of a flywheel that spins at a very high velocity and an integrated electrical apparatus that can operate either as a motor to turn the flywheel and store energy or as a generator to produce electrical power on demand using the energy stored in the flywheel. The use of magnetic bearings and a vacuum chamber helps reduce energy losses. A proper match betwee n geometry and material characteristics influences optimal wheel design. As a result, engineers have focused on th e development of materials with high working strength-to-density ratios. Flywheels have been proposed to improve the range, performance and energy efficiency of electric vehicles. Development of flywheels for utilities has been focused on power quality applications [20,23].

Superconducting Magnetic Energy Storage (SMES): A SMES system stores energy in the magnetic field create d by the flow of direct current in a coil of superconducting material. To maintain the coil in its superconducting state , it is immersed in liqui d helium contained in a vacuum-insulated cryostat. The energy output of a SMES system is much less dependent on the discharge rate than batteries. SMES systems also have a high cycle life and, as a result, ar e suitable for applications that require constant, full cycling and a continuous mode of operation. Although research i s being conducted on larger SMES systems in the range of 10 to 100 MW, recent focus has been on the smaller micro -SMES devices in the range of 1 to 10 MW. Micro-SMES devices are available commercially for power qualit y applications [20,22,23].

Advanced Electrochemical Capacitors: Supercapacitors (also known as ultracapacitors or supercapacitors) are in the earliest stages of development as an energy storage technology for electric utility applications. An electrochemical capacitor has components related to both a battery and a capacitor. Consequently, cell voltage is limited to a few volts. Specifically, the charge is stored by ions as in a battery. But, as in a conventional capacitor, no chemical reaction takes place in energy delivery. An electrochemical capacitor consists of two oppositely charged electrodes, a separator , electrolyte and current collectors. Presently, very small supercapacitors in the range of seven to ten watts are widel y available commercially for consumer power quality applications and are commonly found in household electrica l devices. Development of larger-scale capacitors has been focused on electric vehicles [24]. Currently, smal-scal e power quality (<250 kW) is considered to be the most promising utility use for advanced capacitors.

Table 1 summarizes the key features of each energy storage system. Batteries, flywheels, SMES and advance d electrochemical capacitors lend themselves to distributed utility applications while pumped hydro and CAES are large, centralized installations. All cost estimates are for complete systems with power conditioning subsystems (PCS) , controls, ventilation and cooling, facility, and other balance of plant components.

Research & Development

The Electric Power Research Institute, since its inception in 1972, has pioneered development of energy storage . Current programs are focusing on deployment of SMES, CAES, and batteries; and further assessments of the flywheels and super capacitors. The U.S. Department of Energy, through its Energy Storage Systems (ESS) Program, has focused almo st exclusively on battery systems for the last decade for a variety of reasons, including technology versatility , applicability to customer needs, modular construction, and limited funds. Recently, the program has been expande d to include SMES, flywheels and advanced electrochemical capacitors. The ESS Program today performs collaborative research with industry on system integration and field testing, component development, and on systems analysis . Pumped hydro development was performed by the U.S. Army Corps of Engineers, flywheel development was don e by the Department of Transportation, and SMES development was sponsored by the Department of Defense. Advanced electrochemical capacitors were investigated by the Department of Energy Defense Program s


Installed (U.S. total)

Facility Size Range

Potential/Actual Applications

Commercially Available

Selected Manufacturers

Estimated System Costs ($1997)

Pumped Hydro

22 GW at 150 facilities in 19 states

• Load Leveling

• Spinning Reserve


Allis-Chalmers, Combustion Engineering, General Electric, North American Hydro, Westinghouse

500-1,600 $/kW


110 MW in Alabama

• T&D Applications

• Spinning Reserve


Dresser Rand, Westinghouse, ABB

350-500 $/kW (commercial plant estimates)


More than 70 MW installed by utilities in 10 states

• Spinning Reserve

• Integration with


• T&D Applications

• Peak Shaving Transportation

Yes (Flooded Lead-Acid, VRLA)

(Zinc/Bromine, Lithium)

AC Battery Corp, C&D, Delco-Remy, Delphi , GE Drive Systems, GNB, Precise Power Corp., SAFT America, Yuasa-Exide, ZBB

(20-40 MW, 0.5 hr) 400-600 $/kW (2 MW, 10-20 sec)


1-2 demo facilities, no commercial facilities


Electricity • Power Quality Transportation Defense

(steel, low rpm) No (advanced composite)

American Flywheel Systems, Boeing, Int'l Computer Products, SatCon, US Flywheel Systems

Advanced: 6,000 $/kW (~1 kW) 3,000 $/kW (~20 kW) Steel:

500 $/kW (1 MW, 15 sec)


5 facilities with approx. 30 MW in 5 states

From 1-10 MW (micro-SMES) to 10-100 MW


• T&D Applications

• Power Quality

(micro-SMES) No (larger units)

Superconductivity, Inc.

1,000 $/kW (1-2 MW, 1 sec)




Millions of units for standby power; 1 defense unit

7-10 W commercial

10-20 kW prototype

Electricity • Power Quality Consumer Electronics Transportation Defense

Yes (low-voltage, standby power)

(power quality)

Evans, Maxwell, NEC, Panasonic, Pinnacle, Polystor, Sony


Sources: References 1, 20, 22-25

Sources: References 1, 20, 22-25

and Office of Transportation Technologies, although it appears that only defense applications are currently bein g pursued.

This report is focused on renewable energy generation technologies. The most appropriate storage systems for suc h applications presently appear to be batteries. Batteries have been installed in stand-alone PV and wind systems fo r more than two decades throughout the U.S. Worldwide sales of batteries attached to PV installations in 1995 were estimated at 3,000 MWh, with total installed of over 10,500 MWh. U.S. sales of PV batteries in 1995 were estimated at 340.5 MWh [26]. These annual sales statistics include both new installations and replacements. They ar e significant when considered against the amount of PV generating capacity in operation. By 1996, the U.S. PV industry had installed a total of 210 MW of PV generating capacity worldwide [16].

Batteries support renewable generation in at least three size ranges: (a) 1-4 kW residential, (b) 30-100 kW commercial, industrial, or village, and (c) > 1 MW generation or grid-support. Much of the activity funded by the PV industry has focused on residential-scale applications with oversized (many hours of) battery back-up, while much of the activit y funded by the battery manufacturers has focused on the industrial-scale applications with low battery back-up. Fo r example, EPRI and Sandia National Laboratories are completing an analysis of a 2.4 kW PV array and 7-hour battery operating in a grid-connected home in the Salt River Project service area [8].

Opportunities for PV are appearing in geographic zones previously excluded from consideration. The Nationa l Renewable Energy Laboratory (NREL), assisted by the State University of New York (SUNY) at Albany, has derived a new measure of effective PV capacity. The effective load-carrying capacity is the ability of any generator t o effectively contribute to a utility's capacity to meet its load. While the intensity of solar insolation is critical to PV , it is less important than PV's relationship to load requirements [9]. SUNY researchers have developed a complementary measure of the minimum amount of back-up or stored energy needed to ensure that all utility load s above a threshold are met by the PV/storage system. The minimum buffer energy storage measure found that a smal l amount of storage could yield an increased capacity credit for PV.

The following technology characterization proceeds from the SUNY premise, examining an integrated 30 k W PV/30 kWh battery system connected to the electric grid.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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