Energy storage encompasses a plethora of options which can be deployed at many different points along the electricity production and delivery value chain for bulk or wholesale transmission, substation support, distribution-system support, microgrids, customer-end services, and intermittent and peaking capacity. Thumbnail sketches of the competing technologies, presented below, can help the novice come up to speed quickly.
Pumped hydroelectric storage (PHS) is the granddaddy of all the technologies. Essentially water flows between an upper reservoir and a lower reservoir. During periods of low grid demand, electricity is consumed by a motor driving a pump (moving water uphill). During period of high demand, water is gravity fed through the same units acting in reverse as turbine/generators. PHS units can respond within minutes to put hundreds of megawatts of capacity online. Capital costs are high and permitting is arduous, two reasons why none have been built in the deregulation era.
Compressed-air energy storage (CAES) is similar to PHS, except that the working fluid is air, compressed and expanded to/from storage using compressor/turbine motor/generator units. Most large CAES facilities are envisioned for use with naturally occurring or solution-mined underground caverns, although aquifers have been evaluated. Smaller CAES units employ above-ground storage such as tanks or series of long horizontal pipes. Only the underground-cavern version has been demonstrated to date in the US. Capital costs typically approximate those for a combined-cycle facility.
Lead-acid batteries. Two large grid-connected systems were demonstrated over a decade and a half ago, and more recently several projects in the 10- to 40-MW range have come online. Several suppliers are qualified for grid-scale systems but the technology is limited by depth of discharge, number of charge/discharge cycles, and environmental issues associated with lead (even though 90% of the lead typically is recycled).
Thermal energy storage is a well-proven option that seems to get little love among energy-storage enthusiasts. TES has been around for decades. Notably, 25+ MW of rooftop ice storage units were selected as part of Southern California Edison’s recent 2220-MW solicitation to fill in the capacity gap created by the forced closing of the San Onofre Nuclear Generating Station. In sum, 264 MW of storage was selected as part of that solicitation. At night, excess electricity is used to make ice, which then is used for space cooling. Hot-water heaters also are popular for thermal storage.
Lithium-ion batteries essentially are scaled-up versions of the powerful batteries used in IT equipment and for automotive applications. The largest battery project in North America recently commissioned by SCE is of the lithium-ion variety. To give an idea of scale, it comprises over 600,000 of the same battery cells used in the Chevrolet Volt electric car. Several grid-scale technologies are being demonstrated through the DOE program
Sodium-sulfur batteries. Technically a flow battery, this chemistry appeared to be a leading contender five years ago for utility applications but has been plagued with cost, performance, and thermal-management issues. Regarding the last point, unlike other battery chemistries, sodium-sulfur requires a high operating temperature—around 600F. Several grid-scale systems have been demonstrated in the US and Japan.
Flywheels are mechanical devices. Energy is stored in an ultra-high-speed permanent magnet-type motor/generator with a rotor made of advanced lightweight composite material. It is maintained in a “charged” state at a very high rate of speed, until the process is reversed during discharge when the rotor is decelerates at a controlled rate with the generator. The rotor slows to zero when all energy is released. Commercial demonstration of the leading flywheel technology for grid applications resulted in high-profile failures at a DOE-funded site, but the company, and the technology, appears to be making a comeback, with new commercial facilities announced and coming online.
Flow batteries. Energy is stored and released through a reversible electrochemical reaction between two electrolytes. Because of the tanks and external fluid handling equipment, these systems look less like batteries and more like a compact chemical process plant. Regenerative fuel cells are included in the flow-battery category. Grid-scale facilities typically are built-up using individual modules rated less than 500 kW. Several different configurations have been demonstrated at small scale over the years, but in general the category has a checkered history with respect to performance and cost.
