its own compressor.24 CAES is now a viable option for providing 100–300 MWe or more of electric power for up to 10 hours. Before 2020, CAES will be the only viable option, aside from pumped hydro, for storing hundreds to thousands of megawatts of energy. Both are dependent on specific features being available (caverns or hills where reservoirs can be built), which greatly limit their applicability.

For distribution systems, storage at lower power ratings (10 MW and below) and lower discharge times (hours to minutes, depending on the application) can be used to improve power quality and security. Distributed storage can help to regulate the system and improve system stability, including reducing the risk of system collapse by supporting islanding and restoration following a disruption. Some battery-storage technologies for these applications, such as lead-acid and sodium-sulfur batteries, have been demonstrated and are currently available for deployment (Bjelovuk, 2008). Batteries are modular and not site specific, meaning they can be located close to intermittent generation sites, near the load, or at T&D substations. However, current battery technologies are expensive and have high losses and reliability issues.

In the longer term, battery storage technology at larger capacities (in the 100 MW range) may help to accommodate variable renewable energy sources, but further R&D is needed before more widespread deployment is likely. Given the large potential in the electric vehicle market for lithium-ion, nickel metal hydride, and other types of batteries, much R&D is now in progress. Advanced batteries with lower cost, high energy density, and higher charge-discharge cycles could also be used for storage in the T&D systems. They may be available for deployment in T&D systems after 2020.

Other longer-term possibilities for energy storage in the grid include supercapacitors, superconducting energy storage, and flywheels. None of these technologies is currently suitable for grid use because of high costs and low energy-storage density. Flywheel storage units are being installed for first-of-a-kind experience with power capacity in the MW range that can smooth out short variations of wind power. However, the technology is a long way from economic deployment on a large scale that would affect daily peaks and day-to-day variations. If advances are made, particularly in materials, all these technologies may become

24

Conventional gas turbines use about two-thirds of their output to operate their compressors; thus only a third of the turbine’s output power is available to produce electricity. By moving the compression to off-peak hours when power costs are low, output of the turbine can be approximately tripled and sold at the much higher peak rate.



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