As calls mount for an electricity future substantially reliant on renewable energy, how can we cope with the variability of solar and wind power? Households, businesses, and industries all want to access power in a timely way – not just when the sun is shining or the wind blowing.
Demand-response measures can modulate electricity consumption for uses as small as household appliances and as large as commercial heating and cooling. Time-of-use electric rates can provide a financial incentive for utility customers to shift key uses to non-peak hours. In addition, utilities can offer discounts to customers who grant them a degree of remote control over the operation of power-consuming functions such as air conditioning, electric vehicle charging, and commercial refrigeration.
Smartly timed use of electricity can play an important role in stabilizing a grid reliant on renewable energy, but a robust investment in energy storage will also be essential. Solar power today accounts for a modest 2.3% of U.S. electricity; wind provides about 6.5%. Making the leap from these modest numbers to a mid-21st century America powered primarily by renewable electricity will require development of safe and affordable ways to store vast amounts of power.
A brief overview of energy storage
Energy storage has been used for decades to accommodate fluctuations in electricity demand that baseload power plants – particularly those running on coal and nuclear – cannot ramp up quickly enough to address. Pumped storage is one technology that meets this need, taking water from a lower-elevation reservoir or a flowing water source and pumping it to an upper basin where it becomes a source of supplemental hydropower. Pumped storage plants total 22.9 gigawatts in capacity nationwide – more than any other energy storage resource. Yet most of these plants were built between 1960 and 1990, and no new ones are under development. Siting challenges are among the obstacles they face.
Far less commonly used are utility-scale flywheel systems, only three of which now operate in the U.S. Flywheels convert electricity to stored kinetic energy that can be released within milliseconds to ensure stable voltage on the grid. They cannot, however, deliver a sustained stream of energy over longer periods – multiple minutes, hours, or days. That constraint, along with the bankruptcy of one industry leader, has slowed flywheel deployment, currently yielding just a few tens of megawatts of storage capacity.
Pumping compressed air into underground cavities is another storage technology that has drawn some attention, but only one such facility has been built in the U.S. Molten salt is used as a storage medium for heat captured by sun-tracking mirrors at a small number of concentrating solar power plants in the Southwest. However, both of these storage technologies face high operating costs and technical hurdles that have dimmed their prospects for broader introduction.
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