Published Feb. 13, 2010

A passing cloud can be curtains for a solar-electric system, suddenly cutting generation dramatically. Production shoots back up as soon as the cloud is past. Then it ends at sundown, only to resume at dawn. Wind energy is also intermittent.

Electric utilities can accommodate these fluctuations in modest amounts, but adding lots of distributed solar generation on rooftops and in backyards could complicate a system designed to keep electrical current flowing steadily.

If large numbers of people, including solar owners, acquire plug-in vehicles, their hefty batteries could potentially serve as grid intermediaries when connected to "smart" charging equipment and digital meters. The batteries could store electricity most of the time and also remain on call to dispense it if needed.

But battery usage and charging patterns will be subject to the habits of vehicle owners. If plug-in vehicles take off, large numbers of used batteries will start to become available in five to 10 years. These used batteries will be unable to rapidly accelerate a 3,000-pound car but may still be capable of significant power storage, perhaps enough to run a typical household for a day. For more information, please see our story, "With Plug-In Cars, Second Life for Batteries May Save the Day."

Utilities and government agencies are investigating how to develop and use this potential power reservoir and other battery systems while solar and other renewables still make up a small fraction of grid electrical generation.

In California, the state Energy Commission has announced a $2 million award to Pacific Gas & Electric to study large-scale energy storage using sodium sulfur batteries. The three-year demonstration project will be the largest stationary storage system for battery energy in California.

"This is a great example of building on previous public-interest energy research the Energy Commission has sponsored," said James Boyd, vice chairman of the commission, in a news release. "Energy storage will become critical as we migrate to California's future 'smart grid' and integrate renewable-energy sources, manage peak congestion, and relieve transmission-line congestion."

Sodium sulfur batteries have high energy density – power storage capacity per unit of weight – and other advantageous characteristics, including a relatively low cost of materials. Because they operate at very high temperatures and contain highly corrosive chemicals, they aren't suitable for use in a vehicle, but are considered to have strong prospects for grid-energy storage.

The Energy Commission's news release notes: "Energy storage technologies have the potential to increase the reliability and transmission of California's energy supply. Building a portfolio of energy storage options balances the development of newer, distributed storage technologies."

The commission says the sodium sulfur system is highly efficient and has a lifetime of about 15 years. The $2 million grant is to be used along with investments of up to $13 million from PG&E to set up a 4-megawatt system with a capacity for storing 28 megawatt-hours of electricity. That capacity is enough to power about 1,400 typical California households for a day.

The project calls for monthly reports, surveys and assessments by the utility company and its subcontractors.

"The goal is to deploy, demonstrate and document a sodium sulfur battery energy storage system that could integrate renewable energy resources in an existing utility delivery system," PG&E's news release says. "The capabilities of the battery storage system offer several potential market uses, such as providing emergency power during outages, leveling the energy demand load, and providing reserves when needed."

The money is to be disbursed from the Public Interest Energy Research program, which supports research and development intended to improve the quality of life by helping to bring environmentally safe, affordable and reliable energy services and products to the market.

PG&E is one of the nation's largest utilities in the number of customers served. More than 20,000 residential, commercial and nonprofit or governmental solar arrays have been installed in its Northern and Central California service area since 2007.