The challenge for green energy: how to store excess electricity

For years, the stumbling block for making renewable energy practical and dependable has been how to store electricity for days when the sun isn't shining and the wind isn't blowing. But new technologies suggest this goal may finally be within reach.

“Why are we ignoring things we know? We know that the sun doesn’t always shine and that the wind doesn’t always blow.” So wrote former US Energy Secretary James Schlesinger and Robert L. Hirsch last spring in the Washington Post, suggesting that because these key renewables produce power only intermittently, “solar and wind will probably only provide a modest percentage of future US power.”

Never mind that Schlesinger failed to disclose that he sits on the board of directors of Peabody Energy, the world’s largest private-sector coal company - a business with much to lose if a solar- and wind-powered future arrives. But at least he and his co-author got it partly right. The benefits from wind and solar are mostly intermittent - so far. But the pair somehow missed the fact that a furious search for practical, affordable electricity storage to beat that intermittence problem is well underway.

For decades, “grid parity” has been the Holy Grail for alternative energy. The rap from critics was that technologies like wind and solar could not compete, dollar-for-dollar, with conventional electricity sources, such as coal and nuclear, without large government tax breaks or direct subsidies. But suddenly, with rapid technological advances and growing economies of manufacturing scale, wind power is now nearly at grid parity - meaning it costs roughly the same to generate electricity from wind as it does from coal. And the days when solar power attains grid parity may be only a half-decade away.

So with grid parity now looming, finding ways to store millions of watts of excess electricity for times when the wind doesn’t blow and the sun doesn’t shine is the new Holy Grail. And there are signs that this goal - the day when large-scale energy storage becomes practical and cost-effective - might be within reach, as well. Some technologies that can store sizeable amounts of intermittent power are already deployed. Others, including at least a few with great promise, lie somewhere over the technological horizon.

New storage approaches include improvements to existing lithium ion batteries and schemes to store energy as huge volumes of compressed air in vast geologic vaults. Another idea is to create a network of small, energy-dense batteries in tens of millions of homes. Under such a “distributed storage” scheme, utility computers could coordinate electricity flows over a “smart grid” that continually communicates with - and adjusts the flow of power to and from - local batteries. This would even include batteries in future plug-in hybrid or all-electric vehicles.

And one 2008 breakthrough could even fulfil chemists’ long-held dreams of producing a squeaky-clean and storable fuel by using excess electricity generated from renewable sources to cheaply produce hydrogen, which could then be used in fuel cells to power homes and cars.

In a world run mainly on fossil fuels, finding ways to store electricity was not a pressing concern: power plants across a regional electrical grid could simply burn more fuel when demand was high. But large-scale electricity storage promises be an energy game-changer, unshackling alternative energy from the constraints of intermittence. It would mean that if a wind or solar farm were the cheapest and cleanest way to generate power, it wouldn’t matter when the sun shone or the wind blew.

One storage approach seems obvious: to improve battery technologies. Picture efficient, enormous batteries that can store tens of millions of watt-hours of juice. Today, the vast majority of new rooftop solar photovoltaic panels are connected to the grid, using it as a giant battery, pushing excess power onto the grid when solar panels provide excess power. The building then draws power from the grid when the sun doesn’t shine, with its meter spinning backward and forward with the ebb and flow of power. With relatively few solar roofs yet in play, utilities manage any ebb and flow by drawing down and ramping up generation at conventional power plants designed to balance fluctuating supply and demand.

A more robust world of solar and wind power might be better served by some sort of giant battery - or, more likely, many of them, widely distributed. The basic concept has been proven. Since 2003, the world’s largest battery backup has been storing energy for an entire city: Fairbanks, Alaska. Isolated as it is, and not part of any regional electricity grid, the metropolitan area of about 100,000 residents needs an electricity backstop more than most: in its sub-zero winters, pipes can freeze solid in as little as two hours. Six years ago, the city installed a huge nickel-cadmium battery, the same technology used for years in laptop computers and other portable devices.

Housed in a giant warehouse, the 1,300-metric ton battery is larger than a football field, and can crank out 40 million watts of power. Still, the Fairbanks battery provides only enough electricity for about 12,000 residents for seven minutes. That was enough to prevent 81 blackouts in the city in the battery’s first two years of operation.

Yet effective storage of electricity from solar or wind arrays that generate power equivalent to one large coal plant implies batteries on a breathtaking scale - hundreds of units the size of the Fairbanks array.