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Giant batteries are coming to a power grid near you. In fact, they're already starting to appear on the grid in California.
That's because California is planning to rely increasingly on power supplies that aren't necessarily available every minute of every day. The state plans to get one-third of its electricity from wind and solar energy by 2020.
Utilities in the state are trying to figure out how they can cope with that uncertain power supply. Batteries aren't a panacea, but they could help.
Pacific Gas & Electric Co. is already starting to figure out how to make the most of batteries with a test at its Vaca-Dixon substation, near the Northern California town of Vacaville.
"Unfortunately there are no dancing bears, no mice running on wheels, so it's not that exciting," jokes PG&E's Dave Fribush, as he leads us to two gray cabinets the size of moving vans.
Inside is enough battery power to store the amount of energy that two large wind turbines generate over the course of seven hours. It's only a small fraction of California's energy need, but it's part of a bigger experiment that the California Public Utilities Commission has launched. The commission has called for hundreds of batteries of this scale to be connected to the grid over the next seven years, with a potential price tag of $5 billion.
Here's the catch. Nobody really knows how the batteries can best smooth out the irregular power supply from wind and solar power.
"There are many possible different uses for a battery on an electric grid," says Todd Strauss, senior director for energy policy planning and analysis at PG&E. "And the question becomes, how does it get used in practice in those different ways? What are the relative costs of actually using a battery in those different ways?"
Looking at the batteries looming over us, Strauss adds, "This is one attempt to try to get some sense of that."
You can think of a fully charged battery as a source of energy, ready to sell its product to the electric grid, just the way a power plant does. For that to work, battery owners would need to buy electricity to charge the battery when the price is low, and then sell that electricity back to the grid when the price is high.
But that idea turns out to be a dud.
"I think in the hour we did it the battery made $9," say Fribush, who manages battery storage integration at PG&E. "So we have a long way to go. We're not even making [San Francisco's] minimum wage with the system yet!"
That's partly because batteries aren't very efficient. Batteries waste about 25 percent of the energy in the process of being charged and discharged. These sodium-sulfur batteries need to be heated to 600 degrees Fahrenheit to work.
Considering this battery setup cost $10 million, they need to find a more valuable use of its time.
Strauss expects batteries could be useful in keeping the overall electric grid reliable — and the grid operators will pay for that service, too. Electricity supply needs to match electricity demand second by second, and batteries can provide some of that essential fine-tuning.
"So there's no doubt," Strauss says, "that as we get more renewables — particularly more wind, and especially more solar photovoltaics — on the system, there needs to be a lot more flexibility on the system." Power supply and demand will need to be adjusted — hour by hour, as well as minute by minute — to deal with passing clouds and fickle winds.
And batteries can't fill in if the wind stops blowing for days or weeks, as sometimes happens. Instead, PG&E will fall back on its gas-fired power plants during those stretches, or it might import more energy from out of state.
"And also we'd look to the customer side," Strauss says.
Industrial customers can sometimes scale back their power demands. And — maybe, someday — home appliances will be able to reduce their power consumption automatically when supply is lean.
Batteries aren't the only storage option for short-term needs. PG&E owns two reservoirs in the High Sierra that are connected to one another. When there's abundant power on the grid, the utility pumps water uphill. And when the demand for power rises, they can let the water flow through turbines to generate electricity.
Utilities are also experimenting with flywheels as a way to store energy. And PG&E is thinking about pumping compressed air into underground caverns. They can then release that pressurized air to power a turbine. And thermal solar energy is a method of storing energy in the form of heat — molten salt.
Strauss says they have lots of choices. "It all seems technologically possible," he says, "but at the end of the day the question is, 'What will it cost?' "
Batteries will find some role, but they are still very expensive. Strauss says for batteries to succeed, the technology needs the kind of revolution that brought down the price of solar panels by about 75 percent over the past decade.
"If we get that for batteries, terrific," he says. "If battery costs remain what they are today, that's not so good and we're likely to look for different kinds of technology."
And there's not much time to figure this out. Utilities like PG&E are preparing for a huge surge of on-and-off power supplies in the next six years, as California's wind and solar industries ramp up to meet the state's 2020 renewable energy goal.
The lessons learned in California can inform the decisions of many other states, as they ponder how to deploy their own renewable energy resources.