Sometimes, the wind doesn't blow. Sometimes, the sun doesn't shine.
That's why engineers who understand the country's power needs say wind and solar can't make significant gains without some form of back up storage.
It's also why experts from a variety of industries — battery manufacturers, utility engineers and even automakers — are looking for storage solutions. They want to pave the way for wind and solar power to step up to the nation's electrical grid in a big way. All that's needed, they say, is one grand vision.
“If you tie thousands of batteries together and provide adequate cooling, you can store the power generated by wind and solar cells,” says David Cole, director of the Center for Automotive Research, which has worked with the auto industry on the concept. “You could take that power and use it when it's needed, during the peak demand times.”
To be sure, the visions are varied and many. Some foresee a day when, as Cole predicts, long strings of batteries will capture gigawatts of power. Others envision huge battery cells — as large as 1,000 ft3 each — providing electrical current for thousands of homes. Still, others favor so-called “flow batteries” and superconducting storage rings.
Undeniably, those scenarios are still a stretch. But engineers who understand the situation say the technology doesn't need to be ready tomorrow.
“Having no form of storage is not a problem right now, because only (about 2) percent of our power comes from wind and solar,” says George Crabtree, senior scientist and Distinguished Fellow in Argonne National Lab.'s Materials Science Div. “But that won't work if you intend to get really serious and create 30 percent to 40 percent of your power that way. Every wind and solar source has to have a backup.”
A Need for Balance
Indeed, most experts believe the magic number is somewhere around 10 percent. At that point, wind and solar become a more significant and indispensable part of the power grid. As a result, they need a so-called “balancing” power source.
The reason for that need is extraordinarily simple but often unappreciated. Wind turbines generate power when and only when the wind blows; solar cells make power only when the sun is shining brightly. Like all other sources of power — coal, nuclear, hydro — the electrical current created by wind and solar is used immediately. With only a few minor exceptions, utilities don't have a way of storing that electricity for later use.
That's why “balancing resources” are so important. Without them, utility customers face the prospect of blackouts, as happened in a wind-dependent area of west Texas earlier this year when the wind died down. Or they face the opposite problem — too much generation — as occurred in New York state recently when wind turbines had to be shut down because they caused “grid congestion.”
“As we put more 'intermittency' — wind and solar — into our system we need the balancing resources for those times when they are not available,” says Arshad Mansoor, vice president of power delivery and utilization for the Electric Power Research Institute (EPRI). Mansoor says the most common balancing resources today are gas turbines and coal plants. In other words, utilities must balance large amounts of wind and solar with the very CO2-spewing sources they are trying to eliminate.
Still, experts say that without some form of storage, they have little choice. “If you can't store it, then it's no good,” says Donald Sadoway, the John F. Elliott Professor of Materials Chemistry at MIT. “Name me someone who will put a company in an area where they have unreliable sources of power.”
Most experts agree the U.S. could easily add wind farms today without an immediate need for storage, largely because wind and solar now comprise only about 2 percent of the country's total power. But for wind and solar to make an impact in the way described by this year's presidential candidates — and for wind and solar to impact the global warming crisis — storage will be necessary.
“Wind proponents claim that the need for storage is rubbish because Denmark is at 20 percent and they don't have storage,” says Tim Hennessy, CEO of VRB Power Systems Inc. “But Denmark has a huge battery connected to it. It's called Germany.”
Bring on the Batteries
To be sure, solutions already exist. So-called pumped hydro — in which power is used to pump water up a hill in off-peak hours — has potentially been available for decades. Technically, pumped hydro is considered viable because it allows utilities to employ the potential energy of the pumped water to spin a generator and create electricity at times of peak demand. Problem is, pumped hydro proposals have run into “siting” challenges. No one, it seems, wants trillions of cubic feet of water in their backyard.
Similarly, utilities have proposed the use of compressed air energy storage (CAES) in caverns below ground. Using the energy produced by wind and solar facilities, utilities say they could pump air at pressures ranging from 800 to 1,000 psi into the underground caverns, then use the compressed air to spin an electric generator and — voila! — re-gather the power originally created by the wind and sunlight.
In the next five to eight years, pumped hydro and CAES are still the best bets, utility engineers say. But in the long term, especially as more wind farms come online, many believe batteries will play a bigger role. Cole of the Center for Automotive Research says automakers are now talking with utilities about employing used lithium-ion batteries from plug-in hybrid vehicles, such as the Chevy Volt, stringing thousands of them together and creating low-cost battery farms. The lithium-ion batteries would be ideal, he says, because they would easily outlive the vehicles, possibly by as much as 20 years.
“The battery life of a vehicle like the Volt could be as much as 20 to 30 years,” Cole says. “You'd have your central power-generating facilities and then distributed throughout the grid, you'd have these battery farms with the proper electronics to convert ac to dc and dc back to ac.” The farms, he says, could store the power from wind until it's needed at peak demand times.
Moreover, automakers have even discussed using the batteries as a balancing resource while they are still installed in the vehicles. “If you have a plug-in hybrid in the garage that you're not using and the power company has peak demands, it could buy back some of that power that it sold to you earlier,” Cole says. “You could even make a profit because the cost of off-peak power is so low.”
Some energy storage solutions are already being tested by utilities. VRB Power Systems Inc., for example, has installed its vanadium-based fuel cells into solar and wind applications in Ireland, Japan, Denmark, Germany, South Africa and Alaska. Known as a Vanadium Redox Flow Battery, VRB's product chemically stores energy in different ionic forms of vanadium and pumps it into “flow cells” across a proton exchange membrane. The resulting reaction is reversible, enabling the battery to be charged and discharged as it stores energy and releases it.
Hennessy of VRB compares the technology to a car's engine and fuel tank. Increasing the size of the engine produces more power, while making the fuel tank bigger results in greater range and time. “The key with the flow battery is duration,” he says. “The battery comes in a fixed size. The size determines the power. If you want more time, you add more liquid.” Hennessy adds that VRB has created 10 MW batteries by employing 50 of the battery units, each measuring about 2 x 3 x 8m.
A handful of manufacturers around the world are developing other technologies aimed at large-scale power storage. NGK Insulators Ltd., for example, has created a sodium-sulfur battery for utility “load leveling and peak shaving.” Known as the NAS battery, it comes in a module containing multiple cells, each about the size of a 1l soda bottle. The sealed battery operates by applying 300C heat to the liquid metal (sodium) and liquid non-metal (sulfur), which are separated by a ceramic material. By connecting many of the modules, NGK says it can create a battery capable of storing “several megawatts.”
Similarly, Wisconsin Power and Light says it plans to connect a superconducting magnetic energy storage (SMES) system — a giant battery of sorts — to its Cedar Ridge wind farm this December as a means of “bringing more stability to the grid.” SMES, in development for many years at the University of Wisconsin, stores electricity by flowing direct current over a coil that is cryogenically cooled to a superconducting temperature. Ultimately, researchers hope huge SMES rings — possibly the size of a football field — could be used to store gigawatts of power. Such rings, highly expensive because of the costly refrigeration systems needed to reach superconducting temperatures, would offer the benefit of 95 percent efficiency and minimal time delay in charging and discharging.
With a few exceptions, however, most of the battery-based storage technologies are years from fruition and/or faces serious cost, as well as technical hurdles. Unlike nuclear plants, which typically produce 1.5 to 1.6 GW of power, a lead-acid battery storage farm would be hard-pressed to produce 100 MW. Most battery-based facilities today are capable of far less than that.
“It's a challenge to store that much energy right now,” says Crabtree of Argonne. “There is no good way to do it and this has been recognized for 20 years.”
“It's a really, really tough problem,” adds Sadoway of MIT, who is working on the design of his own massive liquid-metal battery, which could have cells measuring 10 x 10 x 10 ft. “The issue isn't simply the amount of storage. It's the ability to uptake charge and deliver it quickly. When the peak demand arrives, you have to be able to deliver the energy.”
Still, experts say wind and solar can't reach truly significant levels without some form of storage. In essence, the problem is that wind and sunlight are variable and utility companies have problems matching variable resources to variable customer demands.
“The whole argument that you put more wind in to give the utility a 'greener' footprint runs into a very real technical challenge,” Hennessy says. “It's the reason utilities push back against wind farms.”
Still, most experts are squarely behind the idea of storage. Without it, they say, the country would have to turn to more coal and gas turbines as a means of balancing the renewables.
“If you have to go to more coal and more gas to balance the wind, then environmentally you're defeating the purpose,” says Mansoor of EPRI. “That's why storage is so important. It's the ideal balancing resource.”