How Grid Energy Storage Works

On any ordinary day, electric power companies plan how much electricity to generate on the next day. They try to predict what customers will do, mainly by reading historical records of usage on the same day of the previous year. Then they adjust those figures to the current weather forecast for the following day.

"It's impossible to exactly predict what the demand for power will be at a given moment," says John Boyes, who manages the Energy Storage Program at Sandia National Laboratories. This scenario sets utilities up to make more or less electricity than customers use. The mismatch sends ripples through the grid, including variations in AC frequency, which, if not controlled, can damage electronics. Regional electricity managers, or independent system operators (ISOs), swoop in and try to close the gap by asking some power plants to change how much electricity they generate. But nuclear and fossil fuel plants can't do that quickly. Their slowness worsens the mismatch between electricity supply and demand.

Now, consider what happens on a sweltering day in Los Angeles when people citywide are running their air conditioners. These are peak demand conditions, when the most customers use the most electricity, which happens for a few hours on five to 10 days each year. On these days, facilities known as peaker plants are called into action. These expensive fossil-fuel plants sit idle all year and can emit more air pollution than a large coal-fired plant. "We wouldn't like to do it in a [smoggy] city like Los Angeles, but we do it anyway," says Imre Gyuk. If the peaker plants fall short, utilities pay large customers like aluminum smelters to use less electricity. "If nothing works, you have brownouts and rolling outages," says Gyuk.

Meanwhile, old substations are overloading. They're carrying more current than they're meant to handle, and the metal structures heat. "That's not recommended practice," says Boyes.

If the electric grid sounds stressed, you haven't seen anything yet. Read on.

Maybe it's not an ordinary day. Maybe a tree falls on a power line or lightning strikes it. These disruptions will knock the line's voltage off of the intended amount. Voltage variations reset computers. Now your alarm clock is blinking 12:00. Or worse: "For all automated manufacturing processes, if the computer resets, it shuts down the process. If you're a plastics manufacturer, and your machines cool down, plastic solidifies in your machines," says Boyes.

And what if a day's events exceed utilities' efforts to compensate? Yes, you guessed it -- you're facing a blackout. It certainly happened across the Northeast in 2003.

With the grid already scrambling, it's hard to imagine adding more renewables, like wind and solar power, because they are intermittent sources of power. We know customers are unpredictable, but now, so is the electricity. When the wind dies unexpectedly, a wind farm can lose 1,000 megawatts in minutes and must then quickly buy and import electricity for its customers.

The alternative then is to use a peaker-style fossil-fuel plant, but that adds air pollution to clean electricity. Or nature can reign. On wind farms in Texas, the wind blows almost exclusively at night while demand is low, and the price of electricity becomes negative. "That means you have to pay the grid to put electricity on it," says Gyuk. "I talked to someone who runs his air conditioning all night to chill the house because he gets it for free. Then he shuts the windows."

According to Gyuk, these problems will worsen as we use more electronics and more electricity. So what could be the answer to these problems? Grid energy storage.

Before we dive into the topic, it's important to understand what it means to store energy. The job of the grid is to deliver electricity to every customer at 120 volts and 60 hertz. This is accomplished by adding or removing current from the grid. A storage device helps by adding or removing current exactly when needed.

Read on to learn how energy storage can strengthen the grid.

Pumped hydroelectric stations use falling water to make electricity. An example of this can be seen at Raccoon Mountain in Tennessee. At the foot of the mountain, the Tennessee Valley Authority (TVA) made a lake by siphoning some of the Tennessee River.

When customers aren't using much electricity, TVA diverts electricity from other power stations to a power house inside the mountain. The electricity spins the house's turbines backwards, pushing lake water up a tunnel in the mountain to the top. After 28 hours, the upper basin is full. To make electricity, TVA opens a drain in the upper basin. Water falls straight through the center of the mountain and spins the turbines forward, generating electricity. It falls for 22 hours, steadily outputting 1,600 megawatts of electricity, matching the output of a large coal-fired plant. TVA adds this electricity to the contribution from its other plants on days of high demand [source: TVA].

Pumped hydroelectric stations are operating worldwide, outputting between 200 megawatts and 2,000 megawatts of power on peak demand days [source: Cole]. They emit no air pollution, and once charged, are online in 15 minutes, faster and greener than a peaker plant. The only problem is "we're running out of good sites for it," says Gyuk.

Compressed air energy storage (CAES) is storage for natural-gas power plants. Normally, these plants burn natural gas to heat air, which pushes a turbine in a generator. When natural gas plants are near an underground hole, like a cavern or old mine, they can use CAES. On slow days, the plant can make electricity to run a compressor that compresses outside air and shoves it into the hole underground. On days when customers need maximum electricity, the power plant can let the compressed air rush out against the turbine, pushing it, along with the normal heated air. This compressed air can help for hours, steadily adding 25 megawatts to 2,700 megawatts of electricity to the plant's output on peak demand days [source: Cole].

Keep reading to learn where else we can store energy on the grid.

Now it's time to look at storage that supplies a big burst of big electricity or less for longer. These systems can't send big electricity to customers all day, like pumped hydroelectric and CAES can.

Flywheels store energy by spinning. The fastest ones consist of a motor, a levitating magnet, a vacuum to nix friction and a shell for safety. When there's extra electricity available on the grid, it can run the motor, which spins the magnet. When electricity is needed, the flywheels can spin it out in minutes to hours, as the situation requires.

On the electric grid, flywheels make good quality controllers. They're good at steadying frequency, which, as we've mentioned, wobbles above and below 60 hertz in the U.S. today. It spikes when utilities make more electricity than customers use and dips when utilities make less. Flywheels change the situation because ISOs can control them directly -- eventually, they'll be automatic -- so that no one has to call Jane at power plant A and wait for her to raise or lower generation to correct the frequency problem. With fast response, the frequency can be leveled before the customer feels it. In fact, several U.S. I.S.O.s are testing flywheel pads [source: Beacon Power 1, Beacon Power 2, Beacon Power 3].

Another use for flywheels is steadying voltage on the grid. What could possibly change the voltage on those sturdy high-voltage lines? Try domino effects from power outages, downed trees and electric trains. When subway or light rail trains brake, they generate electricity, raising voltage and making current surge locally. When trains accelerate out of the station, they draw electricity, making the voltage dip and sucking current from elsewhere. Flywheels can absorb and release the current, leaving the rest of the grid undisturbed. In fact, they've been tested on New York City's subway trains [source: Kennedy].

Flywheels are also great for wind farms, where they can spin up extra electricity during gusts and spit it out during die-downs, so customers don't suffer the fluctuations.

Supercapacitors, even speedier than flywheels, store energy by separating charges. They're "super" because they store more energy than traditional capacitors, but they work the same way. When there's extra electricity, it can be used to push charges off of some metal plates and onto others, leaving some positively and others negatively charged. When electricity is needed, the plates neutralize, and charge flows, making a current. In Madrid, Beijing and other cities, cabinets full of supercapacitors buffer electric trains [source: Siemens].

Superconducting magnetic energy storage, or SMES, is another way to get rid of voltage dips and spikes on the grid. During spikes, loops of wire take up extra current, and during dips, the loops return the current to the grid. Because the wire has almost no resistance, it stores current with almost no loss.

Next up -- power storage systems many of us use on a daily basis: batteries.

Batteries are like Lego sets for the grid. They come in many types, can be stacked or enlarged to store more energy and can drive electricity for seconds to hours. On the longevity end, you'll find trailer-sized flow batteries like vanadium redox and zinc-bromide and high-temperature batteries like sodium-sulfur. These can supply up to 20 megawatts of power for hours [source: Gyuk]. On the burst-of-power end, lead-acid batteries are commonly used today. Other batteries include metal-air, lithium-ion, nickel-cadmium and lead-carbon. All batteries use and release energy through chemical reactions.

Batteries are all over the U.S. electricity grid, usually on the customer side, where factories, and maybe the computers in your office, use an uninterruptible power supply, or UPS to run electronics running during outages.

But batteries also back up the guts of the grid. In Charleston, W. Va., a substation used to overheat every time too many customers drew current through it. Then American Electric Power installed a battery to supply electricity on peak demand days, and the substation stopped overheating. Alaskans used to suffer outages with every glitch on the power line between Anchorage and Fairbanks until they installed a soccer-field-sized battery to cover the line during failure and repair.

Batteries can also help wind farms in places where wind blows only at night and customers use energy during the day.

There's talk of one day using plug-in hybrid electric cars, or PHEVs, with batteries that charge by plugging into the wall socket, for commercial electricity. With the right wiring in your house, your parked car could run your dishwasher. In the far future, many cars plugged into many garages could send electricity to wherever it's needed on the grid in an application called vehicle to grid, or V2G. But it's many years off, since the wall socket can't take electricity from the battery, and the cars aren't commercial.

Does this sound practical? Keep reading to find out how much it all costs.

"When it comes to actual costs, energy storage is not cheap," says Imre Gyuk.

We can see where costs stand today, but they'll drop as more storage goes onto the grid. Let's start with storage at power plants. As we learned earlier, an electric company may store energy at a power plant to supply power on high-demand days. The plant will need big power all day, and only compressed air and pumped hydroelectric can supply that. For every $700 it pays for a compressed air system, the utility gets 1 kilowatt of electricity, supplied for more than 20 hours, enough to run one coffee maker all day [source: EAC, NSTAR]. Pumped hydroelectric costs more -- $2,250 per kilowatt.

For power that lasts minutes to hours, lithium-ion batteries cost $1,100 per kilowatt (or coffee maker), flywheels cost $1,250 per kilowatt, flow batteries cost $2,500 per kilowatt, and high-temperature batteries like sodium-sulfur cost $3,100 per kilowatt [source: EAC]. And storage in supercapacitors costs even more.

But, according to Gyuk, we get a lot for our investment into storage. We get a grid able to handle more wind and solar power plants, without supply nightmares. We get fewer peaker plants, which means less carbon dioxide emissions and air pollution. And we get protection against outages, which, according to Gyuk, cost 33 cents out of every dollar we spend on electricity [source: Gyuk 2008].

Electric power companies and ISOs will pay for storage, if they decide to install it. "The price of storage is coming down. The price of solving the problems in other ways is going up. Pretty soon, these prices are going to cross," notes Boyes, suggesting cost could spur the addition of storage to the grid.

Will consumers' electricity rates fall in the end? Maybe. With enough storage, utilities will be able to generate electricity in a more controlled manner. They'll better use the hardware in the grid, like transmission lines and substations, instead of replacing or enlarging them.

Even if consumers' electricity rates rise, "We'll get a better system," says Gyuk.