It’s a question that inventors have been tackling for decades. No one wants the fridge, or the hospital, going on the blink when demand surges or the power plant needs repairs.
It turns out to be a surprisingly tricky question to answer. Today, with the rise of green energy sources like solar and wind, the need for industrial-scale energy storage is becoming ever more vital to make sure there’s power even after the sun sets or the breeze dies down.
It’s usually (but not always) still too impractical to string together enough traditional batteries — those powered by chemical reactions, like the ones in smoke alarms and Teslas — to do the job. Instead, with remarkable ingenuity, technicians have relied on a host of physical forces and states such as temperature, friction, gravity and inertia to keep energy locked up for later release.
That’s why in Wales a power company engineered a special lake on a mountaintop. And in Germany a utility pumps underground caverns full of compressed air. Here’s how those and other systems — all in use today — work.
Compressed Air in a Cavern
Back in the 1970s, a German utility wanted to build a flexible storage plant that could respond to sudden peaks in electricity demand, since its conventional plants — mainly coal — weren’t designed to dial up or down quickly.
It didn’t have the hilly terrain needed for a hydroelectric plant, which can start operating much more quickly when demand surges. But here’s what it did have: ancient, underground salt deposits.
Borrowing a technique commonly used to store natural gas and oil deep underground, it piped water into the salt beds to dissolve the salt and create two caverns roughly a half-mile below the grassy fields in Huntorf. The plant, which opened in 1978, uses electricity from the grid, when it’s cheap because demand is low, to compress and store air in the salt caves.
Then, when electricity demand surges, a motor pushes the air to the surface and into a combustion system, where it burns natural gas that spins a turbine to produce electricity. Compressing the air allows it to deliver more oxygen to the turbines, making them more efficient.
A similar plant opened in 1991 in McIntosh, Ala. Several energy companies, mainly in the United States and Europe, are exploring mining their salt deposits for storage as well.
Molten Salt to Stockpile the Sun’s Rays
Out in the desert of Tonopah, Nev., about 200 miles northwest of Las Vegas, an enormous spiral of mirrors surrounds a concrete tower roughly 55 stories tall. Topped with a 100-foot heat exchanger formed of tubes, it’s not a relic of some mystical pagan rite, but the Crescent Dunes Solar Energy Facility.
It is the world’s first utility-scale concentrating solar power plant that uses extremely hot salt to extend the use of solar energy way past sundown.
Rather than using solar panels to produce electricity, the plant has more than 10,300 billboard-size mirrors that focus the sun’s heat on the heat exchanger, melting the salt into millions of gallons of 1,050-degree liquid that is stored until electricity is needed. The salt, which can stay liquid at higher temperatures than some other fluids like water, then flows through a steam-generating system that drives a turbine, producing enough electricity for 75,000 homes for as long as 10 hours past sundown — in essence, allowing the sun to shine at night.
Spinning Wheels That Power a Crane
On Kodiak Island in Alaska, the local electric cooperative got an unusual request from the shipping company that operates the port: Could it install an electric crane?
The company wanted to replace its aging diesel-powered crane with a new, and faster, electric one. It would be able to service larger ships and higher container stacks, making the shipping operations more efficient.
At first, the utility balked. The powerful crane would need to suck up tremendous amounts of electricity in short bursts. The local grid wasn’t really set up to handle that.
But after studying the proposal and potential solutions, it settled on the flywheel, which uses a rotor spinning in a vacuum to act as both a motor and a generator. Operating since 2015, the system uses grid electricity to accelerate the flywheels, which maintain their speed through inertia. When the crane lifts, the system converts the momentum of the rotors to electricity. And when the crane lowers, it recharges the flywheels, feeding power back to speed them up again.
The installation also helps the electric company balance energy fluctuations on the grid from its wind turbines, which provide about a quarter of the island’s electricity.