Green energy has a storage problem. Wind and sunshine are intermittent, which means they don't produce energy we can utilize at will. Sometimes the sun just doesn't shine or the wind doesn't blow, and other times we have an overabundance of both. This doesn't correspond to our own, fluctuating demand for energy, which is why we need to store more energy to balance our grid.

Innovations in battery technology are advancing quickly to increase their efficiency and storage capacity, but so far not quickly enough to cover the scores of renewable energy projects rising up across the world. That’s why scientists and companies are developing alternative ways to store energy—and many of these large-scale options involve retrofitting, or reimagining, existing energy storage technologies for the 21st century. 

Deep under the surface of Texas and Louisiana, massive salt caverns store hundreds of millions of barrels of petroleum. This is the Strategic Petroleum Reserve of the United States. Created in the 1970s after the oil embargo, it remains one of the largest stores of petroleum in the world.

The reserve of course isn't something the public would usually associate with a sustainable energy transition, but the way in which it was built might offer a solution to decarbonize our energy system. The salt caverns that are today pumped full of dirty petroleum might tomorrow house compressed air or excess wind and solar power that’s been converted into green hydrogen. 

"Salt has some very useful characteristics if you want to store energy," says Oliver Duffy, research scientist at the University of Texas at Austin. "It serves as an excellent form of isolation."

These salt layers can be massive—the size of Mount Everest—and are generally located between .3 and 1.2 miles (or 500 meters and 2 kilometers) deep. To make a cavern in these deposits, which acts like a storage tank, engineers start by drilling into the ground. When they reach the salt layer, they inject a stream of fresh water that dissolves the salt around the drill hole. Afterward, they pump out the resulting brine, which leaves a cavern in its place.

"These caverns can range dramatically in size," Duffy says. "The largest ones can engulf the Empire State Building, and a given salt structure may be large enough to host several caverns of this size."

Building these caverns, and their ultimate use, does pose some environmental dangers. A lot of water is needed to dissolve the salt, and disposing of the brine in an environmentally friendly way is challenging. Additionally, if these projects are badly managed, they can contaminate water supplies, leaks can occur, and a ground subsidence—or cave-in—can happen. These caverns might also store blue, gray, and brown hydrogen, which are produced with energy that isn’t 100 percent renewable. 

The world’s largest green hydrogen storage facility—which relies on salt caverns—is currently in development in Utah, partially funded by the US Department of Energy. The hydrogen stored here will be converted from excess wind and solar energy to service western states during the wind and solar off seasons. 

And hydrogen isn’t the only renewable energy source that might be stored in salt caverns: Compressed air is also an option. As this stored compressed air is released, it runs through a turbine and generates energy. Salt caverns offer a more efficient option than storing compressed air above ground because constructing tanks of this size would be very expensive. In 2022, China opened its first salt cavern storing compressed air.

Despite its myriad applications, the rate at which the renewable energy sector is growing may outpace the number of salt caverns available. "There needs to be a mind shift in the industry to scale up the development and use of the technology," says Duffy.

Another old energy storage technology that's getting a fresh look is pumped storage hydropower. Instead of letting water pass only one time through a dam, as with traditional hydropower, it is partly collected in a basin at the bottom for reuse. If energy needs to be stored, powerful pumps move it back up toward the reservoir in front of the dam. When energy is needed, that same water can flow through the dam again.

"Pumped storage hydropower is a mature technology", says Professor Sebastian Sterl of the Free University of Brussels in Belgium, where he specializes in hydropower. "We know how much it costs and produces. There's also very little loss of energy."

At the border between Virginia and West Virginia, the largest facility of this kind—Bath County Hydro Pumped Storage Facility—has operated since 1985. It is often nicknamed “the world's largest battery.” And in 2018, the Los Angeles Department of Water and Power announced it would convert California's Hoover Dam into a pumped storage hydropower installation. 

Across the world, interest in pumped storage hydropower is also booming. In 2022, Switzerland completed an installation with the same energy storage capacity as 400,000 car batteries. Spain, Bulgaria, and Finland have all launched similar projects in the last few months alone.  

A future with more droughts and decreased water supplies might hinder these developments. Yet pumped storage hydropower is less vulnerable than regular hydropower because the former recycles water. Pumped storage hydropower also only needs two (possibly artificial) lakes with a height difference to work. It doesn't always require a river with a strong flow.

Pumped storage hydropower works so well that one start-up wants to transplant these systems of pumps and turbines to the ocean floor—where water supply and pressure are certainly no issue.

In the Netherlands, engineers at the University of Groningen began developing what they call an ocean battery for the bottom of the sea in 2014. In 2018, they spun the work out into a start-up called Ocean Grazer. 

Their idea is to build reservoirs on the bottom of the ocean under high pressure, like a submarine. When there is a surplus of energy, the reservoirs can be emptied against the water pressure, using pumps. When energy is needed, the reservoirs are opened, and pressure pushes water inside, powering a turbine to generate electricity.

"The basic technology is not the biggest challenge we're facing," says Marijn van Rooij, cofounder and chief technology officer of Ocean Grazer. "The challenge is how to build the reservoirs on the seabed as cheaply and scalably as possible."

Ocean Grazer first hopes to build some smaller installations at the bottom of lakes, and within five years to start building on the seafloor. The idea is to put these ocean batteries close to offshore wind farms, which increasingly dot the nearby North Sea. Surplus wind energy can, in this way, be stored close to its source.

"We needed this kind of technology yesterday," van Rooij says. "A lot of energy is already lost today because we cannot store it. We need more energy storage, and we need it now."