How Hydrostor Is Solving One of Clean Energy’s Biggest Challenges

The world needs all the renewable energy it can get as nations begin to move away from a carbon-heavy power supply and toward a clean energy future. Yet even today, a surprising amount of that much-needed clean energy goes to waste. When a wind or solar farm makes more energy than it can use or sell elsewhere, for example, it may deactivate some of its capacity.

This practice, called curtailment, is an example of a growing and urgent challenge. When renewable sources such as solar, wind, geothermal, and hydropower make up a minority of the energy supply, they can integrate with the power grid with relative ease. But when there is ample clean energy to dominate supply, utilities and governments run into a problem: A power grid made for the constant, predictable nature of burning fossil fuels doesn’t match with more intermittent renewable sources.

To overcome this incongruity, and to enable an economy that doesn’t run on fossil fuels, the world needs all kinds of technological innovations to store clean energy when there’s a surplus and use it later when it’s needed. A suite of solutions, called long-duration energy storage (LDES), is already making it possible to use solar energy when it’s not sunny and wind power when the breezes are calm. Even more useful would be the ability to save energy for not only hours but days in order to make a clean energy grid as consistent and reliable as possible. This is the vision behind Hydrostor.

Hydrostor’s technology, a patented form of advanced compressed air energy storage (A-CAES), relies on elemental forces like compressed air, heat, and water to store energy for days on end. “It's really using tried-and-true generation techniques, but also some of the gifts that we get with Mother Nature, to be able to use water and air in a different way to create a clean electricity solution,” says Hydrostor’s Scott Bolton.

A-CAES begins with a large underground cavern built out of rock, one that is filled with water when the system is not in a “charged” state (when the battery is empty, so to speak). To store a large amount of surplus energy, Hydrostor runs compressors that create hot, pressurized air. The heat is extracted and stored aboveground for later, while the cooled compressed air is pumped underground. There, the air pressure lifts water out of the cavern and into a small above-ground reservoir. When it’s time to discharge the energy and give it back to the power grid, the Hydrostor system runs in reverse. The water is released from the above-ground reservoir, at which point, with gravity’s help, it pushes the compressed air back to the surface. Once the compressed air is reunited with the stored heat, it can expand through a turbine to generate electricity.

Rendering of Hydrostor's advanced compressed air energy storage (A-CAES) technology. Hydrostor

What Is and What’s to Come

LDES technology comes in many forms, and all of them will be needed to meet ambitious clean energy goals, such as the International Renewable Energy Association's prediction that the world will meet 90 percent of its needs with renewables by 2050. Widely deployed technologies include lithium-ion batteries like those inside electric vehicles.

Such systems have already provided grids with the ability to store some solar energy during a sunny afternoon to use at nighttime, when people return from work and demand spikes. But they come with key limitations. Battery manufacturing is a resource-intensive process that requires rare and expensive materials, which are typically imported from foreign countries. And similar to your cell phone’s battery, degradation means a battery used for grid storage must be replaced every decade or so. Although batteries are useful for four to six hours of energy storage, they aren’t a great solution for 8 to 10 hours, if not more.

“We're seeing pretty rapid deployment of short-duration lithium-ion batteries today, but that need will only grow into longer durations and the need for more diversity of technologies to support the grid,” says Bolton. “You don't want to rely on just one technology such as lithium-ion.”

Hydrostor’s Goderich facility. Hydrostor

The need is clear in places that have reached a renewable energy tipping point. California has spent years trying to figure out solutions to its infamous “duck curve” problem, one that some are now calling a “canyon curve” with increasing severity: Solar production peaks in midday, when the state sometimes produces more than it can use, but falls off in the evening because the sun sets just as overall energy demand ramps up. LDES would help to smooth out that mismatch, saving solar in the afternoon to use in the evening and overnight hours. That’s why California has mandated that its utilities acquire LDES solutions, including a gigawatt of 12-hour storage and another gigawatt of multi-day storage. The goal is not only to smooth out the duck curve but also to make the state’s increasingly renewable power supply consistent enough that California could one day retire its remaining natural gas plants.

Because it can store energy for days with minimal losses, Hydrostor’s A-CAES is an ideal solution to fit these needs, and the company is already developing a 500-megawatt facility to be sited in California. And A-CAES has other advantages beyond existing tech. Earlier forms of compressed air energy storage used some of the same principles as Hydrostor’s A-CAES to store energy underground. However, those technologies required taking advantage of existing salt chambers, which limited where they could be built, and depended upon burning natural gas during the process of returning saved energy to the grid.

Rendering of Willow Rock project in California. Hydrostor

Instead, A-CAES uses underground caverns that are purpose-built out of rock, which allows the system to be constructed in many more high-value areas — such as right next to an energy-hungry data center. Hydrostor’s method for saving heat means no fossil fuels are used through the process. A 500-megawatt facility requires only about 100 acres of land and can last for 50 years. The reservoir must be filled with water only once, a crucial fact at a time when water supplies are becoming more precious.

Leading the Way

It’s not just solar- and wind-heavy places such as California and Texas that need LDES. Across the United States, renewables are closing in on natural gas as the top energy source in the country. At the same time, the world is making a major electrification push. Replacing gasoline cars with EVs and gas stoves with electric ones will lead to rising demand for clean electricity, as will the growth of energy-hungry applications such as cryptocurrency and AI-supporting data centers.

This acceleration makes it ever more pressing that states lead the way in setting energy storage goals to ensure a renewable-dominated grid can be just as reliable as that of the fossil fuel era. By setting ambitious targets for their utilities, states could spark the same kind of revolution for LDES that happened for solar energy in America over the past few decades.

“Long duration energy storage is really following the same policy pathway that we saw with solar and wind going back 15, 20 years ago with the establishment of renewable portfolio standards,” Bolton says. “So having that policy framework to encourage that commercial deployment of that next technology—and the next technology being storage—is a natural evolution of that state policymaking.”