In a world in desperate need to go carbon-free, solar, wind, and renewables are the future. But our present ability to store their excess energy falls short. In fact, most of the time, that power goes missing. Of the many solutions swirling around, air—or rather, compressed air— seems to be sweeping some in the energy sector off their feet. Compressed air energy storage, or CAES, takes advantage of air’s ability to be stored in the form of elastic potential energy until we’re ready to use it. Lauded as a sustainable way to make renewables the world’s go-to energy source, this storage system could be the one we’ve all been waiting for. But we’re currently grappling with modernizing the world’s power grid. A major challenge is to make renewables reliable in every situation, not just on sunny, cloud-free days. These energy sources need to be “firmed,” meaning that they give energy on-demand. As of present, large-scale lithium-ion batteries are the most developed technology used to store and distribute this energy, and are already being readily adopted by renewable power sites across the globe. But as we probably all know by now, work remains to be done to prolong the lifespan of these batteries and make them less expensive. So as the search for long-duration storage solutions continues, what have we got so far? Well, one contender is compressed air storage. The technology is kind of like a giant version of the canned air that blows crumbs loose from your keyboard, except that instead of storing gases in liquid form, CAES involves converting electrical energy into high-pressure air, which is later released to drive a turbine. Theoretically, CAES could be a cheaper, more sustainable alternative to batteries. And since its based off technology that has long been used by the oil and gas industry, the foundational knowledge is there, ready to be worked on and improved. But there are a few things holding it back. Conventional CAES systems require having a place, like an existing or solution-mined rock cavern, that’s big and stable enough to store compressed air at a high pressure. This geographical limitation has meant that the technology can’t always live where grid needs are greatest. Then there’s the issue of conventional CAES systems not storing the thermal energy that’s generated during charging. As air is compressed, it heats up, and as air expands, it cools down. If the thermal energy generated during compression isn’t stored, a large chunk of energy during charging is lost to the atmosphere. To circumvent this problem, conventional CAES systems heat the air prior to expansion by burning natural gas, which not only deteriorates the system’s efficiency but seems utterly pointless if we want a greener system. For these reasons, CAES hasn’t taken off—there’s actually only been two plants built to date. But the Canadian company Hydrostor still thinks that renewables and CAES— or rather, what they call “Advanced” CAES—make a perfect pair. While their core idea is nothing new, their approach is different. Rather than relying on salt caverns to store air like conventional CAES systems, Hydrostor can also use hard-rock caverns, which can be built at practically any site. Using subsurface caverns that are connected to a ground-level water reservoir, compressed air is sent down to the cavern to be stored, displacing water up a shaft to the reservoir. When there’s a power need, water is allowed to flow back into the cavern, forcing the compressed air back to the facility. This technique improves the system’s efficiency and reduces the amount of space needed to store the compressed air. It also means that the system can be deployed near places where energy is most needed, like cities. And to avoid those pesky thermal energy losses? Hydrostor’s technology stores the heat generated during charging and reintroduces it to the air prior to discharge, eliminating the need to burn gas altogether. A Hydrostor plant can currently recover about 6 units of electricity for every 10 that it stores. That’s about a 60% efficiency—significantly better than the 40% of traditional CAES systems. While work remains to be done, Hydrostor already has a 1MW demonstration facility up and running in Toronto, Ontario, and their 2MW commercial contracted facility in Goderich, Ontario was just completed. They’re now bidding to expand out with much larger projects— ranging in capacity from 20-500MW—in the U.S., Canada, Chile, and Australia. So will Hydrostor’s take on CAES be the storage technology that renewables need to kick fossil fuels for good? Time and investment in the tech will soon tell whether the idea behind compressed air storage is just a bunch of hot air, or a system that will let us draw from 100% renewables, 100% of the time. For more innovative tech like this, check out this video on a company that’s capturing carbon and storing it underground. If you liked this episode, let us know in the comments below. Don’t forget to subscribe for more Seeker and thanks for watching.