Energy Storage Wars

Compressed air technology is an underdog to watch

In the battle to scale up intermittent energy sources such as wind and solar, the team with the best storage strategy wins. While many companies are banking on new battery technologies to fill the gaps when the wind dies and the sun sets, a few are working on storing energy using a cheap, abundant resource—the very air around us.

Compressed air energy storage, or CAES, is elegant and simple in concept: use excess power from the electricity grid to compress some air, then let it expand and turn a turbine when you need the electricity back. “It’s a pretty simple idea,” says Steve Crane, the CEO of LightSail Energy, one company refining compressed air technology with the hope of making a dent in the storage market soon. “Like a lot of simple ideas, it is easy to describe, and it is fiendishly difficult to actually implement.”

It’s an idea long touted as a transformative energy-storage method. But early attempts at CAES have been held back in part by those pesky laws of thermodynamics: compressing air into a tighter space than it would prefer generates heat—a lot of heat. The heat losses translate into low efficiency—you need to put a lot of energy in to get just a bit of energy out, with the rest lost as heat.

To deal with the heat, technologies from new companies including LightSail, SustainX, and General Compression keep the compressing air at a near-constant temperature with the simple addition of a fine water mist. Water heats up far more slowly than air; instead of an increase of around 1,000 degrees Celsius with just air, the temperature may go up by only 40 degrees or so. And with that, CAES is no longer too hot to handle.

When it’s time to send electricity back to the grid, compressors run backward to expand the cool air, using the slightly warmed water to help the process. The expanding air then runs a generator or turns a turbine, and the electricity grid can send power off to homes just as it would from a power plant.

Another factor limiting CAES development in the past has been geology. To date, industry has managed to build only two full-scale compressed air facilities—one in McIntosh, Alabama, and one in Huntorf, Germany. In both cases, the air is stored in large underground caverns, salt formations from which the salt has been leached. Such geologic formations aren’t uncommon, but they are not evenly distributed around the world; just because you’re building a wind farm somewhere—say, Iowa—doesn’t mean you’ll find a handy cavern nearby. Alabama’s plant was built specifically because of the suitability of the salt formation underground, not because any particular power source was nearby.

LightSail and SustainX address the siting issue with modularity. Instead of using one big underground storage cavern, they store the air in smaller tanks or pipes which can be added or deleted as needed. The Alabama and Germany facilities both exceed 100 megawatts in size, whereas the new companies’ systems start in the one- or two-megawatt range and can build up from there. As it turns out, smaller storage units may be better suited to the needs of many renewable-energy projects. “We think for a 100-megawatt wind farm, 10 to 20 megawatts of storage capacity is probably the right place to be,” says Richard Brody, SustainX’s vice president of business development. “It provides firmer, more schedulable power to the grid.”

Despite their size, the modular tanks pack a bigger punch with higher energy density, a measure of how much electricity they can store in a given volume. In fact, the densities achieved in the newer systems can rival those of some expensive batteries.

General Compression’s version of CAES is something of a middle ground between the massive caverns and the megawatt-scale, pipe-and-tank systems. In December 2012, they commissioned a two-megawatt facility in West Texas that combines the new, water-based methods of keeping temperatures down with the older idea of using caverns (which happen to be abundant in the region). Company founder David Marcus says they are still ramping up to full capacity, but all is running smoothly so far.

Other creative compressed air solutions are taking off as well. For example, Thin Red Line Aerospace, a company that also makes shielding materials for spacecraft, wants to store air deep under water—in balloons.

CAES has drawn significant attention over the past few years. LightSail has received funding from some impressive sources, including PayPal cofounder Peter Thiel, Bill Gates, and other high-profile venture capitalists, while SustainX has attracted money from General Electric and the Department of Energy, among others. General Compression has raised around $100 million from a variety of sources. All three companies are aiming to start selling the technology to clients within the next year or so.

CAES backers think the technology might fit into a very sweet spot when it comes to energy storage. Batteries have their place, but they tend to be expensive and can cycle only a few thousand times before needing to be replaced. Other storage ideas all have significant drawbacks, from siting requirements to stubbornly high costs. CAES systems using common industrial air compressors would last at least 20 years, much more in line with the needs of the grid, and costs are likely to be very reasonable once the technology is mature. Research out of Sandia National Laboratories in 2011 found CAES was “the least expensive long-duration storage,” given “an appropriate geologic site.” Remove the geology requirement, and CAES may be on its way. ❧

—Dave Levitan

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