Compressed Air Energy Storage Makes a Comeback

Only a handful of compressed-air energy storage (CAES) plants have been installed since the 1970s. This week, SustainX is bringing the technology back to the U.S. electricity grid, albeit in a vastly different form.

The startup, based in Seabrook, New Hampshire, began operating a full-scale demonstration system that stores energy as compressed air in pipes and supplies 1.65 megawatts of power. The company will use the machine to gather data on performance and and to show off the technology to potential investors and customers. The project was funded by $5.4 million from the Department of Energy and at least that much from SustainX, according to a representative.

Conventional compressed-air energy storage uses a compressor to pressurize air and pump it into underground geological formations. The first two plants of this type put into operation—one in McIntosh, Alabama in 1991, and the other in Huntorf, Germany in 1978—use salt caverns as storage tanks, pumping compressed air in at night, when energy demand is lowest. During the day, the air is released, heated with natural gas, and forced through a turbine to generate power. The appeal of this technology is that it’s relatively low cost and can store many kilowatt-hours of energy.

SustainX takes a different tack: it uses compressed air as the energy storage medium, but holds the air in large pipes, the same used in natural gas pipelines. That means utilities or even commercial customers could place a storage device in a range of industrial locations, rather than only where there’s an underground formation available.

At the base of SustainX’s machine, called the S165, is the bottom half of a diesel engine normally used to propel ships. To store energy, a permanent magnet motor-generator turns the engine's crankshaft, driving six pistons located above it. The pistons, each of which is taller than a full-grown man, compress a combination of air and foamy water, which is then pumped into storage tanks. When power is needed, the air is released, driving the pistons and turning the generator to create a current.

A key difference between SustainX's technique and conventional CAES technology is that the compression and expansion of air are done at near-constant temperature and the process doesn’t require natural gas. And unlike conventional batteries, this system can vary the amount of energy independent of the power output. In other words, you can expand the amount of energy it stores simply by installing bigger pipes. That's different from a battery designed to deliver, say, 1 megawatt for 2 hours. If you wanted four hours of storage, you'd have to buy another battery--a more expensive approach, the company says. 

The target market for these systems is renewable energy project developers looking to firm up the output of wind and solar farms or to store excess wind energy at night for sale during peak times. Utilities could elect to install air energy storage instead of making upgrades to transmission and distribution lines to meet growing demand. SustainX expects that a typical configuration will offer between 10 and 20 megawatts and store four to six hours of energy, according to a representative.

The first customers are likely to be in Asia, in places where the price of natural gas prices is higher than it is in the U.S., electricity demand is growing rapidly, and there are problems integrating renewable energy, says Richard Brody, the vice president of business development. He expects the first systems to be installed next year in China. At volume, he projects price of energy can get to $500 per kilowatt-hour and the system can operate for 20 years.

There are at least two other compressed-air energy startups looking to get a toehold in the market for long-duration energy storage. Berkeley, California-based startup LightSail Energy is building a device designed to store compressed air in steel tanks, while General Compression of Newton, Mass., has a compressor operated by a wind turbine that stores air in underground caverns. Late last year, General Compression opened a two-megawatt CAES facility connected to a wind turbine in Gaines, Texas.

All of these companies hope to demonstrate that their technologies cost less and last longer than a host of battery alternatives. With few commercial systems for bulk storage currently online, it’s hard to evaluate which technologies will win out. But many companies are betting they can achieve a technical breakthrough that will make multi-hour energy storage cost-effective. 

Image: SustainX

Updated 17 September 2013: Changed to reflect the opening of a CAES plant last year. 

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