Editor’s note: “Storing That Power” is a seven-part series detailing technologies capable of reserving power obtained from renewable sources. Read each week to learn more about pumped hydroelectric, industrial-scale batteries, flow batteries, flywheels, compressed air energy storage, gravel batteries and molten salt.

Water is not the only material that can be stored and harnessed to be used for energy storage. Air can also be used as an energy storage medium with what is known as Compressed Air Energy Storage (CAES). Several different methods are being explored to use this type of energy storage, although the earliest examples have been in place for decades.

Using underground caves to store compressed air is a technology that dates back to the 1970s in Huntorf, Germany, where a 290 megawatt facility was built using a salt dome. A similar, though smaller (110 megawatt) facility was built in Alabama in the early 1990s. These facilities are both still in operation, but very little was done with the technology for many years.

One of the problems that compressed air storage faces is a thermodynamic effect where a gas heats up as it is compressed, which makes it increasingly difficult to store additional compressed air.

Although the storage conversion efficiency of CAES can be fairly high, the compressed air needs to be air as it is released, which typically requires an outside fuel source. The McIntosh plant in Alabama ends up burning an amount of natural gas to reheat the compressed air equal to about 1/3 of what it would use for direct combustion power generation. This makes CAES far less environmentally friendly than many other storage options. Overall, the efficiency of compressed air systems is also lower than many other storage systems, ranging from 45-70 percent.

Underground CAES systems also are reliant on location, much like pumped hydro systems. Suitable geological formations are necessary to have underground caverns that can be used to contain the pressurized air. However, it turns out these same formations are also desirable for natural gas storage, and, in the last decade, developers who were interested in setting up new CAES facilities often found that the gas industry was already using the available underground structures. Competition for access to suitable sites has also been a hindrance to development of CAES systems. But not all CAES systems are dependent upon locations with suitable geology.

Another compressed-air system being developed by a company called SustainX uses proprietary technology for isothermal compression (ICAES), which avoids the high temperatures and thermal losses. SustainX claims far lower costs than “conventional” compressed air storage, as well as almost 95 percent storage efficiency.

Instead of using caverns or geological formations, the SustainX method uses conventional, industrial, high-pressure bottles to store the compressed air. This enables the SustainX system to be installed anywhere, rather than relying on locations with salt domes or other usable geological formations. Because the process is isothermal, the air does not need to be heated or cooled, and, most importantly, there is no need to burn natural gas, which makes this approach preferable, particularly when it comes to storing power from clean, renewable sources. As yet, there are no installations of the ICAES system, but SustainX plans construction of a demonstration pilot plant to be completed in 2013.

Another especially interesting version of compressed air storage is the Energy Bag. Whereas in most instances, high pressure vessels or rock formations are used to contain the air under high pressure, the energy bag is a lightweight (75 kilograms or 165 pounds) bag that can store pressurized air sufficient to provide 70 megawatt hours of storage. The trick is that it does so deep underwater, where water pressure from the surrounding ocean provides the containment.

A 20-meter (65.6-foot) diameter Energy Bag at a depth of 600 meters (2,000 feet) needs no additional structure to withstand the great pressure of air stored within. The greatest drawback to the Energy Bag is that its application is limited to locations with ready access to fairly deep water, making it best suited for use with off-shore wind turbines. However, there may be some near-shore wave and tidal power generating applications that could also take advantage of this system, as well.

Like SustainX, the Energy Bag is a developmental system still going through testing. An initial investigation was begun off the coast of Scotland in the summer of 2011. Initial studies indicate that the Energy Bag offers a potential storage efficiency of more than 85 percent.

Main photo: Energy Bag, credit Thin Red Line Aerospace
Secondary images: CAES-Huntorf, credit U.S. Dept. of Energy; CAES diagram, credit Sandia National Labs