The Pacific Northwest National Laboratory has completed a study that comes up with two ways to use compressed air technology to store wind energy in underground chambers, the national lab said Monday. The two ways both use data and computer modelling to figure out the best sites that could successfully bank wind energy to be used at a later time.

Compressed air, as its name suggests, makes use of an electrically powered air compressor that sends pressurized air into a storage facility, which can be man-made or an underground reservoir. The pressurized air is let out later to run a turbine and generator to produce electricity. As much as 80 percent of the electricity used to compress air can be recovered when the pressurized air is used to generate energy, the lab said. Power losses are common when converting one form of energy to another.

Power in under ground caves

Utilities in the Northwest have a good reason for taking a look at energy storage technology. Wind power makes up about 13 percent (8.6 GW) of the power supply for the Northwest, the national lab said. Wind power tends to be most plentiful at night, when demand is at the lowest. Storing wind power for use during the day would help utilities meet their customers’ demand and manage their grids, which run smoothly when there is a balance of supply and demand. That prompted the Bonneville Power Administration to work with the lab to look into whether compressed air would be a good fit.

Many U.S. utilities or power producers have done preliminary studies or even pilot projects to check out different types of energy storage technologies, including various types of batteries. Often their regulators require them to gradually increase the amount of renewable energy they supply to their customers. Wind and solar have been popular choices, but they don’t generate a steady supply of electricity around the clock. Here is where energy storage comes in handy to help utilities manage their supply and demand.

The researchers were looking for two suitable underground sites for storing compressed air. They used data from gas exploration in Washington state and a computer model that simulates the flow of fluids underground. The idea is to see how much air a site can hold and how easy it’d be for the air to be harvested for power generation. For the study, an ideal underground storage would be at least 1,500 feet deep and 30 feet thick, and it should be close to transmission lines, the lab said.

Pacific Northwest

They found two locations, a place by the Columbia River, just across from Boardman, Ore., and another one in the Yakima Canyon that is roughly 10 miles north of Selah, Wash.

Sasquatch watches over the lobby of Facebook’s data center in Oregon

The scientists then sketched out two different processes for storing and re-using energy. At the Columbia River location, which is close to a natural gas pipeline, a compressed air storage plant can use natural gas to heat the compressed air and in the process boost the amount of electricity that can be produced.

At the Yakima location, the facility can use geothermal heat to run a chiller, which will in turn cool the air compressor to make it run more efficiently. Geothermal energy also can heat up the compressed air when it’s released from storage.

Bonneville will now take the results of the $790,000 study and do a round of cost-and-benefit analysis to figure out if compressed air makes for a good business case.

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Think of water rushing in through the open hatch of a submarine, and you get an idea of the forces at work underwater. Atmospheric pressure and the weight of the water combine to create pressures that compound with increasing depth. At a depth of 400 meters (almost a quarter mile), for example, the pressure is that of 40 atmospheres, one atmosphere being the pressure we experience at sea level. Subhydro envisions installing large concrete tanks at depths of 400-800 meters, and the deeper the better for maximizing energy generation.

When the “hatch” is opened, water is allowed to flow into the tanks through a turbine that drives an electric generator. The more and larger the tanks, the longer the generation can go on. When the tanks are filled, the turbine can be reversed to pump out the water, a process that draws on the power grid and consumes energy. In this way, the pumped storage plant functions like an underwater battery that can be re-charged, much like a hydroelectric plant on dry land pumps water into an upper reservoir after it has passed through a turbine.

According to Subhydro, the efficiency of the underwater plant is about 80 percent, comparable to efficiencies achieved at conventional plants. Integrating the pumped storage plant with wind or solar farms could create a grid storage system that harnesses excess renewable energy generation to pump out the tanks and flood them during peak hours of production.

Another approach to underwater grid storage is in the works at a depth of 80 meters in Lake Ontario, just off shore of Toronto. There, Hydrostor will begin building underwater tanks that will hold compressed air. Surplus energy from renewables (wind, solar) will provide the energy to compress air from the atmosphere and pump it in to the tanks. To put energy back into the grid, the air is allowed to surface, driving generators as it expands back into the atmosphere.

Hydrostor is partnering with Toronto Hydro to build the 1MW/4MWh compressed air energy storage demonstration facility. The system will run at 70 percent efficiency, according to Hydrostor. Earlier this month MaRS Cleantech Fund announced an investment in Hydrostor’s tech.

Clearly, there are still some hurdles to overcome before energy companies everywhere take the plunge. The environmental impact of offshore submerged facilities will need to be considered, as will the building materials themselves. To withstand the underwater pressure, Subhydro is working with research partners to develop thin concrete reinforced with steel fibers, while Hydrostor’s system will use inflatable polyester bags to hold compressed air. Building underwater facilities is itself energy-intensive, so whether the process can be made cost and energy-effective will determine whether cleantech is ready to get its feet wet.

Image via Knut Gangåssæter/Doghouse

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The technology might sound like some boring power infrastructure system — and it is. Basically. But air energy storage technology could also enable grid storage cheaply enough to help the solve the problem of adding variable clean power to the grid. Solar and wind power can only be generated at certain times of the day. If wind farms and solar projects were coupled with cheap energy storage like compressed air tech, it could make them a lot more economical.

That’s one of the reason that entrepreneurs, investors and power companies are looking to develop the next generation of this “air battery” technology. While the idea behind compressed air energy storage has been around for years, a new crop of startups has been working on making the systems more efficient and lower cost. Here’s three startups and one consortium that are working on this next-gen tech:

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Compressed air is a decades-old technology which takes excess energy from a power plant or renewable energy and uses it to run air compressors, which pump air into tanks or underground caverns where it’s stored under pressure. When the air is released, it powers a turbine, creating electricity. There’s only a handful of compressed air energy storage projects in the world, including one in Alabama and one in Germany.

But there’s a new group of startups building the next-generation of this technology, including SustainX and competitor General Compression. A key to the new technology is a water spray that keeps the air at a constant temperature through the compression and storing process. In January SustainX announced a patent for that technology, and says it holds three patents around its technology. SustainX’s Kepshire says the company uses off-the-shelf parts and can offer energy storage at a disruptively low cost.

SustainX has already built a 40 kW project at its headquarters in West Lebanon, N.H., which Zarrella said has proven the technology. To build the pilot, the company has now raised $14.4 million from investors including GE, Cadent Energy Partners, RockPort Capital, Polaris Venture Partners, and Angeli Parvi. In late 2009, the Department of Energy awarded SustainX a $5.39 million grant to help it reach that commercialization goal.

General Compression has been working on a similar technology, and says its technology has a 70-75 percent round-trip efficiency. Because the units can respond in less than 30 seconds and cycle between compression and expansion quickly, they could be used to back up wind farm power output, which is the company’s main focus. General Compression also raised a $17 million Series A round of funding and $9.9 million back in 2007. Investors include US Renewables Group and Duke Energy, a utility with a lot of wind power to back up.

After these two companies commercialize their technologies, we’ll see how widely power companies will adopt the tech. The idea is that compressed air energy storage can be cheaper — over the lifetime of the system — than other energy storage options like batteries, and SustainX estimates the market for grid-scale energy storage will be $18 billion by 2015.

Image courtesy of SustainX.

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In an interview on Monday, SustainX CEO Thomas Zarrella and founder Dax Kepshire told me that by the middle of next year, the company will start construction on a 1 MW compressed air energy storage project, likely at a coal plant, in conjunction with power company AES, its first customer. In late 2009, the Department of Energy awarded SustainX a $5.39 million grant to help it reach that commercialization goal.

General Compression, too, says it will soon start construction on its first commercial project, and is shooting to start in the fall of 2011, according to its website. I chatted with a General Compression exec at the ARPA-E Summit earlier this year (it received a $750,000 grant from ARPA-E), and the company said it has proven its technology via a pilot project.

In the world of compressed air energy storage, these moves toward commercialization are a big deal. Compressed air is a decades-old technology which takes excess energy from a power plant or renewable energy and uses it to run air compressors, which pump air into tanks or underground caverns where it’s stored under pressure. When the air is released, it powers a turbine, creating electricity. There’s only a handful of compressed air energy storage projects in the world, including one in Alabama and one in Germany.

SustainX has already built a 40 kW project at its headquarters in West Lebanon, N.H., which Zarrella said has proven the technology. To build the pilot, the company raised money from RockPort Capital, Polaris Venture Partners, Angeli Parvi and recently received attention from GE via its Ecomagination Challenge.

SustainX says its secret sauce is in how it uses a water spray to keep the air at a constant temperature through the compression and storing process. In January, the company announced a patent for that technology, and says it holds three patents around its technology. SustainX’s Kepshire says the company uses off-the-shelf parts and can offer energy storage at a disruptively low cost.

General Compression has been working on a similar technology, and says its technology has a 70-75 percent round-trip efficiency. Because the units can respond in less than 30 seconds and cycle between compression and expansion quickly, they could be used to back up wind farm power output, which is the company’s main focus. General Compression also raised a $17 million Series A round of funding and $9.9 million back in 2007. Investors include US Renewables Group and Duke Energy, a utility with a lot of wind power to back up.

After these two companies commercialize their technologies, we’ll see how widely power companies will adopt the tech. The idea is that compressed air energy storage can be cheaper — over the lifetime of the system — than other energy storage options like batteries. The systems also don’t require potentially toxic chemicals like batteries do, and aren’t site specific like pumped hydro is (you need a hill).

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Entrepreneurs and VCs are now looking for innovative ways to tweak this decades-old technology and startup General Compression has come up with an effective way to use compressed air for energy storage without burning natural gas. On Tuesday, the company announced the close of a $17 million Series A round of funding to help it build a commercial-scale unit to test the proposition.

The round was led by US Renewables Group and included Duke Energy, a utility with a lot of wind power to back up. It comes on top of $9.9 million the Newton, Mass.-based startup raised in 2007, according to Mass High Tech.

General Compression’s “GCAES” units use isothermal compression and expansion to generate power without burning any fuel, according to the company’s web site. The company claims its 2-megawatt modular units can store power at 70-75 percent round-trip efficiency. Because the units can respond in less than 30 seconds and cycle between compression and expansion quickly, they could be used to back up wind farm power output, which is the company’s main focus. As for a working model, General Compression has set an early 2011 date to start building its first commercial project — where and for which customer, it doesn’t say.

Nor does it list costs for its energy storage systems, either in kilowatt or kilowatt-hour terms. In general, big (100-300 megawatt) underground gas-fired CAES storage costs about $600-$750 per kilowatt of storage capacity built, according to the Electric Power Research Institute. Smaller scale (10-20 megawatt) above-ground CAES costs about $1,000-$1,800 per kilowatt and $200-$250$250 to $450 per kilowatt-hour, EPRI reported — still cheaper than pumped hydro to build, if not to operate, and cheaper in kilowatt-hour terms than the battery technologies EPRI surveyed in its 2008 cost comparison. (Here’s a handy primer on various energy storage technologies.)

General Compression isn’t the only startup competing to deliver low-cost, no-fuel CAES. UK-based Isentropic Energy says that its method of heating and cooling adjacent tanks of gravel and capturing the stored energy via a heat pump can pull costs down to $80 per kilowatt-hour. Right now there are only two working CAES plants in the world, one in Huntorf, Germany and another in McIntosh, Ala., so new technologies have a lot to prove.

It’s certainly the right time to be bringing new energy storage technologies to market. Any big push to make wind and solar power account for more than a tiny fraction of the world’s energy supply will have to deal with the intermittency problem — the wind doesn’t always blow and the sun doesn’t always shine, but the grid’s power demands follow predictable peaks that have to be met.

The Department of Energy’s $4.5 billion in smart grid grants late last year included $185 million for 16 energy storage projects. Among the batteries, flywheels, fuel cells and other technologies that got a boost were some CAES projects — Pacific Gas & Electric got $25 million to help build a 300-megawatt CAES plant near Bakersfield, Calif., and West Lebanon, N.H.-based SustainX won $5.39 million to develop its own CAES technology. New York State Electric & Gas is working on a CAES facility using an underground salt cavern, and municipal utilities in Iowa have been planning a CAES project since 2003, though construction hasn’t started yet.