Tikalon Blog by Dev Gualtieri

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Compressed Air Energy Storage

July 24, 2013

One problem with the various sources of renewable energy is the need to store energy when it's in surplus to use it at other times. The prime example is solar energy , which you can harvest only when the Sun shines, although you would like to use it at night. Wind power is also notoriously intermittent, although it's become a very popular renewable energy source.

On July 4, which is the anniversary of the US independence from Great Britain , the United Kingdom was asserting its own energy independence with the inauguration of the London Array wind farm located offshore in the Thames Estuary . This wind farm, which has a planned gigawatt capacity, is presently producing 630 MW . This makes it the world's largest offshore wind farm.[1]

Aeolus

Aeolus , the god of the winds in Greek mythology , is said to have had six sons and six daughters.

(Photograph of a marble relief by Ed Stevenhagen, via Wikimedia Commons.)

In this blog I've reviewed many means of energy storage, but few of these are applicable to grid energy storage . Surprisingly, some exotic batteries , such as the sodium-sulfur battery , are up to the task, and they have been deployed in systems at least up to 70 megawatt-hours.[2] I've written about another grid energy storage possibility, the flywheel , in a previous article (Flywheel Energy Storage, July 21, 2011).

One of the more interesting energy storage ideas is a virtual energy storage scheme that uses cold storage warehouses . I mentioned this system in an older article (Peak Power, March 2, 2007). Sietze van der Sluis of the Netherlands Organization for Applied Scientific Research calculated that the equivalent heat capacity of all large cold storage warehouses in Europe is nearly 10,000 megawatt-hours per degree Celsius .[3]

Letting the temperature of these warehouses drop by a degree during the night, and then letting them rise by a degree during the day, would allow storage of 50,000 megawatt-hours of energy.[3] All this is done without melting the food . Since the warehouses are already there, there's no new capital investment , although the longer continuous running time for the refrigeration units at night might lead to more required maintenance .

Another way to store energy, as I described in an earlier article (Potential Energy Storage, February 15, 2012), is to use the Earth's gravitational field as a source of gravitational potential energy . When power is available, you can pump water up a height, and later use the downwards flow to run a turbine-generator when power is needed.

Gravitational potential energy U in joules is given simply as

U = mgh

where m is the mass in kilograms , g is the gravitational acceleration (9.8 m/sec/sec), and h is the difference in height between the initial and final states in meters . A megawatt-hour is 3.6 x 10⁹ joules, so a height differential of a hundred meters would require 3.67 million kilograms of water, or 970,427 gallons, to store a megawatt of energy.

The main problem with this type of energy storage is the need to have juxtaposed reservoirs at a higher and lower height. One other option is to pump a compressible fluid, such as air, to pressurize it. Compressed air technology is quite advanced, since a substantial fraction of electricity is presently used to create compressed air for various industrial applications. Compressed air has been proposed as a vehicle power source .

Along with the cost of the pressure vessel and pumping apparatus , compressed air has the additional problem that it has a low energy density as an energy storage medium . A reasonable pressurization for air is about 4,500 psi (30 MPa ), and this results in an energy density of 50 Watt-hour per liter . This is comparable to a lead-acid battery , about a fifth as dense as a lithium ion battery , and only a half percent of the energy density of gasoline when converted to energy at a hundred percent efficiency .

Although volume energy density is a problem in a vehicle , it isn't a limiting number when you have a lot of liters to spare. When the salt deposited in geological structures called salt domes are dissolved away, huge caverns are formed that are often used for storing oil and natural gas . A salt dome at Huntorf, Germany , was used to create a 290 MW compressed air storage system in 1978 (see figure).

Huntorf, Germany, compressed air energy storage facility

The 290 MW Huntorf, Germany, compressed air energy storage facility stores air in a salt cavern solution-mined specifically for that application.

Multi-stage compressors are used for air injection, and reheat cycle combustion turbines are used for power production.

(Fig. 59 of ref. 4.)[4]

The US Pacific Northwest has an abundance of renewable energy sources, including hydroelectric and wind. The availability of cheap electrical power has lured many Internet giants to site data centers there.[5] At times there's actually too much power, and the federally-run Bonneville Power Administration has at times told wind farms there to generate less power than they're capable of generating.[6]

The region can produce 8,600 megawatts of wind power, but the peak production occurs in the same seasonal time period as mountain snow melt and an upsurge in hydroelectric capacity. A long-term energy storage solution is definitely needed, so scientists from the Bonneville Power Administration and the Department of Energy's Pacific Northwest National Laboratory have just completed a study that recommends compressed air energy storage in the porous rock structures in the region.[4]

Unlike the salt dome cavern approach, the compressed air would be stored in porous subterranean volcanic rock .[6] If this seems to be an unlikely scenario, you just need to remember the 1980 eruption of Mount St. Helens in Washington state . The study identified two sites with a storage capacity equivalent to powering 85,000 homes for a month .[6] The average US home uses about 920 kilowatt-hours of electricity each month, or 11 megawatt-hours annually.[7]

Similar formations have held natural gas under pressure for millions of years, and porous rock formations are common throughout the world. California's San Joaquin Valley also has geology suitable for compressed air energy storage, including depleted natural gas fields.[6] Some regions of Washington state could be used for a combined geothermal and compressed air facility, with the geothermal energy heating the expanding air and providing energy to cool the compression equipment .[6]

There's no question that the economics are there. As reported in National Geographic , the Bonneville Power Administration was selling electrical power in late May , when there's an overabundance of renewable energy, for a dollar per megawatt-hour[6] My residential electrical cost is more than a hundred dollars per megawatt-hour.

Climate change , with possible changes in the amount of rainfall and snowpack , may influence hydroelectric production, making storage of renewable energy that much more important.[6]

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