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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, 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 10
9 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).
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]
References:
- London Array Web Site.
- Erik Spek, "Battery of Possibilities," Letter to New Scientist (3 February 2007), issue no. 2589, p. 21.
- Declan Butler, "Fridges could save power for a rainy day," Nature Online, February 7, 2007, doi:10.1038/news070205-9.
- BP McGrail, CL Davidson, DH Bacon, MA Chamness, SP Reidel, FA Spane, JE Cabe, FS Knudsen, MD Bearden, JA Horner, HT Schaef and PD Thorne, "Techno-economic Performance Evaluation of Compressed Air Energy Storage in the Pacific Northwest," Report No. PNNL-22235 of the Pacific Northwest National Laboratory, February, 2013.
- James Glanz, "Data Barns in a Farm Town, Gobbling Power and Flexing Muscle," The New York Times, September 23, 2012.
- Josie Garthwaite, "Too Much Wind Energy? Save it Underground in Volcanic Rock Reservoirs," National Geographic, July 1, 2013.
- Matthew Lynley, "SolarReserve Snags $737M Loan Guarantee for Solar Power Tower," The New York Times, May 20, 2011,
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