Compressed Air Underground Battery for Wind Farms
by John Laumer, Philadelphia on 01. 3.06
Wind energy may become even more important if the Iowa Association of Municipal Utilities succeeds in launching an innovative wind-powered compressed air energy storage (CAES) facility project they are working on. CAES technology uses off-peak wind turbine generated electricity to pump compressed air into an underground aquifer for use in later generation. The concept of using stored compressed-air energy to help generate electricity is more than 30 years old. Two plants currently exist—an 11-year-old plant in McIntosh, Ala., and a 23-year-old plant in Germany, both with the compressed air stored in caverns created by salt deposits. For you non-engineers, there's a step-by-step explanation after the fold. Caveat: from an environmental standpoint, CAES is not suitable for every wind farm. Saturating free-moving groundwater with pressurized air would change the redox state while the added carbon dioxide would dissolve calcite, with the combined effect of mobilizing metals. The result could be unfortunate for nearby well owners and would lessen energy efficiency. And CAES is definitely not something to try around a deep waste injection well that has previously accepted volatile hazardous wastes. CAES technology, in otherwords, is suitable for locations where natural underground vaults, essentially places where groundwater can be put under pressure and not flow outward due to that pressure, already exist and have not been contaminated. Having said that, this is a pretty cool innovation. For once Iowa beats Minnesota.
CAES steps. This is essentially a peaking power design.
Wind turbines generate electricity which can flow directly to grid or, alternatively, power the site's air compressor bank.
Compressors run at "off peak" times when wind happens to be blowing strongly, but regional electricity demand is low, driving air down into a subterranean cavern of sorts.
Compressed air driven underground both dissolves in, and temporarily displaces, groundwater. The horizonatally displaced water is "contained" by surrounding aquitards, however, so the air remains under pressure for extended periods, ready to be let back up the pipes to the surface when needed. When air pressure is reduced, the previously displaced groundwater flows back toward the zone of lowered pressure, which is now under the "dome". This is the "battery-like" part of the design.
Air flowing back up the pipes, toward the non-wind turbines, is pre-heated by the combustion of natural gas...Our reading of the design narrative indicates this heating is from in-situ' combustion, not requiring a heat exchanger..., further increasing the air's pressure, prior to it's passing through the turbine blade chambers.
The hot compressed air turns the turbine blades just as would flowing water as it passes through a hydroelectric generator.
The turbines turn dynamos that generate electricity for the regional grid at peak demand periods.
"Rinse and repeat".
whats wrong with storing the energy as hydrogen again?
I like that last post. Use electrolysis to get hydrogen to store and burn later. Or storing the energy in a giant flywheel? I know that some companies working on flywheels for that very purpose. Seems that this idea, while perhaps fairly efficient, has pretty limited potential. Now you have to be where the wind is good, where there's transmission lines, and where there's these acquifer/vaults that have all those enviro-characteristics.
===authors' response follows ====
yes I think so too. This design uses off the shelf commodity equipment so that there's no financial risk for designers, parts are common, and insurability is easier. Typical of utilities in the US: no guts no glory. The other aspect is that something of this scale calls for a big wind farm and would be of no use for a two or three-fer projec.
hey (ahem!) why'd that come out anonymous?
in ontario there's been talk of building up reservoirs and turning the electricity into potential water pressure... i don't think its all that bad erik-- you dont need to be near a transmission line, you can fill up fuel cells and then ship 'em down to the city on a fuel cell powered or biodiesel-hybrid train....
fly wheels are, um, pretty fly. think those and ultracapicitors have been hugeley underrated.... i have heard that hydrogen is just highly efficient (compare it to all the losses you have, like, in the grid by the time the juice actually comes down to the cities the losses are INCREDIBLE
Turning it to hydrogen will certainly be one of the top choices once we have a better infrastructure to use it, I suppose.
One of the biggest problem with renewables is that they have unreliable power - wind may blow at 3 a.m. when you don't need it and may not when you do. Because of this, only an estimated 20% of power can come from current renewables (how this estimate was made, i don't know). Whether it's storage by hydrogen, gigantic carbon nanotube flywheels on superconducting magnets, or NaS batteries, some kind of energy storage is needed. All the above technologies are highly uncertain (for example, hydrogen that's stored is bound to leak and could lead to further degredation of the ozone layer, and platinum last i checked is expensive and in short supply), and one thing I've learned about renewable energy is that designers should use environmental conditions to their advantage and use their imagination in finding clever ways to find the best solution at any one site. This is a great example of that: a solution good for some sites, bad for others. The more diversity in design there is, the better shot we have at curbing global warming.
===== author's response follows =====
Excellent insights. Thank you for the reply. One note of caution however. To the best of my knowledge the risk posed to the stratospheric ozone layer by elemental hydrogen is speculative. ALthough it has high fugacity and may be reactive with elemental oxygen, I have not heard of this being modeled in a global context. Until that has been accomplished we should be cautious about giving less rational people in the media a basis on which to cast doubt on the merits of alternative energy forms: i.e. identify it as speculative. I will stand corrected if the modeling has been done.
might be true... but if you ask Amory Lovins, about that Caltech H=noO3, he says; "One cited source, a work by M.A. Zittel and M. Altmann, mentions 10 percent leakage only as a crude worst-case example. Zittel and Altmann actually wrote that hydrogen systems in Germany now leak as little as 0.1 percent, as compared to about 0.7 percent leakage from that country's natural gas system."
...that being said, i do agree that there will be BIOREGIONAL solutions to energy storage. Diversity in this regard will be as beneficial as diversity is in, well, nature!
Very good to hear, thanks for the clarification. I am today one tiny bit more optimistic about the future.
Yay Karls more optimistic!!!!!
..wanna be a littel bit more optimistic? consider this (from EMag)a microcosmic version of our discussion... not only is it a happnin, but the guy seems to toally rock!
did that link not work?? ok, well here, consider this then:
http://www.emagazine.com/view/?2992
The reason we don't make hydrogen or use flywheels instead of using CAES is that CAES is much cheaper (than both) and more efficient (than hydrogen). Flywheels and fuel cells/electrolysers are still expensive and CAES, where the suitable geological formations exist, is cheaper. Also, here's the big rub for hydrogen:
Electrolysis is only 65-70% efficient. Then you have to compress or liquidify the hydrogen to store it. I'm not sure the losses for compression but lets say 10%. Then you have to run the hydrogen back through a fuel cell to get electricity at 40-60% efficiency. Lets be on the high end and say 70%*90%*60% = 37.8%. That means for every megawatt of off-peak wind power used to make hydrogen, you only get .378 megawatts of electricity out when you need it.
Compare this to CAES which gets 82% total efficiency at the old plant in Alabama. CAES is thus over twice as efficient as electrolysing hydrogen and then running it through fuel cells...
==== author's response follows ====
Concur with this statement but for one point. There are several types of fuel cells, some far more efficient than others. 95% total fuel efficiency is possible in some configurations. The number you cited might be a low one.
Oh, and number9plastic, the losses from transmissions, as they are described in that link you posted, are only 10%. That's not insignificant but its not "INCREDIBLE". And you will get very comperable transmission losses from piping hydrogen around the country if we end up using it as an energy carrier.
Sorry, the units should have been megawatt-hours not megawatts in my previous comments since we're talking about energy storage. My bad...
Has anyone investigated underground storage tanks for compressed air? Could they be old metal or plastic tanks surrounded by steel reinforced concrete? The advantage would be higher compression (100-200+ psi) and on-site location. Pressure valves would insure safety. Compressed air could run almost anything that takes a motor.
Energy storage technology are best thought or as "shock absorbers" for a power system. By helping to balance system requirement, the overall system cost can be reduced. Although the commodity value of the stored electricity is important, the ability to reduce capital expenditures is very important to project economics. For large wind farms, the idea is not to absorb all of the power, just enough to gurantee a minimal supply when the wind is not blowing so the wind farm can sell relaible power.
For small wind installations (island grids) many times they need storage installations to minimize the use of diesel generators. BTW - CAES: Developers also envision utilizing salt dome storage (like an underground natural gas storage facility - as the Alabama and Germany facilities use) or possibly an abandoned hard-rock mine.
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