From wind to compressed air

[OKIsItJustMe]

http://www.grcblog.com/?p=258

March 21st, 2008 . 02:56 pm by Stephanie Hoffmann

From wind to compressed air

Wind blows night and day. You only get one chance to extract the energy of a passing breeze by a wind turbine. In an effort to make wind energy more like traditional fossil generation, large-scale energy storage to deliver the energy when needed would be highly beneficial.

The concept of adiabatic Compressed Air Energy Storage (CAES) is a promising candidate, representing an emission-free storage technology with a high storage efficiency, a large power capacity and a potentially competitive cost. The idea is to store inexpensive base-load power during off-peak periods in the form of compressed air and deliver this power during peak demand.

Existing CAES plants in Huntorf (Germany) and in McIntosh (Alabama) are “diabatic” concepts, where co-firing of natural gas is needed in order to compensate for the lost compression heat. Adiabatic CAES, on the other hand, uses a thermal energy store that recovers the heat of the compressed air during the charge of the air reservoir (typically an underground cavern). During the discharging phase, the compressed air exiting the reservoir is reheated in the thermal energy store, and is then expanded through an air turbine to produce power.

Adiabatic CAES offers significant improvements in cycle efficiency compared to the diabatic concept, and as no fuel is used, it generates no carbon dioxide. There are planned projects to implement adiabatic CAES, but so far nothing has been built as some challenges remain: how to design a cost-effective compression train and turbine train that meet the required fast ramp rates and good part-load performance? How to build a large thermal energy store with very limited heat losses over daily cycles and that operates at high pressures and temperatures?

...

[usrbs]

1. It sounds very cool

But could you explain this part?


Existing CAES plants in Huntorf (Germany) and in McIntosh (Alabama) are “diabatic” concepts, where co-firing of natural gas is needed in order to compensate for the lost compression heat. Adiabatic CAES, on the other hand, uses a thermal energy store that recovers the heat of the compressed air during the charge of the air reservoir (typically an underground cavern). During the discharging phase, the compressed air exiting the reservoir is reheated in the thermal energy store, and is then expanded through an air turbine to produce power.

What is the lost compression heat? How does adiabatic CAES get around it? I should be able to understand this, but I never excelled in physics.

[EmperorHasNoClothes]

2. When the compressed air decompresses, it cools

They're talking about adding heat to make up for that.

[OKIsItJustMe]

3. I'm not an expert, but I'll try to explain this as I understand it

When was the last time you pumped air into a tire? As you may recall, in the process, the air gets hotter. Naturally, it cools off in a little while.

Similarly, if you let the air out of a tire, you may notice the air coming out feels cool. (It's not just the "wind chill" you're felling.)


Okay, so... let's pump a bunch of air into the ground instead of a tire. As we pressurize it, it gets hotter. Naturally, it tends to pass that heat along to the ground around it (we're putting energy into warming the ground.)

Fine, so, let's pressurize the air, but before we pump it into the ground, let's pump it through some heat exchangers, to cool it down (ideally to the same temperature as the ground) and save that heat. The pressurized air is now at the same temperature as its container, so it won't lose any heat to it. Meanwhile, we've storing that heat we recovered using the heat exchangers.

Okay, now, when we reverse the process, we use the heat exchangers to warm the air, recovering the stored heat. (Or at least some/most of it. No process is perfect.)

[Fledermaus]

4. Internal combustion engines compresses air before combustion.

The air is precompressed and stored for later use. The stored compress air is later fed into the turbine, mixed with fuel and burned.

Yes compressing air heats it up and, decompressing it cools it. Apparently, the air looses some of its heat during storage and must be warmed back up.

[usrbs]

5. Thanks to all 3 of you that have explained the cooling and restoration.

Ok, so the air gets heated when compressed and cooled when released, now I have a follow-up question that's probably very stupid.

Are they using the heat exchange as energy, or the compressed air as a driver? (I think I saw a car engine designed to use just compressed air.) If they are using the air and not the heat difference, then why bother heating up the released air? Can't cold air work just as well?

If they are using the heat delta, then I assume the energy lost when heating the air up first to counteract the cooling is a lot less than the energy gained.

[OKIsItJustMe]

8. They're using both

The heat is restored to the air to drive a turbine.

Check out the "combined gas law."

    P is pressure
    V is volume
    T is temperature

Heat is a measure of the kinetic energy of molecules. (Essentially, molecules bouncing off one another is called "heat.") When the air is compressed, the molecules are closer to each other, and so they bounce off each other more (they're hotter.)

When the air is decompressed, the molecules move apart. They bounce off each other less. (They're cooler.)


So if you compress the air, but let the heat escape into the surroundings, you're losing kinetic energy that could be used to turn a turbine when you release the pressurized air. (The air will be more energetic if you restore that heat to it.)

[usrbs]

12. Good explanation. Thanks! n/t

[NNadir]

6. Actually in something called "the scientific literature," the external cost of wind driven

compressed air was covered.

In Environ. Sci. Technol. 2005, 39, 1903-1911, Paul Denholm, wind powered CAES advocate, found that compressed air storage of wind energy - which is industrially practiced in zero places on earth - must be pretty good, since it's external cost was "only" ten times higher than that of nuclear energy.

I've been hearing about compressed air storage here for years, and I first heard it about it from the Walmart flake greenwasher Amory Lovins in 1976, when he said it was "just around the corner."

I quote the Walmart quality flake directly from his 1976 paper:

In industry, wind-generated compressed air can easily (and, with due care, safely) be stored to operate machinery: the technology is simple, cheap, reliable and highly developed. (Some cities even used to supply compressed air as a standard utility.) Installing pipes to distribute hot water (or compressed air) tends to be considerably cheaper than installing equivalent electric distribution capacity.

Lovins, Amory, "The Road Not Taken," Foreign Affairs Summer 1976, page 83.

Sullen little anti-nuke thugs on this site assume that their appeals to ignorance and indifference are new. Basically these kinds of delusions are at this point, an unjustifiable and contemptible excuse for doing nothing which is, I suppose, a matter of some indifference at this point since the damage that has been done is now irretrievable.

Ignorance has already killed, and this on a vast scale we can only now begin to barely imagine.

[gear_head]

7. how many air-tight caves are there, in the US? n/t

[leftupnorth]

9. Depends on where you are in the country

Deep underground storage is a possibility depending on geology.

[OKIsItJustMe]

10. An example project

http://blogs.chron.com/newswatchenergy/archives/2007/07/power_for_when.html

July 30, 2007

Power for when the wind won't blow

TXU's power generation business, Luminant, is teaming up with Royal Dutch Shell's wind business to build a new 3,000 megawatt wind turbine project in the Texas Panhandle.

The size of the project alone is noteworthy because it's almost equal to the 3,352 megawatts of wind capacity the state currently has. But it's also going to try to use a technology that could answer one of the biggest shortcomings of wind power, namely the variability of output, or the fact wind doesn't always blow when you need the power the most.



...

[oldhippie]

11. That's an interesting project .........

But I note that it was written about 9 months ago. I wonder if anything has happened since. If anyone has any knowledge of what has happened to this particular project, I'd like to see it. I like the concept.

I'm not as pesimistic as some around here, but I have been watching this (alternative, or renewable enegy) business for 40+ years also. There's just way too many things we hear about that just disappear. (And no, I don't believe in the big oil company quashing all the inventions conspiracy.) I've seen so many things that were "just around the corner" or "just a year from production" that never pan out.

[kristopher]

13. Here is a fuller explanation of that project. Construction slated to begin in 09

http://www.mast.udel.edu/628/Lect10a-wk-storage.pdf.
"Using gas turbines to enhance the value of wind power"
By Jeffery B Greenblatt, Princeton University

An insert from the the Greenblat article:
Two CAES plants operating worldwide, at least two more planned for US
A compressed-air energy storage (CAES) project essentially is a reconfiguration of a standard gas turbine (Fig 1). The viability of the CAES concept is well documented by the positive operating histories of the world’s two operating facilities in Huntorf, Germany, and McIntosh, Ala. At least two more CAES plants are planned for the US—one in Norton, Ohio, the other in central Iowa.

The Huntorf facility, operated by E.ON Kraftwerke AG, was commissioned in 1978 and the first CAES plant to operate commercially. It is designed to produce 290 MW for up to three hours. Huntorf typically is used today in what its owners refer to as “minute reserve” service: Provide short-term emergency power until a conventional power station can be brought online to fill the capacity shortfall. It also is used for peak shaving in the evening when no more pumped-storage hydro is available and as an alternative to purchasing peak power from outside suppliers.

Alabama Electric Cooperative Inc’s McIntosh plant has provided 110 MW of reliable peaking capacity within 15 minutes of startup — accomplished remotely — since commissioning in mid1991. The facility can go from 50% load to full rated capacity in less than 15 seconds. Unit heat rate is less than 4100 Btu/kWh (LHV) and emissions are half that of a standard simple-cycle GT, based on
electrical output. The plant is operated primarily at part load to provide system support and ancillary services for the co-op—including reactive power, voltage support, and load following. Stable operation is possible at loads as low as 10 MW. Summer on-peak starting reliability is in excess of 98%. Annual starts average more than 300. The Norton Energy Storage Project continues
to progress as planned. The facility, which will produce at least 600 MW in its first phase, is owned by CAES Development Co LLC, a wholly owned subsidiary of Haddington Ventures LLC, Houston. Sargent & Lundy, Chicago, is the owner’s engineer. Compressed air will be stored in an inactive limestone mine, which at nearly 340 million ft3, is large enough to support up to 2700 MW of generating capability. The developer has all major permits for the project. Electricity produced by the plant will have multiple paths to market. Norton will have the ability to supply American Electric Power Co’s (Columbus) 765-kV backbone as well as First Energy Corp’s (Akron) 345- and 138-kV circuits.

Iowa’s municipal utilities, working proactively to address energy concerns in the Midwest, have contributed financially to support development of the proposed Iowa Stored Energy Plant. The project, which is in the economic justification/pre-permit stage, would integrate renewable generation, CAES, and gas storage. Preliminary plans call for compressed air and gas storage in separate aquifers, 200 MW of CAES capability, and from 70 to 100 MW of wind generation.
—Bob Schwieger


And a old press report with more information on the Iowa/Dallas project:
Wind energy/storage plant slated for Dallas County
By DAVID ELBERT
REGISTER BUSINESS EDITOR

January 5, 2007



Iowa’s municipal utilities announced plans Friday to build a $200 million power plant west of Dallas Center that will store wind energy in the ground and use it to generate up to 268 mega-watts of electricity.

The announcement culminates more than four years of study and research, although operation of the plant is still several years away.

Construction of the Iowa Stored Energy Park would begin in 2009 with completion of the plant expected in 2011, said John Bilsten, general manager of Algona Municipal Utilities and vice president of the newly formed Iowa Stored Energy Plant Agency. 

Financing would be similar to methods used to build other utility power plants. The agency would pre-sell contracts to municipal utilities and others interested in buying the power and then use those contract to sell bonds to provide the money needed to cover construction costs, Bilsten said.

The site is roughly 40 acres of farmland located between U.S. Highway 169 and Dallas Center. Roughly 3,000 feet below the farmland is a porous rock structure that extends out for about a mile or more and has the capacity to hold compressed air pumped into the ground. That capacity is the key to the storage facility. 



Backers of the project had originally considered sites with similar geological properties in the Fort Dodge area but decided the structures in that area would not work as well as the Dallas County rock formations, Bilsten said. 

The Dallas County site has not yet been purchased but landowners in the area are agreeable to the location, Dallas Center Mayor Mitch Hamilton said at a news conference announcing the project. 



Bilsten described how the project will work.

He said electricity will be generated by wind energy farms at remote sites and will be carried by transmission lines to giant compressors located at the storage site. 

The compressors will pump the air into the ground, where it will be stored under pressure in the porous rock. The pressure is created by displacing air that is already in the rock. Air is contained within the rock by a surrounding solid rock cap. 



The air can be converted back into electricity by releasing the pressure, allowing the air to drive turbines that create electricity. 

The project would employ 300-400 workers during construction and create about 20-40 permanent jobs. The permanent jobs would be skilled jobs with good pay, Bilsten said 

Only two similar wind storage plants are currently in existence. One is in Germany and the other in Alabama. Both are about half the size of the plant planned for Dallas County, Bilsten said.

Business Editor David Elbert can be reached at [phone] or [e-mail]

http://desmoinesregister.com/apps/pbcs.dll/article?AID=/20070105/BUSINESS/70105031/1001