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The Future of Wind Power May Be Underground
Hugh Pickens writes "When the wind is blowing, it is usually the cheapest peaking power available. However utilities need consistent always-on power from large, cheap coal and nuclear power plants that are the backbone of the electric grid. Wired reports that operators are looking at Compressed Air Energy Storage (CAES) using abandoned mines and sandstones of the Midwest to store compressed-air. This converts the intermittent motions of the air into a steady power source by using it to run air compressors to pump air into an underground cave where it's stored under pressure. The first CAES plant in the United States actually went online in McIntosh, Alabama in 1991 where engineers created a geological pocket 900 feet long and up to 238 feet wide in a dome by pumping water into it to dissolve the rock salt. When the (briny) water was pumped back out, the salt resealed itself and they had an air-tight container."
Instead, build long tunnels between major cities, evacuate them down to between 0 and 3 psi, and run high speed trains through them. The trains would need very little energy to run thru the extremely thin atmosphere, and the pressure diffential can be used to generate electricity when needed. 2 birds, 1 stone.
Another solution for the large scale storage of electricity is the inflation of airtight bags deep under water. Since water is so heavy, it exerts a lot of pressure against the air, leading to a cheap method of energy storage. The problem with all compressed-air systems is that have losses due to the non-isothermal nature of the process. That means some energy is lost as heat during compression, and you don't gain it all back thanks to Carnot. The energy density by volume is quite low, unfortunately, but in this application, that's basically irrelevant.
For the curious, the energy density of compressed gas, is 100*P*ln(P/A) kJ/m^3, where P is the maximum pressure and A is the ambient pressure. That m^3 term is in the volume when compressed.
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It may or may not be more efficient. Its a hell of a lot cheaper. Efficiency is not really the be-all and end-all. Cost is.
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The problem with these energy storage techniques for renewables is every single one of them would be more economical if they were used as load leveling systems (suck extra energy during down times, release in peak hours) rather than supply leveling systems (suck extra energy in high production hours, release it in low production hours).
The reason for this is day-to-day and monthly power consumption is a very easy thing to predict, so we know very well how much storage we need and if it will or will not be enough. Using these systems we can level the load and allow the greenest power sources (nuclear, followed by hydro) to produce the vast majority of power we need (because they can run at near 100% 24/7).
The wind is a very much harder thing to predict. So how much storage is needed? Who knows. What we DO know is that every single wind power station is going to need gas turbine backups for when a) the wind doesn't blow, b) demand is high and c) storage is depleted.
Using energy storage to allow nuclear and hydro to run most economically is a far better choice than using it to level the output of wind power.
The first question I thought of was, "Why not just use pumped hydro power?" Then, oddly enough, I read TFA and found the answer in it:
And, as noted in the summary, compressed air energy storage (CAES) been tried and it works:
steveha
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No. It's far *less* efficient. Li-ion batteries have round-trip efficiencies in the 90s (some chemistries in the upper 90s). Compressed air storage has a round trip efficiency generally under 50%. Sometimes significantly.
There was an interesting article the other day about storing electricity in molten aluminum/alumina -- basically, turning today's electrolysis method of making aluminum into a reversible process. They claim to already have better than lead-acid prices, but far longer cycle life, as well as li-ion energy density. Could be interesting, although I haven't seen an efficiency stat. Also, since it runs hotter than a Zebra cell, I doubt it'd scale down well. But who knows.
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This is the Slashdot-misunderstood version of the Wired dumbed-down version. Here's some of the more serious stuff.
Wind Operations Dispatching Training: This is the grid system operator's view of wind power.
There's a lot going on. Since electricity deregulation, the power distribution companies don't own much generation capacity. They buy power from generating companies. So there's a market system and contracts in place. The contracts are now more long-term; the "auction every half hour" scheme California had for a few years is out of favor. Now, the planning horizon is about one day.
There's a whole series of PJM online courses, and if you go through some of the basic ones, you'll be able to talk about electric power intelligently.
I assume we should reverse those first numbers: we spend 1,000 watt-hours to gain 870 watt-hours later. Cool to see that it beats pumped hydro.
http://www.nytimes.com/1991/09/29/business/technology-using-compressed-air-to-store-up-electricity.html
And it's cheaper than pumped hydro!
Interesting. Of course, if you use this with a wind farm, you don't get this benefit; the plant discussed here is a coal plant, with plenty of waste heat.
The above article is from 1991. Despite all these advantages, the idea never took off before now. It saved money, but not a huge amount. But since the wind blows when it blows, not when you want it to blow, I can see this being a useful thing for a wind farm.
steveha
lf(1): it's like ls(1) but sorts filenames by extension, tersely
No. It's far *less* efficient.
There was an interesting article the other day about storing electricity in molten aluminium/alumina
We already have fuel cells that consume aluminium. They're only about %40 efficient, but they are 100-1000 times cheaper than hydrogen fuel cells. So, without any technology development, the "aluminium economy" is %25 efficient (%70 percent efficient electrolysis, %40 percent efficient fuel cell). I think if you re-designed an aluminium fuel cell, you could get 90 percent efficiency, so you would have overall %60 efficiency. Not great, but it works. My idea is to use the ZEBRA electrolyte, (or maybe another electrolyte like it) to avoid corrosion and inefficiency in the al-air fuel cell.
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Yes, but eventually, you want to store more air than the free stuff can store, so you want to use the bags. The bags are useful for off-shore wind farms.
Ha! wind bags. I knew they had to be good for something.
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Your right that compressed air is a less energy efficient storage medium than Li-ion batteries, but only for the first couple of years. Li-ion battery storage capacity decreases at about 20% a year because of natural degradation. Consider the cost to frequently manufacture, replace and dispose of batteries compared to the wear cycles of a compressed air container which is probably measured in decades.
My point here is that the maintenance cost for compressed air energy storage is quite low compared to other options. You also have to consider the cost of making the storage devices. Steel tanks are mostly hollow and we are already really good at making them. We are good at recycling steel too. Air storage, unlike fuel cells or batteries options which consume lots of metals and require complex electronics to regulate, compressed air is extremely cheap and simple.
If our choice is cheap simple but supposedly inefficient storage of 50% via compressed air or storing 0% via other supposedly more efficient but unaffordable and unsustainable methods the choice is pretty simple.
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I mean, the weak link would definitely be the seal (one would think).
I, for one, think that the weak link would be the compressors. Most gas pumps just aren't especially efficient. If only someone would invent a pump that's better than current designs, the world's energy problems could be quickly solved.
Here's what the N.Y. Times article said:
The chamber in Alabama could hold 5,500 psi, but the pump is only capable of 1,100 psi. Design a better pump, and the cavern could store significantly more air.
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Wind -> Electricity ->compressed air ->electricity. That should give some serious losses. On top of that, windmills have gearboxes, brakes and all kinds of complicated crap to make them run perfectly in sync with the phase of the power grid. So question is, would it not be cheaper to mount a basic compressor in the nacelle and have it run directly on the axle, then pump the air through a set of pipes. Yes pipes have losses too, but remember the main cost of the windmill is its purchase, so a cheaper design might pay off?
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The problem is that trains need people on board who in general want to breath, spoiled brats they are.
So, the train would need an oxygen supply on board, added weight and explosion risk and a LOT of oxygen because people do a lot of breathing. It would also need to scrub the CO2 out, because it is after all a closed system.
Then the train needs to enter a normal area to let people in and out without explosive decompression.
It can be done, but is just not worth the hassle, especially when aerodynamics don't matter all that much for a train. The nose after all is only a small part of a LOOOOOOOOOOOONG train. The carriages don't add much to wind resistance, you can in a way decrease air-resistance per carried passenger by just carrying more passengers.
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No. You could use anything from an efficient spinning wheel with a lot of potential energy and very little friction (think super heavy pottery wheel with an engagable generator/motor) to a super-conductive coil (or looped superconductive powerline) to just stash the energy for a bit. And these will almost certainly be more efficient.
The larger problems is that we don't have enough wind to care right now, and the problem of energy storage has nothing to do with wind. It's a modular problem that simply deals with electricity on the grid, if electricity storage is needed for the inconsistent power on the grid, then it's needed. The fact that it's needed for wind power isn't something of any consideration. Such problems should have a healthy amount of encapsulation.
The total amount of battery power on the planet could power our electrical needs for ten minutes. That's not enough. It's a problem, who cares where the power comes from. This crap reminds me of that stupid idea of building another power grid for renewable power so people could know the power they get is from renewable sources. WTF.
If compressed air works well as a battery it works well as a battery, my guess is that it almost certainly doesn't work well as a battery and the failure that is the air car is quite telling of that point. Even when you can control for everything (unlike a hole in the ground (see carbon capture)) you still can't compress and get power back at anything close to efficient enough to give it a second thought.
It is no longer uncommon to be uncommon.
Sadly, tunnels large enough to carry trains, as modern subways will prove, are prohibitivley expensive.
however, compressed air is a good energy storage medium.
Assuming a 900 foot by 300 foot by 300 foot cavern was filled with compressed air with a pressure of 300 bars, would have a potential energy of roughly 50 gigawatt hours. (source: http://www.tinaja.com/glib/energfun.pdf) Or enough to run the entire united states for about an hour. This is a massive pool of energy, and significantly more cost effective than a battery.
HOWEVER, there lies a rub. When you compress air, you generate a massive amount of heat as the thermal energy stored in the air is highly compressed. This heat energy, unless properly reclaimed and stored (I.E. In a molten salt bath) just leaks away, stealing a huge chunk of the potential energy with it. When the air is uncompressed, there is significantly less heat energy stored in the air, and thus the expanded gas is very cold. This limits how far it can expand again, and creates a formidable problem in the form of condensation.
What you need to do to get EFFICENT compressed air storage, is either store the heat in an efficent manner, and add it back to the compressed air. OR you can gradually warm it back up to room temperature through a heat exchanger as it expands.
All in all, the challenges to attaining decent efficency are considerable.
What might be an easier way to achieve the same energy storage using similar principles, is to turn that same cavern they created into a giant hydro dam. Basically, create an enclosure of equal size below it. When energy needs to be stored, pump the water up to the higher cavern. When energy needs to be released, release it through hydro turbines into the lower cavern.
People have been storing electrical energy using water for a long time (over a century). The basic idea is the same, but in the case of water and hydroelectric dams, the solution is easier (you just run the turbines as pumps, putting water into the resevoir instead of letting it drain out). According to the wikipedia article on Pumped-storage hydroelectricity :
In 2009 the United States had 21.5 GW of pumped storage generating capacity, accounting for 2.5% of baseload generating capacity. PHS generated (net) -6288 GWh of energy in 2008
In 2007 the EU had 38.3 GW net capacity of pumped storage out of a total of 140 GW of hydropower and representing 5% of total net electrical capacity in the EU.
And, yes, people have considered using pumped-storage hydroelectric to even out the variation in wind power.
I myself doubt that compressed air storage would ever amount to more than a fraction of pumped hydro-electric storage, but it might be useful in very dry or very flat regions.
rough guess-- you lose 50% of the wind energy coming and going.
Rough answer, you are wrong, RTFA and RTF Thread, particularly (#31435384) http://hardware.slashdot.org/comments.pl?sid=1578760&cid=31435384
You can do better by pumping water uphill, where you don't have the compressive losses.
no, you can't, again, RTFA
here it is 2010, and I'm still using cutesy acronyms from the early 1990s, seriously though RTFA has never been a more appropriate response