The Future of Wind Power May Be Underground 223
Posted
by
samzenpus
from the blow-from-down-below dept.
from the blow-from-down-below dept.
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."
Unwater Bags (Score:5, Informative)
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.
Compared to pumped hydro (Score:5, Informative)
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
The real info about dispatching wind power (Score:5, Informative)
This is the Slashdot-misunderstood version of the Wired dumbed-down version. Here's some of the more serious stuff.
Wind Operations Dispatching Training [pjm.com]: 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 [pjm.com], and if you go through some of the basic ones, you'll be able to talk about electric power intelligently.
Numbers from second article (Score:5, Informative)
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 [nytimes.com]
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
Re:Load leveling Vs. Supply leveling (Score:3, Informative)
"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."
The amount needed depends on many factors such as the amount of demand control too.
So it's a grave error to think that all wind supply needs 100% callable backup, IMHO.
Rgds
Damon
compressors are the weak link (Score:4, Informative)
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 [wikipedia.org], 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.
Re:Generate a Vacuum (Score:3, Informative)
Re:Load leveling Vs. Supply leveling (Score:3, Informative)
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.
Many studies [wikipedia.org] have been done on this subject. You appear to be a bit confused as to the purpose of load levelling systems in proposed green energy schemes.
Re:Been thought off and rejected as to complex (Score:3, Informative)
3psi is about atmospheric pressure at 40,000 feet - roughly where commercial airliners fly. Inside a commercial airliner the pressure altitude is around 8000-10000 feet, or about 11-10psi.
It's not entirely a solved problem, but it's not as bad as you think.
advantages and disadvantages of compressed air (Score:5, Informative)
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 [tinaja.com]) 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.
Doing it with water (Score:3, Informative)
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 [wikipedia.org] :
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 [cam.ac.uk] 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.
Re:advantages and disadvantages of compressed air (Score:5, Informative)
Um, 50 gigawatt hours is about 1.8 * 10^14 joules. That is about 43 kilotons of energy. Now think catastrophic failure. Here [wikipedia.org] is an example 1/10 the size.
All energy storage systems...especially physical storage systems...suffer from the same problem. In order to store a useful amount of energy, they need to exist on a potentially catastrophic scale. Pump storage...where is the flood plane. Compressed air...what is the blast radius, where will the supercooled plume go, will it reach aviation altitudes? Flywheel storage...reference mythbusters with the CD on a die grinder. And while not a storage system, even geothermal power plants seem to cause geological instability.
A few years ago, I did some modelling development for people doing salt mining for compressed air storage. (IAAMechEngineer.) At the time, I remember thinking what must the hoop stresses on a 100m cavern look like at a few hundred atmospheres? And that is rock and dirt and salt holding it together. Nothing in that system tends to behave elastically. So pressurizing and depressurizing it has to induce crack growth and eventually some geological instability. How do you do in-situ inspection of your "pressure vessel"?
In my mind, some electrochemical process is far safer, even if it uses nasty chemical. Because you can keep the chemical apart (with 100-ft high berms if need be) until it is time to react them.
Re:Generate a Vacuum (Score:3, Informative)
It's expensive to run all those lines and make all those towers, but the overall cost is less. If you can plug wind power into this sort of system (which is a huge if) then the overall system can be even better.
Re:Generate a Vacuum (Score:3, Informative)