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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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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.
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.