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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.
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 think the losses in the CAES system are due to the fact that it is a non-adiabatic process (a diabatic process?, i.e. one where heat can be lost from the system). When you compress the air, the temperature rises, and some heat is lost to the surrounding ground. But if the cycles are fast enough, those losses may be reduced -- i.e., you allow the air to re-expand, which cools it, and it sucks heat back from the ground. Since heat moves slowly through the ground, you may be able to get a lot of it back before it goes anywhere. The innovation in the Alabama system was to use waste heat in the turbine's exhaust gases to replace this lost heat as well.
I think the solution you propose is isobaric (constant pressure) and isothermal (constant temperature), but still not adiabatic. Some of the energy used to compress the air is converted to heat, and that heat would be lost to the ocean instead of raising the temperature of the air.
A better solution might be to use pre-inflated air bags (or air boxes?) attached to pulleys on the bottom of the ocean. Use a motor to pull the other end of the rope, and you would draw the air bag downward, storing energy. Play out the rope and the rising air bag would turn the motor (now acting as a generator), generating electricity. You could also do this with stones or bags of silt/gravel, just raising and lowering them from the surface.
The problem is, you would need a lot of air bags or stones to store any significant amount of energy. If the stones or gravel have a density of 2000 kg/m^3 (similar to "Gravel, wet" according to http://www.simetric.co.uk/si_materials.htm, higher than "Clay, wet excavated" (1600) but lower than concrete (2400)), then they will have a net weight in water of about 1000 kg/m^3 (i.e., a downward force of about 10000 Newtons per m^3). Air bags would exert a similar force upward. If you can find a near-shore location with a depth of 1 km, you could store 10000 N * 1000 m = 10 MJ of energy per cubic meter of material, which is about 3 kWh/m^3. A 100 MW wind farm (presumably closer to shore) would generate 100,000 kWh of electricity per hour when the wind is blowing, so if you wanted to store 6 hours of energy from this wind farm, you would need to raise and lower about 6 * 100,000 / 3 = 200,000 cubic meters of stone or air (e.g., 200 large chunks, each 10 meters across). I suppose it could be done...
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