this would be a good use of coal mines
http://en.wikipedia.org/wiki/Compressed-air_energy_storageAdiabatic storage retains the heat produced by compression and returns it to the air when the air is expanded to generate power. This is a subject of ongoing study, with no utility scale plants as of 2010. Its theoretical efficiency approaches 100% for large and/or rapidly cycled devices and/or perfect thermal insulation, but in practice round trip efficiency is expected to be 70%.<2> Heat can be stored in a solid such as concrete or stone, or more likely in a fluid such as hot oil (up to 300 °C) or molten salt (600 °C).
Diabatic storage dissipates the extra heat with intercoolers (thus approaching isothermal compression) into the atmosphere as waste. Upon removal from storage, the air must be re-heated prior to expansion in the turbine to power a generator. A natural gas fired burner for utility grade storage or with a heated metal mass for a large Uninterruptible power supply. The lost heat degrades efficiency, but this approach is simpler and is thus far the only system which has been implemented commercially. The McIntosh CAES plant requires 0.69 kilowatt-hours (2,400 BTU) of electricity and 4,100 British thermal units (1.2 kW·h) lower heating value (LHV) of gas for each kilowatt-hour (kW–h) of power output.<3> A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 6,293 BTU (LHV) of gas per kW–h generated,<4> a 54% thermal efficiency comparable to the McIntosh 6,455 BTU, at 53% thermal efficiency.
Isothermal compression and expansion approaches attempt to maintain operating temperature by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. The theoretical efficiency of isothermal energy storage approaches 100% for small and/or slowly cycled devices and/or perfect heat transfer to the environment. In practice neither of these perfect thermodynamic cycles are obtainable, as some heat losses are unavoidable.
A different, highly efficient arrangement, which fits neatly into none of the above categories, uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. Early compressed air torpedo designs used a similar approach, substituting seawater for air. The venturi warms the exhaust of the preceding stage and admits this preheated air to the following stage. This approach was widely adopted in various compressed air vehicles such as H. K. Porter, Inc's mining locomotives<5> and trams.<6> Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.
Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders'<7> or air engines driving electrical generators to produce electricity.
The storage vessel is often an underground cavern created by solution mining (salt is dissolved in water for extraction)<8> or in an abandoned mine. Plants operate on a daily cycle, charging at night and discharging during the day.
Compressed air energy storage can also be employed on a smaller scale such as exploited by air cars or wind farms in steel or carbon-fiber tanks.
very technical sounding, but very interesting.
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