Compression of air generates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, the efficiency of the storage improves considerably.
There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermal:
- Adiabatic 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, but a German project ADELE is planned to enter development in 2013. The theoretical efficiency of adiabatic storage approaches 100% with perfect insulation, but in practice round trip efficiency is expected to be 70%. 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 solutions (600 °C).
- Diabatic storage dissipates much of the heat of compression with intercoolers (thus approaching isothermal compression) into the atmosphere as waste; essentially wasting, thereby, the renewable energy used to perform the work of compression. Upon removal from storage, the temperature of this compressed air is the one indicator of the amount of stored energy that remains in this air. Consequently, if the air temperature is low for the energy recovery process, the air must be substantially re-heated prior to expansion in the turbine to power a generator. This reheating can be accomplished with a natural gas fired burner for utility grade storage or with a heated metal mass. As recovery is often most needed when renewable sources are quiescent, fuel must be burned to make up for the wasted heat. This degrades the efficiency of the storage-recovery cycle; and while this approach is relatively simple, the burning of fuel adds to the cost of the recovered electrical energy and compromises the ecological benefits associated with most renewable energy sources. Nevertheless, this is thus far the only diabatic system which has been implemented commercially. The McIntosh, Alabama CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value (LHV) of gas for each megajoule of energy output, corresponding to an energy recovery efficiency of about 27%. A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 6.6 MJ (LHV) of gas per kW–h generated, a 54% thermal efficiency compared to the McIntosh 6.8 MJ, at 27% 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 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 pump 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 and trams. 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' 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) or by utilizing 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 and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks. However, when compressed air is stored at room temperature this stored air, in general, contains the same amount of energy per pound as uncompressed room temperature air. The considerable amount of energy used to compress this air is not stored there if the air is allowed to reduce to room temperature. Therefore, to obtain substantial energy from the expansion of this stored room temperature compressed air a heat reservoir must be provided to supply the needed energy. This can be challenging in mobile applications.
US Patent #8347628, awarded in January 2013, describes a turbo-expander machine that offers both high efficiency energy recovery and high power output. The turbo-expander utilizes no fuel combustion, but instead is powered by the recovery of atmospheric heat (e.g., the wasted heat of compression as described in the Diabatic classification). When combined with renewable energy powered air compression, such an energy recovery machine would enable a compressed air storage and recovery methodology that is both cost-effective and eco-friendly.
Read more about this topic: Iowa Stored Energy Park
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