Compressed Air Energy Storage (CAES) refers to the compression of air to be used later as energy source. At utility scale, it can be stored during periods of low energy demand (off-peak), and for use in meeting periods of higher demand (peak load). Alternatively it can be used to power tools, or even vehicles.
Types
Compressed air energy storage can be done adiabatically, diabatically, or isothermally:
- With adiabatic storage, the heat that appears during compression is also stored, then returned to the air when the air is expanded. This is a subject of ongoing study, but no utility scale plants of this type have been built. The theoretical efficiency of adiabatic energy storage 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%. 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 a molten-salt (600 °C).
- With diabatic storage, the extra heat is removed from the air with inter coolers following compression (thus approaching isothermal compression), and is dissipated into the atmosphere as waste. Upon removal from storage, the air must be re-heated (usually in a natural gas fired burner for utility grade storage or with a heated metal mass for large Uninterruptible Power Supplies) prior to expansion in the turbine to power a generator. The heat discarded in the intercoolers degrades efficiency, but the system is simpler than the adiabatic one, and thus far is the only system which has been implemented commercially. The McIntosh CAES plant requires 0.69 kW·h (2,355 btu) of electricity and 4,100 btu (LHV) of gas for each 1.0 kW·h of electrical output . A GE 7FA 2x1 combined cycle plant, one of the most efficient non-CAES natural gas plants in operation, uses 6,293 btu (LHV) of gas per kW·h generated, a 54% thermal efficiency comparable to the McIntosh 6,455 btu, a 53% thermal efficiency.
- Isothermal compression and expansion approaches (which attempt to maintain operating temperature by constant heat exchange to the environment) are only practical for rather low power levels, unless very effective heat exchangers can be incorporated. 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 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 (or seawater as in early compressed air torpedo designs) over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. This warms the exhaust of the preceding stage and admits this preheated air to the following stage. This was widely practiced 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, expansion with turbo 'expanders' or air engines driving electrical generators to produce electricity.
Air is stored in mass quantity in underground in a cavern created by solution mining (salt is dissolved away) or an abandoned mine. Plants are designed to operate on a daily cycle, charging at night and discharging during the day.
Compressed air energy storage can also be used to describe technology on a smaller scale such as exploited by air cars or wind farms in steel or carbon-fiber tanks.
History
City-wide compressed air energy systems have been around since the early 19th century (1870). In the early days, cities as Paris, Birmingham, Rixdorf, Offenbach, Dresden and Buenos Aires were equipped with these systems. The first systems were constructed by Victor Popp for powering clocks (by sending a pulse of air every minute to change the pointer), yet quickly evolved as a means to deliver power to industry and to homes. As of 1896, the Paris system had 3,000 hp (2,200 kW) of generation distributed at 80 psi (550 kPa) in 30 miles (50 km) of air pipes whose use for motors in light as well as heavy industry. Usage was measured with meters. The systems were the main source of (house-delivered) energy in these days and were also used to power the machines of dentists, seamstresses, printing facilities and bakeries.
Physics of isothermal compressed air storage
One type of reversible air compression and expansion is described by the isothermal process, where the temperature remains constant. Compressing air heats it up and the heat must therefore be able to flow to the environment during compression for the temperature to remain constant. In practice this is often not the case, because to properly intercool a compressor requires a compact internal heat exchanger that is optimized for high heat transfer and low pressure drop. Without an internal heat exchanger, isothermal compression can be approached at low flow rates, particularly for small systems. Small compressors have higher inherent heat exchange, due to a higher ratio of surface area to volume. Nevertheless it is useful to describe the limiting case of ideal isothermal compression of an ideal gas:
The ideal gas law, for an isothermal process is:
By the definition of work, where A and B are the initial and final states of the system:
where, P A V A = P B V B , and so,
This amounts to about 2.271 ln( P A / P B ) kJ at 0 degrees Celsius (273.15 kelvins) or 2.478 ln( P A / P B ) kJ at 25 °C (298 K), per mole, or simply 100 ln( P A / P B ) kJ/m³ of gas (at 0.1 MPa = approx. atmospheric pressure).
An isothermal process is thermodynamically reversible, so to the extent the processes are isothermal, the efficiency of compressed air storage will approach 100%. The equation above represents the maximum energy that can be stored. In practice, the process will not be perfectly isothermal and the compressors and motors will have heat-related energy losses.
When gas is compressed adiabatically, some of the compression work goes into heating the gas. If this heat is then lost to the surroundings, and assuming the same quantity of heat is not added back to the gas upon expansion, the energy storage efficiency will be reduced. Energy storage systems often use large natural underground caverns. This is the preferred system design, due to the very large gas volume, and thus the large quantity of energy that can be stored with only a small change in pressure. The cavern space can be compressed adiabatically and the resulting temperature change and heat losses are small.
Practical constraints in transportation
Energy density and efficiency
Compressing air heats it up and expanding it cools it down. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless it is useful to describe the maximum energy storable using the isothermal case, which works out to about 100 ln{ P A / P B ) kJ/m 3 . Thus if 1.0 m 3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bars (20 MPa), the potential energy stored is 530 kJ (or 0.15 kW·h). A highly efficient air motor could transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 200-bar (20 MPa) pressure completely down to 1 bar (0.10 MPa) (bottle completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to nonlinear energy storage and the thermodynamic considerations. If the bottle above is emptied down to 10 bars (1.0 MPa), the energy extractable is about 300 kJ at the motor shaft. The efficiency of isothermal compressed gas storage is theoretically 100% but in practice the process is not isothermal and the two engines (compressor and motor) have additional types of losses.
A standard 200-bar (20 MPa) 5 liter steel bottle has a mass of 7.5 kg, a superior one, 5 kg. Bottles reinforced with, or built from, high-tensile fibers such as carbon-fiber or Kevlar can be below 2 kg in this size, consistent with the legal safety codes. One cubic meter of air contained inside such a full bottle has a mass of 1.225 kg (at 20 °C). Thus, theoretical energy densities are from roughly 70 kJ/kg at the motor shaft for a plain steel bottle to 180 kJ/kg at the motor shaft for an advanced fiber-wound one, whereas practical achievable energy densities for the same containers would be from 40 kJ/kg to 100 kJ/kg. Comparing to the data given for rechargeable batteries, this makes the advanced fiber-reinforced bottle example comparable to the lead-acid battery in terms of energy density and advanced battery systems are several times better. Batteries a
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