Currently there are two commercial CAES plants worldwide; the Huntorf plant in Germany and the McIntosh plant in Alabama. Both of these run diabatic systems in which off-peak grid electricity is used to compress air which is then mixed with natural gas during expansion and combusted. The compression is done in several stages coupled with inter-cooling to dump the heat of compression. The compressed air is stored in large underground salt caverns. During peak times, compressed air is then mixed with natural gas and combusted in a gas turbine. The Huntorf plant uses a 310,000m3 cavern at a depth of 600m with a pressure tolerance between 50 – 70 bar, converted from a solution mined salt dome. It runs on a daily charging cycle of 8h providing a peak output of 290MW for 2 hours. The McIntosh plant has a 538,000m3 salt cavern at a depth of 450m with a pressure tolerance between 45-76 bar. Originally it provided an output of 110MW for 26 hours but in 1998 two extra generators were added and its total capacity is now 226MW.
Figure 1.2: Schematic of diabatic CAES system. The Huntorf plant, commissioned in 1978 to become the world’s first CAES plant, uses 0.8kWh of electricity and 1.6kWh of gas to produce 1kWh of electricity. The McIntosh plant incorporates a recuperator and uses 0.69kWh of electricity and 1.17kWh of gas to produce 1kWh of electricity . Calculating efficiency is problematic however as either one can take the approach that the 0.69 kWh of electricity input for the McIntosh plant probably required 1.725 kWh of gas (assuming a gas turbine efficiency of 40%), implying an overall efficiency of 1/(1.17+1.725) = 34.5%, or equally it could be suggested that 1.17 kWh of gas would have produced 0.468 kWh of electricity, and therefore the efficiency should be 1/(0.69+0.468) = 86.4%! Nevertheless both plants are commercially viable. As with PHS, CAES also requires favourable geography to provide the underground air storage caverns. Although there are many suitable sites for creating caverns, the artificial creation of caverns specifically for CAES is considered to be prohibitively expensive. Crucially, diabatic CAES is part energy storage and part natural gas combustion. Hence it has similar emissions to conventional gas turbines. There is recent interest in CAES variants that function without the need for natural gas. Adiabatic CAES, Isothermal CAES and absorption enhanced CAES are all new ideas in Research and Development. These are discussed in Fuelless CAES.
CAES Performance Characteristics and Applications
CAES systems have traditionally been designed as centralised storage facilities, to cycle on a daily basis and to operate efficiently during partial load conditions. This design approach allows CAES units to swing quickly from generation to compression modes. This flexibility means that they are well suited to ancillary services markets, providing frequency regulation. Their ability to operate on a (intra) daily cycle means that they are also useful for load-following/peak shaving. It is likely that they would be well-suited to electricity markets in which the loads and electricity prices vary significantly throughout the day. Although there are only two CAES plants in existence, and despite the fact that no new CAES plants have been built in excess of 20 years, there is speculation of an emerging market for medium-size CAES (around 100MW) to provide storage for wind and solar generation facilities where supplying enough battery storage would be cost prohibitive.
Summary of characteristics
|Typical Capacity||Typical Power||Efficiency (%)||Storage Duration||$/kWh||$/kW||Lifespan||Cycling capacity||Comments|
|500MWh – 2.5GWh||50 – 300MW||n/a||Hours – days||4-7 2-50 60 – 120 ||300-600 400-800 1000-1250 ||20-40 years||High||Requires favourable geography, though many more sites available than PHS|
Table: CAES characteristics
As with PHS, CAES costs are very site specific. The costs of mining a suitable underground cavern or creating an above-ground equivalent are at present prohibitive, whereas conversely alternatively a naturally occurring cavern can offer a very attractive price in $/kWh. It should be noted that the inlet pressure (45-76 bar) for the CAES high pressure turbine is much higher than the equivalent for a typical gas turbine (about 11 bar) so a typical gas turbine can only be used as the low pressure expander. The high pressure turbine at Huntorf is based on a small-intermediate steam turbine design. It is also worth noting that the Huntorf costs were considerably less than the equivalent cost in gas turbines in Germany at the same price date .
 Kaldellis, J. K. & Zafirakis, D., 2007. Optimum energy storage techniques for the improvement of renewable energy sources-based electricity generation economic efficiency.. Energy, Volume 32, p. 2295–2305.