Compressed Air:

Here again the concept is simple. In times of energy use where production exceeds demand and hence there is an energy surplus, then use that surplus to compress air and store that compressed air. The energy density in compressed air is quite high, much higher than batteries, and so it is a good choice for energy storage in princple.

In practive, however, there are rather few natural caverns or containers for compressed air that are located near an electricity production site. In principle the system would look as follows:





In practice only 2 facilities in the world were ever built.

• 290 MW in Germany late 1970s
• 110 MW in Alabama 1991

The Iowa Stored Energy Park project has been cancelled. This was to build a wind farm on top of an evacuated aquifer to compressed air storage. A good idea - apparently good ideas' don't pan out ...

Summary:

The table below is worth studying as it provides the relative advantages for various technologies in terms of energy storage vs power storage.

• A technology which has a good "energy application" means that it stores power which is then discharged over a large amount of time.
  
  
• A technology which has a good "power application" means that it can store a large amount of power but it discharges it pretty fast and therefore is not a good energy application. Most batteries fall in this category as do flywheels.