Mechanical Storage

There is increasing discussion of linking wind power systems with pumped storage hydro facilities, especially in Norway and Canada, directly via the grid. This was effectively done with Danish wind power, a large amount of which has been generated at periods of low demand, necessitating export to Norway and Sweden. Pumped hydro storage alone will not be enough to meet Denmark’s future renewable energy targets.

Pumped hydro storage is one of the most mature storage technologies with 100 sites operating worldwide. However, it is probably more viable to retro furbish or re-power an existing hydro storage plant to improve its efficiency than build new plants. These plants have very significant adverse impacts on the environment and require a suitable terrain.
During ‘off peak’ demand wind can be used to pump water to an upper reservoir. At times of peak demand the water would be released through a small hydro turbine to generate electricity. This would also generate a commercial gain for utilities from the net difference in power rates.

Locations for wind-powered pumped hydro storage are limited. Sites must have a geographic elevation difference of more than 400 feet. The site’s storage capacity is limited by the water capacity of the reservoirs.

Storing energy by compressing air in empty underground aquifers is another possible solution to wind’s intermittency. Compressed air storage systems typically operate at 80% efficiency.

Diabatic CAES stores compressed air in salt caverns, depleted gas fields or aquifer areas. Gas turbines are used to heat the compressed air, when energy is needed. This technology is mainly used in Germany and the USA.

Adiabatic CAES stores both the compressed air and heat from the compression process. The heat is reused when energy is needed without the need for gas turbines.