Anyone who looks at using renewable energy sources like wind and solar, knows that a longstanding problem is what to do when the sun doesn't shine or the wind doesn't blow. During such times very little energy is produced, making large scale energy production unreliable.
The approach to this problem has been to develop efficient, large scale ways to store energy, so that during bright sunshine and heavy winds energy could be stored for use when production is low. There are many competing technologies looking at energy storage, with the two most researched and most promising ones being high tech batteries, such as lithium batteries, or hydrogen fuel cell. There are dozens of companies competing in this field and great strides forward are being made.
Two major hurdles to using batteries and fuel cells to store energy is the capacity to store a lot of energy and hold it for a long time, and the price. This is improving all the time, but it is still very expensive. Storing enough energy to power a car or maybe even a house is one thing, but using batteries or fuel cells to store energy for a city could be much more economically challenging.
Enter a very simple idea . The world's first large scale facility that uses underwater balloons to store energy is being built in Toronto, Canada. The idea is simply to take the electricity you want to store and use it to pump air into giant underwater balloons. Since these balloons are about 180 feet underwater, they sit under heavy pressure. When you want your energy, you open a valve and the water pressure alone drives the air back up. This air flow is used to turn a turbine and generate electricity again.
The technology is new and has yet to prove itself, but the fact that it is being tried on a large scale means that studies so far show it to be promising enough to invest in it. A pilot project was done off the coast of Scotland about twelve years ago The benefit this has over using batteries and fuel cell is that it's much cheaper to build. Also, it doesn't use some of the very toxic materials which are used in making batteries and fuel cells, so it is much better for the environment.
A couple of link you can look at for this are below
http://www.digitaltrends.com/cool-tech/hydrostor-underwater-balloons/
http://www.thin-red-line.com/140714_ThinRedLine_OSES2014.pdf