Another nice gravity based one: http://www.heindl-energy.com/gravity-storage/idea-function.html http://www.heindl-energy.com/gravity-storage/idea-function.h...

Another nice gravity based one: http://www.heindl-energy.com/gravity-storage/idea-function.html http://www.heindl-energy.com/gravity-storage/idea-function.h... A piston of rock carved out of the land. For now just a concept though.