Sealing the Rock Piston
The rock mass that has been excavated is lifted by pumping water underneath the rock piston, using large electrically-driven water pumps. The pressure depends on the height of the rock piston. With smaller systems, the pressure can be approximately 25 bar; with very large systems up to 100 bar. These pressures can be attained easily with pumps that are currently available on the market, such as those used in Alpine pumped storage power plants.
To function properly, the gravity storage requires a seal that seals the water chamber below the piston in overpressure conditions. In principle, the situation is similar to conventional hydraulic piston. However, there are a few substantial differences.
- Manufacturing accuracy: Whereas conventional hydraulic components can be produced on a lathe with an accuracy of a few thousandths of a millimeter, the piston outer wall, even with the greatest diligence, can be clad with an accuracy of approximately up to a few centimeters.
- Elastic deformation: Because the piston will have a diameter of 100 or more meters, factors come in play that are unimportant in normal hydraulics. The piston becomes smaller under the pressure of the water because it is elastically deformed, in that it is compressed. This effect can amount to a few centimeters in the radius and depends strongly on the size of the system. This reduction, in the case of a simple seal, would immediately lead to water flowing out at the seal.