Gravity Storage is a concept with which unprecedentedly large quantities of power can be stored for a long time of 6-14 hours, and can be made available again.
The fundamental principle is based on the hydraulic lifting of a large rock mass. Using electrical pumps, as already used today in pumped storage power plants, water is pumped beneath a movable rock piston, thereby lifting the rock mass. During times of insufficient generation of renewable power, the water which is under high pressure from the rock mass, is routed to a turbine, as in conventional hydroelectric plants, and generates electricity using a generator.
The capacity of energy storage depends on the mass of the rock piston and the height by which it can be lifted. For pistons that are exactly as high as the diameter of the piston, the mass is proportional to the cube of the radius, r3. Because the height by which the piston can be lifted, depends on its own height (the piston can be lifted at most by half of its height, otherwise it could tilt), the storage capacity increases with the fourth power of the radius, r4.
The construction costs however only increase with the square of the radius, r². This means that the construction costs increase substantially more slowly than the storage capacity. Thus, very low costs per kilowatt hour of storage capacity are possible. Strictly speaking, the price per kilowatt hour of storage capacity decreases with 1/r². This is the outstanding competitive advantage of this storage concept.
The rock piston should have a diameter of at least 100 meters in order to be competitive with pumped storage power plants. The real costs will vary for each site. But we calculate with costs of about 200 Euro/kWh capacity at a size of 250 meter diameter.
The storage requires mainly two central seals:
1. Sealing the piston and the exterior walls
2. Sealing ring around the Piston to withstand hydraulic pressure
The piston and its surroundings are sealed with geomembranes, so that no water can leak out of the displaced volume.
A rolling membrane sealing is fixed between piston and cylinder. Here in more detail.
Methods known from mining and tunneling are used for the construction.
Suggested reading: Idea and function