Basic Concept
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Basic Concept
The fundamental idea of the hydraulic potential energy store, Gravity Storage, is based on the hydraulic lifting of a very large rock mass using water pumps. The rock mass acquires potential energy and can release this energy when the water that is under pressure is discharged back through a turbine.
The decisive variable with such energy storage lies in the storage capacity. It 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.
If a piston is selected for the Gravity Storage having a radius r and a length l=2r, then the piston can be lifted to the height h=r. The height h=r results from the consideration that the seal must lie somewhat above the center of gravity, thus at a distance r above the bottom of the cylinder so that the cylinder is hydrostatically stable while floating.
The storage capacity E is given by the density ρr of the rock and the density ρw of water, and the gravitational acceleration g:
E = (2ρr – 3/2ρw)πgr4
The last term is decisive – the radius to the fourth power. This has two important consequences. First, the storage capacity can increase 16-fold by doubling the radius, and second, the construction costs only increase by approximately the square of the radius. Therefore due to geometrical rules, the relative cost per energy unit decreases proportionally to 1/r².
In Tab. 1 the values for the storage capacity are given for a rock density of 2600 kg/m³.
A diameter of 250 meters would already result in a storage capacity of 8 GWh, which is comparable to the largest pumped storage power station in Goldisthal, Germany (8.4 GWh).
Storage capacity with different sizes
The efficiency of a Gravity Storage, at approximately 80%, is comparable with that of a pump storage.
The storage requires mainly two central seals:
- Sealing the piston and the exterior walls
- Sealing ring around the Piston to withstand hydraulic pressure
The piston and its surroundings are sealed with concrete and steel, 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.