# Idea and Function

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. 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 *ρ _{r }*of the rock and the density

*ρ*of water, and the gravitational acceleration

_{w}*g*:

E = (2ρ_{r }- 3/2ρ_{w})πgr^{4}

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

Usable Capacity [GWh] | 1,0 | 3,0 | 8,0 | 80 |

Number of people supplied for 24 h* | 250.000 | 750.000 | 2 Mill | 20 Mill |

Diameter [m] | 150 | 200 | 250 | 500 |

Volume of water [1000 m³] | 1,340 | 2,380 | 5,990 | 38,580 |

Pressure [bar] | 41 | 67 | 71 | 105 |

*based on demand of 4 kWh per person per night. |

The efficiency of a Gravity Storage, at approximately 80%, is comparable with that of a pump storage.

An overview of the terms of the individual elements of the storage, as used here on the website, can be found here.