The task of the potential energy storage is to store huge amounts of energy for power supply. In our current electricity grid, there is no storage device that can compensate for a supply gap. The only stores that are available serve as a reserve for the failure of a large power plant. Thus, eg. the Schluchsee, a hydro power pumping plant, can deliver instantaneously power in the range of one gigawatt (GW). The storage capacity of the Schluchsee is approximately 10GWh, the failure of a power plant with a capacity of 1GW be compensated for 10 hours.
Germany uses permanent 60GW[Google public data] of electricity. The US uses permanent 460GW[Google public data] of electricity. In the case, that solar and wind power are the only generators of electricity, as many future concepts say, we need at least a storage device for seven days [Popp2010 S.136] this impies a capacity of 10TWh for Germany and 77TWh for the USA (Calculation: 7*24h*60GW = 10080GWh). Precisely for this problem large potential energy storage is optimal. A memory with 1 km in diameter and 1 km depth can store 1600GWh.
The pure construction costs of the storage device, assuming 500m radius and 1600GWh capacity, are in the range of one billion ($1,000,000,000). In addition the cost of the pumps, generators and turbins have to be paid. A complete calculation tells us, that the dominating costs result from the electrical devices. Although the construction price seems to be highm this has to be compared with other storage devices. The cost per kWh is in this concept below 10$, compared to more tha 100$ in the case of a pumpung power station, and incredible smal if compared to a lithium battery that cost about 1000$ per kWh. Goldisthal example pump storage power plant 71€/kWh.
A smal device ist possible, but the price per kWh is growing very fast if the system raduis is small. A storage device with 50m radius is priced at 300$ per kWh, this is already higher tha in a pumped storage power plant. If the radius is further reduced to 10m, the price per kWh is up to 10.000$ and that´s ten times the price of a lithium power system. Not to mention the space requirements.
There could be a interesting solution with a small radius and a very deep hole, may be this is an economic alternative.
The striking advantage is the energy density!
A conventional pumping power station with 400m and 10m water level change can be store about 10kWh per m² lake surface. The hydraulic potential energy storage performs per square meter with 2.000kWh, about 200 times more, so you need much less land for the reservoirs.
Reservoirs overflow large areas of land that can no longer be used. In the case of the potential energy storage device, the vegetation will remain on the mountain. If you wanted to build a reservoir, which has the same capacity as a rock cylinder of 500m radius, you need a lake with 160 km ², twice the size of the Chiemsee. There is no space in Germans low mountain ranges. Moreover, the huge dams would represent a huge risk.
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