DThis idea sounds daring: The "Hambacher Loch", which has been created since 1978 by the extraction of around 40 million tons of brown coal every year, is to become a kind of gigantic water battery in the future. It is intended to store surplus wind and solar power and thus compensate for the disadvantage of these regenerative power generation technologies, which do not provide electricity continuously.
As the two protagonists of this storage technology, the retired physics professors Horst Schmidt-Böcking from Frankfurt and Gerhard Luther from Saarbrücken, Germany, the operator of the largest German opencast mine in the Rheinische Revier near Düren, RWE Power AG, could send a signal for the energy transition and help her to make a breakthrough.
The fact that this concept is not fancy is shown by successful tests with a concrete hollow sphere of almost three meters in diameter. The was released by scientists Fraunhofer Institute for Wind Energy and Energy Systems (IWES) in Kassel in November 2016 on the bottom of Lake Constance at a depth of about 100 meters. The underlying principle, which was first reported here in April 2011, is simple: With "excess current", the ball is pumped empty with the help of a turbine built into the outer shell. If the stored energy is to be used again at a later time, a valve is opened. Water flows powerfully back into the concrete ball due to the depth of the water pressure and drives the turbine and generator.
Numerous details could be clarified with the experiments on the Bodensee-Kugel. For example, if the ball can be emptied completely at all, how the pressure changes affect the material and whether air must be supplied if the water is to be pumped out of the ball. This is not necessary, noted the Kassel researchers, who also demonstrated the high efficiency of the recovery of their concept. Accordingly, it achieves almost 90 percent, while in the power-to-gas process, for example, this figure is a meager 30 percent. In this storage concept, which has been discussed for some time, an electrolyzer is operated with renewable electricity, and the hydrogen produced, or the methane produced in a further step, is fed into the gas network or into large earth caverns.
In order to use the Hambacher Loch as energy storage, a 100 to 200 meter high concrete structure with the largest possible internal cavity is to be placed in the 450-meter-deep lignite mine with a bottom surface of about four square kilometers. Then it is flooded. In order for the building to withstand the pressure on it, it is made up of several cells. At the bottom of the concrete structure, a piping system ensures that the individual segments are connected, allowing the water to flow to the lowest point installed turbines, which ensure that the water can be pumped quickly from the hollow body into the surrounding lake. or can flow back for later recovery of electricity.
The two professors calculated roughly what storage potential would be possible with such a water battery: For a base area of the box of four square kilometers (1000 by 4000 meters) and a height of 100 meters – at a medium depth (water surface of the lake to the middle of the concrete body) of 400 meters – the potential energy stored in the water is 300 million kilowatt hours (kWh). At a depth of 400 meters, the potential energy of one cubic meter of water is around one kWh. This would allow more than 270 gigawatt-hours (GWh) of electrical energy to be stored in one cycle, which is about eight times that of what all classic German pumped-storage power plants can store.
The inventors of the water-battery concept therefore see the opportunity in the Hambacher Loch for a huge plant for storing green electricity by converting it into potential energy. And they continue to calculate: With a height of the concrete hollow box of 200 meters, the storage capacity amounts to around 480 GWh. If you took it one step further and would deepen the hole to 1000 meters and widen the sole to two kilometers (ie eight square kilometers), a storage capacity of 4000 GWh would result from a 300 meter high hollow box. With 100 filling cycles a year, more than 250 billion kWh could be stored, which is four times the current power production from Hambach.
According to the professors, the costs would be between one and two cents per kilowatt hour, depending on the size of the extension. Their talks with the concrete pavers of Hochtief, who were already responsible for the ball dropped in Lake Constance, showed their interest in the development of such a large-scale project, it is reported. But there are still a lot of details to clarify, such as where the storage water should come from.