Questions on the technology
- Can the rock piston jam or get stuck in the event of any unevenness?
The gap between the piston and wall, 2-3 meters, is much greater than the greatest inaccuracies that cannot be avoided when working the faces of the wall.
The idea of a rock piston weighing millions of tons becoming canted in a large shaft is not very pleasant. But this won't happen: as long as the rock's centre of gravity is below the sealing line, the rock cannot become canted. There is a simple physical reason for this that is known from the principle of buoyancy in ships. As long as the water line is above the centre of gravity, a ship will not capsize (details: Wikipedia). In the case of the rock piston it also has to be remembered that the rock lies in "very quiet" water and thus experiences no moments of tilt.
- Is a rock of this size inherently stable? For example, could crevasses lead to fractures in the piston?
This is a key question for the feasibility of the project, and one which we are looking into intensively. Stresses and tensile forces in principle act on every rock and they change if the rock is exposed or lifted out of its natural environment [Tarbuck2009]. Nature is able to produce steep faces that are stable for over 1000 m. (Highest steep face: Mount Thor 1250 m).
If a rock is changed artificially the tensile forces thus have to be investigated and estimated beforehand. In order to calculate the behavior, numerical models are calculated that take this geological situation at each potential location into account. We are assuming that the rock piston will be secured by anchors that prevent any brittleness. Furthermore, all surfaces will be sealed against water.
A rock of this size will most certainly be crevassed. But these cannot burst open naturally since the cylinder is sealed against water. If larger cavities appear in the edge area they can be filled with concrete under pressure, something that is common practice in tunneling.
- What happens in the event of an earthquake?
There are two types of waves in an earthquake: shear waves (S) and compression waves (P). The shear waves cause the most damage to buildings since the ground moves horizontally and buildings collapse on account of their inertia. If this kind of S-wave hits the rock piston, that floats in water, it will react like a baby in a womb; it will thus move with its surroundings because the water transmits the pressure perfectly as an incompressible fluid. However, given the dimensions, the local movement has to be taken into account so that it is conceivable that there may be some contact between the wall and cylinder. This is not very critical though because the piston moves extremely slowly. An exact estimate of the possible consequences of earthquakes, in particular with realistic amplitudes, can be provided by seismic studies.
- What would happen if a seal fails?
A complete failure of the seal is very unlikely (see the explanations of the seal). If leaks do appear in the seal, water may escape in the form of small fountains. In the event of larger failures correspondingly more water will shoot upwards. However, in the event of a complete failure the rock will only settle very slowly and in the event of a leak the release of the water will be initiated.
Questions on the economics
- What does the storage cost?
The costs of the hydraulic rock storage depend to a large extent on the location and fluctuate globally from region to region. The rough cost blocks comprise the preliminary testing, planning, construction of the piston itself, the seal, the machines (turbines, pumps etc.) and the grid connection. Particular cost factors here are the parameters of the geology, the availability and storage of the required water, the distance to the next grid connection and the current prices of raw materials. We estimate that for a location in Germany the economic viability in the form of a comparable cost structure with nine pumped storage power plants would probably be a radius of around 100 meters.
The higher the radius, the more economical the storage since the storage capacity increases with the fourth power of the radius.
- What profits can be made?
The hydraulic rock storage makes its profits depending on the constitution of the respective energy market of the country in which the storage is used. The basic business model for a grid-connected storage consists of its charging at low electricity prices and release at a higher electricity price than the cost price. This business model is currently in a crisis, as the economically difficult situation of pumped storage power plants shows. This is because the differences in prices on the electricity markets occur more rarely and more irregularly and above all the "midday peak" in prices diminishes through a high availability of electricity.
Since a commercial use of the hydraulic rock storage still lies many years in the future it is in principle questionable whether the current design of the electricity market and its price formation will still be relevant for the profits from the hydraulic rock storage.
Relative to today's European or German electricity market, the storage could be operated in both a classic spot market or to avoid electricity grid charges and to maintain frequencies in the electricity grid. It can be used on the electricity balancing market as well as any possible future capacity market. This flexibility is a big advantage of the storage.
- Is the Gravity Storage more advantageous than normal pumped storage power plants?
A comparison of the two concepts is difficult inasmuch as the Gravity Storage is a concept in its development stage whereas pumped storage power plants represent a mature technology. Both have as comparably high efficiency of 80-85%. However, the Gravity Storage may develop decisive advantages over pumped storage power plants in future:
- Energy density: the energy density of rock is much higher than that of water.
- Land use efficiency: with the higher energy density and the omission of a second basin, the Gravity Storage has a much higher land use efficiency than a pumped hydro storage of a comparable size.
- Water required: for a Gravity Storage is only one quarter of that for a comparable pumped storage power plant.
It should be noted that the experiences gained from the construction and operation of pumped storage power stations may make a valuable contribution to the realization of the Gravity Storage.
- Has a Gravity Storage already been built ?
No Gravity Storage has been built as yet. The concept was only developed in 2010. Our goal is to build a pilot project by around 2020. The necessary feasibility studies are currently being prepared and the international search is on for locations for a pilot project.
- How long will it take to build?
This depends on the size and the local conditions. We are currently assuming at least 2 years for planning and 3-4 years for the construction.
- What environmental impacts will its construction have?
The most obvious environmental impact is the up and down movement of the rock piston and its visibility in the landscape. A piston with a radius of 100 meters when fully charged would protrude approx. 100 meters above ground level. It would be raised and lowered at 1mm/second and is thus hardly perceivable with the naked eye.
The impact on the landscape can be minimized by building a 20-30 meter high earth wall, for example, around the piston and planting trees on top of this so that after a few years part of the piston would "disappear" behind the trees.
When the piston is extended it will throw a shadow. Its construction near settlements that would be affected by the shadow is unrealistic.
The impacts in the construction phase depend to a large extent on the location. Like all large underground constructions, there will probably be a temporary lowering of the groundwater level during the construction phase. During operation, however, these will probably not be necessary since the rock piston will be sealed off against water.
The noise during the construction phase will be within the normal levels for such projects. What's more, the removal of the overburden will lead to a temporary high volume of truck traffic.
No noise emissions need be feared during operation since the pumps and turbines will work underground.
- Is such a storage conceivable in densely populated Germany?
The gravity storage will probably be used in thinly populated areas that have an infrastructure with pre-existing conditions. Potential locations exist in Germany and other European countries. Although the concept was developed in Germany and is based on German quality standards in mining and mechanical engineering, the hydraulic rock storage will be used worldwide.