METHOD AND SYSTEM FOR GRAVITATIVE ENERGY STORAGE

Description

[0001] The invention relates to a method for manufacturing and operating a system for underground gravitational energy storage according to claim 1. Furthermore, the invention relates to the system and an arrangement comprising several of these systems for underground gravitational energy storage according to claims 4 and 8, respectively, as well as a use of the system according to claim 11.

[0002] Energy storage devices of the type mentioned above serve to store fluctuating energy supplies, for example, renewable energy, preferably kinetic or electrical energy. Existing, preferably kinetic, energy is reversibly converted into a storable form of energy, i.e., preferably potential energy, in order to be converted back into kinetic energy, preferably the originally existing form of energy, when required. Energy is only temporarily converted into a storable form of energy.

[0003] Pumped-storage power plants have been known for a very long time. In these plants, a storage medium, usually water, is pumped to a higher level in a reservoir such as a dam and stored there. The potential energy stored in the reservoir can then be accessed when needed. It can be converted back into kinetic energy by releasing water through a turbine, and from there into electrical energy via a generator. So-called pump-turbines are used in this process. These consist of a turbomachine that can be used as both a turbine and a pump, mechanically coupled to an electric machine that can drive the pump and generate the turbine. Depending on the operating state, pump-turbines function as either pumps or generators. However, such pumped-storage power plants require suitable topographical conditions and a considerable amount of space, especially for reservoirs.

[0004] Furthermore, energy storage systems are also common, in which a compressible storage medium is typically compressed in a sealed, e.g., underground, cavity and, when needed, converted back into energy via a turbine or other expansion machine, with or without a generator. Energy is thus reversibly converted into storable, elastically compressed potential energy and temporarily stored. This design does not require the topographical constraints of a pumped-storage power plant and, above all, is also mobile.

[0005] German patent DE 10 2010 034 757 A1 discloses a technology for hydraulic energy storage commonly known as "gravity storage." In this technology, a cylindrical rock body is guided within a cylindrical rock opening and lifted (energy storage) by injecting a hydraulic storage medium. The stored energy is then recovered by releasing the pressurized storage medium from beneath the rock body, with the pump used for this purpose acting as an energy converter. A sealing ring is located between the rock body and the rock opening. However, the patent does not address important details for practical implementation, nor does it mention solutions such as how to avoid solid friction or rockfall during the injection process when moving the rock body within the opening.

[0006] The technology, called Gravity Storage, involves hydraulically lifting a cylindrical rock mass for energy storage and lowering it for energy generation, with the pump acting as a generator. It does not explain how erosion during the necessary injection process will be avoided or how the efficiency-reducing friction on the cylinder wall will be minimized. Without cementation, the emerging cylinder would disintegrate in the ground, thus precluding any further use of the site.

[0007] US 2011 / 0113769 A1 describes an energy storage system particularly suitable for the intermediate storage of large amounts of energy. It proposes an extensive (typically 160,000–250,000 m²) flat storage volume buried near the Earth's surface, which can be filled with water as the storage medium via a turbine. The soil layer above the storage volume serves to generate internal pressure within the volume; a soil thickness of 20–25 m above the storage volume is specified. This thickness can be increased or varied by adding fill material. The storage volume is sealed by a lower and an upper membrane, with the upper membrane (e.g., polymer) being lighter and the lower membrane (e.g., concrete) being heavier than the storage medium.

[0008] EP 3 692 253 B1 also proposes a system for storing energy and / or water in the ground. It involves placing a double polyurethane membrane, welded at its edges, in an excavated area and then backfilling the excavated soil. When water is pumped in from an adjacent pond, preferably located below the double membrane, the membrane forms a cushion-shaped storage volume. This volume raises the soil and thus the ground surface above it and lowers again when the storage medium is drained. During draining, the soil above the storage volume sinks, and the pump acts as a generator. The capacity and efficiency of this gravitational energy storage system are limited by the available excavation depths and widths, as well as by the potential for soil instability above the water cushion. The pump used to fill the storage medium also serves as a generator.The capacity and efficiency of this gravitational energy storage are limited by available excavation depths and widths, as well as by the inherent instability of the soil above the storage volume. While a double polyurethane membrane is easier to control than the membranes described in US 2011 / 0113769 A1, it is less robust.

[0009] The technology proposed by Aquanamic in EP 3 692 253 B1 involves placing a double polyurethane membrane, welded at its edges, in an excavated area and then backfilling the excavated soil. When water is pumped in from an adjacent pond, the double membrane forms a cushion that lifts the soil above it; when water is drained, this soil lowers, and the pump acts as a generator. The capacity and efficiency of this gravitational energy storage are limited by the available excavation depths and widths, as well as by the potential for soil instability above the water cushion. A double polyurethane membrane is more controllable than that of the aforementioned technology, but less robust. Energy is stored in the same way as with Gravity Storage and the aforementioned technology, but with Aquanamic, the capacity is limited by excavation depths, and the land becomes largely unusable.

[0010] The aforementioned underground energy storage systems require extensive sealing of the pressurized storage volume from the surrounding soil. The membranes and sealing materials presented are designed to be manually applied on-site and must be repairable or replaceable in case of damage. Accessibility thus limits the economically viable installation depth of the storage volume. This limited installation depth also means that filling and / or emptying the storage volume causes the ground surface above it to rise or fall, rendering the surface unavailable for further use or development. The shallow cover depths also imply comparatively low energy efficiency, which is related to potentially extensive areas of plastic deposition.

[0011] Furthermore, a method for storing energy is known from EP 2 464 819 B1. It involves fracturing a section of the subsurface, preferably a rock formation, by injecting water and adding synthetic resin, which bonds to the fracture surfaces, preferably at the fracture tips, hardens, and forms a fluid-tight membrane. This creates a pressure-resistant storage volume that can later be used as a fluid pressure storage system for the intermediate storage of energy. The method assumes elastic deformability of the subsurface, which would allow for high efficiencies of up to 70 to 75%. In reality, rock behaves largely inelastically during fracking, and fluid outbursts could not be avoided by adding synthetic resin alone, even and especially with alternating injection and discharge of the fluid.Repeated pressure changes in the storage tank also cause the hardened resin to undergo plastic damage, resulting in deterioration and wear, ultimately leading to leaks in the storage volume.

[0012] The aforementioned technology involves fracturing a section of the subsurface by injecting water and adding resin. The company claims an efficiency of 70 to 75%, based on the assumption that the rock behaves largely elastically. In reality, rock behaves largely inelastically during fracking, and fluid eruptions cannot be prevented by adding resin (without it alone), especially with alternating injection and withdrawal of the fluid.

[0013] Based on this, a Object of the inventionthe aim is to propose a method for the manufacture and operation of an underground gravitational energy storage system that also provides larger storage volumes for intermediate storage, but does not have the aforementioned limitations.

[0014] Another task is to propose a system suitable for the process, as well as an arrangement of several such systems for gravitational energy storage in a subsurface below a terrain surface.

[0015] The problems are solved by a method, a system, an arrangement, and a use with the features of the first, fourth, eighth, and eleventh patent claims, respectively. Dependent claims relating to these describe advantageous embodiments.

[0016] To solve the problem, a gravitational energy storage system based on the following approach is proposed: For gravitational energy storage, a suspension of swelling solid particles is first injected through a ring-tight, ring-enclosed access point near the ground surface and a packer beneath a continuous layer of clay or mudstone. A double membrane forms around the resulting gap through hydraulic consolidation, which is cold-welded at the edge by pressure reduction. After removing the packer, water is pumped into the gap for the first time, lifting the soil above it while a plastic membrane forms at the borehole edge. Subsequent release of water causes the soil to subside, and the associated reduction in its potential energy is converted into electrical energy, similar to a pumped-storage hydroelectric plant. Several such cycles compact loose soil areas and depressurize rock areas.In subsequent cycles with sufficiently low amplitude, the subsoil behaves predominantly elastically, while the soil deformations at the ground surface do not impair its ecological and / or technical function. The radius of the gap is limited so that the soil above it cannot become unstable during hydraulic lifting. Several membranes are arranged side by side in such a way that earth pillars between them are not lifted.

[0017] Specifically, the preferred design is the manufacture and operation of a system for underground gravitational energy storage, encompassing at least some of the following features: (1) Selection of a suitable clay or claystone layer in the subsurface to act as a hydraulic barrier, (2) Sinking of an access shaft from the ground surface to the underside of the barrier according to (1), (3) Insertion of a tensile-resistant lining of the access shaft according to (2) from the ground surface to an upper section of the access shaft, and support of the subsurface by a filter cake of a suspension of swellable solid particles, (4) Insertion of a packer into the access shaft according to (2) inside the barrier according to (1), tensioning of the packer against the barrier and attachment of the packer with a pulley, (5) Injection of a suspension of swellable solid particles through the packer according to (4), which is thereby raised, under the barrier according to (1), so that an approximately radially symmetrical gap is formed, (6) Monitoring of the gap propagation according to (5) via pressure and quantity of the suspension, as well as by means of refraction seismics,(7) Formation of a membrane at the top and bottom of the gap by hydraulic consolidation during its manufacture according to (5), (8) Cold welding of the upper and lower membranes according to (7) along an annular edge strip by temporarily reducing the suspension pressure, (9) Removal of the packer according to (4), insertion of a pump and an electric motor into the upwardly extended wall according to (3) and closing it except for an access for water and electricity, (10) Construction or repurposing of a water reservoir at or near the ground surface above the gap according to (5), (11) Pumping water into the access according to (2) such that the filter cake according to (3) is compacted into a membrane, and that the gap is widened and the subsoil - possibly with water reservoirs according to (10) - is raised above it without the edge according to (8) opening, (12) Discharging water from the gap in such a way that,that the gravitational energy of the ground above decreases again, while the pump acts as a turbine and its motor as a generator feeds energy into the power grid, (13) compacting loose subsurface areas by repeated pumping in and out according to (11) and (12), (14) pumping in and out according to (11) and (12) in the case of rocky subsurface in a stepwise manner to limit the associated seismic activity, (15) repeated lifting and lowering according to (11) and (12) after insertion according to (13) and / or (14) with such small changes in height that the ecological and / or technical functionality of the terrain or the buildings is not restricted, and thus in such a way that the energy loss in the subsurface is negligible, (16) arranging several systems according to (1) to (15) side by side in such a way that the fissures are not hydraulically connected to each other, and in such a way that earth pillars remain between them which are not lifted and lowered,(17) Arrangement of elongated earth pillars according to (16) watercourses crossing under the terrain.

[0018] In a preferred embodiment, a method for manufacturing and operating an underground gravitational energy storage system is described, comprising the following process steps: a) Selection of a clay or claystone layer in the subsurface suitable as a hydraulic barrier. The barrier preferably extends two-dimensionally horizontally and / or planarly in the subsurface. b) Drilling of a (technical) access point from the ground surface to the underside of the barrier. The access point is preferably a borehole drilled with a deep geological drilling tool from the ground surface through the subsurface and through the barrier, and preferably has a circular cross-section. The drilling of the access point is preferably carried out using a suspension containing preferably swellable solid particles, wherein the suspension enters the access point and thus reaches the walls of the access point, where it wets the surrounding subsurface material, penetrates at least superficially, and forms a filter cake with the solid particles, which then hydraulically consolidates into a membrane on the wall of the access point.The membrane is preferably plastically deformable, which means that, in principle, a degree of subsurface flexibility under pressure is tolerable without the membrane rupturing. The preferred swellable solid particles are preferably swellable minerals and are characterized by their ability to absorb and release water, thus increasing or decreasing in density and volume with varying water content. c) Insertion of an annularly rigid lining of the access point from the ground surface to an upper section of the access point, and support of the subsurface by a (pre-mentioned) filter cake made of a suspension of preferably swellable solid particles. The annularly rigid lining is preferably a pipe section that is inserted into the access point as a whole or in interconnected segments and is suitable for withstanding internal overpressure to the outside.This is particularly necessary in the upper section of the access point because the subsoil there, due to its closer proximity to the ground surface, is usually less compacted than in deeper subsoil areas and therefore exhibits a correspondingly higher degree of flexure to applied pressures and forces. With increasing flexure, the risk rises that a consolidated membrane will no longer be able to withstand these forces and will rupture. In contrast, lower flexure is to be expected in the more compacted, deeper subsoil above the barrier; for this purpose, the aforementioned membrane, hydraulically consolidated from a aforementioned filter cake, is sufficient. The wall of the access point in the subsoil area between the ground surface and the barrier is therefore preferably divided into two sections: an upper section with an inserted tensile-resistant lining and a lower section with a aforementioned consolidated membrane.The lining and membrane are preferably fluid-tight, or at least watertight. d) Inserting a packer into the access point within the barrier (i.e., in the penetration through the barrier), tensioning the packer against the barrier, and attaching the packer with a pulley. The term "packer" here also refers to any other sealing elements with at least one opening for an injection line, wherein the injection line is sealed within the packer, or the access point and the opening itself form the injection line. The packer is inserted into a borehole and can be tensioned against the borehole wall to create a seal.e) Injection of a suspension, preferably of swellable solid particles, through the packer beneath the barrier, creating an approximately radially symmetrical gap and lifting the packer within the barrier. This results in the formation of a membrane at the top and bottom of the gap by hydraulic consolidation (as previously described). As the pressure and volume of the suspension increase, the gap volume and thus the gap's extent expand. f) Monitoring the spread of the gap in e) by measuring and controlling the pressure and volume of the suspension, as well as by means of refraction seismics. Preferably, it is proposed to monitor the formation and extent of the gap by measuring the pump rate, the packer's lifting height, and / or the size of the gap volume. g) Cold welding of the upper and lower membranes along an annular edge strip by temporarily reducing the suspension pressure.A reduction or interruption of the suspension supply or pressure causes an interruption of the gap expansion. In particular, the membrane at the circumferential edge strip is not yet fully consolidated due to crack propagation that occurred only immediately prior to this point. Therefore, leaks and increased local flow of the suspension within the gap volume are to be expected there. The standstill thus leads to the rearrangement of solid particles already deposited from the suspension, resulting in filter cake formation, consolidation of this cake, and consequently, cold welding and an increasingly stable connection between the upper and lower membranes in the circumferential edge strip surrounding the gap volume. h) Removal of the packer from the access point.This occurs when the membranes and the cold welding are stable enough for subsequent operation of the system with water instead of the aforementioned suspension, and the gap volume is watertight to the outside.

[0019] The aforementioned process steps a) to h), preferably in the illustrated chronology, serve to produce the gap volume and to line it with a hydraulically consolidated membrane made of solid particles from a suspension. The suspension is therefore a membrane-forming suspension whose solid component preferably consists of or comprises particles of a swellable mineral and / or includes an additive that promotes geoformation.

[0020] The following process steps preferably serve to commission the system, wherein the initial injection of water is carried out slowly and under controlled conditions, so that the expanding upper surface of the double membrane and its edge do not expand further due to hydraulic consolidation of suspended solids remaining in the gap. This initial injection of water into the gap is performed in a sequential process, i.e., in several cumulative loading steps with interruptions and intermediate unloading, whereby the subsoil above the barrier is compacted with increasing loading height and becomes increasingly elastically deformable. The subsoil above the barrier preferably consists of a rock-like formation, which also promotes elastic deformability of the subsoil.

[0021] The aforementioned procedure is further characterized by the following subsequent procedural steps (preferably in the order listed below): i) Inserting a pump-turbine (turbine suitable as a pump and as a drive turbine) with an electric machine (usable as a generator or electric motor) through the access point onto or into the annularly rigid lining, thereby sealing the access point fluid-tight up to a water supply line; j) Constructing or repurposing a water reservoir at or near the ground surface above the gap; and k) Commissioning the system, comprising pumping water via the pump into the access point and the gap, so that the filter cake is compacted into a membrane, the gap is widened, and the subsoil above the gap is raised, thereby storing gravitational energy (potential energy) in the subsoil above the gap without the annular edge strip opening; and releasing water from the gap through the access point via the pump-turbine.in which the stored gravitational energy is converted into electrical energy via the escaping water (and thus via a lowering of the subsurface area resting on the gap volume), as well as compaction of the subsurface above the gap by repeated pumping in and letting out of water according to process features i) and ii).

[0022] A preferred repetition of the driving process according to the aforementioned process feature k) results in a decrease in the changes in the elevation of the ground surface with each pumping and emptying cycle. Preferably, the repetition is carried out until such small changes in elevation occur on the ground surface that the ecological and / or technical functionality of the terrain or the buildings is not impaired, and the energy loss in the subsurface is negligible.

[0023] As with the initial pumping, the procedure also basically includes one or more pumping and releasing during the incremental movement, which results in further compaction of the subsoil moved as a result, in particular the subsoil area above the lateral extent of the gap.

[0024] Furthermore, the method preferably also includes operating the system for the gravitational storage of energy beneath a layer of clay or mudstone suitable as a hydraulic barrier in a subsurface below a ground surface. In this system, water from the aforementioned water reservoir (e.g., a lake) is pumped into the storage volume via the aforementioned pump, turbine, or pump-turbine for energy storage (storage cycle). For energy release (release cycle), the water is preferably discharged from the storage volume back into the water reservoir via the pump-turbine. During the storage cycle, the pump-turbine acts as a pump, and the generator acts as a motor to drive the pump. During a release cycle, the pump-turbine acts as a turbine, transferring its mechanical energy to the generator for conversion into electrical energy.

[0025] An alternative description of the process for manufacturing and operating a system for the gravitational storage of energy in a fissure volume in a subsurface preferably comprises the following process steps: Provision of a clay or mudstone layer suitable as a hydraulic barrier in the subsurface, a drilling tool, an annular tensile-resistant lining, and a packer with an injection line penetrating it; sinking of a technical access point with the drilling tool from a ground surface above the barrier through the subsurface to the underside of the barrier; insertion of the annular tensile-resistant lining starting from the ground surface, at least in the uppermost section of the access point; insertion of the packer into the access point within the barrier and tensioning it against the barrier; injection of a suspension through the injection line and the packer under the barrier, whereby the packer is raised, forming an approximately radially symmetrical gap under the barrier; further pumping of the membrane-forming suspension into the laterally expanding gap.wherein the suspended solid forms a filter cake with the adjacent substrate, which, through hydraulic consolidation of the suspended solid in the gap, forms a watertight plastic membrane on the top and bottom of the gap, and wherein the pumping is stopped or throttled, thereby interrupting further expansion of the gap and a watertight plastic edge of the gap is formed by the formation of a filter cake and further hydraulic consolidation of suspended solid remaining in the gap.

[0026] Preferably, the method includes subsequent removal of the packer from the access point.

[0027] The aforementioned method is preferably characterized in that the sinking of the access is carried out with the aid of the membrane-forming suspension, wherein the suspension enters the access, and further preferably a watertight plastic membrane is formed on the wall of the access below the tensile-resistant lining by the formation of a filter cake and further hydraulic consolidation of the suspended solid.

[0028] A preferred method variant is characterized in that at least one intermediate storage tank for water is created or repurposed above or vertically above the gap and is connected to the access point via pumps or turbines or at least one pump turbine (which can be used as both a pump and a turbine).

[0029] A preferred method variant for subsequent commissioning of the system is characterized in that the initial pumping of water is carried out slowly and in a monitored manner, that the expanding upper surface of the double membrane and its edge do not expand further due to hydraulic consolidation of suspended solid remaining in the gap, and further preferably the initial pumping is carried out in a piling step, whereby the subsoil above the barrier is compacted and thereby becomes predominantly elastically deformable.

[0030] Furthermore, to solve the task, a system proposed for the gravitational storage of energy under a layer of clay or mudstone suitable as a hydraulic barrier in a subsurface below a ground surface, which is preferably, but not necessarily, producible, accessible and / or operable according to the aforementioned method.

[0031] The system comprises a gap volume beneath the barrier with an upper and a lower watertight plastic membrane, as well as a rim around the gap (annular rim strip) made of a hydraulically consolidated solid from a membrane-forming suspension. Preferably, the gap borders directly on the barrier at its upper edge. Furthermore, a subsurface area is arranged above the barrier, which terminates at the ground surface. Consequently, the system includes a subsurface area above the gap between the barrier and the ground surface that exerts a gravitational influence on the gap volume. The lateral extent of the subsurface area preferably corresponds at least to that of the lateral extent of the gap. Preferably, the gap is completely covered by the subsurface area.Furthermore, the system includes a (technical) access from a ground surface above the barrier through the subsoil to the underside of the barrier into the gap volume, wherein a ring-tensile-resistant lining is provided starting from the ground surface at least in the uppermost section of the access.

[0032] A membrane-forming suspension is a suspension with suspended solid particles that are suitable for forming a filter cake on a wall against a substrate, which hydraulically consolidates and develops into a membrane.

[0033] Preferably, but at the latest after a break-in period, the subsurface area is compacted compared to the state before the system was manufactured and is predominantly elastically deformable, i.e., the elastic component of a deformation of the subsurface area during operation of the system exceeds a plastic component in this deformation.

[0034] Furthermore, the system preferably comprises at least one intermediate storage tank for water above or vertically above the cracking volume. The intermediate storage tank, or at least one of them, is connected to the technical access via pumps or turbines (e.g., the aforementioned pump-turbine). A preferred embodiment provides direct connections from each intermediate storage tank to the pumps or turbines, which can be switched individually or in groups as needed. If the at least one intermediate storage tank is located wholly or partially on the ground surface and / or on the subsurface area that exerts a gravitational influence on the cracking volume, such that it is raised or lowered with a change in the fill level of the storage volume, it can also advantageously be used for the additional storage of gravitational energy.

[0035] Furthermore, an arrangement comprising at least two adjacent systems of the aforementioned type is proposed, characterized by the fact that earth pillars are arranged between the gap volumes of the systems, away from the gravitationally acting subsurface areas. Preferably, the gap volumes are spaced apart from one another and not hydraulically connected to each other, such that the earth pillars remain between them.

[0036] Earth pillars, as defined in the application, are the areas of the subsurface above the space between two fissures; unlike the aforementioned subsurface areas above the fissures, they are neither raised nor lowered by them. Therefore, they do not serve to store gravitational energy.

[0037] One design envisions laterally elongated earth pillars, i.e., pillar sections that preferably extend in a straight line or at least follow a route across the terrain surface. It is proposed that this route be positioned along a watercourse and the systems located alongside this route to ensure the desired functional separation of the gravitational energy storage system, or even just individual systems within it, from the watercourse through spatial separation.

[0038] The creation of boreholes (access points) and fissures sealed watertight by a (wax-like) membrane through the injection of a preferably membrane-forming (thixotropic) suspension is a method not yet used in geotechnical engineering or reservoir engineering. Also novel is its use as an energy storage system without excavation, resulting in such minimal subsurface deformation that hardly any energy is dissipated and the functionality of the Earth's surface is not impaired.

[0039] The invention is explained in more detail with reference to exemplary embodiments (preferred embodiments), the following figures, and descriptions. All features shown and their combinations are not limited to these exemplary embodiments and their configurations. Rather, they are intended to be considered representative of further possible configurations that are not explicitly shown as exemplary embodiments. The figures show... Fig. 1a and b a schematic representation of an embodiment of an arrangement of a system with a continuous clay layer as a barrier - case group 1, Fig. 2a and b a schematic representation of an embodiment of an arrangement of a system with a clay layer with faults as a barrier - case group 2 as well as Fig. 3a and b a schematic representation of an embodiment of an arrangement of a system with a clay layer with caprocks as a barrier - case group 3.

[0040] Manufacturing and operation are described below for three geologically defined case groups. Numerical data represent orders of magnitude, and drawings are simplified. General requirements, combinations, and exceptions are subsequently explained. Case group 1: Continuous clay layer (see Fig. 1a and b):

[0041] In sedimentary basins, particularly at depths of approximately 100 to 200 meters, continuous horizontal clay layers of approximately 0.2 to 1.5 meters thickness occur over distances of approximately 1 to 10 kilometers. The outcrops of these layers along faults are smaller than the layer thickness due to minimal tectonic reshaping. The clay particles do not attract each other in saline water; the clay, consolidated by overburden and time, therefore lacks effective cohesion. When exposed underwater, it disintegrates into a suspension after expansion. In open-pit mining—especially for lignite extraction—such clay layers serve as a seal for the drained pit against groundwater pressure from below, while the groundwater level in the distance remains close to the surface. The subsequent backfill is loose and largely saturated with water; depending on the extraction volume and the groundwater level, depressions with residual lakes may remain.

[0042] To manufacture a storage system ( Fig. 1a and b) drilling will be 1 (= access) to a suitable clay layer 2 (= barrier) laid down. The piping 3 The ring-shaped, tensile-resistant lining reaches a maximum depth of approximately 1 / 10 of the clay layer to prevent the surrounding soil (subsoil) from collapsing and being radially displaced by the subsequent overpressure. Below this, a filter cake of thixotropic clay suspension supports the soil. If dissolved substances in the groundwater inhibit gel formation, a gel-promoting additive is added to the suspension. A sealing packer is then used to seal the surface. 4 (cf.) Fig. 1b The suspension is forced under the clay layer with such high overpressure that it is slightly lifted along with the packer and the soil above it (barrier and subsoil above the barrier). This creates a lens-shaped gap a few centimeters high. 5.On its upper and lower surfaces, a waxy membrane a few millimeters thick forms through hydraulic consolidation (similar to process engineering with non-thixotropic suspensions). This membrane is firmer than the filter cake at the borehole edge, yet viscoplastic and not brittle. The membrane forms first in particularly permeable areas of the underlying clay layer due to the flow force acting on mineral particles in the suspension, then in less permeable areas, and finally, and more thinly, on the underside of the clay layer.

[0043] The horizontal edge of the gap is approximately circular at every intermediate stage of its construction. The volume of the gap (gap volume) is equal to that of the suspension injected so far, and its central height is determined by the measured lift of the packer. The mass of clay mineral required to form a watertight membrane is, for each intermediate stage, the product of the gap's surface area and the fixed mineral mass per unit area. The latter, as well as the times required for hydraulic consolidation, are derived from laboratory tests. Based on these results, a gap radius of two to three times the clay layer depth is achieved through the mineral content of the suspension, as well as pumping volumes, pressures, and durations.

[0044] Upon reaching the edge of the gap, the upper and lower membranes are cold-welded together in an annular strip. For this, the suspension pressure is reduced sufficiently and slowly to ensure that the gel component of the pressed membranes moves towards the gap, and pore water is squeezed vertically out of the less consolidated contact zone. The required pressures and their durations are determined through laboratory tests and calculations.

[0045] After removing the packer, the piping is connected to a pump. 6 (cf.) Fig. 1a ,(corresponding to a pump turbine) is connected, and water is pumped in through it. The overpressure required for the production and use of the double membrane prevents radial soil displacement by the casing near the surface. Due to the overpressure and with controlled water flow, the height of the lens-shaped gap initially increases by approximately 1 to 2 meters. Even at the borehole margin, where sand or clay is present, the filter cake forms a waxy membrane that, in particular, prevents the softening and erosion of the clay layer at the borehole. During hydraulic expansion of the double membrane enclosing the gap, its cold-welded edge does not tear because the tensile stress does not increase there due to relaxation caused by the slow expansion, and because membrane areas that become more permeable due to stretching seal themselves through the consolidation of slurry components.

[0046] The gap remains lens-shaped because the soil above its edge is continuously sheared, thus preventing the formation of a step. Even during subsequent emptying and refilling of the gap with water, the deformation of the viscoplastic membrane is so small and slow that it does not tear and therefore remains sealed. After emptying, the upper and lower membrane sections that touch do not adhere to each other because the overpressure is maintained. This occurs with every hydraulic lifting and lowering operation, during which the pump and motor are connected to the borehole (borehole). 1 When the system temporarily functions as a turbine and generator, some of the energy to be stored is dissipated in the ground. This proportion remains below 1% after a few start-up cycles because the gap radii, spacing, and height changes are optimized. Existing or constructed ponds 7 (= water reservoir, cf. Fig. 1aThey serve as intermediate storage for pumped water. Insofar as they lie above the double membrane (= membrane pair on the top and bottom of the gap), they are raised and lowered along with the ground (subsoil with barrier) above them.

[0047] A combined storage system (plant) is created from several boreholes in a grid pattern (not necessarily square), the dimensions of which closely approximate the diameter of the circular slit in plan view. This leaves earth pillars between the double membranes, which are not raised or lowered during operation of the storage system, thus maintaining a stable equilibrium in the raised subsurface area. Uplift and subsidence caused by the operation of the storage system are limited to ensure that soil deformations at the surface do not impair its function. Initially loose soil layers (in the subsurface) are compacted by cyclical shearing during drilling, making them suitable for construction without disrupting groundwater flow. Ponds serve as operational water reservoirs, while flowing watercourses rest on long, interconnected earth pillars and are not raised or lowered.

[0048] For example, a field measuring 2 km in length and width, with a clay layer at a depth of 100 m and an average uplift of up to 1.5 m, yields a storage capacity of approximately 7 x 10⁸ kNm, or about 1500 MWh (at market prices in autumn 2022, this would cost at least €150,000). This is sufficient as a buffer storage system for periods of low wind for approximately 50 wind turbines, each with an average output of 1 MW, under average wind conditions. Case group 2: Clay layer with faults (see Fig. 2a and b)

[0049] In sedimentary basins, clay layers located deeper than those in case group 1 can exhibit dislocations greater than their thickness due to tectonic reshaping along faults. Local shearing then transforms clay in faults into claysmears, whose thickness is approximately 1 / 6 that of the clay layer. When the dislocation exceeds about eight to ten times the thickness of the clay layer, gaps filled with sand or gravel form in the claysmears. Without these gaps, sufficiently thick and continuous clay layers with claysmears are suitable for groundwater sealing during open-pit mining. This can lead to slight uplift of the soil between the clay layer and the pit floor, and rarely to the eruption of water, clay mud, and gas along a fault with a defective claysmear. Faults containing claysmears and silt subdivide the subsurface into hydraulic compartments, the fractal arrangement of which can only be approximately determined through exploration.

[0050] To manufacture a storage system ( Fig. 2a and b ) are drilled into compartments 1 (= access) into suitable sections of the clay layer 8 (each forming a barrier) between faults 9 They are deposited. As in case group 1, they are provided with a grouting to a maximum depth of approximately 1 / 10 of the clay layer. After inserting a packer, a thixotropic suspension of swelling clay mineral in water (with geoforming additive if necessary) is forced under pressure beneath the clay layer, creating a gap beneath it. With a greater overpressure than in case group 1, waxy membranes form around the expanding gap, which, despite their higher strength, are viscoplastic, as in case group 1. At a clay smear 10 will the spread of the gap 5 hindered, especially within the area of ​​the fault 9Thinner clay bands also occur. In sand-filled gaps of a claysmear resulting from large displacements, the further propagation of a fissure is blocked by membrane formation because the fissure growth is no longer guided by a clay cap.

[0051] Due to spatially fractally distributed faults with claysmears, the resulting fissures are no longer circular as in case group 1, but rather circularly polygonal. Nevertheless, after removing the packer and connecting the pump, a [missing information - likely a specific type of fissure] forms. 6 (corresponding to a pump turbine) a wax-like membrane is applied to the casing by pumping water into the borehole rim, which protects the clay layer in particular from softening and erosion. Cold welding of the membrane rim (= circumferential rim) is carried out as in case group 1, even where it is inclined at a claysmear. Spacing of boreholes initially results from hydrogeologically determined compartments (simplified in Fig. 2a and bIf the width of borehole groups is greater than approximately two to three times the depth of the clay layer, earth pillars must remain between double membranes to maintain the stability of the soil above during hydraulic lifting. As in case group 1, a pond serves this purpose. 7 (= water reservoir) as the upper reservoir during storage operation, and with optimal borehole arrangement and uplift, energy dissipation in the ground remains below 1% of the gravitational component per cycle. With sufficiently small uplifts, the ground deformation caused by storage operation does not impair the functionality of the terrain.

[0052] For example, with an average uplift of 2 m and an average clay layer depth of 500 m, a field 2 km long and wide yields a storage capacity of approximately 3 x 10< kNm (market value in 2022 at least €2.5 million). This underground battery is sufficient as a buffer for approximately 500 wind turbines with a capacity of approximately 1 MW each under average wind conditions. Case group 3: Caprocks (see Fig. 3a and b)

[0053] Natural gas accumulates in sandstone beneath claystone layers at depths of approximately 3 to 5 km, provided that faults (which, along with claysmears and due to grain fracturing, contain more fine particles) trap the gas as it rises into caprocks. Since natural gas collects in hydraulically confined compartments caused by faults, it is extracted from such a field using multiple wells. After the production wells are decommissioned, small quantities of gas remain beneath the caprock.

[0054] To manufacture a storage system ( Fig. 3a and b ) drilling will be 1 reactivated or newly formed in compartments between faults 9 to the Caprock 12The borehole is drilled using a thixotropic suspension of swelling clay mineral (with a gel-forming additive if necessary). This suspension not only supports the borehole wall but also drives the drill's turbine and removes the drill cuttings (standard technique). Casing is installed in the overlying unconsolidated rock to a maximum depth of approximately 1 / 10 of the clay cap. After inserting a packer, thixotropic clay suspension is injected from each borehole at a pressure high enough to create fissures with a slight uplift of the caprock. These fissures are enclosed by a waxy membrane of clay mineral. Residual gas is released. 11 Overpressure promotes the formation of the gap. The gap is present despite Claysmears. 10 approximately lens-shaped, and its lateral edge is circularly polygonal.

[0055] After removing the packers and connecting the piping to the pumps 6Water is pumped into the fissure in such a way that the claystone layer rises by approximately 1 to 2 meters. This process also creates a waxy (and therefore ductile) membrane at the borehole edge, which prevents hydraulic weakening of the cap at the claystone opening. During the initial uplift, residual gas escapes, and moderate seismic activity can occur due to the deformation of the raised area. The magnitude of this seismic activity can be reduced by incrementally lifting the fissure. During the lowering phase, water from the fissure drives the pumps as turbines, and the motors become generators. With each cycle, some energy is dissipated into the subsurface, but this loss remains below approximately 2% after a few initial cycles with optimal configuration. Pillars between fissures are not required as long as the radius of the temporarily raised subsurface area is less than two to three times the depth of the claystone layer. A pond serves as the upper water reservoir.The deformation of the terrain surface caused by the operation of the storage system is so small, given the approximately 1000 times greater width, when the amplitude is limited to approximately 1 m to 2 m, that buildings and watercourses are not affected.

[0056] For example, with an average uplift of up to 2 m of a 4 km deep claystone layer, a field of 2 km length and width has a storage capacity of around 10 12< kNm (market value autumn 2022 at least 50 million euros), which is sufficient as a buffer for around 1000 offshore wind turbines with an average output of 10 MW in good wind conditions. Requirements, combinations and exceptions

[0057] The technology according to the invention requires thorough exploration, planning, and monitoring. This includes seismic locating of the intended clay or mudstone layer and its faults. This also makes the gap visible, because shear waves do not propagate through it at all, and pressure waves propagate more slowly than above and below. Although the waxy membrane does not tear when deformed by repeated gap widening and narrowing, a repair using suspension is possible if recorded seepage losses suggest this.

[0058] Suitable claystone layers also occur at shallower depths than in case group 3, and like clay layers of case group 1, they can exhibit larger displacements and clay smears. A double membrane made of hydraulically consolidated, swelling clay minerals can be produced in various combinations from the three case groups and used for energy storage. (Lowland uplift is not a factor.) With earth pillars for stabilization, adjacent double membranes are expanded and contracted so that the deformations of the ground surface remain sufficiently small. As in case groups 1 and 2, ponds serve as reservoirs for process water, which can be raised and lowered along with the membranes, while flowing waters remain at their current level on elongated earth pillars.

[0059] Despite the presence of claysmears, the technology requires sufficiently continuous clay or mudstone caps. Small gaps in claysmears or resulting from previous drilling are bridged or intentionally limited during the fracturing process, but remnants of scour holes and the boundaries of former water bodies are unsuitable. Extensive gas pockets beneath caprocks must be vented beforehand. Sites with oil above gas pockets are unsuitable, as are those with highly active tectonics and / or cold volcanism. Economic benefits:

[0060] In Germany, approximately 110 THW of electrical energy are currently consumed annually. At today's price of at least €300 per MWh, this equates to at least €30 billion. Nuclear power plants, which still contribute about 5%, are scheduled to be decommissioned by spring 2023, lignite-fired power plants by 2035, and those on the left bank of the Rhine by 2030. The electricity demand, including that for electric vehicles, is then to be met by wind and solar power plants, which currently can only cover twice the required output during periods of strong wind and sunshine. The storage of approximately 5 to 10 TWh, required due to fluctuations in wind and solar radiation, is to be achieved using hydrogen. This will hardly be feasible for the vast majority of applications because the efficiency is below 30%, the necessary pressure of 100 bar requires massive containers, and leaks from containers and pipes would produce oxyhydrogen gas.

[0061] The gravitational energy of an Earth mass with a density of 10 kN / m³ (minus buoyancy), a width and length of 2 km, and a depth of 200 m increases by approximately 3 x 10¹¹ kNm when lifted by 2 m. This corresponds to approximately 90 MWh and could be sufficient as a buffer for a wind turbine with 50 1 MW rotors. The storage costs increase by a maximum of about 10% due to the installation and operation of the ground-based battery, because there are approximately 50 weekly and 2 seasonal cycles per year, and the storage system experiences minimal wear (or at most, only easily correctable wear).

[0062] For larger wind turbines with rotors that produce approximately 1 to 10 MW on average under good wind conditions, larger storage systems are required. Reclaimed areas of former open-cast mines or former gas fields are particularly suitable for this purpose, as are populated areas due to the minimal impact on the ground. Dissipative energy losses are lower than with pumped-storage power plants. The costs per unit of energy per year are lower than for pumped-storage power plants due to the less complex installation and maintenance. While it is difficult to find suitable locations for pumped-storage power plants in Germany, for example, undeveloped or less developed sites in sedimentary basins with clay or mudstone layers are sufficient to meet the foreseeable storage requirements after the transition to a climate-friendly energy economy. This also applies to the majority of other countries, but not to tectonically active and / or mountainous regions. Reference symbol list:

[0063] 1. Borehole, access 2. Clay layer, barrier 3. Casing, tensile-resistant lining 4. Packer 5. Gap 6. Pump, pump turbine 7. Pond, water reservoir 8. Clay layer section 9. Fault 10. Clay smear 11. Residual gas 12. Caprock

Claims

1. Method for creating and operating a system for underground gravitational energy storage, characterised by the following method steps: a) selecting a clay or mudstone layer in the subsoil suitable as a hydraulic barrier, b) sinking an access shaft from the ground surface to the underside of the barrier, c) inserting a ring-tensile-resistant lining of the access shaft from the ground surface to an upper portion of the access shaft, and supporting the subsoil by a filter cake consisting of a suspension of swellable solid particles, d) hanging a packer in the access shaft within the barrier, tensioning the packer against the barrier and attaching a pulley to the packer, e) injecting a suspension of swellable solid particles through the packer under the barrier, so that an approximately radially symmetrical gap is formed and the packer is lifted in the barrier, forming a membrane at the top and bottom of the gap through hydraulic consolidation, f) checking the expansion of the gap in e) by measuring and checking the pressure and quantity of the suspension and by seismic refraction, g) cold welding the upper and lower membranes along a ring-shaped edge strip with temporary reduction of the suspension pressure, h) removing the packer from the access shaft, i) inserting a pump turbine with an electric motor via the access shaft onto or into the ring-tensile-resistant lining, the access shaft being sealed at the top in a fluid-tight manner except for a water supply line, j) creating or repurposing a water reservoir at or near the ground surface above the gap, and k) running-in the system, involving i) pumping water via the pump into the access shaft and the gap, so that the filter cake is compressed into a membrane, the gap is widened and the subsoil above the gap is raised, thereby storing gravitational energy in the subsoil above the gap without opening the ring-shaped edge strips, ii) releasing water from the gap via the access shaft by means of the pump turbine, thereby converting the stored gravitational energy into electrical energy via the escaping water and iii) compacting the subsoil above the gap by repeating the pumping-in and releasing of water multiple times according to method features i) and ii).

2. Method according to claim 1, characterised in that pumping-in and releasing are carried out in successive small steps during the running-in phase.

3. Method according to claim 1 or 2, characterised in that the running-in process according to method feature k) is repeated until such small height changes occur on the surface of the terrain that the ecological and / or technical functionality of the terrain or farmland is not restricted, and the energy loss in the subsoil is reduced to a minimum.

4. System for gravitational storage of energy under a clay or mudstone layer suitable as a hydraulic barrier (2) in a subsoil under a ground surface, created and operated by a method according to any of claims 1 to 4, comprising: a) a gap volume (5) under the barrier with an upper and lower watertight plastic membrane and an edge around the gap made of a hydraulically consolidated solid material consisting of a membrane-forming suspension, b) a subsoil region acting gravitationally on the gap volume above the gap between the barrier and the ground surface, and c) a technical access shaft (1) from a ground surface above the barrier through the subsoil to the underside of the barrier (2) into the gap volume, wherein a ring-tensile-resistant lining (3) is provided starting from the ground surface at least in the uppermost portion of the access shaft.

5. System according to claim 4, characterised in that the subsoil region is compacted and largely elastically deformable.

6. System according to claim 4 or 5, further comprising: d) at least one intermediate reservoir (7) for water over or vertically above the gap volume (5) and e) pumps (6) or turbines between the technical access shaft and the at least one intermediate reservoir.

7. System according to claim 6, characterised in that the at least one intermediate reservoir (7) is arranged on the ground surface and / or on the subsoil region acting gravitationally on the gap volume (5).

8. Arrangement comprising at least two systems arranged next to one another according to any of claims 4 to 7, characterised in that earth pillars are arranged between the gap volumes of the systems away from the gravitationally acting subsoil regions.

9. Arrangement according to claim 8, characterised in that the gap volumes are not hydraulically connected to one another, and that earth pillars remain between them which are not raised and lowered with them.

10. Arrangement according to claim 8 or 9, characterised in that the earth pillars are laterally elongated and arranged under a watercourse.

11. Use of a system according to any of claims 4 to 7 for carrying out an operation according to a method according to any of claims 1 to 3.