Improvements in flywheel energy storage

Abstract

Embodiments of a flywheel energy storage system configured to be installed in an existing natural high pressure environment and configured to compress a flywheel rotor using ambient pressure to increase its energy storage capacity are provided. Embodiments of a rotor for a flywheel energy storage system are also provided, the rotor including a housing layer that encases the rotor and is free to rotate to reduce the impact of frictional resistance on the rotor.

Description

Technical Field

[0001] The present invention relates to a flywheel energy storage device, a rotor for a flywheel energy storage device, and a method for generating electricity using a flywheel energy storage device. Background Art

[0002] Flywheels have been used for years to store energy because they are efficient, have a long lifespan, and are very cheap to extract the energy stored in them.

[0003] Typically, a flywheel consists of a solid rotor, usually cylindrical in shape, that is coupled to the rotating shaft of an electric motor. As the electric motor spins the rotor, it converts electrical energy into kinetic energy, which is stored as angular kinetic energy in the flywheel. This rotational energy can then be converted back into electrical energy by using the flywheel to drive the motor, causing the motor to act as a generator.

[0004] To improve the efficiency of the system, the rotor is generally housed in a housing that is in a vacuum or near vacuum. This reduces the frictional resistance acting on the rotor, resulting in reduced energy loss and improved efficiency of the flywheel storage device.

[0005] Because a flywheel's maximum rotational speed, and therefore the maximum kinetic energy that can be stored within it, is a function of the tensile strength of the material used to construct it, flywheel rotors typically need to be made of high-tensile-strength materials in order to store significant amounts of energy. In most commercial environments, this means the rotors are constructed from materials such as steel, aluminum, carbon fiber, or fiberglass—materials that are often rare, difficult to process, highly carbon-intensive, and expensive. Consequently, adding additional storage capacity by increasing the rotor's mass, as needed for long-term storage, can be prohibitively expensive and difficult. This means that while flywheels are widely used for short-term storage of small amounts of energy, they are generally overlooked for long-term storage due to the associated costs. However, nearly all energy storage needs require longer storage capacities than currently available flywheels can achieve, as they aim to mitigate the diurnal fluctuations of renewable energy sources such as solar and wind. Given increasing environmental pressures and the world's increasing shift toward renewable energy, a flywheel energy storage device capable of efficient, long-term energy storage is needed. Furthermore, such a system should be constructed from materials that are abundant, inexpensive, and have a low carbon intensity, making flywheel energy storage devices feasible for manufacture and deployment worldwide.

[0006] Several attempts have been made to increase the storage capacity of flywheel rotors. For example, US Pat. No. 10281003 B2 attempts to reduce the reliance on excessively high-tensile-strength materials by using a flywheel comprising a cylindrical mass body composed of a primary material, such as concrete, with a compressive strength of at least 25 MPa. At least a portion of the outer surface of the body is surrounded by fibers, the fibers comprising a material having a tensile strength of at least 100 MPa. The tension of the fibers wrapped around the body compresses the primary material and causes it to be prestressed. Consequently, the maximum rotational speed of low-tensile-strength concrete is increased because the centripetal force generated by the flywheel's rotation must overcome this compression before the concrete is under net tension. However, as stated above, this compression is achieved in US Pat. No. 10281003 B2 by wrapping the concrete with high-tensile-strength fibers, which negates much of the benefit of using a low-tensile-strength material for the primary body. Similarly, the reliance on relatively scarce high-tensile-strength fibers limits this solution to regions that can afford the expensive material and have the capacity to manufacture and use it.

[0007] US5015940A applies a similar pre-compression principle to known high-tensile-strength rotors. In this case, the additional pressure is applied externally and is independent of the rotational motion of the object itself. For example, US5015940A describes placing the rotor inside a pressure chamber and using gas held at superatmospheric pressure (i.e., pressure greater than 1 atm ≈ 1 bar) within the chamber to compress the rotor, thereby increasing the maximum storage capacity through the same mechanism described above with respect to US10281003B2. However, any benefit of being able to store more energy in a given rotor mass is largely offset by the increased cost and technical difficulty of having to provide a high-tensile-strength pressure chamber to accommodate the superatmospheric pressure of the contained gas.

[0008] Therefore, there is a need for a low-cost flywheel energy storage device, and in particular, a flywheel energy storage device that reduces reliance on scarce, expensive, difficult to process, and carbon-intensive high-tensile strength materials.

[0009] The use of superatmospheric pressure gas to compress the flywheel rotor presents additional difficulties for flywheel energy storage devices because the gas increases frictional resistance on the rotor, thereby reducing the energy storage efficiency. In US5015940A, losses caused by the interaction of the gas with the flywheel can be minimized by using a low-viscosity gas such as hydrogen or helium. Alternatively, a drag reduction system as described in WO2007012267A1 can be used, which consists of n concentric thin-walled rigid cylindrical shells surrounding the flywheel. However, the structural requirements of this drag reduction system make it uneconomical for large-scale production. Therefore, there is a need for an improved drag reduction system for use with a flywheel energy storage device. Summary of the Invention

[0010] The present invention addresses the aforementioned shortcomings of known flywheel storage devices by utilizing an existing ambient high-pressure source to provide compression for a low-tensile-strength rotor, thereby increasing its energy storage capacity and reducing costs. For example, embodiments of the present invention can be installed in high-pressure environments, such as naturally occurring high-pressure environments, such as the ocean floor, lakes, caves, or flooded mines. This reduces the pressure differential across the housing, thereby lowering the tensile strength requirements of the housing.

[0011] According to a first embodiment of the present invention, a flywheel energy storage device is provided, comprising a housing, wherein the housing is sealed to separate the interior of the housing from an external environment; a rotor mounted on an axle within the housing, the rotor being configured to rotate relative to the housing, wherein the rotor is made of a low tensile strength material; a fluid contained within the housing to compress the rotor; and a device for maintaining a pressure of the fluid substantially balanced with a pressure of the external environment.

[0012] According to a second embodiment of the present invention, a flywheel energy storage device is provided, comprising a housing, wherein the housing is sealed to separate the interior of the housing from an external environment; a rotor mounted on an axle within the housing, the rotor being configured to rotate relative to the housing; a fluid contained within the housing to compress the rotor; and a device for maintaining a pressure of the fluid substantially balanced with or greater than a pressure of the external environment; and wherein the housing is a pressure vessel capable of withstanding only a pressure less than the pressure of the fluid. Specifically, the housing of the second embodiment of the present invention can be a pressure vessel capable of withstanding only a pressure less than the in-situ pressure of the contained fluid under atmospheric conditions, i.e., when the second embodiment of the present invention is installed in an ambient high-pressure environment.

[0013] By installing the flywheel energy storage device according to the first or second embodiment of the present invention in a high-pressure environment, the fluid in the housing can be essentially in equilibrium with the environment so that the rotor is in a compressed state, thereby increasing its energy storage capacity and eliminating the reliance on expensive, difficult-to-process, high-tensile-strength materials with high carbon strength. Moreover, since the fluid is in equilibrium with the environment in the first embodiment, the housing itself does not need to act as a pressure vessel to maintain the superatmospheric pressure of the fluid contained therein, and the structural complexity and material cost of the housing can also be reduced. Similarly, as in the second embodiment, if the fluid is operated at a pressure greater than the ambient pressure, the presence of external pressure means that the pressure differential to which the housing is subjected is reduced. Therefore, the total pressure that the housing must withstand is less than the total pressure of the fluid, thereby reducing the structural requirements and associated material costs required for a given operating pressure of the fluid in the housing.

[0014] In addition or alternatively, the energy storage device of the first and second embodiments may also include one or more of the following: an electric motor connected to the wheel axle for storing energy in a flywheel energy storage device; and a generator connected to the wheel axle for extracting energy from the flywheel energy storage device, thereby enabling the energy storage device to store electrical energy in the first and second embodiments.

[0015] In addition, the housing of the first or second embodiment can be made of a low tensile strength material or a material without intrinsic tensile strength. Since the flywheel energy storage devices of the first and second embodiments are intended to be installed in a high-pressure environment, there is no need to accommodate the total pressure of the superatmospheric fluid, so such materials can be used. In more detail, because the housing is prestressed by the ambient pressure of the environment into a highly compressed state, and the pressure of the fluid contained within the housing must first overcome this compression before the housing is under net tension, a material without intrinsic tensile strength (e.g., concrete) can be used for the housing. Furthermore, this allows the housing to use materials that are more readily available, less expensive, easier to process, and have a lower carbon intensity. In this way, the first and second embodiments of the present invention reduce the cost and complexity of the storage device as a whole, while still obtaining the benefit of increased storage capacity by using the superatmospheric pressure of the external environment to compress the rotor.

[0016] Additionally or alternatively, the low-tensile-strength material of one or more of the housing and the rotor of the first and second embodiments may have a tensile strength of less than 200 MPa. Preferably, the low-tensile-strength material may have a tensile strength of less than 150 MPa, 100 MPa, or 75 MPa, and optionally, less than 50 MPa, 40 MPa, 30 MPa, 20 MPa, 10 MPa, or 5 MPa. In some embodiments, the rotor may comprise a material without inherent tensile strength, i.e., a material with a tensile strength of 0 MPa.

[0017] Additionally or alternatively, the rotor of the first or second embodiment can be made of ceramic. For example, one or more of cement, concrete, and clay can be used. Alternatively, the rotor can be made of a granular material enclosed in a sealed container, where the pressure within the container is maintained at a pressure lower than the fluid pressure. Both ceramic and granular materials (such as sand) are dense, inexpensive materials, making it easy to manufacture and use rotors weighing more than one ton at a relatively low cost, thereby reducing the material and manufacturing costs of the system itself and the energy storage costs. Furthermore, by utilizing a very large rotor mass, these materials also improve the long-term energy storage capacity of the system. These materials are also abundant and readily available, enabling the manufacture and installation of flywheel energy storage devices in remote or impoverished areas where specialized high-tensile-strength materials are difficult to obtain and may not be available. Granular materials (such as sand) lack inherent bonding between individual particles and therefore lack tensile strength. These materials are abundant, inexpensive, have low carbon intensity, and can be compacted to form a dense block. Therefore, these materials are ideal for manufacturing high-quality flywheel rotors such as those used in the present invention.

[0018] Additionally or alternatively, the means for maintaining pressure in the first and second embodiments may include one or more of the following: a pressure compensator coupled to the external ambient pressure; and a valve system coupled to a pressurized fluid source. Additionally or alternatively, the means for maintaining pressure may include one or more of the following: a U-shaped tube; and a flexible membrane covering a port connected to the interior of the storage device's housing. The pressure compensator maintains equilibrium between the fluid's pressure and the external environment by automatically compensating for any drops in internal pressure, thereby ensuring that the fluid's pressure is maintained at an optimal level during operation. Alternatively or additionally, the use of a valve system and a pressurized fluid source allows for better control of the housing's internal pressure by allowing the fluid in the housing to exceed ambient pressure. Alternatively, a pump may be used to achieve a pressure within the housing exceeding ambient pressure. Furthermore, embodiments including a pressurized fluid source or pump may be passively or actively controlled to control the housing's internal pressure. For example, active control may include using a pressure sensor and actuator to control the valve system to alter the pressure of the fluid contained within the housing.

[0019] Additionally or alternatively, the housing of the first or second embodiment may be sealed using one or more seals to separate seawater from the fluid, thereby ensuring that the fluid is maintained at an optimal operating pressure and preventing the ingress of seawater that may corrode or damage components within the housing.

[0020] Additionally or alternatively, the fluid used in the first and second embodiments can be a low-density fluid, and optionally, the density of the fluid can be less than one-third the density of the rotor. Using a low-density fluid improves system efficiency by minimizing frictional energy losses in the fluid. Furthermore, by selecting a fluid based on a desired density ratio to the rotor (specifically, at a ratio of one-third of the required density), energy losses can be reduced to an acceptable level while still providing sufficient compression to the rotor. Alternatively, the fluid can be one or more of the following: hydrogen and helium. Alternatively or additionally, the fluid can be one or more of the following: neon, argon, krypton, xenon, or nitrogen.

[0021] Additionally or alternatively, the first and second embodiments may further include an electrical connector comprising a conductor for electrically communicating with the energy storage device, wherein the electrical connector includes means for isolating the conductor from seawater, optionally wherein the means includes one or more of: a seal and a dielectric fluid. The use of means for isolating the conductor from seawater ensures that the storage device can be used underwater while protecting the required electrical connections from the corrosive effects of seawater. Furthermore, the required isolation can also protect the environment from the potential high voltage electrical signals carried by the connector and prevent short circuits and other electrical faults.

[0022] Additionally or alternatively, the first and second embodiments may further comprise control electronics, for example, configured to control at least a motor-generator, and include means for isolating the control electronics from the external environment. Optionally, the means for isolating the control electronics may comprise a sealed air-filled canister for protecting the control electronics from seawater. Alternatively, the canister may be filled with a dielectric. By providing the control electronics as an integral part of the energy storage device, the first and second embodiments may be provided as a "plug-and-play" system, enabling them to be conveniently installed in remote locations and without the need for specialist installation. Furthermore, by providing means for isolating the control electronics from the environment, the first and second embodiments may be conveniently installed underwater without the risk of water damaging the control electronics.

[0023] Additionally or alternatively, the first and second embodiments may further include a lifting attachment for lowering the energy storage device to an underwater location. Providing a lifting attachment allows the device to be easily installed in remote, deep areas of the ocean (or other high-pressure environments) using readily available surface-mounted cranes or lifting tools (e.g., mounted on a vessel for installation). Alternatively or additionally, the lifting attachment may allow the storage device to be lowered using the descent of a buoyancy control device.

[0024] Additionally or alternatively, the first and second embodiments may further include a handle configured to interface with an ROV, allowing installation, inspection, maintenance, and scrap removal tasks, for example, to be performed on-site by a remotely operated vehicle (ROV), thereby reducing the risk to divers who must perform such tasks. Such a handle may also be coupled to an electrical connector, allowing the storage device to be easily connected to a power source via the ROV once installed underwater.

[0025] Additionally or alternatively, the housings of the first and second embodiments can be configured to connect to foundation anchors to support and stabilize the energy storage device in place, and optionally, wherein the foundation anchors are one or more of: a mud mat, a caisson, or one or more piles. The use of foundation anchors allows the storage device to be installed in areas with poor surface conditions and remain secure during operation and rotation at high rotational speeds. Furthermore, the concrete floors typically used for surface flywheel storage devices can exert excessive pressure on the loose seabed soil. Therefore, the use of anchoring devices specifically adapted for underwater use, such as the anchoring devices listed above, ensures that the first and second embodiments can be properly secured and anchored in place on the seabed.

[0026] Additionally or alternatively, the first and second embodiments may further include a housing layer that encases the rotor and is coupled to the axle via bearings, the housing layer having a first shape, wherein the bearings allow the housing layer to rotate about the rotor with its axis of rotation concentric with the axle. Using a housing layer that can freely rotate about the rotor reduces frictional resistance experienced by the rotor by reducing velocity differences in the fluids experienced by the rotor. Thus, the housing layer improves the efficiency of the flywheel energy storage device and reduces energy storage costs.

[0027] According to a third embodiment of the present invention, a rotor for a flywheel energy storage device is provided, the rotor comprising a rotor; an axle coupled to the rotor; and a shell layer encasing the rotor and coupled to the axle via bearings, the shell layer having a first shape, wherein the bearings allow the shell layer to rotate around the rotor about an axis of rotation concentric with the axle, and wherein a cross-section of the shell layer, when drawn in a plane in which the axis of rotation lies, defines a closed, basic convex shape having a top surface and a bottom surface that intersect the axis of rotation, and wherein each of the top and bottom surfaces comprises a continuous curve.

[0028] According to a fourth embodiment of the present invention, a rotor for a flywheel energy storage device is provided, the rotor comprising a rotor; an axle coupled to the rotor; and a shell layer wrapping the rotor and coupled to the axle via bearings, the shell layer having a first shape, wherein the bearings allow the shell layer to rotate around the rotor about an axis of rotation concentric with the axle, and wherein the shell layer defines a closed three-dimensional substantially convex shape having a top surface and a bottom surface intersecting the axis of rotation, and wherein each of the top surface and the bottom surface includes a portion having a continuous curvature.

[0029] As explained in more detail below, the use of a shell layer that can rotate freely around the rotor as provided in the third and fourth embodiments reduces the frictional resistance borne by the rotor by reducing the speed difference of the fluid borne by the rotor. In this way, the shell layer improves the efficiency of the flywheel energy storage device and reduces the cost of energy storage. In addition, the specific shape of the shell layer provided in the third and fourth embodiments enables the shell layer to act as a pressure vessel and withstand the centripetal pressure generated by the fluid within the shell layer as the rotor rotates, without requiring uneconomical and impractical shell layer materials. Therefore, the use of the shell layer provided in the third and fourth embodiments improves the energy storage efficiency of the rotor by reducing the resistance borne by the rotor. The specific shape of the shell defined by these embodiments increases the storage capacity by increasing the maximum speed at which the rotor can rotate before damaging the shell layer. Specifically and as explained below, providing a curved surface on the shell layer reduces the material cost of the shell layer by allowing the pressure to be accommodated through tension in the material rather than bending moment, thereby allowing the use of thinner materials.

[0030] Additionally, any of the previously described embodiments of the present invention may include a plurality of concentrically nested housing layers, wherein each housing layer has a corresponding first shape, each housing layer including a bearing coupled to the axle, wherein the bearing is configured to rotate the housing layer about the rotor and relative to each of the other housing layers. The use of multiple nested housing layers further reduces the drag experienced by the rotor, thereby further improving energy storage efficiency and further reducing energy storage costs.

[0031] Additionally or alternatively, in any of the previously described embodiments of the present invention, the gap between concentric shell layers can be less than 30% of the rotor diameter, preferably no more than 25%, and ideally no more than 20%. By reducing the gap between layers, the drag reduction effect can be more efficient. Furthermore, reducing the gap size allows for smaller shell layer diameters, thereby reducing material costs. Furthermore, the required shell layer thickness at smaller radii is reduced, thereby reducing material costs and reducing the reliance on high-tensile-strength materials.

[0032] Additionally or alternatively, the first shape of the shell layer of any of the previously described embodiments may be defined by a hypergeometric function. The inventors have discovered that the shape of the shell layer is defined using a hypergeometric function specifically as follows:

[0033] ,

[0034] The shell layers can be optimized to maximize strength while reducing the required material thickness, thereby minimizing material cost. Where h(r) is the shape of the shell layer's cross section in the plane of the axis of rotation, r is the radial distance perpendicular to the axis of rotation, R1 is the total radius of the rotor, and 2F1 is the hypergeometric function.

[0035] Alternatively, the first shape of the shell layer of any of the previously described embodiments can be one of the following: a sphere, an ellipsoid, and a cylinder with a domed top surface and a bottom surface. In a manner similar to the hypergeometric function defined above, these shapes of the shell layer can address the problem of centripetal pressure by having curved top and bottom surfaces. The use of curves allows for compressive forces to be resisted by tension rather than bending moments, thereby allowing the use of thinner materials and reducing the material cost of the shell layer.

[0036] Additionally or alternatively, in any of the previously described embodiments, the shape of the rotor may be oriented relative to the first shape of the housing layer to maximize the mass of the rotor, and accordingly the energy storage capacity, for a given volume defined by the housing layer.

[0037] Additionally or alternatively, the shell layers of any of the previously described embodiments may be elastically deformable and include a second shape when at rest, and wherein the shell is configured to change to the first shape upon rotation. For example, when at rest and in the absence of centripetal pressure, the shell layers may sag or "deflate" into the second shape. Then, upon rotation, the centripetal pressure may cause the layers to "expand" into the first shape. In this way, a desired shape may be achieved while accommodating deformation of the shell layers in response to increased centripetal pressure. Moreover, by allowing the shell layers to deform in response to this pressure rather than requiring them to rigidly maintain their shape, materials with lower stiffness that are more plentiful and less expensive may be used to form the shell layers.

[0038] Additionally or alternatively, any of the previously described embodiments may further include support features configured to inhibit deformation of the housing layers toward the rotor. As the rotational speed of the housing layers decreases, and thus the centripetal pressure of the contained fluid decreases, segments of the housing layers may sag under the force of gravity into contact with the rotor. Alternatively, in embodiments with multiple nested housing layers, a given housing layer may sag into contact with the nested housing layers. This contact may generate friction, which in turn may reduce the energy storage efficiency of the energy storage device by converting rotational kinetic energy into heat. Therefore, in some embodiments, support features configured to inhibit deformation of the housing layers toward the rotor may be additionally provided, thereby reducing or completely preventing contact between the rotor and the housing layers when the housing layers are in the second shape. In embodiments with multiple nested housing layers, support features may be associated with each housing layer (e.g., with the same number of support features as housing layers) such that the support features associated with a given housing layer inhibit deformation of the given housing layer toward the nested housing layer.

[0039] Additionally, the support feature may include a portion of the top surface of the casing layer, wherein the portion of the top surface of the casing layer extends away from the rotor to couple to the bearing. By including a portion of the top surface of the casing layer that extends away from the rotor to couple to the bearing, the casing layer is able to better resist deformation toward the rotor under gravity by supporting its weight through tension in the casing layer. This is because, by extending away from the rotor, the casing layer has a component parallel to gravity, which, for this embodiment, acts parallel to the axis of rotation. In this context, the casing layer having a "component parallel to gravity" means that decomposing a tangent to the surface of the casing layer at a point of interest into orthogonal vectors defined as parallel to and normal to the direction of gravity results in a non-zero vector parallel to the direction of gravity.

[0040] Additionally or alternatively, the support feature may include a reinforced section of the casing layer. For example, by reinforcing a section of the casing layer, the stiffness of the section may be increased, thereby increasing its resistance to deformation and inhibiting deformation of the casing layer toward the rotor. For example, in embodiments where the casing layer sags parallel to the axis of rotation under the force of gravity, the reinforced section may be located on the top surface proximate the axis of rotation and may couple the casing layer to the bearing, thereby inhibiting deformation of the top surface toward the rotor under the force of gravity.

[0041] Additionally, the reinforced sections of the shell layer may include one or more of the following: sections of the shell layer with increased thickness; sections of the shell layer made of a different material than the rest of the shell layer; and reinforcing structures coupled to the surfaces of the shell layer. For example, the stiffness of the shell layer can be increased by increasing the thickness of the material of the shell layer. This increased thickness can be located closer to the axis of rotation. The thickness of the material of the top and / or bottom surfaces of the shell layer may increase toward the axis of rotation, for example, reaching a maximum thickness at the section of the top surface closest to the axis of rotation. Alternatively or additionally, the shell layer can be reinforced by constructing sections of the shell from a different material than the rest of the shell layer. For example, sections of the top and / or bottom surfaces close to the axis of rotation can be constructed from a material having a higher stiffness than the material used to construct the rest of the shell layer. As used herein, stiffness is the ability of a material to resist deformation in response to an applied force. Alternatively or additionally, the shell layer can be reinforced by coupling a reinforcing structure to the surface of the shell layer. As used herein, a reinforcing structure is a structure added to the shell layer to increase its stiffness. For example, one or more of reinforcing plates, steel bars, steel beams, reinforcing rods, reinforcing trusses, or reinforcing brackets may be attached to the surface of the shell layer to increase its rigidity, thereby inhibiting deformation and limiting contact between the shell layer and the rotor or nested shell layers. Alternatively, such structures may be embedded within the shell layer. In some embodiments, reinforcing structures may be attached to both the interior and exterior surfaces of the shell layer to sandwich the shell layers.

[0042] Additionally or alternatively, the support feature may include a section of the shell layer having a different material structure than the remainder of the shell layer. As used herein, "material structure" refers to the macroscopic arrangement of the material. By modifying the material structure, the properties of the material can be altered. For example, the stiffness of the material can be increased to reduce the increase in weight compared to simply increasing the material thickness.

[0043] In addition, sections of shell layers having different material structures may include materials that form a repeating unit structure. As used herein, a repeating unit structure is a macroscopic arrangement of materials into a hollow shell pattern. For example, a reinforcement section may include a material that forms a honeycomb structure (i.e., a repeating hexagonal pattern). Alternatively, the material may form a repeating pattern of any suitable shape, such as a triangle, square, pentagon, heptagon, octagon, enneagon, or decagon. Alternatively, the unit structure may be irregular, such as a Voronoi tessellation. Alternatively, the shape of the unit may vary with the repeating pattern. By adopting a repeating unit structure, the stiffness of the material can be increased, while the weight is reduced, compared to a solid material of the same thickness. To further increase the stiffness, the material having a repeating unit structure can be sandwiched between two reinforcing plates.

[0044] Additionally or alternatively, the support feature may include a support structure, wherein the shell layer is configured to contact the support structure when the shell layer is in the second shape, and wherein the shell layer is configured to move away from the support structure when the shell layer changes to the first shape. In this way, because the shell layer sags and contacts the support structure, the shell layer can be prevented from sagging and contacting the rotor or shell layers nested therein, thereby preventing the shell layer from sagging further. However, when the shell layer rotates to a certain speed and changes to the first shape, it decouples from the support feature and can rotate independently of the support structure. By decoupling the shell layer from the support structure, which can rotate with the shell layer as described below, the rotational mass of the shell layer is reduced when the shell layer reaches a certain speed, thereby reducing the kinetic energy stored in the shell layer and improving the storage efficiency of the flywheel energy storage system.

[0045] Alternatively, the support structure can be coupled to the axle and configured to rotate with the housing. For example, the support structure can be coupled to the axle via its own support bearing, separate from the bearings of the housing. In this example, the support structure can rotate independently of the housing, but rotate with it when in contact with the housing, potentially reducing slip and friction between the housing and the support structure. Alternatively, the support structure can be coupled to the same bearings as the housing, ensuring that the support structure and the housing always rotate at the same speed. This can completely prevent slip between the support structure and the housing when in contact.

[0046] Additionally or alternatively, the support feature may include a pair of magnets configured to inhibit deformation of the housing layer via magnetic repulsion between the pair of magnets. For example, one or more magnets may be coupled to the top surface of the rotor, while one or more magnets may be coupled to the interior of the top surface of the housing layer. The poles of the magnets facing each other have the same polarity, so that as the housing layer sags and the magnets approach each other, the magnets repel each other and inhibit further deflection of the housing layer toward the rotor, thereby preventing contact between the housing layer and the rotor. Alternatively or additionally, the same magnet arrangement may be used between a given housing layer and a nested housing layer located within the given housing layer.

[0047] Additionally or alternatively, the support features can be positioned proximate to the axis of rotation. In embodiments where the shell layers deform under gravity in a direction parallel to the axle, the axle itself can be used to support the shell layers and help prevent contact between the shell layers and the rotor in a manner similar to the poles of a tent that support a tent. As used herein, "proximate to the axis of rotation" includes support features that are located within the center 50% of the radius of the shell layers. Alternatively, the support features can be located within the center 40%, 30%, 20%, 15%, 10%, or 5% of the radius of the shell layers. Alternatively, in embodiments where the support feature comprises a portion of the shell layers or where the support feature comprises a reinforced section of the shell layers or where the support feature comprises a different material structure, the support feature can be directly coupled to the bearing such that the support feature couples the remainder of the shell layers to the bearing.

[0048] Additionally or alternatively, the flywheel energy storage device of any of the first and second embodiments may include the rotor of the third and fourth embodiments.

[0049] According to a fifth embodiment of the present invention, there is provided a method of generating electricity using the flywheel energy storage device of any preceding embodiment, the method comprising: using a rotor to rotate a generator coupled to an axle of the storage device. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] By way of example only, a description will now be given with reference to the accompanying drawings, in which:

[0051] Figure 1 shows an external view of an embodiment of a flywheel energy storage device;

[0052] Figure 2 Shown Figure 1 More details on the pressure control device of the flywheel energy equipment system;

[0053] Figure 3 An example connector assembly for a flywheel energy storage device is shown;

[0054] Figure 4Shown Figure 1 A top view of an energy storage device;

[0055] Figure 5 shows a flywheel energy storage device along Figure 4 a cross-sectional view along line AA, illustrating internal components of the storage device;

[0056] Figure 6 a cross-sectional view of a second embodiment of a flywheel energy storage device illustrating an embodiment of a drag-reducing rotor;

[0057] Figure 7 Pictured Figure 6 The optimal shape of the shell layer of the drag-reducing rotor;

[0058] Figures 8A to 8D Pictured Figure 6 Alternative shapes of the shell layer of the drag reducing rotor;

[0059] Figure 9A and Figure 9B The torque acting on the shell layer with increasing radius and decreasing angular momentum is illustrated;

[0060] Figure 10A and Figure 10B The coordinate system used to derive the optimal shape of the shell layer is shown;

[0061] Figure 11 The diagram shows the deformation of the shell layer under the action of gravity;

[0062] Figure 12 illustrates an embodiment of a support feature that includes a portion of a top surface of a housing layer extending away from a rotor to couple to a bearing;

[0063] Figure 13A and Figure 13B The support features of the reinforcement section including the shell layer are illustrated;

[0064] Figure 14A and Figure 14B Illustrated are materials utilizing a repeating unit structure and materials suitable for use as support features for materials utilizing a repeating unit structure;

[0065] Figure 15 illustrates a support feature comprising a support plate configured to rotate with the shell layer;

[0066] Figure 16 illustrates a support feature comprising a pair of opposing magnets that inhibit deformation of the shell layer via magnetic repulsion; and

[0067] Figure 17The illustration shows a rotor and a housing layer configured to rotate about an axis of rotation perpendicular to gravity, thereby allowing the housing layer to support itself as an arch using compression in the curved surface of the housing layer. DETAILED DESCRIPTION

[0068] Figure 1 An external view of a flywheel energy storage device 10 configured to be installed in a high pressure environment according to an embodiment of the present invention is shown. As shown, the storage device includes a housing 20 that houses a flywheel rotor (e.g., Figure 5 The storage device 10 also includes a device 70 for controlling the pressure of the fluid in the housing 20 using the pressure of the external environment. For example, in Figure 1 In the illustrated example, the storage facility 10 includes a pressure compensator for substantially balancing the pressure of the fluid within the housing 10 with the pressure of the external environment. As is understood in the art, a pressure compensator is used in subsea systems to balance the internal pressure of a hydraulic system (in this example, the fluid within the housing 20) with the ambient pressure of the surrounding water to offset the effects of pressure differences at depth.

[0069] Thus, as explained above, the pressure of the external environment can be used to pressurize the fluid contained within the housing, which in turn compresses the flywheel rotor, thereby increasing its energy storage capacity. Furthermore, the presence of the external high-pressure environment reduces the pressure differential experienced by the housing for a given fluid pressure, and accordingly reduces the housing's pressure containment requirements. This means that the housing only needs to be able to withstand pressures lower than the pressure of the contained fluid, and can therefore be constructed from less expensive, more abundant, and less carbon-intensive materials.

[0070] Suitable high pressure environments include the seabed, lakes, caves, flooded mines or any other existing high pressure environments. Preferably, the pressure of the external environment is greater than 10 bar, which can be easily achieved at the seabed at a depth of more than 100 m.

[0071] Also like Figure 1 As shown, the storage device 10 also includes a motor-generator 50 for storing energy in the storage device 10 by rotating the rotor and extracting energy from the storage device 10. Although shown as a combined motor-generator, the storage device 10 may alternatively include a motor and a separate generator. Further, the storage device 10 may include only one of the motor or the generator. To store energy in the storage device 10, the motor-generator operates as a motor by drawing electrical power from an external power source and converting it into the rotational kinetic energy of the flywheel rotor, thereby storing electrical energy as kinetic energy. To extract energy from the storage device 10, the motor-generator 50 operates as a generator by driving it using the rotating flywheel rotor, thereby converting the rotor's kinetic energy back into electrical energy.

[0072] exist Figure 1 In the example shown, the storage device 10 also includes control electronics 55 for controlling the motor generator. Figure 1 While illustrated as forming an integral part of storage device 10, control electronics 55 need not be mounted on storage device 10, but rather can be housed at a remote location and coupled to the motor-generator via cabling or other forms of electronic communication. If, as in the illustrated example, control electronics are to be included in storage device 10 and mounted underwater, storage device 10 includes means for isolating control electronics 55 from the external environment. For example, this may include enclosing the control electronics in a sealed, air-filled tank to protect control electronics 55 from the corrosive effects of seawater.

[0073] like Figure 1 As shown, the storage facility 10 may also include a frame 30, which includes a lifting attachment 40 for raising and lowering the storage facility 10 into position. As will be appreciated, it is necessary to control the pressure of the fluid within the housing 20 while the storage facility 10 is lowered to a submerged position to compensate for the increasing pressure experienced by the storage facility as depth increases. This can be achieved by lowering the storage facility and a pressure vessel (e.g., a pressurized fluid cylinder) coupled to the fluid within the housing 20 via a valve system. When the storage facility 10 is lowered using the lifting attachment 40, a suitable valve system can regulate the pressure of the fluid using the pressure vessel to ensure that the pressure within the housing is the same as the ambient pressure for the duration of the descent operation. Preferably, this can be a passive system, where the valve system opens when the pressure of the fluid within the housing 20 is less than the ambient pressure, allowing the fluid to be appropriately pressurized using the pressure contained within the pressure vessel. However, as will be appreciated, an active system comprising a pressure sensor and an actuator can also be used to achieve the desired pressure control, controlling the valve system coupled to the pressure vessel or a pressure pump.

[0074] The frame 30 may also be configured to be connected to foundation anchors to support and stabilize the storage facility 10 in place. The foundation for a surface-based flywheel storage facility may not be suitable for the types of surfaces found in an underwater environment, and the storage facility 10 may be configured to be secured in place using, for example, mud mats, caissons, or piles driven into the seafloor as a foundation. Figure 1 In the illustrated example, the storage device 10 includes a mud mat 32 and side skirts 34 coupled to the frame 30 .

[0075] Figure 2 More details of the pressure control device 70 and the connection assembly 60 for connecting the motor generator 50 to an external power source are shown. As explained above, Figure 1The pressure control device 70 in the illustrated example is a pressure compensator, a passive mechanism for ensuring that the pressure of the fluid within the housing 20 is substantially balanced with the surrounding environment. Alternatively, or in addition to the illustrated compensator, a valve system coupled to a high-pressure fluid source can be used to control the pressure of the fluid within the housing 20. Such a valve system can be passive and regulate the fluid pressure based on the pressure of the external environment, for example, by opening if the fluid pressure falls below the ambient pressure. Alternatively, the valve system can include a pressure sensor and an actuator to provide active control of the pressure within the housing 20. As indicated above, the valve system uses a high-pressure fluid source to control the pressure within the housing 20, which can be, for example, a pressure vessel (e.g., a high-pressure gas cylinder) or a pump. However, further additionally or alternatively, the pressure control device can include a trap or a U-bend (i.e., a U-shaped portion of a pipe designed to trap a fluid or gas) and a flexible membrane covering a port coupled to the fluid contained within the housing. These alternatives are relatively simple and may be preferable in remote areas where regular maintenance may be more difficult.

[0076] Figure 3 Shown Figure 1 and Figure 2 More details of the electrical connector assembly 60 are shown. Figure 1 As shown, the electrical connector assembly 60 includes an electrical connector 62, a handle 64 configured for use with a remotely operated vehicle (ROV), and an electrical cable 66 coupled to the motor-generator 50. As indicated above, the electrical connector assembly 60 is configured to connect to an external electrical system, from which energy can be stored, and into which energy extracted from the storage device 10 can be injected via the electrical connector 62. The electrical connector 62 can be configured for underwater connection, in which case the electrical connector 62 will include means for isolating the electrical conductors within the electrical connector 62 from the external environment. For example, this can be achieved using one or more seals designed to prevent the ingress of seawater and / or by using a dielectric fluid within the electrical connector 62.

[0077] like Figure 3As shown, the electrical connector assembly may also include a handle 64 configured for use with a remotely operated vehicle. As described above, the storage facility 10 may be installed underwater at depths greater than 100 meters. In such cases, installation, inspection, maintenance, and decommissioning removal tasks associated with the storage facility 10 performed by divers may become dangerous, expensive, and impractical. Therefore, the storage facility 10 may be provided with several ROV intervention points or handles, such as the handle 64 on the electrical connector assembly, which allow these tasks to be performed by an ROV, thereby reducing risk to divers and allowing for more regular and convenient maintenance of the storage facility 10.

[0078] As shown, electrical connector 62 is coupled to motor-generator 50 via cable 66, which may be a high-voltage submarine electrical cable configured for use in a high-voltage environment. For example, cable 66 may include a rugged jacket, such as one constructed of thermoplastic, to withstand the external pressures of the environment and protect the internal cable wiring from damage and abrasion by rocks and other debris that may be present in the area where the storage device is installed. Cable 66 may also contain a dielectric fluid to prevent or quickly quench any electrical discharge that may occur from the conductors within cable 66, for example, if the cable is damaged.

[0079] Figure 5 The diagram shows an embodiment of the present invention. Figure 4 FIG2 is a cross-sectional view taken along line AA of FIG2 , which illustrates in greater detail the internal components of the flywheel energy storage device 10. As can be seen, the storage device 10 includes a rotor 80 mounted on an axle 100 within a housing 20. The axle 100 is supported within the housing via one or more bearings, which in the illustrated example are shown as thrust bearings 90 and radial bearings 92, thereby enabling the rotor 80 to rotate relative to the housing 20. As also shown, the axle 100 is coupled to a motor-generator 50, enabling the motor-generator 50 to rotate the rotor 80 in order to store energy and to rotate the motor-generator 50 when energy is required. A fluid 110 is contained within the housing 20 and is pressurized using a pressure control device 70 to place the rotor 80 in a compressed state, thereby increasing the energy storage capacity of the storage device 10.

[0080] As described above, the present invention aims to reduce the reliance on scarce, expensive, difficult-to-process, and carbon-strong, high-tensile-strength materials for manufacturing flywheel rotor 80, as achieved by prestressing rotor 80 using the pressure of fluid 110. Thus, by initially placing rotor 80 under compression, a wide variety of abundant, inexpensive, and easily processed materials can be used to manufacture rotor 80, thereby reducing the material cost and complexity of the resulting system. For example, various embodiments of the present invention can utilize inexpensive materials with low tensile strength, which is considered to be less than 200 MPa, preferably less than 150 MPa, 100 MPa, or 75 MPa. Alternatively, the tensile strength can be less than 50 MPa, 40 MPa, 30 MPa, 20 MPa, 10 MPa, or 5 MPa. Alternatively, rotor 80 can comprise a granular material, such as sand, which has no inherent bonding between the individual material particles and, therefore, no tensile strength, i.e., a tensile strength of 0 MPa. Such materials are abundant, inexpensive, have low carbon strength, and can be compacted to form dense bulk materials. Therefore, such materials may represent an ideal choice for manufacturing high-quality flywheel rotors used in the present invention.

[0081] The tensile strength of a material can be determined by several standard tests. For example, a uniaxial tensile test can determine the tensile strength of isotropic materials (such as metals and plastics). Specific standard tests that can be used to determine the tensile strength of metals include ASTM E8 / E8M-13, ISO 6892-1, and ISO 6892-2. Specific standard tests that can be used to determine the tensile strength of plastics include ASTM D638, ASTM D828, ASTM D882, and ISO 37. Triaxial shear testing can be used to determine the tensile strength of granular materials (e.g., the sand described above). For example, this may include a consolidated drained test or a consolidated undrained test. Specific triaxial shear test standards include ASTM D7181-11, ASTM D4767-11, ASTM D2850-03a, BS 1377-8, ISO / TS 17892-8, and ISO / TS 17892-9.

[0082] To enable the formation of rotor 80 from granular materials lacking tensile strength, the granular material can be sealed within a membrane, and the air within the membrane can be removed to compress the granular material into a hard, dense, bulk material. Furthermore, the presence of external pressure from fluid 110 compensates for the granular material's inherent lack of tensile strength by initially compressing the rotor. Thus, as the rotor's rotational speed increases, the centripetal force acting on the rotor increases, and this compression must be overcome before the rotor is in net tension. Consequently, rotors comprising materials with low or no tensile strength can be used as flywheel rotors.

[0083] Using the high-availability and easily processable materials described above, rotor masses greater than 0.5 tons (preferably greater than 0.75 tons, and optionally greater than 1 ton) can be readily achieved, thereby reducing the material and manufacturing costs of the system, as well as the energy storage costs. Furthermore, the cost of adding more material to increase rotor mass, and thus energy storage capacity, is significantly reduced compared to the high-tensile strength materials traditionally used to manufacture flywheels.

[0084] like Figure 5 As shown, the housing 20 contains a fluid 110 that is used to compress the rotor 80. Thus, the housing 20 may include one or more seals (not shown) for isolating the fluid 110 from water in the external environment to ensure that the fluid 110 is maintained at an optimal operating pressure and to prevent water from entering the housing that could corrode or damage components within the housing.

[0085] When fluid 110 is maintained at superatmospheric pressure to compress rotor 80, the density of fluid 110 and the frictional effect of fluid 110 on rotor 80 during its rotation increase relative to fluid 110 at atmospheric pressure. Therefore, fluid 110 can be selected to minimize frictional losses, thereby improving the energy storage efficiency of storage device 10. For example, when resistance increases linearly with fluid density, fluid 110 can be selected such that its density at operating pressure is less than that of the rotor. Specifically, the fluid density can be less than half the density of the rotor, and preferably less than one-third, one-quarter, one-fifth, one-sixth, one-seventh, or one-eighth the density of the rotor at operating pressure. For example, inert gases such as helium, neon, argon, krypton, and xenon can be used. Alternatively, diatomic elements such as hydrogen or nitrogen can be used as fluid 110. Furthermore, fluid 110 can be selected to have a low intrinsic viscosity (considered to be less than the viscosity of air, 18.46 μPas) to further improve energy storage efficiency. Furthermore, since the centripetal pressure exerted on the shell layer decreases with lower fluid pressure, the use of a low-density fluid ensures that the following (refer to Figure 6) The shell layer described can be made of a lower tensile strength and thinner material. Therefore, the use of a low-density fluid as defined above also reduces the cost of the energy storage device.

[0086] As indicated, the primary purpose of fluid 110 is to compress rotor 80. To this end, as described above, the pressure of fluid 110 is controlled using pressure control device 70 and the existing pressure of the external environment in which storage device 10 is installed. For example, by using pressure control device 70 (e.g., a pressure compensator) to bring fluid 110 into substantial equilibrium with the external environment, the fluid can be pressurized to greater than 10 bar, which is readily achievable at depths greater than 100 meters below sea level. Examples of fluid 110 being substantially equilibrium include the pressure of fluid 110 being within 20%, preferably within 15%, and optionally within 10% of the pressure of the external environment.

[0087] Steering Figure 6 , illustrates an embodiment of a rotor 120 for further reducing the effects of friction between the rotor and the fluid 110. As shown, the rotor 120 includes Figure 5 The rotor 122 is similar to the rotor 80 shown. In addition, the rotor 120 further includes a first shell layer 123 and a second shell layer 124, which encase the rotor 122 and are coupled to the axle 100 via a first bearing pair 125 and a second bearing pair 126. Figure 6 In the example of FIG, two housing layers are shown, but it will be appreciated that any number of housing layers may be used. The first housing layer 123 and the second housing layer 124 are configured to freely rotate around the rotor 122 about a rotation axis concentric with the axle 100 using a first bearing pair 125 and a second bearing pair 126.

[0088] As will be appreciated, when rotor 120 is rotated by motor-generator 50, energy is stored in both rotor 122 and rotating casing layers 123 and 124. Therefore, it may be desirable to minimize the mass of the casing layers and maximize the efficiency of bearings 125 and 126 in order to minimize energy lost to the casing layers. For example, by using thin, low-mass casing layers (relative to the mass of rotor 122), energy lost to the casing layers may be less than 1% of the total stored energy.

[0089] The following describes how a single cylindrical shell layer positioned equidistantly between the stationary housing 20 and the rotating rotor 122 works to reduce friction experienced by the rotor. The single layer will begin to rotate until the fluid friction on either side is equal, which occurs when the shell layer's speed is half the difference between the rotor speed and the housing speed—that is, half the rotor speed for a shell layer positioned equidistantly between the rotor and housing. Since drag increases with the square of speed, and the velocity difference of the fluid experienced by rotor 122 is now halved, the drag experienced by rotor 122 is reduced to one-quarter by using a single shell layer. As will be appreciated, the equilibrium speed of a single shell layer will vary depending on its position between the rotor surface and the housing. For example, the equilibrium speed of a shell layer will be higher for a shell layer positioned closer to the rotor surface than for a shell layer positioned at the equilibrium distance between the rotor and housing.

[0090] In more detail, when the rotor 122 rotates in the presence of the fluid 110, skin friction drag acts on the surface of the rotor 122. The skin friction exerts a torque τ on the rotor.

[0091]

[0092] where r is the radial distance perpendicular to the axis of rotation, and dF is the differential friction element, which is defined as follows:

[0093] ,

[0094] C f is the friction constant, is the dynamic pressure, and dA is the area element.

[0095] Considering the cylinder, we can write the friction elements for both the sides and the top of the rotor:

[0096] ,

[0097] ,

[0098] The corresponding torque is as follows:

[0099] ,

[0100] ,

[0101] Where ρ is the rotor density, ω is the angular velocity of the rotor over time, H is the total height of the rotor, and R1 is the total radius of the rotor. Based on these equations, the total drag from skin friction can be calculated as:

[0102] .

[0103] C f The value of is derived from experience. For a coaxially rotating cylinder, C f The expression can be obtained based on the cylinder radius, the spacing (s) and the Couette Reynolds number (R e )get.

[0104]

[0105] .

[0106] Here μ is the dynamic viscosity of the contained fluid.

[0107] refer to Figure 6 、 Figure 9A and Figure 9B , the drag reduction system consists of the freely rotating shell layers 123, 124. To illustrate this, we can consider the torque on the side of the cylindrical rotor. Here, we have the following expression, as derived above,

[0108] ,

[0109] where α is a constant that includes a pre-factor that depends on the geometry. Now, if you insert casing layers around the rotor and allow them to rotate freely, equilibrium is achieved. This equilibrium is one where the rotor and casing layers exert equal but opposite torques on each other, and the same is true for the casing layers and the housing. Figure 10A and Figure 10B In the schematic illustration of , the geometry is shown from a downward perspective along the axis of rotation.

[0110] Can be used Set up a system of equations where the subscripts represent the increasing radii of the rotor, separator, and housing. For example, τ 1,2 represents the torque from the casing layer on the rotor, and τ 3,2 represents the torque from the first shell layer on the second shell layer.

[0111] Consider the case of a separator, τ 1,2 =τ 2,1 and τ 2,3 =τ 3,2 For equilibrium, τ 2,1 =τ 2,3 Setting the angular velocity of the shell to zero yields:

[0112] ,

[0113] ,

[0114] Equating these expressions and rearranging them gives:

[0115] ,

[0116] Using this value, the expression for the torque when there is one shell layer is as follows:

[0117] .

[0118] This result shows how the shell layer produces a torque reduction factor. Continuing in a similar manner, the case of two shell layers can be considered. The equilibrium equation can be extended to give τ 1,2 =τ 2,3 =τ 3,4， And the torque is as follows:

[0119] ,

[0120] ,

[0121] ,

[0122] Again, equating these expressions and rearranging them twice yields:

[0123] ,

[0124] Comparing the expressions for ω2 for the one shell case and the two shell case, a clear pattern emerges and a general form for n shells can be derived:

[0125] ,

[0126] And the friction torque becomes:

[0127] ,

[0128] This significantly improves performance in terms of reducing energy losses in the flywheel storage system.

[0129] When the fluid 110 rotates, a centripetal pressure is generated in the fluid 110. Figure 6 In the illustrated layered drag reduction system, each shell layer rotates at a different speed, with the innermost layer rotating the fastest. This means that the centripetal pressure within the innermost layer is greater than the centripetal pressure within the layers surrounding it, and so on. In other words, the centripetal pressure varies from layer to layer, so each layer must accommodate the pressure differential across the layers.

[0130] Typically, the flywheel rotor is cylindrical, e.g. Figure 5As shown, the same is true for known shell layers used with cylindrical rotors. Since pressure acts in all directions, the centripetal pressure produces a net force in a direction parallel to the axis of rotation (i.e., parallel to the axle 100) that needs to be accommodated by the top and bottom surfaces of each shell layer. This can be achieved by simply increasing the thickness of the material used for each shell layer so that each shell layer can resist the centripetal pressure with a bending moment. However, this approach may not be economical in many scenarios because it increases the material cost required for each shell layer. To address this problem, the inventors have found that by providing top and bottom surfaces that include curves, rather than providing the shell layers with flat top and bottom surfaces (such as those surfaces of the shell layers that intersect the axis of rotation), the generated centripetal pressure can be accommodated by tension within the material of the shell layers rather than as a bending moment, thereby reducing the thickness of the material and, in turn, the material cost of the shell layers. Moreover, by reducing the strength required for the shell layers in this way, less expensive and more abundant materials can be used. Such an arrangement as Figure 6 shown.

[0131] In broad terms, improvements in the strength and material cost of the shell layer will be achieved by having a shell layer having a cross-section that, when drawn in the plane of the axis of rotation, defines a closed substantially convex shape having a top surface and a bottom surface, each of which comprises a continuous curve. As used herein, a shape is considered to be substantially convex if the ratio of the enclosed interior area of the shape to the area of the smallest convex set that contains the shape is greater than 70%, preferably greater than 80%, and optionally greater than 90%. As used herein, a point set is a convex set if it contains the entire line segment connecting any given two points in the set. Alternatively, the cross-section of the shell layer, when drawn in the plane of the axis of rotation, can define a closed convex shape having a top surface and a bottom surface consisting of continuous curves.

[0132] Figure 7 An example of the shape of the shell layer defined above is shown, which represents the optimal shape of the shell layer to maximize strength and reduce material cost. The optimal shape is defined as follows:

[0133] ,

[0134] 2F 1 is the hypergeometric function. By evaluating the first few terms of this power series, we get Figure 7 The shape has a height to radius ratio of approximately 0.431.

[0135] To understand how to obtain this shape, refer to Figure 10A and Figure 10B, considering the stresses applied in the azimuthal and polar directions on a shape-asymmetric shell layer with unknown bending shape h(r). Figure 10A and Figure 10B The coordinates of the curved shell layer shape as viewed in cross section in two different planes are shown. Figure 10A Starting with the coordinates of and taking a segment of the shell layer in the normal direction, we can write the equilibrium equation that defines the stress in that direction in the separator. This equilibrium equation takes the following form:

[0136]

[0137] where r = r θ sinθ,r θ is the length AB, is the length AC, t is the shell thickness, is the centripetal pressure, and and σ θ They are The shell layer stresses in the and θ directions. Substituting these definitions yields the equation:

[0138]

[0139] Now, reference Figure 10B , we can get the equilibrium equation in this new direction. The vertical force on the shell layer must be equal to the centripetal pressure in the vertical direction:

[0140] .

[0141] Furthermore, substituting the above definitions into the equation we can get:

[0142] .

[0143] Using two other definitions of difference elements:

[0144] and ,

[0145] It can be concluded that the θ direction and Two equations for the shell layer stress in each of the directions:

[0146]

[0147]

[0148] According to these two equations, the optimal shape will produce constant stress in both directions. As indicated above, a solution of the following form can be obtained:

[0149] .

[0150] Although as defined above Figure 7 The shape of is optimal for reducing material cost and maximizing strength, but alternative shapes are possible. Figures 8A to 8D Four examples of such shapes are shown. While these shapes are not optimal and require increased wall thickness, resulting in higher material costs when manufactured, they can still be used to contain the centripetal pressure generated when the fluid rotates within them and can be economical in certain scenarios. As shown, the cross-section of the shell layer in the plane of the axis of rotation can alternatively form a circle, an ellipse, a channel (i.e., a shape defined by the set of all points located at the same distance from a line of a given length), and a convex polygon (e.g., a rectangle). Alternatively, although Figures 8A to 8D Although not shown, the shell layers can define cylinders with domed top and bottom surfaces. As will be appreciated, the disclosed shell layer technology will also provide improvements in drag reduction for shell layers of any shape, although some shapes are less economical when needed to accommodate the centripetal pressures outlined above. Thus, as a further alternative, each shell layer can be a cylinder with flat top and bottom surfaces.

[0151] Back to Figure 6 In the illustrated example, each of the shell layers 123 and 124 has the same Figure 7 123 . Similarly, rotor 122 defines an outer surface that conforms to the shape of innermost shell layer 123. This arrangement ensures rotor mass, thereby maximizing the rotor's mass and energy storage capacity for a given volume defined by innermost shell layer 123. However, this is not always the case, and because the structural requirements for the rotor differ from those for the shell layer, the rotor's shape may differ from the shape defined by the shell layer. For example, the rotor may be shaped as a Laval disc, whose cross-section decreases with radius to maintain uniform stress across all portions of the disc, while the shell layer may have the optimal shape defined above to maximize its strength while minimizing material cost.

[0152] To reduce the required strength and rigidity of the shell layers while still ensuring that they conform to a desired shape during use, shell layers 123 and 124 can be constructed from an elastically deformable material and define a second shape when at rest. The second shape can be selected such that when the shell layers are rotating and operating under the centripetal pressure generated by the contained rotating fluid 110, the centripetal pressure causes the shell layers to change from the second shape to the first desired shape. This ensures that the shell layers define the desired shape during rotation without having to be constructed from unnecessarily rigid materials to prevent any deformation due to the generated centripetal pressure.

[0153] The centripetal pressure generated by the fluid 110 increases as the rotational speed increases. Thus, at lower rotational speeds, for example, when the flywheel is accelerating or decelerating, the centripetal pressure acting on the shell layer may be lower than the force acting on the shell layer due to gravity. In this case, as Figure 11 As illustrated by the dashed lines in FIG, the shell layers may sag under gravity toward the second shape and toward the rotor 122. Figure 11 As shown at points A and B in FIG, this droop increases the risk of contact between casing layer 124 and rotor 122. This contact may cause friction between rotor 122 and casing layer 124, which may reduce storage efficiency by converting rotational kinetic energy into heat. Furthermore, rubbing between the rotor and casing layer may damage one or both of rotor 122 and casing layer 124.

[0154] Thus, several support features are described below to inhibit deformation of the housing layer toward the rotor, or, in embodiments comprising multiple housing layers, to inhibit deformation between a given housing layer and a housing layer nested within it. While a number of embodiments are described below with respect to housing layers and rotors, each of these is equally applicable to preventing contact between a given housing layer and a housing layer nested within it.

[0155] Figure 12 A first embodiment of a support feature for inhibiting deformation of the shell layer 124 is illustrated. As illustrated, the support feature may comprise a portion of the top surface 250 of the shell layer. Figure 12 In the illustrated embodiment, portion 250 includes a section of the top surface that is deflected away from rotor 122 near the center of rotation. To accommodate this deflection, the shell layer bearing 126 is also moved along axle 100 away from rotor 122. This causes portion 250 to include a component parallel to the direction of gravity, as indicated by arrow G. More precisely, a tangent to a point on the surface of portion 250, when decomposed into orthogonal vectors defined as parallel to and normal to G, will include a non-zero component parallel to G. In this way, the shell layer itself can support the weight of the shell layer as tension in portion 250, thereby inhibiting deformation of the shell layer toward the rotor. Alternatively, portion 250 can include a section of the shell layer that is deflected toward the rotor, allowing portion 250 to support the weight of the shell layer via compression.

[0156] Figure 13A and Figure 13B Other embodiments of support features for inhibiting deformation of the shell layer are illustrated. As illustrated, the support features may include a reinforced section 200 of the shell layer 124. Figure 13AAs shown, the reinforcement section 200 may include a section of the casing layer adjacent to the bearing 126 of increased thickness, thereby increasing the stiffness of the material and causing the reinforcement section to inhibit deformation of the casing layer 124 toward the rotor. As used herein, "thickness" is measured in a direction normal to the surface of the casing layer. Figure 13A In the illustrated example, the reinforcement section comprises a section of uniform thickness; however, in alternative embodiments, the reinforcement section may have a non-uniform thickness. For example, the thickness of the reinforcement section 200 may decrease as the distance from the bearing 126 increases. This decrease in thickness may be linear or non-linear. For example, the thickness of the reinforcement section 200 may decrease gradually, or may decrease proportionally to the square, cube, or any other non-linear distance function of the distance from the bearing.

[0157] Additionally or alternatively, the reinforcement section 200 can be constructed of a different material than the rest of the shell layer, such that the material used for the reinforcement section 200 is stiffer than the material of the rest of the shell layer. As used herein, stiffness refers to the ability of a material to resist deformation in response to an applied force. For example, the reinforcement section can be constructed of a composite material such as fiberglass or carbon fiber.

[0158] like Figure 13B As shown, additionally or alternatively, the reinforcement section 200 may include a reinforcement structure that is coupled to a surface of the shell layer. In the illustrated example, the reinforcement section 200 includes one or more reinforcement plates 220 that are coupled to the top surface of the shell layer adjacent to the bearing 126. The reinforcement plates may be made of the same or a different material than the shell layer 124. Thus, the reinforcement plates may increase the stiffness of the reinforcement section 200 by increasing the thickness of the reinforcement section or additionally by being made of a material that has an intrinsic stiffness that is higher than the stiffness of the rest of the shell layer. By coupling the reinforcement plates 220 to the shell layer, the stiffness of the reinforcement section is increased, thereby suppressing deformation of the shell layer under gravity. Furthermore, as Figure 13B As shown, the reinforcement plates may sandwich the shell layers by being coupled to the outer and inner surfaces of the shell layers adjacent to the bearing 126. Figure 13B Although illustrated as reinforcement plates, the reinforcement structure may take alternative forms. For example, the reinforcement structure may include rebar, steel beams, reinforcing rods, reinforcing trusses, or reinforcing brackets coupled to one or more surfaces of the shell layer 124. Additionally, while illustrated as coupled to a surface of the shell layer, the reinforcement structure may alternatively be embedded within the shell layer.

[0159] Figure 14A and Figure 14BOther embodiments of support features are illustrated, wherein the support feature comprises a section of the shell layer having a different material structure than the remainder of the shell layer. As used herein, a "material structure" is a macroscopic arrangement of materials. Figure 14A As shown, the material structure may include a repeating unit structure. Figure 14A In a specific example, the repeating unit structure includes a repeating hollow tubular hexagonal unit pattern, for example, a "honeycomb" structure. By providing a structure having a repeating unit structure (such as Figure 14A Materials with a repeating unit structure (as shown) can increase the stiffness of the material with less increase in weight compared to corresponding thicker solid materials.

[0160] Figure 14B Examples of materials suitable for use as support features are shown, including Figure 14A As shown, the material 210 having a repeating unit structure is sandwiched between a top plate and a bottom plate 212. Figure 14B As shown in the figure, Figure 14A The axes of the hexagonal tubes defined by the cell structure shown extend perpendicular to the top and bottom plates 212 .

[0161] Figure 15 In the illustrated example, the support structure comprises a support plate 300 coupled to the axle 100 via a support bearing 310 . Figure 15 The shell layer 124 is shown in a first shape (ie, its shape during steady-state operation). Figure 15 As shown, when in the first shape, the shell layer does not contact the support plate 300, thereby allowing the support plate 300 and the shell layer 124 to rotate freely relative to each other. However, when the rotational speed of the shell layer 124 decreases, the centripetal pressure of the fluid contained within the shell layer decreases, and the shell layer begins to sag toward the second shape under the force of gravity. As the shell layer 124 sags, the shell layer is configured to contact the support plate 300, which supports the shell layer 124 and prevents it from further deforming toward the rotor 122. In this way, the support plate 300 prevents the shell layer from contacting the rotor 122. Moreover, because the support plate 300 can rotate freely on the bearing 310, when the plate 300 contacts the shell layer 124, the speed of the plate 300 matches the speed of the shell layer 124, thereby minimizing slippage and friction between the shell layer 124 and the plate 300.

[0162] While FIG14 illustrates that support plate 300 can freely rotate relative to shell layer 124 via bearings 310, in an alternative embodiment, support plate 300 can instead be coupled to bearings 126 and thus configured to rotate with shell layer 124 at all times. Such an embodiment can ensure that when the shell layer sags into contact with support plate 300, there is no sliding or friction between the shell layer and the plate.

[0163] While FIG. 14 illustrates the support structure as a support plate, the support structure may take alternative forms, such as one or more support beams, support rods, support ribs, support trusses, or support brackets.

[0164] Steering Figure 16 In various alternative embodiments, the support feature can include a pair of magnets configured to inhibit deformation of the shell layer via magnetic repulsion between the pair of magnets. As illustrated, the first magnet of the pair can be coupled to the top surface of rotor 122, while the second magnet can be coupled to the top surface of the shell layer, such that the pair of magnets oppose each other. In this embodiment, the magnets are polarized so that similar poles oppose each other. Thus, when shell layer 124 decelerates and begins to sag under gravity, the first and second magnets approach each other and repel each other, thereby inhibiting further deflection of shell layer 124 toward rotor 122.

[0165] In the illustrated example, the first and second magnets of the pair 400 are illustrated as ring magnets and are Figure 16 . However, the first magnet and the second magnet may take any suitable form. For example, each of the first magnet and the second magnet may be an arc magnet, a bar magnet, a rod magnet, a disc magnet, a horseshoe magnet, a slot magnet, or a block magnet. Furthermore, the first magnet and the second magnet may take different forms from each other. Furthermore, the magnets 400 may be permanent magnets, or they may be selectively activatable electromagnets.

[0166] Additionally, embodiments of the present invention may include more than one pair of magnets. For example, multiple opposing pairs of magnets may extend to the top surface of the rotor and housing layer to inhibit deformation of the housing layer toward the rotor over the entire length of the top surface.

[0167] Finally, if Figure 17 As shown, alternatively or additionally, the shell layer can be configured to resist deformation toward the rotor under the influence of gravity by being mounted with its axis of rotation perpendicular to the direction of gravity. For example, axle 100 can be configured to be coupled to housing 20 such that axle 100 is parallel to the plane defined by mud pad 32. By mounting the rotor in this manner, the shell layer acts as an arch, allowing the weight of the shell layer to be supported by compression across the curved surface of the shell layer.

[0168] Although the shell layer with continuous curved top and bottom surfaces has been discussed in Figures 11 to 17 The above support features are described in detail in FIG, but it will be appreciated that the support features are equally applicable to alternative shell layer shapes. For example, the support features described above may be applied to Figures 8A to 8D Each of the illustrated shell layer shapes, as well as alternative cross-sections, such as other convex polygons, e.g., rectangles. Support features can also be applied to shell layers in the form of cylinders with rounded top and bottom surfaces and cylinders with flat top and bottom surfaces.

Claims

1. A flywheel energy storage device comprising: an enclosure, wherein the enclosure is sealed to isolate an interior of the enclosure from an external environment; a rotor mounted on an axle within the housing, the rotor being configured to rotate relative to the housing, wherein the rotor is made of a low tensile strength material; a fluid contained within the housing to compress the rotor; as well as Means for maintaining the pressure of the fluid in substantial equilibrium with the pressure of the external environment.

2. A flywheel energy storage device comprising: an enclosure, wherein the enclosure is sealed to isolate an interior of the enclosure from an external environment; a rotor mounted on an axle within the housing, the rotor being configured to rotate relative to the housing; a fluid contained within the housing to compress the rotor; means for maintaining the pressure of the fluid substantially in equilibrium with or greater than the pressure of the external environment; as well as The housing is a pressure vessel that can only withstand a pressure smaller than the pressure of the fluid.

3. The energy storage device of claim 1 or claim 2, further comprising one or more of the following: an electric motor coupled to the axle for storing energy in the flywheel energy storage device; and A generator is coupled to the axle for extracting energy from the flywheel energy storage device.

4. An energy storage device as claimed in any preceding claim, wherein the housing is constructed of a low tensile strength material.

5. An energy storage device as claimed in any preceding claim, wherein the low tensile strength material of one or more of the housing and the rotor has a tensile strength of less than 200 MPa.

6. An energy storage device as claimed in any preceding claim, wherein the rotor is made of ceramic.

7. The energy storage device of any one of claims 1 to 5, wherein the rotor is made of a granular material enclosed in a sealed container, wherein the pressure within the container is maintained at a pressure less than the pressure of the fluid.

8. An energy storage device as described in any of the preceding claims, wherein the means for maintaining the pressure includes one or more of the following: a pressure compensator, the pressure compensator being coupled to the pressure of the external environment; and a valve system being coupled to a source of pressurized fluid.

9. An energy storage device as claimed in any preceding claim, wherein the housing is sealed using one or more seals for separating seawater from the fluid.

10. An energy storage device as claimed in any preceding claim, wherein the fluid is a low density fluid, and optionally wherein the density of the fluid at operating pressure is less than one third the density of the rotor.

11. An energy storage device as claimed in any preceding claim, further comprising an electrical connector comprising a conductor for electrical communication with the energy storage device, wherein the electrical connector comprises means for isolating the conductor from seawater, optionally wherein the means comprises one or more of a seal and a dielectric fluid.

12. An energy storage device as claimed in any preceding claim, further comprising control electronics and comprising means for isolating the control electronics from the external environment, wherein the means for isolating the control electronics comprises: A sealed, air-filled tank to protect the control electronics from seawater.

13. The energy storage device of any preceding claim, further comprising: A lifting attachment for lowering the energy storage device to a submerged position.

14. The energy storage device of any preceding claim, wherein the housing is configured to connect to a foundation anchor to support and stabilize the energy storage device in place, and optionally wherein the foundation anchor is one or more of: a mud mat, a caisson, or one or more piles.

15. The energy storage device of any preceding claim, further comprising a casing layer encasing the rotor and coupled to the axle via bearings, the casing layer having a first shape, wherein the bearings allow the casing layer to rotate around the rotor about an axis of rotation concentric with the axle.

16. A rotor for a flywheel energy storage device, the rotor comprising: rotor; an axle coupled to the rotor; a casing layer enclosing the rotor and coupled to the axle via bearings, the casing layer having a first shape, wherein the bearings permit the casing layer to rotate about the rotor about an axis of rotation concentric with the axle, and wherein a cross-section of the casing layer, when drawn in a plane within which the axis of rotation lies, defines a closed substantially convex shape having a top surface and a bottom surface, the top and bottom surfaces intersecting the axis of rotation, and wherein each of the top and bottom surfaces comprises a continuous curve.

17. A rotor for a flywheel energy storage device, the rotor comprising: rotor; an axle coupled to the rotor; a casing layer enclosing the rotor and coupled to the axle via bearings, the casing layer having a first shape, wherein the bearings permit the casing layer to rotate about the rotor about an axis of rotation concentric with the axle, and wherein the casing layer defines a closed three-dimensional substantially convex shape having a top surface and a bottom surface intersecting the axis of rotation, wherein each of the top surface and the bottom surface includes a portion having a continuous curvature.

18. The energy storage device of claim 15 or the rotor of claim 16 or 17, comprising a plurality of concentrically nested shell layers, wherein each shell layer has a corresponding first shape, each shell layer comprising a bearing coupled to the axle, wherein the bearing is configured to allow the shell layer to rotate about the rotor and relative to each of the other shell layers.

19. The energy storage device or rotor of claim 18, wherein the gap between the shell layers is less than 20% of the diameter of the rotor.

20. The energy storage device of claim 15 or the rotor of any one of claims 16 to 19, wherein the first shape of the shell layer is defined by a hypergeometric function.

21. The energy storage device of claim 15 or the rotor of any one of claims 16 to 20, wherein the first shape of the housing layer is one of a sphere, an ellipsoid, and a cylinder with domed top and bottom surfaces.

22. The energy storage device of claim 15 or the rotor of any one of claims 16 to 21, wherein the rotor has a shape corresponding to the first shape of the housing layer.

23. The energy storage device of claim 15 or the rotor of any one of claims 16 to 22, wherein the shell layer is elastically deformable and comprises a second shape when at rest, and wherein the housing is configured to deform when rotated into the first shape.

24. The energy storage device or rotor of claim 23, further comprising support features configured to inhibit deformation of the shell layers toward the rotor.

25. The energy storage device or rotor of claim 24, wherein the support feature comprises a portion of the top surface of the casing layer, wherein the portion of the top surface of the casing layer extends away from the rotor to couple to the bearing.

26. An energy storage device or rotor as claimed in claim 24 or claim 25, wherein the support feature comprises a reinforced section of the shell layer.

27. The energy storage device or rotor of claim 26, wherein the reinforced section of the shell layer comprises one or more of: a section of the shell layer having increased thickness; a section of the shell layer constructed of a different material than the remainder of the shell layer; and A reinforcement structure is coupled to a surface of the shell layer.

28. The energy storage device or rotor of any one of claims 24 to 27, wherein the support feature comprises a section of the casing layer having a different material structure to the remainder of the casing layer.

29. The energy storage device or rotor of claim 28, wherein the sections of the shell layer having different material structures include materials forming a repeating unit structure.

30. The energy storage device or rotor of any one of claims 24 to 29, wherein the support feature comprises a support structure, wherein the shell layer is configured to contact the support structure when the shell layer is in the second shape, and wherein the shell layer is configured to move away from the support structure when the shell layer is deformed into the first shape.

31. The energy storage device or rotor of claim 30, wherein the support structure is coupled to the axle and is configured to rotate with the shell layer.

32. The energy storage device or rotor of any one of claims 24 to 31 , wherein the support feature comprises a pair of magnets, wherein the pair of magnets are configured to inhibit deformation of the shell layer toward the rotor via magnetic repulsion between the pair of magnets.

33. The energy storage device or rotor of any one of claims 24 to 32, wherein the support feature is located proximate the axis of rotation.

34. A flywheel energy storage device as claimed in any one of claims 1 to 15, wherein the rotor is a rotor as claimed in any one of claims 16 to 33.

35. A method of generating electricity using a flywheel energy storage device as claimed in any preceding claim, the method comprising: The rotor is used to rotate a generator coupled to the axle of the storage device.

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