Developments for flywheel energy storage

Abstract

[0006] An embodiment of a flywheel energy storage system is provided that is configured to be installed in an existing naturally high pressure environment and that utilizes the pressure of the environment to provide compression by the rotor of the flywheel to increase energy storage capacity. An embodiment of a rotor for use in the flywheel energy storage system is also provided, the rotor comprising a shell layer that houses the rotor and is freely rotatable to reduce the effects of frictional drag on the rotor.

Description

[Technical Field]

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

[0002] Flywheels have been used for energy storage for many years due to their high efficiency, long life and very low cost of retrieving the stored energy.

[0003] A flywheel typically includes a solid rotor, usually cylindrical, connected to the shaft of an electric motor. As the motor rotates the rotor, it converts electrical energy into kinetic energy, which is stored in the flywheel as angular kinetic energy. This rotational energy is then converted back into electrical energy by using the flywheel to drive the electric motor, thereby causing the electric motor to function as a generator.

[0004] To improve the efficiency of the system, the rotor is typically housed in a housing that is maintained under vacuum or near vacuum conditions, which reduces frictional resistance and thus energy loss on the rotor, improving the efficiency of the flywheel storage device.

[0005] Because the maximum rotational speed of a flywheel, and therefore the maximum storable kinetic energy, depends on the tensile strength of the flywheel's manufacturing material, fabricating a flywheel rotor from a high-tensile-strength material is generally required to store substantial energy. In most commercial environments, this means that rotors are fabricated from materials such as steel, aluminum, carbon fiber, or fiberglass, which can be rare, difficult to process, have high carbon density, and have high material costs. As a result, adding storage capacity through the increase in rotor mass required for long-term storage can be prohibitively costly and difficult. While flywheels are widely used for short-term storage of small amounts of energy, they have generally been ignored for long-term storage applications due to the associated high costs. However, due to the need to smooth out diurnal fluctuations in renewable energy sources such as solar and wind power, nearly all energy storage needs require longer storage capacities than can be achieved with currently commercially available flywheels. As the world increasingly transitions to renewable energy sources in light of growing environmental pressures, flywheel energy storage devices capable of efficient, long-term energy storage are needed. Furthermore, such a system should be capable of being constructed from abundant, low cost, and low carbon density materials so that flywheel storage devices can be manufactured and deployed worldwide.

[0006] Various approaches have been attempted to improve the storage capacity of flywheel rotors. For example, U.S. Patent No. 10,281,003 attempts to reduce reliance on expensive, high-tensile-strength materials by using a flywheel with a cylindrical mass containing a main material, such as concrete, with a compressive resistance of at least 25 MPa. At least a portion of the mass's outer surface is covered with fiber, the tensile strength of the material comprising the fiber being at least 100 MPa. The tension generated by wrapping the fiber around the body compresses the main material, prestressing it. Therefore, the maximum rotational speed of low-tensile-strength concrete is increased because the centripetal force generated by the rotation of the flywheel must overcome this compression before the concrete is placed in net tension. However, as noted above, this compression is achieved in U.S. Patent No. 10,281,003 by wrapping the concrete in high-tensile-strength fiber, thereby eliminating many of the benefits of using a low-tensile-strength material as the main material. Similarly, the reliance on relatively hard-to-obtain high tensile strength fibers limits this solution to regions that have both the financial means to purchase the expensive material and the capabilities to manufacture and process it.

[0007] U.S. Pat. No. 5,015,940 applies a similar pre-compression principle to known high-tensile-strength rotors. In this case, the additional pressure is applied by external means and is independent of the rotational motion of the article itself. For example, U.S. Pat. No. 5,015,940 describes placing the rotor in a pressure chamber and using gas in the chamber at superatmospheric pressure (i.e., pressure greater than 1 atm≒1 bar) to compress the rotor and improve its maximum storage capacity through a mechanism similar to that of the aforementioned U.S. Pat. No. 10,281,003. However, the advantage of being able to store more energy for a given rotor mass is largely offset by the increased costs and technical difficulties associated with the need to provide a high-tensile-strength pressure chamber to hold the superatmospheric pressure gas. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, there is a need for low-cost flywheel energy storage devices, especially those that reduce their reliance on high tensile strength materials that are rare, expensive, difficult to process, and have high carbon concentrations.

[0009] Using gas above atmospheric pressure to compress the flywheel rotor poses additional challenges for flywheel energy storage devices because the gas increases frictional resistance on the rotor and reduces energy storage efficiency. U.S. Pat. No. 5,015,940 minimizes losses due to gas-flywheel interaction by using low-viscosity gases such as hydrogen or helium. Alternatively, the drag reduction system described in International Publication No. WO 2007 / 012267 can be employed, which comprises n concentric, thin-walled, rigid cylindrical shells encasing the flywheel. However, this type of drag reduction system imposes structural requirements that make it uneconomical for large-scale production. Therefore, an improved drag reduction system for use in flywheel energy storage devices is needed.

[0010] The present invention addresses the above-mentioned shortcomings of known flywheel storage devices by increasing energy storage capacity and reducing costs by utilizing existing sources of high-pressure environments to compress the low-tensile strength rotor. For example, embodiments of the present invention can be installed in high-pressure environments, including natural high-pressure environments such as the ocean floor, lakes, caves, or flooded mines. This also reduces the pressure differential across the housing, relaxing the tensile strength requirements of the housing. [Means for solving the problem]

[0011] According to a first embodiment of the present invention, there is provided a flywheel energy storage device comprising: a housing, the housing being sealed to isolate an interior of the housing from an external environment; a rotor mounted on a shaft within the housing, the rotor being configured to be rotatable relative to the housing, the rotor being made of a low tensile strength material; a fluid contained within the housing to compress the rotor; and means for maintaining the pressure of the fluid in substantial equilibrium with the pressure of the external environment.

[0012] According to a second embodiment of the present invention, there is provided a flywheel energy storage device comprising: a housing, the housing being sealed to isolate an interior of the housing from an external environment; a rotor mounted on a shaft within the housing, the rotor being configured to rotate relative to the housing; a fluid contained within the housing for compressing the rotor; and means for maintaining a pressure of the fluid at approximately equilibrium with or higher than the pressure of the external environment, wherein the housing is a pressure vessel capable of withstanding only pressures lower than the natural 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 placing the flywheel energy storage device according to the first or second embodiment of the present invention in a high-pressure environment, the fluid within the housing can be brought into near-equilibration with the environment to compress the rotor, thereby increasing energy storage capacity and eliminating reliance on high-tensile materials that are expensive, difficult to process, and have a high carbon content. Furthermore, because the fluid is in equilibrium with the environment in the first embodiment, the housing itself does not need to be a pressure vessel to maintain the contained fluid at a pressure above atmospheric pressure, reducing the structural complexity and material costs of the housing. Similarly, when the fluid is operated at a pressure above ambient pressure, as in the second embodiment, the presence of external pressure means that the pressure differential experienced by the housing is reduced. Therefore, the total pressure the housing must withstand is lower than the total pressure of the fluid, thereby reducing the structural requirements and associated material costs for a given operating pressure of the fluid within the housing.

[0014] Additionally or alternatively, the energy storage device of the first and second embodiments may further comprise one or more of an electric motor coupled to the shaft for storing energy in the flywheel energy storage device, and a generator coupled to the shaft for extracting energy from the flywheel energy storage device, thereby enabling the storage device in the first and second embodiments to store electrical energy.

[0015] Furthermore, the housing of the first or second embodiment may be constructed of a material with low tensile strength or no inherent tensile strength. Such materials can be used because the flywheel energy storage devices of the first and second embodiments are intended to be installed in a high-pressure environment and therefore do not need to maintain a total fluid pressure above atmospheric pressure. More specifically, because the housing is pre-compressed to a state of high compression by the ambient pressure of the environment, a material with no inherent tensile strength, such as concrete, can be used for the housing; the pressure of the fluid contained within the housing must first overcome this compression before the housing is placed in net tension. This allows the housing to be made of a material that is readily available, inexpensive, easy to process, and has a low carbon density. In this way, the first and second embodiments of the present invention reduce the cost and complexity of the overall storage device while still benefiting from the increased storage capacity achieved by compressing the rotor using pressure above atmospheric pressure of the external environment.

[0016] Additionally or alternatively, one or more low tensile strength materials of the housing and rotor of the first and second embodiments may have a tensile strength of less than 200 MPa. Preferably, the tensile strength of the low tensile strength material may be less than 150 MPa, 100 MPa, or 75 MPa, and most preferably less than 50 MPa, 40 MPa, 30 MPa, 20 MPa, 10 MPa, or 5 MPa. In some embodiments, the rotor material may include a material with no inherent tensile strength, i.e., a tensile strength of 0 MPa.

[0017] Additionally or alternatively, the rotor of the first or second embodiment may be constructed of ceramic. For example, any one or more of cement, concrete, and clay may be used. Alternatively, the rotor may be made of a granular material enclosed in a sealed container, with the pressure within the container maintained at a pressure lower than that of the fluid. Because both ceramic and granular materials (such as sand) are dense and inexpensive, rotors with masses exceeding one ton can be easily manufactured and used at relatively low cost, thereby reducing the material and manufacturing costs of the system itself and the energy storage costs. Furthermore, by enabling rotors with very large masses, these materials also improve the long-term energy storage capabilities of the system. The abundance and wide availability of these materials allows for the manufacture and installation of flywheel storage devices in remote or impoverished areas where specialized high-tensile strength materials are not readily available and difficult to process. Granular materials such as sand lack inherent bonds between individual particles and therefore lack tensile strength. Such materials are extremely abundant, inexpensive, and have low carbon consolidation, allowing them to be compressed to form dense bulk materials. Therefore, such materials may be ideal choices for manufacturing high mass flywheel rotors such as those used in the invention.

[0018] Additionally or alternatively, the means for maintaining pressure in the first and second embodiments may include one or more of a pressure compensator coupled to the pressure of the external environment and a valve mechanism coupled to the pressurized fluid source. Additionally or alternatively, the means for maintaining pressure may include one or more of a U-tube and a flexible membrane covering a port coupled to the interior of the housing of the reservoir. The pressure compensator can maintain the fluid pressure in equilibrium with the external environment by automatically compensating for drops in internal pressure, thereby ensuring that the fluid pressure is maintained at an optimal level during operation. Alternatively or additionally, the use of a valve mechanism and a pressurized fluid source can more precisely control the internal pressure of the housing by allowing the fluid within the housing to be elevated above environmental pressure. Alternatively, a pump may be used to increase the pressure within the housing above environmental pressure. Additionally, embodiments including a pressurized fluid source or pump may be passively or actively controlled to control the internal pressure of the housing. For example, active control may include using a pressure sensor and actuator to control a valve mechanism to vary 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 optimum operating pressure and preventing the ingress of seawater which may corrode or damage components within the housing.

[0020] Additionally or alternatively, the fluid used in the first and second embodiments may be a low-density fluid, and optionally, the density of the fluid may be less than one-third the density of the rotor. The use of a low-density fluid improves the efficiency of the system by minimizing frictional energy losses in the fluid. Furthermore, by selecting a fluid based on the required density ratio to the rotor, particularly the one-third ratio described in the claims, energy losses can be reduced to an acceptable level while providing sufficient compressive force to the rotor. Optionally, the fluid may be one or more of hydrogen and helium. Additionally or alternatively, the fluid may be one or more of neon, argon, krypton, xenon, or nitrogen.

[0021] Additionally or alternatively, the first and second embodiments may further comprise an electrical connector comprising conductors for electrically communicating with the energy storage device, the electrical connector comprising means for isolating the conductors from seawater, optionally comprising one or more of a seal and a dielectric fluid. The use of means for isolating the conductors from seawater ensures that the storage device can be used underwater while protecting necessary electrical connections from the corrosive effects of seawater. Furthermore, the isolation means protects the environment from potentially high-voltage electrical signals carried by the connector, as well as from short circuits and other electrical faults.

[0022] Additionally or alternatively, the first and second embodiments may further include, for example, control electronics configured to control at least the electric motor-generator, and may include means for isolating the control electronics from the external environment. Optionally, the means for isolating the control electronics may include a sealed, air-filled canister to protect the control electronics from seawater. Alternatively, the canister may be filled with a dielectric. By providing the control electronics as an integral component of the energy storage device, the first and second embodiments may be provided as a "plug-and-play" system, allowing for easy installation in remote locations and not requiring specialized installation personnel. Furthermore, by providing means for isolating the control electronics from the environment, the first and second embodiments may be easily installed underwater without risk of water damage to 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. Installation of such a lifting attachment may facilitate installation of the device in remote deep-sea areas (or other high-pressure environments), for example, using a general-purpose water crane or lifting tool mounted on a vessel. Alternatively, the lifting attachment may allow the device to be lowered using buoyancy to control the descent of the storage device.

[0024] Additionally or alternatively, the first and second embodiments may further comprise a handle configured to interface with an ROV, for example, to allow installation, inspection, maintenance, and end-of-life removal to be performed in situ by a remotely operated vehicle, thereby reducing the risk of a human being having to dive to perform the work. Such a handle may be coupled to an electrical connector, allowing the storage device to be easily connected to a power source by an ROV after being installed in the water.

[0025] Additionally or alternatively, the housing of the first and second embodiments is configured to be connected to a foundation anchor to support and stabilize the energy storage device in place, and optionally the foundation anchor is one or more of a mud mat, a caisson, or one or more piles. The use of the foundation anchor allows the storage device to be installed in areas with less than optimal ground conditions and to be safely held in place during operation and high-speed rotation. Furthermore, the concrete floors typically used in above-ground flywheel storage devices exert excessive pressure on the loose soil of the seabed. Therefore, the use of anchoring means specifically designed for underwater use, as described above, ensures that the first and second embodiments are properly anchored and positioned on the seabed.

[0026] Additionally or alternatively, the first and second embodiments further include a shell layer housing the rotor and coupled to the shaft via a bearing, the shell layer having a first shape, and the bearing allowing the shell layer to rotate around the rotor about an axis of rotation concentric with the shaft. The use of a shell layer that is freely rotatable around the rotor reduces the frictional resistance experienced by the rotor by reducing the fluid velocity differential experienced by the rotor. This allows the shell layer to improve the efficiency of the flywheel storage device and reduce energy storage costs.

[0027] According to a third embodiment of the present invention, there is provided a rotor for use in a flywheel energy storage device, the rotor comprising: a rotor; a shaft coupled to the rotor; and a shell layer covering the rotor and coupled to the shaft via a bearing, the shell layer having a first shape, the bearing allowing the shell layer to rotate around the rotor about an axis of rotation that is concentric with the axis, the cross section of the shell layer, when drawn in a plane in which the axis of rotation lies, defining a closed generally convex shape having upper and lower surfaces that intersect the axis of rotation, and each of the upper and lower surfaces comprising a continuous curve.

[0028] According to a fourth embodiment of the present invention, there is provided a rotor for use in a flywheel energy storage device, the rotor comprising: a rotor; a shaft coupled to the rotor; and a shell layer covering the rotor and coupled to the shaft via a bearing, the shell layer having a first shape, the bearing allowing the shell layer to rotate around the rotor about an axis of rotation that is concentric with the axis, the shell layer defining a closed three-dimensional generally convex shape having upper and lower surfaces that intersect the axis of rotation, each of the upper and lower surfaces including a portion with a continuous curvature.

[0029] As described in more detail below, the use of a shell layer that can freely rotate around the rotor, as provided by the third and fourth embodiments, reduces the frictional resistance experienced by the rotor by reducing the differential fluid velocity experienced by the rotor. In this way, the shell layer improves the efficiency of the flywheel storage device and reduces energy storage costs. Additionally, the specific shape of the shell layer provided by the third and fourth embodiments allows the shell layer to function as a pressure vessel and withstand the centripetal pressure that occurs as the fluid rotates with the rotor within the shell layer without requiring the shell layer to be uneconomically and impractically thick. Therefore, the use of shell layers according to the third and fourth embodiments improves the energy storage efficiency of the rotor by reducing the resistance experienced by the rotor. The specific shell shape defined by these embodiments improves storage capacity by increasing the maximum speed at which the rotor can rotate before damaging the shell layer. Specifically, as described below, providing a curved surface in the shell layer allows pressure to be maintained by material tension rather than bending moment, thereby enabling the use of thinner material and reducing material costs for the shell layer.

[0030] Additionally, the above-described embodiments of the present invention may include multiple concentric nested shell layers, each shell layer having a corresponding first shape, each shell layer including a bearing coupled to the shaft, the bearings configured to allow the shell layer to rotate about the rotor and relative to each of the other shell layers. The use of multiple nested shell layers further reduces the drag experienced by the rotor, thereby further improving energy storage efficiency and reducing energy storage costs.

[0031] Additionally or alternatively, the above-described embodiments of the present invention may have a gap between the concentric shell layers that is less than 30% of the rotor diameter, preferably no more than 25%, and ideally no more than 20%. Reducing the gap between the layers improves the efficiency of the drag reduction effect. Furthermore, the reduced gap size allows for smaller diameter shell layers, reducing material costs. Additionally, the required thickness of the shell layers in the small radius region is reduced, leading to reduced material costs and a reduced reliance on high tensile strength materials.

[0032] Additionally or alternatively, the first shape of the shell layer of the above-described embodiments of the present invention may be defined by a hypergeometric function. The inventors have found that the shape of the shell layer can be defined by a hypergeometric function, in particular:

number

[0033] Alternatively, the first shape of the shell layer in the above-described embodiments of the present invention may be one of a sphere, an ellipsoid, and a cylinder with domed top and bottom surfaces. Similar to the hypergeometric function described above, shell layers of these shapes can address the problem of centripetal pressure by using curved top and bottom surfaces. The use of curved surfaces allows pressure to be resisted by tension rather than bending moments. This allows for the use of thinner material and reduces material costs for the shell layer.

[0034] Additionally or alternatively, in the above-described embodiments of the invention, the shape of the rotor may correspond to the first shape of the shell layer, thereby maximizing the mass of the rotor for a given volume defined by the shell layer, and therefore maximizing the energy storage capacity.

[0035] Additionally or alternatively, the shell layer of the above-described embodiments of the present invention may be elastically deformable and have a second shape at rest, with the shell layer configured to deform to a first shape upon rotation. For example, at rest and in the absence of centripetal pressure, the shell layer sags or "shrinks" to the second shape. Then, upon rotation, centripetal pressure causes the layer to "expand" to the first shape. In this manner, an ideal shape can be achieved while allowing the shell layer to deform in response to increasing centripetal pressure. Furthermore, by allowing the shell layer to deform in response to this pressure rather than maintaining a rigid shape, more abundant, less costly materials with less rigidity can be used to form the shell layer.

[0036] Additionally or alternatively, the above-described embodiments of the present invention may further include support features configured to inhibit deformation of the shell layer toward the rotor. As the rotational speed of the shell layer decreases, the centripetal pressure of the contained fluid decreases, and portions of the shell layer may deflect under the influence of gravity and contact the rotor. Alternatively, in embodiments with multiple nested shell layers, a particular shell layer may deflect until it contacts a shell layer nested within it. Such contact may create friction and reduce the storage efficiency of the energy storage device by converting rotational kinetic energy into heat. Therefore, in some embodiments, additional support features configured to inhibit deformation of the shell layer toward the rotor may be provided to reduce or completely prevent contact between the rotor and the shell layer when the shell layer is in the second shape. In embodiments with multiple nested shell layers, support features may be associated with each shell layer (e.g., so that there are as many support structures as there are shell layers) to inhibit deformation of the shell layer toward the shell layers nested within the particular shell layer.

[0037] Additionally, the support configuration may include a portion of the upper surface of the shell layer, the portion of the upper surface of the shell layer extending away from the rotor to couple to the bearing. By including a portion of the upper surface of the shell layer extending away from the rotor and coupling to the bearing, the shell layer is improved in its ability to resist deformation toward the rotor under gravity by supporting its weight through tension in the shell layer. This is because, by extending away from the rotor, the shell layer has a component parallel to gravity, which in this embodiment acts parallel to the rotation axis. In this context, the shell layer having a "component parallel to the direction of gravity" means that decomposing the tangent to the shell layer surface at the point of interest into a vector parallel to the direction of gravity and a vector perpendicular to the direction of gravity results in a non-zero vector parallel to the direction of gravity.

[0038] Additionally or alternatively, the support configuration may include a reinforcing portion of the shell layer. For example, reinforcing a portion of the shell layer increases the stiffness of that portion, thereby increasing resistance to deformation and reducing deformation of the shell layer toward the rotor. For example, in an embodiment in which the shell layer deflects in a direction parallel to the rotation axis due to gravity, the reinforcing portion may be located near the rotation axis of the upper surface and connect the shell layer to a bearing, thereby reducing deformation of the upper surface toward the rotor under gravity.

[0039] Additionally, the reinforcement of the shell layer may include one or more of: a portion of the shell layer with increased thickness; a portion of the shell layer constructed of a material different from the remainder of the shell layer; and a reinforcing structure bonded to the surface 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 may be located near the axis of rotation. The thickness of the material on the upper and / or lower surfaces of the shell layer may increase toward the axis of rotation, e.g., with the greatest thickness at the portion of the upper surface closest to the axis of rotation. The shell layer may alternatively or additionally be reinforced by constructing a portion of the shell with a material different from the remainder of the shell layer. For example, the portion of the upper and / or lower surface near the axis of rotation may be constructed with a material having a higher stiffness than the material used to construct the remainder of the shell layer. As used herein, stiffness refers to the ability of a material to resist deformation due to an external force. The shell layer may alternatively or additionally be reinforced by bonding a reinforcing structure to the surface of the shell layer. As used herein, a reinforcing structure refers to a structure added to the shell layer to increase the stiffness of the shell layer. For example, stiffening plates, bars, beams, rods, trusses, or brackets may be bonded to the surface of the shell layer to increase stiffness, thereby restricting deformation and limiting contact between the shell layer and the rotor or an internally fitted shell layer. Alternatively, such structures may be embedded in the shell layer. In some embodiments, stiffening structures may be bonded to the inner and outer surfaces of the shell layer to sandwich the shell layer.

[0040] Additionally or alternatively, the support form may comprise a portion 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 materials. By altering the material structure, the properties of the material can be altered, for example, increasing the stiffness of the material while limiting the weight without simply increasing the thickness of the material.

[0041] Additionally, the portions of the shell layer having different material structures may include material formed into a repeating cell structure. As used herein, a repeating cell structure refers to a macroscopic arrangement of material in a pattern of hollow enclosures. For example, the reinforcement portion may include material formed into a honeycomb structure, i.e., a repeating hexagonal pattern. Alternatively, the material may be formed into a repeating pattern of any suitable shape, such as triangles, squares, pentagons, heptagons, octagons, nonagons, or decagons. Alternatively, the cell structure may be irregular, e.g., a Voronoi tessellation. Alternatively, the cell shape may vary within the repeating pattern. The repeating cell structure may increase the stiffness of the material while reducing its weight compared to a solid material of the same thickness. To further increase stiffness, the material having the repeating cell structure may be sandwiched between two reinforcing plates.

[0042] Additionally or alternatively, the support configuration may include a support structure, with the shell layer configured to contact the support structure when the shell layer is in the second shape and to separate from the support structure when the shell layer transforms to the first shape. In this manner, the shell layer sags and is prevented from contacting the rotor or any shell layers nested therein; instead, the shell layer sags and contacts the support structure, preventing further sagging of the shell layer. However, when the shell layer reaches rotational speed and transitions to the first shape, the shell layer is decoupled from its support function and is able to rotate independently of the support structure. By allowing separation from the support structure, which may later rotate with the shell layer, the rotating mass of the shell layer is reduced when it reaches a constant speed, thereby reducing the kinetic energy stored in the shell layer and improving the storage efficiency of the flywheel storage system.

[0043] Furthermore, the support structure may be coupled to the shaft and configured to rotate together with the shell layer. For example, the support structure may be coupled to the shaft via its own support bearing separate from the bearing of the shell layer. In this example, the support structure can rotate independently of the shell layer but rotates together with the shell layer when in contact, thereby reducing slippage and friction between the shell layer and the support structure. Alternatively, the support structure can be coupled to the same bearing as the shell layer, ensuring that the support structure and the shell layer always rotate at the same speed. In this way, slippage that occurs when the support structure and the shell layer come into contact can be completely avoided.

[0044] Additionally or alternatively, the support configuration may include a pair of magnets configured to prevent the shell layer from deforming toward the rotor due to magnetic repulsion between the pair of magnets. For example, one or more magnets may be attached to the upper surface of the rotor and one or more magnets may be attached inside the upper surface of the shell layer. As a result, when the shell layer deflects and the magnets approach each other, the magnets repel each other, preventing further deflection of the shell layer toward the rotor, so that the poles of the magnets facing each other have the same polarity, thereby preventing contact between the shell layer and the rotor. Alternatively or additionally, the same magnet arrangement may be used between a given shell layer and a nested shell layer disposed inside the given shell layer.

[0045] Additionally or alternatively, the support features may be located near the axis of rotation. In embodiments where the shell layer deforms under gravity in a direction parallel to the axis, the axis itself can be used to support the shell layer and help prevent contact between the shell layer and the rotor in a manner similar to tent poles supporting a tent. As used herein, "near the axis of rotation" includes the support features being located within the central 50% of the radius of the shell layer. Alternatively, the support features may be located within the central 40%, 30%, 20%, 15%, 10%, or 5% of the radius of the shell layer. Alternatively, in embodiments where the support features comprise a portion of the shell layer, a reinforcement for the shell layer, or a different material structure, the support features may be directly coupled to the bearing such that the support features couple the remaining portion of the shell layer to the bearing.

[0046] Additionally or alternatively, the flywheel energy storage device of either the first or second embodiment may include the rotor of the third or fourth embodiment.

[0047] 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 of the above embodiments, comprising using a rotor to rotate a generator coupled to a shaft of the storage device. [Brief explanation of the drawings]

[0048] A description will now be given, by way of example only, with reference to the accompanying drawings in which:

[0049] [Figure 1] FIG. 1 shows an external view of an embodiment of a flywheel energy storage device. [Figure 2] FIG. 2 shows details of the pressure control means of the flywheel energy plant system of FIG. [Figure 3] FIG. 3 illustrates an exemplary connector assembly for a flywheel energy storage device. [Figure 4] FIG. 4 shows an overhead view of the energy storage device of FIG. [Figure 5] FIG. 5 is a cross-sectional view of the flywheel storage device taken along line AA shown in FIG. 4, showing the internal components of the flywheel storage device. [Figure 6] FIG. 6 shows a cross-sectional view of a second embodiment of a flywheel storage device employing a drag reduction rotor embodiment. [Figure 7] FIG. 7 shows the optimum shape of the shell layer of the drag reduction rotor of FIG. [Figure 8A] FIG. 8A shows an alternative configuration for the shell layer of the drag reduction rotor of FIG. [Figure 8B] FIG. 8B shows an alternative shape for the shell layer of the drag reduction rotor of FIG. [Figure 8C] FIG. 8C shows an alternative shape for the shell layer of the drag reduction rotor of FIG. [Figure 8D] FIG. 8D shows an alternative configuration for the shell layer of the drag reduction rotor of FIG. [Figure 9A] FIG. 9A shows the torque acting on the shell layer, whose angular momentum decreases with increasing radius. [Figure 9B] Diagram B shows the torque acting on the shell layer, whose angular momentum decreases with increasing radius. [Figure 10A] FIG. 10A shows the coordinate system used to derive the optimal shape of the shell layer. [Figure 10B]FIG. 10B shows the coordinate system used to derive the optimal shape of the shell layer. [Figure 11] Figure 11 shows the deformation of the shell layer under the influence of gravity. [Figure 12] FIG. 12 shows one embodiment of a support form that comprises a portion of the top surface of the shell layer that extends away from the rotor for coupling to a bearing. [Figure 13A] FIG. 13A shows a support configuration with reinforcement of the shell layer. [Figure 13B] FIG. 13B shows a support configuration with reinforcement of the shell layer. [Figure 14A] FIG. 14A shows a material that utilizes a repeating cell structure and a material suitable for use as a support form that utilizes the repeating cell structure material. [Figure 14B] FIG. 14B shows a material that utilizes a repeating cell structure and a material suitable for use as a support form that utilizes the repeating cell structure material. [Figure 15] FIG. 15 shows a support feature comprising a support plate configured to rotate with the shell layer. [Figure 16] FIG. 16 shows a support mechanism comprising a pair of opposing magnets that suppress deformation of the shell layer by magnetic repulsion. [Figure 17] FIG. 17 shows the rotor and shell layer configured to rotate about an axis of rotation perpendicular to gravity, allowing the shell layer to self-support as an arch structure using compression of the curved surface of the shell layer. DETAILED DESCRIPTION OF THE INVENTION

[0050] FIG. 1 illustrates an exterior view of a flywheel energy storage device 10 configured for installation in a high-pressure environment in accordance with an embodiment of the present invention. As illustrated, the storage device includes a housing 20 that houses a flywheel rotor (shown in FIG. 5) and contains a low-density fluid used to compress the rotor in situ. The storage device 10 further includes a means 70 for controlling the pressure of the fluid within the housing 20 using the pressure of the external environment. For example, in the example shown in FIG. 1, the storage device 10 includes a pressure compensator that substantially equalizes the pressure of the fluid within the housing 10 to the external environment. As those skilled in the art will appreciate, pressure compensators are components used in subsea systems to counteract the effects of pressure differences at depth by balancing the internal pressure of the hydraulic system (in this case, the fluid within the housing 20) with the ambient pressure of the surrounding water.

[0051] In this way, the pressure of the external environment can be used to pressurize the fluid contained within the housing, thereby compressing the flywheel rotor and increasing its energy storage capacity as described above. Additionally, the presence of an external high-pressure environment reduces the pressure differential across the housing at a given fluid pressure, and correspondingly reduces the pressure retention requirements of the housing. This means that the housing can be constructed from less expensive, more readily available materials with lower carbon density because the pressure the housing needs to withstand is lower than the pressure of the contained fluid.

[0052] Suitable high pressure environments include the ocean floor, lakes, caves, submerged mines or other existing high pressure environments. Preferably, the pressure of the external environment is greater than 10 bar, which can be easily achieved on the ocean floor at depths of more than 100 meters.

[0053] As also shown in FIG. 1 , the storage device 10 further includes a motor-generator 50 for storing and extracting energy in and from the storage device 10 by rotating the rotor. While the motor-generator is integrated as shown, the storage device 10 may alternatively include an electric motor and a separate generator. Furthermore, the storage device 10 may include only one of an electric motor and a generator. To store energy in the storage device 10, the motor-generator operates as an electric motor by capturing electrical energy from an external power source and converting it into rotational kinetic energy of the flywheel rotor, thereby storing the electrical energy as kinetic energy. To extract energy from the storage device 10, the motor-generator 50 is driven by the rotating flywheel rotor, thereby operating as a generator, thereby converting the rotor's kinetic energy back into electrical energy.

[0054] In the example shown in FIG. 1 , storage device 10 further includes control electronics 55 for controlling the motor-generator. While shown in FIG. 1 as being integrally formed as a component of storage device 10, control electronics 55 need not be located on storage device 10; instead, it may be housed at a remote location and coupled to the motor-generator via cabling or other forms of electronic communication. When control electronics are built into storage device 10 and located underwater, as in the illustrated example, storage device 10 includes means for isolating control electronics 55 from the external environment. For example, this may include housing control electronics in a sealed, air-filled container to protect control electronics 55 from the corrosive effects of seawater.

[0055] As shown in FIG. 1 , the storage device 10 may also include a frame 30 having a lifting attachment 40 for raising and lowering the storage device 10 into position. As can be appreciated, the pressure of the fluid within the housing 20 needs to be controlled while the storage device 10 is lowered into the water and placed into position to compensate for the increasing pressure experienced by the storage device as it goes deeper. This can be achieved, for example, by lowering the storage device with a pressure vessel (e.g., a cylinder of pressurized fluid) connected to the fluid within the housing 20 via a valve mechanism. While the storage device 10 is lowered using the lifting attachment 40, a suitable valve mechanism regulates the fluid pressure within the pressure vessel to ensure that the pressure within the housing equalizes with the ambient pressure during the lowering operation. Preferably, this is a passive system in which the valve mechanism opens when the pressure of the fluid within the housing 20 is lower than the ambient environment so that the pressure within the pressure vessel can be utilized to appropriately pressurize the fluid. However, as can be appreciated, the necessary pressure control can also be achieved using an active system comprising a pressure sensor and actuator for controlling a valve mechanism or pressure pump coupled to the pressure vessel.

[0056] The frame 30 may be configured to connect to foundation anchors to support and stabilize the storage device 10 in place. Foundations used for land-based flywheel storage devices may not be suitable for the types of surfaces found in aquatic environments, and the storage device 10 may instead be configured to be anchored in place using, for example, mud mats, caissons, or piles driven into the seabed. In the example shown in Figure 1, the storage device 10 includes a mud mat 32 and side skirts 34 coupled to the frame 30.

[0057] FIG. 2 shows details of the pressure control means 70 and the connection assembly 60 for connecting the motor-generator 50 to an external power source. As mentioned above, the example pressure control means 70 shown in FIG. 1 is a pressure compensator, which is a passive mechanism that ensures that the pressure of the fluid within the housing 20 is approximately in equilibrium with the environment. Alternatively, or in addition to the illustrated compensator, a valve mechanism coupled to a high-pressure fluid source can be used to control the fluid pressure within the housing 20. Such a valve mechanism can be passive and adjust the fluid pressure based on the pressure of the external environment, for example, by opening when the fluid pressure drops below the environmental pressure. Alternatively, the valve mechanism can include a pressure sensor and actuator and actively control the pressure within the housing 20. As mentioned above, the valve mechanism uses a high-pressure fluid source, such as a pressure vessel (e.g., a cylinder of high-pressure gas) or a pump, to control the pressure within the housing 20. Additionally or alternatively, the pressure control means may comprise one or more of a trap or U-tube (i.e., a U-shaped section of tubing designed to trap liquid or gas), and a flexible membrane covering a port in communication with the fluid contained within the housing. The simplicity of these alternatives may make them preferable in remote locations where regular maintenance is more difficult.

[0058] FIG. 3 shows details of the electrical connector assembly 60 shown in FIGS. 1 and 2. The electrical connector assembly 60 includes an electrical connector 62, a handle 64 configured for use on a remotely operated vehicle (ROV), and an electrical cable 66 coupled to the motor-generator 50 as shown in FIG. 1. As described above, the electrical connector assembly 60 is configured to connect to an external electrical system capable of storing energy and to inject energy extracted from the storage device 10 into the external electrical system via the electrical connector 62. The electrical connector 62 may be configured for underwater connection, in which case the electrical connector 62 includes means for insulating the conductors within the electrical connector 62 from the external environment. For example, this may be achieved by using one or more seals designed to prevent the ingress of seawater and / or by using a dielectric fluid within the electrical connector 62.

[0059] As shown in Figure 3, the electrical connector assembly may further include a handle 64 configured for use with a remotely operated vehicle. As noted above, the storage device 10 may be located in deep water, at depths of greater than 100 meters, where it may be dangerous, expensive, and impractical for human divers to perform the installation, inspection, maintenance, and end-of-life removal tasks associated with the storage device 10. Therefore, the storage device 10 may be provided with multiple ROV intervention points or handles, such as the handle 64 on the electrical connector assembly, so that the tasks can be performed by an ROV, reducing risks to human divers and allowing for more regular and convenient maintenance of the storage device 10.

[0060] As shown, electrical connector 62 is coupled to motor-generator 50 via cable 66, which may be a high-voltage submarine electrical cable intended for use in high-pressure environments. For example, cable 66 may include a heavy-duty jacket, constructed of, for example, a thermoplastic resin, to withstand the external pressures of the environment and to protect the internal wiring from damage and abrasion caused by rocks and other debris that may be present at the location where the storage device is installed. Cable 66 may also include a dielectric fluid, for example, to prevent or quickly extinguish electrical discharges that may occur from conductors within cable 66 if the cable is damaged.

[0061] FIG. 5 shows a cross-sectional view taken along line AA of FIG. 4 , detailing the internal components of flywheel energy storage device 10 in accordance with an embodiment of the present invention. As can be seen, storage device 10 includes rotor 80 supported on shaft 100 within housing 20. Shaft 100 is supported within the housing via one or more bearings, shown in the illustrated embodiment as thrust bearing 90 and radial bearing 92, which allow rotor 80 to rotate relative to housing 20. As also shown, shaft 100 is coupled to motor-generator 50, which allows motor-generator 50 to rotate rotor 80 for storing energy and for rotor 80 to rotate motor-generator 50 when energy is required for extraction. Fluid 110 is contained within housing 20 and is pressurized by pressure control means 70 to maintain rotor 80 in a compressed state, thereby increasing the energy storage capacity of storage device 10.

[0062] As noted above, an objective of the present invention is to reduce reliance on rare, expensive, difficult-to-machine, and high-carbon-concentration high-strength materials in the manufacture of flywheel rotor 80 by utilizing the pressure of fluid 110 to pre-compress rotor 80. Therefore, by first placing rotor 80 in compression, a wider variety of abundant, inexpensive, and easily machined materials can be used to manufacture rotor 80, thereby reducing the material cost and complexity of the resulting system. For example, embodiments of the present invention may employ inexpensive materials considered to have a tensile strength of less than 200 MPa, preferably less than 150 MPa, less than 100 MPa, or less than 75 MPa. Optimally, the tensile strength is less than 50 MPa, less than 40 MPa, less than 30 MPa, less than 20 MPa, less than 10 MPa, or less than 5 MPa. Alternatively, rotor 80 may comprise a granular material, such as sand, that has no inherent bond between the individual material particles and therefore has no tensile strength, i.e., a tensile strength of 0 MPa. Such materials are abundant, inexpensive, and have low carbon concentrations that allow them to be compressed to form dense bulk materials, making them ideal choices for use in manufacturing high mass flywheel rotors such as those used in the present invention.

[0063] The tensile strength of a material can be determined by several standard tests. For example, uniaxial tensile tests may be used to 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 tests can be used to determine the tensile strength of granular materials such as the sand mentioned above. For example, this includes consolidated drained or undrained tests. 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.

[0064] To allow rotor 80 to be formed from granular materials that lack tensile strength, the granular material may be encapsulated within a membrane and air removed from the membrane to compress the granular material into a hard, dense, agglomerated material. Furthermore, the presence of external pressure from fluid 110 compensates for the lack of inherent tensile strength of the granular material by initially compressing the rotor. In this manner, the acting centripetal force, which increases as the rotor increases its rotational speed, must overcome this compression before the rotor is subjected to a net tension force. Therefore, rotors comprising materials with low or no tensile strength can function as flywheel rotors.

[0065] By using the abundant and easily processed materials described above, rotor masses of 0.5 tonnes or more, preferably 0.75 tonnes or more, and optimally 1 ton or more can be easily achieved, thereby reducing the material and manufacturing costs of the system as well as the energy storage costs. Furthermore, the additional material costs for increasing the rotor mass and thus the energy storage capacity are significantly reduced compared to the high tensile strength materials traditionally used in flywheel manufacturing.

[0066] 5, housing 20 contains fluid 110 that is used to compress rotor 80. Accordingly, housing 20 may include one or more seals (not shown) that separate water in the external environment from fluid 110 to ensure that fluid 110 is maintained at an optimum operating pressure and to protect components within the housing from corrosion or other damage.

[0067] Because the fluid 110 is maintained at a pressure above atmospheric pressure to compress the rotor 80, the density, and therefore the frictional effect, of the fluid 110 against the rotating rotor 80 is increased compared to the fluid 110 at atmospheric pressure. Therefore, the fluid 110 can be selected to minimize frictional losses and improve the energy storage efficiency of the storage device 10. For example, because resistance increases proportionally with fluid density, the fluid 110 may be selected so that its fluid density at the operating pressure is less than the rotor density. In particular, the fluid density may be less than half the rotor density, and preferably less than one-third, one-quarter, one-fifth, one-sixth, one-seventh, or one-eighth of the rotor density at the operating pressure. For example, noble gases such as helium, neon, argon, krypton, and xenon may be used. Alternatively, diatomic elements such as hydrogen or nitrogen may be used as the fluid 110. Furthermore, the fluid 110 may be selected to have a low intrinsic viscosity (considered to be lower than the viscosity of air, 18.46 μPas) to further improve energy storage efficiency. Furthermore, the use of a low density fluid ensures that the shell layer, described below with reference to Figure 6, can be made of a lower tensile strength and thinner material because the centripetal pressure on the shell layer decreases as the fluid pressure decreases. Thus, the use of a low density fluid as defined above further reduces the cost of the energy storage device.

[0068] As noted above, the primary purpose of fluid 110 is to compress rotor 80. To this end, as noted above, the pressure of fluid 110 is controlled using pressure control means 70 and the existing pressure of the external environment in which storage device 10 is located. For example, by bringing fluid 110 into near equilibrium with the external environment using pressure control means 70, e.g., a pressure compensator, the fluid may be pressurized to greater than 10 bar, which can be easily achieved at depths greater than 100 m below sea level. Examples of fluid 110 being in near equilibrium include the pressure of fluid 110 being within 20%, preferably within 15%, and optimally within 10% of the pressure of the external environment.

[0069] Referring to FIG. 6 , an embodiment of the rotor 120 for further reducing the effects of friction between the rotor 120 and the fluid 110 is shown. As shown, the rotor 120 includes a rotor 122 similar to the rotor 80 shown in FIG. 5 . The rotor 120 further includes a first shell layer 123 and a second shell layer 124 that house the rotor 122 and are coupled to the shaft 100 via a pair of a first bearing 125 and a second bearing 125. While two shell layers are shown in the example of FIG. 6 , it can be understood that any number of shell layers can be used. By using the pair of the first bearing 125 and the second bearing 125, the first shell layer 123 and the second shell layer 124 are configured to freely rotate around the rotor 122 about an axis of rotation concentric with the shaft 100.

[0070] As can be seen, when rotor 120 is rotated by motor-generator 50, energy is stored in both rotor 122 and rotating shell layers 123, 124. Therefore, to minimize energy loss to the shell layers, it is desirable to minimize the mass of the shell layers and maximize the efficiency of bearings 125 and 126. For example, by using thin, low-mass shell layers (relative to the mass of rotor 122), energy loss to the shell layers can be less than 1% of the total stored energy.

[0071] The manner in which a shell layer reduces rotor friction is described below with reference to a single cylindrical shell layer evenly spaced between the stationary housing 20 and the rotating rotor 122. The single layer begins to rotate until fluid friction is equal on both sides, which occurs when the shell layer's speed is halfway between the rotor speed and the housing speed, i.e., half the rotor speed, if the shell layer is evenly spaced between the rotor and the housing. Because resistance increases with the square of the speed, if the fluid velocity difference experienced by the rotor 122 is halved, the use of a single shell layer reduces the resistance experienced by the rotor 122 by a factor of four. As can be seen, the equilibrium velocity of a single shell layer varies with position between the rotor surface and the housing. For example, the equilibrium velocity of a shell layer located close to the rotor surface will be faster than that of a shell layer located at the equilibrium distance between the rotor and the housing.

[0072] More specifically, skin friction resistance acts on the surface of the rotor 122 rotating in the fluid 110. This skin friction generates a torque τ on the rotor.

number

number

number

number

[0073] Considering a cylinder, friction force components can be written for both the side and top surfaces of the rotor.

number

number

number

[0074]

number

number

number

[0075] 6, 9A and 9B, the drag reduction system is comprised of freely rotating shell layers 123, 124. To understand this, we can consider the torque acting on the side of the cylindrical rotor, where, as derived above,

number

[0076]

number

[0077] Considering the case of one partition, τ 1,2 =τ 2,1 and τ 2,3 =τ 3,2 In the equilibrium state, τ 2,1 =τ 2,3 Setting the angular velocity of the housing to zero gives us the following equation:

number

[0078] Connecting these equations with an equation and rearranging them gives us

number

[0079] Using this value, the torque when one shell layer is present is given by:

number

[0080] This result shows how a shell layer produces a torque reduction factor. Using a similar approach, we can consider the two-shell case. Extending the equilibrium equation to τ 1,2 =τ 2,3 =τ 3,4 Then the torque is as follows:

number

[0081] Again, equating these equations and rearranging them twice gives us

number

[0082] Comparing the expression for ω2 with the single and double layer cases, a clear pattern emerges and a general form for n layers can be derived as follows:

number

number

[0083] As the fluid 110 rotates, a centripetal pressure is generated. In the stratified drag reduction system shown in Figure 6, each shell layer rotates at a different speed, with the innermost layer rotating fastest. This means that the centripetal pressure in the innermost layer is greater than the centripetal pressure in the surrounding layers, as well as in the outer layers; i.e., the centripetal pressures in each layer are not equal and each layer must maintain a pressure difference across the layer.

[0084] Typically, flywheel rotors are cylindrical, as shown in FIG. 5 , similar to known shell layers used with cylindrical rotors. Because pressure acts in all directions, the centripetal pressure creates a resultant force parallel to the axis of rotation (i.e., axis 100), which must be restrained 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, allowing each shell layer to resist the centripetal pressure through bending moments. However, this approach can be uneconomical in many scenarios due to the increased material costs required for each shell layer. To address this, the inventors discovered that instead of providing shell layers with planar top and bottom surfaces (defined as the surfaces of the shell layer intersecting the axis of rotation), providing curved top and bottom surfaces can absorb the centripetal pressure generated by tensile stresses in the shell layer material rather than bending moments, thereby reducing the material thickness and material costs for the shell layers. Furthermore, reducing the strength required for the shell layers in this way allows for the use of less expensive and more abundant materials. Such an arrangement is shown in FIG. 6.

[0085] Generally, improvements in shell layer strength and material costs are achieved by a shell layer whose cross section, taken in a plane containing the axis of rotation, defines a closed, generally convex shape with top and bottom surfaces each formed by a continuous curve. A shape is considered generally convex herein if the ratio of its enclosed interior area to the area of ​​the smallest convex set containing the shape is 70% or greater, preferably 80% or greater, and optimally 90% or greater. A set of points is convex herein if it contains all the line segments connecting any two points in the set. Alternatively, a cross section of a shell layer taken in the plane of the axis of rotation may define a closed, convex shape with top and bottom surfaces formed by continuous curves.

[0086] An example of the shell layer shape described above, which represents the optimum shape of the shell layer for maximizing strength while reducing material costs, is shown in Figure 7. This optimum shape is defined as follows:

number

number

[0087] To understand how this shape is derived, consider the stresses acting in both the azimuthal and polar directions in an asymmetric shell layer with unknown curvature shape h(r) with reference to Figures 10A and 10B. Figures 10A and 10B show the coordinates of the curved shell layer shape cross-sectioned in two different planes. Starting from the coordinate system of Figure 10A and taking an interval along the normal direction of the shell layer, we can derive the equilibrium equation that governs the stress in this direction. Its form is:

number

number

number

[0088] Referring to Figure 10B, the equation of equilibrium in this new orientation can be derived: The normal force on the shell layer must equal the normal centripetal pressure.

number

[0089] Again, substituting the above definitions, we get:

number

[0090] Using two other definitions using differential elements,

number

number

[0091] The optimum shape produces equal stresses in both directions according to these two equations. As stated above, the solution is obtained in the form:

number

[0092] While the shape shown in FIG. 7 has been found to be optimal for reducing material costs and maximizing strength, as discussed above, alternative shapes are possible. Four examples of such shapes are shown in FIGS. 8A-D. While these shapes are not optimal and require increased wall thickness and associated material costs during fabrication, they maintain the ability to contain the centripetal pressure generated as the internal fluid rotates and may be economical in certain scenarios. As shown, the cross-sectional shape of the shell layer in the plane of the axis of rotation can also form a circle, an ellipse, a slot (a shape defined by the set of all points equidistant from a line of a given length), and a convex polygon such as a rectangle. Alternatively, although not shown in FIGS. 8A-D, the shell layer may define a cylinder with dome-shaped top and bottom surfaces. It will be appreciated that the disclosed shell layer technology provides improved drag reduction for shell layers of any shape, even if certain shapes are less economical when centripetal pressure must be contained, as discussed above. Thus, as a further alternative, each shell layer may be a cylinder with flat top and bottom surfaces.

[0093] Returning to FIG. 6 , in the illustrated example, each shell layer 123 and 124 has a shape substantially identical to the optimal shape described above with reference to FIG. 7 . Similarly, rotor 122 defines an outer surface that matches the shape of innermost shell layer 123. This arrangement ensures that the rotor's mass and energy storage capacity are maximized for a given volume defined by innermost shell layer 123. However, this is not necessarily the case, and the rotor may have a different shape than that defined by the shell layers, since the structural requirements for the rotor are different from those of the shell layers. For example, the rotor may have a Laval disk shape with a radially reduced cross-sectional area to maintain the same stresses across the disk, while the shell layers may have the optimal shape described above to maximize shell layer strength while minimizing material costs.

[0094] Shell layers 123 and 124 may be constructed of an elastically deformable material and define a second shape at rest to reduce the strength and stiffness required of the shell layers while ensuring that the shell layers conform to a desired shape during use. This second shape can be selected such that when the shell layers rotate and operate under centripetal pressure generated by contained rotating fluid 110, the centripetal pressure causes the shell layers to deform from the second shape to the first desired shape. In this way, it is ensured that the shell layers define a desired shape when rotated without having to be constructed of an unnecessarily stiff material to prevent deformation due to the centripetal pressure generated.

[0095] The centripetal pressure generated by the fluid 110 increases as the rotational speed increases. Therefore, during slow rotation, such as when the flywheel is accelerating or decelerating, the centripetal pressure acting on the shell layer may be less than the force of gravity on the shell layer. In this situation, as shown by the dotted line in FIG. 11 , the shell layer may deflect toward the second shape and toward the rotor 122 due to the influence of gravity. As shown by points A and B in FIG. 11 , this deflection increases the risk of contact between the shell layer 124 and the rotor 122. Such contact causes friction between the rotor 122 and the shell layer 124, which may reduce the heat storage efficiency by converting rotational energy into heat. Furthermore, friction between the rotor and the shell layer may damage either or both the rotor 122 and the shell layer 124.

[0096] Therefore, in embodiments including multiple shell layers, several support configurations are described below that inhibit deformation of the shell layers toward the rotor or between a particular shell layer and its nested inner shell layer. Although several embodiments are described below with respect to shell layers and rotors, each of these is equally applicable to preventing contact between a particular shell layer and its nested inner shell layer.

[0097] FIG. 12 illustrates a first embodiment of a support structure for suppressing deformation of the shell layer 124. As illustrated, the support structure may include a portion 250 of the upper surface of the shell layer. In the embodiment illustrated in FIG. 12, the portion 250 includes a portion of the upper surface that is deflected toward the rotor 122 near the center of rotation. To accommodate this deflection, the shell layer bearing 126 is similarly moved along the axis 100 away from the rotor 122. As a result, a component parallel to the direction of gravity is generated in the portion 250, as indicated by arrow G. More precisely, when a tangent to a point on the surface of the portion 250 is resolved into orthogonal vectors parallel and perpendicular to G, it includes a non-zero component parallel to G. Therefore, the shell layer itself can support the weight of the shell layer as tension in the portion 250, thereby suppressing deformation of the shell layer toward the rotor. Alternatively, the portion 250 may include a portion of the shell layer that is deflected toward the rotor, thereby supporting the weight of the shell layer in compression.

[0098] 13A and 13B illustrate another embodiment of a support feature for suppressing deformation of the shell layer. As shown, the support structure may include a reinforcing portion 200 of the shell layer 124. As shown in FIG. 13A, the reinforcing portion 200 may include an increased thickness portion of the shell layer adjacent the bearing 126, thereby increasing the stiffness of the material and suppressing deformation of the shell layer 124 toward the rotor. As used herein, "thickness" is measured in a direction perpendicular to the surface of the shell layer. In the example shown in FIG. 13A, the reinforcing portion includes a uniform thickness portion; however, in other embodiments, the reinforcing portion may have a non-uniform thickness. For example, the thickness of the reinforcing portion 200 may decrease as the distance from the bearing 126 increases. Such a decrease in thickness may be linear or nonlinear. For example, the thickness of the reinforcing portion 200 may decrease in a stepwise manner or may decrease proportionally to the square, cube, or other nonlinear function of the distance from the bearing.

[0099] Additionally or alternatively, the reinforcement 200 may be constructed of a different material than the remainder of the shell layer, such that the material used for the reinforcement 200 is stiffer than the material of the remainder of the shell layer. As used herein, stiffness refers to the ability of a material to resist deformation under an applied force. For example, the reinforcement may be constructed of a composite material such as fiberglass or carbon fiber.

[0100] Additionally or alternatively, as shown in FIG. 13B , the reinforcement portion 200 may comprise a reinforcing structure bonded to the surface of the shell layer. In the illustrated example, the reinforcement portion 200 comprises one or more reinforcing plates 220 bonded to the top surface of the shell layer adjacent the bearing 126. The reinforcing plates may be made of the same material as the shell layer 124 or a different material. Accordingly, the reinforcing plates may increase the stiffness of the reinforcement portion 200 by increasing the thickness of the reinforcement portion or by being made of a material that is significantly stiffer than the remainder of the shell layer. Bonding the reinforcing plates 220 to the shell layer increases the stiffness of the reinforcement portion, thereby reducing deformation of the shell layer under gravity. Furthermore, as shown in FIG. 13B , the reinforcing plates may sandwich the shell layer by bonding to both the outer and inner surfaces of the shell layer adjacent the bearing 126. While the reinforcing structure is shown as a reinforcing plate in FIG. 13B , the reinforcing structure may take other forms. For example, the reinforcing structure may comprise reinforcing bars, beams, rods, trusses, or brackets bonded to one or more surfaces of the shell layer 124. Although the reinforcing structure is shown as being bonded to the surface of the shell layer, the reinforcing structure may additionally be embedded in the shell layer.

[0101] 14A and 14B illustrate another embodiment of a support structure, which comprises a portion of a shell layer having a different material structure than the remaining portions of the shell layer. As used herein, "material structure" refers to the macroscopic arrangement of material. As illustrated by FIG. 14A, the material structure may comprise a repeating cell structure. In the example of FIG. 14A, the repeating cell structure comprises a repeating pattern of hollow tubular hexagonal cells, e.g., a "honeycomb" structure. Providing a material with a repeating cell structure, such as that shown in FIG. 14A, can improve the stiffness of the material while minimizing the weight increase compared to a solid material of a corresponding thickness.

[0102] An example of a material suitable for use as a support structure having the repeating cellular structure shown in Figure 14A is shown in Figure 14B. As shown, a material 210 having the repeating cellular structure is sandwiched between a top plate 212 and a bottom plate 212. As shown in Figure 14B, the axes of the hexagonal tubes defined by the cellular structure shown in Figure 14A extend perpendicular to the top and bottom plates 212.

[0103] FIG. 15 illustrates a support configuration comprising a support structure. In the illustrated example, the support configuration comprises a support plate 300 coupled to the shaft 100 via a support bearing 310. FIG. 15 illustrates the shell layer 124 in a first shape, i.e., a shape during steady-state operation. As shown in FIG. 15, when the shell layer is in the first shape, it does not contact the support plate 300, allowing the support plate 300 and the shell layer 124 to rotate freely relative to each other. However, as 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 deflect toward the second shape due to gravity. As the shell layer 124 deflects, it is configured to contact the support plate 300, which supports the shell layer 124 and prevents further deformation toward the rotor 122. Thus, the support plate 300 prevents the shell layer from contacting the rotor 122. Furthermore, because the support plate 300 can rotate freely around the bearings 310, the plate 300 synchronizes with the speed of the shell layer 124 when it contacts the shell layer 124, thereby minimizing slippage and friction between the shell layer 124 and the plate 300.

[0104] 14 shows that the support plate 300 can freely rotate relative to the shell layer 124 via the bearings 310, in alternative embodiments, the support plate 300 may instead be coupled to the bearings 126 and thus configured to always rotate with the shell layer 124. Such an embodiment may ensure that there is no slippage or friction between the shell layer and the plate as the shell layer flexes and contacts the support plate 300.

[0105] Although FIG. 14 shows the support structure as a support plate, the support structure may take other forms, such as one or more support beams, rods, bars, trusses, or brackets.

[0106] 16 , in an alternative embodiment, the support features may comprise a pair of magnets configured to constrain deformation of the shell layer through magnetic repulsion between the pair of magnets. As shown, a first magnet of the pair may be coupled to the top surface of the rotor 122 while a second magnet is coupled to the top surface of the shell layer, such that the magnets face each other. In this embodiment, the magnets are polarized so that like poles face each other. Thus, as the shell layer 124 decelerates and begins to deflect under gravity, the first and second magnets move closer together and repel each other, thereby constraining further deflection of the shell layer 124 toward the rotor 122.

[0107] In the illustrated example, the first and second magnet pair 400 is shown as a ring magnet, with a cross-sectional view shown in FIG. 16. However, the first and second magnets may have any suitable shape. For example, each of the first and second magnets may be an arc magnet, a bar magnet, a disk magnet, a horseshoe magnet, a channel magnet, or a block magnet. Furthermore, the first and second magnets may have different shapes from each other. Furthermore, the magnet 400 may be a permanent magnet or, alternatively, may be a selectively actuable electromagnet.

[0108] Additionally, embodiments of the present invention may have multiple pairs of magnets, for example, multiple pairs of magnets may extend across the top surface of the rotor and the top surface of the shell layer to restrain the shell layer from deforming toward the rotor along the entire length of the top surface.

[0109] Finally, as shown in Figure 17, the shell layer may alternatively or additionally be configured to resist deformation toward the rotor under the influence of gravity by arranging its rotation axis perpendicular to the direction of gravity. For example, the axis 100 may be coupled to the housing 20 so that the axis 100 is parallel to the plane defined by the mud mat 32. By positioning the rotor in this manner, the shell layer acts as an arch, and the weight of the shell layer can be supported by compression across the curved surface of the shell layer.

[0110] While the support configurations described above are illustrated in Figures 11-17 for shell layers having top and bottom surfaces that include continuous curves, it will be understood that the support configurations can be applied to shell layers of other shapes as well. For example, the support configurations described above can be applied to each of the shell layer shapes shown in Figures 8A-8D as well as alternative cross-sectional shapes such as other convex polygons, e.g., rectangles. Similarly, the support configurations can be applied to cylindrical shell layers with dome-shaped top and bottom surfaces and cylindrical shell layers with planar top and bottom surfaces.

Claims

1. a housing, the housing being sealed to separate an interior of the housing from an external environment; a rotor mounted on a shaft within the housing, the rotor configured to be rotatable relative to the housing, the rotor being made of a low tensile strength material; and a fluid contained within the housing for compressing the rotor; means for maintaining the pressure of said fluid in a state of approximate equilibrium with the pressure of the external environment; A flywheel energy storage device comprising:

2. a housing, the housing being sealed to separate an interior of the housing from an external environment; a rotor mounted on a shaft within the housing, the rotor configured to be rotatable relative to the housing; a fluid contained within the housing for compressing the rotor; means for maintaining the pressure of the fluid at approximately equilibrium with or higher than the pressure of the external environment; wherein the housing is a pressure vessel that can withstand only a pressure lower than the pressure of the fluid.

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

4. The energy storage device of any one of claims 1 to 3, wherein the housing is constructed from a material with low tensile strength.

5. The energy storage device according to any one of claims 1 to 4, 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. The energy storage device according to any one of claims 1 to 5, wherein the rotor is made of ceramic.

7. 6. The energy storage device of claim 1, wherein the rotor is made of a granular material enclosed in a sealed container, the pressure in the container being maintained at a pressure lower than the pressure of the fluid.

8. 8. The energy storage device of claim 1, wherein the means for maintaining pressure comprises one or more of a pressure compensator coupled to the pressure of an external environment, and a valve mechanism coupled to a source of pressurized fluid.

9. 9. The energy storage device of claim 1, wherein the housing is sealed using one or more seals to separate seawater from the fluid.

10. 10. An energy storage device according to any preceding claim, wherein the fluid is a low density fluid, optionally having a density at operating pressure that is less than one third of the density of the rotor.

11. 11. The energy storage device of claim 1, further comprising an electrical connector comprising conductors for electrical communication with the energy storage device, the electrical connector comprising means for isolating the conductors from seawater, optionally comprising one or more of a seal and a dielectric fluid.

12. 12. The energy storage device of any one of claims 1 to 11, further comprising control electronics and means for isolating the control electronics from the external environment, optionally wherein the means for isolating the control electronics comprises a sealed air-filled canister for protecting the control electronics from seawater.

13. The energy storage device of any one of claims 1 to 12, further comprising a lifting attachment for lowering the energy storage device to a submerged location.

14. 14. The energy storage device of any one of claims 1 to 13, wherein the housing is configured to be connected to a foundation anchor to support and stabilise the energy storage device in position, and optionally the foundation anchor is one or more of a mud mat, a caisson or one or more piles.

15. 15. The energy storage device of claim 1, further comprising a shell layer that houses the rotor and is coupled to the shaft via a bearing, the shell layer having a first shape, and the bearing allows the shell layer to rotate around the rotor about an axis of rotation that is concentric with the shaft.

16. 1. A rotor for use in a flywheel energy storage device, comprising: A rotor, a shaft coupled to the rotor; a shell layer covering the rotor and coupled to the shaft via a bearing, the shell layer having a first shape, the bearing allowing the shell layer to rotate around the rotor about an axis of rotation concentric with the axis, a cross section of the shell layer, when drawn in a plane in which the axis of rotation lies, defining a closed, generally convex shape having upper and lower surfaces that intersect the axis of rotation, each of the upper and lower surfaces including a continuous curve; A rotor comprising:

17. 1. A rotor for use in a flywheel energy storage device, comprising: A rotor, a shaft coupled to the rotor; a shell layer covering the rotor and coupled to the shaft via a bearing, the shell layer having a first shape, the bearing allowing the shell layer to rotate around the rotor about an axis of rotation concentric with the axis, the shell layer defining a closed three-dimensional generally convex shape having upper and lower surfaces that intersect the axis of rotation, each of the upper and lower surfaces including a portion having a continuous curvature; A rotor comprising:

18. 18. An energy storage device as claimed in claim 15 or a rotor as claimed in claim 16 or 17, comprising a plurality of concentric nested shell layers, each shell layer having a corresponding first shape, each shell layer comprising a bearing coupled to the shaft, the bearings configured to enable the shell layer to rotate about the rotor and relative to each of the other shell layers.

19. 20. 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 shell layer is one of a sphere, an ellipsoid, and a cylinder with dome-shaped top and bottom surfaces.

22. Energy storage device according to claim 15 or a rotor according to any one of claims 16 to 21, wherein the shape of the rotor corresponds to the first shape of the shell layer.

23. 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 has a second shape at rest, and the shell layer is configured to deform to a first shape upon rotation.

24. 24. The energy storage device or rotor of claim 23, further comprising a support feature configured to restrain the shell layer from deforming toward the rotor.

25. 25. The energy storage device or rotor of claim 24, wherein the support form includes a portion of an upper surface of the shell layer, the portion of the upper surface of the shell layer extending away from the rotor for coupling to the bearing.

26. 26. An energy storage device or rotor according to claim 24 or 25, wherein the support features comprise reinforcements for the shell layer.

27. The reinforcing portion of the shell layer is a portion where the thickness of the shell layer is increased; a portion of the shell layer that is composed of a material different from the remainder of the shell layer; a reinforcing structure bonded to a surface of the shell layer; 27. An energy storage device or rotor according to claim 26, comprising one or more of:

28. An energy storage device or rotor according to any one of claims 24 to 27, wherein the support form comprises a portion of the shell layer having a different material structure to the remainder of the shell layer.

29. 30. The energy storage device or rotor of claim 28, wherein the portion of the shell layer having the different material structure comprises material formed into a repeating cell structure.

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

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

32. 32. An energy storage device or rotor according to any one of claims 24 to 31, wherein the support form comprises a pair of magnets configured to inhibit deformation of the shell layer towards the rotor due to magnetic repulsion between the pair of magnets.

33. An energy storage device or rotor according to any one of claims 24 to 32, wherein the support form is located near the axis of rotation.

34. The flywheel electric energy storage device according to any one of claims 1 to 15, wherein the rotor is a rotor according to any one of claims 16 to 33.

35. A method of generating electricity using a flywheel energy storage device according to any one of claims 1 to 34, comprising the steps of: using a rotor to rotate a generator coupled to the shaft of the storage device.

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