HYDROPNEUMATIC FLYWHEEL FOR ENERGY STORAGE

Description

[0001] The invention relates to a flywheel comprising a hollow rotating body with a rigidly connected shaft along the axis of rotation of the hollow body and means for mechanically coupling the shaft to a rotational energy generator and / or consumer. In particular, the invention relates to a flywheel with a variable moment of inertia, which is varied by the displacement of liquid flywheel mass.

[0002] Such a flywheel can be used for energy storage, for example in electrical power grids. With the increasing shift towards non-fossil and non-nuclear energy generation, conventional power plants are being replaced by renewable energy generators. Most renewable energy is generated by photovoltaic and wind power plants. These two technologies feed their energy into the electrical grid via frequency converters. Other renewable energy generators feed their energy into the grid via rotating generators directly connected to the grid, for example in hydroelectric power plants, biomass power plants, geothermal power plants, and tidal power plants.

[0003] Renewable energy sources are more or less volatile, meaning they are not always available with the same output, and fluctuations in output are not always predictable. Similar volatility exists on the consumer side. In an alternating current (AC) grid, the generated power must equal the consumed power at all times. If this power balance is not maintained, the frequency of the AC voltage changes. Maintaining a constant AC grid frequency is commonly referred to as ensuring grid frequency stability. This requires energy storage systems that can quickly absorb excess energy and release it again quickly when there is a shortage due to increased load.

[0004] A special form of energy storage is the flywheel energy storage system. When mechanically coupled to an AC generator, its inertia can provide what is known as instantaneous reserve power. To eliminate imbalances between generation and consumption in an AC grid, control mechanisms are employed. The fastest control mechanism is primary control, which uses rapidly adjustable power plants to attempt to restore equilibrium. However, primary control is not available at all times and requires a short start-up time. The virtually instantaneous reserve power plant serves to bridge this gap. It is essential to prevent the grid frequency from deviating from normal levels until primary control power plants have adjusted their output.

[0005] One special application of flywheel energy storage is the uninterruptible power supply (UPS) of computer systems in the event of a power outage until emergency generators start.

[0006] Among the well-known advantages of flywheel energy storage systems are their ability to provide significant power for short periods and their virtually unlimited cyclability, meaning that the charging and discharging of flywheels with energy can be repeated any number of times. The known disadvantages lie in the relatively small amount of energy that can be stored per unit of flywheel mass, the resulting high storage costs, the very high self-discharge rate due to friction (up to 15% per hour), and the possibility of catastrophic bursting at high energy levels and material overload. In their potential use in vehicles, the undesirable occurrence of torques perpendicular to the axis of rotation (leading to pitching and rolling movements of the axis of rotation), resulting from the conservation of angular momentum, can also arise.

[0007] Document US 2005 / 077881 A1 describes a grid frequency stabilization system using stationary flywheels, each with a fixed flywheel mass and a known moment of inertia. The system typically consists of multiple flywheels, each of which must increase or decrease its rotational speed to absorb or release energy, respectively. The system includes a control device that monitors the grid state and the charge level of each flywheel and periodically determines which flywheel should be charged and which should be discharged to compensate for fluctuations in the grid.

[0008] Systems based on US 2005 / 077881 A1 are implemented and functional in practice. They require a relatively large number of flywheel energy storage systems located at the same site ("flywheel plant") and that all can be continuously monitored and controlled without time delay.

[0009] If one wants to operate a single flywheel or a large number of flywheel energy storage systems distributed over large areas without them being able to communicate with a control device or with each other, then flywheels with variable inertia (VI) coupled to an AC machine are suitable because they have the fundamental ability to absorb or release energy at a quasi-constant rotational speed. Depending on the number P of pole pairs in the AC machine, a quasi-constant rotational speed of, for example, 3000 / PU / min can ensure that electrical energy is supplied or absorbed at the desired grid frequency f = 50 Hz.

[0010] A proposal for guaranteeing grid frequency stability using VI flywheels can be found in publication WO 2014 / 161030 A1. This publication primarily proposes fixed mass elements whose distance from the axis of rotation can be controlled and varied, allowing the flywheels' moments of inertia to be adjusted. A liquid flywheel mass in a parabolically shaped cavity within the flywheel is also discussed. This liquid remains in the flywheel even when at rest, filling the lower portion of the paraboloid cavity. As the flywheel begins to rotate, the liquid is accelerated towards the outer wall of the cavity and simultaneously rises along this wall. The moment of inertia is increased by this mass displacement.However, the publication provides only very vague information about actual control of the mass distribution of the fluid - especially perpendicular to the axis of rotation; it generally refers to pressure or mechanical means.

[0011] The Van de Ven flywheel described in US patent 8590420 B2 does not specify a predetermined amount of fluid flywheel mass, but allows the flywheel to be filled from a supplied fluid mass reservoir at normal pressure (1013 hPa). As described in Fig. 1 As sketched, the flywheel consists of a cylindrical hollow body (10) in which two gas-tightly separated cavities (30, 40) are arranged one behind the other along the axis of rotation. The partition (50) between the cavities (30, 40) serves as a movable (indicated by arrows on the right of the image) media separator, which is gas-tight and sealed against the inner wall of the cylinder. The volumes of the two cavities (30, 40) are thus variable and only their total volume is constant. A shaft (20) is rigidly connected to the hollow body (10) along the axis of rotation of the hollow body. The shaft (20) is interrupted here, i.e., it does not run as a whole along the axis of rotation, because otherwise it would also have to pass through the movable partition (50) in a gas-tight manner. The shaft (20) serves for the rotatable mounting of the hollow body (10) and for the mechanical coupling to a rotational energy generator and / or consumer (not shown).For coupling, the shaft (20) generally has suitable means, for example at least in sections a hexagonal outer profile or a mounted gear or the like (not shown).

[0012] Specifically, in Van de Ven's invention, a fluid-exchanging channel (60) is provided between the fluid mass reservoir (not shown) and the first cavity (30). A return spring is installed in the second cavity (40), which exerts a force on the partition (50) in the direction of the first cavity (30). This force increases when the partition (50) is moved to reduce the size of the second cavity (40). In his preferred embodiment, Van de Ven implements the return spring by means of a volume of gas enclosed in the second cavity (40), which is under a predetermined elevated gas pressure (symbolized by the P in the circle in the sketch).

[0013] Van de Ven demonstrates in his description that the flywheel of the Fig. 1 The flywheel (10) automatically maintains a stable rotational speed. If additional energy is supplied to the flywheel (10) via the shaft (20), causing an infinitesimally small increase in rotational speed, the pressure of the fluid in the first cavity (30) on the partition (50) increases, particularly at its edge where the fluid has the highest velocity. The partition (50) is displaced towards the second cavity (40), reducing its size. This increases the gas pressure in the second cavity, while fluid is drawn into the first cavity (30) from the reservoir via the channel (60). The supplied energy is simultaneously stored in two forms: (i) as additional kinetic energy in the flywheel storage system, whose moment of inertia is increased by the inflow of fluid at a quasi-constant rotational speed, and (ii) as potential energy in the reduced cavity (40) with increased gas pressure.Energy is extracted in the reverse manner: a reduction in fluid pressure in the first cavity (30) is coupled with a simultaneous gas expansion in the second cavity (40), thus transporting fluid through the channel (60), similar to a syringe. The inherently very simple, self-regulating rotational speed concept of US 8590420 B2, with a liquid flywheel working against a gas pressure to simultaneously store kinetic and potential energy, can also be described as a hydropneumatic flywheel. However, Van de Ven's design has some significant drawbacks: the fluid fills the entire first cavity (30) and rotates as a whole. This requires a much larger fluid mass to generate a given moment of inertia than if the fluid mass were rotating only near the cavity wall. Furthermore, the system is susceptible to pressure-induced deformation.The wall of the hollow cylinder (10) must be very rigid, otherwise the pressure of the rotating fluid will cause radial expansion, leading to leakage at the seals of the partition (50). Avoiding radial expansion in hydropneumatic piston accumulators – as a comparative example – requires an economically undesirable expenditure of material. At the seals of the partition (50), the pressure is always higher on the fluid side than on the gas side, since the centrifugal forces, which increase with the radius, cause the fluid pressure to rise parabolically from the axis of rotation. The gas pressure on the other side of the partition (50), however, is independent of the radius. Thus, the seal is subjected to a pressure gradient on one side only and continuously. To reduce fluid leakage, tightly fitting seals must be used, which generate significant friction and cause wear of the seals after a very large number of load cycles.This results in a very high maintenance effort, and fluid leakage will be almost unavoidable with a permanently charged flywheel.

[0014] Van de Ven also suggested arranging guide vanes inside the first cavity (30) to prevent energy losses due to fluid slippage. This is clearly technically problematic with a movable partition (50) that is supposed to be able to expel all the fluid from this very cavity (30) without leaking. Furthermore, the acceleration of a fluid particle on its way from the channel (60) at the center of rotation to an unpredictable location within the first cavity (30) leads to slippage and turbulence in the fluid. These effects inevitably cause additional energy losses with every change in the state of charge.

[0015] The invention aims to further develop the concept of the hydropneumatic flywheel in such a way as to avoid the aforementioned disadvantages.

[0016] The problem is solved by a flywheel comprising a hollow rotating body (10) with a rigidly connected shaft (20) along the axis of rotation of the hollow body (10) and means for mechanically coupling the shaft (20) to a rotational energy generator and / or consumer, wherein the hollow body (10) encloses at least a first cavity (30) and a second cavity (40) with cavity walls formed by the hollow body (10), wherein the cavities (30, 40) are separated from each other by a partition (50) and are arranged one behind the other along the axis of rotation, wherein the shaft (20) has at least a section of a channel (60) for fluid exchange communication between a reservoir with fluid flywheel mass and the first cavity (30), wherein the second cavity (40) is designed to accommodate a gas overpressure, characterized in that a. the partition wall (50) is immovably and gas-tightly connected to the hollow body (10) and b. a plurality of channels (90) for fluid exchange communication between the first cavity (30) and the second cavity (40) are arranged such that c. the channels (90) open into the areas of the cavity walls furthest from the axis of rotation.

[0017] The dependent claims specify advantageous embodiments.

[0018] The invention is explained below with the aid of figures. These show: Fig. 1 a sketch of the VI flywheel according to Van de Ven (state of the art); Fig. 2 a sketch of a VI flywheel according to the present invention; Fig. 3 a sketch of an alternatively designed VI flywheel according to the invention.

[0019] The invention achieves the desired improvement by recognizing that a movable partition (50) as a media separator is fundamentally disadvantageous and also unnecessary if the compressed gas in the second cavity (40) can only come into contact with fluid that has a correspondingly high fluid pressure due to its rotational speed.

[0020] As in Fig. 2 As shown, the fluid preferentially covers the fastest-moving region of the inner wall of cavity (40) when the fluid enters the second cavity (40), which Van de Ven intended for gas only. The fluid is separated from the gas by its mass due to centrifugal force; that is, no separate media separator is required. In fact, the surface of a fluid mass rotating about an axis running along the direction of gravity forms a paraboloid. However, even at relatively low rotational speeds, e.g., a few hundred rpm, and radii of a few centimeters, the parabolic curve is so steep that one can only approximate a flat surface as shown in Fig. 2 It can turn out to be a bad sign.

[0021] The channels arranged according to the invention between the wall regions of the cavities (30, 40) that are furthest from the axis of rotation are open and can be passed freely by the fluid, but not by the compressed gas. In this way, energy can be supplied, as in the Van de Ven design, by increasing the fluid mass through inflow from a fluid mass reservoir via the channel (60) in the shaft (20), initially into the first cavity (30). According to the invention, the volume of the first cavity (30) is now fixed because the partition (50) is immovably and gas-tightly connected to the hollow body (10). However, fluid flowing in can nevertheless pass through the open channels (90) from the outer edge of the first cavity (30) into the outer edge of the cavity (40), where, due to the rotation of the hollow body (10), it immediately spreads along the cavity wall at the greatest possible distance from the axis of rotation, i.e., from the shaft (20).The gas volume is reduced in proportion to the fluid volume supplied, i.e. the gas pressure in the second cavity (40) increases.

[0022] Due to the free fluid exchange according to the invention via open channels, which connect the fastest-moving areas of the cavity walls of the cavities (30, 40), only a relatively small volume of the first cavity (30) is required. Preferably, its volume is less than one-tenth of the volume of the second cavity (40). The function of the first cavity (30) includes that of an acceleration section for the fluid entering the shaft (20) from the channel (60) and that of supplying the fluid to the outer high-speed area. A fixed, small volume of the cavity (30) avoids unnecessary, slowly rotating fluid inertia in the system and thus reduces slippage and vortex formation. Moreover, a greater moment of inertia is achieved with less fluid inertia.

[0023] The partition (50) between the cavities (30, 40) is rigidly and gas-tightly connected to the hollow body of revolution (10), i.e., it is, for example, welded, fused, or bonded to the inner walls of the hollow body (10), and gas transmission from the second cavity (40) to the first cavity (30) through the partition is impossible. In particular, the partition (50) is designed to withstand a pressure gradient between the cavities (30, 40). The partition (50) stiffens the hollow body (10) and thus counteracts radial expansion. The partition (50) is reversibly expandable within limits, and limited radial expansion of the hollow body (10) in conjunction with the partition (50) can also occur without any negative impact on the gas tightness of the partition (50).

[0024] Since the flywheel according to the invention comprises no moving parts in the interior, maintenance requirements are minimal. Furthermore, guide vanes can easily be arranged on the inner walls of the cavities, ensuring better fluid flow during the rotation of the hollow body (10). The guide vanes can be designed as radial baffles extending through the entire first cavity (30) from the cavity wall to the axis of rotation with the channel (60). The shaft (20) preferably extends completely through the cavities (30, 40) and is flush with the baffles. The guide vanes can also be provided as radial baffles in the second cavity (40) and, by being attached to the continuous shaft (20), further stiffen the hollow body (10).

[0025] Furthermore, it is advantageous to match the number and arrangement of the fluid-exchanging channels (90) to the number and arrangement of the guide vanes. Preferably, the majority of channels (90) for fluid exchange between the first (30) and the second cavity (40) are designed as pipe connections distributed symmetrically to the axis of rotation. Each pipe connection can then open between two guide vanes in both cavities (30, 40), with at least one pipe connection assigned to each area between two guide vanes.

[0026] The guide plates can be made of metal, but are not limited to this. In principle, it appears advantageous if the guide plates are manufactured from the same material as the rotating hollow body (10). This could, for example, be a high-tensile-strength polymer composite, such as a fiber-reinforced composite.

[0027] As already mentioned, a larger moment of inertia can be achieved with less fluid mass. This also implies a lower compression of the gas in the second cavity (40) compared to Van de Ven. To nevertheless achieve a sufficient restoring force on the fluid surface, the gas pressure can be adjusted, in particular increased. It is generally advantageous if the gas pressure in the second cavity (40) can be changed, or at least increased, during operation of the flywheel. A preferred embodiment of the flywheel therefore provides – as in Fig. 2 The drawing shows that the shaft (20) has at least a section of a channel (70) that can be closed off by means of a valve (80) for gas exchange communication between the second cavity (40) and a compressor and / or gas overpressure reservoir, for example a gas cylinder. Setting a predetermined gas pressure in the second cavity (40) sets an operating point for the automatic speed stabilization of the flywheel, thus determining in particular the speed to be maintained autonomously. The valve (80) can be designed to be controllable in order to be able to regulate the gas pressure transiently. It does not have to be, as in Fig. 2 The valve (80) is not shown to be integrated into the flywheel structure, but can be located at any point in the channel (70) or on the supply lines to the channel (70). If a control mechanism for the valve (80) is not desired, it can preferably be designed as a check valve that shuts off the gas flow from the second cavity (40). In this case, integrating the valve (80) into the shaft (20) is preferable so that the channel (70) and the supply lines do not need to be kept under pressure. Refilling the second cavity (40) with gas through the channel (70) may occasionally be necessary to maintain the gas pressure, because the fluid can absorb small amounts of gas and carry them away when the flywheel's charge level changes.

[0028] A suitable fluid for the hydropneumatic flywheel is a mixture of water and glycol with a density of approximately 1000 kg / m³. Fluids with about three times the density are also available, but require suitability testing.

[0029] In this description, the term "hollow body of revolution" should not be understood strictly mathematically as a hollow body of revolution. In fact, mathematical solids of revolution such as cylinders, ellipsoids, or truncated cones are most preferably considered for the hollow body (10) and the cavities (30, 40) it encloses, if only to avoid any imbalance in the flywheel. Preferably, however, the majority of channels (90) for fluid exchange between the first (30) and the second cavity (40) are designed as pipe connections (90) and arranged in a rotationally symmetric distribution with N-fold natural numbers (N).

[0030] As already mentioned, a small volume of the first cavity (30) is definitely advantageous. It is therefore also advantageously possible to design the first cavity (30) in a star shape with the N-fold rotational symmetry of the arrangement of the pipe connections (90). In the extreme case, the first cavity (30) is even enclosed only by linear pipes extending radially outwards from the channel (60), which then seamlessly continue into the channels (90), which in turn open into the second cavity (40).

[0031] The Fig. 3 sketches such a structure with a hollow body of revolution (10) composed of a hollow ellipsoid of revolution (40) and a star-shaped tube assembly (30). If one imagines in Fig. 3 If the pipe connections (90) are removed and the open walls are closed in thought, then the immovable partition (50) between the cavities can be seen as consisting of the underside of the ellipsoid (10) and the pipe walls of the rod, possibly including the air gap between them.

[0032] The device of Fig. 3 It clearly works according to the same principle as the Fig. 2 However, it differs from the prior art in that any arbitrary body of revolution can be used to shape the second cavity (40). This would be impossible with a movable media separator. Fig. 3 The shaft (20) is also exemplarily made from a single piece and extending through the entire flywheel. This offers additional possibilities for providing cross braces in the cavity (40) to support dimensional stability during rotation. As already mentioned, the continuous shaft (20) could also be used in the device of the Fig. 2 They can also be used with additional bracing or bulkheads in the first cavity (30).

[0033] Another advantageous embodiment can consist of arranging the reservoir with the fluid flywheel in a cavity inside the shaft (20). The diameter of the shaft (20) is—especially within the second cavity (40)—a selectable design parameter. In this way, sufficient space can be provided for a fluid flywheel that already rotates at a relatively small radius while still remaining in the reservoir. The reservoir can also be designed, or alternatively, such that the fluid level extends above the top of the hollow body (10), at least when the flywheel is in a low-energy state. In this case, the channel (60) provided for fluid exchange between the reservoir and the first cavity (30) runs entirely within the shaft (20) and—unlike in the figures—has no opening to the outside.Instead, openings can be provided in the shaft (20) for venting the reservoir, ensuring that the fluid remains at normal pressure regardless of the reservoir's fill level. The advantage of this arrangement is that no seals, rotary unions, or moving parts are required when the system is limited to self-regulating operation.

[0034] Depending, if at all, on the arrangement and ventilation of the fluid mass reservoir, the flywheel according to the invention—like that of Van de Ven—does not require the axis of rotation to be aligned along the direction of gravity. This is a significant limitation in most known VI flywheels with fluid mass, which is unnecessary here because the force driving the fluid back inwards originates from the gas pressure in the second cavity (40). The devices sketched in the figures would all function just as well even if turned upside down.

[0035] Finally, it should be noted that the flywheel according to the invention can preferably be operated with an electronic control system that controls the valve (80) for regulating the gas pressure in the second cavity (40) and / or a pump (not shown) arranged in the channel (60) for regulating the flow rate of the fluid mass. Although the flywheel is intrinsically speed-stable at a given operating point, it is desirable, on the one hand, to reach this operating point as quickly as possible when starting the flywheel and, on the other hand, to be able to readjust it precisely if necessary.Moreover, such a control system, in conjunction with precise speed measurement – ​​for example, optically via a stroboscope – can detect the effects of leaks even without sensors inside the flywheel and either compensate by adjusting or – in the case of massive leaks – support the emptying of the flywheel in order to quickly reduce the rotating mass and avoid catastrophic damage.

Claims

1. A flywheel comprising a rotating hollow body (10) having a rigidly connected shaft (20) along the axis of rotation of the hollow body (10) and means for mechanical coupling of the shaft (20) to a generator and / or consumer of rotational energy, wherein the hollow body (10) encloses at least one first hollow space (30) and one second hollow space (40) with hollow space walls formed by the hollow body (10), wherein the hollow spaces (30, 40) are separated from one another by a dividing wall (50) and are arranged one behind the other along the axis of rotation, wherein the shaft (20), at least in sections, has a channel (60) for fluid-exchanging communication between a reservoir with a fluidic flywheel mass and the first hollow space (30), wherein the second hollow space (40) is designed to receive an excess of gas pressure, characterized in that a. the dividing wall (50) is connected to the hollow body (10) in an immovable and gas-tight manner, and b. a plurality of channels (90) are arranged for fluid-exchanging communication between the first hollow space (30) and the second hollow space (40) in such manner that c. the channels (90) respectively lead into the areas of the hollow space walls with the greatest distance to the axis of rotation.

2. The flywheel according to claim 1, characterized in that the first hollow space (30) has less than one tenth of the volume of the second hollow space (40).

3. The flywheel according to any one of the preceding claims, characterized in that the shaft (20), at least in sections, has a channel (70), which can be shut off by means of a valve (80), for gas-exchanging communication between the second hollow space (40) and a compressor and / or an excess gas pressure reservoir.

4. The flywheel according to claim 3, characterized in that the valve (80) is designed as a check valve, which shuts off the gas flow out of the second hollow space (40).

5. The flywheel according to any one of the preceding claims, characterized in that the plurality of channels (90) for fluid-exchanging communication between the first hollow space (30) and the second hollow space (40) are designed as pipe connections and are arranged in an N-numbered rotationally symmetrical distribution.

6. The flywheel according to claim 5, characterized in that the first hollow space (30) is designed star-shaped with the N-numbered rotational symmetry of the arrangement of the pipe connections (90).

7. The flywheel according to claim 6, characterized in that the first hollow space (30) is enclosed by linear pipes extending from the channel (60) in a radially outward direction.

8. The flywheel according to any one of the preceding claims, characterized in that the reservoir with the fluidic flywheel mass is arranged in a hollow space inside the shaft (20).