Flywheel-based energy storage system and method of operating such a flywheel-based energy storage system

Abstract

The present invention relates to a flywheel-based energy storage system (FESS), comprising: • a first rotor module (1) that comprises: • a shaft (5); • at least one vacuum chamber (4) that houses one or more than one rotor (3) that defines a flywheel, wherein the one or more than one rotor is rotatably arranged about a rotation axis (R) that is defined by the shaft extending through opposite walls of the rotor module; • at least two vacuum rotary seals (6) that are arranged at the opposite walls of the rotor module and around the shaft to thereby allow a vacuum inside the vacuum chamber to be maintained during rotation of the one or more than one rotor; and • at least two bearings (7) supporting the shaft on opposite sides outside the vacuum chamber; and • a motor-generator (2) that is rotatably connected to the shaft via a motor-generator-coupling (8), wherein both the motor-generator and the motor-generator-coupling are arranged outside the vacuum chamber of the first rotor module, wherein the system comprises: • one or more than one further rotor module, wherein said further rotor module is similar as defined for the first rotor module; and • wherein the shafts of adjacent rotor modules are coupled by a shaft-coupling (12) that is configured to transfer torque. The invention further relates to a method of operating such a FESS.

Description

[0001] Title: Flywheel-based energy storage system and method of operating such a flywheel-based energy storage system

[0002] Description:

[0003] The present invention relates to a flywheel-based energy storage system, as well as to a method of operating such a flywheel-based energy storage system.

[0004] Different types of energy storage systems exist, each having their own specific advantages and disadvantages regarding aspects such as energy density, power density, lifespan, etc. Energy density refers to the amount of energy a storage system can hold relative to its weight or volume, typically expressed in watt-hours per kilogram [Wh / kg] or watt-hours per cubic meter [Wh / m3]. It indicates how much energy can be stored for a given mass or space. Power density measures how fast a storage system can transfer energy, expressed in watts per kilogram [W / kg] or watt-hours per cubic meter [Wh / m3]. It reflects the rate at which stored energy can be released or charged, indicating how fast the system can provide power. In short, energy density is related to how much energy can be stored, while power density is related to how fast that energy can be transferred.

[0005] Types of energy storage systems are for example batteries, supercapacitors, and flywheel-based energy storage systems (FESS), hereafter also abbreviated as FESS.

[0006] In batteries, energy density is high, meaning they can store large amounts of energy relative to their weight or size, making them ideal for long-term energy storage, such as in electric vehicles. However, their power density is lower, limiting the rate at which energy can be delivered or absorbed, leading to slower charge / discharge times. The operating cycle of energy stored in batteries is typically in the domain of days to weeks, or even longer.

[0007] Supercapacitors, on the other hand, have high power density, allowing them to fast deliver or absorb energy in short bursts, ideal for rapid cycling applications like regenerative braking. However, their energy density is significantly lower than batteries, so they can’t store much energy over extended periods. The operating cycleof energy stored in supercapacitors is typically in the domain of milliseconds to seconds.

[0008] Flywheel-based energy storage systems store energy in the form of rotational kinetic energy. Electrical energy is fed to a motor that accelerates a rotor or disc. The higher the speeds, the more kinetic energy is stored. When energy is needed, the rotor slows down, and the kinetic energy is converted back to electrical energy through a generator. Such flywheel-based systems can be positioned in between batteries and supercapacitors, with higher power density than batteries and moderate energy density. They can deliver energy rapidly and handle frequent cycling with minimal wear, but they store less total energy compared to batteries. Flywheels are therefore excellent for applications requiring both quick response times and high amounts of cycles, such as grid stabilization, but they are not as efficient for long-term energy storage due to energy loss over time. Grid stabilization may be used for grid peak shaving, for example to temporarily store excess energy of solar systems during daytime when the sun shines, to allow the stored energy to be used in between sunset and dawn. The operating cycle of energy stored in such flywheel-based energy storage systems is typically in the range of minutes up to 12 hours.

[0009] Compared to batteries and supercapacitors, FESS are advantageous in scenarios where high power and short-duration storage are needed, when rapid cycling is required, and / or when high reliability and longevity are prioritized. After all, FESS can discharge large amounts of energy in short time frames, and they suffer minimal wear over many charge-discharge cycles. Moreover, FESS are durable and require little maintenance.

[0010] FESS are designed to obtain a round-trip efficiency that is as high as possible. This round-trip efficiency refers to the ratio of the total energy retrieved from the system during discharge to the total energy input during charging. Round-trip efficiency is thus a measure for how effectively the FESS converts electrical energy into kinetic energy (while spinning the flywheel) and back into electrical energy when needed. For example, if a FESS is charged with 100 kWh of energy and later delivers 90 kWh back, the round-trip efficiency would be 90%.

[0011] Losses in efficiency of a FESS typically arise from friction in the bearings, air resistance, and electrical conversion losses. Modern FESS therefore aim to minimize these losses through the use of advanced technologies like magneticbearings to obtain active magnetic levitated flywheels, and vacuum chambers. In this way round-trip efficiencies in the range of 80-90% can be reached. As a result, FESS systems are highly customized, and often tailor-made for specific applications, requiring a lot of engineering efforts for each individual FESS.

[0012] United States patent application US 2014 / 0026564 A1 of Caterpillar Inc. is considered to define the closest prior art. It discloses a flywheel assembly for a turbocharger. The primary purpose of this prior art system is to solve the problem of "turbo lag" by providing a means to rapidly add or remove energy from the turbocharger shaft during transient engine loads. To achieve this, it discloses a single flywheel assembly housed in its own vacuum chamber, separate from the high-temperature environment of the turbocharger. This assembly includes a rotor on a shaft supported by bearings external to the vacuum chamber, and it is connected to an external motor-generator. Relative to US 2014 / 0026564 A1, at least the characterizing features of claim 1 are novel.

[0013] The Chinese Utility Model CN 222141457 U, that discloses a gravity energy storage power recovery system based on an inertia flywheel, is acknowledged as further prior art. This system uses a single flywheel in a vacuum chamber to absorb and release kinetic energy during the start-up and shutdown phases of a larger gravitybased energy storage apparatus, thereby improving its overall efficiency. The flywheel is connected to an external motor-generator.

[0014] An objective of the present invention is to provide a flywheel-based energy storage system (FESS), that is improved relative to the prior art and wherein at least one of the above stated problems is obviated or alleviated. In particular, there is a need for a more versatile flywheel-based energy storage system (FESS), allowing it to be applied to a variety of applications without requiring the engineering efforts for tailor-made and highly customized designs for each individual FESS.

[0015] Said objective is achieved with the flywheel-based energy storage system (FESS) according to claim 1 of the present invention, comprising:

[0016] - a first rotor module that comprises:

[0017] - a shaft;

[0018] - at least one vacuum chamber that houses one or more than one rotor that defines a flywheel, wherein the one or more than one rotor is rotatablyarranged about a rotation axis that is defined by the shaft extending through opposite walls of the rotor module;

[0019] - at least two vacuum rotary seals that are arranged at the opposite walls of the rotor module and around the shaft to thereby allow a vacuum inside the vacuum chamber to be maintained during rotation of the one or more than one rotor; and

[0020] - at least two bearings supporting the shaft on opposite sides outside the vacuum chamber; and

[0021] - a motor-generator that is rotatably connected to the shaft via a motor-generator-coupling, wherein both the motor-generator and the motor-generator-coupling are arranged outside the vacuum chamber of the first rotor module,

[0022] wherein the system comprises:

[0023] - one or more than one further rotor module, wherein said further rotor module is similar as defined for the first rotor module; and

[0024] - wherein the shafts of adjacent rotor modules are coupled by a shaftcoupling that is configured to transfer torque.

[0025] As mentioned above, FESS strike a balance between batteries on the one hand, and supercapacitors on the other hand, with higher power density than batteries and moderate energy density. Moreover, FESS have a very long lifespan. They can undergo many charge-discharge cycles with little degradation, and maintenance is often minimal. Because there are no chemical reactions, wear mainly occurs in mechanical components like bearings. Moreover, FESS provide almost instantaneous energy output, making them suitable for applications requiring rapid response.

[0026] Applicant had the insight that an increased versatility of FESS systems would allow them to be used in a variety of industrial applications that do not allow for the engineering of tailor-made and highly customized designs. According to the invention this increased versatility is obtained by sacrificing part of the round-trip efficiency of the FESS. Instead of the common practice to apply active magnetic levitated flywheels to reduce the friction losses to a bare minimum, the current invention proposes to use a conventional shaft in combination with mechanical bearings. The mechanical bearings provide a radial fixation of the shaft, thereby allowing vacuum rotary seals to be arranged around the shaft. As a result, the vacuuminside the vacuum chamber may be maintained while the rotor, during use, rotates inside the vacuum chamber. However, because the mechanical bearings and the motor-generator are positioned outside the vacuum chamber, this allows the use of standard off-the-shelf components that are readily available. As a result, the design may be highly standardized. Moreover, placing the bearings outside the vacuum chamber allows for efficient lubrication without the challenges of lubricating inside a vacuum. Replacement and maintenance is also easier for components that are outside the vacuum chamber. Finally, vacuum operation significantly lowers the heat convection. By placing components outside the vacuum, a more efficient cooling of said components may be obtained.

[0027] The FESS according to the invention is not only versatile in its component selection but also scalable in its energy storage capacity. This is achieved by providing a modular system architecture. The FESS according to the invention comprises at least a first rotor module and at least one further rotor module, wherein the shafts of adjacent rotor modules are coupled by a shaft-coupling. This modular design allows the total energy capacity of the system to be easily increased or decreased by adding or removing rotor modules, without needing to re-engineer the core components. This provides an unprecedented level of flexibility to tailor the energy storage system to specific customer needs and applications, further enhancing the versatility of the system.

[0028] The present invention thus proposes to make surprising design choices that increase versatility and scalability of the FESS at the expense of some round-trip efficiency. At a first glance, this may be counter intuitive in view of FESS, where all design choices are normally focussed on loss optimization.

[0029] The energy stored in a flywheel can be calculated by Ek = 14 * I * coA2 . Here Ek is the energy stored as kinetic energy, co is the angular velocity (rad / s). The rotational inertia I describes how the mass is distributed relatively to the spinning axis. The SI unit of the rotational inertia is [kg-m2]. This implies that the energy that can be stored in a flywheel is linearly related with the mass of the flywheel and quadratically depended on the rotor speed. Considering the quadratically dependency on the rotor speed, it is a matter of fact that the focus in the development of FESS is on loss optimization, and that this is particularly focussed on obtaining a rotor speed that is as high as possible. Any design choices that sacrifices rotor speed, such a applying aconventional shaft and (mechanical) bearing, instead of applying active magnetic levitated flywheels, is therefore counter intuitive for a skilled person in the field of FESS.

[0030] However, the Applicant has had the insight that the sacrifice of less energy content is in practice not really a sacrifice for some (industrial) applications. A reduction or application tailored energy content will result in a FESS, that is still equipped with a high round trip efficiency. Furthermore, it comes with less system complexity, less complex material composition of the rotor and more robustness against rough environments like construction sites. As such, the FESS according to the present invention not only provides versatility, but is still highly efficient, even though the use of off-the-shelf components allows for a simpler design than the conventional highly customized tailor-made FESS systems.

[0031] According to a preferred embodiment, at least one of the bearings is a mechanical bearing. Although it is conceivable that the bearings are fluid film bearings, active magnetic bearings or high temperature superconductive bearings, it is preferred that the bearing or bearings are of a mechanical type. This allows to use reliable off-the shelf bearings.

[0032] According to a further preferred embodiment, the flywheel-based energy storage system comprises a cooler configured to cool the bearings that are arranged outside the vacuum chamber. Cooling the bearings allows to extend the speed range of the bearings, and also allows to cool down any heat generated as friction between the shaft and the vacuum rotary seals that is required to maintain a vacuum in the vacuum chamber.

[0033] According to an even further preferred embodiment, the cooler comprises cooling with a lubricant, preferably oil. Cooling with a lubricant provides -next to cooling - also lubrication, and thereby increases the life time of the bearings and the reliability of the flywheel-based energy storage system. The lubricant is preferably oil.

[0034] According to an even further preferred embodiment, the shaft of the one or more than one rotor module is oriented in a lying direction, and wherein said shaft is preferably substantially horizontal. A lying orientation of the shaft guarantees that each rotor module may be substantially identical, and also optimized as such. A horizontal arrangement of the flywheel-based energy storage system, wherein theplurality of rotors are arranged next to each other, offers a more predictable and constant load than if the plurality of rotors would be arranged in a vertical stack on an upright shaft. After all, such a vertical stack would cause each rotor to carry the weight of the rotors positioned there above, and consequently a rotor that is positioned at a lower end of the vertical stack would be exposed to higher loads, than a rotor that is positioned at an upper end of the vertical stack.

[0035] The design of a FESS according to the invention not only provides versatility in terms of replacing some highly customized and tailor-made components for readily available off-the-shelf components, allowing the design to be highly standardized. The FESS design according to the invention also provides versatility in another way. After all, the design allows for a modular build, as will be explained below in more detail.

[0036] According to an even further preferred embodiment, the flywheel-based energy storage system further comprises at least one further motor-generator that is rotatably connected to the shaft via a further motor-generator-coupling, wherein both the at least one motor-generator and the further motor-generator-coupling are arranged outside the vacuum chamber of the one or more than one rotor module. The modular build thus allows further motor-generator(s) to be connected to the shaft.

[0037] According to an even further preferred embodiment, the rotor module comprises two rotor module halves that are self-centering to guarantee that the shaft of the respective rotor module is aligned through the opposite walls.

[0038] According to the invention, the flywheel-based energy storage system comprises at least two rotor modules that each comprise a respective shaft, and wherein the shafts of adjacent rotor modules are coupled by a shaft-coupling that transfers torque. In an even further preferred embodiment, said shaft-coupling also provides a flexibility to compensate misalignment between the shafts of adjacent rotor modules.

[0039] According to an even further preferred embodiment, the motor-generator-coupling transfers torque and provides a flexibility to compensate misalignment between a rotation axis of the motor generator and the shaft of the adjacent rotor module.

[0040] According to an even further preferred embodiment, the shaft-coupling and the motor-generator-coupling have an identical design. If the couplings used asshaft-coupling and as motor-generator-coupling are the same, it is sufficient to have one single type of coupling in store. This reduces the number of unique parts required to build a flywheel-based energy storage system. The modular design allows to freely combine one or more than one rotor module, with one or more than one motor generator, wherein the coupling between adjacent rotor modules, and between a rotor module and a motor generator, can all be made with the same type of coupling.

[0041] According to an even further preferred embodiment, the flywheel-based energy storage system is designed for a speed operation of the one or more than one rotor below 20.000 rpm, preferably below 15.000 rpm. In this speed range, it is possible to use off the shelf components like (mechanical) bearings, seals and motor generators, and therefore prevents that these components need to be tailor made. It therefore contributes to a modular design of the FESS. Off the shelf components provide reliable solutions and allow for easy maintenance and / or replacement.

[0042] According to an even further preferred embodiment, at least one rotor module comprises an integrated rotor part that integrates the shaft and the rotor of said respective rotor module.

[0043] According to an even further preferred embodiment, for at least one rotor module, the rotor is made of a first material, and the shaft is made of a second material, wherein:

[0044] - the first material and the second material are different materials; or - the first material and the second material are the same, but exposed to a different material treatment during manufacturing. This allows for different material properties to applied for the rotor and the shaft. For example, the shaft may be hardened, whereas this is not necessary for the rotor. Having different types of material thus allows for optimization, also relating to e.g. energy content, press-fit between the rotor and shaft, etc.

[0045] According to an even further preferred embodiment, wherein the second material of which the shaft is made, is hardened by a hardening treatment. Hardening the shaft allows to provide an optimized seal surface.

[0046] The invention further relates to a method of operating a flywheel-based energy storage system according to the invention, comprising the steps of:- at a first point in time, charging energy from a first system to the flywheel-based energy storage system by increasing a rotation speed of one or more than one rotor in one or more than one rotor module;

[0047] - maintaining the energy over a period of time by allowing the one or more than one rotor to spin in a vacuum chamber wherein it is housed; and

[0048] - at a second point in time, that is less than an hour after the first point in time, discharging at least a part of the energy and feeding this discharged energy back to the first system or to a second system. This method allows for peak shaving for different type of (industrial) applications, as will be explained below. Sometimes the energy is used for the system that is also the primary source, and sometimes for another system.

[0049] According to a preferred embodiment of the method:

[0050] - the first system is a crane, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system with potential energy that is obtained by lowering a load; and

[0051] - at the second point in time, at least a part of said energy is discharged back to the crane for lifting a further load. This principle is also known as peak-shaving.

[0052] The worldwide electrification of port operations presents challenges such as securing ample electricity for the electrification process, from the introduction of electrical cranes, handling equipment, buildings, shore power, etc. Significant modifications to existing energy infrastructure are required to accommodate future power needs. The power profile from crane operation (high and frequent peaks) matches with the FESS configurable operation range. With the FESS according to the invention, power demands can be achieved without upgrades to the local electricity network. By providing an easy and quick to install FESS, the invention overcomes the need for impactful and costly grid upgrades. The freed capacity of the installed base achieved by peak shaving crane operation with the FESS can be re-purposed for further electrification. For example, with the installation of a 80kW 11 kWh FESS a 65% reduction of power demand may be achieved for a crane terminal equipped with 3 cranes demanding a combined power of approximately 600kW.

[0053] Moreover, crane terminals come in a wide variety, depending on: quantity of cranes, individual crane power demand, operation intensity, (micro)gridspecifications, etc.. This variety immediately highlights the value of the easily configurable FESS as presented, to be applied in peak shaving crane terminals in ports.

[0054] According to a further preferred embodiment of the method:

[0055] - the first system is a rail bounded electric vehicle, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system with kinetic energy that is obtained by decelerating the rail bounded electric vehicle to a standstill at a stop location, and wherein said kinetic energy is transferred via an electrical connection to the flywheel-based energy storage system that is arranged at or near said stop location, and outside the vehicle; and

[0056] - at the second point in time, at least a part of said energy is discharged back to the same rail bounded vehicle, or another rail bounded vehicle, that is leaving said stop location.

[0057] For example, the flywheel-based energy storage system may be located at a train, tram or subway station, and energy provided by a first train, tram or subway entering the station, may shortly thereafter be used by another train, tram or subway leaving the station. In this way, the energy demand of the station, and in particular any peak loads the station exposes to the grid, may be significantly reduced. If the energy is obtained from one vehicle and discharged back to said same vehicle, the energy is obtained and discharged to the same first system. However, if the energy is discharged to another vehicle, this other vehicle defines the second system.

[0058] According to an even further preferred embodiment of the method: - the first system is a wind turbine, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system with excess energy generated by the wind turbine that exceeds a predetermined maximum energy level; and

[0059] - at the second point in time, at least a part of said energy is discharged to the grid when the energy generated by the wind turbine is below the predetermined maximum energy level.

[0060] Wind turbines are sensitive to wind speeds. Temporarily too much energy may be generated that cannot be transferred to the grid. The flywheel-based energy storage system allows to temporarily buffer excess energy when the energygenerated by the wind turbine exceeds a predetermined upper limit, and allows to discharge said energy to the grid at the second point in time at which the wind speed is temporarily lower.

[0061] According to an even further preferred embodiment of the method: - the first system is the grid, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system at a charging speed with energy from the grid; and

[0062] - at the second point in time, at least a part of said energy is discharged at a discharging speed, that is higher than the charging speed of the flywheel-based energy storage system at the first point in time, to thereby provide electric energy to or charge an electric apparatus, preferably an electric vehicle, at a higher charging or delivery speed than used for charging the flywheel-based energy storage system at the first point in time.

[0063] In this way, it is possible to fast charge an electric apparatus, such as an electric vehicle, without exposing the grid to the peak loads associated therewith. Instead, the flywheel-based energy storage system may be charged at a relatively slow charging speed at the first point in time, thereby minimizing peak loads on the grid. It is also conceivable that the fast charging of the electric apparatus at the higher charging speed is obtained by combining charging from the flywheel-based energy storage system and simultaneously supplementary charging from the grid.

[0064] The electric vehicle may be an electric car, but may also be an electric rail bounded vehicle , such as a train, tram or subway, that is provided with electrical power during use, e.g. via an overhead line. It is also conceivable that the electric apparatus is a crane, to which electric energy is provided for direct use, instead of charging for later use. In this case, the delivery speed for providing the electric energy may be higher than the charging speed used for charging the flywheel-based energy storage system at the first point in time. In this way, it is for example possible to maintain voltage dips below a predetermined maximum. The FESS can be slowly charged when there is no rail bounded vehicle passing by. When a next rail bounded vehicle passes by at a later stage, the voltage on the overhead line can be temporarily increased by discharging the flywheel-based energy storage system. In this way, theelectric rail bounded vehicle can be charged when it passes while reducing voltage dips.

[0065] According to an even further preferred embodiment of the method: - the flywheel-based energy storage system is part of a hybrid energy storage system that further comprises a chemical based battery energy storage;

[0066] - the flywheel-based energy storage system and the chemical based battery energy storage are independently chargeable and dischargeable;

[0067] - the first system is a micro-grid, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system from the micro-grid with an energy supply having a power frequency above a first power frequency threshold, while simultaneously charging or discharging the battery energy storage with an energy supply having a power frequency below said first power frequency threshold; and - at the second point in time, at least a part of said energy is discharged from the hybrid energy storage system back to the micro-grid, wherein the flywheelbased energy storage discharges to the micro-grid with an energy supply having a power frequency above a second power frequency threshold, while simultaneously charging from or discharging to the micro-grid of the battery energy storage with an energy supply having a power frequency below said second power frequency threshold.

[0068] The hybrid energy storage system that combines the flywheel-based energy storage system with the chemical based battery energy storage is able to apply peak shaving and thereby optimize the system in sense of system sizing and reliability. The flywheel may be used to absorb (high frequency) fluctuations in power, both when charging and discharging. In this way, the flywheel allows the chemical based battery energy storage to be exposed to a more fitting and even charging or discharging load, i.e. a lower C-rate. This prevents excessive wear of the chemical based battery energy storage. The flywheel thus limits (mini-)cycles of the battery energy storage, thereby expanding the life time of the chemical based battery energy storage. Energy stored in the flywheel-based energy storage system and the chemical based battery energy storage may be charged or discharged independently of each other, and also simultaneously. The option to apply a simultaneous discharge of the flywheel-basedenergy storage system and the chemical based battery energy storage reduces the need for a large battery size.

[0069] Preferred embodiments are the subject of the dependent claims.

[0070] The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, and in particular the aspects and features described in the attached dependent claims, may be an invention in its own right that is related to a different problem relative to the prior art.

[0071] In the following description preferred embodiments of the present invention are further elucidated with reference to the drawing, in which:

[0072] Figure 1A is a cross-sectional view of an embodiment of a flywheelbased energy storage system according to a first preferred embodiment;

[0073] Figure 1 B is a detailed view of the encircled section of figure 1 A;

[0074] Figure 1C is a perspective view of the flywheel-based energy storage system of figure 1 A;

[0075] Figure 2A is a cross-sectional view of another embodiment of a flywheelbased energy storage system according to a second preferred embodiment;

[0076] Figure 2B is a perspective view of the flywheel-based energy storage system of figure 2A;

[0077] Figure 3A is an embodiment of elements of the flywheel-based energy storage system according to the present disclosure;

[0078] Figure 3B is another embodiment of elements of the flywheel-based energy storage system according to the present disclosure;

[0079] Figure 4 schematically shows an embodiment of a method of operating a flywheel-based energy storage system according to a first exemplary embodiment of the method;

[0080] Figure 5 schematically shows another embodiment of a method of operating a flywheel-based energy storage system according to a second exemplary embodiment;

[0081] Figure 6 schematically shows yet another embodiment of a method of operating a flywheel-based energy storage system according to a third exemplary embodiment;Figure 7 schematically shows an alternative embodiment of a method of operating a flywheel-based energy storage system according to a fourth exemplary embodiment;

[0082] Figure 8 schematically shows a further embodiment of a method of operating a flywheel-based energy storage system according to a fifth exemplary embodiment; and

[0083] Figure 9 schematically shows yet a further embodiment of a method of operating a flywheel-based energy storage system according to a sixth exemplary embodiment.

[0084] Figure 1 shows an embodiments of a flywheel-based energy storage system 10 according to the present disclosure. The flywheel-based energy storage system 10 of figure 1 comprises two rotor modules 1 and one motor-generator 2.

[0085] The function of the flywheel-based energy storage system 10 is to convert electrical energy into rotational kinetic energy and vice versa. When electrical energy is supplied to the system, the motor-generator 2 accelerates a rotor 3 of the respective rotor module 1, thereby storing energy in the form of rotational kinetic energy. When energy is needed, the rotor 3 is decelerated, and the kinetic energy is converted back into electrical energy by the motor-generator 1. The flywheel-based energy storage system 10 is designed for a speed operation of the respective rotors 3 below 20.000 rpm, and preferably below 15.000 rpm, allowing off the shelf components like (mechanical) bearings, seals and motor generators to be used. Such off the shelf components, as opposed to tailor made components for most prior art FESS, contributes to the modular design of the FESS according to the invention. Off the shelf components also provide reliable solutions and allow for easy maintenance and / or replacement.

[0086] Each rotor module 1 encloses a vacuum chamber 4 that houses one or more than one rotor 3 that defines a flywheel. The one or more than one rotor 3 is rotatably arranged about a rotation axis R that is defined by a shaft 5 extending through opposite walls of the rotor module 1. The shaft 5 of each rotor module 1 is preferably oriented in a lying, substantially horizontal, direction.

[0087] Each rotor module 1 comprises at least two vacuum rotary seals 6 and at least two bearings 7, as shown in more detail in figure 1 B. The vacuum rotary seals6 are arranged at the opposite walls of the rotor module 1 and around the shaft 5, to thereby allow a vacuum inside the vacuum chamber 4 to be maintained during rotation of the one or more than one rotor 3. The vacuum rotary seals 6 that are arranged at the opposite walls may be arranged in, on or near said opposite walls. The bearings 9 support the shaft 5 on opposite sides outside the vacuum chamber 4, wherein at least one, and preferably both, of the bearings 9 is a mechanical bearing. To guarantee that the shaft 5 of the respective rotor module 1 is aligned through the opposite walls, the rotor module 1 comprises two rotor module halves 9A, 9B that are self-centering via a self-centering guide 11.

[0088] The rotor 3 and shaft 5 of a respective rotor module 1 can be designed as an integral rotor part or can be designed as separate elements attached to each other. Figure 3A shows a detailed view of an embodiment of an integrated rotor part that integrates the shaft 3 and the rotor 5 of a respective rotor module 1. Figure 3B shows a detailed view of another embodiment, wherein the rotor 3 and shaft 5 are designed as separate elements. This allows that the rotor 3 is made of a first material and the shaft 5 is made of a second material. The first material and the second material can either be different or can be the same, but exposed to a different material treatment during manufacturing. The second material of which the shaft 5 is made, is for example hardened by a hardening treatment.

[0089] The motor-generator 2 is rotatably connected to the shaft 5 via a motor-generator-coupling 8, wherein both the motor-generator 2 and the motor-generator-coupling 8 are arranged outside the vacuum chamber 4 of the rotor module 1. The motor-generator-coupling 8 transfers torque and provides a flexibility to compensate misalignment between the rotation axis R of the motor generator 2 and the shaft 5 of the adjacent rotor module 1.

[0090] The two rotor modules 1 each comprise a respective shaft 5, wherein the shafts 5 of adjacent rotor modules 1 are coupled by a shaft-coupling 12 that transfers torque and provides a flexibility to compensate misalignment between the shafts 5 of adjacent rotor modules 1. The shaft-coupling 12 and the motor-generator-coupling 8 preferably have an identical design.

[0091] The flywheel-based energy storage system 10 furthermore comprises a cooler 15 configured to cool the bearings 7 that are arranged outside the vacuum chamber 4, wherein the cooler comprises cooling with a lubricant, such as oil. In Figure2A, reference number 15 indicates channels of the cooler 15 via which the lubricant can pass via the bearings 7.

[0092] The design of the flywheel-based energy storage system 10 according to the present disclosure not only provides versatility in terms of replacing some highly customized and tailor-made components for readily available off-the-shelf components, but also provides versatility in another way. After all, the design of the flywheel-based energy storage system 10, in particular the identical design of the shaft-coupling 12 and the motor-generator-coupling 8, allows for a modular build, allowing to freely combine one or more than one rotor module 1 , with one or more than one motor generator 2, wherein the coupling 8, 12 between adjacent rotor modules 1, and between a rotor module 1 and a motor generator 2, can all be made with the same type of coupling.

[0093] The modular build thus allows further motor-generator(s) 2 to be connected to the shaft 5. Figure 2 shows an embodiment of the present disclosure, wherein the flywheel-based energy storage system 10 comprises two motorgenerators 2 and two rotor modules 1, each rotor module 1 arranged for comprising one or more than one rotor 3. The flywheel-based energy storage system 10 according to the embodiment of figure 2 is largely identical to the flywheel-based energy storage system 10 according to the embodiment of figure 1 as described above, but it differs in that the flywheel-based energy storage system 10 according to the embodiment of figure 2 comprises a further motor-generator 2. The further motor-generator 2 is rotatably connected to the shaft 5 via a further motor-generator-coupling 8, wherein both the further motor-generator 2 and the further motor-generator-coupling 8 are arranged outside the vacuum chamber 4 of the respective rotor module 1.

[0094] However, other embodiments that are not shown in the figures are possible. The flywheel-based energy storage system 10 according to present disclosure can comprise any desired number of rotor modules 1 and motor-generators 2. For example, it can comprise one motor-generator 2 and one rotor module 1, two motor-generator 2 and one rotor module 1, one motor-generator 2 and three or more than three rotor modules 1, and two motor-generators 2 and three or more than three rotor modules 1, etc., wherein the shaft 5 of each rotor module 1 is coupled to an adjacent rotor module or to a motor-generator 2 by a respective shaft-coupling 12 or a motor-generator-coupling 8.Furthermore, more than one flywheel-based energy storage systems 10 can be used in cooperation, wherein multiple flywheel-based energy storage systems 10 are configured to be arranged in a standard 10ft. or 20ft. container, with a maximum of three flywheel-based energy storage systems 10 per 10ft. container or eight flywheel-based energy storage systems 10 per 20ft. container.

[0095] The flywheel-based energy storage system 10 as described above, is in particular suitable to be operated in a method 20 according to the present disclosure, for short-term energy storage, wherein the method comprises the steps of:

[0096] - at a first point in time, charging energy 22 from a first system 21 to the flywheel-based energy storage system 10 by increasing a rotation speed of one or more than one rotor 3 in one or more than one rotor module 1 ;

[0097] - maintaining the energy over a period of time by allowing the one or more than one rotor 3 to spin in a vacuum chamber 4 wherein it is housed; and

[0098] - at a second point in time, that is less than an hour after the first point in time, discharging 23 at least a part of the energy back to the first system 21 or to a second system 24.

[0099] Figure 4 shows an embodiment of the method 20, wherein the first system 21 is a crane. At the first point in time, the flywheel-based energy storage system 10 is charged 22 with potential energy that is obtained by lowering a load. At the second point in time, at least a part of the energy is discharged 23 back to the crane, i.e. the first system 21 , for lifting a further load. Furthermore, the flywheel-based energy storage system 10 may additionally be charged 25 with energy from the grid 26, for example at a point in time when the crane is not lowering a load.

[0100] Figure 5 shows another embodiment of the method 20, wherein the first system 21 is a rail bounded electric vehicle. At the first point of time, the flywheelbased energy storage system 10 is charged 22 with kinetic energy that is obtained by decelerating the rail bounded electric vehicle to a standstill at a stop location. The kinetic energy is transferred via an electrical connection 27 to the flywheel-based energy storage system 10 that is arranged at or near the stop location, and outside the vehicle. At the second point in time, at least a part of the energy is discharged 23 back to the same rail bounded vehicle (i.e. the first system 21 ), or to the second system 24, such as another rail bounded vehicle, that is leaving the stop location.Figure 6 shows yet another embodiment of the method 20, wherein the first system 21 is an overhead railway line, which is arranged between two railway stations 28. As a rail bounded electric vehicle moves along the track, a variable electrical load is created on the overhead railway line, causing voltage drops in the voltage levels of the overhead railway line. For mitigating these voltage drops, power substations 29 are connected to the overhead railway lines at predefined positions between the railway stations 28, that are conventionally directly connected to the grid 26. According to an embodiment of the invention, at least one power substation 29 is powered by a flywheel-based energy storage system 10. At the first point of time, when no rail bounded electric vehicle is passing the power substation 29, the flywheel-based energy storage system 10 is charged 22 with energy that is obtained from the overhead railway line. At the second point in time, when a rail bounded electric vehicle is passing the power substation 29, at least a part of the energy is discharged 23 back to the overhead railway line, thereby effectively preventing voltage drops without the need for a grid connection. Alternatively, and not shown, the power substation 29 that comprises the flywheel-based energy storage system 10 may also be connected to the grid 26. In this way, the flywheel-based energy storage system 10 may be charged with energy relatively slowly, and discharged when a rail bounded electric vehicle passes by.

[0101] Figure 7 shows an alternative embodiment of the method 20, wherein the first system 21 is a wind turbine. A wind turbine typically produced an unstable energy output, causing fluctuations in the generated voltage levels. For stabilizing the energy output of the wind turbine, the flywheel-based energy storage system 10 is charged 22 at the first point of time with excess energy generated by the wind turbine that exceeds a predetermined maximum energy level. The second system 24 is the grid 26, wherein, at the second point in time, at least a part of the energy is discharged 23 to the grid 26 when the energy generated by the wind turbine is below the predetermined maximum energy level.

[0102] Figure 8 shows a further embodiment of the method 20, wherein the first system 21 is the grid 26. At the first point in time, the flywheel-based energy storage system 10 is charged 22 at a predefined charging speed with energy from the grid 26 to prevent peak loads on the grid 26. The second system 24 is an electric vehicle charger, wherein, at the second point in time, at least a part of the energy is discharged23 at a discharging speed to the electric vehicle charger. The discharging speed is higher than the charging speed of the flywheel-based energy storage system 10 at the first point in time, In this way it is possible to, in a first example, charge an electric vehicle at a higher charging speed than used for charging 22 the flywheel-based energy storage system 10 at the first point in time. In a second example, it is also possible that the flywheel-based energy storage system 10 is charged 22 via the grid 26, that may suffer from net congestion, at a relatively slow charging speed. At the second point in time, the flywheel-based energy storage system 10 may provide the energy to the second system 24, comprising an electric apparatus such as a crane, at a discharging speed that is faster than the charging speed.

[0103] Figure 9 shows yet a further embodiment of the method 20, wherein - the flywheel-based energy storage system 10 is part of a hybrid energy storage system 35 that further comprises a chemical based battery energy storage 32;

[0104] - the flywheel-based energy storage system 10 and the chemical based battery energy storage 32 are independently chargeable and dischargeable;

[0105] - the first system 21 is a micro-grid 33, and the step of charging energy to the flywheel-based energy storage system 10 at the first point in time comprises charging the flywheel-based energy storage system 10 from the micro-grid 21,33 with an energy supply having a power frequency above a first power frequency threshold, while simultaneously charging or discharging the battery energy storage 32 with an energy supply having a power frequency below said first power frequency threshold; and

[0106] - at the second point in time, at least a part of said energy is discharged from the hybrid energy storage system 35 back to the micro-grid 21, 33, wherein the flywheel-based energy storage 10 discharges to the micro-grid 21, 33 with an energy supply having a power frequency above a second power frequency threshold, while simultaneously charging from or discharging to the micro-grid 21, 33 of the battery energy storage 32 with an energy supply having a power frequency below said second power frequency threshold. In this embodiment, the grid 34 is electrically connected to the micro-grid 21, 33.

[0107] It is mentioned that wording like “one or more than one object is” is to be interpreted as “one object is or more objects are”, and is therefore synonymous to the wording “at least one object is”.Although they show preferred embodiments of the invention, the above described embodiments are intended only to illustrate the invention and not to limit in any way the scope of the invention. Accordingly, it should be understood that where features mentioned in the appended claims are followed by reference signs, such signs are included solely for the purpose of enhancing the intelligibility of the claims and are in no way limiting on the scope of the claims. Furthermore, it is particularly noted that the skilled person can combine technical measures of the different embodiments. The scope of protection is defined solely by the following claims.

Claims

CLAIMS1. A flywheel-based energy storage system (FESS), comprising:- a first rotor module that comprises:- a shaft;- at least one vacuum chamber that houses one or more than one rotor that defines a flywheel, wherein the one or more than one rotor is rotatably arranged about a rotation axis that is defined by the shaft extending through opposite walls of the rotor module;- at least two vacuum rotary seals that are arranged at the opposite walls of the rotor module and around the shaft to thereby allow a vacuum inside the vacuum chamber to be maintained during rotation of the one or more than one rotor; and- at least two bearings supporting the shaft on opposite sides outside the vacuum chamber; and- a motor-generator that is rotatably connected to the shaft via a motor-generator-coupling, wherein both the motor-generator and the motor-generator-coupling are arranged outside the vacuum chamber of the first rotor module,characterized in that the system comprises:- one or more than one further rotor module, wherein said further rotor module is similar as defined for the first rotor module; and- wherein the shafts of adjacent rotor modules are coupled by a shaftcoupling that is configured to transfer torque.

2. The flywheel-based energy storage system according to claim 1, wherein the shaft of the one or more than one rotor module is oriented in a lying direction, and wherein said shaft is preferably substantially horizontal.

3. The flywheel-based energy storage system according to claim 1 or 2, wherein the shaft-coupling that transfers torque also provides a flexibility to compensate misalignment between the shafts of the adjacent rotor modules.

4. The flywheel-based energy storage system according to any of the foregoing claims, wherein the motor-generator-coupling transfers torque and provides a flexibility to compensate misalignment between a rotation axis of the motor generator and the shaft of the adjacent rotor module.

5. The flywheel-based energy storage system according to any of the foregoing claims, wherein the shaft-coupling and the motor-generator-coupling have an identical design.

6. The flywheel-based energy storage system according to any of the foregoing claims, wherein the rotor module comprises two rotor module halves that are self-centering to guarantee that the shaft of the respective rotor module is aligned through the opposite walls.

7. The flywheel-based energy storage system according to any of the foregoing claims, wherein the flywheel-based energy storage system is designed for a speed operation of the one or more than one rotor below 20.000 rpm, preferably below 15.000 rpm.

8. The flywheel-based energy storage system according to any of the foregoing claims, wherein at least one of the bearings is a mechanical bearing.

9. The flywheel-based energy storage system according to any of the foregoing claims, comprising a cooler configured to cool the bearings that are arranged outside the vacuum chamber.

10. The flywheel-based energy storage system according to claim 9, wherein the cooler comprises cooling with a lubricant, preferably oil.

11. The flywheel-based energy storage system according to any of the foregoing claims, further comprising at least one further motor-generator that is rotatably connected to the shaft via a further motor-generator-coupling, wherein boththe at least one motor-generator and the further motor-generator-coupling are arranged outside the vacuum chamber of the one or more than one rotor module.

12. The flywheel-based energy storage system according to any of the foregoing claims, wherein at least one rotor module comprises an integrated rotor part that integrates the shaft and the rotor of said respective rotor module.

13. The flywheel-based energy storage system according to any of the foregoing claims, wherein, for at least one rotor module, the rotor is made of a first material, and the shaft is made of a second material, wherein:- the first material and the second material are different materials; or - the first material and the second material are the same, but exposed to a different material treatment during manufacturing.

14. The flywheel-based energy storage system according to claim 13, wherein the second material of which the shaft is made, is hardened by a hardening treatment.

15. Method of operating a flywheel-based energy storage system according to claim 1, comprising the steps of:- at a first point in time, charging energy from a first system to the flywheel-based energy storage system by increasing a rotation speed of one or more than one rotor in one or more than one rotor module;- maintaining the energy over a period of time by allowing the one or more than one rotor to spin in a vacuum chamber wherein it is housed; and- at a second point in time, that is less than an hour after the first point in time, discharging at least a part of the energy and feeding this discharge energy back to the first system or to a second system.

16. Method according to claim 15, wherein:- the first system is a crane, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging theflywheel-based energy storage system with potential energy that is obtained by lowering a load; and- at the second point in time, at least a part of said energy is discharged back to the crane for lifting a further load.

17. Method according to claim 16, wherein:- the first system is a rail bounded electric vehicle, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system with kinetic energy that is obtained by decelerating the rail bounded electric vehicle to a standstill at a stop location, and wherein said kinetic energy is transferred via an electrical connection to the flywheel-based energy storage system that is arranged at or near said stop location, and outside the vehicle; and- at the second point in time, at least a part of said energy is discharged back to the same rail bounded vehicle, or another rail bounded vehicle, that is leaving said stop location.

18. Method according to claim 15, wherein:- the first system is a wind turbine, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system with excess energy generated by the wind turbine that exceeds a predetermined maximum energy level; and- at the second point in time, at least a part of said energy is discharged to the grid when the energy generated by the wind turbine is below the predetermined maximum energy level.

19. Method according to claim 15, wherein:- the first system is the grid, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system at a charging speed with energy from the grid; and- at the second point in time, at least a part of said energy is discharged at a discharging speed, that is higher than the charging speed of the flywheel-basedenergy storage system at the first point in time, to thereby provide electric energy to or charge an electric apparatus at a higher charging or delivery speed than used for charging the flywheel-based energy storage system at the first point in time, wherein said electric apparatus is preferably an electric vehicle.

20. Method according to claim 15, wherein:- the flywheel-based energy storage system is part of a hybrid energy storage system that further comprises a chemical based battery energy storage;- the flywheel-based energy storage system and the chemical based battery energy storage are independently chargeable and dischargeable;- the first system is a micro-grid, and the step of charging energy to the flywheel-based energy storage system at the first point in time comprises charging the flywheel-based energy storage system from the micro-grid with an energy supply having a power frequency above a first power frequency threshold, while simultaneously charging or discharging the battery energy storage with an energy supply having a power frequency below said first power frequency threshold; and - at the second point in time, at least a part of said energy is discharged from the hybrid energy storage system back to the micro-grid, wherein the flywheelbased energy storage discharges to the micro-grid with an energy supply having a power frequency above a second power frequency threshold, while simultaneously charging from or discharging to the micro-grid of the battery energy storage with an energy supply having a power frequency below said second power frequency threshold.

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