Hollow disk rotor for flywheel power storage device and manufacturing method thereof

Abstract

To provide a hollow disk rotor for a flywheel power storage device and a method for manufacturing the same which remarkably improve a limit peripheral velocity and limit storage energy.SOLUTION: A hollow disk rotor for a flywheel power storage device is composed of a plurality of reinforced fibers wound around a circumference, a matrix material filling a gap between the reinforced fibers, and a carbon nano-tube spreading in an upper direction toward the matrix material starting from the surface of the reinforced fibers.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention is directed to the critical peripheral speed and the limiting speed of a hollow disk (including a cylinder) rotor, which is the main element of the flywheel of a flywheel electric storage device, particularly a "circumferentially wound fiber-reinforced hollow disk rotor." limit A typical example of this rotor is a "carbon fiber reinforced plastic (CFRP)" rotor, which is made by wrapping high-strength carbon fibers around the circumference and then impregnating and solidifying a matrix material such as epoxy resin in the gaps between the fibers. e Examples of rotors include, but are not limited to, carbon fiber reinforced plastics (CFRP) rotors.

[0002] The reinforcing fibers may be boron fibers, glass fibers, aramid fibers, alumina fibers, silicon carbide fibers, or various metal fibers depending on the application, and the matrix agent may be various resins including epoxy resins.

[0003] Hereinafter, "CFRP hollow disk (or cylindrical) rotor" may be abbreviated as "CFRP rotor" or simply as "rotor," and "flywheel" may be abbreviated as "FW." [Background technology]

[0004] The FW power storage device is a power storage device that has the function of storing external electric power as rotational kinetic energy in the FW rotor through a means for converting electrical energy and rotational kinetic energy into each other, and conversely, supplying the stored rotational kinetic energy of the FW rotor to the outside as electric power.

[0005] It has the following outstanding features: (I) it functions stably in both low and high temperature environments; (II) there is almost no deterioration in its characteristics or lifespan even after repeated charging and discharging, or even when left fully charged; (III) the exact charge level can be easily detected; (IV) the input / output density can be freely designed; and (V) its internal resistance is much smaller than that of secondary batteries.

[0006] Generally, when a hollow disk such as a FW rotor is rotated, stronger tensile stress occurs in the circumferential direction than in the radial direction. If the inner radius and outer radius of the hollow disk are a and b, respectively, as the inner / outer diameter ratio λ = a / b (where 0 < λ < 1) increases (approaches 1), the maximum circumferential tensile rotational stress (hereafter simply referred to as "circumferential stress") of the hollow disk increases, while the maximum radial tensile rotational stress (hereafter simply referred to as "radial stress") tends to decrease; in some cases, the maximum circumferential stress value can be two or more orders of magnitude larger than the maximum radial stress value. Note that the maximum circumferential stress value and the maximum radial stress value occur at different radii r.

[0007] In recent years, in place of conventional bulk metal FW rotors, there has been active development of "circumferentially wound fiber-reinforced plastic (circumferentially wound CFRP)" FW rotors, which are formed by winding high-strength fibers such as carbon fibers in the circumferential direction and impregnating the gaps between the fibers with plastic (as a matrix agent). The reason is that this type of rotor has an extremely high tensile yield strength in the circumferential direction.

[0008] However, with this simple circumferentially wound CFRP rotor (first conventional example), there was a problem in that the expected critical peripheral speed or critical stored energy could not be obtained. (The "critical peripheral speed" and "critical stored energy" mentioned here are both basic quantities when evaluating (comparing) the power storage performance of a rotor. The critical peripheral speed is the peripheral speed that a rotor reaches just before it breaks down as it increases its rotational speed, and the critical stored energy is the maximum rotational kinetic energy that can be stored just before the rotor breaks down. It goes without saying that the higher this is, the higher the performance.)

[0009] The reason for this is that the limiting stored energy of a rotating disk is generally (1-λ 4), so in order to increase the limit stored energy, it is desirable to have a rotor with as small an inner / outer diameter ratio λ as possible (close to 0). However, if the inner / outer diameter ratio λ is reduced in response to this requirement, the proportion of maximum radial tensile stress when rotated increases sharply, and in the case of a simple circumferentially wound FRP rotor, radial tensile yielding (hereafter referred to as radial yielding) occurs at a much lower rotational speed than circumferential tensile yielding (hereafter referred to as circumferential yielding).

[0010] The reason why radial yielding occurs first is that the simple circumferentially wound CFRP rotor has no fiber reinforcement in the radial direction, resulting in a significantly lower radial yield strength compared to the circumferential yield strength.

[0011] In order to solve these problems with simple circumferentially wound CFRP rotors, Patent Document 1 below proposes a 3D woven CFRP rotor reinforced with a "three-dimensional woven carbon fiber skeleton" that uses a weaving method to weave reinforcing fibers and orient them not only in the circumferential direction but also in the radial and axial directions, and that has the characteristic of thinning the thickness from the center to the periphery so that the fiber volume fraction in the radial direction remains constant (see Figures 1 and 3 of Patent Document 1).

[0012] However, when a 3D woven CFRP rotor (second prior art) was actually manufactured and a rotation test was conducted, it was reported that the rotor's axial vibration increased as the peripheral speed increased, and eventually it broke at a peripheral speed (around 530 m / s) much lower than the intended limit peripheral speed (Non-Patent Document 1).

[0013] When a 3D woven CFRP rotor is fabricated by weaving a carbon fiber skeleton structure using a weaving method that does not allow for precise finishing, imbalance is unavoidable. This imbalance can be either static imbalance, where the rotor's center of gravity is shifted from the center of rotation, or dynamic imbalance, where rotational stress causes the carbon fiber skeleton to distort unevenly.

[0014] In order to solve the above-mentioned shaft vibration problem, various improvements (see Non-Patent Document 2) have been made, but a 3D woven CFRP rotor with a peripheral speed exceeding 800 m / s has not yet been obtained. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent No. 5239058 [Non-patent literature]

[0016] [Non-Patent Document 1] Junichi Yoshimura, Noboru Hiroshima, Hiroshi Hatta, Takeshi Goto, Yasuo Mukogo, "Development of a high-speed rotating body made of three-dimensional carbon fiber reinforced plastics," CD-ROM Proceedings of the 18th Mechanical Materials and Processing Technology Conference (M&P2010) of the Japan Society of Mechanical Engineers, No. 10-29 [Non-patent document 2] N. Hiroshima, H. Hatta, M. Koyama, J. Yoshimura, Y. Nagura, K. Goto, Y. Kogo “Spin test of three-dimensional composite rotor for flywheel energy storage system” Composite Structures 136 (2016) pp.626-634 Summary of the Invention [Problem to be solved by the invention]

[0017] As mentioned above, conventional simple circumferentially wound CFRP rotors have the fatal drawback of low radial yield strength, while conventional 3D woven CFRP rotors have the serious drawback of being unable to achieve precise rotational balance characteristics. As a result, the high limiting peripheral speed and high limiting stored energy that were potentially expected of CFRP rotors have yet to be realized.

[0018] In view of the above problems, the present invention has an object to provide a hollow disk rotor for a flywheel electric energy storage device, which has a dramatically improved limit peripheral speed and limit stored energy, and a method for manufacturing the same. [Means for solving the problem]

[0019] The inventors of the present application noticed that the axial vibration problem caused by imbalance that occurred in 3D woven CFRP rotors had not been reported for simple circumferentially wound CFRP rotors, and they pondered over whether there was a configuration or method for increasing the radial yield strength while maintaining the basic configuration of a simple circumferentially wound CFRP rotor.They came up with the idea of ​​strengthening the radial yield strength of a simple circumferentially wound CFRP rotor with carbon nanotubes (hereinafter sometimes abbreviated as CNTs) oriented perpendicular to the reinforcing fibers, and as a result of continued intensive consideration and investigation, they succeeded in creating a CNT-CFRP rotor that can achieve extremely high limiting peripheral speeds and limiting stored energy compared to conventional rotors.

[0020] CNTs are widely known as a material that is highly flexible and exhibits extremely high tensile strength in the direction of the growth axis. In the present invention, as will be described later, CNTs are cleverly incorporated so that this property is prominently exhibited in the radial direction of the rotor.

[0021] The invention of claim 1 is a hollow disk rotor of a flywheel for a flywheel energy storage device, characterized in that it is a hollow disk rotor (CNT-FRP rotor) composed of a plurality of reinforcing fibers (FR) wound around the circumference, a matrix material (P) filling the gaps between the reinforcing fibers, and carbon nanotubes (CNT) extending upward from the surface of the reinforcing fibers toward the matrix material.

[0022] The invention of claim 2 is the hollow disk rotor of claim 1, wherein, when the average separation distance between the pair of nearest neighboring single fibers of the reinforcing fibers is l, the average diameter is D, and the average spread distance of the upwardly spread carbon nanotubes is T, there is at least the following relationship between T, l, and D:

[0023]

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[0024]

number

[0025] The invention of claim 3 is characterized in that, in the hollow disk rotor described in claim 1, the base point on the surface of the reinforcing fiber is either a grafted form in which the carbon nanotube is fixed at one end face, or a spot-fixed form in which the side is bonded at one point.

[0026] The invention of claim 4 is the hollow disk rotor of claim 3, wherein the carbon nanotubes have a relationship between an average length dimension u and an average development distance T in the case of the grafting mode.

[0027]

number

[0028]

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[0029] The invention of claim 5 is the hollow disk rotor of claim 1, wherein the average volume fraction V of the reinforcing fibers is F is at least 0.5 (50%) or more, preferably 0.6 (60%) or more.

[0030] The invention of claim 6 is characterized in that, in the hollow disk rotor of claim 1, the reinforcing fibers are made of one or more composite fibers selected from carbon fibers, boron fibers, glass fibers, aramid fibers, alumina fibers, silicon carbide fibers, and various metal fibers.

[0031] The invention of claim 7 is characterized in that, in the hollow disk rotor of claim 6, the diameter of the reinforcing fibers has an average value in the range of at least 3 μm to 9 μm, with a variation within the average value ±1 μm, and preferably has an average value in the range of 5 μm to 7 μm, with a variation within the average value ±0.5 μm.

[0032] The invention of claim 8 is characterized in that, in the hollow disk rotor of claim 1, the matrix material is one selected from thermosetting resins such as thermosetting epoxy resins, unsaturated polyester resins, vinyl ester resins, and phenolic resins, or thermoplastic resins such as polyolefin resins, polyamide resins, and polycarbonate resins.

[0033] The invention of claim 9 is characterized in that, in the hollow disk rotor of claim 1, the carbon nanotubes are single-walled carbon nanotubes having a diameter of 0.5 to 3 nm, or multi-walled carbon nanotubes having a diameter of 5 to 30 nm, or carbon nanotubes that are a mixture of the single-walled carbon nanotubes and the multi-walled carbon nanotubes.

[0034] A tenth aspect of the present invention is characterized in that, in the hollow disk rotor of the ninth aspect, the carbon nanotubes have a weight percent concentration c in the matrix material in the range of c=0.1 wt % to 8 wt %.

[0035] The invention of claim 11 is a method for manufacturing a hollow disk rotor as described in claim 1, characterized in that it comprises a first step of fixing or adhering carbon nanotubes to the reinforcing fibers, a second step of forming a reinforcing fiber bundle by bundling the reinforcing fibers with carbon nanotubes formed in the first step, and a third step of molding a rotor by a filament winding method using the reinforcing fiber bundle formed in the second step and a matrix resin. [Brief explanation of the drawings]

[0036] [Figure 1]FIG. 2 is an explanatory diagram showing a hollow disk rotor according to the present invention. [Figure 2] FIG. 2 is an explanatory diagram showing the positional relationship of reinforcing fibers in the hollow disk rotor of the present invention. [Figure 3] FIG. 2 is an explanatory diagram showing how carbon nanotubes spread from the surface of a reinforcing fiber into a matrix material. [Figure 4] An explanatory diagram showing the reorganization of carbon nanotubes on the surface of a pair of closely spaced reinforcing fibers. [Figure 5] 4 is a table showing the rotation performance of the hollow disk rotor according to the first embodiment. [Figure 6] FIG. 6 is an explanatory diagram showing the configuration of a flywheel according to a second embodiment. [Figure 7] 10 is a table showing the rotation performance of the flywheel according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be described in detail below using the drawings as appropriate. However, in these drawings, the relationship between thickness and planar dimensions, the thickness ratio of each layer, the thickness, length, and density of the reinforcing fibers (F) and carbon nanotubes (CNTs) are exaggerated or deformed to facilitate understanding. Furthermore, identical components are given the same reference numerals and will not be described again. Hereinafter, the cross-sectional shape of the reinforcing fibers will be described as a typical perfect circle (or nearly a perfect circle), but the cross-sectional shape is not limited to a perfect circle.

[0038] Figure 1 is a macroscopic view of a radially carbon nanotube-reinforced, circumferentially fiber-reinforced plastic hollow disk rotor (hereinafter referred to as a CNT-FRP rotor or simply rotor) 1 for a flywheel for a flywheel energy storage device according to the present invention. Figure 1A is a perspective view of rotor 1, Figure 1B is a cross-sectional view of rotor 1 cut along an arbitrary rθ plane, and Figure 1C is a cross-sectional view of rotor 1 cut along the Z axis. For ease of explanation, this rotor 1 is placed so that its axis of rotation coincides with the Z axis of the well-known rθZ cylindrical coordinate system.

[0039] Reference symbol 2 denotes a circular hole (outer diameter a) in the center of rotor 1. Reference symbols b, a, and h respectively denote the outer radius, inner radius, and height of rotor 1. In the figure, r is a radius variable that indicates any position of rotor 1.

[0040] As shown in Figure 3, the material structure of the rotor 1 of the present invention is basically CNT-FRP (reference numeral 3), in which a matrix material (plastic) is reinforced with carbon nanotubes and reinforcing fibers. The rotor 1 is composed of reinforcing fibers (F) 11 arranged in a circumferentially wound laminated configuration, a matrix material 12 filled in the gaps between the reinforcing fibers, and carbon nanotubes (CNTs) 13 extending upward within the matrix material 12 from the surface of the reinforcing fibers as a base point.

[0041] A typical example of the reinforcing fiber 11 is carbon fiber (CF) having high tensile strength, but boron fiber, glass fiber, aramid fiber, alumina fiber, silicon carbide fiber, and various metal fibers may also be used depending on the application. The diameter of the reinforcing fiber 11 of the present invention is at least in the range of 3 μm to 9 μm on average, with a variation within the average value ±1 μm. More preferably, the average value is in the range of 5 μm to 7 μm, with a variation within the average value ±0.5 μm. If the diameter of the reinforcing fiber 11 exceeds 9 μm, the desired tensile strength cannot be obtained. Furthermore, if the diameter is less than 3 μm, the manufacturing productivity of the reinforcing fiber will be significantly reduced. In the following description, the reinforcing fiber (F) 11 will be described as carbon fiber (CF) having an average diameter of 6 μm.

[0042] Figure 2 is an enlarged image of Figure 1B, which depicts the cross section. It shows two possible positional relationships for the nearest neighboring reinforcing fibers 11: a six-fold symmetric arrangement (Figure 2A) and a four-fold symmetric arrangement (Figure 2B). In the figure, D is the diameter of the reinforcing fiber 11, l is the surface-to-surface distance (average separation distance) between the nearest neighboring pair of reinforcing fibers, and L is the center distance (average line-to-line distance) between the pair of reinforcing fibers, where L = l + D. 12 is the matrix material. For convenience, CNTs 13 are not depicted in Figures 1 and 2.

[0043] In order to meet the demand for improving the circumferential yield strength of the CNT-FRP rotor 1 of the present invention, the average volume fraction V of the reinforcing fibers of the rotor is F It goes without saying that it is better to have a value as high as possible. In light of this, it is specifically desirable that the value be at least 0.5 or more, and preferably 0.6 or more.

[0044] Here, the V of the reinforcing fiber when the six-fold symmetrical arrangement (Fig. 2A) and the four-fold symmetrical arrangement (Fig. 2B) are mixed. F The relationship between the diameter D and the separation distance l is as follows: FH From the geometric relationship in Figure 2A,

[0045]

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[0046] Similarly, referring to FIG. 2B, the volume fraction V of the four-fold symmetric arrangement is expressed as FT teeth,

[0047]

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[0048] Assuming that the reinforcing fibers arranged in six-fold symmetry and the reinforcing fibers arranged in four-fold symmetry are mixed in equal proportions in rotor 1, the average volume fraction V F teeth,

[0049]

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[0050] The specification of rotor 1 is V F Once D is determined, the separation distance l can be determined from this equation.

[0051] When the reinforcing fibers with six-fold symmetry and the reinforcing fibers with four-fold symmetry are mixed in a ratio of m:n, use the formula (1) instead.

[0052]

number

[0053] The matrix material 12 is, for example, a thermosetting epoxy resin. Alternatively, it may be a thermosetting resin such as an unsaturated polyester resin, a vinyl ester resin, or a phenolic resin, or a thermoplastic resin such as a polyolefin resin, a polyamide resin, or a polycarbonate resin. The following description will be given assuming that the matrix material 12 is an epoxy resin.

[0054] As is well known, carbon nanotubes (CNTs) are a reinforcing material with a tensile strength of 50 to 70 GPa in the growth axis direction. The CNTs 13 used in the CNT-FRP rotor 1 of the present invention are single-walled CNTs with a diameter of 0.5 to 3 nm, multi-walled CNTs with a diameter of 5 to 30 nm, or a mixture of the single-walled CNTs and the multi-walled CNTs.

[0055] As shown in Figure 3, the CNTs 13 in the rotor 1 of the present invention are connected to the surface of the reinforcing fiber 11 in a manner that "one bottom surface is fixed" or "the side surface is bonded at one point," and are deployed (i.e., spread out and extended) toward the matrix material above, starting from the connection point. The symbol T in the figure is the average CNT deployment distance. Region 14 is the effective deployment area of ​​the CNTs. The effective deployment area 14 is Reinforced Fiber 11 It refers to the spatial region from the CNT deployment distance T.

[0056] The weight percent concentration c of the CNTs 13 contained in the matrix material is in the range of c = 0.1 wt% to 8 wt%. If the concentration c is lower than 0.1 wt%, in most cases, an effective radial matrix reinforcement effect cannot be obtained. If the concentration c is higher than 8 wt%, it becomes difficult for the matrix material to penetrate to the surface of the reinforcing fibers, making production practically difficult. To prevent excessive consumption of CNTs, which are a relatively expensive material, the minimum concentration that satisfies the desired radial tensile yield strength (of the rotor 1) is selected from the above concentration range.

[0057] The CNT deployment distance T means the average height of the deployed end (floating bottom surface) of the CNT 13 measured from the surface of the reinforcing fiber 11. In the rotor 1 of the present invention, T is at least

[0058]

number

[0059]

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[0060] In the above formula, l is the distance between the reinforcing fibers, and D is the diameter of the reinforcing fibers. The right side of formula (2') corresponds to the distance between the midpoint of a pair of carbon fibers located at diagonal positions (= second nearest neighbor positions) in the tetragonal symmetric arrangement (FIG. 2B) and the surface of the carbon fiber.

[0061] To "fix" the CNTs 13 to the surface of the reinforcing fiber 11, for example, the "grafting method (FIG. 3A)" disclosed in JP 2018-12741 A and the like can be used. In the grafting method, extremely fine particulate catalytic metal (for example, Ni) is deposited on the surface of the reinforcing fiber (CF), and CNTs are grown by chemical vapor deposition (CVD) or the like. Then, the CNTs with their bottom surfaces fixed to the CF surface grow vertically. Since they grow vertically from the CF surface, the average length u of the CNTs 13 becomes approximately equal to the CNT deployment distance T,

[0062]

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[0063] On the other hand, to "adhere" a portion of the side of the CNT 13 to the surface of the reinforcing fiber 11, for example, the "spot-attaching method (Figure 3B)" disclosed in JP 2019-60050 A and elsewhere can be used. This method involves attaching CNTs of average length u to the surface of the reinforcing fiber (CF) so that they entangle with each other, then immersing the fiber in an emulsion-type sizing agent in which adhesive (e.g., an epoxy adhesive) is dispersed in spots. This sizing treatment results in the deposition and solidification of adhesive droplets 15, thereby spot-attaching the side of the CNT to the surface of the reinforcing fiber. According to the same document, the minimum size of the adhesive droplets is 0.05 μm. Since CNTs adhered in this way break from the adhesive points and unfold into the matrix material, in order to obtain the desired average CNT unfolding distance T, the average CNT length u must be twice as long as T. This relationship can be expressed mathematically as follows:

[0064]

number

[0065] The parameters of the CNTs 13, such as the deployment distance T, average length u, and concentration c, are determined, for example, by the following procedure. First, the basic requirements of the CNT-FRP rotor 1 are the type of reinforcing fiber to be used and its volume fraction V F Then, the average diameter D of the reinforcing fiber is determined, and V is obtained from the relation (1). F The reinforcing fiber spacing distance l can be calculated using and D. Once l is determined, the CNT deployment distance T can be determined from conditional formula (2) or (2'). Once T is determined, the desired CNT average length u can be determined from relational formula (3) or (3').

[0066] Next, we will explain the manufacturing method of the CNT-FRP rotor 1. First, reinforcing fibers containing CNTs with the average length u determined as described above are manufactured using the grafting method or the spot fixing method. The reinforcing fibers manufactured in this way are made into a tow (bundle), and this tow and a predetermined matrix material (for example, thermosetting epoxy resin) are wound and laminated using the filament winding method. The matrix material is then heated and solidified, and shaped as necessary to complete the rotor 1. The desired V F To achieve this, the filament winding conditions, such as the tension of the tow, are adjusted.

[0067] Furthermore, the CNT concentration c is adjusted to obtain a desired radial yield strength. Instead of adjusting the CNT concentration c, the average CNT length u may be adjusted. Alternatively, both the CNT concentration c and the average CNT length u may be adjusted.

[0068] The relationship between the CNT concentration c and the radial yield strength depends on the filament winding conditions and the matrix material, so readjustment is necessary if these are changed.

[0069] Next, the effects of the present invention and the mechanism by which these effects are achieved will be explained using schematic diagram 4.

[0070] When any pair of nearest-neighbor reinforcing fibers (CF) 11a, 11b, in which CNTs 13 are deployed in uncured matrix material 12, come close enough that their effective deployment areas T overlap (see Figure 4A: conditional formula (1) above), a reconfiguration occurs in the deployment direction of the CNTs 13, resulting in the structure shown in Figure 4B. When matrix material 12 is cured in this state, a structure is created in which the matrix portion between the two reinforcing fibers is continuously reinforced by CNTs 13.

[0071] The direction of reinforcement by the CNTs 13 is perpendicular to the direction in which the reinforcing fibers extend (the circumferential direction of the rotor) and is also in the direction toward the opposing reinforcing fibers (the rZ direction of the rotor). This reinforcement phenomenon occurs simultaneously in all nearest-neighbor reinforcing fiber pairs. The strength of reinforcement increases the more the separation distance l decreases and the higher the CNT concentration. The change in the separation distance l affects the volume fraction V of the reinforcing fibers. F Since the circumferential yield strength is also affected through the CNT concentration, the strengthening in the rZ direction is usually adjusted by the CNT concentration.

[0072] In this way, the CNT-FRP rotor 1 of the present invention is a rotor reinforced by fibers (CF) in the circumferential direction and by carbon nanotubes (CNT) in the radial (r) and rotational axis (Z) directions.

[0073] In this way, the CNT-FRP rotor 1 of the present invention is 11 Because the rotor is also radially reinforced by a large amount of CNTs 13 originating from the surface, the drawback of the conventional simple circumferentially wound FRP rotor, namely, low radial yield strength, can be reduced or eliminated. Furthermore, because the flywheel FRP rotor 1 of the present invention is manufactured using the circumferential winding method (filament winding method), it overcomes the drawback of the conventional 3D woven FRP rotor, namely, the inability to obtain precise rotational balance characteristics. Therefore, it solves the problem that both of these conventional FRP rotors had, namely, "the expected high limit peripheral speed and high limit stored energy have not yet been achieved." First Example

[0074] Using the specifications and manufacturing method described above, a hollow disk made of CNT-added circumferentially wound carbon fiber reinforced plastic (CNT-CFRP) was formed, and tensile test samples were cut out by cutting in the circumferential and radial directions. The average volume fraction of this CNT-CFRP sample was V F= 0.7, and the average spacing distance of the carbon fibers was l = 0.61 μm. The carbon fibers used were T1000G (manufactured by Toray, average radius D = 6 μm), the matrix material was epoxy resin 470-36S (Ashland Inc.), and the carbon nanotubes were multi-walled CNTs with diameters of 5 to 20 nm. In accordance with conditional formula (2) above, the CNT deployment distance was determined to be T = 2 μm, and taking into account relational formulas (3) and (3'), the average CNT length u was adjusted to u = 3 μm for the grafted sample and u = 6 μm for the spot-fixed sample.

[0075] The tensile yield strength σ of the reference sample (first prior art) without CNTs is σ θy =4.5GPa, radial direction is σ ry In contrast, the radial yield strength σ of the CNT-CFRP samples according to the present invention decreased as the CNT concentration c increased. ry A significant increase in σ was observed at c=1.2wt% for the grafted sample and at c=2wt% for the spot-fixed sample. ry On the other hand, the radial yield strength σ θy increased slightly, but σ ry It was found that the increase in the number of cases was almost flat compared to the increase in the number of cases of the same disease.

[0076] Based on the results of this tensile test, the radial yield strength σ ry = 780 MPa, circumferential yield strength σ θy = 4.5GPa (Fig. 1) and the conventional radial yield strength σ ry = 78MPa, circumferential yield strength σ θy Theoretical calculations were carried out on the rotational stress characteristics of a simple circumferentially wound CFRP rotor with a compressive strength of 4.5 GPa. The outer radius of both rotors was b = 0.15 m, the inner radius a = 0.03 m, and the height h = 1 m. The physical parameters of the CFRP used in the calculations were the same as follows: Young's modulus: E θ =2.07×10 11 Pa, E r =1.80×10 10 Pa, Poisson's ratio: ν θ =0.246, density:ρ=1650kg / m3 The subscripts r and θ attached to each parameter symbol specify the direction on the cylindrical coordinate system.

[0077] FIG. 5 is a table comparing the rotation performance of the CNT-CFRP rotor 1, which is a hollow disk rotor of the present invention, with that of a conventional simple circumferentially wound CFRP rotor (first prior art).

[0078] As shown in Figure 5, the CNT-CFRP rotor 1, which is a hollow disk rotor of the present invention, has a critical peripheral speed of 2200 m / s, a critical rotational speed of 141000 rpm, a critical stored energy of 40 kWh, a mass critical stored energy density of 355 Wh / kg, and a failure mode of circumferential yielding. It is clear that the CNT-CFRP rotor 1 of the present invention has an approximately three-fold improvement in critical peripheral speed and an approximately nine-fold improvement in critical stored energy compared to a conventional simple circumferentially wound CFRP rotor (first prior art) that fails due to radial yielding. It can also be confirmed that the CNT-CFRP rotor 1 has similarly improved critical peripheral speed and critical stored energy compared to a 3D woven CFRP rotor (second prior art) that fails due to axial vibration, although this is not shown. Second Example

[0079] The second embodiment of the present invention is an example in which the CNT-CFRP rotor 1 of the first embodiment (Fig. 5) is used to configure a flywheel 4 of a flywheel electricity storage device as shown in Fig. 6. In the figure, 1 is the hollow disk CNT-CFRP rotor of the first embodiment, and 5 is a hub formed integrally with a rotating shaft (not shown), with the radius of hub 5 being 0.03 m, the height being 1 m, and the radius of the rotating shaft being 0.015 m. The material of the hub and rotating shaft is aluminum alloy A7075P (extra super duralumin).

[0080] The physical property parameters of A7075P are Young's modulus: E θ =E r =7.2×10 10 Pa, Poisson's ratio: ν θ =0.3, density:ρ=2800kg / m 3 The tensile yield strength is σ ry =σ θy =5.1×108 It is Pa.

[0081] The results of theoretical calculations are shown in the table in Figure 7. Because the limiting rotational speed of an extra-super duralumin A7075P hub with a radius of 0.03 m is 210,000 rpm, the limiting rotational speed of the flywheel 4 is determined by the limiting rotational speed of the CNT-CFRP rotor 1, not the hub 5, and this value is the aforementioned 141,000 rpm. The rotational energy and mass of the hub 5 (height h = 1 m) rotating at 141,000 rpm, calculated based on the physical properties of A7075P, are 108 Wh and 7.9 kg, respectively.

[0082] 7, the critical peripheral speed of the flywheel 4 is 2200 m / s, the critical stored energy is 39.9 kWh, and the critical mass energy density is 332 Wh / kg. It can be confirmed that the critical peripheral speed and critical stored energy of the flywheel 4 using the CNT-CFRP rotor 1 of the present invention are improved compared to a flywheel using a conventional simple circumferentially wound CFRP rotor (first prior art) that breaks due to radial yielding. [Explanation of symbols]

[0083] 1...CNT-CFRP rotor (hollow disk rotor) 2...Circular hole in rotor 3...CNT-(C)FRP 4...Flywheel with CNT-(C)FRP rotor 5...Hub 11...Reinforced fiber (CF) 12...Matrix material 13...CNT 14...CNT effective deployment area 15... Droplet adhesive b...Outer radius of FW rotor a...FW rotor inner radius h...FW rotor length r...rotor radius (variable) θ...azimuth angle (variable) Z…center axis D: diameter of reinforcing fiber l...Spacing distance of nearest reinforcing fibers L: Distance between the center lines of the nearest reinforcing fibers T...CNT effective deployment distance

Claims

1. A hollow disk rotor for a flywheel for a flywheel energy storage device, characterized in that it is composed of a plurality of reinforcing fibers wound around the circumference, a matrix material filling the gaps between the reinforcing fibers, and carbon nanotubes extending upward from the surface of the reinforcing fibers toward the matrix material.

2. When the average separation distance between the single fibers of the pair of reinforcing fibers located nearest to each other is 1, the average diameter is D, and the average spread distance of the upwardly spread carbon nanotubes is T, there must be at least the following between T, 1, and D: [Equation 1] Relationship between, preferably, [Equation 2] 2. The hollow disk rotor according to claim 1, wherein the following relationship exists:

3. A hollow disk rotor as described in claim 1, characterized in that the base point on the surface of the reinforcing fiber is either a grafted form in which one end face of the carbon nanotube is fixed, or a spot-fixed form in which the side face is bonded at one point.

4. In the case of the grafting mode, the relationship between the average length dimension u and the average expansion distance T of the carbon nanotube is [Equation 3] In the case of the spot-fixing mode, [Equation 4] 4. The hollow disk rotor according to claim 3, wherein:

5. The average volume occupancy V of the reinforcing fibers Ｆ 2. The hollow disk rotor according to claim 1, wherein the ratio of the axial force to the radial force is at least 0.5 (50%) or more, preferably 0.6 (60%) or more.

6. 2. The hollow disk rotor according to claim 1, wherein the reinforcing fibers are one or more composite fibers selected from the group consisting of carbon fibers, boron fibers, glass fibers, aramid fibers, alumina fibers, silicon carbide fibers, and various metal fibers.

7. The hollow disk rotor according to claim 6, characterized in that the diameter of the reinforcing fibers has an average value in the range of at least 3 μm to 9 μm, with a variation within the average value ±1 μm, and preferably has an average value in the range of 5 μm to 7 μm, with a variation within the average value ±0.5 μm.

8. 2. The hollow disk rotor according to claim 1, wherein the matrix material is one selected from the group consisting of thermosetting resins such as thermosetting epoxy resins, unsaturated polyester resins, vinyl ester resins, and phenolic resins, and thermoplastic resins such as polyolefin resins, polyamide resins, and polycarbonate resins.

9. The hollow disk rotor according to claim 1, characterized in that the carbon nanotubes are single-walled carbon nanotubes having a diameter of 0.5 to 3 nm, or multi-walled carbon nanotubes having a diameter of 5 to 30 nm, or carbon nanotubes that are a mixture of the single-walled carbon nanotubes and the multi-walled carbon nanotubes.

10. 10. The hollow disk rotor according to claim 9, wherein the carbon nanotubes have a weight percent concentration c in the matrix material in the range of c=0.1 wt % to 8 wt %.

11. A method for manufacturing a hollow disk rotor according to claim 1, comprising the steps of: a first step of fixing or adhering carbon nanotubes to the reinforcing fibers; a second step of bundling the carbon nanotube-attached reinforcing fibers formed in the first step to form a reinforcing fiber bundle; a third step of molding a rotor by a filament winding method using the reinforcing fiber bundle formed in the second step and a matrix resin.

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