Power coupler for rotating capacitive power transfer with patterned electrodes and electrical machine comprising such power coupler

Abstract

The invention concerns a power coupler for rotating capacitive power transfer, comprising two rotating capacitors (1) each comprising: - A fixed electrode (2) provided to be electrically connected to an AC power source; - A rotating electrode (3) facing the fixed metal electrode (2) and provided to be electrically connected to a rotor of an electrical machine; - An electrolyte (4) located in a gap between the two electrodes (2, 3), said electrolyte (4) comprising a solvent and a dissolved salt; characterized in that at least a portion of the surface (10) in contact with the electrolyte (4) of the two electrodes (2, 3) is roughened with at least one groove (11, 12).

Description

POWER COUPLER FOR ROTATING CAPACITIVE POWER TRANSFER WITH PATTERNED ELETRODES AND ELECTRICAL MACHINE COMPRISING SUCH POWER COUPLER

[0001] The invention is related to a power coupler for rotating capacitive power transfer, and in particular in a rotating electrical machine comprising such power coupler to feed an electromagnet.

[0002] Wired power couplers usually consist of a stationary conductive part made with wire ending in graphite brush in contact with a rotating ring made of copper. Such systems are, however, known to have problems of wear and material loss.

[0003] To avoid those inconveniences, wireless power couplers have been developed.

[0004] Inductive power couplers provide a good power transfer, generally higher than 10kW. However, such couplers occupy high volume due to shielded magnetic cores in order to avoid electromagnetic disturbances. In addition, those installations are expensive.

[0005] Capacitive power couplers avoid those last inconveniences. A capacitive power coupler comprises two capacitors, each one made of two electrodes facing each other and separated by an air gap. The capacitors are powered by an AC power source. In order to raise the power transferred by the coupler, the air capacitors can be replaced but the so-called electrolytic capacitors, where the gap between the two electrodes of each capacitor is filled with an electrolyte. The power transferred is comprised between 2 and 10 kW.

[0006] However, to ensure such power transfer, the AC current frequency is higher than 10 kHz, resulting in the use of electronics more expensive than those available when using AC current frequencies below 10 kHz.

[0007] To overcome this problem, it is known to increase the geometrical area of the electrodes, which results in the increase of capacitance and the decrease ofelectrolyte resistance. However, this also has an impact on the increase in coupler volume.

[0008] To overcome this problem, it is known to roughen the surface of the electrodes by sandpapers or by sandblasting. However, the effect on the increase in capacitance remains limited.

[0009] It is also known to deposit metallic nanoparticles on the electrodes. For example, the so-called platinized platinum electrodes are made by depositing platinum nanoparticles on smooth platinum electrodes. However, this solution is very expensive.

[0010] It is also known to increase the surface roughness by depositing porous conductive layers on top of the electrode layer, like those made with different porous carbons of different type. However, these layers do not have a high mechanical stability and will not withstand the high rotation speed for the application in the rotating machines. Also, the size and connectivity of the semi-opened pores will decrease the movement of ions of the electrolyte and the capacitance will drop strongly at high frequency.

[0011] The aim of the invention is therefore to propose a power coupler for rotating capacitive power transfer easy and cheap to build, which transfers enough power, and which is usable in a wide range of conditions.

[0012] For this purpose, the invention proposes a power coupler for rotating capacitive power transfer, comprising two rotating capacitors each comprising:- A fixed electrode provided to be electrically connected to an AC power source;- A rotating electrode facing the fixed electrode and provided to be electrically connected to a rotor of an electrical machine;- An electrolyte located in a gap between the two electrodes, said electrolyte comprising a solvent and a dissolved salt,at least a portion of the surface in contact with the electrolyte of the two electrodes is roughened with at least one groove.

[0013] The power coupler of the invention may also include the following optional characteristics considered individually or according to all possible combinations of techniques:- The at least portion of the surface in contact with the electrolyte of the two electrodes is roughened with a plurality of grooves- The depth of each groove is comprised between 5pm and 200pm, preferably between 10 pm and 100 pm.- The grooves are evenly distributed on the at least one portion of the surface in contact with the electrolyte of the two electrodes.- The grooves follow a repeating pattern, with a repeating groove separation between two contiguous grooves.- The width of groove separation between two contiguous grooves is comprised between 1pm and 150pm.- The total surface of the considered electrode is roughened with grooves. - The width of each groove is comprised between 20pm and 100pm, the inter-distance between two consecutive grooves is comprised between 20pm and 200pm.- The electrodes are made of metal, preferentially of stainless steel.- At least a portion of each groove has a nano-structured surface.- The solvent of the electrolyte is organic.

[0014] Another object of the invention concerns a method for laser scribing at least a portion of a surface in contact with an electrolyte of the two electrodes of each capacitor of a power coupler described above, the grooves (11, 12) being scribed using a pulsed laser scanning the surface (10) of the electrodes.

[0015] The method of the invention may also include the following optional characteristics considered individually or according to all possible combinations of techniques:- The scanning speed of the laser is comprised between 1 and 30 mm / s, preferentially between 1 and 10 mm / s.- The laser pulse duration is comprised between 80 and 800 femtoseconds.

[0016] Another object of the invention concerns an electrical machine comprising a stator, a DC power source, a DC-to-AC converter electrically connected to the power source, a power coupler as described above, the fixed electrode of each rotating capacitor of the power coupler being fixed to the stator of the electrical machine and electrically connected to the DC-to-AC converter, and the rotating electrode of each rotating capacitor being electrically connected to an AC-to-DC converter which is electrically connected to an electromagnetic rotor of the machine which is mounted on a rotational axis, the rotating electrode of each rotating capacitor being mounted on the rotational axis.

[0017] Other characteristics and advantages of the invention will be apparent in the below descriptions, by way of indication and in no way limiting, and referring to the annexed figures among which:- Figure 1 , which represents a scheme of a rotating capacitor of the power coupler of the invention,- Figure 2, which represents a Scanning Electron Microscopy (SEM) image at a first magnification of one example of patterned electrode of the invention,- Figure 3, which represents a Scanning Electron Microscopy (SEM) image at a second magnification of the example of patterned electrode of figure 2,- Figure 4, which represents a Scanning Electron Microscopy (SEM) image at the second magnification of another example of patterned electrode of the invention, - Figure 5, which represents a cross-section view of patterned electrode showing two consecutive grooves,- Figure 6, which represents variation of the module and the phase of the impedance of a single capacitor of the power coupler with smooth electrodes versus frequency of the AC voltage source,- Figure 7, which represents variation of the real part and the imaginary part of the complex impedance of a single capacitor of the power coupler of figure 4 versus frequency of the AC voltage source,- Figure 8, which represents the variation of capacitance of the single capacitor of two power couplers, one with smooth electrodes and one with patterned electrodes, - Figure 9, which represents the variation of impedance of the single capacitor of two power couplers of figure 6, one with smooth electrodes and one with patterned electrodes,- Figure 10, which represents a scheme of the circuit for estimation of the power transfer from the power source to the motor of the electrical machine,

[0018] First, it is noted that on the figures, the same references designate the same elements regardless of the figure on which they feature and regardless of the form of these elements. Similarly, should elements not be specifically referenced on one of the figures, their references may be easily found by referring oneself to another figure.

[0019] It is also noted that the figures represent mainly one embodiment of the object of the invention but other embodiments which correspond to the definition of the invention may exist.

[0020] Elements in the figures are illustration and may not have been drawn to scale.

[0021] The invention refers to a power coupler for rotating capacitive power transfer, and to an electrical machine comprising such a power coupler. The invention thus allows wireless capacitive power transfer for electrical machines, in particular Wound Field Synchronous Machines.

[0022] The power coupler comprises at least one rotating capacitor 1 as illustrated in figure 1. Each rotating capacitor 1 comprises one fixed electrode 2 (attached to the stator of the electrical machine via an axis 6), a rotating electrode 3 (attached to the rotor of the electrical machine via an axis of rotation 7), and a space or gap between the electrodes 2, 3. As a non-limitative example, the electrodes 2, 3 can be flat plates parallel to each other or concentric cylinders.

[0023] As a non-limitative example, the electrodes 2, 3 can be made of pure metal - like stainless steel - or of a metal alloy, or any type of material known to havesufficient electrical conductivity, such as glassy carbon, conductive metal carbides, or metal-coated ceramics.

[0024] The gap between the two electrodes 2, 3 is filled with a liquid electrolyte 4, said electrolyte 4 comprising a solvent and a dissolved salt. In addition, to allow the rotation of the considered electrode 3 and to avoid liquid electrolyte 4 leakage, one or more sealing rings 5 are needed in the design of such capacitors 1 (see figure 1).

[0025] Advantageously, the solvent is organic. One of the advantages of using an organic solvent is that it remains in a liquid state over a wide temperature range, making electrolyte 4 based on said organic solvent an excellent candidate for use on rotary machines where the temperature of the electrodes 2, 3, of the electrolyte 4 and of the sealing rings 5 can exceed 100°C.

[0026] More precisely, the electrolyte 4 should stay liquid at least down to -30°C, a value agreed for several outdoor applications, including vehicles. On the other hand, the electrolyte 4 should stay liquid at least up to 150°C, since high speed rotating motors can attain such temperature during operation, a value agreed for motors rotating at high speeds, including those used for electrical vehicles.

[0027] The capacitance C of this type of electrolyte capacitive rotating capacitor 1 is known and it is related to the separation of the ions of opposite charge coming from the salt dissolved in the solvent (the electrolyte being the solution of such salt in such solvent) upon the application of a voltage between an electrode and the liquid electrolyte phase.

[0028] This type of rotating capacitor 1 is also known as double layer capacitor. Indeed, in a complete device (that is, with the two electrodes 2, 3), there is one capacitance per electrode, both being in series, the total capacitance of the capacitor 1 being approximately half of the capacitance of each individual electrode / electrolyte interface. The capacitance of the capacitor 1 cannot be predicted, and, hence, it should be measured. Concerning the geometrical dimensions, it is known that the capacitance of this type of capacitor 1 is approximately proportional to the geometrical surface of the electrodes 2, 3 in contact with the electrolyte 4.

[0029] According to the electrochemistry field, for smooth surface electrodes 2, 3, the capacitance per cm2of smooth surface is usually in the range of tens pF / cm2.The capacitance of these capacitors 1 does not depend on the gap between electrodes 2, 3 since the concentration of salt in the solvent is high enough that the so-called diffuse capacitance becomes negligible. On the other hand, these capacitors 1 are also characterized by the electrolyte resistance, Re, which represents a loss for the power transfer, estimated as the well-known ohmic loss. The electrolyte resistance, Re, depends on the geometrical dimensions of the capacitor 1 and it can be estimated with the known formula Re=peg / A, where peis the resistivity of the electrolyte 4, g the gap or distance between the electrodes 2, 3, and A the geometrical surface of the electrodes 2, 3 in contact with the electrolyte 4. The electrodes may be parallel and / or concentric to each other. A constant depending of the shape and edge effects might be added to correct such formula. Therefore, the loss due to the electrolyte resistance, Re, can be reduced by increasing the geometrical area of the electrodes 2, 3 or by reducing the gap between electrodes.

[0030] The electrical rotating machine of the invention comprises a stator, a DC power source, a DC-to-AC converter connected to the DC power source. People skilled in the art perfectly understand that the DC power source connected to the DC-to-AC converter form an AC power source.

[0031] The electrical machine also comprises a power coupler according to the invention which comprises two rotating capacitors 1 , the fixed electrode 2 of each rotating capacitor 1 of the power coupler being fixed to the stator of the machine via an axis 6 and electrically connected to the AC power source via an electrical cable 8, the rotary electrode 3 of each rotating capacitor 1 being linked to the rotor of the machine via an axis of rotation 7 and electrically connected to an AC-to-DC converter via an electrical cable 9. Finally, the AC-to-DC converter is connected to the electromagnetic rotor mounted on the axis of rotation 7.

[0032] In order to provide an efficient power transfer through the capacitive power coupler of the invention, the capacitance of each rotating capacitor 1 should be such that its impedance at the AC frequency of operation is much lower than the equivalent load resistance of the motor. The impedance of an electrolytic capacitor comprises a real impedance part Zreai, which at high frequencies is practically theelectrolyte resistance Re, and an imaginary impedance part Zimag, which at high frequencies is practically the capacitance C. For electrolytic capacitors made of smooth electrodes the capacitance C is around 1 - 10 pF / cm2). For electrolytic capacitors, the electrolyte resistance Re is proportional to the geometrical area in contact with the electrolyte, inversely proportional to the gap, and it depends on the electrolyte composition.

[0033] For example, for a capacitor of about 30 cm2of geometrical area, 1 mm of gap, and an electrolyte whose resistivity peis at most 150 Q.cm, the electrolyte resistance Re is below a few tenths of ohms. For an efficient power transfer, the impedance of each rotating capacitor 1 should be at least 10% lower than the equivalent load resistance of the motor. For example, if the equivalent motor load resistance is 8 ohms, the impedance of each capacitor should be below 8x10% / 2 = 0.4 ohms (the factor 2 is because of the two capacitors). For capacitors with smooth surface and sufficient geometrical area having a capacitance greater than, for example, 50 pF, it is necessary to use an operating frequency greater between 10 kHz and 100 kHz. If a similar capacitor can enhance its capacitance by a factor of 10, then the operating frequency can be decreased to the range 1 kHz and 10 kHz, making possible a less expensive electronic system. With electrolytes 4 and capacitances respecting all these conditions, a power transfer over 5kW can be expected.

[0034] Regarding the invention and in reference to figures 1 to 5, at least a portion of the surface 10 in contact with the electrolyte 4 of the two electrodes 2, 3 of each capacitor 1 of the power coupler is roughened with at least one groove, more preferentially with a plurality of grooves 11, 12 scribed by a femtosecond laser, said grooves 11, 12 being evenly distributed on the at least one portion of the surface 10 of electrodes 2, 3 in order to facilitate scribing and in order to enhance repeatability and reproducibility. Indeed, homogeneous scribing not only simplifies the femtosecond laser scribing process, but also maximizes the scribed surface 10 and therefore the roughness factor.

[0035] More preferentially, the total surfaces 10 in contact with the electrolyte 4 of both electrodes 2, 3 are roughened with grooves 11, 12.

[0036] It is known that in an electrolytic capacitor, there are in fact two capacitors in series (one per electrode / electrolyte interface). As they are in series, the one with the smaller capacitance prevails. Since it is known that the capacitance depends on the surface of the electrodes, in a very advantageous manner, both electrodes of the capacitor are roughened in the same way, so that they have the same surface roughness. This precise and controlled scribing of the surface of each electrode 2, 3 could be accomplished using the femtosecond pulsed laser.

[0037] The pulsed laser used to scribe the surface 10 of each electrode 2, 3 may be, for example, a solid-state Nb or Nb:YAG laser. Advantageously, the laser used for the scribing of the electrodes 2, 3 of the invention is a fiber laser, and in particular those which is normally pulsed in the femtosecond range. The spot size is much smaller than that obtained for example with a CO2 laser (around 100 times smaller), enabling smaller, more precise scribing. In addition, femtosecond lasers avoid any melting effect, which would smooth the groove walls. In other words, the femtosecond laser scribing allows the apparition of a nanostructure effect inside the grooves 11 , 12, and therefore, such a great increase in roughness, and therefore, in capacitance of the device 1.

[0038] In order to further facilitate scribing and in order to further enhance repeatability and reproducibility, the groves 11 , 12 are scribed in the surface 10 of the electrodes 2, 3 following a repeating pattern 13.

[0039] In a non-limitative way, two types of patterning geometry are carried out: parallel grooves 11 in a single direction, and crossed grooves 11 , 12 in two directions, one perpendicular to the other, as represented in figures 2 and 3 wherein the pattern 13 is a rectangle. More precisely, figure 2 represents a Scanning Electron Microscopy (SEM) image with 30 times magnification of a patterned electrode 2, 3, while figure 3 represents a Scanning Electron Microscopy (SEM) image with 500 times magnification of the patterned electrode 2, 3. Other geometries can be carried out, for example, inclined cross-grooves, 3 cross-grooves, or any other geometry since the depth is enough to raise the area of the surface 10 in contact with the electrolyte 4 of the electrodes 2, 3. However the pattern 13 may be triangular, or even parallelepipedal or more generally a curved section of any shape.

[0040] Concerning the profile of the grooves, this may be triangular, or even parallelepipedal or more generally a curved section of any shape.

[0041] Figure 5 gives an overview of the profile of a groove 11 , 12 and the dimensions associated with the grooves 11 , 12. Each groove 11 , 12 comprises a bottom 25, two opposite walls 24a, 24b and two opposite edges 26a, 26b. The interdistance ID between two consecutive grooves 11 , 12 is the distance between the centers 25 of those two consecutive grooves 11 , 12.

[0042] The range of groove 11 , 12 inter-distance ID is between 20pm and 200pm, and more preferentially between 50pm and 180pm. The width W of each groove 11 , 12 is between 20pm and 100pm, and more preferentially between 25pm and 90pm. The width D of the groove separation 23 (i.e the non-scribed surface) between two contiguous parallel grooves 11 , 12 is between 1pm and 150pm, and more preferentially between 20pm and 150pm.

[0043] The speed for the laser scan during electrodes scribing is between 1 and 50 mm / s, and preferentially between 1 and 10 mm / s, in order to obtain enough depth for the grooves 11 , 12, since the deeper the groove 11 , 12 is, the higher the surface of the groove 11 , 12 is. Several femtosecond lasers were tried, each of them with different laser power in the laser spot of the electrode surface 10.

[0044] The depth H of each groove 11 , 12 is between 5pm and 200pm, and more preferentially between 20pm and 100pm. Assuming that the surface inside the grooves 11 , 12 is smooth, we estimated the increase of surface for a groove 11 , 12 with rectangular cross-section (which will give the maximum increase of surface as compared to a smooth non-patterned surface).

[0045] In the case of a rectangular section, and for a pattern 13 of grooves 11 , 12 in only one direction, the increase of surface with respect to a non-patterned electrode (also called roughness factor) is the area of the vertical walls of one groove times the number of grooves 11 , 12. For the case of crossed-grooves 11 , 12 (in particular, for perpendicular directions with the same conditions for each direction: a rectangle pattern 13), the increase in surface with respect to a non-patterned electrode is the area of the vertical walls of one groove 11 , 12 times the number of grooves 11 , 12 minus the area of the vertical walls of the crossed squares times thenumber of squares 13. A roughness factor from 2 to 5 was obtained in these conditions.

[0046] Scribing the surface 10 of the electrodes 2, 3 with a femtosecond laser enables much deeper cavities than those obtained by random sandblasting, and also increase drastically nano-porosity of the surface 10 of the scribed electrodes inside the grooves 11, 12 : see figure 4 which represents another SEM image with x500 magnification showing nano-structure of the walls 24a, 24b, bottom 25 and edges 26a, 26b of the grooves 11 , 12. This is significantly increasing the total area of the electrodes 2, 3, while ensuring controlled scribing, with a roughness factor significant rising.

[0047] Electrolyte resistance Re and capacitance C was inferred by electrochemical impedance spectroscopy (EIS) using a Gamry Reference 600 impedance meter, using a sinusoidal voltage signal of 0.1 Vrms amplitude and a frequency, f, scan from 1 Hz to 1 MHz. The measurements are done at a constant temperature of 22°C, since both electrolyte resistance and capacitance change with temperature. The electrolyte resistance is the value of the impedance in the zone of high frequencies where the impedance phase is close to zero (so the imaginary part of the impedance is negligible versus the real part).

[0048] The measurements with the impedance meter directly give values of the impedances Z of the capacitor 1 as a function of the frequency, either as the impedance module Zmod and phase Ze, or as the real and imaginary parts of the impedance (Zreai and Zimag). The equivalent capacitance C of the capacitor 1 is obtained assigning the Zimag to a capacitor 1 , that is, by doing C=1 / (2irf Zimag).

[0049] An example using a capacitor with smooth metal electrodes (without laser scribing) of values of the complex impedance Z is shown in Figure 6 and 7, in two representations. Figure 6 shows Z module and phase versus frequency of the AC voltage with a capacitor with smooth metal electrodes in an electrolyte EMIM OMs -GBL (salt 1-ethyl-3-methylimidazolium methanesulfonate (EMIM OMs) dissolved in gamma-butyrolactone (GBL) solvent) using a sinusoidal voltage signal of 0.1 Vrms amplitude (Volts Root Mean Square). Figure 7 shows Z real and imaginary parts versus frequency of the AC voltage with the same capacitor with smooth metalelectrodes and the same electrolyte as in Figure 6, using a sinusoidal voltage signal of 0.1 Vrms amplitude.

[0050] As shown in Figure 6, the electrolytic capacitor 1 does not behave as an ideal capacitor, since the impedance phase is not constant at -90° and the real part of the impedance, which should have been equal to the electrolyte resistance, is not constant either. It is known from the electrochemistry field that electrolytic cells have no constant capacitance. It has been modelled as a circuit component having in series a “Constant Phase Element” (CPE) and an equivalent serial resistance (ESR). The ESR is mainly due the electrolyte resistance. The CPE has a resistor and capacitor in series, where their values decrease with increasing frequency. The phase of a CPE element according to the model is constant. This model is an approximation, since the data of Figure 6 or Figure 7 cannot be fitted to such model on the entire frequency range. Also, their fitting parameters cannot be predicted.

[0051] Figure 7 shows the same impedance data as Figure 6 but represented as real and imaginary parts of the impedance. It teaches us several things: First, the imaginary part (from which the equivalent capacitance can be extracted following the formula C=1 / (2rrf Zimag)) decreases with frequency, which means that the advantage of high capacitance of electrolyte capacitors 1 is lower at high frequencies. Second, above a certain frequency, the imaginary part Zimag is much smaller than the real part Zreai, as the latter is practically constant.

[0052] The capacitance C as a function of frequency f is extracted from the values of the imaginary part Zimag of the impedance, using the above-mentioned formula.

[0053] Figure 8 gives example of plots of C versus frequency for two electrolytic capacitors (made with flat electrodes) with an organic solvent. One capacitor has smooth electrodes (non-patterned) and the other capacitor has scribed electrodes 2, 3 (patterned). It clearly appears that the capacitance C of the capacitor with patterned electrodes 2, 3 is much higher than the capacitance C of the capacitor with non-patterned electrodes for frequencies between 1 Hz and 10kHz. It thus appears that capacitors with scribed electrodes may be used with an AC power source with lower frequencies and still keeping a sufficient capacitance C.Example

[0054] In reference to figure 9, an example will now be described.

[0055] In this example, the electrolyte capacitor 1 is made of two metal flat electrodes 2, 3 of rectangular shape, with a Teflon spacer of 1 mm thickness placed in the edge of the electrodes 2, 3, the surface of the electrodes 10 being scribed with a femtosecond pulsed laser over the entire area in contact with the organic electrolyte 4. Measurement of impedance have been made in static conditions.

[0056] More precisely, each tested electrolytic capacitor 1 was made using two stainless steel plates of 35x35mm size. The plates of each of these devices were separated by a Teflon spacer of 1mm thickness. The gap of the devices 1 is filled with the organic electrolyte 4. In this particular case, it was an electrolyte 4 made with gamma-butyrolactone (GBL) solvent and a 1-ethyl-3-methylimidazolium methanesulfonate (EMIM OMs) salt, with a salt mass percentage (mass of salt over mass of salt plus solvent) of 36%. Other organic solvents, other salts, and other salt mass percentages can have a similar behaviour.

[0057] Pressure between the plates was applied to keep the liquid electrolyte 4 inside the gap of the devices 1, avoiding electrolyte leakage. The electrode geometrical area in contact with the liquid electrolyte 4 (also called active area) was ~ 7.5 cm2(~ 30x25mm). Impedance spectroscopy was carried out using a Gamry Reference 600 impedance meter, using a sinusoidal voltage signal of O.IVrms amplitude and a frequency, f, scan from 1 Hz to 1 MHz. The measurements were done at an ambient temperature of 22°C, and in static conditions, that is, without no speed of rotation.

[0058] Several pattern geometries and laser scribed parameters were tested. Parallel single or crossed (perpendicular) linear grooves were laser scribed with a typical groove inter-distance in the range from 50pm to 200pm. One example of pattern is shown in Figures 2, 3 and 4 as images obtained with a Scanning Electron Microscope (SEM) at two magnifications.

[0059] Table 1 below shows twelve different pattern geometries tested and one reference with smooth electrodes (non-patterned). The code n.a is for “non applicable”.Table 1

[0060] Regarding column “groove geometry”, the groove geometry was either parallel grooves 11 , 12 in a single direction (X) or two sets of parallel grooves 11, 12 in two directions perpendicular between them (XY). The approximated roughness factor was calculated by assuming that the groove cross-section shape was a perfect rectangle 13, and that the groove surfaces (edges, walls and bottom) were as smooth as the non-patterned parts of the electrode 2, 3. In such case, this hypothetical roughness factor under these conditions is lower than 3.

[0061] Table 2 below shows the values of the capacitance (in pF) of several electrolytic capacitor devices 1 at three frequencies, of 200Hz, 1 kHz and 10kHz, with the capacitance normalized to a geometrical active area 1 cm2(obtained just dividing by the real geometrical area of 7.5 cm2). The table also shows the ratio of capacitance at each of the three frequencies against the capacitance of the capacitor with the reference electrodes, which are the ones with smooth non-patterned surface. The table 2 also shows the imaginary part of the impedance of the devicesat the same three frequencies, expressed in ohms (Q) and taking into account the real geometrical of 7.5 cm2.Table 2

[0062] All the devices, except device #1 , have a ratio of capacitance C increase versus the reference device (smooth non-patterned surface) of more than 5, in the frequency range between 200Hz and 10kHz. Device #1 was made with a small groove depth H (~1 pm) and a high laser scan speed of 50mm / s. For these devices (except device #1), such ratio of C increase ranges from >5 up to around 200, depending on the device and the frequency. This C ratio increase is due to an additional roughness of the groove surface due to nano-structures in the walls 24a, 24b, edges 26a, 26b and bottom 25 of the grooves, 11, 12, which is indeed observed when looking to the Scanning Electron Microscope (SEM) image of figure 4.

[0063] The table 2 also shows the values of the imaginary part of the impedance for the devices with the geometrical active area of 7.5cm2at three frequencies. As an example of an expected efficient power transfer, the values which have an imaginary impedance below 0.3 O and, at the same time, a real part of the impedance below -15% of the value of the electrolyte resistance are marked in bold.At 10kHz, all devices (except device #1 ) fulfill such condition, and at 1 kHz seven out of ten (device #1 excluded) fulfill such condition.

[0064] An example of impedance variation versus AC voltage frequency is given in figure 9. Impedance variation of the capacitor with non-patterned electrodes and Impedance variation of the capacitor device #4 (tables 1 and 2) are plotted in this figure 9, using a sinusoidal voltage of 0.1 Vrms amplitude. The electrolyte is EMIM OMs - GBL.

[0065] It clearly appears that the impedance of the capacitor device #4 is ten times lower than the impedance of the capacitor device with non-patterned electrodes 2, 3. More precisely, both the values of real and imaginary parts Zreai, Zimag are lower for the device #4 than for the non-patterned electrode device. This means that the resistance of the device #4 is lower than that of the non-patterned electrode device, and that the capacitance of the device #4 is higher than that of the non-patterned electrode device.

[0066] The roughening of the surface 10 of electrodes 2, 3 in contact with the electrolyte 4 intends to increase the capacitance by a factor much higher than usual sandblasting, allowing the use of lower frequencies for the AC voltage (below 10kHz), reducing the cost of the converter used in the whole capacitive power transfer system.

[0067] According to the scheme 14 of figure 10, the estimation of the electrical power transfer can be simplified by considering only the transfer of alternative voltage (AC) from a source 15 (i.e the alternative power source of the electrical machine) to the motor of the electrical machine, which is simulated as a load resistance 17, and the impedance 16 of two capacitors of the same size and same capacitance of the power coupler of the invention. For the impedance of the two similar capacitors, we take the experimental values of the real and imaginary parts Zreai, Zimag of the single capacitor 1 , multiplied by two since the power coupler of the invention consists of two similar capacitors 1 in series.

[0068] As the experimental values of impedance of the capacitor is taken in the estimation, this impedance will take into account the electrolyte resistance Re, the capacitance C, and the possible inductance from the plates 2, 3. The scheme 14 forthis simple estimation of the power transfer from the source to the load is shown in Figure 10. The equations for calculation of the power are those used in the well-known domain of electric circuits. The power on electrical components under a sinusoidal electrical signal is averaged over a period. It is zero for the imaginary impedance parts Zimag and the product of the voltage by the current in the real impedance parts Zreai. This is what is called active power. These calculations also allow to estimate the voltage drop over the imaginary part of the capacitor components, to check that this voltage is under the limit of the electrochemical stability window of the system.

[0069] In general, the active power of a circuit component of impedance Z, is the product of the rms voltage across such component, multiplied by the rms current module going through that component, and multiplied by the power factor, which is the ratio of the real part of the impedance over the module of the impedance of such component. In order to calculate the active power of the load resistance 17 of the schematic circuit 14 of Figure 10, one has to calculate the total impedance of the circuit, consisting of the power coupler 15 in series with the load resistance 16. The power coupler 15 impedance has real and imaginary parts (which can be named Zcp,reai and Zcp mag), and the load resistance 16 only a real part (named Ri_oad). The calculation of the real and imaginary parts of the total impedance (named Ztot eai, Ztot.imag) is done by addition of the respectively real and imaginary parts of 16 and 17. Then, this is converted into module (or amplitude) and phase of the total impedance of the circuit, with the well-known formulas of square root of the addition of the squares of the real and imaginary parts for the module, and arctangent of the imaginary part over the total module for the phase. This allows to get the total current amplitude or module (in rms) going through the circuit 14, as the voltage amplitude in rms divided by the module of the total impedance. The phase of the current would be the phase of the total impedance but with opposite sign. Once the current calculated, the voltage and the active power of each of the two components of the circuit 14, that is the load resistance 17 and the power coupler 16, can be calculated using the well-known formulas of the Ohm law (voltage = current x impedance) but using complex numbers or the module-phase notation.

Claims

CLAIMS1. Power coupler for rotating capacitive power transfer, comprising two rotating capacitors (1) each comprising:- A fixed electrode (2) provided to be electrically connected to an AC power source;- A rotating electrode (3) facing the fixed electrode (2) and provided to be electrically connected to a rotor of an electrical machine;- An electrolyte (4) located in a gap between the two electrodes (2, 3), said electrolyte (4) comprising a solvent and a dissolved salt;characterized in that at least a portion of the surface (10) in contact with the electrolyte (4) of the two electrodes (2, 3) is roughened with at least one groove (11 , 12).

2. The coupler according to the preceding claim, characterized in that the at least portion of the surface (10) in contact with the electrolyte (4) of the two electrodes (2, 3) is roughened with a plurality of grooves (11 , 12).

3. The coupler according to claim 1 or claim 2, characterized in that the depth (H) of each groove (11, 12) is comprised between 5pm and 200pm, preferably between 10 pm and 100 pm.

4. The coupler according to claim 2 or claims 2 and 3, characterized in that the grooves (11 , 12) are evenly distributed on the at least one portion of the surface (10) in contact with the electrolyte (4) of the two electrodes (2, 3).

5. The coupler according to any of claims 1 to 4, characterized in that the grooves (11 , 12) follow a repeating pattern (13), with a repeating groove separation (23) between two contiguous grooves (11 , 12).

6. The coupler according to claim 4 or claim 5, wherein the width (D) of the groove separation (23) between two contiguous grooves (11 , 12) is comprised between 1pm and 150pm.

7. The coupler according to any of claims 1 to 6, characterized in that the total surface (10) of the considered electrode (2, 3) is roughened with grooves (11 , 12).

8. The coupler according to any of claims 1 to 7, characterized in that the width (W) of each groove (11 , 12) is comprised between 20pm and 100pm, in that the interdistance (ID) between two consecutive grooves (11 , 12) is comprised between 20pm and 200pm.

9. The coupler according to any of claims 1 to 8, characterized in that the electrodes (2, 3) are made of metal, preferentially of stainless steel.

10. The coupler according to any of claims 1 to 9, characterized in that the at least a portion of each groove (11 , 12) has a nano-structured surface.11.The coupler according to any of the preceding claims, wherein the solvent of the electrolyte (4) is organic.

12. A method for laser scribing at least a portion of a surface (10) in contact with an electrolyte (4) of two electrodes (2, 3) of each capacitor (1) of a power coupler according to any of claims 1 to 11 , characterized in that the grooves (11 , 12) are scribed using a pulsed laser scanning the surface (10) of the electrodes (2, 3).

13. The method according to the preceding claim, characterized in that the scanning speed of the laser is comprised between 1 and 30 mm / s, preferentially between 1 and 10 mm / s.

14. The method according to claim 12 or claim 13, characterized in that the laser pulse duration is comprised between 80 and 800 femtoseconds.

15. An electrical machine comprising a stator, a DC power source, a DC-to-AC converter electrically connected to the power source, a power coupler accordingto any of claims 1 to 10, the fixed electrode (2) of each rotating capacitor (1) of the power coupler being fixed to the stator of the electrical machine and electrically connected to the DC-to-AC converter, and the rotating electrode (3) of each rotating capacitor (1) being electrically connected to an AC-to-DC converter which is electrically connected to an electromagnetic rotor of the machine which is mounted on a rotational axis (7), the rotating electrode (3) of each rotating capacitor (1) being mounted on the rotational axis (7).

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