Power conversion device, safety system, and wireless optical power transmission system

Abstract

A wireless optical power transmission system includes a transmitter and a receiver. The transmitter includes a laser that emits a beam, a scan mirror that steers the beam to a receiver, and a control unit that receives signals from a detection unit in the receiver and controls the beam power and the scan mirror. The receiver includes a photovoltaic cell with a bandgap energy of 0.75 to 1.2 eV, the photovoltaic cell having a plurality of conductors on its beam-receiving surface. A cover layer of a material that blocks illumination at wavelengths outside the laser wavelength is disposed on the photovoltaic cell. The cover layer may have an anti-reflective coating on its top and bottom surfaces. The detection unit thereby generates a signal representing the power of the laser beam impinging on the receiver, independently of illumination other than the wavelength of the laser beam. The control unit thereby can maintain the laser power impinging on the receiver.

Description

Technical Field

[0001] The present invention relates to the field of photovoltaic cells, and more particularly to a photovoltaic cell adapted to convert a beam from a laser power source into electrical energy in a wireless power system.

Background Art

[0002] Photovoltaic cells, known as PV cells, are designed and used in many systems to convert light (visible or invisible) into electrical energy. In a wireless power supply system, a photovoltaic cell is used to convert laser light directed from a transmitter to the cell into usable electricity. In such a system, accurately measuring the amount of laser light received by the cell is important for many reasons, such as maintaining the correct target of the beam, maintaining the safe operation of the system, and efficiently converting the power generated by the photovoltaic cell into stable power that can be used.

[0003] In a typical photovoltaic cell, light is converted into electrical energy using one or more pn junctions and extracted from the cell using two electrodes. The lower electrode is usually coated with a metal, and typically, there are two options for the upper electrode. (i) A metal grid typically made of aluminum, silver, or gold. This metal grid has low series resistance but reflects a portion of the light hitting the grid itself, thus preventing incidence on the cell. (ii) A continuous, substantially transparent conductive coating such as indium tin oxide (ITO). This conductive coating transmits most of the light incident on the cell but has high resistance losses because it has a higher resistance than a metal grid structure.

[0004] According to the metal grid, there is typically a trade-off between resistance loss and optical loss due to reflection from the metal grid. The resistance loss is V 2It is measured by / R. Here, V is the voltage of the cell and R is the series resistance. This resistive loss decreases as the metal coverage ratio of the cell increases, while the optical loss increases as the metal coverage ratio of the cell increases, as known as the "shading effect". Typically, there is an optimal balance between these two factors. This balance typically depends on the predicted cell illumination and current of the photovoltaic (PV) cell.

[0005] Non-Patent Document 1 describes that a PV cell achieving a conversion efficiency of 44.5% from sunlight to electricity has been developed.

[0006] According to Green et al. (Non-Patent Document 2), as published in Non-Patent Document 3 presented at the 29th PV Solar Energy Conference and Exhibition held in Amsterdam, the Netherlands in September 2014, the highest reported PV efficiency was 46%.

[0007] Non-Patent Document 4 proposes a composite glass "lenslet" designed to minimize grid losses by concentrating light away from the conductor grid. Such a composite lenslet pattern requires an exact arrangement with respect to the metal grid and is not widely used.

[0008] Unless such a composite "lenslet" optical system is used, there is a normal trade-off between resistive loss and shading loss, so typically the optical loss results in the range of 2% - 10%, and the same resistive loss for high-concentration cells.

[0009] In a review by Non-Patent Document 5 covering currently available PV optical systems, it is stated that if the trade-off between the conductor ratio and optical shading can be eliminated without using a composite structure optical system that requires an exact arrangement with respect to the metal grid, an improvement in cell efficiency of 3% - 20% in photovoltaic cells would be possible.

[0010] Non - Patent Document 6 describes a system for minimizing shading loss. Instead of typically flat rectangular - shaped conductors, the use of rounded finger - shaped conductors has been proposed. Due to the finger shape, much of the incident light reflected from the PV cell surface is returned to and absorbed by the PV cell surface.

[0011] To optimize prior - art PV cell conductors, an optimization screen is used by PV designers. An example of such a grid calculator is shown below. https: / / www2.pvlighthouse.com.au / calculators / Grid%20calculator / Grid%20calculator.aspx

[0012] When using such a grid calculator, the user can choose different shapes of conductive fingers (rectangular, triangular, elliptical, pseudo - rectangular), different materials, widths, and heights. However, since the user cannot choose the optical cover layer, such a coating cannot be optimized. By using this tool to optimize the grid, it becomes possible to compare different trade - offs between the metal coverage ratios of different cells. The same website defines shading loss as "Shading loss is caused by the presence of metal on the upper surface of a solar cell that prevents light from entering the solar cell." Shading loss is determined by the transparency of the said upper surface. This transparency is defined as the ratio of the upper surface covered by metal for a flat upper surface. Transparency is determined by the width of the metal lines on the surface and the spacing between the metal lines. An important practical limit is the minimum line width associated with a particular metallization technology. For the same transparency, a technology with a narrow line width can make the finger spacing denser, thus reducing the emitter resistance loss.

[0013] The current conventional approach to optimizing conductor grids can also be learned from Non-Patent Document 7. Page 62 of the online reference states that "the optimal width (Wb) of the bus bar occurs when the resistive losses of the bus bar are equal to its shadowing losses," and different well-known methods in the industry for reducing such losses are shown. Different options for increasing light trapping within the cell are described, but no option for recycling reflected light is mentioned.

[0014] However, while the PV cells described in such references provide high efficiency, these PV cells are generally optimized for maximum efficiency when converting solar energy into electrical energy. This may have different technical requirements from converting the power of a laser beam into electrical energy.

[0015] Therefore, there is a need for a photovoltaic cell that converts a wireless transmission laser beam into power to overcome at least some of the drawbacks of prior art systems and methods.

[0016] The disclosure of each publication referred to in this section and other sections of this specification is hereby incorporated herein by reference in its entirety.

Prior Art Documents

Non-Patent Documents

[0017]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0018] This disclosure describes a novel exemplary system for use within a wireless laser power transmission system. The system includes a PV cell having a cover layer used when converting the power of a laser beam into electrical energy. The configurations described in this disclosure seek to improve the photovoltaic efficiency, improve safety, and improve the accuracy of power measurements made using these PV cells by simultaneously achieving several of the following objectives. a) Reduction of the dependence between the metal finger coverage rate and shading. b) Reduction of the amount of reflected light from the cell. c) Reuse of the reflected light as electrical energy. d) Reduction of resistive losses in the series resistance of the conductor. e) Improvement of the accuracy of laser power measurement using a photovoltaic cell when illuminated from different angles. f) Improvement of the accuracy of laser power measurement using a photovoltaic cell when used under different lighting conditions such as outdoors, indoors, or near a heat source. g) Confirmation that the directed transmitted laser power reaches the intended receiver at the intended power level. That is, confirmation that no unreasonable level of power has “strayed” into the path.

[0019] Often, the beam aiming mechanism of a wireless laser power transmission system, and / or the safety mechanism of the system, rely on an accurate measurement of the laser power received by a photovoltaic cell. For example, if the laser aiming is adjusted until the maximum power from the PV is generated, accurate aiming can be impaired by sunlight that can bias the measurement at a fraction of time, such as when the cell is illuminated at one instant but not the next. In systems where safety depends on power measurement, illumination from different directions should be optimized to give similar power measurement results, and sunlight or heat sources should not bias the measurement results beyond a small safety margin. The devices of the present disclosure allow the measurement to be essentially independent from the illumination direction and other light sources of different wavelengths that can illuminate the system.

[0020] When the light reflected by the photovoltaic cell is collected from both the front surface and the conductor and directed towards the cell area not covered by the metal grid, it is possible to reduce the resistive loss by increasing the coverage ratio of the metal grid, while at the same time reducing the shading effect and allowing an accurate optical power estimation based on the generated power in a manner independent from the beam direction and even from the beam uniformity. When using a narrow conductor as in prior art cells and the beam illuminates only a portion of the cell, the narrow conductor results in a greater resistive loss than in the case of uniform illumination, making the optical power estimation based on power or current inaccurate. On the other hand, according to the configuration of the present description, the resistive loss can be low, making the optical power estimation based on power measurement accurate.

[0021] However, light is reflected from different directions onto the photovoltaic cell and is reflected by the grid in a complex pattern that can include several orders of typical Bragg reflection caused by the periodic structure of the grid. This effect causes many problems. First, the efficiency is reduced due to the loss of reflected light. Second, there is a safety hazard in the reflected light. Third, since the amount of reflected light depends on the illumination direction, the optical power measurement by the photovoltaic cell is inaccurate and its usefulness for safety and aiming purposes is reduced. In many cases, the safety mechanism of the system depends on the accurate measurement of the laser power received by the photovoltaic cell.

[0022] In a typical radio power system that does not include PV reorientation when the beam direction changes, i.e., the PV does not automatically tilt to maximize power, the laser can illuminate the cell from almost any direction. Such a system typically operates at angles up to 60 degrees from the normal to the surface of the cell. As a result, the light reflected from the conductor is reflected at different angles, spreading to about ±70 degrees or more from the normal of the cell, and the main reflection is at an angle opposite to the incoming beam. When a symmetric field of view is maintained so that the beam can enter from either side of the PV, any collection systems located inside the field of view must be transparent (including the support structure of the optical elements), otherwise they will prevent the cell's ability to collect light from those directions.

[0023] An efficient collection system must also cover a significant portion of the various directions within this field of view in order to be efficient. So, when the beam is incident from one direction and generates a reflected beam in another direction, in order for the collection system to capture and collect the reflected beam in that other direction, since the collection system is opaque, it inevitably has to block the incoming light from that other direction. In other words, expressed in terms of the field of view provided, any incident angle included in the field of view generates a reflection in some direction. When reusing the light (or reflected light) going in this direction, it is necessary to effectively exclude this direction from the field of view by placing an opaque collection system in the direction of the reflection.

[0024] The object of the present invention is to introduce such a system and improve the efficiency, aiming accuracy and safety of the system.

[0025] In one exemplary implementation, the shapes of the plurality of conductors in the PV are selected such that the conductors reflect light in a broad pattern. The typical structure mainly depends on the manufacturing ability and the predicted field of view of the incident light, that is, the angle at which light can enter. As a general concept, a common flat surface, a square or rectangular conductor grid is not preferred, while round and triangular grids are advantageous.

[0026] The present system can use a PV having a plurality of conductors optimized to prevent shading loss. The shapes of the plurality of conductors in the PV can be selected such that they reflect light in a broad pattern. For example, the plurality of conductors may be in a rounded finger shape or a triangular shape. Thereby, the reflected light from the conductors is directed towards the absorbing PV cells or is totally internally reflected by a cover layer covering the cells, improving the efficiency of the system. Furthermore, this prevents dangerous reflected light that could damage the eyes from being reflected from the PV cells. This is because the light reflected from the conductors is diffused and thus not reflected in a uniform manner.

[0027] The PV configuration of the present disclosure is significantly different from prior art PVs in that the cover layer can be coated with anti-reflection (AR) coatings on both of its interface surfaces, namely an outer AR coating between the cover layer and the outside air and an inner AR coating between the cover layer and the light-absorbing surface of the PV cell material.

[0028] The outer anti-reflection (AR) coating is essentially adapted to allow infrared to transmit only the wavelength of a typical laser beam and reflect all other wavelengths that collide, i.e., wavelengths that are likely to be present in external illumination and affect the ability to measure the laser wavelength of the PV cell. That is, the AR coating does not need to reflect wavelengths to which the PV cell is insensitive, such as wavelengths in the mid- and far-infrared or UV, for example, because it does not affect the measurement at the laser wavelength.

[0029] In addition, the cover layer should include a material that absorbs wavelengths other than the wavelength of the beam. As a result, any wavelengths not reflected by the AR coating are absorbed by the cover layer and do not reach the pn junction of the PV cell, so that an inaccurate reading of the amount of laser power transmittance is not caused. Therefore, the upper layer of the PV cell should also be absorptive to visible wavelengths that can be incident on the upper layer of the semiconductor PV cell material itself due to not being reflected by the AR coating, similar to the cover layer itself. This is achieved by selecting an upper layer of a PV cell material having a sufficiently low bandgap.

[0030] One purpose of blocking wavelengths other than the wavelength of the laser beam from reaching the pn junction of the PV absorption cell is that when the receiver is acting as a safety power meter for the transmitter, this receiver needs to give an accurate reading of the amount of the colliding laser beam. That is, other incident illumination should be blocked before colliding with the absorption PV cell because it must not interfere with this measurement.

[0031] Furthermore, the cover layer needs to have a high refractive index. This is to lower the critical angle of total internal reflection (TIR) and capture as much of the laser beam incident on the cover layer and reflected from the surface and the conductor as possible. This effect will be explained in detail below. This is aimed at maximizing the amount of illumination reaching the absorbing PV cell. This is because a portion of the beam reflected from the conductor is re-reflected from the cover layer, preventing leakage from the PV cell that would have occurred in the absence of the cover layer.

[0032] This also has the advantage that any light emerging from the PV cell, whether from multiple reflections inside the cover layer between the PV cell surface and the conductor or from total internal reflection from the top of the cover layer, will be in different "random" directions and positions. This is in contrast to prior art PV configurations that reflect the beam in a uniform manner.

[0033] The top of the absorbing PV layer at the bottom of the cover layer may also be covered with an AR coating that maximizes the absorption of the light impinging from the cover layer into the PV cell material.

[0034] According to a further implementation example, and using the advantages provided by the double reflection prevention coated cover layer, a wireless optical power transmission system including a transmitter, a receiver, and a control unit is described. Here, a) The transmitter includes a laser adapted to emit a beam and a scan mirror adapted to steer the beam towards the receiver, b) The receiver includes a photovoltaic cell having at least one junction with a bandgap energy of 0.75 eV to 1.2 eV and a detection unit adapted to detect the power of the laser beam impinging on the receiver independently of illumination at wavelengths outside the range of the wavelength of the laser beam, c) The control unit receives a signal from the detection unit and is adapted to interact with at least one of (i) the power of the beam emitted by the laser and (ii) the position of the scan mirror.

[0035] The detection unit then generates a signal representing the power of the laser beam impinging on the receiver. This signal is advantageously independent of illumination at wavelengths outside the wavelength of the laser beam.

[0036] The control unit can then use this signal to operate in two functionally distinct modes. In one implementation, the control unit uses this signal to control at least one of (i) the power of the beam and (ii) the position of the “scan” mirror, such that they are adjusted to provide an optimal aiming output at the receiver. This is the conventional role of a control system, which operates to actively control or “tune” the system to provide an optimal power input at the ultimate target of the system, namely the receiver.

[0037] The term “scan mirror” is used herein to denote any movable mirror. This term is intended to include mirrors with fixed positions, or mirrors that move slowly to direct the aim of a charged beam towards the receiver and maintain that aim, rather than scanning the surroundings, and provides an alternative description of “scan” mirrors with different functions.

[0038] According to the second configuration, the control system can also act as a safety verification system to ensure that the laser power beam is not deflected in an unintended direction where it could cause damage. This system warns about a situation where the laser beam is supposed to direct a beam of a given power towards a receiver, but the receiver is not receiving all or part of that power, indicating that at least a part of the beam power is being deflected in an unintended direction. This is achieved by using the control system in a role opposite to its normal operating role, i.e., for the purpose of verifying the system state rather than controlling the system. In this mode, the control system inputs data from the laser regarding the output power that the laser is instructed to give or is actually giving, as measured at the transmitter, and inputs data from the scan mirror regarding the direction in which the beam is aimed. These data are then combined in the controller and compared with the signal generated in the detector unit regarding the actual laser power received there. A deviation greater than a predetermined error or loss margin between the predicted laser powers from these two sets of data is used to trigger a safety warning state regarding the beam transmission in the forward direction or regarding the presence of an obstacle in the beam path that is reducing or deflecting the intended beam.

[0039] Cover layer

[0040] The cover layer can be advantageously applied on a conductor and this cover layer typically has a high refractive index greater than 1.5, preferably greater than 1.6 or 1.65, or has a semiconductor coating layer with a refractive index greater than 2, but surely has a refractive index greater than 1.3.

[0041] In one implementation example, the cover layer consists of a polymer layer or a dielectric layer covered by a glass layer. The cover layer is an optical layer of uniform composition but typically has an upper volume and a lower volume and may consist of many different transparent or translucent layers and has the following properties.

[0042] The upper volume of the optical cover layer is in contact with the ambient environment, which is typically air, but may also be in contact with other materials. The upper volume is often a protective hard glass layer that is more protective than the soft inner volume of the cover layer. Additionally, the upper volume can be selected to have a higher refractive index than the main volume to increase the probability that internal light undergoes total internal reflection. The lower part of the optical cover layer contacts both the surface of the PV (typically including an AR coating) and the conductors described above. The cover layer needs to be as transparent as possible for the incoming beam wavelength, requires an optical density of less than 2 for the beam wavelength, and an optical density of less than 1 or less than 0.5 is preferred. Some materials may tolerate an optical density of less than 0.1 or less than 0.01, but nevertheless, the cover layer needs to be configured to block, absorb, or reflect most other wavelengths except the beam wavelength. This prevents other wavelengths that may enter from sunlight or other illumination sources from reaching the pn junction. Therefore, it is necessary to have an optical density of at least 0.5 for these wavelengths. The upper part of the PV is usually covered by an AR coating. The AR coating is typically adapted to minimize reflection between air (refractive index of about 1) and the PV (refractive index typically 2 - 4) in prior art PVs.

[0043] In the case of the cells of the present disclosure, the lower AR coating needs to be designed to minimize reflection of the laser wavelength between the PV (refractive index 2 - 4) and the optical cover layer (refractive index 1.5 - 2) for the beam wavelength, but at the same time, it is also necessary to reflect wavelengths shorter and longer than this away from the cell. Both AR coatings can be used to remove unwanted wavelengths. The upper part of the cover layer may be covered by an AR coating adapted to minimize reflection of the beam between air (refractive index of about 1) and the upper part of the cover layer while increasing reflection of other wavelengths.

[0044] The cover layer "shields" the PN junction from light other than the laser beam itself. This is important for safety and aiming measurement accuracy. This can be achieved by absorbing such unwanted light or by reflecting it from the PV cell. As described above, since the AR coating of the cover layer reflects external illumination outside the laser band, the cover layer further absorbs what is not reflected. In many cases, since the PN junction is inefficient for very long wavelengths, there is no need to block such wavelengths.

[0045] The overall structure of the grid itself

[0046] The density of the grid lines needs to be selected according to various parameters, as will be described below.

[0047] It is advantageous when the spacing between the grid lines is greater than the value obtained by dividing the operating wavelength by twice the refractive index of the cover layer. d > λ / 2n Here, d is the spacing between the conductor lines, λ is the operating wavelength in vacuum, and n is the refractive index of the cover layer.

[0048] Also, it is advantageous when the spacing between the grid lines is less than 100 times the operating wavelength in vacuum. d < 100λ

[0049] It is also preferable that the optical cover layer is selected to absorb short wavelengths even with minimum absorption for the operating wavelength.

[0050] Coating the conductor with a diffusion coating can relax the above dimensional constraints.

[0051] Physical explanation

[0052] When light is reflected from the grid of a prior art photovoltaic cell, the light is reflected at an unknown angle and is usually irrecoverable. Constructing a light collection system that will not interfere with the incoming beam is difficult, if not impossible. At the same time, when a prior art photovoltaic cell is placed under direct sunlight, 100 mW / cm of sunlight 2 is converted into power, resulting in inaccurate reading of laser power and potentially affecting safety and aiming operations.

[0053] The photovoltaic cells of the present disclosure utilize four distinct design features that overcome many of the above drawbacks of prior art PV cells. 1. The incoming beam strikes the conductor with a reduced field of view due to the high refractive index of the cover layer used. According to Snell's law, the beam angle is reduced by only sin(θ1)=n1 / n2×sin(θ2), so light reflected at an angle greater than the maximum field of view within the cover layer can be manipulated to be reflected towards the cell without blocking the field of view. 2. The grid structure serves to increase the amount of light reflected outside the field of view of the incoming beam, due to its dense grid lines and the structure of each conductor. 3. The cover layer serves as a collection and reflection system. This is because most of the beam undergoes total internal reflection by the cover layer and is reflected back towards the photovoltaic cell. 4. Sunlight is absorbed / reflected by the cover layer and its associated AR coating, so it does not affect the power measurement value.

[0054] By using the cover layer and the features described above, the optical losses of the laser beam can be reduced. The ability to reuse some of the light allows for a reduction in resistance losses due to an increase in the covered area of the conductor, an increase in the fill factor of the cell, and an increase in the maximum power point voltage and current, thus further improving the device.

[0055] To facilitate efficient reflection to the photovoltaic cell, the thickness of the cover layer needs to be less than the width of the photovoltaic cell. If it is thick, light will escape from the side of the cover layer.

[0056] All of the above design features enable the construction of a grid with many "shadings" as compared to the normal shading described in the prior art.

[0057] The shape of a single conductor line needs to be selected to maximize the higher-order reflections from the grid, which is typically as described above.

[0058] The upper volume needs to have an AR coating for air or for a predicted ambient material such as water or vacuum.

[0059] Advantages when illuminating the cell with an infrared laser

[0060] The features described herein are particularly useful for a photovoltaic cell designed to convert an infrared laser into electrical energy. This is due to several advantages provided by such a cell.

[0061] First, such a cell is typically illuminated by a power 10 to 100 times per unit area compared to the one-sun condition to which a conventional solar cell is typically exposed. Compared to a CPV cell, the present design has a wide field of view and intentionally reduces the efficiency with respect to sunlight.

[0062] Second, an infrared laser relies on low-energy photons, which require a low bandgap in the pn junction that generates a lower voltage and a higher current compared to a normal sunlight spectrum-optimized cell. Since the current is high, the resistive losses become significant.

[0063] The pn junction in such a cell is tuned to "ignore" long-wavelength photons that are longer than the wavelength of the laser beam by selecting a material with a sufficiently low bandgap. As a result, when a receiver is placed near a heat source such as a heating element or an incandescent lamp, the output parameters such as power, current, and voltage are much less affected by long-wavelength infrared photons.

[0064] Thirdly, unlike typically uniform sunlight radiation across the entire area of the cell, laser radiation forms concentrated spots and tends to "underfill" the cell. The laser beam generally has an approximately circular, elliptical or rectangular shape, often with a Gaussian or higher-order Gaussian attenuation power curve in many cases. The non-uniform power curve complicates the optimization of the conductor thickness, and what a PV-centered power curve typically means by a scanning mirror is that electrons generated at the center of the cell where most of the optical power is concentrated generate a high current, so that compared to the conductor required for photons absorbed near the periphery of the cell where there is less optical power present, a conductor of greater thickness and thus lower resistance is required.

[0065] Fourthly, specular reflection from the cell of a high-power laser can pose safety concerns and either blocking or diffusing is required. In any case, the reflection from the cells of the present disclosure is of low power and significantly reduces safety concerns.

[0066] Advantages of Illuminating a Cell Using a Wireless Power Beam Radiation System

[0067] Current cells have many advantages when used as components of a wireless power system. Typically, such a system can power a portable electronic device equipped with a battery having a capacity of 1 to 100 watts-hour. Typically, the required charging power is on the order of 0.5 to 10 W, which requires a charging beam of 1 to 30 W to be used.

[0068] The typical diameter of the beam is a few millimeters, and the smaller the beam, the more difficult it becomes to continue focusing in the range of 1 to 10 m. Thus, non-diffusive reflection of 2 to 10% of the beam is associated with a significant risk (20 mW to 1 W). Therefore, it is essential to reduce the total amount of light reflected by the conductor and at the same time diffuse that light. It is also essential to reduce reflection from the cell. This cell typically gives a reflection of up to 1% instead of the usual 2 to 10%.

[0069] Thus, according to an exemplary implementation of the device described in the present disclosure, there is provided a power conversion device that converts optical power into power suitable for wireless optical power transmission using a laser beam, and this power conversion device (i) a photovoltaic cell having a plurality of conductors on a surface adapted to receive a laser beam, the photovoltaic cell having at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, (ii) a cover layer disposed on the photovoltaic cell and includes, the cover layer includes a material adapted to limit the transmission of illumination having a wavelength outside the range of the wavelength of the laser beam by either absorption or reflection to transmit the laser beam to the photovoltaic cell, the laser beam has a wavelength in the range of 700 nm to 1500 nm, the wavelength of the illumination outside the range of the wavelength of the laser beam is in the range of 550 nm to 700 nm, the transmittance of the cover layer for the wavelength of the laser beam is at least 50% higher than the transmittance of the cover layer for wavelengths in the range of 550 nm to 700 nm, and the power conversion efficiency of the power conversion device for power at the wavelength of the laser beam is at least 2.5 times higher than the power conversion efficiency for power at a wavelength of 550 nm.

[0070] In such a power conversion device, the bandgap energy can be tuned to the wavelength of the laser so that the power conversion efficiency of the optical power to power for any wavelength that is at least 25% longer than the wavelength of the laser is more than 4 times smaller than the conversion efficiency at the laser wavelength. In any of the devices described above, the cover layer further (i) a first anti-reflection coating disposed on the surface of the cover layer away from the photovoltaic cell, the first anti-reflection coating adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and transmit the laser beam to the cover layer, or (ii) A second anti-reflection coating disposed between the surface of the photovoltaic cell and the cover layer, including at least one of the second anti-reflection coatings adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and transmit the laser beam to the photovoltaic cell.

[0071] Still further implementations according to the present disclosure are directed to a safety system for a wireless optical power transmission system including a transmitter, a receiver, and a control unit. Here, (i) The transmitter includes a laser adapted to emit a beam, and a scan mirror adapted to steer the beam to the receiver and (ii) The receiver includes a photovoltaic cell having a plurality of conductors on a surface adapted to receive a laser beam, the photovoltaic cell having at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, and a cover layer disposed on the photovoltaic cell wherein the cover layer includes a material adapted to limit illumination having a wavelength outside the range of the wavelength of the laser beam by either absorption or reflection and transmit the laser beam to the photovoltaic cell, (iii) The control unit receives first data representing the position of the scan mirror, receives second data representing the power of the beam emitted by the laser from the transmitter, determines a predicted power incident on the photovoltaic cell from the first data and the second data, compares the predicted power with the power of the laser beam impinging on the receiver measured by the photovoltaic cell, and is adapted to indicate a safety issue that may occur when the predicted power deviates from the measured power by more than a predetermined level.

[0072] According to still other embodiments of the present application, a wireless optical power transmission system including a transmitter and a receiver is provided, wherein the transmitter includes a laser adapted to emit a beam, A scanning mirror adapted to steer the beam towards a receiver, A control unit adapted to receive a signal from a detection unit in the receiver and to control at least one of (i) the power of the beam emitted by the laser and (ii) the position of the scanning mirror and comprising The receiver comprises A photovoltaic cell having a plurality of conductors on a surface adapted to receive the laser beam, the photovoltaic cell having at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, the photovoltaic cell being adapted to detect the power of the laser beam reaching the photovoltaic cell, The receiver comprises a cover layer disposed on the photovoltaic cell, the cover layer comprising a material adapted to absorb or reflect illumination having a wavelength outside the range of the wavelength of the laser beam and to transmit the laser beam to the photovoltaic cell, (i) A first anti-reflection coating disposed on the surface of the cover layer remote from the photovoltaic cell, the first anti-reflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and to transmit the laser beam to the cover layer, or (ii) A second anti-reflection coating disposed between the surface of the photovoltaic cell and the cover layer, the second anti-reflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and to transmit the laser beam to the photovoltaic cell, at least one of and comprising The detection unit generates a signal representative of the power of the laser beam impinging on the receiver independently of other illumination other than the wavelength of the laser beam, and the control unit is adapted to control at least one of (i) the beam and (ii) the position of the scanning mirror to maintain the power impinging on the receiver.

[0073] A further power conversion device adapted to convert the optical power adapted for optical wireless power transmission using the laser beam described in the present disclosure into electrical power, A power conversion device having at least one junction with a bandgap energy of 0.75 eV to 1.2 eV, which may include a power conversion device having an outer layer through which laser light is transmitted to the one junction. The outer layer is configured to transmit at least a first wavelength to the one junction with an efficiency of at least 80% when illuminated from any direction within ±20° with respect to the normal of the surface of the outer layer, where (i) the conversion efficiency of the power conversion device is at least 30% and the first wavelength is a near-infrared wavelength of 700 nm to 1500 nm; (ii) the outer layer of the power conversion device is configured to reflect or absorb a portion of the incident illumination of a second wavelength of 550 nm to 700 nm, such that less than 60% of the illumination of the second wavelength reaches the at least one junction when illuminated through the outer layer from any direction within ±20° with respect to the normal of the surface of the outer layer, and the conversion efficiency of the power conversion device for the second wavelength is less than 20%; (iii) the outer layer of the power conversion device is configured to absorb or reflect at least a third wavelength of 300 nm to 550 nm, such that at least 50% of the power of the third wavelength is absorbed before reaching the at least one junction when irradiated through the outer layer from any direction within ±20° with respect to the normal of the surface of the outer layer, and the conversion efficiency of the power conversion device for the third wavelength is less than 10%; (iv) the conversion efficiency of the power conversion device for a fourth wavelength of 1500 nm to 2000 nm is less than 5% and is at least one of.

[0074] As described in the present disclosure, further other embodiments of a power conversion device that converts an optical power beam into electric power are (i) a semiconductor device having a pn junction adapted to absorb the optical power beam; (ii) an upper conductor and a lower conductor in electrical contact with the semiconductor device, where the upper conductor is an upper conductor and a lower conductor covering a portion of the upper surface of the semiconductor device. (iii) An optical layer disposed on the upper surface of the semiconductor device, the optical layer including an upper volume and a lower volume, the lower volume being optically in contact with the upper surface of the semiconductor device and optically in contact with the upper conductor of the semiconductor device, and the upper volume being optically in contact with air, the optical layer may include (a) the upper conductor is adapted to reflect at least 30% of the light impinging on the upper conductor, (b) the optical layer has an optical density of less than 2 with respect to the optical power beam, (c) the upper conductor is adapted to direct at least 25% of the light reflected by the upper conductor at an angle greater than sin -1 (1 / refractive index of the lower volume).

[0075] In such a power conversion device, at least a portion of the light reflected by the conductor may be reflected at an angle of total internal reflection from the upper surface of the upper volume. The upper volume of the optical layer itself may be an anti-reflection coating adapted to reduce the reflection of the optical power beam incoming from a medium having an approximate refractive index of 1. The upper volume of the optical layer may further be a scratch-resistant coating. In the case of the anti-reflection coating, it may be further adapted to reduce the reflection of the optical power beam over an angle of at least -10 degrees to +10 degrees with respect to the normal to the upper surface.

[0076] In addition, in any of the power conversion devices described above, the coverage ratio of the upper surface covered by the upper conductor may be at least 4%. The conductor itself may be made of metal and may include at least partially aluminum, gold, silver or copper.

[0077] Furthermore, in the power conversion device described above, the area of the geometric protrusion of the portion of the conductor aligned at an angle of at least sin -1 (1 / refractive index of the lower volume) with respect to the upper surface of the semiconductor device may be at least 25% of the area of the semiconductor device multiplied by the coverage ratio of the upper surface covered by the upper conductor.

[0078] According to yet another implementation of such a power conversion device, laser reflections from the power conversion device can be diffused. In such a case, the diffuse reflection from the power conversion device can have a viewing angle of at least 1.5 milliradians. Further, m 2 Multiplying the area of the semiconductor device measured in m 3 units by the bandgap of the junction measured in square joules and then multiplying the result by the cube of the maximum design power of the cell measured in watts gives P 2 A < 214 × 10 -30 such as 214 × 10 -30 less than.

[0079] In any of the power conversion devices described above, the upper conductor may include a conductive grid having a finger-shaped or triangular-shaped contour.

[0080] Finally, according to yet another implementation described in the present disclosure, there is provided a power conversion device that converts optical power into power suitable for optical wireless power transmission using a laser beam. This power conversion device is a photovoltaic cell having a plurality of conductors on a surface adapted to receive a laser beam, the photovoltaic cell including at least one junction having a bandgap energy of 0.75 eV to 1.2 eV and a cover layer disposed on the junction. Here, the cover layer includes a material adapted to perform at least one of absorption or reflection of illumination having a wavelength in the range of 550 nm to 700 nm and to transmit the laser beam toward the photovoltaic cell. The laser beam has a wavelength of 700 nm to 1500 nm, the transmittance of the cover layer with respect to the wavelength of the laser light is at least 50% higher than the transmittance of the cover layer with respect to wavelengths in the range of 550 nm to 700 nm, the power conversion device has a conversion efficiency to power at the wavelength of the laser beam that is at least 2.5 times as large as the conversion efficiency to power at a wavelength of 550 nm.

Brief Description of the Drawings

[0081] The present invention will be more fully understood and appreciated from the following detailed description when used in conjunction with the drawings.

[0082]

Figure 1

Figure 2

Figure 3

Figures 4A - 4C

Figure 5

Figures 6A - 6B

Figure 7A

Figure 7B

Figures 7C - 7D

Figures 8A - 8B

Figure 8C

Figure 9

DETAILED DESCRIPTION OF THE INVENTION

[0083] Here, refer to FIG. 1 which schematically shows a typical structure of a photovoltaic cell. The photovoltaic cell is made from a p-type semiconductor 13 attached to an n-type semiconductor 11 or, in some cases, close to the n-type semiconductor 11 but still separated by other layers to form a pn junction 12. A complete metal contact is arranged on the back of the cell 14, and a partial metal contact in the form of a grid (or similar structure) 15 is arranged on the front of the cell. This allows light to penetrate the space between the conductors. Incident illumination generates a current 16 through the load.

[0084] Referring now to FIG. 2, the first two orders of Bragg reflection of an incident beam 23 in a prior art rectangular grid photovoltaic cell 21 are schematically shown. The incoming beam 23 that impinges on the metal grid 22 of the photovoltaic cell 21 reflects approximately 2 - 10% of the power in the beam 21 by the conductor grid 22 in the Bragg pattern. This includes several orders such as the zero order 24 and the first order 25 shown in FIG. 2.

[0085] Referring now to FIG. 3, such typical multiple reflections 31 from the surface 32 of a prior art photovoltaic cell are shown. At least 20 reflections are visible. The power in each "order" depends very strongly on the angle of illumination, causing safety and aiming problems. Total reflection extends from the zero order reflection over approximately 5 degrees, and the zero order reflection is the strongest reflection. The higher the order of reflection, the smaller the power contained compared to lower orders. This is particularly different from the cells of the present invention where reflection into higher order modes above the critical angle of the optical cover layer is preferred.

[0086] Reference is now made to FIGS. 4A, 4B, and 4C. Alternative grid contours used in PV cells are schematically shown. FIG. 4A shows the reflection from a grid of rectangular conductors. This reflection is generated at an angle opposite to the incident beam angle. FIG. 4B shows the reflectivity from a triangular reflector. This triangular reflector splits the beam into two wide-angle beams and functions well in a small field of view (FOV). FIG. 4C shows the reflection from a rounded conductor that spreads over a wide angle. A preferred implementation (not shown in FIGS. 4A - 4C) is a conductor covered by a diffusive top layer, which can spread the radiation in a very wide pattern.

[0087] The structures shown in FIGS. 4B and 4C, as well as the diffusive coating on the conductor, enhance the higher-order reflections from the grid, thus enhancing the "reuse" of the reflected light. It should be understood that it is generally not possible to manufacture a complete geometric shape, and such illustrations serve as simple examples. It is preferred to select a pattern that spreads light over a wide FOV, such as a diffusive, circular, or triangular shape. A shape that reflects a significant percentage of the light at wide angles, such as a triangle with a reflector tilt greater than the threshold angle, is preferred. The conductor is typically made of a reflective material such as a metal that reflects at least 30% of the incident light. Typically, a material that reflects at least 90% of the light, such as aluminum, silver, or gold, is selected. The conductor may be covered by a diffusive coating.

[0088] Referring now to FIG. 5, the terms used in this disclosure are explained. The angular dependence of the reflected light is shown as a function of the incident light on a finger-shaped conductor in PV51 having a back electrode and a pn junction in addition to conductor 52. Angle 55 is the local angle formed by the intersection of line 57, which is tangent to the surface of the finger conductor at point 56 on the conductor, and the normal 58. The area projected onto the upper surface 59 of the PV by conductor 52 is shown below the PV and is numbered 53. The area projected onto the PV surface by a portion of conductor 52 where the angle is less than the threshold value is shown below the PV and is numbered 54.

[0089] Referring now to FIGS. 6A and 6B, as proposed in the present application, the difference between the incoming beam angle 61a of a known PV cell 62 without a high refractive index cover layer and the incoming beam angle 61b of a cell 64 with a high refractive index cover layer 63 is shown. Each is illuminated by a light beam either of beam 61a in FIG. 6A or beam 61b in FIG. 6B. Beam 61b passes through a cover layer 63 which is a transparent coating layer and is referred to as an optical cover layer. The upper part of the optical cover layer 63 may be a anti-reflection coating that reduces the reflection of beam 61b, and most of the reflection is allowed to enter the optical cover layer 63. The lower surface 65 of the optical cover layer 63 is in contact with both the upper part of the PN junction of the cell 64 and a conductor (not shown) of the cell 64. There is an anti-reflection coating between the lower part 65 of the optical cover layer 63 and the cell 64. This is adapted to reduce the reflection of the light beam 61b when traversing from the refractive index of the lower part 65 of the optical cover layer 63 to the upper part of the cell 64. The optical cover layer 63 typically has a high refractive index and preferably has an optical density that allows at least 80% of the power to pass through and has an optical density of less than 2 when measured with respect to the optical beam 61b. Due to the high refractive index of the coating layer 63, the light beam 61b reaches the upper surface of the cell 64 at an incident angle smaller than the angle at which the beam 61a parallel to 61b is incident on the cell 62. The thickness of the coating layer 63 should be less than the width of the cell 64.

[0090] Referring now to FIG. 7A, there is shown a photovoltaic cell 71 similar to that shown above in FIG. 5 with a cover layer 78 added. This figure shows the different paths that an incident beam can take when it strikes the PV structure. Beams 79a, 79b, 79c, and 79g are shown to be absorbed by cell 71 when present in a known PV cell having the same conductor coverage ratio. On the other hand, beam 79d is shown to be reflected by the side surface of conductor 72 and immediately absorbed by cell 71. Beam 79e is shown to be reflected by the upper part of conductor 72 and leak out of the device in a manner similar to what occurs when striking a prior art rectangular conductor. That is, the reflection is lost and not converted to electrical energy. Beam 79f is shown to be reflected from the conductor at an angle such that the beam is lost in a prior art PV cell. However, since the current PV is covered by cover layer 78, beam 79f strikes the inside of the upper surface of cover layer 78 at an incident angle greater than the critical angle with respect to the upper surface boundary and is thus reflected back towards the PV surface as beam 79f' and, instead of being lost, is absorbed by the cell even after additional reflections as typically shown in FIG. 7A. The optical conductor 72 is wider than a similar conductor in a prior art cell and thus has lower resistance and higher reflectivity compared to the prior art conductor. The shape of the optical conductor 72 is designed to maximize the projected area of the portion of the conductor that is inclined at an angle exceeding 80% of the critical angle calculated based on the refractive index of the lower part of the optical cover layer 78. Reflection at such an angle is more likely to ultimately reach the cell again and thus be absorbed by the cell and converted to power. Conductor 72 typically has a reflectivity exceeding 80 to 90%, but in some cases may have a lower reflectivity. Conductor 72 may be made of a metal such as aluminum, silver, gold, molybdenum, copper, nickel, or tungsten and may be coated with a diffusive coating such as an opal coating. Alternatively, the conductor may have small reflective structures or particles deposited thereon. The plurality of conductors 72 are spaced such that the reflection from the conductors is maximized up to an order exceeding 80% of the critical angle calculated based on the refractive index of the lower part of the optical cover layer 78.Such a spacing is typically greater than 0.5 wavelengths / refractive index and less than 100 wavelengths.

[0091] Although the conductors in FIGS. 7A and the previous FIG. 5 and the subsequent FIGS. 7B and 8C are shown as finger-shaped, it should also be understood that these conductors can also have any other suitable profile, such as the triangular shape of FIG. 4B, that provides multiple internal reflections of the beam within the cover layer 78.

[0092] Referring now to FIG. 7B showing the PV cell of FIG. 7A according to the present disclosure, anti-reflection coatings 73, 74 are applied to the upper and lower surfaces of the cover layer 78.

[0093] Referring now to FIG. 7C, the significance of the viewing angle of the apparent light source is shown. This is the angle subtended by the apparent light source when viewed from a point within the space. Specifically, the resolution of the human eye is such that a typical retinal photoreceptor views the surrounding environment at an angular resolution of approximately 1.1 milliradians. This calculation is based on the fact that the outer segment of a single retinal photoreceptor cell has a radius of approximately 25 microns and the effective focal length of the human eye is approximately 17 millimeters in an aqueous environment and approximately 22 millimeters in air. A light source with a viewing angle of 1.1 milliradians can be focused on a single retinal cell, so the total power from the light source can be absorbed by the same cell. A light source with a smaller viewing angle than this is still absorbed by a single photoreceptor. However, due to low optical quality, a light source with a large viewing angle cannot be focused to a small spot. That is, it is impossible to focus such a light source on a single biological cell, and as a result, the risk posed by a light source with a large viewing angle is reduced. In other words, a single retinal cell occupies approximately 4×10 -6 steradians. Radiation from a diffused light source with a viewing angle of 1.5 steradians is expected to be distributed over at least two retinal cells, so the retinal risk is approximately halved and is safe.

[0094] Referring now in detail to FIG. 7C, PV71a reflects a portion of the light illuminating the PV in a diffused manner. A lens 75 disposed 100 mm from the surface of the window in front of PV71a converges the light beam to a light spot 77 at the focus of the lens. If PV71a reflects a high-quality light beam similar to a TEM 00 laser, the lens 75 focuses the light to a diffraction-limited spot. The angle 70 subtended between the top of the image, the center of the lens, and the bottom of the image is close to zero.

[0095] Another possible scenario is shown in FIG. 7D. Here, PV71a is diffusely reflecting the image instead of preserving the optical quality of the original laser beam. In such a case, the lens 75 does not produce a diffraction-limited spot, but rather produces an image 76 of PV71a, and the angle 70 is significantly larger.

[0096] The lens 75 typically positions the point of minimum viewing angle of the beam by moving around the PV cell, approaching and moving away from the PV cell, and determines the viewing angle of the beam at a given distance.

[0097] In addition to the above requirements, it is important that the PV responds to changes in the illumination level. The higher the beam power, the more responsive the photovoltaic cell must be to allow for a safety system based on detection of the light level. It has been found that to increase the responsiveness of the PV, the cell structure needs to be adapted to match the intended power level according to the following equation. 10 27 × (bandgap) 2 A / d < 214 / P 3

[0098] Here, d is the thickness of the layer in the photovoltaic cell that absorbs the photons of the beam, measured in meters.

[0099] The bandgap is the bandgap energy of the pn junction, measured in joules.

[0100] A is meters2 It is the area of the photovoltaic cell measured in units.

[0101] If the cell used is too large, the responsiveness of the cell decreases and it cannot respond quickly enough to changes in the illumination level.

[0102] Since d is usually less than 300 microns in thickness and always less than 1000 microns in thickness, this can be simplified as follows. 10 30 × (bandgap) 2 A < 214 / P 3

[0103] Or, in a more convenient form, it is as follows. P 3 × (bandgap) 2 A < 213 × 10 -30

[0104] Now refer to FIGS. 8A and 8B. FIG. 8A shows a reference PV cell 81a having known characteristics, and FIG. 8B shows the effect of the reflected collimated laser light on the human eye. In FIG. 8A, the light beam 84 illuminates the PV cell 81a, and most of the light is absorbed by the cell 81a, while a part of the incident beam is reflected by the cell towards the eye 85. The cell 81a typically reflects about 2 - 10% of the light impinging on the cell, and most of it is a primary reflection. Here, the light hitting the flat surface creates a mirror-like reflection, and the angle of incidence is equal to the angle of return. The reflected light is collected by the lens 88 of the human eye 85 and forms several separate small lines. These lines are images of the conductors on the retina 86.

[0105] As shown in FIG. 8B, the cornea and lens can focus a strip of light 89 from a collimated beam into a small point, which can cause significant local damage. The divergence of the reflection from the collimated beam is minimal, and since the beam is refracted by both the cornea 87 and the lens 88, the result is a powerful beam of light energy that can converge on sensitive eye tissue as shown in FIG. 7C. Such a focused beam has a significant potential to cause damage 80 at the point 80 where the beam strikes the retina 86.

[0106] Now, refer to FIG. 8C, which schematically shows the PV cell 81b according to an implementation of the present disclosure. In contrast to the PV cell 81a, the PV cell 81b uses a non-rectangular, highly convexly contoured conductor 82b and a cover layer 83. The cell 81b includes a conductor 82b that generally reflects up to about 10% of the light, which is wider than a typical rectangular conductor. However, since the conductor 82b reflects the beam at a certain angle, most of the primary reflection is above the total internal reflection angle of the optical cover layer, and thus is reflected back towards the cell, where about 90% of it is absorbed. Approximately 0.04% - 2% of the original beam is reflected a second time by the conductor, and these light rays 89b can be reflected outside the optical cover layer 83, but here they are reflected in a diffused manner. The eye lens 88 can collect a small amount of the diffused emitted light 89b and form an image of the cell 81b on the retina. However, since the power per unit area of the image is significantly small, the risk to the retina is also significantly small. The highest collection of light by the eye occurs at the minimum distance. Since the minimum focal distance of the human eye is about 100 - 150 mm, the most dangerous collection position exists at a distance of approximately 100 - 150 mm from the cell.

[0107] Comparing the reflected light from the reference PV cell in FIG. 8A with the implementation of the present disclosure schematically depicted in FIG. 8C, the beam 84 approaches the cell 81b at the same angle as shown to impinge on the cell 81a. In contrast, in FIG. 8C, after crossing the cover layer 83, the beam is reflected by the conductor 82b at an angle greater than 80% of the critical angle, as calculated based on the reflectivity of the lower part of the cover layer 83. The reflected light rays are absorbed by the cell 81b with higher efficiency compared to the cell 81a, and the percentage of the beam that is finally emitted through the upper part of the optical cover layer 83 is low compared to the percentage reflected by the conductor 82a. Further, the light rays 89b emitted from the upper part of the optical cover layer 83 are diffused and each is emitted in a different direction. Thus, the power of the light impinging on the eye 85 in FIG. 8C is lower compared to the power of the light collected by the eye 85 in FIG. 8A. In FIG. 8C, since these light rays do not travel in the same direction, they do not converge at the same position and thus form a diffused image on the retina of the eye 85 with a much lower risk to the retina, as shown in FIG. 7D.

[0108] The devices described herein typically enable cells of 1 cm×1 cm size to reflect diffused retroreflection, so TEM 00 The reflection of the beam of the laser from the conductor forms a minimum image subtending at least 1.5 mRad, typically much larger than that, of its 1 / e diameter when focused by a f = 25 mm lens placed 100 mm from the surface of the cell. Thus, the risk to the retina is much less. Further, by diffusing the beam, the cell configuration of the present invention enables a typically centrally weighted beam received from a laser source to be less concentrated and more uniform. That is, by improving the uniformity of illumination, current can flow from a short distance to the current collector at the periphery of the cell, enabling full utilization of the cell. The cell of the present invention can also thicken the metal conductive fingers, resulting in lower resistance losses, which is an advantageous feature in the case of high beam fluxes and applies to most laser power converters.

[0109] Referring now to FIG. 9, a safety control system 90 is shown that is configured to provide a warning in the case of a situation where a transmitter 95, typically a laser, is assumed to be directing a beam of a predetermined power in the direction of a receiver 91, but the receiver is not receiving all or part of that power, indicating that the beam is being directed in an unintended direction. The control system 90 indicates or warns that the transmitted laser beam may pose a danger to the surroundings depending on whether it has reached the intended receiver or has reached the correct receiver but the power has been excessively reduced. The controller unit 97 is most advantageously located in the transmitter 95, but as shown in FIG. 9, it may also be located elsewhere in the system or in the space served by the transmitter and receives a signal input 92 from a detection unit 91 at the receiver. This is generally a PV cell at the receiver. The signal 92 that can be sampled from the power output 93 of the PV cell represents that portion of the power level of the illumination having the laser wavelength impinging on the PV cell, even in situations where a significant level of power from other light sources such as sunlight 94 can impinge on the PV cell 91. The control unit 97 is also configured to receive a signal from a laser power supply 95 at the transmitter. Such a signal indicates a signal indicating the power of the laser beam emitted from the position of the scan mirror 96 by the laser, and the settings of both of these system components are determined by the operating requirements for supplying the beamed laser power to the receiver. If the laser power setting and the mirror scan position are such that a predetermined power level is predicted from the detector unit and the actual input to the control unit indicates a power level below the predicted level by exceeding a predetermined amount, a safety warning by the system is activated since it is assumed that part of the beam is obstructed and has not reached the intended receiver target.

[0110] It should be understood that the control system can also operate in its conventional manner, i.e., in the reverse direction, to optimize the settings of the scan mirror, keep the laser beam focused on the receiver PV in accordance with the power measured by the detector unit, and control the laser to supply the intended laser power.

[0111] As will be apparent to those skilled in the art, the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes both combinations and sub - combinations of the various features described above, as well as variations and modifications which, upon reading the above description, will occur to those skilled in the art but which do not exist in the prior art.

Claims

1. A power conversion device that converts optical power adapted for wireless optical power transmission using a laser beam into electric power, a photovoltaic cell having a grid of a plurality of conductors on an upper surface adapted to receive the laser beam, the photovoltaic cell having at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, and a cover layer disposed on the grid of the plurality of conductors of the photovoltaic cell comprising, at least some of the plurality of conductors having a finger-shaped contour or a triangular contour or a rounded contour, the cover layer comprising a material adapted to limit the transmission of illumination having a wavelength outside the range of the wavelength of the laser beam by either absorption or reflection and transmit the laser beam to the photovoltaic cell, the laser beam having a wavelength in the range of 700 nm to 1500 nm, the wavelength of the illumination outside the range of the wavelength of the laser beam being in the range of 550 nm to 700 nm, the transmittance of the cover layer for the wavelength of the laser beam being at least 50% higher than the transmittance of the cover layer for wavelengths in the range of 550 nm to 700 nm, the power conversion device having a conversion efficiency to electric power at the wavelength of the laser beam that is at least 2.5 times higher than the conversion efficiency to electric power at a wavelength of 550 nm.

2. The power conversion device according to claim 1, wherein the bandgap energy is tuned to the wavelength of the laser beam such that the conversion efficiency of optical power to electric power for any wavelength that is at least 25% longer than the wavelength of the laser beam is at least 4 times lower than the conversion efficiency at the wavelength of the laser beam.

3. A first antireflection coating disposed on a surface of the cover layer away from the photovoltaic cell, the first antireflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and transmit the laser beam to the cover layer, or a second antireflection coating disposed between the surface of the photovoltaic cell and the cover layer, the second antireflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the laser beam and transmit the laser beam to the photovoltaic cell The power conversion device according to any one of claims 1 and 2, further comprising at least one of them.

4. A safety system for a wireless optical power transmission system including a transmitter, a receiver, and a control unit, wherein (i) the transmitter includes a laser adapted to emit a beam, and a scanning mirror adapted to steer the beam towards the receiver and wherein (ii) the receiver includes a photovoltaic cell having a plurality of conductors on a surface adapted to receive the beam, the photovoltaic cell having at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, and a cover layer disposed on the photovoltaic cell and wherein the plurality of conductors have a pattern such that a portion of the beam reflected from the plurality of conductors spreads over a wide field of view, wherein the cover layer includes a material adapted to limit illumination having wavelengths outside the range of wavelengths of the beam, either by absorption or reflection, to transmit the beam to the photovoltaic cell, wherein (iii) the control unit receives first data representing the position of the scanning mirror, receives second data representing the power of the beam emitted by the laser from the transmitter, determines a predicted power incident on the photovoltaic cell from the first data and the second data, compares the predicted power with the power of the beam impinging on the receiver measured by the photovoltaic cell, and is adapted to indicate a safety issue that may occur if the predicted power deviates from the measured power by more than a predetermined level, a safety system. **Claim 5** A wireless optical power transmission system including a transmitter and a receiver, including a laser adapted to emit a beam, a scanning mirror adapted to steer the beam towards the receiver, and a control unit adapted to receive a signal from a detection unit in the receiver and control at least one of (i) the power of the beam emitted by the laser and (ii) the position of the scanning mirror and wherein the receiver includes a photovoltaic cell having a plurality of conductors on a surface adapted to receive the beam, the photovoltaic cell including at least one junction having a bandgap energy of 0.75 eV to 1.2 eV, wherein the photovoltaic cell is adapted to detect the power of the beam reaching the photovoltaic cell, wherein the receiver includes a cover layer disposed on the photovoltaic cell, wherein the cover layer A material adapted to absorb or reflect illumination having a wavelength outside the range of the wavelength of the beam and transmit the beam to the photovoltaic cell, (i) a first antireflection coating disposed on the surface of the cover layer away from the photovoltaic cell, the first antireflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the beam and transmit the beam to the cover layer, or (ii) a second antireflection coating disposed between the surface of the photovoltaic cell and the cover layer, the second antireflection coating being adapted to reflect illumination having a wavelength outside the range of the wavelength of the beam and transmit the beam to the photovoltaic cell, at least one of the second antireflection coatings and The detection unit generates a signal representing the power of the laser beam impinging on the receiver independently of other illumination other than the wavelength of the beam, The control unit is adapted to control at least one of (i) the beam and (ii) the position of the scanning mirror to maintain the power impinging on the receiver, a wireless optical power transmission system.

6. A power conversion device adapted to convert optical power adapted for optical wireless power transmission using a laser beam into electric power, including a photovoltaic cell having a grid of a plurality of conductors on an upper surface adapted to receive the laser beam, at least some of the plurality of conductors having a finger-shaped contour or a triangular contour or a rounded contour, The photovoltaic cell has at least one junction having a bandgap energy of 0.75 eV to 1.2 eV and a cover layer disposed on the junction, The cover layer is disposed on the upper surface of the photovoltaic cell, The cover layer is a material adapted to perform at least one of absorption or reflection of illumination having a wavelength in the range of 550 nm to 700 nm, and includes a material adapted to transmit the laser beam toward the photovoltaic cell, The laser beam has a wavelength of 700 nm to 1500 nm, The transmittance of the cover layer for the wavelength of the laser beam is at least 50% higher than the transmittance of the cover layer for wavelengths in the range of 550 nm to 700 nm, The power conversion device has a power conversion efficiency from light to electric power at the wavelength of the laser beam that is at least 2.5 times as large as the power conversion efficiency from light to electric power at a wavelength of 550 nm.

Images