Patent Application
20230174254
Many of the current generation of unmanned aerial vehicles (UAVs) are electrically powered. Most electrically powered small UAVs are battery-powered, such as the RAVEN, WASP III, and PUMA AE by AeroVironment, Inc. of Monrovia, Calif., SCANEAGLE by Boeing of Seattle, Wash., and the MAVERIC UAS by PRIORIA ROBOTICS of Gainesville, Fla. The endurance (i.e., total flight time of the vehicle with a full battery charge) of the current generation of small, electrically-powered unmanned aerial vehicles (UAVs) is limited by power consumed by the UAV and the energy storage capacity of the battery. For example, the endurance of the RAVEN UAV is limited to approximately 90 minutes of flight time. The limit on endurance of small UAVs reduces the operational effectiveness of the small UAVs because it limits the time the UAV can spend over a target of interest, and limits a distance range for targets.
A High-Altitude Long Endurance (HALE) UAV is an airborne vehicle which functions optimally at high altitude (e.g., at least 30,000 feet or 9,000 meters above sea level) and is capable of flights which last for considerable periods of time (e.g., greater than 24 hours) without recourse to landing. Generally, recent generations of HALE UAVs are capable of operating at high altitudes and longer flight times than prior generations. Some examples of HALE UAVs are GLOBAL HAWK by Northrop Grumman Corp. of Falls Church, Va., ALTUS II by General Atomics Aeronautical Systems Inc. of San Diego, Calif., PHANTOM EYE by Boeing of Seattle, Wash., and ZEPHYR by Airbus Defense and Space of Farnborough, UK. Recently some HALE UAVs, such as ZEPHYR, have been produced that can fly at a maximum altitude 70,000 feet. For some types of HALE UAVs the need to refuel can set a limit on the maximum flight time or endurance of the UAV. For some types of HALE UAVs that are powered exclusively by solar cells, a reduction in the mass of the solar cells could increase the payload capacity of the HALE UAV.
A solar sheet is provided. The solar sheet includes thin film solar cells and a flexible polymer sheet disposed across a light receiving surface of the plurality of thin film solar cells. The flexible polymer sheet has a plurality of prismatic structures formed in the top surface thereof to improve light collection efficiency. A bottom surface of the flexible polymer sheet is disposed across the plurality of solar cells.
A method of manufacturing a thin film solar sheet is provided. The method includes forming a plurality of prismatic structures on a first side of a flexible polymer sheet. The method also includes attaching a second side of the flexible polymer sheet that opposes the first side of the flexible polymer sheet to a plurality of thin film solar cells.
A flexible device to enhance light collection efficiency of a plurality of thin film solar cells is provided. The flexible device includes a flexible polymer sheet having a textured top side and a bottom side. The textured top side includes prismatic structures formed therein to improve light collection efficiency. The bottom side is configured to overlay a plurality of thin film solar cells.
A method of improving flight time in an unmanned aerial vehicle (UAV) is provided. The method includes providing a component of a UAV. The component includes a plurality of thin film solar cells and a flexible polymer sheet disposed across a light receiving surface of the plurality of thin film solar cells. The flexible polymer sheet has prismatic structures formed in a top surface thereof to improve light collection efficiency and increase power production in the plurality of thin film solar cells to enable longer flight times for the UAV. The flexible polymer sheet has a bottom surface disposed across the plurality of thin film solar cells. The method further includes providing a power conditioning system configured to operate the plurality of thin film solar cells within a desired power range and configured to provide power in the form of a voltage compatible with an electrical system of the UAV. The method further includes installing the component in the UAV. The method further includes connecting the power conditioning system with the electrical system of the UAV.
An unmanned aerial vehicle (UAV) is provided. The UAV includes a solar sheet installed on a surface of the UAV or on a surface of a component of the UAV. The solar sheet includes a plurality of thin film solar cells and a flexible polymer sheet disposed across a light receiving surface of the plurality of thin film solar cells. The flexible polymer sheet has a plurality of prismatic structures formed in a top surface thereof to improve light collection efficiency. The flexible polymer sheet includes a bottom surface disposed across the plurality of solar cells. The UAV includes a power conditioning system configured to operate the plurality of thin film solar cells within a desired power range and configured to provide power in the form of a voltage compatible with an electrical system of the UAV.
The foregoing and other objects, features and advantages of the invention will be apparent from the following description, and from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles of the invention and are not to scale.
In accordance with various embodiments described herein, systems and methods of the present disclosure include solar sheet devices having a textured coversheet that increases light collection efficiency. The light collection efficiency is particularly improved over that of conventional solar cells or solar sheets for light arriving from high incident angles. The improved collection efficiency results in, for example, greater instantaneous power production as well as greater accumulated power production over the course of a period of time, e.g., one day or multiple days.
While the advantages of greater light collection efficiency are relevant in any solar cell or solar sheet system, greater light collection efficiency in the field of UAVs can improve endurance of the UAVs. The endurance of small UAVs and some HALE UAVs is typically limited due to the operational power requirements for the UAV and the limited energy storage capacity of the battery (e.g., the endurance of the RAVEN small UAV is 60-90 minutes). Increasing endurance enhances the operational effectiveness of a small UAVs and HALE UAVs because a UAV with enhanced endurance can spend more time over the target of interest and/or can travel to targets further away. Adding additional batteries may increase the endurance of a UAV; however the additional batteries would substantially increase the weight of the UAV, thereby reducing its payload or degrading its aerodynamic characteristics.
In many use cases, light (e.g., sunlight) is incident upon solar sheets installed on a UAV at high incident angles, e.g., greater than 60°, particularly for flights during the morning or evening hours or for flights at high latitudes. When light strikes an interface between two media with different refractive indices, reflection and transmission of light at the interface is described by the Fresnel equations. According to these equations, the reflection and transmission of light has an angular dependence, and the light transmission drops significantly and approaches zero as the incident angle approaches 90 degrees. Thus, conventional solar sheets experience reflective losses at any air-material interface. Systems and methods described herein include prismatic structures designed to reduce Fresnel losses at the air-material interface.
For a variety of solar cell applications, it is advantageous to improve efficient collection of light at high zenith angles. For example, solar-powered stratospheric UAVs that are designed to fly continuously for multiple weeks can benefit from more efficient light collection at high zenith angles at the morning sun break (especially at high latitudes). In addition, efficient collection of light at high zenith angles provides additional energy harvesting during the day. Solar sheets and flexible coversheets of the present disclosure provide marked improvement in collection of light at high zenith angles.
One of the problems addressed by some embodiments described herein is how to substantially increase the endurance of a UAV (e.g., a small battery-powered or fuel cell powered UAV such as the RAVEN or a HALE UAV) without substantially increasing its size or weight. Some embodiments address this problem by providing a kit to equip a UAV with lightweight, flexible, high efficiency solar cells (e.g., one or more solar cell strings or sheets of solar cells) that supply additional power to the UAV, thereby significantly increasing the endurance of the UAV as compared to a UAV without solar cells. Other embodiments address this problem by providing one or more solar sheets to be installed on a UAV either after the UAV has been produced or during production. Because the solar cells have relatively small mass per unit area, they do not add significant weight to the UAV. In some embodiments, the solar cells have a high specific power (power to mass ratio) providing significant power generation for relatively little added weight. For both small UAVs (e.g., portable UAVs that may be transported or deployed by a single person in the field) and HALE UAVs it is particularly important that the solar cells do not significantly increase the overall weight of the UAV, which could degrade the performance of the UAV and decrease its endurance.
For solar powered UAVs, a payload capacity of the UAV can be increased by replacing solar cells currently used in the UAV with solar cells or solar sheets capable of generating more power per unit mass of the solar cell or solar sheet. Some solar powered UAVs that incorporate higher specific power solar cells or higher specific power solar sheets can have increased endurance. Higher specific power solar cells and higher specific power solar sheets may enable more solar cells and solar sheets to be incorporated into a UAV without significantly increasing the weight of the UAV.
Systems and methods described herein are not limited to use with aerial vehicles but can also be used in conjunction with ground-based applications. The disclosed systems and methods are particularly advantageous in applications where the surface of the solar panels does not track the motion of the sun. Such applications can include fixed-mount solar installations or portable solar sheets that can be laid on the ground or draped over a human or object.
As used herein, the term “small UAV” includes portable UAVs that may be carried by a single person. The term small UAV includes what may be referred to elsewhere as micro UAVs and mini UAVs and larger portable UAVs. Some non-limiting examples of small UAVs include the RQ-11B RAVEN UAV system with a weight of 1.9 kg and a wingspan of 1.4 m, the WASP Micro Air Vehicle (MAV) with a weight of 0.43 kg and a wingspan of 72 cm, and the RQ-20A PUMA with a weight of 5.9 kg and a wingspan of 2.8 m, the MAVERIC UAV with a 72 cm in wingspan and a loaded weight of about 1.1 kg, and the SCANEAGLE with a 3.1 m wingspan and an 18 kg.
As used herein, the term “HALE UAV” refers to an aircraft that functions at high altitude (i.e., greater than approximately 30,000 feet or 9,000 meters) and is capable of flights which last for considerable periods of time (e.g., longer than approximately 18 hours) without recourse to landing. Some non-limiting examples of HALE UAVs include, but are not limited to the GLOBAL HAWK, ALTUS II, PHANTOM EYE, and ZEPHYR UAVs.
As used herein, the term “areal mass” refers to mass per unit area. For example, the areal mass of a solar cell is the mass of solar cell per unit area of the solar cell. As another example, the areal mass of a solar sheet is the mass of solar sheet per unit area of the solar sheet.
As used herein, the term “areal power” refers to power produced per unit area. For example, the areal power of a solar cell is the power produced by the solar cell under a specified illumination divided by the area of the solar cell. As another example, the areal power of a solar sheet is the power produced by the solar sheet under a specified illumination divided by the area of the solar sheet.
As used herein, the term “specific power” refers to the power produced per unit mass. For example, the specific power of a solar cell is the power produced by the solar cell under a specified illumination divided by the mass of the solar cell. As another example, the specific power of a solar sheet is the power produced by the solar sheet under a specified illumination divided by the mass of the solar sheet. The specific power can also be defined as the areal power divided by areal mass.
As used herein, the term “solar sheet” refers to a plurality of solar cells and one or more polymer layers to which the solar cells are affixed or attached. The solar sheet can also include interconnects that electrically connect at least some of the plurality of solar cells. The solar sheet can also include an adhesive that adheres the solar cells to the one or more polymer layers. The solar sheet can also include an adhesive to adhere the solar sheet to an underlying surface (e.g., a surface of a component of an UAV to which the solar sheet is to be attached). The solar sheet can be flexible to conform to an underlying rounded surface (e.g., the surface of a wing or the surface of a fuselage of a UAV).
As used herein, the term “zenith angle” refers to the angle between the zenith and the center of the sun's disc. The zenith angle represents the sun's apparent altitude and is a complementary angle to the “elevation angle.” The elevation angle as used herein refers to the angle between the horizon and the center of the sun's disc. As an example, the zenith angle is approximately 90° and the elevation angle is approximately 0° during a sunrise or sunset. As used herein, “textured” refers to a surface upon which regular, patterned, random, or periodic structures are formed for the purpose of improving light collection efficiency through the surface.
As used herein, “disposed on” or “disposed over” is not limited to objects in direct contact with but can also encompass a structural relationship including intervening layers.
The outward-facing top surface 110a of the flexible coversheet 110 is textured using the prismatic structures 112 to improve transmission of light through the surface 110a and into the solar sheet 100. Each prismatic structure in the plurality of prismatic structures 112 has a particular geometry. The geometry of the prismatic structure can include an inverted prism structure, a non-inverted prism structure (i.e., the structure projects outward from the surface), a corner-cube (i.e., three-sided) prism structure, a pyramidal (i.e., four-sided) prism structure, a linear prism structure, a curvilinear prism structure, or any other suitable one-dimensional or two-dimensional shape. Here, “inverted” refers to a structure that is a depression into the surface while “non-inverted” refers to a structure that projects from the surface. The geometry of the prismatic structure can include curved walls, straight walls, or a combination of curved and straight elements. Embodiments of the textured surface 110a having one-dimensional or linear prismatic structures can include a sidewall topology that varies predominantly in one dimension (e.g., x-direction) across the surface 110a but that does not vary (within manufacturing variances) in the other dimension (e.g., y-direction). Embodiments of the textured surface 110a having two-dimensional prismatic structures can include a sidewall topology that varies in both dimensions (e.g., x and y dimensions). For polygonal prismatic structures, each structure can have three, four, five, or more facets. In some embodiments, the prismatic structures 112 can be at least partially defined by a sidewall angle 114 (see
In general, the light capturing performance of prismatic structures at high zenith angle is improved relative to low zenith angles as the sidewall angle is increased. However, the transmission at normal incidence (i.e., zenith angle of 0°) can begin to degrade by several percent at high sidewall angles. In some embodiments, geometry of each prismatic structure in the plurality of prismatic structures 112 can be selected to improve light collection efficiency at particular ranges of zenith angles. For example, selection of large sidewall angle 114 may be advantageous for applications where the solar sheet is rarely expected to receive light at low zenith angle. In some embodiments, the geometry of each prismatic structure can be identical across the entire flexible coversheet. In some embodiments, the geometry of each of the prismatic structures 112 can be selected individually. In such embodiments, the variation in geometry of prismatic structures 112 can be random or patterned across the sheet. For example, the geometry (including sidewall angle 114) of the prismatic structures 112 can vary from one end of the solar sheet 100 to the other end or from the ends or perimeter to the middle. For example, the coversheet 110 may have prismatic structures 112 with large sidewall angle 114 at a first edge of the coversheet (e.g., the first edge will be mounted at a leading edge of the UAV wing) and prismatic structures 112 with smaller sidewall angle 114 at a second edge of the coversheet opposite the first edge (e.g., the second edge will be mounted at a trailing edge of the UAV wing). The change in sidewall angles transitioning from the first edge to the second edge can be monotonic or non-monotonic and continuous or punctuated. In some embodiments, it may be advantageous to provide a first subset of prismatic structures 112 having a first geometry on portions of the solar sheet intended to lie on flat portions of a UAV wing and a second subset of prismatic structures 112 having a second geometry on portions of the solar sheet intended to lie on curved portions of the UAV wing.
The flexible coversheet 110 can be formed of one or more polymer species. In various embodiments, the flexible polymer sheet 110 can be formed of one or more materials selected from the group of fluorinated ethylene propylene (FEP); ethylene tetrafluoroethylene (ETFE); ethylene chlorotrifluoroethylene (ECTFE); polychlorotrifluoroethylene (PCTFE); a copolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV); and polyethylene terephthalate (PET). The flexible coversheet 110 can include acrylic-based materials in some embodiments. The coversheet 110 can include glass materials in some embodiments. The coversheet 110 can include silicone materials such as polyimides or polydimethylsiloxane (PDMS) in some embodiments. In some embodiments, selection of polymer species for the flexible polymer sheet 110 can be driven by application-specific factors. For example, the polymer species can be selected based on light transmissivity at a particular wavelength or range of wavelengths in some embodiments. In some embodiments, the polymer species can be selected based upon the manufacturing process to be used in processing the flexible polymer sheet 110. Appropriate manufacturing processes can include thermal forming, molding, casting, forming, embossing, roughening, lithographic processes, or other suitable patterning processes. Some manufacturing processes may be more suitable for certain materials. For example, fluorinated ethylene propylene has a relatively high melting temperature (about 260° C.) that may make it difficult to effectively emboss. In some embodiments, roughening techniques can create a matte-like finish with random prismatic surface structures that can enhance light collection as disclosed herein.
The prismatic structures 112 can be described in terms of a characteristic dimension in some embodiments. For example, the characteristic dimension can be the distance from a peak of a first prismatic element to a peak of a neighboring prismatic element or can be a length or width dimension corresponding to a base of a pyramidal prismatic structure. The characteristic dimension can be a height of the prismatic structures 112. In some embodiments, the characteristic dimension of the prismatic structures 112 can be longer than a wavelength of incident light 152. In some embodiments, the height of each of the prismatic structures 112 can be in a range from approximately 10 micrometers to approximately 100 micrometers. For example, the height of one of the prismatic structures 112 can be measured from a tip or apex to a base of the prismatic structure. In some embodiments, the flexible coversheet 110 is configured to encapsulate the light receiving surface of the plurality of solar cells 130. In some embodiments, the flexible coversheet 110 can have a thickness in a range from approximately 10 micrometers to approximately 150 micrometers, or in a range from approximately 20 micrometers to approximately 50 micrometers. In some embodiments, the thickness of the flexible coversheet 110 can be selected to provide a desired mass density of the solar sheet. Some polymeric materials can be about half as dense as glass, which may impact the thicknesses selected based upon material type. In embodiments intended for space-based applications, the thickness of the flexible coversheet 110 can be, as a non-limiting example, in a range from 200 microns to 350 microns.
In some embodiments, the prismatic structures 112 can increase collection and power conversion efficiencies of the solar sheet 100 through improved optical coupling into the solar cell 100 by reducing the amount of light that is reflected at the air-material interface.
In some embodiments, a kit including solar cells or a solar sheet with the textured flexible solar sheet 110 (e.g., an embodiment of solar sheet 100) and a power conditioning system is used to increase endurance of a UAV. For example,
In some embodiments, the solar cells (e.g., one or more solar cell strings or solar sheets) may be installed on a surface of a previously-produced UAV (e.g., as a post-manufacturing modification). For example, solar sheets of a kit may be applied to the wings of a previously-produced UAV. The power conditioning system and associated electrical wiring may be installed in the wings and fuselage of the previously-produced UAV and interfaced with the existing electrical system of the previously-produced UAV. In some embodiments, the kit may be an upgrade, a retrofit, or an aftermarket kit for installation on a previously-produced UAV. In some embodiments, the solar cells (e.g., solar sheet(s)) may be mounted on or incorporated into a surface of a component of a UAV. The power conditioning system and associated electrical wiring (e.g., electrical harness) and connectors of the kit may be installed in the component. For example,
In some embodiments, the component with the solar sheet(s) (e.g., wing assembly 13) is used to replace a similar component in a previously-produced UAV as a post-manufacturing modification (e.g., as a retrofit or as an aftermarket modification). For example, a wing assembly including an installed kit may be used to replace a wing component in a previously-produced UAV.
In some embodiments, the component with the solar sheets (e.g., the wing assembly) is used during an initial manufacturing process of a UAV (e.g., as an upgrade). For example, a wing assembly with an installed kit may be incorporated into a UAV during initial manufacturing or assembly of the UAV as opposed to adding the solar cells and/or the power conditioning system to a previously-produced UAV.
Some embodiments may include an upgrade kit, a retrofit kit, or an aftermarket kit, for existing UAVs, such as the RAVEN UAV, the Wasp III UAV, the PUMA AE UAV, the MAVERIC UAS, GLOBAL HAWK, ALTUS II, PHANTOM EYE, and ZEPHYR. Different embodiments of kits can be used with different types or different models of UAVs.
In some embodiments, the solar sheets of the present disclosure can be adhered to a surface of a portion of a UAV. In some embodiments, the solar sheets can connect to a power conditioning system included in the UAV instead of being provided in a kit with a power conditioning.
In some embodiments, the UAV may be designed with parts and connections configured for the incorporation of flexible, light weight, high efficiency solar cells or flexible, light weight, high specific power solar sheets. Incorporation of the solar cells or solar sheets into the UAV design may result in better aerodynamics, more robust electrical connections, and reduced additional weight to due to the solar cells, packaging and wiring harness. Some embodiments include UAVs specifically designed for hybrid battery/solar operation, such as UAVs that are primarily battery powered with a secondary solar power system including flexible, lightweight, high-efficiency solar cells. Some embodiments include electric UAVs whose primary power source is solar and that include one or more rechargeable batteries or fuel cells.
In some embodiments, solar sheets are provided that are configured to adhere to a surface of a portion of a UAV. In some embodiments, embodiments of the solar sheets of the present disclosure can connect to a power conditioning system included in the UAV instead of being provided in a kit with a power conditioning. In some embodiments, the solar cells or solar sheets are used with a UAV that was designed to have solar power as its primary power source or run exclusively on solar power (e.g., the ZEPHYR HALE UAV).
In the embodiments depicted in
In the embodiment depicted in
In the embodiments of
The use of a textured polymer cover sheet (e.g., an embodiment of the coversheet 112) can increase the collection and power conversion efficiencies through improved optical coupling into the solar cell by reducing the amount of reflected light. The relative efficiency benefits of texturing are most significant at large solar zenith angles. The improved optical coupling enabled by texturing is demonstrated in
In conventional structures, some light that passes into the coversheet may still be lost due to reflection from the metal grid lines or reflection off of the solar cell surface itself. The active layers 130 of the solar cell are typically coated with multi-layer antireflection coating 125, which reduces but does not eliminate all reflections. In some embodiments, retroreflection may arise when incident light 152a is reflected back from grid lines or the surface of the active layers undergoes total internal reflection at the interface between the flexible coversheet and the surrounding medium (e.g., air). The light is then redirected back to the active layers 130 where there is another opportunity for absorption. The internal reflection of the light from the prismatic structure shown in
Recapture may arise if incident light 152b reflects from a first prismatic structure on the top surface of the coversheet at a trajectory that causes the light to strike a second prismatic structure where the light can be directed toward the solar cell layers. This recaptured light would have otherwise been lost without the presence of the prismatic structures.
By providing additional chances to capture each photon, the overall loss is reduced and the conversion efficiency is increased. The recapture and retroreflection of photons occurs across the range of zenith angles. In some embodiments, the recapture or retroreflection of photons that can occur at nearly overhead angles (e.g., zenith angles in a range from 0° to 20°) means that the prismatic structures provide an overall improvement in conversion efficiency both when the sun is high in the sky and when the sun is close to the horizon (high zenith angle) as described above.
Embodiments of the flexible polymer sheet (e.g., embodiments of the polymer sheet 110) can be formed of one or more materials. The material(s) can be selected to impart particular desirable properties such as optical transmissivity at particular wavelengths or wavelength ranges, enhanced durability, or enhanced machinability in forming the prismatic structures. In some embodiments, the materials can include one or more selected from the group of fluorinated ethylene propylene (FEP), ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene (ECTFE), and polychlorotrifluoroethylene (PCTFE). ETFE and ECTFE in particular are fluorinated polymers with optical transmission over a wide range of wavelengths as well as high durability and resistance to degradation under long-term ultraviolet light exposure.
Modeling has shown that the prismatic structures can also improve solar array performance by several percent at normal incidence by “recapturing” the small percentage of light that is reflected. This occurs when the light is reflected off one prism facet and is transmitted into an adjacent prism or when light reflects from the solar cell/top electrode surface but is recaptured upon total internal reflection from the prism.
The plurality of solar cells may be single-junction solar cells, multi-junction solar cells (e.g., dual-junction solar cells, triple junction solar cells) or any combination of single-junction solar cells and multi-junction solar cells. Although triple junction solar cells generally have a higher efficiency than that of single junction or dual-junction solar cells, triple junction solar cells are generally more complicated to produce and may have a narrower wavelength range for high efficiency performance. The efficiency of the dual-junction and single-junction cells is less sensitive to the spectrum of the incident light than that of a triple-junction cell, so more energy may be obtained from dual-junction or single-junction cells when the cells are exposed to scattered light, rather than to direct sunlight. Accordingly, in some embodiments it may be desirable to use dual-junction or single-junction cells on the underside of the wings or the fuselage where the ratio of scattered light to direct sunlight is greater than for a top side of the wings.
The solar cells, and any solar sheets into which the solar cells are incorporated, must be flexible to conform to an underlying curved aerodynamic shape of a surface of UAV or of a UAV component onto which they will be mounted or into which they will be incorporated. Solar cells for a small UAV may need to be more flexible than solar cells for a large UAV due to the higher curvatures present in surfaces of small UAVs. Further, flexible solar cells are more durable than similar non-flexible or less flexible (i.e., more brittle) solar cells during installation, and during use.
As noted above, the solar cells and the solar sheets that include the solar cells should have a total mass that is relatively small compared to the mass of the UAV and should have a relatively low mass per unit area.
Because additional mass tends to increase the power required to operate a UAV, the power supplied by the solar cells must more than compensate for the increase in the UAV mass due to the presence of the solar cells or solar sheets into which the solar cells are incorporated to increase endurance of a UAV. Thus, only solar cells having sufficient specific power (power per unit mass) would increase the endurance of a UAV.
For UAVs that is designed to incorporated solar cells and solar sheets using higher specific power solar cells or higher specific power solar sheets can increase the payload capacity of the UAV by increasing the available power for a given mass of solar cells or solar sheets incorporated into the UAV. Using higher specific power solar cells or higher specific power sheet may reduce the mass of solar cells or solar sheets incorporated into the UAV to generate a given power.
For a given solar cell, the efficiency for one spectrum of light is generally different than the efficiency for another spectrum of light. Parameters which depend on the efficiency of the solar cell can be specified for different types of illumination. For example, the specific power of solar cells or solar sheets can be specified under air mass coefficient 1.5 (AM1.5) light which is typically used to characterize low altitude or terrestrial based solar cells. The specific power of solar cells or solar sheets can alternatively or additionally be specified under air mass coefficient 0 (AM0) light, which corresponds to high altitude conditions or light conditions above the atmosphere.
Various types of commercially available solar cells as well as ELO IMM triple-junction ((Al)InGaP/GaAs/InGaAs) solar cells and IMM cells with more than three junctions made by MicroLink Devices, Inc. can be used in conjunction with embodiments of the present disclosure. In some embodiments, aluminum can be included in the first junction (e.g., AlInGaP/GaAs/InGaAs) and in some embodiments aluminum was not included in the first junction (e.g., InGaP/GaAs/InGaAs) depending on the application or use. For example, the triple-junction InGaP/GaAs/InGaAs solar cell performs well under AM1.5. But, under AM0, AlInGaP/GaAs/InGaAs can be used because the Al can help the first junction to be better tuned to the high UV content of AM0 as compared to AM1.5.
The commercially available solar cells include single-junction polycrystalline silicon solar cells, single-junction single crystal silicon solar cells, triple junction gallium arsenide solar cells on germanium, triple-junction solar cells on germanium, and single-junction copper-indium-gallium-selenide (CIGS) solar cells. Both polycrystalline silicon solar cells and single crystal silicon solar cells are rigid solar cells, (i.e., not flexible solar cells). CIGS solar cells are grown on glass, polymers and metal sheets. CIGS solar cells are flexible, but they typically have a lower efficiency compared to silicon or GaAs-based solar cells. The GaAs solar cells, which are grown on the Ge substrate, are both rigid and fragile; however, due to their high efficiency, they are often used for space solar arrays.
In some embodiments not including textured coversheets, a specific power of the plurality of solar cells is at least a threshold value (e.g., at least 1000 W/kg, at least 1500 W/kg, at least 2000 W/kg, at least 2500 W/kg, for AM1.5). The threshold value may alternatively, or additionally be specified with respect to AM0 (e.g., at least 1220 W/kg, at least 1870 W/kg, at least 2520 W/kg, or at least 3150 W/kg, under AM0). In some embodiments, the specific power of the solar cells falls within a specified range (e.g., 1000-4500 W/kg, 1500-4500 W/kg, 2000-4500 W/kg, 2500-4500 W/kg, or 1500-6000 W/kg under AM1.5). The range may alternatively, or additionally, be specified with respect to AM0 light (e.g., 1220-5680 W/kg, 1870-5680 W/kg, 2520-5680 W/kg, 3150-5680 W/kg, or at least 1870-7000 W/kg, under AM0).
The specific power of a solar cell depends on the efficiency of the solar cell (electrical energy produced divided by solar energy absorbed for a unit area of the solar cell) and the mass per unit area of the solar cell (i.e., the areal mass). Thus, a solar cell with a relatively high specific power has a relatively high efficiency and/or a relatively low areal mass. Solar cells free of a substrate (e.g., solar cells produced using epitaxial lift off (ELO)) may be particularly well suited for use on a UAV because they have a reduced mass per unit area and greater flexibility as compared to solar cells attached to an underlying substrate.
In general, if the materials of a solar cell remain the same, decreasing the thickness of the solar cell increases the flexibility of the solar cell. As noted above, increased flexibility allows the solar cell to conform to an aerodynamic shape of a UAV surface or of the surface of a UAV component and increases the durability of the solar cell. In some embodiments, the solar cell (including components thereof such as the flexible polymer sheet) can flex in two dimensions to a bend radius down to 1 centimeter. In some embodiments, each solar cell may have a thickness of less than a specified thickness (e.g., less than 40 μm, less than 25 μm, less than 13 μm, or less than 5 μm). In some embodiments, each solar cell may have a thickness that falls in a specified range (e.g., 2-40 μm, 2-30 μm, 2-15 μm).
The areal mass of a solar cell is independent of the light spectrum used for power generation (i.e., AM1.5 or AM0). In some embodiments, the areal mass of a solar cell may have a value that falls in a specified range (e.g., 70-280 g/m2, 165-250 g/m2, 95-165 g/m2, 70-95 g/m2). The areal mass of the solar cell can be reduced by reducing the mass of one or more components of the of solar cell without reducing the area of the solar cell. For example, for solar cells that include a backing layer, such as ELO IMM solar cells including a backing layer, reducing the thickness of the backing layer of the solar cell can reduce the areal mass of the solar cell. In some embodiments, the solar cell includes a metal backing layer. In some embodiments, the metal backing layer may have a thickness of less than a specified thickness (e.g., less than 30 μm, less than 15 μm, or less than 5 μm). In some embodiments, the backing layer can include metal and polymer.
Areal power of a solar cell is dependent on the efficiency of the solar cell. The areal power of a solar cell is greater under AM0 than under AM1.5. This is due to the fact that AM0 light inherently has more power to begin with because, unlike the AM1.5 light, the AM0 light has not been filtered by atmospheric conditions. In some embodiments, the efficiency of the solar cells under AM0 varies by 2.5% from efficiency of the solar cells under AM1.5. For example, if the solar cell has 25% efficiency under AM0, it has about 27.5% efficiency under AM1.5. In some embodiments, the solar cell has 29% efficiency under AM1.5 which results in an areal power of 290 W/m2 under AM1.5, and the solar has 26.5% efficiency under AM0 which results in an areal power of 360 W/m2 under AM0. In another embodiment, the efficiency of the solar cell can increase to 30% under AM0 resulting in an areal power of 410 W/m2 under AM0 and as a result, the efficiency of the solar cell can increase to 32.5% under AM1.5 resulting in an areal power of 325 W/m2 under AM1.5.
In some embodiments, the areal power of the solar cell may be in the range of 260-360 W/m2 under AM1.5. In some embodiments, the areal power of the solar cell may be in the range of 325-450 W/m2 under AM0.
As noted above, at least some solar cells (e.g., embodiments of solar cells 130) may be incorporated into a flexible solar sheet (e.g., embodiments of solar sheets 100). For example, in some embodiments, lightweight solar cells (or strings of solar cells) are disposed under a polymer film or between polymer films to form flexible solar sheets to aid in easier handling and installation, and to provide greater protection of the solar cells. The flexible solar sheets conform to curved aerodynamic surfaces. In some embodiments the flexible solar sheets provide robust waterproof packaging. The flexible solar sheets may be applied to or incorporated into a surface of a UAV or of a component of a UAV.
The areal mass of a solar sheet includes the encapsulating materials that form the solar sheet ready to be installed on a UAV. As noted above, decreasing the mass of solar sheet increases the specific power of the solar sheet. The main factor for reducing the areal mass of the solar sheet is the reduction in thickness of the encapsulating materials by substituting lighter materials and eliminating redundant materials. In some embodiments, the areal mass of the solar sheet may have a value that falls in a specified range (e.g., 120-570 g/m2, 120-300 g/m2, or 120-160 g/m2).
The areal power of a solar sheet is dependent on the efficiency of the solar cells in the solar sheet as well as how tightly the solar cells are packed together in an array in a solar sheet. One way to increase the areal power of the solar sheet is by reducing or minimizing the spacing or the lateral gaps between adjacent solar cells in the solar sheet. In one embodiment, the solar cells were spaced 2 mm or more from each other resulting in a sizable amount of area on the solar sheet that was not active and did not contribute power to the whole solar sheet. The areal power of the solar sheet was measured to be 230 W/m2 under AM1.5. In another embodiment, the solar cells were packed with less than 1 mm spacing between adjacent cells. The areal power of the solar sheet was measured to be 260 W/m2 under AM1.5 and 330 W/m2 under AM0.
The overall increase in mass of the UAV due to installation of a kit or due to installation of solar sheets should be small relative to the total weight of the UAV. For example, in some embodiments the installed kit or the installed solar sheets increase the weight of the UAV by less than 2%, by less than 5%, by less than 10%, by less than 15%, or by less than 20%. As noted above, this requirement may be more challenging for small UAVs than for large UAVs.
Solar cells for the kit or solar cells used with embodiments employing solar sheets may be based on any number of suitable semiconductor materials like III-V semiconductor materials (e.g., GaAs-based materials, InP-based materials, etc.) and Si-based materials. The solar cells may be single junction solar cells, multi-junction solar cells (e.g., double-junction, triple-junction), or a combination of single junction and multi-junction solar cells. In general, higher efficiencies can be obtained with multi-junction solar cells than with single junction solar cells, however, multi-junction solar cells are more complicated to make and can be more expensive. Examples of solar cells having relatively high efficiencies include triple junction inverted metamorphic (IMM) solar cells, which may be produced using ELO or using methods that do not employ ELO. As a specific example, triple junction IMM solar cells with an (Al)InGaP/GaAs/InGaAs grown inverted on GaAs by the inventors demonstrated efficiencies of greater than 29% under AM0.
Further information regarding III-V semiconductor solar cells produced by ELO (e.g., single junction, multi-junction and IMM solar cells), and how to manufacture III-V semiconductor ELO solar cells may be found in U.S. Pat. No. 7,994,419 to Pan et al. issued Aug. 9, 2011, which is incorporated by reference herein in its entirety. Further information regarding InP-based solar cells produced by ELO (single junction, multi-junction and IMM) and how to manufacture InP-based ELO solar cells may be found in U.S. patent application Ser. No. 13/631,533, filed Sep. 28, 2012, which is incorporated by reference herein in its entirety.
For embodiments that include a kit, the kit includes a power conditioning system configured to operate the plurality of solar cells within a desired power range and configured to provide a specified voltage to an electrical system of the UAV.
In some embodiments, the power conditioning system may also be configured to charge an energy storage device (e.g., a battery, fuel cell) of the UAV.
In
Electrical connections (e.g., power bus lines, wiring harness) connecting the solar cells, the power conditioning system, the electrical system of the UAV and the energy storage device (e.g., battery, fuel cell) of the UAV may be integrated into one or more components of the UAV (e.g., the wings or the fuselage).
Some embodiments include a solar sheet configured for installation on a component of a UAV. The solar cell may be included in a kit with a power conditioning system or may be provided without a power conditioning system.
The solar sheet 90 also includes a polymer layer 98 to which the plurality of solar cells 94 are attached. The polymer layer 98 can be planar (as depicted in
In some embodiments the solar sheet 90 includes a first adhesive layer 92. In some embodiments, the first adhesive layer 92 is configured to attach the solar sheet 90 to a component of a UAV. In some embodiments, the first adhesive layer 92 is in contact with a bottom surface 92 of each solar cell. The adhesive can be any suitable adhesive (e.g., NT 1001 pressure sensitive adhesive (PSA) from Forza Power Technologies). In some embodiments, the thickness of the first adhesive layer 92 is in a range of 8 microns to 15 microns. In some embodiments, the thickness of the first adhesive layer is in a range of 8 microns to 25 microns. In some embodiments, the bottom surface 91 of each of the solar cells 94 is in contact with the first adhesive layer 92.
In some embodiments the solar sheet 90 includes a second adhesive layer 96 that attaches the plurality of solar cells 94 to the polymer top sheet 98. In some embodiments, the second adhesive layer 96 is in contact with the top surface 93 of each of the plurality of solar cells 94. The second adhesive layer 96 can be any suitable adhesive (e.g. a PSA such as NT 1001). In some embodiments, the thickness of the second adhesive layer 92 is in a range of 8 microns to 15 microns. In some embodiments, the thickness of the second adhesive layer is in a range of 8 microns to 25 microns.
Although solar sheet 90 depicted in
The nanostructures 212 formed in the top surface 210a of the coversheet 210 differs from the prismatic structures 112 described previously by having a significantly smaller characteristic dimension. The nanostructures can take the form of “moth eye” texturing. The moth eye texturing can have antireflective properties in some embodiments. The nanostructures 212 can have a characteristic dimension (e.g., period) shorter than the wavelengths of incident radiation. As a result, the nanostructures 212 can reduce Fresnel reflective losses. In some embodiments, the characteristic dimension can be below 500 micrometers. In some embodiments, the characteristic dimension can be less than the wavelength of visible light. In some embodiments, the nanostructures 212 can create a graded effective index of refraction in the flexible sheet 210 between the surrounding medium (e.g., air or vacuum) and the one or more thin-film solar cells 130. The index gradation reduces the Fresnel losses by minimizing the discontinuity in refractive index that leads to reflections.
The use of nanostructures 212 can enable reduction in thickness of the flexible coversheet 210, which thus results in reduced weight and increased specific power. In some embodiments, prismatic structures 112 as described in relation to
For embodiments not including textured coversheets, each of the solar cells in the solar sheet has a specific power of at least a specified value (e.g., at least 1000 W/kg, at least 1500 W/kg, at least 2000 W/kg, at least 2500 W/kg, under AM1.5). The specific power of the solar cells in the solar sheet may additionally or alternatively be described in terms of AM0 light (e.g., at least 1270 W/kg, at least 1870 W/kg, at least 2520 W/kg, at least 3150 W/kg, under AM0). In some embodiments, each of the solar cells has a specific power falling within a specified range (e.g., 1000-4500 W/kg, 1500-4500 W/kg, 2000-4500 W/kg, 2500-4500 W/kg, 1500-6000 W/kg, under AM1.5). The specific power of the solar cells in the solar sheet may additionally or alternatively be described in terms of AM0 (e.g., 1270-5680 W/kg, 1870-5680 W/kg, 2520-5680 W/kg, 3150-5680 W/kg, 1870-7000 W/kg, under AM0). Textured coversheets can improve specific power numbers for solar sheets over those ranges given above for non-textured solar sheets.
In some embodiments not including textured coversheets, the solar sheet has a specific power of at least a specified value (e.g., at least 400 W/kg, at least 800 W/kg, at least 1000 W/kg, under AM1.5). In some embodiments the solar sheet has a specific power falling with within a specified range (e.g., 400-2350 W/kg, 800-2350 W/kg, 1000-2350 W/kg, 1020-3000 W/kg, under AM1.5). The specific power of the solar sheets may additionally or alternatively be described in terms of AM0 (e.g., at least 510 W/kg, at least 1020 W/kg, at least 1270 W/kg or in a range of 10-3000 W/kg, 1020-3000 W/kg, 1270-3000 W/kg, 1020-4000 W/kg, under AM0).
As noted above, the areal mass of the solar sheet includes the encapsulating materials that form the solar sheet ready to be installed on a UAV. Decreasing the mass of solar sheet, increases the specific power of solar sheet. In some embodiments, the areal mass of the solar sheet may have a value that falls in a specified range (e.g., 70-280 g/m2, 120-570 g/m2, 120-300 g/m2). The areal power of a solar sheet is dependent on the efficiency of the solar cells as well as how tightly the solar cells are packed together in an array. In some embodiments, the areal power of the solar sheet may have a value that falls in a specific range (e.g., 260-330 W/m2, 200-330 W/m2 under AM1.5 or 325-450 W/m2, 260-410 W/m2 under AM0).
In some embodiments, the solar sheet is configured to be attached to a wing of a UAV. In some embodiments, the solar sheet is a flexible solar sheet. In some embodiments the plurality of solar cells includes solar cells produced using an epitaxial lift-off process.
In some embodiments, each of the plurality of solar cells includes a backing layer. The backing layer can be formed of metal in some embodiments. In other embodiments, the backing layer can be formed of a polymer or a combination of metal and polymer. In some embodiments, the thickness of the metal backing layer is less than 30 μm, less than 15 μm, or less than 5 μm. In some embodiments, the metal backing layer has a thickness in a range of 2 to 30 microns. In some embodiments, the metal backing layer has a thickness in a range of 2 to 15 microns.
Due to added mass of polymer materials in solar sheets, a solar sheet of a plurality of solar cells has a lower specific power than the specific power of the solar cells themselves. Also, if the solar sheet has a top layer, the top layer may reduce the efficiency of the solar sheet (e.g., by absorbing or reflecting some of the incident light before it reaches the solar cell). Alternatively, the textured top sheet in accordance with some embodiments of the present disclosure can improve the efficiency of the solar sheet (e.g., by recapturing light that has reflected from the solar cell surface and redirecting this light back to the surface). In some embodiments, a solar cell has a specific power of at least a specified value (e.g., at least 800 W/kg, or at least 1000 W/kg, under AM1). Additionally or alternatively the threshold for specific power may be described in terms of AM0 light (e.g., at least 1020 W/kg, at least 1270 W/kg, under AM0). In some embodiments, a solar sheet has a specific power falling within a specified range (e.g., 800-2350 W/kg, 1000-2350 W/kg, 1000-3500 W/kg, under AM1.5). Additionally or alternatively the range for specific power may be described in terms of AM0 light (e.g., 1020-3000 W/kg, 1270-3000 W/kg, 1020-4000 W/kg, under AM0).
Some embodiments include methods of increasing an endurance of a battery-powered or fuel cell powered UAV. For example, in method 1000 of
Method 1010 of
As noted above, in order to increase the specific power of a solar sheet, the areal mass of the solar sheet can be decreased. For example, in some embodiments, portions of first adhesive layer of the solar sheet include cutouts to reduce the mass of the solar sheet.
In some embodiments, the second adhesive layer includes a plurality of cutouts, each corresponding to a position of a solar cell in the solar sheet. In some embodiment, both the first adhesive layer and the second adhesive layer include a plurality of cutouts, each corresponding to a position of a solar cell in the solar sheet.
In some embodiments, the plurality of solar cells are integrated into a component of a UAV. For example, solar sheets may be produced as described above and then the solar sheets incorporated into a wing as the wing is produced using a molding process.
The inventors made and tested an example solar sheet having a planar coversheet (e.g., devoid of prismatic structures such as polymer layer 98 shown in
The layers of example solar sheet A generally corresponded to the layers of solar sheet 90 described above with respect to
Table 1 below lists the different layers of example solar sheet A and the materials used for each layer. In addition, Table 1 shows the areal mass of each layer and contribution of the mass of each layer to the total mass.
The inventors made and tested an improved solar sheet having a planar coversheet (e.g., devoid of prismatic structures such as polymer layer 98 shown in
The specific power of example solar sheet B was significantly increased as compared to that of example solar sheet A. In order to decrease the areal mass of the solar sheet, the inventors reduced the thickness of the polymer layer top sheet, omitted the polymer bottom sheet and reduced the thickness of metal backing layer in the solar cells. More specifically, the inventors reduced the thickness of the top polymer layer (i.e., the TEFLON sheet), from about 50 microns (as in solar sheet A) to about 25 microns. The thicknesses of the first and second adhesive layers were reduced from 25 micron to 12 microns. In addition, the thickness of the metal backing layer in the solar cells was reduced from 25 to 13 microns. The areal mass was about 240 g/m2. The areal power of solar sheet was measured to be about 260 W/m2 under AM1.5 and 330 W/m2 under AM0. The specific power of example solar sheet B was 1080 W/kg under AM1.5 and 1380 W/kg under AM0; this is a significant increase over the specific power of example solar sheet A. As the thickness of the solar cells and solar sheets was reduced, it became more challenging to handle the solar cells and components of solar sheets during the assembly of the solar sheets. For example, due to the reduction in thickness, various components of the solar sheet tended to curl easily, increasing the difficulties in making the solar sheets.
Table 2 shows the different layers of example solar sheet B and the materials used for each layer. In addition, Table 2 shows the areal mass of each layer and the contribution of the mass of each layer to the total mass.
Example Solar Sheet C with Frame Adhesive Layer
Example solar sheet C having a planar coversheet (e.g., devoid of prismatic structures such as sheet 98 shown in
Although some embodiments are described herein with respect to battery-powered UAVs, one of ordinary skill in the art will recognize that this disclosure also applies to UAVs incorporating other types of devices for storing electrical energy (e.g., fuel cells). Thus, kits and methods for increasing the endurance of electrically-powered UAVs (e.g., fuel-cell powered UAVs, battery-powered UAVs) fall within the scope of this disclosure.
While the present invention has been described with reference to illustrative embodiments thereof, those skilled in the art will appreciate that various changes in form in detail may be made without parting from the intended scope of the present invention as defined in the appended claims.
As may be recognized by those of ordinary skill in the pertinent art based on the teachings herein, numerous changes and modifications may be made to the above-described and other embodiments of the present disclosure without departing from the spirit of the invention as defined in the appended claims. Accordingly, this detailed description of embodiments is to be taken in an illustrative, as opposed to a limiting, sense. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the described herein. Such equivalents are intended to be encompassed by the following claims.
This application claims priority to U.S. Provisional Application No. 63/253,936, filed Oct. 8, 2021, the entire contents of this application being incorporated herein by reference.
This invention was made with government support under Contract No. FA8730-19-P-0042 awarded by the Air Force Research Laboratory (AFRL). The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 63253936 | Oct 2021 | US |
