Abstract:
An airborne photovoltaic solar system, comprising a parabolic reflector-concentrator ( 110, 1210 ), spread by wind, a solar panel ( 102, 1202 ) raised to high altitude using at least one lighter than air balloon ( 103, 1201 ), tracking the sun using aerodynamic surfaces and/or changes in buoyancy, relying on wind and/or cold air to cool the solar panel in one embodiment, as well as related methods and variations and alternatives.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a continuation of PCT Application No. PCT/US13/40666, filed 10 May 2013, which claims the benefit of U.S. Provisional Applications No. 61/646,316, filed 13 May 2012, No. 61/683,783, filed 16 Aug. 2012, and No. 61/706,123, filed 27 Sep. 2012 by the same inventor as herein, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Airborne photovoltaic solar panels are used as the power source in some unmanned planes, such as NASA Pathfinder and QinetiQ Zephir. An idea of an airborne photovoltaic solar device, transmitting energy to the ground over a power cable was proposed by a team consisting of Aglietti, Redi, Tatnall and Markvart ( Aglietti, Redi, Tatnall and Markvart, Harnessing High - Altitude Solar Power, IEEE Transactions on Energy Conversion,  Vol. 24, No. 2, June 2009.) In it, solar cells are patched to a surface of an aerostat on one side, and this patch is turned toward the sun at all times. This provides an advantage in the amount of light hitting the surface of the cells compared with the ground solar installation. Nevertheless, the advantage is negated by the costs of the aerostat. 
         [0003]    Another airborne photovoltaic solar system is discussed in the U.S. Pat. No. 7,997,532 by Tillotson. It uses a solar panel, suspended from an airborne balloon. Nevertheless, it is still not attractive economically because it requires relatively large area of solar cells, which are expensive. 
         [0004]    Currently, solar energy is not cost competitive with fossil fuels or even with wind energy, even in the south of the US. There is need in a cost effective photovoltaic solar energy system. This invention is directed to teaching such a system. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention is generally directed toward airborne solar energy conversion and related technology. 
         [0006]    One embodiment of the invention is a system for converting solar energy into electric energy, comprising: at least one airborne platform; a solar panel, comprising multiple solar cells, attached to the airborne platform; a reflector, focusing solar radiation on the solar panel; an actuator for rotating the reflector; a power cable, connecting the solar panel to an electric grid or an electricity consumer on the ground; an electronic control system. 
         [0007]    Examples of the airborne platform include: a lighter than air moored balloon; a wing or a system of wings, creating aerodynamic lift using wind; a wing or a system of wings, creating aerodynamic lift using wind by moving cross wind; a kytoon; a rotor with blades, powered by electric energy, generated by the solar panel. The rotor might be similar to a helicopter rotor, driven by an electric engine. 
         [0008]    The reflector can be airborne as well. The reflector can be made of thin fabric, having reflective coating on one side. The reflector can receive and/or maintain its form by the wind pressure. An inverter-transformer can be installed in proximity to the solar panel and connected to the power cable. The power cable can be optionally attached to a second airborne platform, connected to the first airborne platform by another cable. The actuators for rotating solar panel can take form of aerodynamic surfaces, attached to the airborne platform and controlled automatically in response to apparent movement of the sun and/or changes in the direction of the wind and/or changes in the force of the wind and/or motion of the platform. The gas inside of the balloon or the kytoon can be any of the following: hydrogen, methane, helium, hot air or their combinations and mixes with other gases. A system, described above, can be elevated to sufficient altitude and controlled in such a way, that some combination of the following is true: i) cold air and natural winds at the altitude cool down the solar cells; ii) the solar panel is above clouds most of the time; iii) the solar panel is above substantial part of atmosphere, thus reducing solar radiation reflection, diffusion and absorption by the atmosphere; iv) the solar panel is above flying sand and debris and surface induced turbulence. The solar panel and the reflector may be placed at altitude of 3,000 meters above sea level or higher, which satisfies to large degree the conditions i-iv) in most geographic locations. 
         [0009]    Another embodiment is a method of converting solar energy into electric energy, comprising steps: elevating a solar panel, comprising multiple solar cells, above the ground, using either buoyancy or aerodynamic lift or both buoyancy and aerodynamic lift; focusing solar radiation on the solar panel; using either cold air or natural wind or both to cool down the solar cells; using the solar panel to convert solar radiation into electric energy; transmitting the electric energy to a ground based installation through a power cable. 
         [0010]    Further, an airborne reflector can be used for focusing solar radiation on the solar panel. The airborne reflector can be rotated to follow apparent sun motion. The position of the airborne reflector can be optimized to minimize wind drag. The position of the solar panel can be optimized to minimize wind drag. The solar panel and the reflector can be rotated in one or more of the following ways: to track the sun in order to direct maximum or optimum radiation at the solar panel; to follow the wind in order to minimize aerodynamic drag; to follow the wind in order to ensure sufficient aerodynamic forces to keep the reflector inflated and/or solar panel airborne; to direct maximum radiation at the solar panel while ensuring that aerodynamic drag and aerodynamic lift are within pre-defined limits. The solar panel is preferably placed in the focus of the reflector. 
         [0011]    Another embodiment of the invention is a device for converting solar energy into electric energy, comprising: a lighter than air balloon; a solar panel, comprising multiple solar cells, attached to the balloon; a reflector-concentrator, attached to the balloon; an electric cable, connecting the solar panel to an electric grid or an electricity consumer on the ground; an electronic control system. 
         [0012]    In the various embodiments of this invention it is suggested to take into account not only solar radiation, but also the wind, that is almost always present at an altitude. Moreover, methods are provided to use benefits, provided by the wind while avoiding its dangers. 
         [0013]    Various objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings illustrate the invention. The illustrations omit details not necessary for understanding of the invention, or obvious to one skilled in the art, and show parts out of proportion for clarity. In such drawings: 
           [0015]      FIG. 1  is a schematic view of one embodiment of an airborne solar system, showing two balloons, a reflector-concentrator and ground installations and attachments. 
           [0016]      FIG. 2  shows the reflector-concentrator with a coordinate system. 
           [0017]      FIG. 3A  is a sectional view of the reflector-concentrator and the solar panel in plane YZ when the sun is in zenith. 
           [0018]      FIG. 3B  is a view of the same, when the sun lower on the horizon. 
           [0019]      FIG. 4  is a cross sectional view of the reflector-concentrator and the solar panel in plane XZ. 
           [0020]      FIG. 5A  is a view of the solar panel from below, showing flow augmentation diffusers. 
           [0021]      FIG. 5B  is a cross sectional front view of the same. 
           [0022]      FIG. 6  is a view of turbulator strips on the surface of solar panel. 
           [0023]      FIG. 7A  is a perspective view of the reflector-concentrator. 
           [0024]      FIG. 7B  is a projection of the reflector-concentrator into plane XY. 
           [0025]      FIG. 7C  is a projection of the reflector-concentrator into plane YZ. 
           [0026]      FIG. 7D  is a projection of the reflector-concentrator into plane XZ. 
           [0027]      FIG. 7E  is a sectional view the reflector-concentrator in plane XZ. 
           [0028]      FIG. 8  is a schematic view of alternative cables arrangement in the reflector-concentrator in plane YZ. 
           [0029]      FIG. 9  shows details of the balloon, carrying the solar panel, in this embodiment. 
           [0030]      FIG. 10  is a top view of an example of a maneuver of the balloon in this embodiment. 
           [0031]      FIG. 11  is a scheme of a control system. 
           [0032]      FIG. 12  is a perspective view of another embodiment of the invention, in which a single balloon carries both a solar panel and the reflector-concentrator. 
           [0033]      FIG. 13A  is a sectional view of some details of this embodiment in the plane YZ, when the sun is in zenith. 
           [0034]      FIG. 13B  is the previous view, when the sun is not in zenith. 
           [0035]      FIG. 13C  is a sectional view of some details of this embodiment in the plane XZ. 
           [0036]      FIG. 14  is a top schematic view of the reflector-concentrator in this embodiment. 
       
    
    
     GLOSSARY 
       [0037]    The term ‘solar panel’, as used here, includes ‘solar array’. 
         [0038]    Kytoon—a lift creating device, combining lift of lighter than air gas, with an aerodynamic lift in presence of wind. Preferably, kytoon comprises a balloon with a surface, generating at least a part of the useful lift. 
         [0039]    Words ‘reflector’, ‘concentrator’ and ‘reflector-concentrator’ are used interchangeably and mean reflector-concentrator. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]      FIG. 1  shows general view of one embodiment of the invention. There is a balloon  101 , carrying a solar panel  102 . There is another balloon  103 , carrying a mount  104 , which carries weight of an aluminum cable  105  and of three tethers  106  that attach balloon  103  to the ground and keep it in a fixed position in any wind. Tethers  106  are anchored at the ground in the vertices of an equilateral triangle. Tethers  106  are made of strong and light synthetic fibers, such as aramids, para-aramids or ultra-high molecular density polyethylene. Balloons  101  and/or  103  can have streamlined form to decrease wind forces, acting on them. An inverter-transformer  107  can be attached to mount  104 . Cable  108  connects balloon  101  to the top of cable  105 . Cable  108  comprises an aluminum conductor that conducts current from solar panel  102  to cable  105 . Cable  105  is attached to ground electrical infrastructure element  109 , which distributes electricity to ultimate consumers. Ground infrastructure element  109  can include an inverter, converting DC current, generated by the solar cells, into AC current, suitable for feeding into a grid (in absence of airborne inverter-transformer  107 ). Cable  108  also comprises a light but strong tether, made of synthetic fibers, and attached to mount  104 . A large solar reflector-concentrator  110  soars in the air, attached to mount  104  by a cable  111 . Reflector-concentrator  110  provides of its own lift or buoyancy, compensating most or all of its weight. It also has its own stabilization and control surfaces, allowing control system  112  to slowly change its orientation in the air along two axis. Balloon  101  also possess stabilization and control means. These control means allow control system  112  to slowly change it position in the space within maximum reach of cable  108  and to change angles of solar panel  102  to the horizontal plane. Reflector-concentrator (or simply concentrator)  110  has reflecting upper surface and reflects solar radiation and concentrates it on solar panel  102 . Concentration ratios can be 10-1000 times. Concentrator  110  is straight downwind from mount  104  and balloon  103 , balloon  101  is within the downwind hemisphere; cable  108  and cables  111  are attached in such way that they can move more than 360 degrees around mount  104 , not fouling and unimpeded by balloon  103 . Aluminum cable  105  can run along a mechanical cable, made of synthetic fibers, reinforcing it. Solar panel  102  is made conventionally: smaller solar modules or solar cells, connected both in series with diodes and in parallel. Solar panel  102  can be equipped with air flow augmenting surfaces, increasing air flow speed in the vicinity of the panel and creating turbulence, thus improving cooling. It can be also equipped with heat sinks. Heat conducting surfaces can be combined with air flow augmenting surfaces and/or turbulators in order to save mass. 
         [0041]    Using  FIG. 2 , it is convenient to introduce axis: longitudinal axis X, lateral axis Y and normal axis Z. It is also convenient to designate the edge, closest to mount  104 , as a leading edge  201 , the opposite edge as a trailing edge  202 , and two other edges as a left tip  203 L and a right tip  203 R. Leading edge  201  is always facing the wind. Length L of solar panel  102  approximately equals length (measured along axis X) of concentrator  110 . The surface of concentrator  110  is a parabola in each cross section in the plane YZ, and it is a straight line in each cross section in the plane XZ; it can become slightly curved under force of wind in some variations. 
         [0042]    The system is operated in the daylight according to the following algorithm: 
         [0043]    axis X of concentrator  110  is kept parallel or at small angle to the wind in order to minimize wind forces, acting on its surfaces (in some embodiments—also to provide some lift) 
         [0044]    concentrator  110  is rotated around axis X in such way, that its axis Z points to the projection of the sun onto plane YZ (the projection is parallel to axis X); for example, when the sun is in the zenith, axis Y is horizontal; the angle of axis Y can change from −85° to +85°, depending on the position of the sun and angle between the horizontal projection of the direction to the sun and wind direction 
         [0045]    there is a strip of approximately length L, parallel to axis X, into which concentrator  110  reflects the solar radiation that hits it; this strip is in the focus of the parabolic section of concentrator  110 ; balloon  101  is maneuvered to put solar panel  102  into this strip and to turn it perpendicular to the reflected sun beams. Thus, maximum solar energy is converted into electricity throughout a day and a year. 
         [0046]    These are sample parameters of the system: 
         [0047]    Voltage, DC: 10,000 V 
         [0048]    Inverter-transformer  107 : not used because DC current has sufficient voltage 
         [0049]    Altitude: 5,000 m 
         [0050]    Cable  105  sectional area: 10 cm 2    
         [0051]    Total weight of cables  105  and  106 : 15,000 kg 
         [0052]    Volume of balloon  103 , hydrogen filled: 30,000-50,000 m 3    
         [0053]    Reflecting area (perpendicular sunbeams): 500,000 m 2  (L×W, where L=250 m, W=2,000 m) 
         [0054]    Height of the reflecting surface  110  (along axis Z): 350 m 
         [0055]    Focus distance: f=W/2.83=700 m 
         [0056]    Concentration ratio: 50:1 
         [0057]    Solar panel area: 5,000 m 2  (250 m×40 m) 
         [0058]    Solar cell efficiency: 15% 
         [0059]    Peak output power: &gt;100 MW 
         [0060]    Required minimum wind speed: 2 m/s 
         [0061]    The embodiment or embodiments, described above, have multiple advantages over ground based photovoltaic solar systems: 
         [0062]    1) It allows to use much smaller amount of expensive solar cells and much smaller solar panels surface, compared with direct photovoltaics. Most of the expensive solar cells and supporting structures of solar panels are replaced by an inexpensive reflecting fabric, which is self-supporting in the presence of the wind. Further, efficiency of solar cells increases for concentrated light, allowing to produce more electricity. 
         [0063]    2) It allows to cool the solar cells much more efficiently, because of the lower air temperature at the altitude and presence of the wind most of the time. In those rare times, when the wind becomes too slow, energy production may become less efficient. If wind&#39;s speed falls even lower, below a pre-defined threshold, the system can direct reflected light away from the solar panel, preventing it from damage by high temperature. 
         [0064]    3) There is more energy in the solar radiation at the altitude, and larger part of it comes in the form of direct beams that can be efficiently concentrated (rather than diffused radiation, that cannot be concentrated). The concentrator and the solar panel can be also above most of the weather and clouds. Thus, this system can be deployed in the areas, where ground based systems are not efficient because of clouds or fog. 
         [0065]    4) The system can track sun at lower angles, than the ground based tracking systems, and generate energy with lower losses at such time. This allows generating substantial power even in the high latitudes and throughout the whole day. This covers the late afternoon peak in the energy consumption, existing in many countries. 
         [0066]    The system is least efficient, when the sun is very low and horizontal projection of the direction to the sun is close to the direction of the wind (or opposite to it). 
         [0067]    Therefore, the location and altitude for such system should be preferably selected in such way, that this combination does not happen frequently. More solutions for this problem are proposed in the additional embodiments of this invention below. 
         [0068]    5) Because the concentrator is above most of the weather, energy production is regular and predictable. This allows to overcome the problem of the intermittency, which plagues scalable renewable energy sources. 
         [0069]    6) The solar panel and the solar concentrator are protected from sand, dust, flying debris, rain, wind gusts, vandalism and other harmful impacts, that exist on the ground. This allows to make it lighter and decreases need for periodic maintenance. Thus, solar concentrator&#39;s surface can be made almost entirely of a thin fabric, covered in a reflecting film (for example rip-stop nylon, covered with aluminum foil, or of metalized BoPET). Such airborne solar concentrator can be scaled to a very large size, which is impossible for ground based concentrators. 
         [0070]    7) A single device, described above, would have negligible consequences for local climate. For a ground observer, it will have an effect of a single cloud, quickly passing in the sky. Nevertheless, a number of such devices, placed in proximity one to another, can be used for local climate improvement, such as creating an oasis in a desert. It is achieved because of decrease of direct solar irradiation, reaching ground. The density of such devices should decrease from the center to the periphery, in order to decrease effect of the temperature gradient near the ground, and resultant winds. 
         [0071]    8) It should be noted that this system is not dependent on the wind in the same way as wind energy conversion device are, because it can operate at very low winds, which are almost always present. Because of its ability to operate in the winds, in which wind energy conversion devices cannot operate or have very low output, it is complementary to the wind energy. 
         [0072]      FIG. 3A  shows sectional view of reflector-concentrator  110 , solar panel  102  and balloon  101  in plane YZ, when the sun is in zenith. A property of a parabola is that it reflects light beams, parallel to its axis, into its focus F. Sectional form of solar panel  102  is a semi-circle with a center in the point F. Preferably, solar panel  102  completely shields balloon  101  from concentrated sun beams. 
         [0073]      FIG. 3B  shows rotation of the subsystem, consisting of concentrator  110  and solar panel  102 , when the sun is at the angle 45° to the horizon. This rotation can be achieved most easily by rotating concentrator  110  around its center of mass in plane YZ, shifting solar panel  102  sideways and down. Balloon  101  and solar panel  102  are rotated around their common center of mass. The direction of the wind is perpendicular to the viewed plane. 
         [0074]      FIG. 4  shows cross sectional view of the same details in plane XZ. It shows, how light beams bounce off the surface of concentrator  110  and hit solar panel  102 . Horizontal position of balloon  101  with solar panel  102  can be adjusted by control system  112  to ensure precise hit. 
         [0075]      FIG. 5A  shows a view from below of solar panel  102  with air flow augmentation diffusers  501 . These devices increase speed of air flow over both surfaces of solar panel  102  and create turbulence, increasing heat dissipation.  FIG. 5B  is one of a cross sectional views of the same. 
         [0076]      FIG. 6  shows solar panel  102  with turbulator strips  601  on its surface that create turbulence and increase heat dissipation. 
         [0077]      FIG. 7A-7E  show one possible implementation of concentrator  110 .  FIG. 7A  is a perspective view.  FIG. 7B  is the projection into plane XY; it shows most details.  FIG. 7C  is the projection into plane YZ.  FIG. 7D  is the projection into plane XZ.  FIG. 7E  is a sectional view in the plane XZ.  FIG. 7C  omits the stabilizer, and each figure omits some cables to decrease clutter. 
         [0078]    Concentrator  110  comprises a large sheet  700  (that can be stitched from a number of smaller sheets) with reflecting upper surface, its leading edge is reinforced with a cable  701 , its trailing edge is reinforced with a cable  702 , its left and right edges are reinforced with cables  703 L and  703 R correspondingly. Cylindrical balloons  704  with lighter than air gas are attached to the left and the right edges of sheet  700  and create buoyancy, which keeps sheet  700  in the air under all conditions. Internally, sheet  700  can be reinforced by cables  706 . There is a device  705 , attached to cable  111  on one side and terminating front lines  707  and back lines  708 . Front lines  707  are attached to the leading edge of sheet  700 , while back lines  708  are attached to the trailing edge of a horizontal stabilizer  710 , which is attached to the trailing edge of sheet  700  by multiple cables  709 . Stabilizer  710  is implemented as a strip of synthetic fabric, which is much stronger than sheet  700 . Control sub-system  712  is carried by device  705 . Additionally, device  705  contains mechanisms to pull in and let out at least back lines  708 . The trailing edge of sheet  700  is slightly higher than the leading edge, allowing the wind to inflate sheet  700 , spreading its sides. In other words, axis X is slightly inclined to the horizontal plane; the angle increases in a weaker wind and decreases in a stronger one (by pulling in or letting out back lines  708 ). Lateral cables  711  (shown only in  FIG. 7C ) are connected at different heights and throughout the length of sheet  700  and ensure that sheet  700  keeps the form of parabola in the plane YZ. In this embodiment, sheet  700  is slightly curved, as shown in  FIG. 7D , but the curvature is insignificant, and does not contribute much to the concentrating effect in this embodiment. It creates a negative lift along axis Z, which is compensated by pull of lines  707  and  708 , and buoyancy of balloons  704 . Balloons  704  are equipped with means to increase lift, decrease lift and keep the balloon at a defined altitude.  FIG. 7E  is a sectional view in the plane XZ. It shows the form of stabilizer  710 , position of cables and joint form of sheet  700  and stabilizer  710 . Surface of stabilizer  710  is non-reflective. In certain implementations there is a danger, that back lines  708  will hit lateral cables  711 . To prevent this, an alternative arrangement of lateral cables  711  is shown in  FIG. 8 . Control sub-system  712  of control system  112  rotates sheet  700  in the plane YZ by commanding balloons  704  to increase or decrease their lift, and to stay at a defined altitude. Control sub-system  712  changes angle between axis X and direction of the wind by pulling in or letting out back lines  708 . 
         [0079]      FIG. 9  shows some details of balloon  101 . It comprises a horizontal stabilizer-rudder  901  in the tail, a vertical stabilizer-elevator  902  in the tail, a horizontal rudder  903  in the nose, a vertical elevator  904  in the nose, a ballonet  905  for changing the buoyancy, a motorized pulley  906  for pulling in and letting out cable  108 , optional counter-weights and a control sub-system  912 . Employing aforementioned actuating devices in the presence of the wind, control sub-system  912  ensures that solar panel  102  stays in the focus of concentrator  110  and faces it straight. It should be noted, that direction of the wind can change, but balloon  101  and concentrator  110  change their positions with the wind, “following the wind”, so their relative positions can be analyzed as if the wind direction does not change.  FIG. 10  shows, how the tail and the nose rudders are used to move balloon  101  sideways in the horizontal plane (solid line—position of balloon  101 , maintained by the deflected rudders). It should be noted, that there is always imprecision in the form of concentrator  110 , so it reflects the sun radiation over the whole surface of solar panel  102 , rather than at a single line. 
         [0080]      FIG. 11  shows an example control system  112 , comprising processing logic  1101 , plurality off sensors  1102  and plurality of actuators  1103 . Typically, some elements of control system  112  are airborne and some are ground based, but the whole system can be airborne. Control system  112  directs and controls all aspects of the system operation, including sub-systems  712  and  912 . It can comprise multiple sub-systems, attached to different parts of the construction. Control system  112  comprises one or more central processors or microcontrollers, sensors, communication means for communication between the sub-systems and with the outside world, and actuators. Possible communication means is a wireless network. The sensors may include anemometer, barometer, radar, hygrometer, thermometer, GPS, cable tension meter, cameras, light sensors, pressure sensors, altitude meters and other. Control system  112  can be connected to the Internet to receive general weather information, especially warnings of extreme weather events. 
         [0081]    In more embodiments, cable  105  can be supported by more than one set of tethers  106  and balloons  103  at different altitudes. The reflecting surface of concentrator  110  can be covered by a substance, transparent for light waves in the length diapasons that can be absorbed and converted to electricity by solar cells, and diffusing or absorbing other light. This would allow to further decrease heating of solar panel  102 . Balloon  101  can carry fans that would create air flow over the surface of solar panel  102 , when there is no sufficient wind. Fans would work from electricity, generated by the solar panel itself. Balloon  101  can also employ propellers, driven by electric motors, to adjust its position in the space. Multiple concentrators  110  can be used with a single solar panel  102 . Kytoons can be used in place of balloons  101  and/or  103 . In other embodiments, sheet  700  can be stretched inside of a rigid frame, consisting of tubular members, used instead of cables  701 ,  702 ,  703 R and  703 L. These members can be made of fiberglass or carbon fiber. In a variation of this embodiment, only rigid members are in the leading and the trailing edges. Sheet  700  is kept stretched by the drag of stabilizer  710 . In more embodiments, sheet  700  is perforated in order to minimize local pressure differences on two sides of the sheet and decrease wear. In more embodiments, a double sheet is used instead of single sheet  700 . The space between the sheets is divided into cells, like in a foil kite, and the cells are inflated by the ram air. Such arrangement can be more durable, than a single sheet. In a variation of this embodiment, the cells are closed and inflated by lighter than air gas, eliminating need in balloons  704 . In more embodiments, concentrator  110  can have a form of elliptical paraboloid with ellipse&#39;s longer axis along axis X and the shorter axis along axis Y. In another embodiment, the angle of axis Y to the horizon can be changed by a leap to 45° degrees or more, skipping the range of angles in which sheet  700  would experience high aerodynamic lift from the wind, and jumping into the range of lower drag forces. This is especially effective, when the sun is low and the wind is blowing from the direction of the sun. When the wind is weak, the angle of axis Y can be changed gradually through all the range. Further, in some embodiments, solar panel  102  can be fixed in the focus of concentrator, being rigidly fastened to its frame. A single balloon will carry the whole construction. Tracking will be achieved using aerodynamic forces, developed by concentrator  110  in the wind. The frame of concentrator  110  can be equipped with aerodynamic control surfaces, helping it to change angle to receive lift from the wind. In more embodiments, concentrator  110  can be sharded, and this allows almost arbitrary large scale shape of concentrator  110 . 
         [0082]      FIG. 12  shows a general view of another embodiment of the invention. There is an elongated balloon  1201 , filled with hydrogen or another lighter than air gas, carrying a solar panel  1202  attached on cables. Solar panel  1202  comprises one or more solar modules. Balloon  1201  has a balloon empennage  1203 , comprising a vertical stabilizer, a horizontal stabilizer, a rudder, elevators, or their functional equivalents. One or more power electronics blocks  1204  are attached to balloon  1201 . A power electronics block  1204  contains a solar inverter DC-to-AC with maximum power point tracking means. Power electronics blocks  1204  also comprise one or more transformers, increasing voltage of the AC current in order to decrease losses caused by the wire resistivity. It should be noted that at lower temperature at the altitude the transformers can be cooled easier and may not require liquid cooling, allowing making the transformers lighter. Each inverter is electrically connected to one or more solar modules on panel  1202 . The inverters are connected in series and/or in parallel by internal wires  1205 . Tether  1207  is made of strong and light synthetic fibers with aluminum wires inside of it. Tether  1207  attaches balloon  1201  to the ground and delivers generated electric energy from the above mentioned transformer(s) to the ground infrastructure. Ground connection can comprise a converter from high frequency AC (that allows to make transformers, carried by balloon  1201 , lighter) to grid standard 50-60 Hz. Multiple inclined slats  1206  are connected to solar panel  1202  and are used for increasing air flow around solar panel  1202  and for creating lift in the presence of wind. A large solar reflector-concentrator  1210  is carried by balloon  1201 , attached by cables  1208  to solar panel  1202  and balloon  1201  itself. Reflector-concentrator (or simply concentrator)  1210  is made of a sheet of a flexible fabric, having light reflecting upper surface. Concentrator  1210  reflects solar radiation and concentrates it on solar panel  1202 . Balloon  1201  can be equipped with an optional wing  1209 , which can be ram air inflatable. Concentrator  1210  has approximately the same length or is slightly shorter than solar panel  1202 , both are shorter than balloon  1201 . Concentrator  1210  is equipped with an additional empennage  1211 , having width and height, close to those of concentrator  1210 . Empennage  1211  comprises at least a vertical and horizontal stabilizers and, optionally, a rudder, elevators, or their functional equivalents. Empennage  1211  can be ram air inflatable. Empennage  1211  is connected to the trailing edge of concentrator  1210  in multiple points of both the empennage and the concentrator (not shown on the picture to reduce clutter), and to balloon  1201  in at least one point. Empennage  1211  provides stability to concentrator  1210  and can be used to control it. Concentrator  1210  is also equipped with control surfaces  1213  along its left and right edges. There is a control system  1212 , carried on balloon  1201 , which controls all aspects of the system operation. Concentrator  1210  can be made of rip-stop nylon, covered with aluminum foil, or of metalized BoPET, among other options. 
         [0083]    In many respects, construction and operation of this embodiment are similar to those of the embodiment in  FIG. 1  (balloon  1201  is similar to balloon  101 , panel  1202  is similar to panel  102 , concentrator  1210  is similar to concentrator  110 , control system  1212  is similar to control system  112 ). 
         [0084]    Using a coordinate system, similar to one in  FIG. 2 , axis X of concentrator  1210  has a small angle of 2-5° to the wind, so that the wind “inflates” the fabric of concentrator  1210 , giving it a form of a parabola in each section in the plane YZ. Because of small angle between the surface of concentrator  1210  and direction of the wind the wind pressure on concentrator  1210  is small, and the fabric can be made thin and light. Solar panel  1202  is parallel to the axis X of concentrator  1210 . 
         [0085]      FIG. 13A  shows sectional view of some details of this embodiment in the plane YZ, when the sun is in zenith. Plurality of cables  1208  attaches concentrator  1210  to panel  1202  and balloon  1201 . Lengths and distribution of cables  1208  are selected in such way, that concentrator  1210  has parabolic section with a focus in the point F slightly behind the face of panel  1202  (when concentrator  1210  is fully stretched by the wind). No buoyancy elements are used in concentrator  1210 —it is fully supported by cables  1208 . When angle of the sun on the horizon changes, the construction is rotated around wind vector (which is close to the axis X), so that direction to the sun remains parallel to the axis of the parabola, as shown in  FIG. 13B . Angles of rotation are +/−90°. To achieve the rotation, control system  1212  determines desirable angle of the axis Z to the vertical and issues commands to the actuators of control surfaces  1213  and other movable control surfaces that may be present in empennage  1203  and/or empennage  1211  to change orientation of the system. It should be noted, that the fabric of concentrator  1210  can be very light, and aerodynamic forces, acting on it are balanced by aerodynamic forces, acting on panel  1202  and balloon  1201 , so only a small force is required to turn the device in space. Balloon  1201  gets in the position with its long axis along the wind by the action of the wind, without need for active control. The long axis of balloon  1201  remains constantly aligned with the wind. If wing  1209  is present, its lateral axis remains horizontal and it creates vertical lift, as will be explained below.  FIG. 13C  shows sectional view of some details of this embodiment in the plane XZ. Besides these details, marked by their numbers, it shows reflection of the light beams in the plane XZ. When the sun is low and horizontal projection of direction to the sun is close to the wind vector (+/−180°), sun beams falls onto concentrator  1210  and reflect from it at low angles ( FIG. 13C  shows an angle of about 40°). This is why the lengths of concentrator  1210  and solar panel  1202  are significantly larger than the distance between them. Even at low angles, most of the light, falling on the concentrator  1210 , is reflected onto the panel  1202 . In this embodiment, the position of concentrator  1210  relative to panel  1202  is fixed.  FIG. 13C  also shows slats  1206 . These slats  1206  are flat shields, made of fiberglass or aluminum, attached to solar panel  1202  and inclined at 15-45° to the horizon in the direction of axis X. They serve two functions: increase air flow around the solar panel, especially at its back side, and create aerodynamic lift in the positive direction of axis Z, which compensates negative lift, created by the surface of concentrator  1210 . Unlike it, the aerodynamic lift by wing  1209  is usually directed upward, and serves to compensate downward force, resulting from the sum of drag, created by the wind, and pull by tether  1207 . Wing  1209  is shown symbolically. Instead of it, three or more short wings can be attached in different points around circumference of vertical section of balloon  1201  and controlled in such a way, as to create upward lift. Also, wing  1209  can be omitted, in which case balloon  1201  will tend to descend to lower altitude in stronger winds, until forces of wind, buoyancy and pull of tether  1207  come into balance. 
         [0086]      FIG. 14  is a schematic top view of concentrator  1210 , showing work of control surfaces  1213 . To move concentrator  1210  to one side, control system  1212  rotates control surfaces  1213  on that side, and the wind force causes movement to that side, as shown by an arrow near concentrator  1210 . Control surfaces  1213  on the opposite edge remain parallel to the edge. 
         [0087]    This embodiment has all the benefits of the embodiment in  FIG. 1 , while being simpler and better scalable down. 
         [0088]    These are example parameters of the system: 
         [0089]    Voltage, AC: 50,000 V 
         [0090]    Power electronics  1204 : 8×500 kW inverters and 1×4 MW transformer 
         [0091]    Altitude: 6,000 m 
         [0092]    Length of balloon  1201 : 300 m 
         [0093]    Diameter of balloon  1201 : 15 m 
         [0094]    Volume of balloon  1201 : 45,000 m 3    
         [0095]    Airborne mass: up to 25,000 kg 
         [0096]    Reflecting area (perpendicular to sunbeams): 20,000 m 2  (L×W, where L=250 m, W=80 m) 
         [0097]    Concentrator&#39;s Focus distance: f=40 m 
         [0098]    Concentration ratio: 20:1 
         [0099]    Solar panel area: 1,000 m 2  (250 m×4 m) 
         [0100]    Solar modules in panel  1202 : 8 units 
         [0101]    Solar module efficiency: 15% 
         [0102]    Peak output power (at 1.25 sun): 3.75 MW 
         [0103]    Required minimum wind speed: 1 m/s 
         [0104]    Additionally, a Fresnel prism can be installed in front of solar panel  1202 , dispersing concentrated light from concentrator  1210 . Then, two kinds of solar cells can be used, each for the part of spectrum, in which it is more effective. For example, polycrystalline silicon cells with bandgap 1.1 eV can be used in infrared and red parts of spectrum, and amorphous silicon cells with bandgap 1.7 eV can be used in the parts of spectrum with shorter wavelengths, including most of the visible light. Such system will have increased efficiency and lower parasitic heating, without using expensive multi junction cells. When the sun is low on the horizon, the wind blows from the direction of the sun and the wind is light, control system  1212  can increase angle between axis X and the wind from 2-5° to 15-30° and even 60°. This will increase amount of light, falling on concentrator  1210  and reflected onto solar panel  1202 . For better stability, parts of concentrator  1210  can have holes in the fabric. In another embodiment, concentrator  1210  can taper toward its trailing edge. In other words, the further to the trailing edge the section in the plane YZ is take, the shorter focus distance of the parabola becomes in this embodiment. With such form, concentrator  1210  is kept inflated even when its axis X is parallel to the wind. In this embodiment, solar panel  1202  should be placed at angle to axis X, so that the solar panels remain near the focus over all its length. In more embodiments, solar panel  1202  and concentrator  1210  can be moved relative to one another along axis X in order to fully catch reflection from concentrator  1210  onto solar panel  1202 . In other embodiments, inverters and transformers in power electronics blocks  1204  can be omitted. It should be also noted that the parabolic form of reflector-concentrator can be approximated by other curved surfaces. 
         [0105]    Thus, an airborne photovoltaic solar device and system are described in conjunction with one or more specific embodiments. While above description contains many specificities, these should not be construed as limitations on the scope, but rather as exemplification of several embodiments thereof. Many other variations are possible and contemplated.