Abstract:
A photovoltaic array with improved thermal performance includes a photovoltaic array, an electro-optic shutter disposed on the photovoltaic array, and a control system connected to at least the electro-optic shutter. The control system, based upon input from a sensor, switches the electro-optic shutter between transmissive and reflective conditions so as to control exposure of light to the photovoltaic array.

Description:
TECHNICAL FIELD 
     The present invention is related to airships or any other surfaces which might support a photovoltaic array. More specifically, the present invention is related to an electro-optic shutter which is associated with the photovoltaic array so as to regulate the temperature of the array and any supporting structure. 
     BACKGROUND 
     Generally, lighter-than-air vehicles, hereinafter airships, utilize photovoltaic arrays, also referred to as solar arrays, for the generation of power. This power is utilized to control operation of propulsion systems, communications equipment, and also surveillance instruments that may be carried by the airship. 
     Photovoltaic, or solar, arrays comprise a plurality of cells that are fundamentally designed to have high absorptivity in the solar visible and near visible infrared light spectrums. A fraction of this absorbed spectral energy is converted to electromotive power, also referred to as electrical energy, which is the inherent function of a solar or photovoltaic cell. Much of the remainder of the absorbed solar spectrum is converted to heat, thereby increasing the temperature of the cell and, via conduction, any underlying structures. Skilled artisans will appreciate that this extra heat can have detrimental effects on the underlying structures. 
     In certain applications of the photovoltaic array, such as in space and near-space regenerative power systems and for building roof or curtain-wall solar arrays, there may be times of the day or year when the benefits of photovoltaic electric generation are out weighed by the deleterious effects of sub-structural heating. For example, in a high-altitude airship, when the solar arrays are adequately powered and all energy storage capacity is utilized, absorption of extra heat in the airship may cause the supporting structure to over-heat and adversely effect the control and positioning of the airship. It is also conceivable that the extra absorbed heat could adversely effect any communications or optical equipment carried by the airship. Therefore, there is a need in the art to provide a way to reduce the impact of the excessive heat by controlling the amount of solar radiation that is exposed through the photovoltaic array when part or all of the solar array are not needed for electrical energy generation.  
     SUMMARY OF THE INVENTION 
     In light of the foregoing, it is a first aspect of the present invention to provide an optical shutter for improved photovoltaic array thermal performance. 
     It is another aspect of the present invention to provide an improved photovoltaic array comprising a photovoltaic array, an electro-optic shutter disposed on the photovoltaic array and a control system connected to the electro-optic shutter, the control system switching the electro-optic shutter between transmissive and reflective conditions so as to control exposure of light to the photovoltaic array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other features and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein: 
         FIG. 1  is an elevated perspective view of an airship employing an optical shutter for improved photovoltaic thermal performance according to the present invention; 
         FIG. 2A  is a schematic cross-sectional view of a supporting structure which carries a photovoltaic array and electro-optic shutter according to the present invention, wherein the shutter is in a transmissive mode; 
         FIG. 2B  is a schematic cross-sectional view of a supporting structure which carries a photovoltaic array and electro-optic shutter according to the present invention, wherein the shutter is in a reflective mode; and 
         FIG. 3  is a schematic diagram of a control system utilized to operate the electro-optic shutter and other components associated with the structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and in particular to  FIG. 1 , it can be seen that an airship according to the present invention is designated generally by the numeral  10 . The airship  10  may be employed to carry surveillance equipment, communications equipment or other payloads or cargo. In a specific embodiment, the airship may be used in high altitude configurations so that the airship may attain altitudes in excess of 60,000 feet. The airship includes a hull and one or a plurality of stabilizing fins, wherein the hull is generally identified as an envelope  12  which contains pressurized air and pressurized lighter-than-air gas for the purpose of moving the airship to a desired altitude. The airship could also be non-pressurized or rigid.  
     The envelope  12  includes a laminate material  14  which has attributes that are ideally suited for high-altitude flight. As such, the laminate material  14  may be a multi-layered construction which is strong, light weight, resistant to ultra-violet light and is able to withstand the rigors of extended deployment. Exemplary laminate materials are disclosed in U.S. Pat. Nos. 6,979,479 and 7,354,636; and U.S. patent application Ser. Nos. 11/443,327 filed May 30, 2006; 11/991,499 filed Mar. 4, 2008; and 12/011,509 filed Jan. 28, 2008, all of which are incorporated herein by reference. 
     Directional and altitude control of the airship may be provided by a plurality of propulsion systems  16  which are carried and otherwise secured to the airship envelope  12 . Skilled artisans will also appreciate that other devices, such as blowers, may be associated with the airship so as to control the amount of air retained within the airship for the purpose of adjusting the altitude thereof as needed. 
     A photovoltaic array, designated generally by the numeral  20 , which may also be referred to as a solar array, is disposed on the laminate material  14 . The photovoltaic array  20  is utilized to generate electrical power which is used by the propellers, blowers and surveillance and electronic communications equipment in a well known manner. The solar array may also be used to charge rechargeable batteries if carried by the airship. The array  20  is configured to include a large number of individual cells, all of which are designed so as to absorb as much solar radiation for the purpose of generating electrical power. That solar radiation which is not absorbed for the generation of electrical power also generates significant amounts of heat which is dissipated into the atmosphere surrounding the airship, absorbed by the supporting structures such as the laminate material and potentially absorbed by the communications equipment and other cargo carried by the airship. If needed, there may be additional layers between the laminate  14  and array  20 . These layers may include, but are not limited to, adhesive and insulator layers. 
     An electro-optic shutter, which is designated generally by the numeral  30 , is disposed selectively over the solar array  20  and may also extend over portions of the envelope which do not carry the array  20 . The electro-optic shutter  30  is configured to be switchable between reflective and transmissive modes. It will further be appreciated that in certain embodiments the electro-optic shutter may be controlled in sections so that certain array cells or panels may be exposed to solar radiation while others are not. 
     Referring now to  FIGS. 2A and 2B , it can be seen that the electro-optic shutter  30  is disposed on and covers the photovoltaic array  20  which is supported by the laminate  material  14 . As mentioned previously, the laminate material  14  encloses air and/or lifting gas in a manner well known in the art. Although the present embodiment is discussed as being in association with an airship, it will be appreciated that the concepts of the present invention are equally applicable to any supporting structure which carries a photovoltaic array and wherein it is desired to control the amount of solar radiation exposed to the array. In any event, the electro-optic shutter includes a pair of opposed substrates  32 A and  32 B which face one another. In most embodiments the substrates  32  are constructed of a flexible plastic material which is light weight so as to minimize weight of the airship. However, in some embodiments the substrates  32  could be glass or any other material commonly associated with electro-optic shutters. A gap is formed between the substrates which may be provided by polymeric materials or spacers such as beads, cylindrical rods or polymer wall configurations which maintain a desired spacing between the substrates. Each substrate includes an electrode  34 A and  34 B on respective facing surfaces of the substrates  32 A and  32 B. An electro-optic material  38  is disposed between the substrates. The material is selected so that it is electrically controllable and the electro-optic material is selected such that it is switchable between a transmissive condition and a reflective condition. Although any type of electro-optic or electrochromic material can be used, it will be appreciated that liquid crystal materials such as a guest-host dye configuration, cholesteric liquid crystal materials or any other material that is switchable between transmissive and reflective modes by application of a voltage across the electrodes may be utilized. Accordingly, application of a voltage across the electrodes  34  may switch the material between reflective and transmissive conditions. The substrates may be provided with alignment layers so as to control the orientation of the liquid crystal materials and to optimize the ability to reflect selected radiation wavelengths and absorb the desired radiation wavelengths so that is the wavelengths are received by the photovoltaic array. 
     An ultra-violet/infrared filter  40  may be disposed over the electro-optic shutter so as to prevent harmful radiation from adversely affecting the shutter  30 , the array  20 , and/or the laminate material  14 . 
     As seen in  FIG. 2A , radiation, also referred to as sun light, from the sun if projected onto the airship and the electro-optic shutter is switched into a transmissive condition so that the radiation reaches the photovoltaic array for the generation of electrical energy. In  FIG. 2B  it can be seen that the electro-optic shutter is configured into a reflective condition so that orientation of the electro-optic material is changed and reflects the radiation or sunlight. Skilled artisans will appreciate that the representation of the electro-optic  material is for the understanding of the reader and that any orientation of the material may be utilized to achieve the desired effect. 
     Referring now to  FIG. 3 , an airship control system, designated generally by the numeral  50 , is utilized so as to control operation of the components associated with the airship. The control system  50  includes a controller  52  which may be any processor-based device that incorporates the necessary hardware, software and memory components to coordinate multiple components carried by the airship. The controller shown is specifically for the purpose of operating the components discussed herein, but it may also be used for other features maintained by the airship. Connected to the controller  52  is a rechargeable battery  54  wherein the controller observes the amount of power retained by the battery. The battery may be in the form of a closed-loop regenerative fuel cell, lithium-ion battery or any other electrical storage device that can be associated with the airship or other structure. The control system may be used with any support structure that carries a photovoltaic array and an electro-optic shutter. Connected to both the controller  52  and the rechargeable battery  54  is an electric load  56  which may be any component utilized or carried by the airship. As noted previously, the airship utilizes propellers, surveillance and communications equipment that require electrical energy, and all of these components are identified as the electric load. 
     The solar array  20  is utilized to generate electrical energy which is transmitted to the rechargeable battery  54  for the purpose of storing the energy and wherein the controller utilizes that stored energy and controls distribution of power from the battery to the electrical load as needed. The electro-optic shutter  30  is positioned over the solar array to control the amount of radiation exposed to the solar array as previously described. As such, the controller  52  may control selected portions of the electro-optic shutter so as to control the amount of electrical energy generated as needed. To assist in determining whether the solar array should be exposed to solar radiation or not, a sensor  58  is associated with the solar array  20 . The sensor  58  may be placed between the solar array and the laminate material or it may be directly incorporated into the solar array or in close proximity thereto. In any event, the sensor  58  provides temperature information which is critical to the operation of the airship or associated structure. As such, when the sensor detects that the airship and/or the solar array is in an over-heated condition, that information is transferred to the controller which can make a determination as to whether sufficient power is stored and, as such, allow the electro-optic shutter to be activated into a reflective mode when electrical energy is not required by the rechargeable battery  54 . In  this manner, the airship control system can optimize operation of the airship and/or any other supporting structure associated with the photovoltaic array for a desired operation. As such, if the rechargeable battery is in a low condition, the electro-optic shutter can be in a totally transmissive condition and allow for maximum utilization of the solar array. It will further be appreciated that the electric-optic shutter could be used in areas that do not cover the solar array so as to reflect solar radiation and maintain other portions of the airship in a condition that does not allow for excessive generation of heat. When the rechargeable battery is fully charged, this information can also be relayed to the controller and the electro-optic shutter can be activated so as to more easily dissipate heat when electrical power is not required or is not needed to be stored. 
     In view of the foregoing it will be appreciated that there are several advantages to the airship control system  50  and the combined electro-optic shutter and photovoltaic array. Such a configuration is advantageous in that it allows for selective use of the solar array so as to prevent over charging of a rechargeable battery and to prevent overheating of the entire assembly when the batteries no longer require charging. Such a configuration is also advantageous in that it allows for optimal use of the photo-voltaic array without adding excessive weight to the overall airship structure. 
     Thus, it can be seen that the objects of the invention have been satisfied by the structure and its method for use presented above. While in accordance with the Patent Statutes, only the best mode and preferred embodiment has been presented and described in detail, it is to be understood that the invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention, reference should be made to the following claims.