Abstract:
A wireless power transmission system for use in a mobile asset comprising a host transmitter for providing at least one of a microwave or a lightwave energy signal, a receiver configured to receive said signal, a converter for converting said signal to a storable energy form, and a controller to control the transfer of storable energy from said converter to at least one energy storage device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to wireless energy transmission, more specifically to a wireless energy transmission and storage system. 
     BACKGROUND 
     The operation of many mobile devices, including vehicles, is limited by the amount of onboard energy they are able to store. For example, in battery or gasoline powered vehicles, the weight and/or size of the batteries, fuel, or storage units thereof are limiting factors on the effective operating range of the asset. These operating range limitations, as well as costly and time consuming refueling or recharging procedures, can severely limit the performance of these assets. 
     Unmanned Aerial Vehicles (UAVs), for example, can carry critical intelligence, surveillance, and reconnaissance (ISR) payloads, such as cameras or video recorders, but their flight time is limited by the amount of onboard energy resources. Increasing these resources for long duration flights adds significant weight to the aircraft, thus reducing performance. Moreover, when operating a UAV from ships at sea, the UAV must be landed to be refueled or recharged. This is generally accomplished by catching the UAV in a net, typically resulting in damage to the asset. These landings also disrupt the ship&#39;s operations. Likewise UAV&#39;s associated with a moving convoy need to depart the convoy and return to a landing field to be refueled. Accordingly, it would be advantageous to allow a UAV to remain airborne without landing to refuel onboard energy sources. 
     While the above describes typical problems associated with UAVs, other types of assets, including many types of land and sea based vehicles, may not have immediate access to fuel or other energy supplies, and suffer similar reductions in performance as increased energy payloads are added to improve range. 
     Accordingly, a method of remotely supplying energy to these assets is desired. 
     SUMMARY 
     In one embodiment of the present invention, a wireless energy transmission and storage system is provided. The system includes a first microwave transmission source and a second lightwave transmission source. The output of the first and second sources are received by a hybrid array arranged on an asset and configured to convert the received microwaves and lightwaves into direct current. An energy storage device is operatively connected to the hybrid array and configured to store the power delivered therefrom. A controller is provided and configured to control the supply of power from the hybrid array to the energy storage device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a wireless energy transmission and storage arrangement according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a wireless energy transmission and storage arrangement used in a hybrid asset application. 
         FIG. 3  shows an exemplary embodiment of the present invention comprising a UAV being charged by a host ship. 
         FIGS. 4   a - c  show various embodiments of receiving array antennas. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     With reference to  FIG. 1 , an embodiment of the power transmission and storage system is shown as it may be applied to a mobile asset, such as a UAV. A host platform  10  is provided and preferably comprises at least one microwave transmitting, or transmitting and receiving, source  11 , for example a focused radar antenna, and at least one lightwave transmitting source  12 , such as a laser transmitter. In a preferred embodiment, the microwave transmitting source  11  operates on L, X, or S bands, such as those used in conventional radar systems, and the lightwave transmitting source  12  operates to provide electromagnetic radiation in the form of, for example, a visible light laser, infrared laser, or ultraviolet laser. The host platform  10  maybe located on the ground, aircraft, sea ship, or any suitable location depending on the application. It is also envisioned that the host platform  10  may comprise a mobile arrangement. 
     The microwave and lightwave sources  11 , 12  preferably operate in parallel to transmit respective energy signals to at least one array  13 . In a preferred embodiment, the at least one array  13  is located on a mobile asset, for example, a UAV used for surveillance and intelligence gather purposes. However, it is envisioned that a similar array  13  could be placed on any air, land, or sea vehicle, as well as on any other suitable portable devices. 
     As shown in  FIG. 4   a , the array  13  may comprise two or more distinct receiving elements, such as a microwave antenna  50  and an arrangement photovoltaic (PV) cells  51  for receiving each of the microwave and lightwave transmissions. Alternatively, the array may comprise a single hybrid array configured to receive both microwave and lightwave transmissions. In either embodiment, it is preferred that the array  13  also convert the received signals to a form of usable power, for example, direct current. 
     The array  13  may comprise a hybrid arrangement of photovoltaic cells for receiving and converting lightwaves into direct current, and a diode-based rectifying antenna (rectenna) for receiving and converting microwaves into direct current. In an alternate embodiment, the rectenna may be replaced with any suitable microwave receiving antenna and a separate rectifying circuit provided for the production of direct current. 
     This hybrid array  13  may be formed by any known method in the art. For example, as shown in  FIG. 4   b , it is envisioned that the array may be formed on a flexible substrate, as is typically used in roll-to-roll electronics. A metallic pattern of microwave antenna elements  52  could be applied to the substrate, defining voids which allow the passage of visible light therethrough. A PV cell array  53  may be disposed within these voids to absorb the lightwave transmission. This array arrangement  13  could be mounted to, for example, the underside of the UAV. In another embodiment shown in  FIG. 4   c , the microwave antenna elements  54  and PV cell array  55  comprise and interleaved arrangement. 
     Arranging the PV cells within the microwave receiving portion of the array  13  aids efficient energy transfer. Specifically, laser and other types of lightwaves are transmitted in a narrow, focused beam. Thus, this beam needs to be accurately aimed onto PV cells to achieve ideal energy transfer. In a preferred embodiment, in addition to providing a power signal, the microwave transmitter  11  may be configured to track the asset, and provide a positional reference for the accurate transmission of lightwaves. To facilitate this beam steering, an RF link  35  may be provided between the array  13  and the host platform  10 , conveying, for example, positional information of the UAV, more specifically, the position of the array  13 . 
     Power provided by the array  13  is supplied to an energy storage device, for example, a capacitor  20 , battery  21 , or a combination thereof. In a preferred embodiment, the converted power is stored in a capacitor  20  during a charging cycle, and slowly discharged to a battery  21  during and/or after the charging cycle has been completed. The capacitor  20  may comprise an ultracapacitor or a nano-tube enhanced capacitor for increased storage capacity. 
     Applying the power to a capacitor provides added benefits over charging a battery directly. Notably, a capacitor may be charged at a significantly higher rate of speed than conventional batteries. Accordingly, an asset would only be required to be in range of the host platform  10  for short periods of time during a charging cycle. For example,  FIG. 3  shows an exemplary UAV  200  in a flight path circling a host ship  201 . Arranged on the ship  201  are microwave and lightwave transmission sources  211 , 212 , which provide the above-described energy transmission signal to the UAV  200 . Once the capacitor  20  is charged, the UAV  200  or other asset would be free to leave the range of the host platform  201 , and the capacitor could be discharged into the battery at an optimal charging rate as the asset continues on its mission. 
     In an alternate embodiment, the battery  21  may be eliminated, and the capacitor  20  retained as the sole method of storing power received from the array  13 . This embodiment may prove especially advantageous as capacitor technology improves, and capacitor power densities rise. Similarly, a simplified arrangement may provide only a battery or plurality of batteries for storing the power received from the array  13 . Any of the above-described arrangements may be implemented depending on a number of considerations, such as the cost, weight, or functional requirements of a particular asset. 
     Energy stored in the capacitor  20  or battery  21  may be used to power an asset&#39;s drive system, such as an electric motor  25 . Moreover, this power may be used by any of the asset&#39;s subsystems including but not limited to: sensors  26 , surveillance devices such as cameras or video recorders, additional antennas  27  for transmitting and/or receiving data or control signals, as well as positioning or control systems. 
     The charging of the energy storage device is controlled by a control system  30 . Specifically, the control system  30  controls the power output of the array  13 . For example, impedance, voltage, and/or current levels may be monitored and/or controlled in order to ensure proper charging of the energy storage device. Moreover, any number of suitable devices, including voltage converters, amplifiers, and filters may be included in the array  13 , control system  30 , or additional circuits (not shown) in order to properly condition the output of the array  13  to be received by the energy storage device. In the embodiment in which a capacitor  20  is initially charged, and the power stored therein later applied to a battery  21 , the control system  30  may act to control the rate of discharge of the capacitor  20 , and therefore the rate of charge of the battery  21 . 
     The control system  30  may be operatively connected to at least one of the capacitor  20 , battery  21 , and the above-mentioned sub-systems of the asset. In this way, the control system  30  may monitor the voltage levels of the capacitor  20  and/or battery  21  in order to determine when a full charge has been reached. Once a completed charging cycle has been detected, the control system  30  may discontinue power transmission from the array  13 . Likewise, the control system  30  may provide a signal to the operator of the asset, through, for example, an RF antenna provided on the asset, giving notice of the competed charge cycle. Similarly, the control system  30  can provide continuous, real-time system power level and consumption data to an operator. 
     Because the control system  30  may be tied to both the power consuming devices of the asset, as well as the energy storage device(s), the control system  30  may monitor both power usage and current power levels in order to predict expected battery life. This information made be forwarded to the asset&#39;s operator, and/or used to alter the asset&#39;s power usage in real-time, for example, reducing or eliminating power applied to non-critical systems in order to extend the operating range of the asset. 
     The control system  30  also may be operatively coupled to a transponder and/or GPS system of the asset. In this way, the control system  30  may convey positional information, for example, through the RF link  35  between the array  13  and the host platform  10  in order to facilitate accurate targeting of the array  13  by the transmitters  11 , 12 . This positional information may likewise be used to determine the asset&#39;s proximity to a given host transmitter  10 . In this way, the control system  30  may provide an operator with a power level warning that varies according to the asset&#39;s distance from the host platform  10 . 
     In an alternate embodiment shown in  FIG. 2 , the energy transfer system may be applied to a hybrid-powered asset. In this embodiment, charging of the capacitor  120  and/or battery  121  is achieved in the same fashion described above with respect to the previous embodiment, with the control system  130  operating to control the charging of the energy storage device(s). The energy supplied by the host platform  110  may be used to power a portion of a hybrid drive system. For example, it is envisioned that the asset may possess alternate energy supplies, such as an onboard fuel tank  129  for the storage of liquid or gas fuels. In the case of a UAV, these fuels may be used to power an engine  128  for all or a portion of a flight. For example, an internal combustion engine  128  may be used to propel an asset to a desired altitude, wherein the electric motor  125  could take over. This would eliminate the heavy power consumption associated with the climb. The motor  125  could also operate as a generator, driven by the engine  128 , for providing additional power to the energy storage device(s). The motor  125  could also be implemented in situations were quiet operation is required, for example during covert reconnaissance missions in hostile areas. 
     In yet another embodiment, electrical power may be used to power the asset&#39;s electrical systems, such as control and communication systems, rather than its propulsion system. For example, the asset&#39;s engine  128  may run on liquid fuel, and the control systems, data recording and storage devices, and communications systems may be provided power from the battery  121  and/or capacitor  120 . In this way, the range of the asset may be increased, as the onboard engine  128  would not be required to run a generator for supplying power to these subsystems. 
     In any of the above-described embodiments, the control system  130  may operate in a similar manner to that described with respect to  FIG. 1 . Specifically, the control system  130  may monitor and/or control the power output of the array  113  to the capacitor  120  or battery  121 , the discharge of the capacitor  120  to the battery  121 , the battery and/or capacitor levels, the power usage of the system, in addition to provide positioning information to the host platform  110  for accurate aiming of the transmitters  111 , 112 , as well as regulate between operation of the asset under power of the engine  128  or the motor  125  depending on desired performance characteristic, and/or power or fuel levels. 
     The following describes basic energy transmission principles as well as estimations for the performance of the above-described systems. 
     The effective power transmitted from the host transmitting array is equal to the transmit power (P t ) multiplied by a transmit gain (G t ):
 
Effective Power=P t G t  
 
     Power density (P d ) is equal to effective power divided by a function of the transmission distance, specifically:
 
 P   d =Effective Power/4π R   2  
 
     Power received by the receiving array is a function of the power density, multiplied by the effectiveness of the receiving array (A e ).
 
Received Power= P   d   A   e  with  A   e   =G   r λ 2 /4π
 
     G r  denotes receiver gain which takes into account transmission losses, array inefficiency, and ohmic losses. According, power received (P r ) is equal to:
 
 P   r   =P   t   G   t   G   r λ 2 /(4π R)   2   =P   t   G   t   A   e /(4π R   2 )
 
     The table below indicates estimated transmitted and received power levels based on a 50 meter transmission range, a receiving array having an area of approximately 1.77 square meters, and typical or assumed values of L, S, and X band microwave transmissions: 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 L Band 
                 S Band 
                 X Band 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Avg Rad. Pt (kW) 
                 3.75 
                 2.0 
                 0.5 
               
               
                   
                 Gt (dB) 
                 40 
                 34 
                 31.7 
               
               
                   
                 Ae (m2) 
                 1.77 
                 1.77 
                 1.77 
               
               
                   
                 R (m) 
                 50 
                 50 
                 50 
               
               
                   
                 Avg Pr (kW) 
                 2.1 
                 0.53 
                 0.31 
               
               
                   
                 Conv. Eff. 
                 0.7 
                 0.7 
                 0.7 
               
               
                   
                 Power Out (kW) 
                 1.5 
                 0.37 
                 0.22 
               
               
                   
                   
               
             
          
         
       
     
     While the foregoing describes exemplary embodiments and implementations, it will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention.