Abstract:
A resonant, contactless, RF power coupling suitable for high power-density applications and for use in an ocean environment is disclosed. In the illustrative embodiment, the power coupling includes a transmit coupling and a receive coupling, each of which include a resonant element. A high-powered RF generator is coupled to the transmit coupling and a rectifier circuit is coupled to the output coupling. Each of the resonant elements is disposed in its own electrically-conductive canister and advantageously potted in an appropriate insulating dielectric. Each canister has an open end to facilitate inductive coupling between the two resonant elements. In order to exclude seawater from the interface between the canisters, a seal of compliant material is disposed therebetween.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case claims priority to U.S. Provisional Patent Application Ser. No. 61/079,666, filed Jul. 10, 2008, which is incorporated by reference. 
     In addition, the underlying concepts, but not necessarily the language, of the following cases are incorporated by reference:
         (1) U.S. patent application Ser. No. 12/396,349, filed Mar. 2, 2009; and   (2) U.S. patent application Ser. No. 12/411,824, filed Mar. 26, 2009.       

     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power transmission in general, and, more particularly, to contactless power transmission. 
     BACKGROUND OF THE INVENTION 
     Electrically powered vehicles are attractive in many application areas including civilian transport, military transport, long-life sensor platforms, undersea vehicles, airborne vehicles, and watercraft. In many cases, however, the operating time of these vehicles is short due to a drain on their storage systems by power intensive activities. As a result, their useful operating time is dictated by the ability to resupply them with electrical power. Electrical power can be supplied from either onboard power generation equipment or power transfer and storage of externally generated energy. On-board power generation is faced with many challenges, however. As a result, power transfer and storage systems are typically employed for most electrically powered vehicle systems. 
     In most cases, the capacity of the storage systems used to power these vehicles is limited; therefore, it is typically necessary to recharge these systems frequently. The time required to recharge an storage system can rival the operational time of the vehicle between charges. As a result, the use of electrically powered vehicles remains fairly limited. To further complicate matters, in many cases, the vehicle must be recharged without removing it from its environment, such as extended underwater sensor systems and Autonomous Undersea Vehicles (AUVs). 
     The transfer of externally generated electrical energy requires an ability to couple the external power source to an storage system on board the vehicle through a power coupling. Although underwater power couplings have been in use for a variety of underwater applications (e.g., oil industry, ships, submarines, towed arrays, etc.) for close to one hundred years, there are drawbacks to all known approaches. 
     Traditional contact-type power couplings (e.g., plug-and-socket connectors) suffer from a combination of complex connector geometries. Further, they are highly susceptible to corrosion when exposed to seawater. Although this type of coupling has been relied upon for many years, there is need for improvement in both the reliability of the power connection and its ease of use. 
     A variety of non-contact-type power couplings are known in the art, such as capacitive couplings, inductive couplings, radio frequency (RF) transformers, and resonant RF power couplings. Capacitive couplings generally suffer from relatively high impedance, which limits their power transfer efficiency. In addition, capacitive couplings require frequencies in excess of 100 megahertz to over a gigahertz to achieve kilowatt levels of power transfer. 
     Inductive (transformer) power couplings are more amenable to high power levels, but are based on very heavy core materials and require large amount of copper. As a result, inductive power couplings tend to be unwieldy and expensive to implement. 
     Radio Frequency (RF) transformers are much lighter than inductive couplings, but their transfer efficiency in a seawater environment is severely degraded by the conductivity of seawater itself. 
     Resonant RF power transfer has proven attractive for the transfer of electrical power over long distances. For example, resonant RF power transfer has been demonstrated to produce as high as 30 percent efficiency at multi-meter ranges in air. Unfortunately, the efficiency of resonant RF power transfer in seawater is also severely degraded by the conductivity of seawater. Further, the efficiency of prior-art resonant RF power coupling systems is reduced due to their reliance on open resonators, which radiate RF energy in many directions. 
     SUMMARY OF THE INVENTION 
     The invention provides a way to transfer externally generated electrical power to an storage system that avoids some of the costs and disadvantages of the prior art. 
     Embodiments of the present invention are suitable for rapidly charging storage systems, such as those used to store power for electrically powered vehicles including terrestrial vehicles, autonomous robotic systems, airborne or waterborne craft, such as AUVs, underwater vehicles, unmanned underwater vehicles (UUVs), unmanned aerial vehicles (UAVs), and the like. 
     The illustrative embodiment of the present invention is a resonant RF power coupling suitable for high power transfer applications. In the illustrative embodiment, the power coupling includes a transmit coupling and a receive coupling, each of which include a resonant element. A high-power RF generator is coupled to the transmit coupling and an storage system is coupled to the output coupling. In some embodiments, efficient transmission of power to an storage system within a vehicle submerged in seawater is enabled. Embodiments of the present invention overcome some of the inefficiencies typically associated with resonant power transfer in seawater. 
     RF power transfer in an ocean environment is complicated by the conductivity of seawater. In order to operate the resonant RF power coupling in an ocean environment, therefore, the RF energy must be shielded and isolated from the seawater. In the illustrative embodiment, this is accomplished by disposing each resonant circuit in a separate electrically conductive canister and potting the resonator in an appropriate insulating dielectric. Each canister has an open end to facilitate inductive coupling of the two resonant elements. In order to exclude seawater from the interface between the canisters, a seal of compliant material is disposed therebetween. When the canisters are aligned with one another and pressed together, the seal expresses sea water out of the region between the canisters. 
     In some embodiments, RF power is transferred between a pair of helical resonators with their open ends abutted. The helical resonators resonate at the operating frequency of the RF power transfer system, as well as at harmonics of this frequency. As a result, multiple frequencies of an RF signal are transmitted from the transmit coupling to the receive coupling, thereby enabling high-efficiency power transfer. 
     The inventive power coupling is relatively less sensitive to misalignment than prior art systems. Further, the present invention enables hermetic sealing of both the power source and a submerged instrumentation package. Embodiments of the invention are lighter and more robust than non-resonant, non-direct contact (no metal-to-metal contact) power coupling systems. Further, embodiments of the present invention have much higher power density than prior-art capacitive power coupling systems. A key advantage of embodiments of the present invention is the ability to avoid corrosion (galvanic and otherwise) and/or the complex and problematic seals that are required for use with direct-contact power coupling systems. 
     Although particularly well-suited for use in an ocean environment, resonant RF power couplings described herein can be used to advantage in any environment where contactless power transfer is desired. In particular, and among other benefits, embodiments of the invention: (1) enable rapid power transfer; and (2) enable improved isolation of power equipment from power spikes and transients, such as lightning and EMP bursts. 
     An embodiment of the present invention comprises: a transmit coupling, wherein the transmit coupling includes a first resonant circuit having a first resonant frequency; and a receive coupling, wherein the receive coupling includes a second resonant circuit having a second resonant frequency; wherein the transmit coupling receives an RF signal having a first frequency and a second frequency that is a harmonic frequency of the first frequency, and wherein the transmit coupling passes each of the first frequency and second frequency to the receive coupling when the transmit coupling and receive coupling are in a coupling relationship. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic drawing of an RF power coupling system in accordance with an illustrative embodiment of the present invention. 
         FIG. 2  depicts operations of a method for providing power to an AUV in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts transmit coupling  112  mated with receive coupling  114 . 
         FIG. 4  depicts a plot of RF signal  108 . 
         FIG. 5  depicts a schematic drawing of an RF generator in accordance with the illustrative embodiment of the present invention. 
         FIG. 6  depicts sub-operations of a sub-method suitable for providing an RF signal. 
         FIG. 7  depicts a power transfer system in accordance with a first alternative embodiment of the present invention. 
         FIG. 8  depicts a power transfer cable in accordance with a second alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a schematic drawing of an RF power coupling system in accordance with an illustrative embodiment of the present invention. System  100  enables rapid charging of an storage system included in an AUV while the AUV is submerged in a seawater environment. System  100  comprises RF generator  104 , cable  110 , transmit coupling  112 , and receive coupling  114 . Although the illustrative embodiment comprises an RF power coupling system that is configured for charging a submerged AUV, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention that are suitable for charging battery-powered land-based vehicles, surface-based nautical vessels, and aircraft. 
     RF generator  104  is mounted on ship  102 , which is located at the surface of ocean  124 . RF generator  104  is an RF generation system capable of generating hundreds of kilowatts (kW) of RF power at a frequency of 30 MHz. RF generator  104  generates RF signal  108 , which is conveyed to transmit coupling  112  on cable  110 . In some embodiments, RF generation system is located on a floating or fixed-position deep-sea platform, such as a tension-leg platform, floating platform, or moored platform. In some embodiments, the RF generation system is located in a terrestrial system, such as a port facility. 
     AUV  106  is an unmanned submarine that comprises receive coupling  114  and storage system  120 . Storage system  120  comprises a conventional battery-based storage system and signal conditioning apparatus (i.e., bridge rectifiers, etc.) for converting RF signal  108  into electrical energy suitable for charging the batteries. In some embodiments, storage system  120  comprises alternative energy storage systems other than, or in addition to, batteries. Systems suitable for use in storage system  120  include, without limitation, fuel cells, ultracapacitors, flow batteries, and the like. In some embodiments, energy storage system  120  stores energy in a form other than electrical energy, such as mechanical, thermal, magnetic, chemical, etc. It will be clear to one skilled in the art, after reading this specification, how to specify, make, and use AUV  106  and storage system  120 . 
       FIG. 2  depicts operations of a method for providing power to an AUV in accordance with the illustrative embodiment of the present invention. Method  200  begins with operation  201 , wherein transmit coupling  112  and receive coupling  114  are aligned to enable efficient power transfer through system  100 . 
     When transmit coupling  112  and receive coupling  114  are suitably aligned, the input impedance of the power coupling system is substantially matched to the impedance of RF generator  104 . As a result, power can be transferred through the mated couplings with little or no back-reflection or scattered energy. 
     When transmit coupling  112  and receive coupling  114  are misaligned, the input impedance of the power coupling system is substantially different than the impedance of RF generator  104 . In some embodiments, system  100  comprises a back impedance detection system to detect proper alignment of transmit coupling  112  and receive coupling  114 . In some embodiments, system  100  detects RF power that is reflected from transmit coupling  112  and uses this detected power to indicate proper coupling alignment. Since impedance detection can be done at low energies, such detection schemes do not significantly degrade the overall efficiency of the power transfer system. 
       FIG. 3  depicts transmit coupling  112  mated with receive coupling  114 .  FIG. 3  is described with continuing reference to  FIGS. 1 and 2 . 
     Transmit coupling comprises canister  302 - 1 , resonator  304 , and flange  306 . Canister  302 - 1  is an electrically conductive canister having a diameter of approximately five inches and one open end. Canister  302 - 1  is electrically connected to ground potential and acts as an RF shield for resonator  304 . Canister  302 - 1  also isolates resonator  304  from the effects of seawater. 
     Receive coupling comprises canister  302 - 2 , resonator  310 , and flange  312 . Canister  302 - 2  is analogous to canister  302 - 1 . In similar fashion to canister  302 - 1 , canister  302 - 2  acts as an RF shield for resonator  310  and isolates it from the effects of seawater. 
     Resonator  304  is a helical resonator having a resonant frequency of 30 MHz. Resonator  304  has a length of approximately seven inches and has 15 turns of fine, multi-stranded, individually insulated wire. One end of resonator  304  is electrically connected to canister  302 - 1  at point d 1 . The other end of resonator  304  is open to the open end of canister  302 - 1 . RF generator  104  is electrically connected to resonator  304  at point d 2 . 
     Transmit coupling  112  is characterized by an input impedance of approximately 700 Ohms and a quality factor (Q) of approximately 1500. In some embodiments, impedance matching is provided to facilitate power transfer between transmit coupling  112  and the external circuitry to which it is electrically connected (i.e., RF generator  104 ). In some embodiments, this impedance matching is provided by forming point d 2  within approximately 1.25 turns from point d 1 . 
     Resonator  310  is also a helical resonator having a resonant frequency of 30 MHz. In some embodiments, the resonant frequencies of resonators  304  and  310  are matched to facilitate power transfer between transmit coupling  112  and receive coupling  114 . Resonator  310  has a length of approximately seven inches 15 turns of fine, multi-stranded, individually insulated wire. One end of resonator  310  is electrically connected to canister  302 - 2  at point d 3 . The other end of resonator  310  is open to the open end of canister  302 - 2 . Storage system  120  is electrically connected to resonator  310  at point d 4 . 
     Receive coupling  114  is characterized by an impedance of approximately 700 Ohms and a Q of approximately 1500. In some embodiments, point d 4  is formed within approximately 1.25 turns from point d 3  to facilitate impedance matching between receive coupling  114  and storage system  120 . In some embodiments, impedance matching devices, as are well known in the art, are used to improve power transfer between transmit coupling  112  and receive coupling  114  and reduce standing wave ratios. 
     In some embodiments, the resonant frequency of at least one of resonators  304  and  310  is tuned. This can be accomplished using tuning devices, as are well known in the art. Typical tuning devices for resonant circuits include varactors, variable inductors, or variable capacitors. Automatic tuning circuits are also known in the art. They are particularly useful since environmental variations, coupler alignment, and the like can affect the precise resonant frequencies of the couplers. Loading of the couplers will reduce the effective Q of the circuits and thus broaden the resonance. This simplifies the process of matching the frequencies of the two halves of the RF coupler. 
     Although the illustrative embodiment comprises resonators that are helical resonators, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention that comprise a resonator that is other than a helical resonator. Resonators suitable for use in the present invention include, without limitation, helical resonators, inductor-capacitor (LC) circuits, tunable resonators, and the like. 
     Resonators  304  and  310  are potted within canisters  302 - 1  and  302 - 2 , respectively, by dielectric  308 . Dielectric  308  is an insulating dielectric compound suitable for use as a potting compound for the resonators. It will be clear to one skilled in the art how to specify, make, and use dielectric  308 . 
     Flanges  306  and  312  collectively form a mating system that physically connect transmit coupling  112  and receive coupling  114 . Flanges  306  and  312  substantially align resonators  304  and  310 , as well as bring the open ends of the resonators into close proximity with one another, which facilitates inductive coupling between the resonators. 
     At optional operation  202 , transmit coupling  112  and receive coupling  114  are sealed by mating flanges  306  and  312 . While mating flanges  306  and  312 , transmit coupling  112  and receive coupling  114  are drawn toward one another. This reduces the gap between the resonators and enables each canister to form a substantially watertight seal with seal  116 . 
     One skilled in the art will recognize that seawater located between the resonators will reduce the efficiency with which RF signal  108  couples between resonators  304  and  310 . To mitigate the effects of seawater between the resonators, therefore, seal  116  comprises a shape that enhances the expression of seawater from the region between resonators  304  and  310 . As transmit coupling  112  and receive coupling  114  are drawn toward one another, seal  116  is compressed and expresses seawater out of this region. 
     Seal  116  is formed of a compliant material and has a shape that is similar to that of a convex lens, wherein it is relatively wider near its midpoint and narrower at its ends. As a result, as transmit coupling  112  and receive coupling  114  are drawn toward one another, the midpoint of seal  116  is pressed against the exposed faces of dielectric  308 . As canisters  302 - 1  and  302 - 2  are drawn further toward one another, the contact area between seal  116  and dielectrics  308  expands forcing seawater away from the center of the exposed surface of the dielectric. In addition to improving the efficiency of the transfer of power from transmit coupling  112  to receive coupling  114 , the removal of seawater from the coupling system reduces or eliminates corrosion (e.g., galvanic, etc.) that degrades the lifetime and reliability of prior-art contact-type power coupling systems. 
     One skilled in the art will recognize that once the couplings are mated, any thin layer of seawater that remains between the exposed faces of transmit coupling  112  and receive coupling  114  is vaporized during the RF power transfer process. Further, it will be clear that the design of seal  116  is application dependent and that in some applications seal  116  is unnecessary. 
     One skilled in the art will also recognize that the specific designs of transmit coupling  112  and receive coupling  114  are highly dependent on a number of factors, such as application, cable lengths, environment, and operating frequency, among others. It will be clear to one skilled in the art, therefore, after reading this specification, how to make and use alternative embodiments of the present invention that:
         i. operate at frequencies other than 30 MHz; or   ii. comprise a different RF shield or a shield of different dimensions; or   iii. comprise a resonator other than a helical resonator; or   iv. comprise a helical resonator other than resonators  304  and  310  (e.g., different length, number of windings, different wire diameter, etc.); or
 
are characterized by any combination of i, ii, iii and iv.
       

     It is an aspect of the present invention that transmit coupling  112  and receive coupling  114  enable the transmission of the fundamental frequency of RF signal  108  as well as harmonic frequencies of the fundamental frequency. The present invention derives this advantage over the prior art through the use of resonators that are resonant at each of these frequencies. 
     It should be noted that when transmit coupling  112  and receive coupling  114  are mated, canisters  302 - 1  and  302 - 2  form a substantially continuous electrical shield around resonators  304  and  310 . As a result, the present invention enables a power coupling system that substantially isolates the power generation equipment and storage system from power spikes such as those caused by lightning strikes or electromagnetic pulse attacks. 
       FIG. 4  depicts a plot of RF signal  108 . Plot  400  depicts fundamental frequency  402 , first even harmonic frequency  404  and first odd harmonic frequency  406 . As evinced by plot  400 , the amplitude of RF signal  108  is substantially at a maximum for both fundamental frequency  402  and first odd harmonic frequency  406  at the free end, L 1 , of resonator  304 . As a result, resonator  304  resonates for both of frequencies  402  and  406  and substantially all the RF power associated with them is passed from resonator  304  to resonator  310 . The efficiency of RF power transfer for embodiments of the present invention, therefore, can be much higher than RF power transfer systems of the prior art. This improved efficiency affords several advantages for the present invention over prior-art systems, including:
         i. reduced sensitivity to misalignment; or   ii. operation at higher power levels; or   iii. reduced charging time; or   iv. reduced heating and component degradation due to power transfer inefficiency; or   v. reduced galvanic corrosion; or   vi. any combination of i, ii, iii, iv, and v.       

     At operation  203 , RF generator  104  provides RF signal  108  to transmit coupling  112  on cable  110 . 
       FIG. 5  depicts a schematic drawing of an RF generator in accordance with the illustrative embodiment of the present invention. RF generator  104  comprises power supply  502 , switch bank  508 , clock  514 , controller  516 , and diode  518 . 
       FIG. 6  depicts sub-operations of a sub-method suitable for providing an RF signal. Sub-method  600  is suitable for use in operation  203  of method  200 . Sub-method  600  begins with sub-operation  601 , wherein power supply  502  provides a constant voltage signal on cable  504 . 
     Cable  504  is electrically connected to input node  506  of switch bank  508 . Switch bank  508  comprises switches  510 - 1  through  510 - 32  (collectively referred to as switches  510 ), each of which is operates as a 2 MHz chopper. Switches  510  are interconnected as pairs of choppers that are electrically connected in series. Each switch pair is electrically connected with the remaining switch pairs in parallel between input node  506  and output node  512 . 
     At operation  602 , clock  514  and controller  516  collectively provide control signals to switch bank  508  to sequence the opening and closing of switches  510 . As a result of this sequencing of switches  510 , a square wave of frequency 30 MHz (i.e., RF signal  108 ) appears at output node  512 . Transmit coupling  112  is electrically connected to output node  512  (and diode  518 ); therefore, transmit coupling  112  receives RF signal  108 . 
     Although the illustrative embodiment comprises an RF generator that is a chopper-based, switched-mode power supply, it will be clear to one skilled in the art, after reading this specification, how to specify, make, and use alternative embodiments of the present invention that comprise a different type of RF generator, such as a class “D” power supply, a digital oscillator, an analog oscillator and linear amplifier, an analog oscillator without a linear amplifier, a non-linear analog oscillator, and the like. 
     At operation  204 , RF signal  108  is coupled between transmit coupling  112  and receive coupling  114 . 
     At operation  205 , electrical energy based on RF signal  108  is conveyed to storage system  120  on power cable  118 . 
       FIG. 7  depicts a power transfer system in accordance with a first alternative embodiment of the present invention. System  700  comprises AUV  702  and power node  706 . AUV  702  is depicted as coupled with power node  706 , which is located on sea floor  720 . 
     In many cases, remote devices, such as remote sensors, AUVs, and the like, can be difficult to access for direct power transfer from an external source. An energy transport vehicle, such as AUV  702 , enables such devices to be recharged more easily. 
     AUV  702  comprises receive coupling  114 , storage system  120 , RF generator  704 , and transmit coupling  112 . AUV  702  is an underwater vehicle that is capable of transporting electrical energy between an external source (e.g., ship  102 ) and a remote power node. 
     RF generator  704  is analogous to RF generator  104 . 
     Power node  706  is a remote hub for providing power to a plurality of remote devices, such as remote sensors, AUVs, and the like. Power node  706  comprises coupling  708 , switch  710 , storage system  712 , impedance detector  714 , and RF generator  716 . Power node  706  is capable of receiving power at coupling  708  from an energy transport vehicle, as shown. Alternatively, power node  706  can be reconfigured so that it can provide power at coupling  708  (e.g., to an energy transport vehicle, AUV, etc.). 
     Coupling  708  is analogous to receive coupling  114 ; however, one skilled in the art will recognize, after reading this specification, that a receive coupling can act as a transmit coupling to transmit an RF signal to another coupling when the coupling is properly configured. 
     Switch  710  is a three-way switch that enables coupling  708  to be selectively interconnected with storage system  712 , impedance detector  714 , or RF generator  716 . 
     Storage system  712  is analogous to storage system  120 , and comprises signal conditioning circuitry, a controller for managing interconnectivity between storage system  712 , other power nodes, and remote sensors  718 - 1  throucih  718 -n. 
     Impedance detector  714  is a conventional low-power impedance detector. When interconnected to coupling  708  through switch  710 , impedance detector  714  detects when a vehicle has properly mated to coupling  708 . 
     RF generator  716  is analogous to RF generator  104 . 
     Remote sensors  718 - 1  through  718 - n  are sensors for detecting seismic activity, sonar signals, temperature, pressure, and the like. 
     In order to transfer electrical energy from AUV  702  to storage system  712 , storage system  120  is recharged as described above and with respect to  FIGS. 1-6 . Once storage system  120  has stored a desired amount of electrical energy, AUV  702  travels to the location of power node  706 . In anticipation of receiving electrical power, switch  710  interconnects impedance detector  714  and coupling  708 . Once proper alignment of transmit coupling  112  and coupling  708  is detected, switch  710  disengages impedance detector  714  and connects storage system  712  and coupling  708 . In some embodiments, it is not necessary to disconnect impedance detector  714  prior to transferring power through coupling  708 . Once the couplings are aligned, RF generator  704  provides an RF signal to transmit coupling  112 , in analogous fashion to operation  203 . This RF signal is coupled into storage system  712  through coupling  708 . Storage system  712  conditions the RF signal, thereby providing electrical energy for charging its storage batteries. This stored electrical energy can then used to power remote sensors  718 - 1  through  718 -n. 
     Alternatively, power node  706  can be configured to provide electrical energy to an AUV or other vehicle. In order to provide electrical energy at coupling  708 , switch  710  connects coupling  708  and RF generator  716 . Once coupling  708  is suitably connected with the receive coupling of an AUV, an RF signal is transmitted between coupling  708  and the receive coupling of the AUV. Although not depicted in  FIG. 7 , RF generator  716  derives its energy from storage system  712 . In some embodiments, power node  706  is a self-contained power generation system that does not require periodic recharging by an AUV or other vehicle. Examples of stand-alone power generation systems suitable for use in power node  706  include, without limitation, energy scavenging systems and geo-thermal energy conversion systems, such as those described in U.S. patent application Ser. No. 12/396,349, filed Mar. 2, 2009, and U.S. patent application Ser. No. 12/411,824, filed Mar. 26, 2009, each of which is incorporated herein by reference. 
     It should be noted that the number of remote sensors to which power node  706  is interconnected is limited only by the capacity of storage system  712  and the rate at which the storage system can be recharged. Further, it will be clear to one skilled in the art, after reading this specification, that a plurality of power nodes  706  can be interconnected to provide multiple storage systems from which any of the plurality of remote sensors can draw energy. Each of these power nodes also provides an access point at which electrical energy can be received or provided. 
     Although the alternative embodiment depicts an AUV comprising a separate transmit and receive coupling, it will be clear to one skilled in the art, after reading this specification, that a switched system, such as that included in power node  706 , is also suitable for use in an AUV or other vehicle. 
       FIG. 8  depicts a power transfer cable in accordance with a second alternative embodiment of the present invention. Cable  800  comprises receiver  802  and transmitter  804 . Cable  800  is analogous to an electrical extension cord or optical fiber jumper cable. Cable  800  enables the transfer of electrical energy to a remote location. Cable  800  receives RF energy at a receiving end, converts it to a DC signal, and transmits it along an electrically conductive cable to a transmitting end. In some embodiments, cable  800  enables more efficient transfer of electrical energy through a medium that would significantly attenuate an RF signal, such as salt water. 
     Receiver  802  comprises receive coupling  114 - 1  and rectifier  806 . Receive coupling  114 - 1  receives RF signal  108  from a transmit coupling  112 - 1  and passes it to rectifier  806 . Rectifier  806  converts the RF signal into DC signal  808  on DC cable  810 . 
     Transmitter  804  comprises RF generator  716  and transmit coupling  112 - 2 . Transmitter  804  receives DC signal  808  at RF generator  716 , which converts the DC signal into RF signal  812 . In some embodiments, RF signal  812  is substantially identical to RF signal  108 . Transmit coupling  112 - 2  transmits RF signal  812  to a receive coupling  114 - 2 , when these couplings are mated. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.