Patent Publication Number: US-2021166855-A1

Title: Transmission coil and power transmission apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/498,964, filed Sep. 27, 2019, which is a National Stage Entry of International Pat. Appl. No. PCT/JP2018/009107, filed Mar. 8, 2018, which claims the benefit of Japanese Pat. Appl. No. 2017-069070, filed Mar. 30, 2017. The disclosure of each of the above-identified documents, including the specification, drawings, and claims, is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a transmission coil and an electric power transmission apparatus configured to transmit electric power in water. 
     BACKGROUND ART 
     In related art, it is known that an underwater base station, serving as an electric power transmission apparatus, transmits electric power to an underwater vehicle, serving as an electric power reception apparatus, in a non-contact manner using a magnetic resonance method (see, for example, Patent Literature 1). The electric power transmission apparatus includes an electric power transmission resonance coil, a balloon, and a balloon control mechanism. The electric power transmission resonance coil transmits electric power to an electric power reception resonance coil of the electric power reception apparatus in the non-contact manner by the magnetic field resonance method. The balloon contains the electric power transmission resonance coil therein. The balloon control mechanism expands the balloon during electric power transmission, thereby discharging water between the electric power transmission resonance coil and the electric power reception resonance coil. 
     An antenna apparatus is known, which transmits electric power and data to an IC-mounted medium by using an electromagnetic induction method using a 13.56 MHz frequency band (see, for example, Patent Literature 2). It is disclosed that the antenna apparatus includes at least one electric-power-supplied loop antenna, which is supplied with a signal current, and at least one non-electric-power-supplied loop antenna, which is not supplied with the signal current, a signal current is also generated in the non-electric-power-supplied loop antenna through using a magnetic field generated by the electric-power-supplied loop antenna, thereby enlarging a communication range of the electric-power-supplied loop antenna. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-2015-015901 
     Patent Literature 2: JP-A-2005-102101 
     SUMMARY OF INVENTION 
     Technical Problem 
     A Q factor (quality factor) of a coil used for electric power transmission is one index that indicates transmission efficiency of electric power transmission of the magnetic field resonance method. A plurality of transmission coils, including at least an electric power transmission resonance coil and an electric power reception resonance coil, are used in the electric power transmission of the magnetic field resonance method. 
     In a case where the transmission coil is immersed in water (for example, fresh water or sea water), the water may enter between coil wires (between windings). For example, fresh water or sea water has a high relative permittivity of 70 and has properties of serving as a dielectric. Therefore, when the water enters between the coil wires, dielectric loss is likely to occur. The fresh water also has a relatively large electrical conductivity, for example, 0.02 (S/m) in a case of tap water. Further, in a case of sea water, the electrical conductivity is 3.5 (S/m), which is extremely large. Therefore, electric loss may occur between windings of a coil CL due to an eddy current caused by water, and the electric loss due to the eddy current increases particularly when the coil CL is disposed in sea water. Therefore, the Q factor of the transmission coil can be reduced, and the transmission efficiency of the electric power can be reduced during non-contact electric power transmission (wireless power supply). 
     The present disclosure is made in view of the above circumstances, and provides a transmission coil and an electric power transmission apparatus which can inhibit the reduction in the transmission efficiency during underwater non-contact electric power transmission. 
     Solution to Problem 
     A transmission coil of the present disclosure is configured to transmit electric power in water, and includes: an annular electric wire through which an alternating current flows; and a first cover which includes non-conductive resin or non-magnetic resin and seals a periphery of the electric wire. The electric wire transmits the electric power via a magnetic field generated by flowing of the alternating current. 
     Advantageous Effects of Invention 
     According to the present disclosure, the reduction in the transmission efficiency during the underwater non-contact electric power transmission can be inhibited. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing an example of an environment in which an electric power transmission system according to a first embodiment is placed. 
         FIG. 2  is a block diagram showing a configuration example of the electric power transmission system. 
         FIG. 3A  is a cross-sectional view showing a structure of a coil according to the first embodiment. 
         FIG. 3B  is a cross-sectional view showing a structure of a coil of a comparative example. 
         FIG. 4A  is a table showing measurement conditions of inductance (L) and Q factor. 
         FIG. 4B  is a table showing measurement results of the inductance (L) and the Q factor of a coil without a cover member at a frequency of 40 kHz. 
         FIG. 4C  is a table showing measurement results of the inductance (L) and the Q factor of a coil that includes the cover member at the frequency of 40 kHz. 
         FIG. 4D  is a table showing measurement results of the inductance (L) and the Q factor of the coil without the cover member at a frequency of 80 kHz. 
         FIG. 4E  is a table showing measurement results of the inductance (L) and the Q factor of the coil that includes the cover member at the frequency of 80 kHz. 
         FIG. 5A  shows a configuration of a transmission efficiency measuring circuit for measuring electric power transmission efficiency. 
         FIG. 5B  is a plan view showing shapes of an electric power transmission coil and an electric power reception coil. 
         FIG. 5C  is a cross-sectional view showing the shapes of the electric power transmission coil and the electric power reception coil as viewed from a direction of arrow E-E in  FIG. 5B . 
         FIG. 6A  is a table showing transmission efficiency measurement conditions. 
         FIG. 6B  is a table showing measurement results of the electric power transmission efficiency at the frequency of 40 kHz in the case where the cover member is provided and in the case where the cover member is not provided. 
         FIG. 6C  is a table showing measurement results of the electric power transmission efficiency at the frequency of 80 kHz in the case where the cover member is provided and in the case where the cover member is not provided. 
         FIG. 7  is a cross-sectional view showing a structure of a coil according to Modification 1 of the first embodiment. 
         FIG. 8  is a cross-sectional view showing a structure of a coil according to Modification 2 of the first embodiment. 
         FIG. 9  is a cross-sectional view showing a structure of a coil according to a second embodiment. 
         FIG. 10  is a cross-sectional view showing a structure of a coil according to Modification  1  of the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the drawings. Unnecessarily detailed descriptions may be omitted. For example, a detailed description of a well-known matter or a repeated description of substantially the same configuration may be omitted. This means to avoid unnecessary redundancy in the following description and to facilitate understanding of those skilled in the art. The accompanying drawings and the following description are provided for those skilled in the art to facilitate thorough understanding of the present disclosure, and are not intended to limit the claimed subject matters. 
     First Embodiment 
     [Configurations and the Like] 
       FIG. 1  is a schematic diagram showing an example of an environment in which an electric power transmission system  10  according to a first embodiment is placed.  FIG. 2  is a block diagram showing a configuration example of the electric power transmission system. The electric power transmission system  10  includes an electric power transmission apparatus  100 , an electric power reception apparatus  200 , and a coil CL. The electric power transmission apparatus  100  wirelessly (no contact point) transmits electric power to the electric power reception apparatus  200  via a plurality of coils CL in accordance with a magnetic resonance method. The number of the disposed coils CL is n and can be set to any number. 
     The coil CL is formed in an annular shape, for example, and is insulated by a resin cover. The coil CL is, for example, a helical coil or a spiral coil. The helical coil is an annular coil wound in the same plane. The spiral coil is an annular coil which is wound not in the same plane but in a spiral shape along a transmission direction of electric power in the magnetic resonance method. The coil CL is formed of, for example, a cab tire cable. The coil CL includes an electric power transmission coil CLA and an electric power reception coil CLB. The electric power transmission coil CLA is a primary coil, and the electric power reception coil CLB is a secondary coil. 
     The coil CL may include one or more booster coils CLC disposed between the electric power transmission coil CLA and the electric power reception coil CLB. The booster coils CLC are disposed substantially parallel to each other, and half or more of opening surfaces formed by the booster coils CLC overlap with each other. An interval between a plurality of booster coils CLC is ensured, for example, to be larger than a radius of the booster coil CLC. The booster coil CLC assists electric power transmission of the electric power transmission coil CLA. The booster coil CLC may be a non-electric-power-supplied coil. 
     The electric power transmission coil CLA is provided in the electric power transmission apparatus  100 . The electric power reception coil CLB is provided in the electric power reception apparatus  200 . The booster coils CLC may be provided in the electric power transmission apparatus  100  or in the electric power reception apparatus  200 , or may be provided separately in the electric power transmission apparatus  100  and the electric power reception apparatus  200 . A part of the booster coils CLC may be provided in the electric power transmission apparatus  100 , and the other part of the booster coils CLC may be provided in the electric power reception apparatus  200 . 
     The electric power transmission apparatus  100  is installed in a watercraft  50 . The electric power reception apparatus  200  is installed in a movable underwater vehicle  60  (for example, a submarine  70  or an underwater excavator  80 ) or an electric power reception apparatus that is fixedly installed (for example, a seismometer, a monitoring camera, or a geothermal power generator). Each coil CL is disposed in water (for example, in sea). 
     For example, the submarine  70  may include a remotely operated vehicle (ROV), an unmanned underwater vehicle (UUV), or an autonomous underwater vehicle (AUV). 
     Apart of the watercraft  50  is above a water surface  90  (for example, a sea surface), that is, on the water, and the other part of the watercraft  50  is below the water surface  90 , that is, underwater. The watercraft  50  is movable on the water and can move freely to, for example, the water of a data acquisition location. An electric wire  20  is connected between the electric power transmission apparatus  100  and the electric power transmission coil CLA of the watercraft  50 . The electric wire  20  is connected to, for example, a driver  151  (see  FIG. 2 ) in the electric power transmission apparatus  100  via a connector on the water (not shown). 
     The underwater vehicle  60  is in the water or on a water bottom  95  (for example, a sea bottom) and travels in the water or on the water bottom  95 . For example, the underwater vehicle  60  can move freely to a data acquisition point according to an instruction from the watercraft  50  on the water. The instruction from the watercraft  50  may be transmitted by communication via each coil CL, or may be transmitted by other communication methods. 
     Each coil CL is connected to a connecting body  30  and the coils CL are arranged, for example, at equal intervals. A distance between adjacent coils CL (coil interval) is, for example, 5 m. The coil interval is, for example, about half of a diameter of the coil CL. A transmission frequency is, for example, 40 kHz or less and is preferably less than 10 kHz in consideration of an attenuation amount of a magnetic field intensity in the water (for example, in fresh water or in sea). The transmission frequency may also be 40 kHz or more. In a case where electric power is transmitted at a transmission frequency of 10 kHz or more, it is necessary to perform a predetermined simulation based on regulations of the Radio Act, and this operation can be omitted in cases of less than 10 kHz. An electric power transmission distance increases as the transmission frequency becomes lower, and the coil interval increases as the coil CL becomes larger. 
     The transmission frequency is determined based on coil characteristics such as an inductance of the coil CL, the diameter of the coil CL, the number of turns of the coil CL. The diameter of the coil CL is, for example, from several meters to several tens of meters. An electric resistance in the coil CL and electric power loss is lowered as a size of the coil CL increases, that is, as a wire diameter of the coil CL increases. Electric power transmitted via the coil CL is, for example, 50 W or more, and may be 1 kW or more. 
     Although the number of the connecting bodies  30  is three in  FIG. 1 , the present invention is not limited thereto. A weight  40  is connected to an end portion of the connecting body  30  on the side of the electric power reception coil CLB. A buoy  45  is connected to an end portion of the connecting body  30  on the side of the electric power transmission coil CLA. 
     Movement of the connecting body  30  can be restricted by the weight  40 , and movement of each coil CL fixed to the connecting body  30  can thus be restricted. Therefore, even if a water flow is generated in the water, the movement of each coil CL is restricted by the weight  40 , so that reduction in efficiency of electric power transmission using the coil CL can be inhibited. 
     In the connecting body  30 , the weight  40  is connected to the end portion on the electric power reception coil CLB side, and the buoy  45  is connected to the end portion on the electric power transmission coil CLA side, so that the weight  40  is located on a water bottom side while the buoy  45  is located on a water surface side, and a posture, in which the connecting body  30  is substantially perpendicular to the water surface  90 , can be maintained. Therefore, surfaces defined by each coil CL are substantially parallel to the water surface  90 , and the electric power can be transmitted in a water depth direction (a direction substantially orthogonal to the water surface) by magnetic field resonance method. 
     The weight  40  may be detached from the connecting body  30  during transportation of the connecting body  30 , and the weight  40  may be attached to the connecting body  30  when the transportation of the connection body  30  is completed and the connection body  30  is installed at a predetermined position. Accordingly, the transportation of the connecting body  30  is facilitated. 
     As shown in  FIG. 2 , the electric power transmission apparatus  100  includes a power supply  110 , an AC/DC converter (ADC)  120 , a central processing unit (CPU)  130 , an information communication unit  140 , and an electric power transmission circuit  150 . 
     The ADC  120  converts AC power supplied from the power supply  110  into DC power. The converted DC power is transmitted to the electric power transmission circuit  150 . 
     The CPU  130  generally controls operations of each unit of the electric power transmission apparatus  100  (for example, the power supply  110 , the ADC  120 , the information communication unit  140 , and the electric power transmission circuit  150 ). 
     The information communication unit  140  includes a modulation and demodulation circuit  141  configured to modulate or demodulate communication data communicated between the information communication unit  140  and the electric power reception apparatus  200 . The information communication unit  140  transmits, via the coil CL, control information from the electric power transmission apparatus  100  to the electric power reception apparatus  200 , for example. The information communication unit  140  receives, via the coil CL, data from the electric power reception apparatus  200  to the electric power transmission apparatus  100 , for example. This data includes, for example, data of exploration results obtained by underwater exploration or water bottom exploration of the electric power reception apparatus  200 . Through the information communication unit  140 , data communication can be quickly performed between the information communication unit  140  and the underwater vehicle  60  while the underwater vehicle  60  is performing operation such as data collection. 
     The electric power transmission circuit  150  includes the driver  151  and a resonance circuit  152 . The driver  151  converts the DC power from the ADC  120  into an AC voltage (pulse waveform) having a predetermined frequency. The resonance circuit  152  includes a capacitor CA and the electric power transmission coil CLA, and generates a sinusoidal waveform AC voltage based on the pulse waveform AC voltage from the driver  151 . The electric power transmission coil CLA resonates at a predetermined resonance frequency in accordance with the AC voltage applied from the driver  151 . The electric power transmission coil CLA is impedance-matched with an output impedance of the electric power transmission apparatus  100 . 
     The predetermined frequency according to the AC voltage obtained by the conversion of the driver  151  corresponds to the transmission frequency of the electric power transmission between the electric power transmission apparatus  100  and the electric power reception apparatus  200 , and corresponds to the resonance frequency. In the present embodiment, the transmission frequency is set based on a Q factor of each coil CL. 
     As shown in  FIG. 2 , the electric power reception apparatus  200  includes an electric power reception circuit  210 , a CPU  220 , a charge control circuit  230 , a secondary battery  240 , and an information communication unit  250 . 
     The electric power reception circuit  210  includes a rectifier circuit  211 , a regulator  212 , and a resonance circuit  213 . The resonance circuit  213  includes a capacitor CB and the electric power reception coil CLB, and receives AC power transmitted from the electric power transmission coil CLA. The electric power reception coil CLB is impedance-matched with an input impedance of the electric power reception apparatus  200 . The rectifier circuit  211  converts AC power induced in the electric power reception coil CLB to DC power. The regulator  212  converts DC voltage transmitted from the rectifier circuit  211  to a predetermined voltage suitable for charging the secondary battery  240 . 
     The CPU  220  generally controls operations of each unit of the electric power reception apparatus  200  (for example, the electric power reception circuit  210 , the charge control circuit  230 , the secondary battery  240 , and the information communication unit  250 ). 
     The charge control circuit  230  controls charging of the secondary battery  240  according to a type of the secondary battery  240 . For example, in a case where the secondary battery  240  is a lithium ion battery, the charge control circuit  230  starts charging the secondary battery  240  at a constant voltage using DC power from the regulator  212 . 
     The secondary battery  240  accumulates the electric power transmitted from the electric power transmission apparatus  100 . The secondary battery  240  is, for example, a lithium ion battery. 
     The information communication unit  250  includes a modulation and demodulation circuit  251  configured to modulate or demodulate communication data communicated between the information communication unit  250  and the electric power transmission apparatus  100 . The information communication unit  250  receives, via the coil CL, the control information from the electric power transmission apparatus  100  to the electric power reception apparatus  200 , for example. The information communication unit  250  transmits, via the coil CL, the data from the electric power reception apparatus  200  to the electric power transmission apparatus  100 , for example. This data includes, for example, data of exploration results obtained by underwater exploration or water bottom exploration of the electric power reception apparatus  200 . Through the information communication unit  250 , data communication can be quickly performed between the information communication unit  250  and the watercraft  50  while the underwater vehicle  60  is performing operation such as data collection. 
     Similarly to the electric power transmission coil CLA and the electric power reception coil CLB, the booster coil CLC forms a resonance circuit together with a capacitor CC. That is, in the present embodiment, since the resonance circuit is arranged in multiple stages in the water, the electric power is transmitted by the magnetic resonance method. 
     Next, electric power transmission from the electric power transmission apparatus  100  to the electric power reception apparatus  200  will be described. 
     In the resonance circuit  152 , when a current flows through the electric power transmission coil CLA of the electric power transmission apparatus  100 , a magnetic field is generated around the electric power transmission coil CLA. Oscillation of the generated magnetic field is transmitted to the resonance circuit including the booster coil CLC that resonates at the same frequency or the resonance circuit  213  including the electric power reception coil CLB. 
     In the resonance circuit including the booster coil CLC, a current is excited in the booster coil CLC by the oscillation of the magnetic field, the current flows, and a magnetic field is further generated around the booster coil CLC. Oscillation of the generated magnetic field is transmitted to a resonance circuit including another booster coil CLC that resonates at the same frequency or the resonance circuit  213  including the electric power reception coil CLB. 
     In the resonance circuit  213 , an alternating current is induced in the electric power reception coil CLB by the oscillation of the magnetic field of the booster coil CLC or the electric power transmission coil CLA. The induced alternating current is rectified, converted to a predetermined voltage, and charged to the secondary battery  240 . 
     [Coil Structure] 
     Next, a structure of the coil CL will be described. 
     When the coil CL is used, the coil CL is submerged in water and disposed in water. The coil CL preferably adopts a structure that can prevent reduction in the Q factor of the coil CL as much as possible, so as to transmit electric power by a non-contact transmission method (in the present embodiment, the magnetic resonance method). 
     According to the coil CL described below, a waterproof structure is adopted, which inhibits water from entering space between windings (electric wires) of the coil CL in the water, so that the reduction in the Q factor of the coil CL can be inhibited, and reduction in efficiency of underwater electrical power transmission using the coil CL can be inhibited. 
       FIG. 3A  is a cross-sectional view showing a structure of a coil CL 1  (an example of the coil CL). The coil CL 1  has a structure in which electric wires w 1  wound in a ring shape (annular shape), a tubular surrounding member hi 1  surrounding outer peripheral surfaces of the electric wires w 1  in a radial direction, and a self-bonding tape mt 1  wound around an outer peripheral surface of the surrounding member hi 1  are concentrically overlapped with each other. A litz wire may be used as the electric wire w 1 . 
     The litz wire may be a wire in which a plurality of enamel wires, which are coated metal wires, are twisted so that a high-frequency current can easily flow through. The litz wire is a relatively expensive wire. Through using a plurality of enamel wires to increase a surface area, an electric resistance is reduced, due to a skin effect, when the high-frequency current flows to a surface. 
     The surrounding member hi 1  is formed of a material having at least one of a non-magnetic property and a non-conductive property. Since the surrounding member hi 1  is non-magnetic, in the coil CL 1 , strength of a magnetic field generated due to an alternating current flowing in the electric wire w 1  and a magnetic field generated due to resonance with a magnetic field of another coil CL can be inhibited from being absorbed by the surrounding member hi 1 . Therefore, by maintaining the strength of the magnetic field, the coil CL 1  can inhibit the reduction in transmission efficiency when the electric power is transmitted via the magnetic field. Since the surrounding member hi 1  is non-conductive, in the coil CL, a current flowing through the electric wire w 1  can be inhibited from being transmitted to the water, which has high electric conductivity, via the surrounding member hi 1 . Therefore, the coil CL 1  can maintain magnitude of the current, maintain the strength of the magnetic field generated on the basis of the current, and inhibit the reduction in the transmission efficiency when the electric power is transmitted via the magnetic field. 
     A surrounding member hi may have flame retardancy so that the surrounding member hi is difficult to burn even when the electric wire w 1  generates heat when electric power is supplied to the electric wire w 1 . That is, the surrounding member hi 1  may be formed of a material having flame retardancy. Accordingly, safety is improved with respect to combustion of the coil CL. 
     The surrounding member hi 1  may be formed of a flexible or elastic rubber (an example of resin). Accordingly, since the surrounding member hi 1  has flexibility or elasticity, the surrounding member hi can be easily processed to fit with a shape of the coil CL 1  (for example, a shape wound in a circular shape). 
     As a specific material of the surrounding member hi 1 , for example, chloroprene rubber, vinyl chloride resin, polyethylene resin, or silicone resin may be used. A heat insulating material for air conditioner piping may be used as the surrounding member hi 1 . Accordingly, a general-purpose product can be used in the coil CL 1  to surround the electric wire w 1 , and a cost for obtaining the waterproof structure can be reduced. Materials other than the materials exemplified above may be used as the material of the surrounding member hi 1 . 
     The surrounding member hi 1  may have a thickness equal to or greater than ½ of a diameter of the electric wire w 1 . For example, as in  FIG. 7  described below, the electric wire w 1  may have a diameter of 18.3 mm. A size (outer diameter) of the surrounding member hi 1  surrounding outside the electric wire w 1  may be 41.5 mm. In this case, the thickness of the surrounding member hi 1  is 11.6 mm (=(41.5−18.3)/2), which satisfies the requirement that the thickness of the surrounding member hi 1  is equal to or greater than ½ of the diameter of the electric wire w 1 . Accordingly, the surrounding member hi 1  can serve as a spacer, and a distance from an outer surface of the electric wire w 1  to the water can be increased. Therefore, the magnetic field generated by flowing of the alternating current through the coil CL 1  hardly leaks into the water, and the coil CL 1  can inhibit the reduction in the efficiency of the electric power transmission via the magnetic field. 
     In order to insert the electric wire w 1  into the surrounding member hi 1  from a radial direction outer side along a longitudinal direction of the surrounding member hi 1  (a direction in which the surrounding member hi 1  extends), a notch ct 1  extending from outside to inside in the radial direction of the surrounding member hi 1  is formed in the surrounding member hi 1  (see  FIG. 3A ). The electric wire w 1  is covered with the tubular surrounding member hi 1  by opening a portion of the notch ct 1  and accommodating the electric wire w 1  inside the surrounding member hi 1 . 
     The self-bonding tape mt 1  is formed of a non-magnetic and non-conductive material. When the self-bonding tape mil is wound around the surrounding member hi 1 , for example, butyl rubber of an adhesive layer flows out over time, so that the self-bonding tape mt 1  has a function of filling space existing in a portion where the self-bonding tape mt 1  and the surrounding member hi 1  overlap with each other. Therefore, when the coil CL 1  is submerged in the water, the self-bonding tape mt 1  can inhibit the water from entering an inner side of the surrounding member hi 1  through the notch ct 1  formed in the surrounding member hi 1 . That is, the self-bonding tape mt 1  has a function of serving as a waterproof tape that closes a gap formed by the notch ct 1 . The self-bonding tape mt 1  also has a function of serving as a reinforcing tape that prevents the electric wire w 1  from going out of the surrounding member hi 1 . A thickness of the self-bonding tape mt 1  is, for example, 100 micron or more, and may be 0.25 mm, 0.1 mm, or the like. 
     The surrounding member hi 1  surrounding the electric wire w 1 , and the self-bonding tape mt 1  are also collectively referred to as a cover member cv 1 . The cover member cv 1  seals a periphery (including a radial direction outer periphery) of the electric wire w 1 . The coil CL 1  is manufactured by winding the electric wires w 1  covered with the cover member cv in a helical manner or spiral manner. When the coil CL 1  is submerged in the water, since the cover member cv 1  covers the periphery of the electric wire w 1 , a thickness of the cover member cv 1  serves as a spacer, and a distance between the water in the water and the electric wire w 1  is increased. 
     Therefore, in the coil CL 1 , the magnetic field is more difficult to leak into the water, and the reduction in the electric power transmission efficiency can be inhibited. The coil CL 1  can inhibit dielectric loss caused by water (for example, sea water or fresh water) having properties of serving as a dielectric. Therefore, the coil CL 1  can inhibit the reduction in the Q factor of the coil CL 1 , and can inhibit the reduction in the transmission efficiency of the non-contact electric power transmission. 
       FIG. 3B  is a cross-sectional view showing a structure of a coil CL 10  of a comparative example. The coil CL 10  of the comparative example is a coil in which electric wires w 10  wound in a ring shape (annular shape) are bundled by a spiral tube st 10 . Since the spiral tube st 10  has a plurality of notches from outside to inside in the radial direction, water may enter from notched portions. Therefore, the coil CL 10  does not have the waterproof structure. 
     In the coil CL 10 , since the electric wire w 10  covered with a coating layer of an enamel wire directly contacts the water, the coil CL 10  is easily short-circuited with the water, which is a conductor. Since there is no distance from the electric wire w 10  to the water in the water, the magnetic field leaks easily. In a case where the water is sea water, electric power loss, caused by an eddy current of the sea water, easily occurs. Dielectric loss caused by the water easily occurs. When the water contacts the electric wire w 10 , the electric wire w 10  is easily corroded and deteriorated, and a life of the electric wire w 10  may be shortened. 
     Next, performance of the coil CL 1  of the present embodiment and the coil CL 10  of the comparative example will be described. 
     First, inductance (L) and Q factors of the coils CL 1  and CL 10  are compared and discussed.  FIG. 4A  is a table showing measurement conditions of the inductance (L) and the Q factors. The inductance (L) and the Q factors are measured in space (in air), fresh water (in fresh water), sea water (in sea water). Temperatures (° C.) of the space, fresh water, and sea water may be 24.5 degrees, 25.7 degrees, and 24.8 degrees, respectively. Conductivity of the coil may be 0.01224 (S/m) in the freshwater, 5.16 (S/m) in the seawater. A salt concentration of the sea water may be 3.4%. 
     The measurement results of  FIGS. 4B to 4E  are results of each value measured under the measurement conditions shown in  FIG. 4A . In  FIGS. 4B to 4E , alternating currents of predetermined frequencies (for example, 40 kHz, 80 kHz) were applied to the electric wires w, w 10  of the coils CL 1 , CL 10 , and each value was measured. 
       FIG. 4B  is a table showing measurement results of inductance (L) and a Q factor of the coil CL 10  which is not provided with the cover member cv 1  at a frequency of 40 kHz. The inductance (L) of the coil CL 10  is 135 pH 145 pH, and 145 pH, respectively, in the space, fresh water, and seawater. The inductance (L) of the coil CL 10  slightly increases in the freshwater and seawater as compared with the inductance (L) in the space. The Q factor of the coil CL 10  is 361, 282, 195, respectively, in the space, fresh water, and sea water. The Q factor decreases in the fresh water and is further decreased in the sea water, as compared with the Q factor in the space. This is considered to be a result of the fact that electric power loss (dielectric loss) is caused by capacitive components of the fresh water or the sea water, which have the property of serving as dielectrics. Specifically, it is considered that capacitance is generated due to water entering space between windings of the coil CL 10 , dielectric loss components of the sea water increases a series resistance of the coil CL 10 , so the electric power loss of the coil CL 10  is increased, and the transmission efficiency is reduced. Since the conductivity of the sea water is higher than the conductivity of the fresh water, it is considered that there is more electric power loss due to eddy currents. 
       FIG. 4C  is a table showing measurement results of the inductance (L) and the Q factor of the coil CL 1  that includes the cover member cv 1  at the frequency of 40 kHz. The inductance (L) of the coil CL 1  is 135 pH, 135 pH, and 134 pH, respectively, in the space, fresh water, and sea water. The inductance of the coil CL 1  is substantially the same in the space, fresh water, and sea water. The Q factor of the coil CL 1  is 371, 381, and 301, respectively, in the space, fresh water, and sea water. The Q factor is slightly higher in the fresh water and slightly lower in the seawater as compared with the Q factor in the space. This is considered to be a result of the fact that the fresh water or sea water does not reach the electric wire w 1  due to the cover member cv, and a distance is secured from the electric wire w 1  to the water in the water or the sea water in sea, so that influence of the fresh water and sea water is reduced. The influence of the fresh water and sea water may include dielectric loss caused by contacting the coil CL 1  with the water, and eddy current loss caused by approaching the coil CL 1  to the water (particularly, sea water). 
       FIG. 4D  is a table showing measurement results of the inductance (L) and the Q factor of the coil CL 10  which is not provided with the cover member cv 1  at a frequency of 80 kHz. The inductance (L) of the coil CL 10  is 136 pH, 147 pH, and 148 pH, respectively, in the space, fresh water, and seawater. The inductance of the coil CL 10  slightly increases in the freshwater and sea water as compared with the inductance in the space. The Q factor of the coil CL 10  is 663, 177, and 164, respectively, in the space, fresh water, and sea water. The Q factor decreases in the fresh water and is further decreased in the sea water, as compared with the Q factor in the space. This is considered to be a result of the fact that large electric power loss (dielectric loss) is caused by the capacitive components of the fresh water or the sea water, which have the property of serving as dielectrics. Specifically, it is considered that capacitance is generated due to water entering space between windings of the coil CL 10 , dielectric loss components of the sea water increases a series resistance of the coil CL 10 , so the electric power loss of the coil CL 10  is increased, and the transmission efficiency is reduced. Since the conductivity of the sea water is higher than the conductivity of the fresh water, it is considered that there is larger electric power loss due to eddy currents. 
       FIG. 4E  is a table showing measurement results of the inductance (L) and the Q factor of the coil CL 1  that includes the cover member cv 1  at the frequency of 80 kHz. The inductance (L) of the coil CL 1  is the same value 135, 135, and 135 pH, respectively, in the space, fresh water, and sea water. The Q factor of the coil CL 1  is 670, 679, and 341, respectively, in the space, freshwater, and seawater. The Q factor is slightly higher in the freshwater and significantly lower in the sea water as compared with the Q factor in the space. This is considered to be a result of the fact that the fresh water or sea water does not reach the electric wire w 1  due to the cover member cv 1 , and a distance is secured from the electric wire w 1  to the water in the water or the sea water in sea, so that influence of the fresh water and sea water is reduced. The influence of the fresh water and sea water may include dielectric loss caused by contacting the coil CL 1  with the water, and eddy current loss caused by approaching the coil CL 1  to the water (particularly, seawater). 
     Next, transmission efficiency of non-contact electric power transmission using the coil CL 1  of the present embodiment and the coil CL 10  of the comparative example will be described. 
       FIG. 5A  shows a configuration example of a transmission efficiency measuring circuit  300  for measuring the transmission efficiency of the non-contact electric power transmission. The transmission efficiency measuring circuit  300  measures the transmission efficiency for the non-contact electric power transmission. The transmission efficiency measuring circuit  300  includes an electric power transmission circuit  300 T and an electric power reception circuit  300 R. Here, as an example, a case where the transmission efficiency is measured at electric power transmission frequencies of 40 kHz and 80 kHz is shown. The electric power transmission frequency is lowered as the diameter of the coil increases. For example, if the diameter of the coil is longer than 5 m, the electric power transmission frequency is about 1 kHz or less. For example, if the diameter of the coil is equal to or less than 5 m, the electric power transmission frequency is higher than about 1 kHz. 
     The electric power transmission circuit  300 T includes a high-frequency power supply circuit  310 , capacitors C 1 , C 2 , and an electric power transmission coil Lr 1 . The high-frequency power supply circuit  310  is input with a DC voltage and generates an electric power transmission signal having a frequency of 40 kHz. The capacitors C 1 , C 2 , and the electric power transmission coil Lr 1  form an LC resonance circuit that resonates at 40 kHz. Inductance of the electric power transmission coil Lr 1  is, for example, 145 pH. 
     The electric power reception circuit  300 R includes a load apparatus  320 , capacitors C 3 , C 4 , and an electric power reception coil Lr 2 . The load apparatus  320  incorporates a load resistor RL. The load resistor RL is, for example, 10Ω. Similarly to the electric power transmission circuit  300 T, the capacitors C 3 , C 4  and the electric power reception coil Lr 2  form an LC resonance circuit that resonates at 40 kHz. Inductance of the electric power reception coil Lr 2  is, for example, 145 μH. 
     The transmission efficiency measuring circuit  300  includes a power analyzer (not shown) that analyzes transmission efficiency of electric power. The power analyzer acquires transmitted electric power in the electric power transmission circuit  300 T, acquires received electric power in the electric power reception circuit  300 R, and derives the transmission efficiency based on the transmitted electric power and the received electric power. 
     For example, the transmission efficiency measuring circuit  300  may detect a current I 1  flowing through a resistor R included in the high-frequency power supply circuit  310  and calculate and acquire an electric power value (transmitted electric power value) represented by (I 1 ) 2 ×R. The transmitted electric power may be electric power of a point Pin of the high-frequency power supply circuit  310 . For example, the transmission efficiency measuring circuit  300  may detect a current I flowing through the load resistor R L  and calculate and acquire an electric power value (received electric power value) represented by I 2 ×R L . That is, the received electric power may be electric power of a point Pout of the load apparatus  320 . 
     The transmission efficiency measuring circuit  300  may calculate the transmission efficiency using Equation (1) based on the transmitted electric power and the received electric power. 
       Transmission Efficiency=Received Electric Power/Transmitted Electric Power×100(%)  (1)
 
     Specifically, in  FIG. 5A , assuming that the electric power transmission is performed in the sea water, the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  are accommodated in a container  350  containing the sea water. A distance dl between the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may be 80 mm. 
     A coupling coefficient k between the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may be 0.099. The coupling coefficient k may be represented by Equation (2). 
         k=M /( L 1× L 2) 1/2   (2)
 
     Here, M refers to mutual inductance, L 1  refers to self-inductance of the electric power transmission coil Lr 1 , and L 2  refers to self-inductance of the electric power reception coil Lr 2 . The self-inductance of the electric power transmission coil Lr 1  and the self-inductance of the electric power reception coil Lr 2  are both 145 μH. The coupling coefficient k is a value ranging from a maximum value k=1 to a minimum value k=0. 
       FIG. 5B  is a plan view showing shapes of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2 .  FIG. 5C  is a cross-sectional view showing the shapes of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  as viewed from a direction of arrow E-E in  FIG. 5B . The electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may have the same specification. The number of windings N of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may be 22. An outer diameter, an inner diameter, and a thickness of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may be φ174 mm, φ134 mm, and 21.6 mm, respectively. A φ0.05 mm×1200 litz wire may be used as an electric wire of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2 . 
     Sizes of the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  shown in  FIG. 5C  may be sizes that do not include the cover member cv 1  and the spiral tube st 10 . That is, the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  may be the wound wires w 1 , w 10 . 
       FIG. 6A  is a table showing measurement conditions of the transmission efficiency of the non-contact electric power transmission. Similar to the measurement conditions of the inductance (L) and the Q factor, the transmission efficiency is measured in the space (in the air), fresh water (in the fresh water), sea water (in the sea water). Temperatures (° C.) of the space, fresh water, and sea water may be 24.5 degrees, 25.7 degrees, and 24.8 degrees, respectively. Conductivity of the fresh water may be 0.01224 (S/m) and conductivity of the sea water may be 5.16 (S/m). A salt concentration of the sea water may be 3.4%. 
     Measurement results of  FIGS. 6B and 6C  are results of each value measured under the measurement conditions shown in  FIG. 6A  using the transmission efficiency measuring circuit  300  of  FIG. 5A . In  FIGS. 6B and 6C , alternating currents of predetermined frequencies (for example, 40 kHz, 80 kHz) were applied to the electric wires w 1 , w 10  of the coils CL 1 , CL 0 , and each value was measured. The coil CL 1  including the cover member cv 1  may be configured such that the cover member cv 1  is mounted on the electric wire w 1  shown in the electric power transmission coil Lr 1  or the electric power reception coil Lr 2 . The coil CL 10  which is not provided with the cover member cv may be configured such that the spiral tube st 10  is mounted on the electric wire w 10  shown in the electric power transmission coil Lr 1  or the electric power reception coil Lr 2 . 
       FIG. 6B  is a table showing measurement results of the electric power transmission efficiency at the frequency of 40 kHz in a case where the cover member cv 1  is provided (covered by the cover member cv) and in a case where the cover member cv 1  is not provided (not covered by the cover member cv 1 ). When the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  which were not provided with the cover member cv 1  were used, the transmission efficiency was 94.4%, 90.6%, and 88.7%, respectively, when measured in the space, fresh water, and sea water. Meanwhile, when the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  including the cover member cv 1  were used, the transmission efficiency was 94.8%, 95.1%, and 94.0%, respectively, when measured in the space, fresh water, and sea water. Referring to the measurement results, it is understood that the transmission efficiency is lowered in the fresh water or sea water in the case where the coil CL 10  which is not provided with the cover member cv 1  is used, as compared with the transmission efficiency in the space. Meanwhile, in the case where the coil CL 1  including the cover member cv 1  is used, the transmission efficiency in the fresh water or sea water is not much different from the transmission efficiency in the space. That is, in the case where the coil CL 1  including the cover member cv 1  is used, the reduction in the transmission efficiency in the fresh water or sea water is inhibited as shown in  FIG. 4C , as compared with the case where the coil CL 10  which is not provided with the cover member cv 1  is used. 
       FIG. 6C  is a table showing measurement results of the electric power transmission efficiency at the frequency of 80 kHz in a case where the cover member cv 1  is provided and in a case where the cover member cv 1  is not provided. When the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  which were not provided with the cover member cv 1  were used, the transmission efficiency was 96.0%, 89.0%, and 84.3%, respectively, when measured in the space, fresh water, and sea water. Meanwhile, when the electric power transmission coil Lr 1  and the electric power reception coil Lr 2  including the cover member cv 1  were used, the transmission efficiency was 96.1%, 96.8%, and 93.3%, respectively, when measured in the space, fresh water, and sea water. Referring to the measurement results, it is understood that even when the frequency is 80 kHz, the transmission efficiency is lowered in the fresh water or sea water in the case where the coil CL 0  which is not provided with the cover member cv 1  is used, as compared with the transmission efficiency in the space. Meanwhile, in the case where the coil CL 1  including the cover member cv is used, the transmission efficiency in the fresh water or sea water is not much different from the transmission efficiency in the space. That is, even when the transmission frequency is 80 kHz, in the case where the coil CL including the cover member cv 1  is used, the reduction in the transmission efficiency in the fresh water or sea water is inhibited as shown in  FIG. 4E , as compared with the case where the coil CL 10  which is not provided with the cover member cv 1  is used. 
     In this way, referring to  FIGS. 4B to 4E, 6B and 6C , it can be understood that when the coil CL 1  is used, in which the cover member cv 1  is mounted on the electric wire w 1 , characteristic change of each value at the transmission frequency of 40 kHz or more is smaller than the case where the coil CL 10  is used, in which the cover member cv 1  is not mounted on the electric wire w 10 . It is considered that reduction in the characteristic change includes dielectric loss (tan δ) caused by the water entering the space between the windings of the coil. 
     In this way, in the coil CL 1  of the first embodiment, the waterproof structure is realized by surrounding and sealing the electric wire w 1  with the cover member cv 1 . Accordingly, the coil CL 1  can inhibit the water from entering the space between wires of the electric wire w 1  of the coil CL 1 , so that reduction in the Q factor of the coil CL 1  can be inhibited. By making the coil CL 1  waterproof, it is difficult for the water to enter the coil CL 1 , thereby making it difficult for the electric wire w 1  to short-circuit, thus voltage endurance of the coil CL 1  can be improved. Accordingly, the coil CL 1  can transmit high electric power with an increased voltage. The coil CL 1  can improve long-term reliability of the electric wire w 1  by preventing water or sea water from entering the electric wire w 1 . 
     By securing a thickness of the tubular surrounding member hi to a certain extent (for example, ½ or more of the diameter of the electric wire w 1 ), a long distance can be secured between the electric wire w 1  and the water in the water by a thickness of the cover member cv 1 . Therefore, the coil CL 1  can inhibit the electric power loss caused by the eddy current in the sea water. Therefore, the coil CL 1  can further inhibit the reduction in the electric power transmission efficiency when the non-contact electric power transmission is performed. 
     Modification 1 of the First Embodiment 
       FIG. 7  is a cross-sectional view showing a structure of a coil CL 2  (an example of the coil CL) according to Modification 1 of the first embodiment. The coil CL 2  has a structure in which electric wires w 2  wound in a ring shape (annular shape), the tubular surrounding member hi 1  surrounding outer peripheral surfaces of the electric wires w 2  in a radial direction, and the self-bonding tape mt 1  wound around the outer peripheral surface of the surrounding member hi 1  are concentrically overlapped with each other. A size of the coil CL 2  exemplified in  FIG. 7  may be the same as a size of the coil CL 1  described in the first embodiment. Sizes shown in  FIG. 7  may be measurement (simulation) sizes, and the electric power transmission coil CLA, the electric power reception coil CLB, and the booster coil CLC used actually in the electric power transmission apparatus  100  and the electric power reception apparatus  200  may be larger than the sizes. 
     In the coil CL 2 , a single wire is used as the wire instead of the litz wire. The single wire is inexpensive and available as compared with the litz wire. The single wire has a stronger mechanical strength than a twisted wire. Therefore, the coil CL 2  is difficult to break and is easy to process. 
     The electric wire w 2 , which is a single wire, may have a diameter of 18.3 mm. A size (outer diameter) of the cover member cv 1  surrounding the electric wire w 2  may be 41.5 mm. The thickness of the cover member cv 1  may be 11.6 mm (=(41.5−18.3)/2). Therefore, when the coil CL 2  is submerged in water, a distance from the electric wire w 2  to the water in the water is equal to or greater than 11.6 mm (the thickness of the cover member cv 1 ), and is equal to or greater than ½ of the diameter of the electric wire w 1  (=18.3/2). Accordingly, the coil CL 2  can inhibit the electric power loss caused by the eddy current in the sea water even when a high frequency current flows through the coil CL 2 . 
     In this way, according to the coil CL 2  of Modification 1, by using the single wire electric wire w 2  instead of the litz wire electric wire w 1 , the electric wire w 2  and the coil CL 2  can be manufactured relatively inexpensively. The material of the coil CL 2  is available, and the coil CL 2  can be easily processed. 
     Modification 2 of the First Embodiment 
       FIG. 8  is a cross-sectional view showing a structure of a coil CL 3  (an example of the coil CL) according to Modification 2 of the first embodiment. The coil CL 3  includes a tubular surrounding member hi 2 , which includes ferrite fg 1 , instead of the tubular surrounding member hi 1 . That is, a cover member cv 2  is formed by the surrounding member hi 2  and a self-bonding tape mt 1 . 
     The ferrite fg 1  includes iron oxide as a main component, and is a soft magnetic material having high magnetic permeability. By mixing the granular ferrite fg 1  into the tubular surrounding member hi 2 , a magnetic field generated by an AC current flowing through the annularly wound electric wire w 1  is concentrated so as to pass through the ferrite fg 1  scattered inside the cover member cv 2 . As a result, the magnetic field hardly leaks to a radial direction outer side of the coil CL 3 , and the magnetic field is enlarged. 
     As described above, according to the coil CL 3  of Modification 2, a magnetic material such as the ferrite is made granular and mixed into the cover member cv 2  that covers the electric wire w 1 , so that the magnetic field generated in the coil CL 3  is concentrated inside the cover member cv 2 , and hardly leaks to the radial direction outer side of the coil CL 3 . As a result, a Q factor of the coil CL 3  is improved, and the reduction in the transmission efficiency is inhibited. Therefore, the coil CL 3  can maintain the transmission efficiency and improve waterproof performance. 
     The magnetic material mixed in the surrounding member hi 2  may be a magnetic material other than the ferrite. For example, silicon steel, permalloy, sendust, or the like may be used. The tubular surrounding member hi 2 , in which the magnetic material such as the ferrite is mixed, may also be used in the coil CL 2  using the electric wire w 2  of Modification 1 and the electric power transmission efficiency is improved similarly. 
     Second Embodiment 
     In the first embodiment, the case where the waterproof structure is realized by surrounding the electric wire with the cover member cv 1  is shown. In the second embodiment, the waterproof structure is realized by molding the electric wire with resin. The coil CL of the second embodiment is manufactured by a helical winding that is spirally wound in an electric power transmission direction. 
     In the helical winding, the coil CL is wound in two or more turns so as to be overlapped in the transmission direction. Each electric wire is molded with resin so that an outer peripheral surface thereof is covered, so that sea water does not enter space between the electric wires in the coil CL. The number of turns of the wound electric wire is, for example, five turns, and may be any number of turns as long as the number of turns is two or more. 
       FIG. 9  is a cross-sectional view showing a structure of a coil CL 4  (an example of the coil CL) according to the second embodiment. In  FIG. 9 , in the coil CL 4 , each electric wire w 3  is covered with a molded portion md 1  and is wound by five times. The coil CL 4  may be integrated by winding each electric wire w 3  covered with the molded portion md 1  by five times and then tying the electric wires w 3  by a tying band. The coil CL 4  may also be integrated by winding the electric wires w 3  by five turns and arranging the electric wires w 3  of each winding in one line at intervals, and then molding the electric wires w 3  with a resin mold. 
     The molded portion md 1  is molded by filling a resin mold material, for example. A material that is non-magnetic and non-conductive is used as the mold material. Chloroprene rubber, vinyl chloride resin, or the like, which has excellent insulating property and is easy to process, may be used as the material. Further, a flame retardant mold material may be used, which does not melt even when heated. 
     As in the first embodiment, the wire of each electric wire w 3  may be a litz wire or a single wire. The electric wire w 3  may be covered or may not be covered. Even in this case, for example, water is inhibited by the molded portion md 1  from entering the space between the windings of the electric wire w 3 , so that the reduction in the transmission efficiency can be inhibited, and short circuit between the windings can be inhibited. 
     As in the first embodiment, a thickness of the molded portion md 1  in a radial direction may be equal to or larger than ½ of a diameter of the electric wire w 3 . In this case, the molded portion md 1  can serve as a spacer, and a distance from an outer surface of the electric wire w 3  to the water can be increased. Therefore, a magnetic field generated by flowing of alternating current through the coil CL 4  hardly leaks into the water, and the coil CL 4  can inhibit the reduction in the efficiency of the electric power transmission via the magnetic field. Since the molded portion md 1  has the thickness described above, the coil CL 4  can inhibit heat generation caused by eddy currents of another electric wire (for example, an electric wire of a second turn) adjacent to a wound electric wire (for example, an electric wire of a first turn), and the transmission efficiency can be improved. 
     As described above, according to the coil CL 4  of the second embodiment, the molded portion md 1  is formed so as to cover the outer peripheral surface of each electric wire w 3  which is a bundle having two or more turns. Therefore, the coil CL 4  can be manufactured relatively inexpensively. Since the electric wire w 3  is surrounded by the molded portion md 1 , a waterproof property and a mechanical strength can be improved. 
     In the coil CL 4 , the waterproof structure is realized by surrounding and sealing the electric wire w 3  with the molded portion md 1 . Accordingly, the coil CL 4  can inhibit the water from entering the space between the windings of the electric wire w 3  of the coil CL 4 , so that reduction in a Q factor of the coil CL 4  can be inhibited. By making the coil CL 4  waterproof, it is difficult for the water to enter the coil CL 4 , thereby making it difficult for the electric wire w 3  to short-circuit, thus voltage endurance of the coil CL 4  can be improved. Accordingly, the coil CL 4  can transmit high electric power with an increased voltage. The coil CL 4  can improve long-term reliability of the electric wire w 3  by preventing water or sea water from entering the electric wire w 3 . 
     By securing a thickness of the molded portion md 1  to a certain extent (for example, ½ or more of the diameter of the electric wire w 3 ), a long distance can be secured between the electric wire w 3  and the water in the water. Therefore, the coil CL 4  can inhibit the electric power loss caused by the eddy current in the sea water. Therefore, the coil CL 4  can further inhibit the reduction in the electric power transmission efficiency when the non-contact electric power transmission is performed. 
     Although the helical coil is mainly exemplified here, the coil CL 4  of the second embodiment can be similarly applied to a case where the coil CIA is manufactured by a spiral winding that is wound in a horizontal direction perpendicular to the electric power transmission direction. 
     Modification of the Second Embodiment 
       FIG. 10  is a cross-sectional view showing a structure of a coil CL 5  (an example of the coil CL) according to Modification 1 of the second embodiment. In the coil CL 5 , a molded portion md 2  which includes ferrite fg 2  is used instead of the molded portion md 1 . The ferrite fg 2  includes iron oxide as a main component, and is a soft magnetic material having high magnetic permeability. The molded portion md 2  is made through mixing the granular ferrite fg 2  into resin, a magnetic field generated in the electric wire w 3  which is wound in an annular shape is concentrated so as to pass through the ferrite fg 2  scattered inside the molded portion md 2 . As a result, the magnetic field hardly leaks to a radial direction outer side of the coil CL 5 , and the magnetic field is enlarged. 
     According to the coil CL 5  of Modification 1, a magnetic material such as the ferrite fg 2  is contained in the molded portion md 2  that covers the electric wire w 3  which is a bundle having two or more turns, so that the magnetic field generated in the coil CL 5  is concentrated inside the molded portion md 2 , and hardly leaks to the water on the radial direction outer side of the coil CL 5 . As a result, a Q factor of the coil CL 5  is improved, and the reduction in the transmission efficiency is inhibited. Therefore, the coil CL 5  can maintain the transmission efficiency and improve waterproof performance. 
     The magnetic material mixed in the molded portion md 2  may be a magnetic material other than the ferrite. For example, silicon steel, permalloy, sendust, or the like may be used besides the ferrite. 
     Although various embodiments are described above with reference to the drawings, it is needless to say that the present disclosure is not limited to such examples. It will be apparent to those skilled in the art that various changes and modifications can be conceived within the scope of the claims, and it should be understood that such changes and modifications also belong to the technical scope of the present disclosure. 
     The transmission coil and the electric power transmission apparatus of the above embodiments will be summarized. 
     A coil CL (an example of the transmission coil) transmits electric power in water. The coil CL includes: an annular electric wire w 1  through which an alternating current flows; and a cover member cv 1  (an example of a first cover), which includes non-conductive resin or non-magnetic resin and seals a periphery of the electric wire w 1 . The electric wire w 1  transmits the electric power via a magnetic field generated by flowing of the alternating current. 
     Accordingly in the coil CL, since the cover member cv 1 , which surrounds the annularly wound electric wire w 1 , is non-conductive or nonmagnetic, the magnetic field generated by the coil CL can be inhibited from being transmitted to outside of an underwater electric power transmission path. For example, since the cover member cv 1  is non-magnetic, in the coil CL, strength of a magnetic field generated due to an alternating current flowing in the coil CL and a magnetic field generated due to resonance with a magnetic field of another coil CL can be inhibited from being absorbed by the cover member cv 1 . Therefore, by maintaining the strength of the magnetic field, the coil CL can inhibit the reduction in transmission efficiency when the electric power is transmitted via the magnetic field. For example, since the cover member cv 1  is non-conductive, in the coil CL, a current flowing through the electric wire can be inhibited from being transmitted to the water, which has high electric conductivity, via the cover member cv. Therefore, the coil CL can maintain magnitude of the current, maintain the strength of the magnetic field generated on the basis of the current, and inhibit the reduction in the transmission efficiency when the electric power is transmitted via the magnetic field. 
     Since inside of the coil CL is sealed by the cover member cv 1 , the coil CL can inhibit water (for example, sea water or fresh water) from entering the electric wire w 1  inside the cover member cv. Since the coil CL is surrounded and sealed by the cover member cv 1 , direct contact between the coil CL and the water can be inhibited, thus dielectric loss caused by the water can be inhibited. For example, the water can be inhibited from entering space between adjacent electric wires of the wound electric wire w 1  (for example, an electric wire of a first turn and an electric wire of a second turn), thus occurrence of the dielectric loss can be inhibited. Therefore, reduction in a Q factor of the coil CL can be inhibited, and reduction in electric power transmission efficiency can be inhibited. 
     In a case where a voltage applied to the electric wire w 1  is a high voltage, the coil CL is more likely to become short-circuited (beak) in the water, which has conductivity, than in the air (space). However the coil CL is hardly short-circuited since a waterproof structure is provided, thus voltage endurance of the coil CL can be improved. Since the coil CL has the waterproof structure, long-term reliability of wire material of the electric wire w 1  can be improved. In this way, the coil CL can inhibit the reduction in transmission efficiency during underwater non-contact electric power transmission. 
     The cover member cv 1  may include a notch ct 1  along a radial direction of the cover member cv 1 , and may include a surrounding member hi 1  (an example of a second cover) which surrounds a radial direction outer peripheral surface of the electric wire w with non-conductive resin or non-magnetic resin, and a non-conductive or non-magnetic self-bonding tape mt 1 . 
     Accordingly in the coil CL, a relatively available member, such as the same material as a heat insulating material for an air conditioner pipe, can be used as the surrounding member hi 1  surrounding the electric wire w 1 , thus versatility of the coil CL can be improved. Even in this case, the coil CL can inhibit the water from entering from the notch ct 1  by sealing the surrounding member hi 1  surrounding the electric wire w 1  with the self-bonding tape mt 1 , thereby securing a waterproof function. 
     The cover member cv 1  may include a molded portion md 1  that seals the radial direction outer peripheral surface of the electric wire w 1  with non-conductive resin or a non-magnetic resin. The electric wire w 1  and the cover member cv 1  may be spirally formed along a transmission direction of electric power transmission using the coil CL. 
     In this way, by surrounding the electric wire w 1  with the molded portion md 1 , the periphery of the electric wire w 1  can be sealed without being provided with a notch portion for inserting the electric wire w 1 . Therefore, the waterproof function of the coil CL can be improved. By spirally winding the electric wire w 1 , a wide space can be secured inside the coil CL. That is, a wide area can be secured in the coil CL, which enables the electric power to be supplied while inhibiting the reduction in the transmission efficiency. 
     The cover member cv 1  may have flame retardancy. 
     Accordingly, even when a high voltage is applied to the coil and when heat is generated due to a short circuit or the like, the coil and a periphery thereof are difficult to be combusted. Therefore, safety of the coil CL can be maintained, and a life of the coil CL can be extended. 
     The cover member cv 1  may contain a magnetic material such as ferrite fg 1 . 
     Accordingly, the magnetic field generated by the coil CL is concentrated inside the cover member cv 1 , and is unlikely to leak to outside. Therefore, even if the non-conductive or non-magnetic cover member cv 1  or molded portion md 1  surrounds the electric wire w 1 , the magnetic field for electric power transmission can be easily transmitted through the non-conductive or non-magnetic cover member cv 1  or molded portion md 1 . Therefore, the coil CL can secure the waterproof function and improve the electric power transmission efficiency. 
     An electric power transmission apparatus  100  transmits electric power to an electric power reception apparatus  200  including an electric power reception coil CLB in water. The electric power transmission apparatus  100  includes: one or more coils CL (an example of a transmission coil) including an electric power transmission coil CLA that transmits electric power to the electric power reception coil CLB via a magnetic field; a driver  151  (an example of an electric power transmission unit) that transmits AC power to the electric power transmission coil CLA; and a capacitor CA that is connected with the coil CL and forms a resonance circuit  152  that resonates with the coil CL. The coil CL is anyone of the coils CL described above. 
     Accordingly in the electric power transmission apparatus  100 , since the cover member cv 1 , which surrounds the annularly wound electric wire w 1 , is non-conductive or nonmagnetic, the magnetic field generated by the coil CL can be inhibited from being transmitted to the outside of the underwater electric power transmission path. For example, since the cover member cv 1  is non-magnetic, in the coil CL, strength of a magnetic field generated due to an alternating current flowing in the coil CL and a magnetic field generated due to resonance with a magnetic field of another coil CL can be inhibited from being absorbed by the cover member cv 1 . Therefore, by maintaining the strength of the magnetic field, the coil CL can inhibit the reduction in transmission efficiency when the electric power is transmitted via the magnetic field. For example, since the cover member cv 1  is non-conductive, in the coil CL, a current flowing through the electric wire can be inhibited from being transmitted to the water, which has high electric conductivity, via the cover member cv 1 . Therefore, the coil CL can maintain magnitude of the current, maintain the strength of the magnetic field generated on the basis of the current, and inhibit the reduction in the transmission efficiency when the electric power is transmitted via the magnetic field. 
     Since the inside of the coil CL is sealed by the cover member cv 1 , the coil CL can inhibit water (for example, sea water or fresh water) from entering the electric wire w 1  inside the cover member cv 1 . Since the coil CL is surrounded and sealed by the cover member cv 1 , direct contact between the coil CL and the water can be inhibited, thus dielectric loss caused by the water can be inhibited. For example, the water can be inhibited from entering space between adjacent electric wires of the wound electric wire w 1  (for example, an electric wire of a first turn and an electric wire of a second turn), thus occurrence of the dielectric loss can be inhibited. Therefore, the coil CL can inhibit the reduction in the Q factor of the coil CL, and inhibit the reduction in the electric power transmission efficiency. 
     In a case where a voltage applied to the electric wire w 1  is a high voltage, the coil CL is more likely to become short-circuited (beak) in the water, which has conductivity, than in the air (space). However the coil CL is hardly short-circuited since a waterproof structure is provided, thus voltage endurance of the coil CL can be improved. Since the coil CL has the waterproof structure, long-term reliability of wire material of the electric wire w 1  can be improved. Through using such a coil CL, the electric power transmission apparatus  100  can inhibit the reduction in the transmission efficiency in the underwater non-contact electric power transmission. 
     In the above-described embodiments, a processor may be physically configured in any way. Through using a programmable processor, since processing content can be changed by changing programs, a degree of design flexibility of the processor can be improved. The processor may be configured with one semiconductor chip, or may be configured with a plurality of semiconductor chips physically. Ina case where the processor is configured with a plurality of semiconductor chips, each control of the above-described embodiments may be realized by another semiconductor chip. In this case, it can be considered that one processor is constituted by the plurality of semiconductor chips. The processor may be configured with the semiconductor chip and a member having a different function (such as a capacitor). One semiconductor chip may be configured to realize a function of the processor and other functions. One processor may be configured with a plurality of processors. 
     Although the present disclosure is described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure. 
     The present disclosure is based on Japanese Patent Application No. 2017-069070 filed on Mar. 30, 2017, the contents of which are incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable for a transmission coil and an electric power transmission apparatus which can inhibit reduction in transmission efficiency during underwater non-contact electric power transmission. 
     REFERENCE SIGNS LIST 
     
         
           10  Electric power transmission system 
           20  Electric wire 
           30  Connecting body 
           40  Weight 
           45  Buoy 
           50  Watercraft 
           60  Underwater vehicle 
           70  Submarine 
           80  Water bottom excavator 
           90  Water surface 
           95  Water bottom 
           100  Electric power transmission apparatus 
           110  Power supply 
           120  ADC 
           130  CPU 
           140  Information communication unit 
           141  Modulation and demodulation circuit 
           150  Electric power transmission circuit 
           151  Driver 
           152  Resonance circuit 
           200  Electric power reception apparatus 
           210  Electric power reception circuit 
           211  Rectifier circuit 
           212  Regulator 
           220  CPU 
           230  Charge control circuit 
           240  Secondary battery 
           250  Information communication unit 
           251  Modulation and demodulation circuit 
           300  Transmission efficiency measuring circuit 
           300 T Electric power transmission circuit 
           300 R Electric power reception circuit 
           310  High-frequency power supply circuit 
           320  Load apparatus 
           350  Container 
         C 1 , C 2 , C 3 , and C 4  Capacitor 
         CL, CL 1  CL 2 , CL 3 , CL 4 , CL 5  and CL 10  Coil 
         CLA Electric power transmission coil 
         CLB Electric power reception coil 
         CLC Booster coil 
         CA, CB and CC Capacitor 
         ct 1  Notch 
         fg 1  and fg 2  Ferrite 
         hi 1  and hi 2  Surrounding Member 
         Lr 1  Electric power transmission coil 
         Lr 2  Electric power reception coil 
         md 1  and md 2  Molded portion 
         mit 1  Self-bonding tape 
         st 10  Spiral tube 
         w 1 , w 2 , w 3  and w 10  Electric wire