Patent Publication Number: US-2011049978-A1

Title: Self-resonant coil, non-contact electric power transfer device and vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a self-resonant coil to be used in a non-contact electric power transfer device adapted to transfer electric power by using magnetic field resonance, a non-contact electric power transfer device having the self-resonance coil, and a vehicle equipped with the non-contact electric power transfer device. 
     BACKGROUND ART 
     Today, a considerable amount of attention is riveted to electric vehicles, such as an electric automobile and a hybrid car, as environment-friendly vehicles. These vehicles are equipped with an electric motor for generating a traveling drive force and a rechargeable power storage device for storing electric power to be supplied to the electric motor. Examples of the hybrid car are a vehicle equipped with an internal combustion engine as a power source in addition to the electric motor, and a vehicle equipped with a fuel battery as a direct current source for driving the vehicle in addition to the power storage device. 
     In some known hybrid cards, an in-vehicle power storage device can be charged by a power source outside of the vehicle as with electric automobiles. An example of such a hybrid cars is, what is called, a “plug-in hybrid car”, in which a household power source can be used to charge the power storage device such that a vehicle charging port is connected to a plug socket provided in a house by a charging cable. 
     Meanwhile, a power transmission method attracting attention in recent years is wireless power transmission in which neither a power supply code nor a power transmission cable is used. Three known technical methods are prevalently employed in the wireless power transmission; power transmission by using electromagnetic induction, power transmission by using electromagnetic wave, and power transmission by using a resonance technique. 
     The resonance technique is a non-contact power transmission technology wherein a pair of resonators (for example, a pair of self-resonant coils) is resonated in an electromagnetic field (near field) so that electric power is transferred by way of the electromagnetic field. This technique enables the transmission of such a large power as a few kW over a relatively long distance (for example, a few meters) (see the Patent Document 1 and Non-Patent Document 1). 
     Japanese Patent Laying-Open No. 2008-87733 (Patent Document 2) recites a non-contact power feeding device for transmitting power based on the mutual dielectric effect of electromagnetic induction. The non-contact power feeding device feeds electric power from a primary coil for feeding power to a secondary coil for receiving power, wherein the primary coil and the secondary coil both have a circular shape in cross section. 
     Patent Document 1: Japanese Patent Laying-Open No. 2008-87733 
     Patent Document 2: WO 2007/008646 
     Non-Patent Document 1: Andre Kurs a al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances”, [online], Jul. 6, 2007, SCIENCE, Volume 317, p. 83-86, [Searched on Sep. 12, 2007], Internet &lt;URL; http://www.sciencemag.org/cgi/reprint/317/5834/83.pdf&gt; 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In wireless power transmission and reception devices in which the resonance technique is employed, a self-resonant coil for transferring power by way of an electromagnetic field is provided. The self-resonant coil has a circular shape in its cross section perpendicular to a direction where the self-resonant coil extends. 
     The transmission and the reception of electric power generates a high-frequency electric flow in the self-resonant coil. It is a technical common knowledge that a current density is higher on a surface of the coil than other parts and lower with more distance from the surface (skin effect) when a high-frequency electric flows in the coil. 
     Due to the effect, the primary coil and the secondary coil recited in Japanese Patent Laying-Open No. 2008-87733 (Patent Document 1) have a rather small current-carrying area, adversely increasing an electrical resistance. 
     The wireless power transmission and reception devices are often loaded and used in, for example, a vehicle. Therefore, it is highly necessary to form these devices in a compact size. 
     The present invention was carried out in view of these conventional disadvantages. A main object of the present invention is to provide a self-resonant coil achieving the reduction of an electrical resistance and formed in a compact size, a non-contact electric power transfer device having the self-resonant coil, and a vehicle equipped with the non-contact electric power transfer device. 
     Means for Solving the Problems 
     A self-resonant coil according to an aspect of the present invention is a self-resonant coil to be used in a non-contact electric power transfer device for transferring electric power by using magnetic field resonance. As a virtual coil is used a coil having a circular shape in its cross section perpendicular to a direction where the coil extends, wherein a circumferential length that defines the cross section is equal to a length of a line segment that defines a cross-sectional outer peripheral edge of the self-resonant coil when the self-resonant coil is observed in its cross section perpendicular to a direction where the self-resonant coil extends. At least one of a radial width and an axial length of the self-resonant coil in its cross section perpendicular to the direction where the self-resonant coil extends is smaller than a cross-sectional diameter of the virtual coil. 
     A self-resonant coil according to another aspect of the present invention is a self-resonant coil to be used in a non-contact electric power transfer device for transferring electric power by using magnetic field resonance. The self-resonant coil has first and second main surfaces facing each other. In a cross section of the self-resonant coil, at least a part of a center line passing through between the first main surface and the second main surface extends so as to intersect with a virtual axis line extending in a radial direction of the self-resonant coil. 
     A self-resonant coil according to still another aspect of the present invention is a self-resonant coil to be used in a non-contact electric power transfer device for transferring electric power by using magnetic field resonance. A cross section of the self-resonant coil perpendicular to a direction where the self-resonant coil extends has a shape obtained by bending or curving a plate-shape member having main surfaces disposed in an axial direction of the self-resonant coil in the axial direction of the self-resonant coil. 
     A self-resonant coil according to still another aspect of the present invention is a self-resonant coil to be used in a non-contact electric power transfer device for transferring electric power by using magnetic field resonance. A cross section of the self-resonant coil perpendicular to a direction where the self-resonant coil extends has a substantially U shape or a substantially V shape. 
     The cross section of the self-resonant coil thus having the substantially U shape or the substantially V shape preferably forms a channel portion open in one of axial directions of the self-resonant coil, wherein the channel portion accommodates at least a part of the self-resonant coil axially adjacent to a position of the channel portion. 
     A curvature of a bottom section of the channel portion is preferably progressively smaller from an end side of the self-resonant coil in one of axial directions toward an end side of the self-resonant coil in the other axial direction. 
     The self-resonant coil is preferably provided with a dielectric member disposed between the first main surface and the second main surface. 
     A non-contact electric power transfer device according to the present invention is provided with the self-resonant coil, and a primary coil for transferring electric power to and from the self-resonant coil by using electromagnetic induction. 
     Effects of the Invention 
     The self-resonant coil, the non-contact electric power transfer device and the non-contact electric power transfer device according to the present invention enable the reduction of an electrical resistance and downsizing of the coil per se. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall view of a power feeding system according to a preferred embodiment of the present invention. 
         FIG. 2  is a diagram for describing the rationale of power transmission in which a resonance technique is employed. 
         FIG. 3  is a graph illustrating a relationship between a distance from a current source (magnetic current source) and an electromagnetic field intensity. 
         FIG. 4  is a perspective view schematically illustrating a secondary self-resonant coil  110 . 
         FIG. 5  is a sectional view of second self-resonant coil  110  in a cross section thereof perpendicular to a direction where secondary self-resonant coil  110  extends. 
         FIG. 6  is a sectional view of a part of secondary self-resonant coil  110  in a cross section thereof along the direction of a center axis line O 1 . 
         FIG. 7  is a sectional view illustrating a modified embodiment of a spirally-wound state of secondary self-resonant coil  110 . 
         FIG. 8  is a sectional view illustrating a first modified embodiment of a cross-sectional shape of secondary self-resonant coil  110 . 
         FIG. 9  is a sectional view illustrating a second modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . 
         FIG. 10  is a sectional view illustrating a third modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . 
         FIG. 11  is a sectional view illustrating a fourth modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . 
         FIG. 12  is a sectional view illustrating a fifth modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . 
     
    
    
     DESCRIPTION OF THE REFERENCE SIGNS 
     
         
         
           
               100  electric vehicle,  110  secondary self-resonant coil,  120  secondary coil,  130  rectifier,  140  converter,  150  power storage device,  170  motor,  190  communication device,  200  power feeding device,  210  alternating current source,  220  high-frequency electric power driver,  230  primary coil,  240  primary self-resonant coil,  250  communication device,  310  high-frequency power source,  317 ,  320  primary coil,  330  primary self-resonant coil,  340  secondary self-resonant coil,  350  secondary coil,  360  load,  404  capacitor,  420 ,  421  main surface,  422 ,  425 ,  426  bottom section,  423 ,  424 ,  427 ,  428  axially-extending section,  430  non-contact power reception device,  440  virtual circular coil,  441  virtual rectangular coil,  445  dielectric member,  446  channel portion,  500  center line. 
           
         
       
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention is described in detail referring to the drawings. Any identical or corresponding constitutive elements are simply shown with the same reference symbols to avoid redundant description. 
       FIG. 1  is an overall view of a power feeding system according to a preferred embodiment of the present invention. Referring to  FIG. 1 , the power feeding system is equipped with a non-contact electric power reception device (non-contact electric power transfer device) provided in an electric vehicle  100 , and a power feeding device (non-contact electric power transfer device)  200  provided outside of the vehicle. The non-contact electric power reception device is provided with a secondary self-resonant coil  110 , a secondary coil  120 , a rectifier  130 , a DC/DC converter  140 , and a power storage device  150 . Electric vehicle  100  is provided with, in addition to the power reception device, a power control unit (hereinafter, may be referred to as “PCU”)  160 , a motor  170 , a vehicle ECU (Electronic Control Unit)  180 , and a communication device  190 . 
     Secondary self-resonant coil  110  is provided in a lower section of the vehicle, however, may be provided in an upper section of the vehicle as far as power feeding device  200  can also be provided in the upper section of the vehicle. Secondary self-resonant coil  110  is an LC resonant coil in which both ends are open (non-contact). Secondary self-resonant coil  110  resonates with a primary self-resonant coil  240  (described later) of power feeding device  200  by way of an electromagnetic field so that electric power is received from power feeding device  200 . In this description, a coil stray capacitance is a capacitance component of secondary self-resonant coil  110 , or a capacitor to be connected to the both ends of the coil may be provided otherwise. 
     The number of windings of secondary self-resonant coil  110  can be suitably set so that a Q value indicating a resonance strength between primary self-resonant coil  240  and secondary self-resonant coil  110  (for example, Q&gt;100) and K indicating a degree of coupling therebetween show larger values based on a distance between secondary self-resonant coil  110  and primary self-resonant coil  240  of power feeding device  200  and resonance frequencies of primary self-resonant coil  240  and secondary self-resonant coil  110 . 
     Secondary coil  120  is provided coaxial with secondary self-resonant coil  110  and can be magnetically coupled with secondary self-resonant coil  110  through electromagnetic induction. Secondary coil  120  retrieves electric power received by secondary self-resonant coil  110  by using electromagnetic induction and outputs the retrieved power to rectifier  130 . Rectifier  130  rectifies an alternating current retrieved by secondary coil  120 . 
     DC/DC converter  140  converts the electric power rectified by rectifier  130  into a voltage level of power storage device  150  based on a control signal transmitted from vehicle ECU  180  and outputs a conversion result thereby obtained to power storage device  150 . In the case where electric power is received from power feeding device  200  while the vehicle is traveling (power feeding device  200  is provided in the upper section or either of side sections of the vehicle in that case), DC/DC converter  140  may convert the power rectified by rectifier  130  into a system voltage and directly send a conversion result thereby obtained to PCU  160 . DC/DC convert  140  is not an indispensable constitutive element, and the alternating current retrieved by secondary coil  120  may be rectified by rectifier  130  and then directly imparted to power storage device  150 . 
     Power storage device  150  is a rechargeable direct current power source, including a lithium-ion or nickel-hydrogen secondary battery. In power storage device  150 , electric power supplied from DC/DC converter  140  and regenerative electric power generated by motor  170  are stored. Then, power storage device  150  supplies the power stored therein to PCU  160 . A capacitor having a large capacitance can be used as power storage device  150 , and any electric power buffer is usable as far as it can temporarily store therein the power supplied from power feeding device  200  and the regenerative electric power generated by motor  170  and supply the stored power to PCU  160 . 
     PCU  160  drives motor  170  using the electric power outputted from power storage device  150  or the electric power directly supplied from DC/DC converter  140 . Further, PCU  160  rectifies the regenerative electric power generated by motor  170  and outputs the rectified regenerative electric power to power storage device  150  in order to charge power storage device  150 . Motor  170  is driven by PCU  160 , and a vehicle drive force thereby generated is outputted to driving wheels. Motor  170  generates electric power using a kinetic energy received from driving wheels and an engine not shown, and outputs the generated regenerative power to PCU  160 . 
     When the vehicle is traveling, vehicle ECU  180  controls PCU  160  based on a traveling status of the vehicle and a state of charge in power storage device  150  (hereinafter, may be referred to as “SOC”). Communication device  190  is a communication interface for wirelessly communicating with power feeding device  200  outside of the vehicle. 
     Power feeding device  200  includes an alternating current power source  210 , a high-frequency electric power driver  220 , a primary coil  230 , a primary self-resonant coil  240 , a communication device  250 , and an ECU  260 . 
     Alternating current power source  210  is a power source provided outside of the vehicle, for example, a system power supply. High-frequency electric power driver  220  converts electric power received from alternating current power source  210  into high-frequency electric power and supplies the converted high-frequency electric power to primary coil  230 . The high-frequency electric power generated by high-frequency electric power driver  220  has a frequency in the range of, for example, 1 MHz-10-odd MHz. 
     Primary coil  230  is provided coaxial with primary self-resonant coil  240 , and can be magnetically coupled with primary self-resonant coil  240  through electromagnetic induction. Primary coil  230  supplies the high-frequency electric power from high-frequency electric power driver  220  to primary self-resonant coil  240  by using electromagnetic induction. 
     Primary self-resonant coil  240  is provided near the ground, or may be provided in the upper section of the vehicle in the case where electric vehicle  100  is supplied with power from the upper section of the vehicle. Primary self-resonant coil  240  is also an LC resonant coil in which both ends are open (non-contact). Primary self-resonant coil  240  resonates with secondary self-resonant coil  110  of electric vehicle  100  by way of an electromagnetic field so that electric power is transmitted to electric vehicle  100 . In a manner similar to the earlier description, a capacitance component of primary self-resonant coil  240  corresponds to a coil stray capacitance. However, a capacitor to be connected to the both ends of the coil may be provided. 
     The number of windings of primary self-resonant coil  240  can also be suitably set so that the Q value (for example, Q&gt;100) and κ indicating the degree of coupling shows larger values based on the distance between primary self-resonant coil  240  and secondary self-resonant coil  110  of electric vehicle  100  and resonance frequencies of primary self-resonant coil  240  and secondary self-resonant coil  110 . 
     Communication device  250  is a communication interface for wirelessly communicating with electric vehicle  100  to be fed with power. ECU  260  controls high-frequency electric power driver  220  so that electric power received by electric vehicle  100  reaches a target value. More specifically, ECU  260  obtains the power received by electric vehicle  100  and its target value by using communication device  250 , and controls outputs of high-frequency electric power driver  220  so that the power received by electric vehicle  100  is equal to the target value. ECU  260  can transmit an impedance value of power feeding device  200  to electric vehicle  100 . 
       FIG. 2  is a diagram for describing the rationale of power transmission in which a resonance technique is employed. According to the resonance technique illustrated in  FIG. 2 , two LC resonant coils having an equal natural frequency resonate with each other in an electromagnetic field (near field) in a manner similar to the resonance of two tuning forks, so that electric power is transmitted from one of the coils to the other by way of the electromagnetic field. 
     More specifically, primary coil  320  is connected to high-frequency electric power source  310  so that primary self-resonant coil  330  magnetically coupled with primary coil  320  by the electromagnetic induction is fed with electric power having such a high frequency as 1 MHz to ten-odd MHz. Primary self-resonant coil  330  is an LC resonator constructed from its own inductance and stray capacitance, resonating with secondary self-resonant coil  340  having a resonance frequency equal to that of primary self-resonant coil  330  by way of the electromagnetic field (near field). As a result, an energy (electric power) is transferred from primary self-resonant coil  330  to secondary self-resonant coil  340  by way of the electromagnetic field. The energy (electric power) transferred to secondary self-resonant coil  340  is retrieved by secondary coil  350  magnetically coupled with secondary self-resonant coil  340  by the electromagnetic induction and imparted to load  360 . The power transmission by means of the resonance technique can be carried out when a Q value indicating a resonance strength between primary self-resonant coil  330  and secondary self-resonant coil  340  is, for example, larger than 100. 
     Describing a correspondence relationship between  FIGS. 1 and 2 , alternating current source  210  and high-frequency electric power driver  220  illustrated in  FIG. 1  correspond to high-frequency power source  310  illustrated in  FIG. 2 . Further, primary coil  230  and primary self-resonant coil  240  illustrated in  FIG. 1  respectively correspond to primary coil  320  and primary self-resonant coil  330  illustrated in  FIG. 2 , and secondary self-resonant coil  110  and secondary coil  120  illustrated in  FIG. 1  respectively correspond to secondary self-resonant coil  340  and secondary coil  350  illustrated in  FIG. 2 . Rectifier  130  and other constitutive elements behind it illustrated in  FIG. 1  are collectively illustrated as load  360 . 
       FIG. 3  is a graph illustrating a relationship between a distance from a current power source (magnetic current source) and a electromagnetic field intensity. Referring to  FIG. 3 , the electromagnetic field includes three components. A curve k 1  is a component in inverse proportion to a distance from a wave source, generally called “radiation field”. A curve k 2  is a component in inverse proportion to the square of the distance from the wave source, generally called “induction field”. A curve k 3  is a component in inverse proportion to the cube of the distance from the wave source, generally called “electrostatic field”. 
     The “electrostatic field” is a region where an electromagnetic intensity sharply drops over the distance from the wave source. The resonance technique leverages a near field (evanescent field) where the “electrostatic field” is dominant in order to transfer an energy (electric power). More specifically, a pair of resonators having an equal natural frequency (for example, a pair of LC resonant coils) is resonated in the near field where the “electrostatic field” is dominant, so that the energy (electric power) is transferred from one of the resonators (primary self-resonant coil) to the other resonator (secondary self-resonant coil). The “electrostatic field” does not transmit the energy over a long distance. According to the resonance technique, therefore, the power transmission can be accomplished with less energy loss than in the electromagnetic wave that transmits the energy (electric power) using the “radiation field” in which the energy is transmitted farther. 
     Non-contact electric power reception device  430  includes secondary self-resonant coil  110  and secondary coil  120  illustrated in  FIG. 1 . The vehicle is equipped with a non-contact electric power reception device for receiving electric power from a power transmission coil for transmitting electric power using power supplied from a power source outside of the vehicle. 
       FIG. 4  is a perspective view schematically illustrating secondary self-resonant coil  110 . As illustrated in  FIG. 4 , secondary self-resonant coil  110  is formed in a spirally-wound shape with its center on a center axis line O 1 .  FIG. 5  is a sectional view of second self-resonant coil  110  in its cross section perpendicular to a direction where secondary self-resonant coil  110  extends. As illustrated in  FIG. 5 , a cross section  450  perpendicular to the direction where secondary self-resonant coil  110  extends has a substantially U shape. 
     A virtual circular coil  440  illustrated with a dashed line in  FIG. 5  extends in a spiral shape in a manner similar to secondary self-resonant coil  110 , and a cross section of virtual circular coil  440  perpendicular to a direction where it extends has a circular shape. A circumferential length of virtual circular coil  440  that defines a cross-sectional outer peripheral edge thereof is equal to a length of a line segment that defines an outer peripheral edge of cross section  450  of secondary self-resonant coil  110 . It is a technical common knowledge that, when a high-frequency current is passed through a coil wire, the current mostly runs on a surface of the coil wire (skin effect). The cross-sectional circumferential length of virtual circular coil  440  is equal to the length of the cross-sectional outer peripheral edge of secondary self-resonant coil  110 . Therefore, an electrical resistance generated when the high-frequency current runs through virtual circular coil  440  is equal to an electrical resistance generated when the high-frequency current runs through secondary self-resonant coil  110 . 
     As is clearly known from  FIG. 5 , an area of cross section  450  of secondary self-resonant coil  110  is set to be smaller than an area of the cross section of virtual circular coil  440 . Thus, secondary self-resonant coil  110  is reduced in size as compared with virtual circular coil  440 . More specifically, the cross-sectional shape of secondary self-resonant coil  110  is reduced in size as compared with the cross-sectional shape of virtual circular coil  440  in both a radial width and an axial height. 
     Comparing secondary self-resonant coil  110  to the virtual circular coil in which the cross section perpendicular to the direction where the coil extends has an area equal to that of cross section  450 , the line segment that defines the outer peripheral edge of cross section  450  of secondary self-resonant coil  110  is longer than the line segment that defines the cross-sectional outer peripheral edge of the virtual circular coil. 
     Accordingly, the electrical resistance generated in secondary self-resonant coil  110  when the high-frequency current is passed therethrough can be controlled to be smaller than the electrical resistance of the virtual circular coil. 
     It is known from the description given so far that secondary self-resonant coil  110  having the U shape contributes to reduction of the coil dimensions and alleviation of the electrical resistance generated by the high-frequency current. 
     Secondary self-resonant coil  110  has a shape obtained by axially curving both ends of a virtual rectangular coil  441  in a radial direction illustrated with a broken line in  FIG. 5 . 
     Virtual rectangular coil  441  is a coil having a spirally-wound shape in a manner similar to secondary self-resonant coil  110 . A cross section of virtual rectangular coil  440  perpendicular to a direction where virtual rectangular coil  441  extends has such a rectangular shape that a main surface  442  and a main surface  443  are disposed in the direction of center axis line O 1 . 
     Secondary self-resonant coil  110  has the shape obtained by axially curving the ends of virtual rectangular coil  441  in the radial direction as described earlier. Therefore, a length of a line segment that defines a cross-sectional outer peripheral edge of virtual rectangular coil  441  is equal to the length of the line segment that defines the outer peripheral edge of cross section  450  of secondary self-resonant coil  110 . Accordingly, the skin effect makes an electrical resistance of virtual rectangular coil  441  in response to the high-frequency current become equal to the electrical resistance of secondary self-resonant coil  110 . 
     At the same time, secondary self-resonant coil  110  is formed such that at least one of the radial ends of virtual rectangular coil  441  is bent or curved in the direction of center axis line O 1 . Therefore, a width of cross section  450  of secondary self-resonant coil  110  in a radial direction L 2  is smaller than a radial width of virtual rectangular coil  441 . Thus, secondary self-resonant coil  110  is radially downsized. 
     The downsizing is particularly attained in the radial dimension of cross section  450  because the cross section of secondary self-resonant coil  110  has the substantially U shape, and the radial ends of secondary self-resonant coil  110  on both sides are bent in the direction of center axis line O 1 . 
     Secondary self-resonant coil  110  is provided so that a main surface  420  and a main surface  421  thereof face each other in the direction of center axis line O 1 . Main surface  420  and main surface  421  are both curved in an arc shape, and main surface  420  constitutes a channel portion  446 . Channel portion  446  is formed to be open in an axial direction L 1  included in the direction of center axis line O 1 . 
       FIG. 6  is a sectional view of a part of secondary self-resonant coil  110  in its cross section along the direction of center axis line O 1 . 
     As illustrated in  FIG. 6 , a dielectric member  445  fills a space between main surface  420  of secondary self-resonant coil  110  that forms channel portion  446  and main surface  421  thereof adjacent to main surface  420  in the axial direction L 1 . Accordingly, a stray capacitance having a predetermined capacitance can be obtained without separately providing a capacitor, and the stray capacitance can be used as the capacitance component of secondary self-resonant coil  110 . A material such as silicon is used for the dielectric member. 
     Secondary self-resonant coil  110  is formed so that a curvature that defines a bottom section of channel portion  446  is progressively smaller from the end thereof in the axial direction L 1  toward the other end thereof. More specifically, a bottom section P 1 , a bottom section P 2  and a bottom section P 3  are serially aligned from the end of secondary self-resonant coil  110  on the side of axial direction L 1  toward the other end side, and these bottom sections are formed so that respective curvature radiuses R 1 , R 2  and R 3  are increased in the order. Therefore, an opening width of channel portion  446  is increased from the side of axial direction L 1  toward the other end side. 
     According to the structure, channel portion  446  can accommodate at least a part of secondary self-resonant coil  110  closer to the side of axial direction L 1  than channel portion  446 . Since a part of secondary self-resonant coil  110  can be thus accommodated in channel portion  446 , secondary self-resonant coil  110  can have a reduced dimension in the direction of center axis line O 1 . In the case where secondary self-resonant coil  110  is provided in a floor panel, such a smaller dimension thus obtained in the direction of center axis line O 1  can prevent the coil from overly protruding from the floor panel. 
     In the example illustrated in  FIG. 6 , secondary self-resonant coil  110  is formed so that a part thereof is contained in channel portion  446 . However, secondary self-resonant coil  110  may be spirally wound such that a part thereof is not contained in channel portion  446 . 
       FIG. 7  is a sectional view illustrating a modified embodiment of the spirally-wound state of secondary self-resonant coil  110 . As illustrated in  FIG. 7 , secondary self-resonant coil  110  is formed in such a spirally-wound shape that respective winds are spaced from one another in the direction of center axis line O 1 . Accordingly, main surface  420  and main surface  421  are both exposed outward, allowing heat to be released outward from main surface  420  and main surface  421 . 
     A dielectric member  445  may fill a space between main surface  420  and main surface  421  as illustrated with a broken line in  FIG. 7 . In that case, side parts of a surface of dielectric member  445  in the radial direction of secondary self-resonant coil  110  are exposed outward. Then, heat transmitted to dielectric member  445  from main surfaces  420  and  421  of secondary self-resonant coil  110  to dielectric member  445  is released outward from the side surfaces of dielectric member  445 . 
     In the examples illustrated in  FIGS. 5 and 7 , secondary self-resonant coil  110  has main surfaces  420  and  421  facing each other and exposed outward, and at least a part of a center line  500  passing through between main surface  420  and main surface  421  extends so as to intersect with a virtual axis line O 2  extending along the radial direction of secondary self-resonant coil  110 . 
     In sections of secondary self-resonant coil  110  where center line  500  extends in the direction where it intersects with the virtual axis line O 2 , vector components in the radial direction are lessened, consequently leading to reduction of the radial width of secondary self-resonant coil  110 . 
     In the examples illustrated in  FIGS. 5 and 7 , center line  500  extends in the direction where it intersects with virtual axis line O 2  in any sections of secondary self-resonant coil  110  other than bottom section  422 . As a result, a remarkable reduction of the radial width can be accomplished. Another advantage is that main surfaces  420  and  421  exposed outward can release heat outward directly or by way of the other members, for example, the dielectric member. 
       FIG. 8  is a sectional view illustrating a first modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . As illustrated in  FIG. 8 , secondary self-resonant coil  110  may be formed to have a M letter in its cross section. In the example illustrated in  FIG. 8 , a plurality of bottom sections  422 ,  425  and  426  are formed, and axially-extending sections  423 ,  424 ,  427  and  428  extending in a direction where they intersect with virtual axis line O 2  are formed at positions radially adjacent to bottom sections  422 ,  425  and  426 . 
     When virtual rectangular coil  441  is bent or curved a plurality of times in the direction of center axis line O 1 , the radial width can be decreased, while the dimension in the direction of center axis line O 1  is still prevented from increasing. 
       FIG. 9  is a sectional view illustrating a second modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . As illustrated in  FIG. 9 , virtual rectangular coil  441  is not necessarily curved, and a shape obtained by bending virtual rectangular shape  441  may be used. 
       FIG. 10  is a sectional view illustrating a third modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . As illustrated in  FIG. 10 , secondary self-resonant coil  110  is not necessary formed by deforming virtual rectangular coil  441 . A shape obtained by deforming a virtual coil having an oval shape or an elliptical shape in its cross section may be used. 
       FIG. 11  is a sectional view illustrating a fourth modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . As illustrated in  FIG. 11 , a shape obtained by tilting virtual rectangular coil  441  so that center line  500  of virtual rectangular coil  441  intersects with virtual axis line O 2  is also an option. 
     When virtual rectangular coil  441  is thus tiltingly deformed, the radial width of secondary self-resonant coil  110  can be smaller than the radial width of virtual rectangular coil  441 , resulting in reduction of the radial dimension. 
       FIG. 12  is a sectional view illustrating a fifth modified embodiment of the cross-sectional shape of secondary self-resonant coil  110 . In the example illustrated in  FIG. 12 , a plurality of recessed portions (dented portions) or protruding portions are formed on an outer peripheral surface of secondary self-resonant coil  110 . Secondary self-resonant coil  110  thus formed can have a cross-sectional area smaller than that of virtual circular coil  440 . As a result, secondary self-resonant coil  110  can be produced in a compact size. 
       FIGS. 4 to 12  illustrate the possible shapes of secondary self-resonant coil  110 . These shapes of secondary self-resonant coil  110  can be applied to primary self-resonant coil  240  as well. 
     The non-contact electric power reception device described in the respective embodiments can be loaded in a variety of electric vehicles. Examples of the electric vehicles are a series/parallel hybrid car capable of splitting a mechanical power of an engine using a power split device and transmitting the split mechanical powers to driving wheels and a motor generator, in addition to hybrid cars of other types. More specifically, the present invention can be applied to such hybrid cars as a generally-called series hybrid car where an engine is exclusively used for driving a motor generator, and a vehicle drive force is generated solely by the motor generator, a hybrid car where, of a kinetic energy generated by an engine, a regenerative energy alone is collected as an electric energy, and a motor-assisted hybrid car where an engine is used as a principal mechanical power with occasional assistance from a motor whenever necessary. 
     The present invention is also applicable to an electricity-driven automobile where no engine is provided, a fuel battery car provided with a fuel battery as a direct current power source in addition to a power storage device, and an electric vehicle where no boost converter is provided. 
     The embodiments disclosed in this specification are merely the illustration of examples in all aspects and should not restrict the present invention by any means. The scope of the present invention is based on not the description of embodiments but the appended scope of claims, and it is intended to cover all of such modifications as fall within the scope of the appended claims and the meaning and scope of equivalent.