Best Paper Award
Miscellaneous Electromagnetic Phenomena in Wireless Power Transfer under Water
Ikuo Awai, Yuichi Sawahara, Kazuya Yamaguchi, Masashi Hotta, Toshio Ishizaki
[Trans. Commun.iJPN Edition), Vol. J96-B No.11, Nov. 2013]

Ikuo Awai
Yuichi Sawahara
Kazuya Yamaguchi
Masashi Hotta
Toshio Ishizaki
  Since the coupled-resonator wireless power transfer (WPT) system is expected to have a great potential for midrange uses, it is drawing fervent attention from automobile, architecture, consumer electronics, medical, marine and other businesses, and its research and development is being carried out in accordance with the demands of each field. The approach by those groups, however, is rather poorly backed by the basic theories such as electric circuit, semiconductor circuit or electromagnetic theory, and thus requires support from those specialists. The present authors have studied underwater WPT systems, aiming at unearthing the problems in it experimentally as specialists of electromagnetic theory, and reported the result in the present paper.
  First of all, it is to be noted that they found quantitatively that water has property of a conductive solvent of electrolytes, and the real part of the permittivity is not affected by the electrolyte whereas the imaginary part is increased appreciably. The real part influences the resonant frequency, while the imaginary part, the loss, and the latter eventually deteriorates the power transfer efficiency, as expected. Neither part of permittivity, however, was found to affect the coupling coefficient between resonators.
  Secondly, although those effects are decreased by the use of magnetic coupling, it is known that confinement of the electric field becomes less effective owing to the induced conduction current through the magnetic field at higher salinities.
  The 3rd result is the refraction of the electric and magnetic fields at the material boundary (produced by aligning PET bottles filled with water), whereby focusing effect of fields are induced, resulting in increase of power transfer efficiency by the gfield guideh, which resembles a waveguide. It will help extend the outreach of WPT systems.
  All the results above are somehow understandable, if not explained perfectly theoretically, but the 4th result is difficult to understand. The authors have found that power transfer loss increases according to the increases in salinity, but it decreases once to its minimum and then again steadily increases.
  It is exciting that these strange phenomena have remained unknown in the peripheral research fields, and thus engineers/researchers will enthusiastically attempt to identify and explain them.

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