Wireless Energy Transfer

The term Wireless power or Wireless energy transfer would not exist if wired power transmission systems would not be that familiar and self-evident. In order to precisely define wire­less power transmission, a brief overview of power transmission systems throughout recent history is given first.

In early industrial environments, the power to be transmitted was mostly me­chanical in nature, and hence were the transmission systems. Drive shafts, gears, hydraulic and pneumatic connections are examples of mechanical power transmis­sion systems that are still encountered in today's machines. The transportation of energetic compounds (fuel) allowed long-range energy transfer, as it still does to­day. With the emergence of electric machines, lighting and communication during the 19^lh century, more and more power was demanded in electric form. Unlike me­chanical power, electrical power is fit for long-distance transmission, which proved to be an important virtue. The generation and distribution of three-phase alternat­ing current, as it is known today, made its advent at the end of the 19^th century. In this way, remote energy sources become available to any consumer connected to the grid.

Conductive cables were, and remain to be the norm in electric power transmis­sion because of their ability to guide an electromagnetic energy flow precisely to where it is required. It is only at elevated frequencies, in the infrared and optical region of the spectrum, that dielectric waveguides are better suited for guiding an electromagnetic energy flow. Also in electric communication systems, electromag­netic power is transmitted from sender to receiver. The best performance is hence achieved over a wired channel, be it a conductive cable or an optical fiber. The first long-distance communication link based on electric signaling is the electric tele­graph, dating back from 1831.

Wireless power transmission implicitly refers to electric power. The term "wire­less" indicates that the wire connection normally used is left out. The power trans­mission still occurs through an electromagnetic field, but is no longer guided by a cable. Broader definitions of wireless power transmission may be encountered, in which the absence of any solid connection is the criterion. According to this latter definition, acoustic links for instance qualify as wireless powering systems. In this text however, the nature of the transmission is assumed to be electromagnetic. This definition is further narrowed down to the non-ionizing frequency range, in which electromagnetic fields and waves can be treated in a classical way.

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