Nikola Tesla in his Colorado Laboratory with magnifying transmitter

Nikola Tesla in his Colorado Laboratory with magnifying transmitter in action generating 20 million volts of electricity

Introduction to Wireless Power Transmission (WPT)

The wireless power transmission (WPT) refers to the efficient transmission of electric power from one point to without use of wire or any other substance.

This technology can be used where conventional wires are unaffordable, inconvenient, expensive, hazardous, unwanted or impossible. It can also be used for applications where either an instantaneous amount or a continuous delivery of energy is needed. The power can be transmitted using microwaves, millimetre waves or lasers.  WPT is a technology that can transport power to locations, which are practically impossible to reach.

Famous Tesla Tower erected in Shoreham

The Famous Tesla Tower erected in Shoreham, Long Island, New York was 187 feet high, the spherical top was 68 feet in diameter. The Tower, which was to be used by Nikola Tesla is his "World Wireless" was never finished.

Maxwell’s theory of electromagnetism was published in 1865 which stated that  electromagnetic waves travels at the speed of light, and the light is  itself just  such  a  wave. In 1886  Hertz successfully performed a experiment with pulsed wireless energy transfer. He produced an apparatus which produced and detected microwaves in the UHF region.

In 1899 Tesla performed experiments in the field of pulsed wireless energy transfer. Tesla’s Magnifying Transmitter, an early type of Tesla Coil that measured 16 meters in diameter, was able to transmit tens of thousands of watts without wires.

In 1897, he filed his first patents dealing with Wardenclyffe tower. This tower was supposed be a pilot plant for his “World Wireless System” to broadcast energy around the globe. But he was not able to make it fully operational due to economic problems.

The Raytheon Company performed the first successful WPT experiment in 1963. During that experiment energy was transmitted with a DC-to-DC efficiency of 13%. In1975 the Jet propulsion lab of NASA carried out an experiment and demonstrated the transfer of 30 kW over a distance of 1 mile using an antenna array erected at the Goldstone facility. This test proved the possibilities of wireless power outside the laboratory.

Rockwell International and David Sarnoff Laboratory operated in 1991 a microwave powered rover at 5.86 GHz. Three kilowatts of power was transmitted and 500 watts was received.


The WPT technology

Electricity is needed to be transformed into a suitable energy form for its transportation. For wireless transmission, this has to be a form that can travel trough air.

Microwave frequencies hold this ability. The microwave spectrum is defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies.

Most common applications are within the 1 to 40 GHz range.

Apparatus for transmitting Electrical Energy by Nikola Tesla

Apparatus for transmitting Electrical Energy by Nikola Tesla

A complete microwave transmission system consists of three essential parts:

  1. Electrical power to microwave power conversion
  2. Absorption antenna that captures the waves
  3. (Re) conversion to electrical power

The components include a microwave source, a transmitting antenna and a receiving antenna. The microwave source consists of an electron tubes or solid-state devices with electronics to control power output. The slotted waveguide antenna, parabolic dish and microstrip patch are the most popular types.

Due to high efficiency (>95%) and high power handling capacity, the slotted waveguide antenna seems to be the best option for power transmission.

The combination of receiving and converting unit is called rectenna which is a rectifying antenna that is used to directly convert microwave energy into DC electricity. Rectenna includes a mesh of dipoles and diodes for absorbing microwave energy from a transmitter and converting it into electric power. Its elements are usually arranged in a multi element phased array with a mesh pattern reflector element to make it directional.

One of the disadvantages is that microwaves have long wavelengths that exhibit a moderate amount of diffraction over long distances. The Rayleigh criterion dictates that any beam will spread (microwave or laser), become weaker, and diffuse over distance. The larger the transmitter antenna or laser  aperture,  the  tighter  the  beam  and  the  less  it  will  spread  as  a function of distance (and vice versa).

Therefore, the system requires large transmitters and receivers. The used power density of the microwave beam is normally in de order of 100 W/m2. This is relative low compared to the power density of solar radiation on earth (1000 W/m2) and chosen this way for safety reasons.

Download Complete patents of Nikola Tesla in PDF (40Mb)

WPT Applications

Power transfer, bridging applications

Using a powerful focused beam in the microwave or laser range long distances can be covered. There are two methods of wireless power transmission for bridging application. First is the direct method, from transmitting array to rectenna. A line of sight is needed and is therefore limited to short (< 40 km) distances. Above 40 kilometers, huge structures are needed to compensate for the curvature of the earth.

The second method is via a relay reflector between the transmitter and rectenna. This reflector needs to be at an altitude that is visible for both transmitter and rectenna. This method is not discussed further. Next three bridging applications of WPT are discussed.


WTP for space solar

The largest application for microwave power transmission is space solar power satellites  (SPS).  In  this  application,  solar  power  is  captured  in space and converted into electricity. The electricity is converted into microwaves and transmitted to the earth. The microwave power will be captured with antennas and converted into electricity. NASA is still investigating the possibilities of SPS.

One of the problems is the high investment cost due to the space transport.


Bio-effects of Microwaves

The long-term exposure to low  levels  of microwaves might be unsafe and even could cause cancer. The scientific research indicates that heating of humans exposed to the radiation is the only known effect.

There are also many  claims  of  low-level  non-thermal  effects,  but  most  of  these  are difficult  to replicate  or  show  unsatisfying  uncertainties.

A clearly relevant bio-effect is the effect of microwave radiation on birds, the so-called “fried bird effect“.

Research is done on such effect at 2.45 GHz. The outcome showed slight thermal effects that probably are welcome in the winter and to be avoided in the summer. Larger birds tend to experience more heat stress then small birds.


author-pic

Bipul Raman - Bipul Raman, B.E.(Electrical & Electronics) had presented two research papers in National Conferences. He has worked as Project Trainee at HAL, NTPC and BSEB Substation. He has successfully completed a project to design an Automated CB Test-Rig for Light Combat Aircraft at Aircraft Research and Design Center (HAL). His areas of interests are Renewable Energy Sources, SCADA and Circuit Breakers. Visit his complete profile at http://bipul.in/about-me/.



6 Comments


  1. Rushikesh Deshmukh
    Mar 10, 2014

    Im 3rd year electrical engg student…Im trying to build my practical knowledge with ur articles.

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  4. hanuman.chinnari
    Aug 20, 2012

    Hiiii……everybody….This is prasad chinnari. I have complted my B- tech 1 year back and now presently working in a power plant as a Executive Engineer. I basically very interesrted in reading all technical related data. I have gone through many web sites that gives technical awareness, but I didnot find that technical data is readable except this site. Finally I suggest you all that follow this site for your career growth


    • Edvard
      Aug 20, 2012

      Thank you very much!! EEP is created for this purpose, education and learning, so I’m really glad to hear that you find it usefull!

      If you have any suggestion or remark, please write it down.

      Thank you again!


      • disha.karnataki
        Jun 02, 2013

        i am disha.karnataki here, now presently i am in 1st year in electronics electrical branch…
        this site has really expanded my knowledge about electrical field…
        my question is whether in wireless power transmission there is some improvement in efficiency or is it the same as before???
        like ex as given above that if 3kw of power was transmitted the received power was only about 500w
        so is there change in efficiency till today or it’s the same as before..

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