05-09-2014, 12:38 PM
A SEMINAR REPORT ON WIRELESS ELECTRICITY (WiTricity)
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INTRODUCTION
Today we do live in the “wireless age”, in which the air that we breathe probably contains more information than oxygen. However, this is also an age where mobile phones, MP3 players, laptop computers and domestic robots exist alongside old-fashioned power wires and bulky batteries. Unlike information, electrical energy is still physically confined to these borderline anachronistic appliances. Overcoming these last obstacles would finally make this a truly wireless world. It all started a few years ago when Marin Soljac`´ic´, a physicist at the Massachusetts Institute of Technology(MIT) in the US, was driving back home one cold winter night and he heard an unfriendly beep from his mobile phone. It was the annoying reminder that the battery was running out, once again. It then suddenly occurred to Soljac`´ic´how great it would be if the mobile phone could take care of its own charging. The next morning, he returned to his office at the MIT determined to find a solution to the problem. An exhaustive literature search soon revealed that wireless transmission of power was not an original idea. Back in the 1890s Nikola Tesla, one of great pioneers of electromagnetism, was the first to envisage that electricity, then a newly found form of energy, should be delivered to every house, in every city, in every country on the planet. However, Tesla did not foresee that people would be willing to drag wires around the entire globe to use electricity. Instead, he dreamed of a way of transferring electrical energy wirelessly over long distances. This would be achieved using big, coupled electromagnetic resonators able to generate very large electric fields, which were meant to propagate most likely either via conduction through the ionosphere (presumably including gigantic sparks) or through the Earth (possibly via intermediate coupling to the Earth’s charge resonances, so-called Schumann resonances). The epitome of Tesla’s efforts to achieve his goal was Wardenclyffe Tower, a 57 m high structure in Long Island that was meant to deliver electricity to the entire planet. The construction was interrupted in about 1905, not because the method was considered impractical or dangerous, but because the funder, the famed financier and banker J P Morgan, was concerned that there would be no way to bill remote electricity users. Nowadays, more than a century after Tesla, electricity reaches nearly every home through a global electrical grid. Nevertheless, J P Morgan’s objections meant a premature end to the first attempt at wireless electricity. In the near future, wireless electricity could replace the ubiquitous power cable.
WHY DO WE GO FOR WIRELESS POWER TRANSMISSION (WPT)SYSTEM?
2.1-LOSSES IN THE WIRED SYSTEM
Professor Rauscher showed that the earth’s magnetosphere contains sufficient potential energy (at least 3 billion kilowatts) so that the resonant excitation of the earth-ionosphere cavity can reasonably be expected to increase the amplitude of natural “Schumann” frequencies, facilitating the capture of useful electrical power. Tesla also knew that the earth could be treated as one big spherical conductor and the ionosphere as another bigger spherical conductor, so that together they have parallel plates and thus, comprise a “spherical capacitor.” Dr. Rauscher calculates the capacitance to be about 15,000 microfarads for the complete earth-ionosphere cavity capacitor. W.O. Schumann is credited for predicting the “self-oscillations” of the conducting sphere of the earth, surrounded by an air layer and an ionosphere in 1952, without knowing that Tesla had found the earth’s fundamental frequency fifty years earlier.In comparison to the 3 billion kW available from the earth system, it is possible to calculate what the U.S. consumed in electricity. In 2000, about 11 Quads (quadrillion Btu) were actually used by consumers for electrical needs, which is equal to 3.2 trillion kWh. Dividing by the 8760 hours in a year, we find that only 360 million kW are needed on site to power our entire country. This would still leave 2.6 billion kW for the rest of the world! The reallyshameful U.S. scandal, unknown to the general public, is that out of the total electrical power generated using wire transmission (about 31 Quads), a full 2/3 is totally wasted in “conversion losses.” (Ref. Electricity Flow Chart 1999, which contains US DOE/EIA data, updating the Toby Grotz article in this book.) No other energy production system of any kind in the world has so much wastefulness. Instead of trying to build 2 power plants per week (at 300 MW each) for the next 20 years (only to have a total of additional 6 trillion kWh available by 2020), as some U.S. government officials want to do, we simply need to eliminate the 7 trillion kWh of conversion losses in our present electricity generation modality. Tesla’s wireless transmission of power accomplishes
HISTORY OF WPT
• 1820: André-Marie Ampère develops Ampere’s law showing that electric current produces a magnetic field.
• 1831: Michael Faraday develops Faraday’s law of induction describing the electromagnetic force induced in a conductor by a time-varying magnetic flux.
• 1864: James Clerk Maxwell synthesizes the previous observations, experiments and equations of electricity, magnetism and optics into a consistent theory and mathematically models the behavior of electromagnetic radiation.
• 1888: Heinrich Rudolf Hertz confirms the existence of electromagnetic radiation. Hertz’s "apparatus for generating electromagnetic waves" was a VHF or UHF "radio wave" spark gap transmitter.
• 1891: Nikola Tesla improves Hertz-wave wireless transmitter RF power supply or exciter in his patent No. 454,622, "System of Electric Lighting."
• 1893: Tesla demonstrates the wireless illumination of phosphorescent lamps of his design at the World's Columbian Exposition in Chicago.
Fig-2 (wardencliffe tower)
• 1894: Hutin & LeBlanc, espouse long held view that inductive energy transfer should be possible, they received U.S. Patent # 527,857 describing a system for power transfer at 3 kHz.
• 1894: Tesla wirelessly lights up single-terminal incandescent lamps at the 35 South Fifth Avenue laboratory, and later at the 46 E. Houston Street laboratory in New York City by means of "electrodynamic induction," that is to say wireless resonant inductive coupling
THEORIES ON WPT
Near field theory
Near field is wireless transmission techniques over distances comparable to, or a few times the diameter of the device(s), and up to around a quarter of the wavelengths used. Near field energy itself is non radiative, but some radiative losses will occur. In addition there are usually resistive losses. Near field transfer is usually magnetic (inductive), but electric (capacitive) energy transfer can also occur.
Electrical conduction principle
Electrical energy can be transmitted by means of electrical currents made to flow through naturally existing conductors, specifically the earth, lakes and oceans, and through the upper atmosphere starting at approximately 35,000 feet (11,000 m) elevation — a natural medium that can be made conducting if the breakdown voltage is exceeded and the constituent gas becomes ionized. For example, when a high voltage is applied across a neon tube the gas becomes ionized and a current passes between the two internal electrodes. In a wireless energy transmission system using this principle, a high-power ultraviolet beam might be used to form vertical ionized channels
Electrostatic induction principle
The "electrostatic induction effect" or "capacitive coupling" is an electric field gradient or differential capacitance between two elevated electrodes over a conducting ground plane for wireless energy transmission involving high frequency alternating current potential differences transmitted between two plates or nodes. The electrostatic forces through natural media across a conductor situated in the changing magnetic flux can transfer energy to a receiving device (such as Tesla's wireless bulbs).Sometimes called "the Tesla effect" it is the application of a type of electrical displacement, i.e., the passage of electrical energy through space and matter, other than and in addition to the development of a potential across a conductor.
Experimental demonstration of evanescent coupling at MIT
Radiative modes of omni-directional antennas (which work very well for information transfer) are not suitable for such energy transfer, because a vast majority of energy is wasted into free space. Transmitting electricity within a room, namely over distances a few times greater than the size of the receiving devices themselves (what engineers define as mid-range distances), is a challenge for most modern applications. Achieving this goal with satisfactory efficiency, safety and low cost remains an unsolved problem. That was the challenge for Soljac`´ic´ and his collaborators at the MIT labs: John Joannopoulos, Peter Fisher, Andre Kurs, Robert Moffatt . The energy of any resonator naturally decays due to intrinsic energy-loss mechanisms (friction for mechanical resonances, radiation and resistive absorption for electromagnetic resonances, collisions with phonons and spontaneous emission for atomic resonances). Losses are typically quantified by the number of oscillation periods that it takes for the energy to decay by a factor of 2.72. This number, represented by the “quality factor” Q, is an intrinsic property of resonators and depends on the strength of the loss mechanisms. (As a simple analogue, water inside a bucket with a hole will leak out at a rate that depends on the size of the hole.) If two equal resonators exchange energy, it also takes a characteristic number of oscillation periods to transfer the energy from resonator A to resonator B, which is proportional to a constant that quantifies the strength of the coupling between the resonators, Qk. (If water is pumped from one bucket to another via a hose, then the transfer time depends on the strength of the pump.) Clearly, for energy transfer to be efficient, Qneeds to be much larger than Qk, i.e. the rate at which energy is being transferred needs to far exceed the rate at which energy is being lost. (Water will be efficiently transferred between two leaking buckets if the pump is faster then the leaks from the holes.)
Far field theory
Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). With radio wave and optical devices the main reason for longer ranges is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated Laser Beam) thereby delivering almost all emitted power at long ranges. The maximum directivity for antennas is physically limited by diffraction.The Raytheon Company did the first successful WPT experiment in 1963. In this experiment energy was transmitted with a DC-to-DC efficiency of 13%. This company also demonstrated a microwave-powered helicopter in 1964 [2]. The Jet propulsion lab of NASA carried out an experiment and demonstrated the transfer of 30 kW over a distance of 1 mile in 1975. They used an antenna array erected at the Goldstone facility. This test demonstrated 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 .
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. The current rates on the Space Shuttle run between $7,000 and $11,000 per kilogram of transported material. Recently the idea of Space Solar Power caught again the public attention e.g. by the Obama transition team and The Economist .
Compatibility with other radio services and applications
It is assumed that WPT systems working with microwaves use frequency bands around 2.45 GHz or 5.8 GHz. These bands are already allocated in the ITU-R radio regulation to a number of radio services. They are also designated for industrial science and medical (ISM applications). The ISM band is, as presently defined, for local use only. The 2.45 GHz is further more used for radio LAN and microwave ovens. The 5.8 GHz is also used heavily for various applications like Radiolocation service and DSRC (Dedicated Short-Range Communications). More investigation is needed to get an image of the possible influencing between the systems
OVERVIEW OF FUTURE WITH WIRELESS TRANSMISSION OF ELECTRICITY
As Tesla himself said,
“In the near future we shall see a great many uses of electricity…we shall be able to disperse fogs by electric force and powerful and penetrative rays…wireless plants will be installed for the purpose of illuminating the oceans…picture transmission by ordinary telegraphic methods will soon be achieved…another valuable novelty will be a typewriter electrically operated by the human voice…we shall have smoke annihilators, dust absorbers, sterilizers of water, air, food and clothing…it will become next to impossible to contract disease germs and country folk will go to town to rest and get well
CONCLUSION
This technology is a big impediment to development in the retail sector right now.
The wireless transfer of electricity has been a sci-fi dream up to this point ,and truly, if electricity could simply be in the air, in the same way radio waves and wi-fi signals are, it would change the world