27-12-2012, 12:19 PM
Seminar Report Witricity
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ABSTRACT
The technology used for wireless power transmission is known as witricity. Wireless power transmission is not a new idea, Nikola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. Tesla's work was impressive, but it did not immediately lead to wide spread practical methods for wireless power transmission. Since then many researchers have developed several techniques for moving electricity over long distances without wires. Some exist only as theories or prototypes, but others are already in use. In 2006 researchers at Massachusetts Institute of Technology led by Marine Soijacic discovered an efficient way to transfer power between coils separated by a few meters. They have dubbed this technology as witricity. Witricity is based upon coupled resonant objects. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while not interchanging the surroundings. The researchers demonstrate the ability to transfer 60W with approximately 40% efficiency over distance in excess of 2 meters. Currently the project is looking for power transmission in the range of 100watts. As witricity is in the development stage, lots of work is to be done in improving the range of power transmission and efficiency.
INTRODUCTION
If we are particularly organized and good with tie wrap then also a few dusty power cord tangles around our home. We have even had to follow one particular cord through the seemingly impossible snarl to the outlet hoping that the plug pull will be the right one. This is one of the downfalls of electricity. While it can make people's lives easier, it can add a lot of clutter in the process. For these reasons, scientists have tried to develop methods of wireless power transmission that could cut the clutter or lead to clean sources of electricity. Wireless power transmission is not a new idea. Many researchers developed several methods for wireless power transmission. But witricity is a new technology used for wireless power transmission. By the use of this technology transmission of electrical energy to remote objects without wires can be possible. The inventors of witricity are the researchers from Massachusetts Institute of Technology (MIT). They developed a new technology for wireless electricity transmission and this is based upon the coupled resonant objects. In this resonant magnetic fields are used. So the wastage of power is reduced. The system consists of witricity transmitters and receivers. The transmitters and receivers contain magnetic loop antennas made of copper coils and they are tuned to the same frequency.
METHODS USED FOR WIRELESS POWER TRANSMISSION
INDUCTION (INDUCTIVE COUPLING):
This is the first method used for wireless power transfer. The simplest example for wireless energy transfer using this method is the electrical transformer. In this the primary and secondary circuits are
electrically isolated from each other. The transfer of energy takes place by electromagnetic coupling through mutual induction. The main draw back of this method is the short range. For efficient working of a system which uses this method, the receiver must be in very close proximity to the-transmitter. A larger, stronger field can be used for energy transfer over lorjg distance, but this process is extremely inefficient. Since magnetic field spreads in all direction, making a large wastage of energy.
RESONANT INDUCTION (EVANESCENT WAVE COUPLING)
In 2006 MIT researchers discovered an efficient method to transfer power between coils separated by few meters. They extend the distance between coils in inductive coupling system by adding resonant. They demonstrated by sending electromagnetic waves around in a highly angular waveguide, evanescent waves are produced, which carry no energy. An evanescent wave is a near field standing wave exhibiting exponential decay with distance. Evanescent waves are always associated with matter, and are most intense within one-third wavelength from any radio antenna. Evanescent means tends to vanish, the intensity of evanescent waves decays exponentially with the distance from the interface at which they are formed. If a proper resonant waveguide is brought near the transmitter, the evanescent waves can allow the energy to tunnel to the power drawing wave guide. Since the electromagnetic waves would tunnel, they would not propagate through the air to be absorbed or dissipated and would not disrupt .electronic devices or cause physical injury like microwave or radio waves transmission.
In resonant induction method induction can take place a little differently if the electromagnetic fields around the coils resonate at the same frequency. In this a curved coils of wire uses as an inductor. A capacitance plate which can hold a charge attaches to each end of the coil. As electricity travels through this coil the coil begins to resonate. Its resonant frequency is a product of the inductance of the coil and the capacitance of the piate. Unlike multiple layer secondary of non-resonant transformer single layer solenoids with closely spaced capacitor plates on each end as shown in figure 1 is used as transmitter and receiver.
RADIO AND MICROWAVE
If resonance is incorporated or not, induction generally sends wireless power over relatively short distance. For very long distance power transmission radio and microwaves are used. Japanese researcher YAGI developed a directional array antenna known as YAGI antenna for wireless energy transmission. It is widely used for broadcasting and wireless telecommunications industries. While it did not prove to be particularly useful for power transmission. Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna is a rectifying antenna, an antenna used to convert microwaves into DC power. Being that an antenna refers to any type of device that converts electromagnetic waves into electricity or vice versa. A rectenna is simply a microwave antenna. Inverse rectennas convert electricity into microwave beams, rectennas suitable for receiving energy beamed from solar panels in geocentric orbit would need to be several miles across. Although power densities of such an arrangement would be low enough to avoid any damage to people or the environment. Rectifying antennae are usually made an array of dipole antennae, which have positive and negative poles. These antennae connect to semiconductor diodes. Rectenna conversion has an efficiency of about 95%.
WITRICITY IN HISTORY
Wireless power transmission is not a new idea. Nickola Tesla demonstrated transmission of electrical energy without wires in early 19th century. Tesla used electromagnetic induction systems. William C Brown demonstrated a micro wave powered model helicopter in 1964. This receives all the power needed for flight from a micro wave beam. In 1975 Bill Brown transmitted 30kW power over a distance of 1 mile at 84% efficiency without using cables. Researchers developed several technique for moving electricity over long distance without wires. Some exist only as theories or prototypes, but others are already in use.
CONCLUSION
Witricity is in development stage, lots of work is to be done to use it for wireless power applications. Currently the project is looking for power transmission in the range of 100w. Before the establishment of this technology the detailed study must be done to check whether it cause any harm on any living beings.