PRESENTED BY
SRINIVAS ARUN TEJ P

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Abstract -.The technology for wireless power transfer (WPT) is in the forefront of electronic development. Applications involving microwaves, solar cells, lasers, and resonance of electromagnetic waves have had the most recent success with WPT. The main function of wireless power transfer is to allow electrical devices to be continuously charged and lose the constraint of a power cord. Although the idea is only a theory and not widely implemented yet, extensive research dating back to the 1850’s has led to the conclusion that WPT is possible.
The three main systems used for WPT are microwaves, resonance, and solar cells. Microwaves would be used to send electromagnetic radiation from a power source to a receiver in an electrical device. The concept of resonance causes electromagnetic radiation at certain frequencies to cause an object to vibrate. This vibration can allow energy to be transmitted between the two vibrating sources. Solar cells, ideally, would use a satellite in space to capture the suns energy and send the energy back to Earth. This concept would help to solve the major energy crisis currently concerning most of the world. These ideas would work perfectly in theory, but converting the radio frequencies into electrical power and electrical power to radio frequencies are two main problems that are withholding this idea to become reality.
This paper will explore the technological applications of microwaves, resonance, and solar cells in WPT and explain the basic technique of transmitting power wirelessly. It will also include problems encountered during experimentation and recent advances in the field. The paper will also include the futuristic applications of WPT and its ability to solve the energy crisis.
The Beginning Of WPT
Electricity by today’s standards is considered an essential to life. Electricity has been the fuel for technological development since its first applications dating back to the late 16th century. This marvellous phenomenon, however, comes with a price. The cost of making electricity is harmful to the environment. The Energy Information Administration’s records show that nearly 50% of all electrical plants are high polluting coal plants. Major changes in the environment have occurred over the last 30 years that are detrimental to the future of this planet. If this path is left unchanged, scientists have predicted that certain parts of the world could be uninhabitable by 2050. The solution is to reduce greenhouse gas emissions into earth’s atmosphere through alternative power generation. One sustainable technology leading this charge is wireless power transfer (WPT).
The concept of WPT has been around since the mid 17th century. WPT is exactly what the name states; to transfer electrical power from a source to a device without the aid of wires. The founder of AC electricity, Nikola Tesla, was first to conduct experiments dealing with WPT. His initial experiment of lighting gas discharge lamps from over 25 miles away, wirelessly, was a success. His idea came from the notion that earth itself is a conductor that can carry a charge throughout the entire surface. Although his idea of a world system of WPT could never be properly funded, his initial research sparked the scientific world into a whole new theory of power generation. While Tesla’s experiments were not creating electricity, but just transferring it, his ideas can be applied to solve our energy crisis. His experiments sparked new ideas such as applications involving microwaves, lasers, resonance and solar cells. Each application has its respective drawbacks but also has the potential to aid this planet in its dying need for an alternative to creating power.
Today, portable technology is a part of every day life. Most commonly used devices no longer need to draw power from the supply continuously. But from portability emerges another challenge: energy. Almost all portable devices are battery powered, meaning that eventually, they all must be recharged–using the wired chargers currently being used. Now instead of plugging in a cell phone, PDA, digital camera, voice recorder, mp3 player or laptop to recharge it, it could receive its power wirelessly–quite literally, “out of thin air”.
How Does WPT Work
Wireless power transfer is a varied and complicated process. There is more than one system that works to complete the process. Three more scientifically sound ideas are space solar cells, lasers, and resonating electro-magnetic waves. While each process varies in the way the energy is collected and used, the mechanisms of converting from RF energy to DC energy and vise versa are the same for all WPT systems.
The process of converting DC to RF starts with the power- that power to be transmitted is first tapped from the main power grid at about 50Hz AC. The voltage is then reduced to a viable load for rectifying into DC.
The energy then is supplied to an oscillator-fed magnetron and electrons are emitted from the central terminal. A positively charged anode surrounds the inner cathode to attract the electrons. Due to the current flowing through the magnetron the, the magnetic field produced causes the electrons to experience the cyclotron effect.
The circling electrons pass resonating cavities of the magnetron and create a pulsating magnetic field which constitute an electromagnetic radiation in microwave frequency range. The voltage coming out of the rectifier that connects the AC grid to the magnetron controls the magnetron anode DC voltage.
Since the anode is attracting the electrons into it (the cyclotron effect), the DC voltage that is supplied to it will determine the strength of the magnetic field. The stronger the magnetic field the greater the force on the electrons through the resonating cavities. Although frequency of the radiation can be adjusted by varying the inductance or capacitances of the resonant cavities, the experimental transmitting frequencies with the highest success rate are 2.45GHz and 5.8GHz.
The process of ‘catching’ the energy for it to be used in the conversion back to DC has different obstacles than the process of transmission. A problem with transmitting RF energy long distances is that it will lose its strength due to free space propagation. In order to compensate for this loss, antennas are connected in arrays. One such device called the energy harvesting circuit, patented by University of Pittsburgh’s Dr. Marlin Mickle, consists of multiple antennae each tuned to a different portion of the frequency spectrum. This increases the RF energy absorbed thus increasing the efficiency of the transmission. A series-parallel assembly of Scohttky diodes (rectenna) are then used at the receiving end to convert the microwave power back into DC. These diodes contain a low standing power rating but RF qualities enabling it to rectify the incoming microwaves into usable energy.
Using resonating electromagnetic waves is the system that will most likely be seen in the near future in applications that demand less than 10m of transmission. When two objects vibrate at the same frequency, they create larger amplitude together, rather than standing alone. If an antenna resonating with a particular frequency is brought within a few meters of a receiving antenna resonating at the same frequency, then the energy can be ‘tunneled’ through space and into the receiving antenna to be rectified. The quantum phenomenon of tunneling allows the energy to travel through space without being propagated. In a sense, the energy being tunneled is able to cross the potential gap between the two antennae without losing any energy. This resonating causes electromagnetic waves to vibrate through space. The energy is then used to recharge a battery inside the device. Since no energy is lost during the transfer, any surrounding circuitry is not harmed.
Solar power is a truly unlimited energy supply. Using the resonating electromagnetic waves system in coordination with outer space solar cells, takes wireless power transfer to a new scale. In geosynchronous orbits, solar satellites would be illuminated by the sun’s rays 99% of the time. A constant transmission of energy from the satellites down to earth would prove that there would be no need for costly storage devices to hold excess energy. The theory includes massive outer space panels attached to these satellites that would continuously absorb the suns rays. The energy would be beamed back to Earth using the electromagnetic wave system. Wasted heat cause from the absorption and transmission can be radiated right back into space, eliminating the potential for overheating. This has been looked into extensively, especially for the reasons of the energy problems in the world today. Particular problems however occur when trying to implement a WPT system able to sustain such a demand as currently needed. Besides the cost and complexity to build such a large scale system, a recent study by the DOE shows that the solar collectors would cover many square miles just in space. The receiving collectors on earth would cover close to 50 square miles. Despite the setbacks, this form of WPT is receiving currently the most attention in the science world due to the fact that it can transmit energy at close to 85% efficiency.
The use of lasers to transfer energy is a much different process from those above. This process involves transferring energy from a source to a receiver by beaming a laser to an object with a solar cell receiver. This idea is possible but is highly inefficient. The laser would need a direct line of sight to the object it is charging. Also, converting electricity to a laser and back to electricity causes a loss of energy. Energy from the laser is absorbed into the atmosphere also causes a loss of energy. In theory the system would work, but it is not an efficient form of wireless power transfer and would not be worth the trouble that the system would cause.
WPT Today
WPT is becoming a world renowned idea and is in a position to change the society in countless ways. Each country is doing its part to contribute to the cause. Over the past 50 years countries such as the United States, Canada, France, Russia and Japan have brought feasible and scientifically sound ideas to this field .
Currently in the U.S., Dr Joseph Hawkins and William Brown are experimenting with a fully operational microwave powered helicopter that was publicly demonstrated in 1995 at a conference for WPT in Japan. Applying similar concepts as their helicopter project, the team has developed the concept of ESPAM, or electronically steerable phased array module. Proposed applications of such systems include power transfer from ground to air or ground to space through the use of microwaves. This could provide power to long-haul space flights, orbiting spacecraft and orbiting facilities.
France is in the forefront of European research on WPT. With France being dependent on relatively low polluting nuclear energy, the idea of having an additional cleaner source of power sparked the interests of many scientists. The most recent project is to extend utility grade power (~100kW) across a three km ravine on Reunion Island using WPT, where utility cable would be hazardous to the environment. Although this does not demonstrate a space to ground application, the concepts of transmitting and converting RF energy is independent of the distance needed to travel. On top of thier research, France also hosts world summit meetings to discuss space solar power, every two years.
Russian Professor Vladimir Vanke has made the most significant contribution to solving the hurdles of WPT. In order for microwaves to be used as electricity, the radiant energy needs to be converted into DC power. Vanke solved this problem with the invention of the cyclotron wave converter.