Wireless power transmission is the process of transmitting electrical energy to an electrical load, from a source emitting power without interconnecting wires. The wireless transfer of electromagnetic energy as in different forms like audio format, video format and data format is common nowadays but wireless power transfer technology is still years ahead. Even though the idea was developed 100’s of years ago and scientists are still working on the topic an efficient way for wireless transfer of power is yet to be developed. The development of highly efficient WPT technology will be a breakthrough in the history of wireless transmission as it allows lots of portable devices to be charged without plugging on to a socket or docked to a charging station.
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The development of WPT serves to be useful in many ways, as transmission of power in hazardous environment is inefficient and dangerous. The WPT differs from the wireless transmission of telecommunication signals as the WPT takes into consideration the efficiency parameter where as in the latter the energy parameter is significant only if the signal is distorted in such away that it cannot be distinguished. The WPT is economical if and only if the receiver receives the same amount of energy transmitted by the source.
The direct induction method is the most common method used for the WPT and is being used for short distance WPT, but it had disadvantages and was overcome by later technologies developed like the resonant magnetic induction method, electromagnetic radiation methods like microwaves and lasers. The modern era where the ideology of high efficiency in practical circuits comes to existence, the WPT technology proves wrong due to the low efficiency of the developed technologies. The WPT technology in industries by electromagnetic radiation is possibly only remote in the design and most of the industries are looking forward for better designs for better efficiency in power transfer. Most of such design lags the property of safe human environment for living and the economical barriers. A design of WPT system which overcomes these factors is still to be worked out and is being experimented throughout the world.
Why Wireless Power Transmission (WPT)?
The studies throughout the world has proven that most of the electrical energy transfer is done through the interconnection wires and lots of energy is lost during the transmission due to various internal and external factors like the resistance of the wires the material of the conduction wires, the size of the wires, the atmospheric conditions like temperature, humidity etc. On an average basis the loss of power due to transmission through wires is calculated to exceed to be more than 30%. This is where the WPT comes into action , the WPT is highly reliable, fast , low cost for the maintenance and can also be used for short range and long range distance transmissions. These factors pop-in the idea for WPT.
History of WPT
The WPT history can be traced from the early 19th century. The first electromagnet was developed in the year 1825 by William Sturgeon which paved the way for the discovery of principles of magnetic induction in the year 1831. This was followed by the demonstration of transmission and reception of electrical energy without wires to connect the point of origin and termination by Nicholas Joseph Callan using the above two developments. These experiments even though demonstrated was successful but in practical cases was always kept under suspicion which was due to the interpretance that it was not possible for long distance transmission and fear of safety.
The invention of transmission of very and ultra high frequency radio waves dedicated to Heinrich Hertz followed the revolutionary development of the wireless electricity transfer by Nikola Tesla in the year 1891, on which he took the patent-ship for illuminating the bulbs wirelessly. Nikola Tesla is called as the pioneer of the induction techniques as his technique was based on induction method. His vision of “WORLD WIRELESS SYSTEM” which inculcated the idea of people accessing the free energy. He even planned to set working a 187 feet tall tower for broadcasting energy but was put down due to lack of funds. The experimentation in early 20th century was successful in transmitting power over a distance wirelessly but was not able solve the problem of power loss due to hindrances. In the 19th century WPT passed through various phases of technological developments. Even after 100 years the idea of Tesla is being used by the MIT scientists led by Soljacic for their project named “WiTricity”.
Different technologies of WPT
Near-field techniques: The techniques for short distance transmission of power wirelessly. Mostly the induction methods are used for near-fields. These techniques are as follows.
(1)Inductive coupling
Electromagnetic induction is a process whereby a conductor placed in a changing magnetic field results in the production of voltage across a conducting plate. This voltage in-turn generates an electrical current called the induced current because the current on the second conductor is induced by the first. In the inductive coupling technique the primary and the secondary are not connected by wires but the energy is transferred by mutual induction. Mutual induction refers to the generation of EMF in circuit due to a change of current in nearby circuit.
Electromagnetic increases with the increase in current, the voltage in the conducting plate responsible for the magnetic fields and also the increase in the frequency. The energy is transmitted from the conducting plates which is responsible for the fields to another conductor termed the secondary where the fields impinge on. A part of the energy on the primary is passed inductively through space decreasing the energy in the primary and imparting energy into the secondary. High frequency currents are not liable to pass the current for long distances but transfers energy by induction rapidly to the adjacent or neighboring conductors. Lower the frequency of the current less preponderant becomes the effects of induction. The phenomenon is classified more local, if the energy decreases rapidly in the circuit causing the current in the circuit to die-out. In the space outside the conductor the phenomena results in dielectric stress and steady magnetic condition for continuous current and alternating for alternating current input. For the telecommunication channels the magnetic and electric fields outside conductor is only considered for the transmitting and receiving of messages. Transformer working is the simplest example for WPT.
A current which is varying in the primary generates a varying magnetic flux in the core of the transformer which results in the varying magnetic field in the secondary. This field induces a varying voltage or EMF(electromotive force) in the secondary. This is termed as mutual induction. It should be noted that most of the energy transferring devices are usually air-cored. There are many examples of air-cored devices, such as the wireless charging pads, electric brushes etc.
Induction coupling is the underlying principle of charging of an electric toothbrush. The input current through the primary winding creates a magnetic flux thus a magnetic field. When the toothbrush is placed in the charger, a current is induced in the secondary by the magnetic field, which is connected to the batter of the brush and thus recharges the battery.
The wireless charging of the portable devices of great advantage as it is comfortable to use, shock proof and less usage of wires which is more economical.
The figure() shows the picture of an wirelessly charged toothbrush.
The principle to recharge several devices at a time is similar to that of the charging of electric brush. The Splashpower recharging mat and Edisions Electric PowerDesk are few examples. The WCP (wireless charging pads) are devices that are devised for the charging the battery automatically when placed on it. There is no connection with the wires between the charging pad and the device battery to be charged. The technique of induction to transfer energy is utilized in this and air acts as core.
Resonance Inductive Coupling
It is the combination of resonance principles along with the inductive coupling principles. When a relatively low frequency close to the same frequency of vibration as the natural frequency of system being resonated produces large amplitude of vibration it is called resonance. Two objects or bodies are made to strongly interact with each other during the time of resonance. It is also noted that an objects physical structure determines the natural frequency of vibration of the object. Objects vibrate more easily at resonant frequency than at other frequencies. We know that magnetic flux developed due to the passage of current in the primary creates a magnetic field which induces an EMF in the secondary and thus current was the principle of inductive coupling.
Working principles of Resonance Inductive Coupling.
The coil in the fig() provides the inductance when the circuit is supplied with voltage. The induction takes place in a slight different manner when the electromagnetic fields surrounding the coils start to resonate in the similar frequencies. The inductor used here is a curved coil wire. We also see a capacitance plate attached to each end of the coil to hold the charge, that is a capacitance, is connected in parallel to the coil on both sides that is the primary and the secondary coil. When electricity is passed through the coil the coil begins to resonate. The resonating frequency is the product of the capacitance of the plates and the inductance of the coil.
Electricity which travels alongside the electromagnetic wave tunnels from the source coil to the target coil, if both of the resonant frequencies are the same, where resonance is a function of the product of the capacitance connected in parallel and the inductance of the coil. Loss of energy or power due to radiation is negligible in this case.
If the two coils to be linked are out of range of one another, there occurs no transmission of power as the fields are not strong enough to affect the surroundings. This is the same affect when two coils which are resonating at different frequencies exists. Considering two coils resonating at the same frequency to be in the same range, energy streams moves from the transmitting to the recipetant. This theory also provides the functionality of more than one receiving coil in the same range operating at the same resonant frequency to function. This type of energy is termed as “NON-RADIATIVE ENERGY TRANSFER” as it involves the static fields surrounding the coils more than the fields spreading in all directions.
Advantages of Resonance Inductive Coupling (RIC) over Inductive Coupling
Compared to inductive coupling the efficiency of RIC is very high.
The range of RIC is much greater than compared to the inductive coupling, range for inductive coupling is below 5 cm whereas for the RIC it in the range of few meters.
The directivity of the RIC is larger compared to the inductive coupling.
RIC can implemented in one-to-many format, but the implementation of inductive coupling is generally one-on-one format.
The devices implemented using RIC are easily portable compared to that of the inductive coupling gadgets. This is because of the small size of the component structure and the ease to use.
Air Ionization
Ionization of air refers to the process of imparting charges that is negative or positive charge to the air molecules by the application of high voltage. In this method of transferring the electricity, the charged particles that are the anions or cations move from the source to the target terminal through the atmosphere, by imparting charges to the air molecules. This technique even though seems to be simple in construction, it is the most toughest and un-economical method of transfer of power in the near field techniques.
The disadvantages are really more in this technique because minimum field strength of 2.11 Megavolt is required to transfer the electricity over a distance of 1 meter, which is very high potential difference to be created. There should be a static atmospheric condition for this technique to be implemented and mostly the dust particles in the atmosphere can also affect the transfer of electric power, from one terminal to the other terminal.
The natural example is the lightening which occurs. So taking into consideration all the negative points which overcome the advantages, it can be concluded that Air ionization method for wireless energy transfer is not feasible for implementation.
Advantages of near-field techniques
The absence of wires is being considered as the greatest advantage.
There is no presence of e-waste.
The need for battery is suppressed to a certain extent.
Efficient energy transfer using the RIC up-to a distance of few meters is possible.
The methods are harmless, if the strength of the fields used are under safety levels prescribed by the standards prescribed.
The maintenance cost is considerably low for the devices in the near field techniques.
Disadvantages of near-field techniques
One of the main disadvantages of the near-field techniques is the distance. This technique implies the wireless transfer of energy only for a short distance.
It has to be taken care of that, the strength of the fields developed should be below the safety level so that it does not affect the human safety.
The initial capital for setting the circuit is considerably high.
The tuning of the RIC circuit is a difficult task and consumes more time.
The signals to the supply should be having high frequency because the induction is proportional to the frequency of the signal supply.
The air ionization process for the near-field technique is not a feasible technique that can be implemented for the wireless transmission of energy.
Far-field techniques for wireless power transmission
Far-field techniques for WPT refer to transfer of power from one terminal to another terminal which is kept far apart. The techniques used for near-field transfer are not viable because the efficiency will be tending to zero or almost zero if implemented by those techniques. That is the power received at the receiving terminal will be zero.
There are mainly two types of techniques used for the energy transfer in far-field techniques they are as follows
Microwave wireless power transfer
The electromagnetic waves that are arranged in between the infrared spectrum and the radio wave spectrum are termed as microwave. Their frequency ranges in from 300 Mega hertz to 300 Giga hertz. They are used for a wide range of applications such as Bluetooth, wifi, microwave Owen etc. The frequency range of microwave is in between 1 and 40 GHz which is generally used for applications.
For long distance transfer of power, microwave is one of the best medium used. The microwave band is generally used because antennas of convenient sizes that are able to transmit and receive the microwave signals and also the metal waveguides for carrying the radio power works are available. Besides this the technologies in electronics where millimeter waveband is required the microwave proves to be helpful. This is because in millimeter wavelengths the radio waves are attenuated while the microwaves are not. The microwave was used for transmission of power for various applications such as running of the unmanned helicopter developed by Americans, powering the outer space vehicles etc.
There are mainly three steps involved in the transmission and reception of power by MPT.
Step1: The conversion of electrical energy into microwave. After this process the microwave is emitted with very-high power emitters called the cavity magnetrons.
Step2: The microwave is then captured using a rectenna. The rectenna is a combinational circuit consisting of rectifier and an antenna.
Step3: Microwave energy is converted to electrical by the rectenna.
In the process of transmitting microwave, the AC supply needs to be converted to DC as it is not possible to convert the AC directly to microwave. The process of conversion of DC to microwave is done by magnetron. The magnetron is a highly powered vacuum tube structure that is responsible for the production of coherent microwaves. The rectenna can also be called as rectifying antennae as it converts the microwave directly into DC electricity. Rectenna consists of elements that are sorted in multi element array and consists of pattern reflector element mesh to give directionality. It is easily constructed by placing Schottky diode in between the antennae dipoles. The DC is converted back in the next process using an inverter circuit. Considering the receiver to be a photovoltaic cell, the microwave system for earthbound applications exceeding an area limit of diameter size 10 Km the arrays in the receiving station has large power levels in total , limiting the exposure of electromagnetic radiations which is considered for human safety. A density of power equals to or less than 1mW/cm2 is considered safe for human environment, and with this density of power for a diameter of 10 km 750 megawatts of power can be generated. This standard value is utilized by several modern power plants in the world.
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Laser transmission of power
A device emitting electromagnetic radiation by the process of optical amplification on the basis of stimulated emission is termed as laser. Laser has high directivity and high degree of temporal and spatial coherence which is a unique property of laser that cannot be attained by any other technologies. The laser beams does not get dispersed for long during the time of transmission. The disadvantage of the laser is that it gets attenuated while propagating through the atmosphere because of hindrance due to the dust particles prevailing in the atmosphere.
The receiver designed for the laser power transmitter and receiver is simple in design and construction. The laser system is a cost efficient system due to the simple design and structure. These points form positive side for the implementation of the laser system. The receiver in the case of laser system is a photovoltaic cell.
The laser system is mainly used when electromagnetic radiation in the spectrum near to visible region that is 10s of nm or microns are to be transmitted, power is transmitted changing electricity in the form of laser beams and then projected onto a photo-voltaic cell which acts as the receiver. The receiver then turns it back to electricity. This is called the “power beaming” mechanism. This is so called because power is forced or beamed at the receiving end to convert the power into useful electric energy. The fig() shows the concept how laser is used for transferring the power to an object in the sky. Here it represents flying object’s battery being charged by controlled transmission of laser beams, tracking the object with the help of the radar, and powering the target with the help of laser generating source.
Comparison of Laser and Microwave techniques
Laser method requires antennae of smaller sizes compared to that of the microwave technique of power transfer.
Microwaves interferes with each other where as laser is highly monochromatic and does not interfere (two frequencies used for microwave power transfer are 2.45GHz and 5.4 GHz).
Laser gets attenuated and also diffracted due to the particles present in the atmosphere, but microwave is resistant to this.
Transmission of energy over a long range is possible due to property of collimated monochromatic propagation of wave-front in lasers but the range of microwave is small considered to lasers.
The receiver size of the laser technique is small considered to the microwave method because of the compact size of the photovoltaic components.
The laser beams does not affect or interfere the normal radio communication due to the large difference in their wavelengths, but microwave as it has wavelengths closer to that of the radio communication channel wavelength the possibility of interference is more.
The photovoltaic cells in the laser receiver is not as efficient as in the microwave receiver as in laser, only 40%-50% efficiency is only acquired.
Advantages of far-field energy transfer
Far-field energy transfer is more efficient.
The need for setting substations, grids etc are eliminated due to the large distance transfer ability.
These systems require less maintenance cost compared to the wired system of energy transfer.
The systems become more and more effective if the transmitter and the receiver are along the line of sight.
This system can be implemented in remote locations where access is difficult.
Disadvantages of far-field energy transfer techniques
This is radiative in nature so the losses are prevalent due to the radiations.
In order to be more effective in working the transmitting and receiving stations must be in line of sight.
When lasers are used: (1) conversion to electrical energy is inefficient at the receiver. (2)absorption loss is high due to the atmospheric particles which causes attenuation and diffraction in lasers(3) difficult to use in human environment.
When microwaves are used (1) interference of two power signals (2) standards should considered for the sake of healthy living environment.
Applications
Near-field energy transfer
Electric automobile charging
The automobiles running on electricity are more common nowadays as it more environmental friendly, but the storage capacity of these vehicles are limited as it cannot afford huge batteries, so similar to the petroleum bunks, rises the concept of recharging stations. But these recharging stations cannot afford large number of plug-ins, as the density of vehicles increases besides it is a waste of time working with plug-ins in this busy and fast world, so wireless recharging of the vehicles is done in these situations to avoid waste of time and energy by near-field techniques, besides all these many vehicles can be recharged at a time using resonance inductive coupling.
Consumer electronics
In the world of consumer electronics, the technology changes are really fast. Each technology brings in more compacability and introduces more functionality into the products being manufactured. Such a world essentially requires the introduction of the wireless charging, because it reduces the cost of the wires plugged on to it, increases the reliability of use of the equipment, reduces the total size of the equipment and increases the ease of portability etc. Inductive coupling or resonance Inductive coupling techniques prove to be useful in these cases. Example is electronic toothbrush charger, splash power recharging mat etc.
Industrial applications
In industries there are processes involving high temperature, hazardous environment created by chemical reactions etc. In such an environment if wires are used for the transfer of electricity if required, it is highly inefficient and at times dangerous, so in such cases only the wireless transfer of electricity is possible, the methods of inductive coupling and resonance inductive coupling proves to be efficient because the hazardous environment or the high temperature does not affect the transfer of energy wirelessly.
Far-field energy transfer
Solar power satellites
In this era of growing need of energy, our mother earth with all its resources may not be able to supply enough energy for the utilization for mankind, besides the energy earned from coal, oil etc are causing pollution besides this, these non renewable sources of energy. So in order to satisfy human needs one of the latest technologies for energy harvesting is the solar power satellites. These are machineries working on the basis of the microwave and laser power transmission methods. These helps to efficiently make use of the solar energy, and are placed in geostationary orbits for the efficient working.
Solar energy is captured using the solar cells on the satellite; where in 400 million photocells are present in each solar power satellite. The energy is converted to microwaves or laser and transmitted to the earth and received at the earth station using the rectenna or photovoltaic cell converts it into electrical energy. The efficiency of the microwave system is more than 95% usually, but for the laser power transfer system the efficiency is around 40% to 50%.
The diagram in the fig() shows the working of the solar power satellite and how it transfers power from the supply terminal to the receiver terminal by microwave method of power transfer. This is also called “BEAMING DOWN” process.
Energy to remote areas
In remote location that is places that cannot be accessed easily by people, the transfer of energy by using grids are considered to be waste of money, time and energy. In such cases the transfer of energy through wireless media is only possible. Certain factories which produce hazardous wastes, confidential or secret services offices and plants of nations etc are sometimes located in remote locations are the examples. There are villages to be supplied with electricity located in remote locations government of nations are planning and implementing far-field techniques of power transfer to meet these tasks.
Energy broadcasting globally
The concept of energy broadcasting globally had bought in nations to work together in projects dealing with more efficiently trapping of solar energy to power up their resources. As solar energy is renewable source and available in plenty the only step left behind is more efficient way of trapping the solar energy. The achievement of this would certainly result in reducing the crisis of energy, and to broadcast the energy globally.
One of the famous projects undertaken is based on Kalam’s (former Indian president) vision which calls in the program for space-farming to contribute substantially for the global energy broadcasting. One such proposal is Pan African e- network.
Projects established on the basis of wireless power transfer.
Wi-Tricity
This was a project done in Massachusetts Institute of Technology on wireless transfer of energy based on the resonance inductive coupling method. The team was led by Marin Soljacic. The team was able to transfer energy wirelessly for a distance of 2m. The coils used for the experiment was helical in shape and they never used any capacitor in the experiment. The energy received at the receiving terminal was almost 40% of the total input power from the supply.
The frequencies used for the transmission of power were 1MHz and 10MHz. The field strengths were safer at 1 MHz frequency for the human environment. The usage of 10MHz frequency, created fields of strength above the ICNIRP (International Commission on Non-Ionizing Radiation Protection) standards.
Rectenna in US
The rectenna build in USA worked on the microwave method of wireless energy transfer. The rectenna which was spread over one and half mile in was diameter was able to generate electric power of 5000MW. The range of frequency of microwave used is compactable to the ICNIRP standards.
Alaska ’21
WPT can be used for supplying power to rural areas. Alaska ’21 project was presented in the year 1993, for supplying power to villages in Alaska. The other sources of energy were not possible to be used because of the limited infrastructure. The price estimate of $40/kWh was done for energy produced by other sources, which is too high to accommodate. Even cabling to this area was not possible due to extremes of climatic conditions. So a pilot project was conducted which was passed on WPT, not only to supply energy but also to avoid pollution. The system consisted of 2.45 GHz design array and could bridge around 1 to 15 miles. Present status is unknown.
Grand Basin project
This project aims in supplying electricity to an isolated mountain village in the remote area of the La Reunion Island. This project was developed for making Grand Basin to a tourist village. The project was given complete support by CNES, which is the French space centre. WPT was used as the underlying basis of the project. Planning to build a microwave link which operates at 2.45GHz frequency working over a distance of 700 meters delivers 10kW and had an efficiency of 57%. The whole system worked in combination with the photovoltaic panels and the batteries. Capital of 1 million dollars for 10 kW was proposed and the project was put down.
WPT Demonstration in Hawaii
In the year 2008 on moth of May a demonstration based on WPT was held on the islands of Hawaii. The demonstration was organized by one of the US based companies named Managed Energy Technologies. They were able to demonstrate wireless power transfer up to a distance of 148 km. They were only able to transfer 20 watts of power, which would barely light a fluorescent lamp, because most of the energy were lost during the time of transmission. The limitation of this demonstration was not the science but the capital involved. This project involved less than a million dollar. If the company could have afforded more number of solar panels and more phased transmitter array and receivers more than 60% of efficiency could have been acquired.
Munich electronica exploration
The Munich ELECTRONICA exhibition organized by the automotive, embedded, electronica and the ZEVI forums in the month of November from 9th to 12th in Germany was a real trade off for the electronics products. There were 2595 companies participating in the exchange and almost 70000 visitors took part in the exhibition. The exhibition is being conducted every two years alternatively to the Productronica. There were participation of various companies dealing with different types of electronic goods like the displays, connectors and interconnects, semiconductors, automotive, sensors, passive components, assemblies and sub-systems, wireless, embedded systems, power supplies, test and measurement etc. It is one of the best places where we come to know about different technologies, products and even test the new product to confirm our expectation. The fair also is business area where companies come for advertising and selling their products. I was able to visit stalls of 37 companies dealing with power supply devices and gathered information’s which I was able to present on the topic wireless transfer of power. Certain companies were doing research works on this topic but were not able to introduce their products because certain defects were yet to be cleared before manufacturing.
Companies visited
RRC Solutions
The company was established in the year 1989, with the excellent management has become one of the leading companies in the supplier of power supplies and battery chargers. It has a good platform for the professional applications in medical, industrial, military and consumer products. The company has good wealth of expertise engineers creating new and innovative ideas. The company has its branches in various locations of the world with its headquarters in south west Germany. Its branches are located in USA, Japan, China, Hong Kong etc
The company’s products involve battery chargers, power supplies, integrated system solutions, products
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