Witricity, standing for wireless electricity, is a new system for transmitting power, is a term coined by MIT researchers, to describe the ability to provide electricity to remote objects without wires. Using self-resonant coils in a strongly coupled regime, efficient non-radiative power transfer over distances of up to eight times the radius of the coils can be done. Witricity is based upon coupled resonant objects to transfer electrical energy between objects without wires. The system consists of a Witricity transmitter (power source),and devices which act as receivers (electrical load).It is based on the principle of resonant coupling and microwave energy transfers. The action of an electrical transformer is the simplest instance of wireless energy transfer. There are mainly two types of transfers i.e. short range and long range transmission. The short range are of 2-3 metres where as the long range are of few kilometres.
Keywords- wireless electricity transfer, resonance coupling, magnetic induction ,coupled resonators,
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1. Introduction
Wireless transmission is ideal in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous, or impossible. The tangle of cables and plugs needed to recharge today’s electronic gadgets could soon be a thing of the past. The concept exploits century-old physics and could work over distances of many metres. Consumers desire a simple universal solution that frees them from the hassles of plug-in chargers and adaptors.”Wireless power technology has the potential to deliver on all of these needs.”However, transferring the power is the important part of the solution.
With wireless energy transfer, the efficiency is a more critical parameter and this creates important differences from the wireless data transmission technologies. To avoid the conflicts like recharging and carrying its appliances of electrical and electronic devices, wireless power transmission is desirable.
Wireless power transmission was originally proposed to avoid long distance electrical distribution based mainly on copper cables. This can be achieved by using microwave beams and the rectifying antenna, which can receive electromagnetic radiation and convert it efficiently to DC electricity. 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. Magnetic resonance was found a promising means of electricity transfer because magnetic fields travel freely through air yet have little effect on the environment or, at the appropriate frequencies, on living beings and hence is a leading technology for developing witricity. In our present electricity generation system we waste more than half of its resources. Especially the transmission and distribution losses are the main concern of the
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present power technology. Much of this power is wasted during transmission from power plant generators to the consumer. The resistance of the wire used in the electrical grid distribution system causes a loss of 26-30 percent of the energy generated. This loss implies that our present system of electrical distribution is only 70-74 percent efficient. We have to think of alternate state of art technology to transmit and distribute the electricity. Now a day’s global scenario has been changed a lot and there are tremendous development in every field. If we don’t keep pace with the development of new power technology we have to face a decreasing trend in the development of power sector. The transmission of power without wires may be one noble alternative for electricity transmission.
1. 1The Technologies Available
In this remarkable discovery of the “True Wireless” and the principles upon which transmission and reception, even in the present day systems are based, Dr. Nikola Tesla shows us that he is indeed the “Father of the Wireless.”The most well known and famous Tesla Tower was designed and constructed mainly for wireless transmission of electrical power, rather than telegraphy. The most popular concept known is Tesla Theory in which it was firmly believed that Wardenclyffe (see fig 1.1) would permit wireless transmission and reception across large distances with negligible losses. In spite of this he had made numerous experiments of high quality to validate his claim of possibility of wireless transmission of electricity. But this was an unfortunate incidence that people of that century was not in a position to recognise his splendid work otherwise today we may transmit electricity wirelessly and will convert our mother earth a wonderful adobe full of electricity.
Fig.1.1Tesla Tower (height 187 feet)
The modern ideas are dominated by microwave power transmission called Solar power satellite to be built in high earth orbit to collect sunlight and convert that energy into microwaves, then beamed to a very large antenna on earth, the microwaves would be converted into conventional electrical power.
2. WiTricity Technology: The Basics
Electricity: The flow of electrons (current) through a conductor (like a wire), or charges through the atmosphere (like lightning). A convenient way for energy to get from one place to another. Magnetism: A fundamental force of nature, which causes certain types of materials to attract or repel each other. Permanent magnets, like the ones on your refrigerator
and the earth’s magnetic field, are examples of objects having constant magnetic fields. Oscillating magnetic fields vary with time, and can be generated by alternating current (AC) flowing on a wire. The strength, direction, and extent of magnetic fields are often represented and visualized by drawings of the magnetic field lines.
Fig.2.1Erth’s Magnetic Filed
As electric current I flows in a wire, it gives rise to a magnetic field B which wraps around the wires shown in fig 2.2 .When the current reverses direction, the magnetic field also reverses its direction.
Fig 2.2: magnetic field by electric current
The blue lines represent the magnetic field that is created when current flows through a coil. When the current reverses direction, the magnetic field also reverses its direction.
Figure 2.3: magnetic field through coil
Fig.2.4 Electric Transform
An electric transformer is a device that uses magnetic induction to transfer energy from its primary winding to its secondary winding, without the windings being connected to each other. It is used to “transform” AC current at one voltage to AC current at a different voltage.
2.1 Magnetic Induction: A loop or coil of conductive material like copper, carrying an alternating current (AC), is a very efficient structure for generating or capturing a magnetic field. If a conductive loop is connected to an AC power source, it will generate an oscillating magnetic field in the vicinity of the loop. A second conducting loop, brought close enough to the first, may “capture” some portion of that oscillating magnetic field, which in turn, generates or induces an electric current in the second coil. The current generated in the second coil may be used to power devices. This type of electrical power transfer from one loop or coil to another is well known and referred to as magnetic induction. Some common examples of devices based on magnetic induction are electric transformers and electric generators.
2.2Energy/Power Coupling: Energy coupling occurs when an energy source has a means of transferring energy to another object. One simple example is a locomotive pulling a train car, the mechanical coupling between the two enables the locomotive to pull the train, and overcome the forces of friction and inertia that keep the train still and the train moves. Magnetic coupling occurs when the magnetic field of one object interacts with a second object and induces an electric current in or on that object. In this way electric energy can be transferred from a power source to a powered device. In contrast to the example of mechanical coupling given for the train, magnetic coupling does not require any physical contact between the object generating the energy and the object receiving or capturing that energy.
3.The Invention of WiTricity Technology: The story starts late one night a few years ago, with MIT Professor Marin Soljacic (pronounced Soul-ya-cheech), staring at his cell phone. It was probably the sixth time that month that he was awakened by his mobile phone beeping to let him know that he had forgotten to charge it. At that moment, it occurred to him: “There is electricity wired all through this house, all through my office everywhere. This phone
should take care of its own charging!” But to make this possible, one would have to find a way to transfer power from the existing wired infrastructure to the cell phone-without wires. Soljacic started thinking of physical phenomena that could make this dream a reality.
3.1 Resonance: Resonance is a property that exists in many different physical systems. It can be thought of as the natural frequency at which energy can most efficiently be added to an oscillating system. A playground swing is an example of an oscillating sys-
tem involving potential energy and kinetic energy. The child swings back and forth at a rate that is determined by the length of the swing. The child can make the swing go higher if she properly coordinates her arm and leg action with the motion of the swing.
The swing is oscillating at its resonant frequency and the simple movements of the child efficiently transfer energy to the system. Another example of resonance is the way in which a singer can shatter a wine glass by singing a single loud, clear note. In this example, the wine glass is the resonant oscillating system. Sound waves travelling through the air are captured by the glass, and the sound energy is converted to mechanical vibrations of the glass itself. When the singer hits the note that matches the resonant frequency of the glass, the glass absorbs energy, begins vibrating, and can eventually even shatter. The resonant frequency of the glass depends on the size, shape, thickness of the glass, and how much wine is in it.
3.2Resonant Magnetic Coupling: Magnetic coupling occurs when two objects exchange energy through their varying or oscillating magnetic fields. Resonant coupling occurs when the natural frequencies of the two objects are approximately the same. Two idealized resonant magnetic coils, shown in yellow. The blue and red colour bands illustrate their magnetic fields. The coupling of their respective magnetic fields is indicated by the connection of the colour bands.
Fig.3.1: Magnetic coupling.
3.3 Coupled Resonators: To achieve wireless power transfer in a way that is practical and safe, one needs to use a physical phenomenon that enables the power source and the device (in this case, the mobile phone) to exchange energy strongly, while interacting only
weakly with living beings and other environmental objects, like furniture and walls. The phenomenon of coupled resonators precisely fits this description. Two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off-resonant objects. A child on a swing is a good example of a resonant system. A swing exhibits a type of mechanical resonance, so only when the child pumps her legs at the natural frequency of the swing is she able to impart substantial energy into the motion of the swing. Another example involves acoustic resonances; If an opera singer enters that room and sings a very loud single note, the glass having the corresponding resonant frequency can accumulate enough energy to shatter, while the other glasses are unaffected.
3.4 WiTricity Technology is born:The experimental design consisted of two copper coils, each a self-resonant system. One of the coils, connected to an AC power supply, was the resonant source. The other coil, the resonant capture device, was connected to a
60 watt light bulb. The power source and capture device were suspended in midair with nylon thread, at distances that ranged from a few centimetres to over 2.5 meters (8.2 ft). Not only was the light bulb illuminated, but the theoretical predictions of high efficiency over distance were proven experimentally. By placing various objects between the source and capture device, the team demonstrated how the magnetic near field can transfer power through certain materials and around metallic obstacles. Thus Prof. Soljacic’s dream of finding a method to wirelessly connect mobile electric devices to the existing electric grid was realized. WiTricity Corp. was soon launched to carry this technology forward from the MIT laboratories to commercial production.
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4. How wireless energy could work
4.1WiTricity Technology: WiTricity power sources and capture devices are specially designed magnetic resonators that efficiently transfer power over large distances via the magnetic near-field. These proprietary source and device designs and the electronic systems that control them support efficient energy transfer over distances that are many times the size of the sources/devices themselves.
Fig. 4.1: Witricity power transfer.
The WiTricity power source, left, is connected to AC power. The blue lines represent the magnetic near field induced by the power source. The yellow lines represent the flow of energy from the source to the WiTicity capture coil, which is shown powering a light bulb. Note that this diagram also shows how the magnetic field (blue lines) can wrap around a conductive obstacle between the power source and the capture device.
4.1 Electrical conduction:
Electrical energy can also be transmitted by means of electrical currents made to flow through naturally existing conductors, specifically the earth, lakes and oceans, and through the atmosphere a natural medium that can be made conducting if the breakdown voltage is exceeded and the 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 practical wireless energy transmission system using this principle, a high-power ultraviolet beam might be used to form a vertical ionized channel in the air directly above the transmitter-receiver stations. The same concept is used in virtual lightning rods, the electro laser electroshock weapon and has been proposed for disabling vehicles.
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4.2 Short range power transmission and reception:
Resonant induction: By designing electromagnetic resonators that suffer minimal loss due to radiation and absorption and have a near field with mid-range extent (namely a few times the resonator size), mid-range efficient wireless energy-transfer is possible. The reasonment is that, if two such resonant objects are brought in mid-range proximity, their near fields (consisting of so-called ‘evanescent waves’) couple (evanescent wave coupling) and can allow the energy to transfer from one object to the other within times much shorter than all loss times, which were designed to be long, and thus with the maximum possible energy transfer efficiency. Since the resonant wavelength is much larger than the resonators, the field can circumvent extraneous objects in the vicinity and thus this mid-range energy transfer scheme does not require line-of-sight. By utilizing in particular the magnetic field to achieve the coupling, this method can be safe, since magnetic fields interact weakly with living organisms.
“Resonant inductive coupling” has key implications in solving the two main problems associated with non-resonant inductive coupling and electromagnetic radiation, one of which is caused by the other; distance and efficiency. Electromagnetic induction works on the principle of a primary coil generating a predominantly magnetic field and a sec-
ondary coil being within that field so a current is induced within its coils. This causes the relatively short range due to the amount of power required to produce an electromagnetic field. Over greater distances the non-resonant induction method is inefficient and wastes much of the transmitted energy just to increase range. This is where the resonance
wireless-power-7
Fig. 4.2: Resonant induction recharging.
comes in and helps efficiency dramatically by “tunnelling” the magnetic field to a receiver coil that resonates at the same frequency. Unlike the multiple-layer secondary of a non-resonant transformer, such receiving coils are single layer solenoids with closely
spaced capacitor plates on each end, which in combination allow the coil to be tuned to the transmitter frequency thereby eliminating the wide energy wasting “wave problem” and allowing the energy used to focus in on a specific frequency increasing the range. Some of these wireless resonant inductive devices operate at low milliwatt power levels and are battery powered. Others operate at higher kilowatt power levels. Current implantable medical and road electrification device designs achieve more than 75 percent transfer efficiency at an operating distance between the transmit and receive coils of less than 10 cm.
4.3 Long distance power transmission and reception:
Microwave 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 may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95 percent have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered. Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large. These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets. For earthbound applications a large area 10 km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1 mW/cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants.
5.WiTricity Features and Applications
5.1 Features of Witricity:
Highly Resonant Strong Coupling Provides High Efficiency Over Distance: WiTricity mode of wireless power transfer is highly efficient over distances ranging from centimetres to several meters. Efficiency may be defined as the amount of usable electrical energy that is available to the device being powered, divided by the amount of energy that is drawn by the WiTricity source. In many applications, efficiency can exceed 90 percent. And WiTricity sources only transfer energy when it is needed. When a WiTricity powered device no longer needs to capture additional energy, the WiTricity power source will automatically reduce its power consumption to a power saving idle state.
Energy Transfer via Magnetic Near Field Can Penetrate and Wrap Around Obstacles: The magnetic near field has several properties that make it an excellent means of transferring energy in a typical consumer, commercial, or industrial environment. Most common building and furnishing materials, such as wood, gypsum wall board, plastics, textiles, glass, brick, and concrete are essentially transparent to magnetic fields-enabling
WiTricity technology to efficiently transfer power through them. In addition, the magnetic near field has the ability to wrap around many metallic obstacles that might otherwise block the magnetic fields. WiTricity applications engineering team will work with you to address the materials and environmental factors that may influence wireless energy transfer in your application.
Non-Radiative Energy Transfer is Safe for People and Animals: WiTricity technology is a non-radiative mode of energy transfer, relying instead on the magnetic near field. Magnetic fields interact very weakly with biological organisms-people and animals and are scientifically regarded to be safe. Professor Sir John Pendry of Imperial College London, a world renowned physicist, explains: The body really responds strongly to electric fields, which is why you can cook a chicken in a microwave. But it doesn’t respond to magnetic fields. As far as we know the body has almost zero response to magnetic fields in terms of the amount of power it absorbs.” Evidence of the safety of magnetic fields is illustrated by the widespread acceptance and safety of household magnetic induction cooktops. Through proprietary design of the WiTricity source, electric fields are almost completely contained within the source. This design results in levels of electric and magnetic fields which fall well within regulatory guidelines. Thus WiTricity technology doesn’t give rise to radio frequency emissions that interfere with other electronic devices, and is not a source of electric and magnetic field levels that pose a risk to people or animals.
Limits for human exposure to magnetic fields are set by regulatory bodies such as the FCC, ICNIRP, and are based on broad scientific and medical consensus. WiTricity technology is being developed to be fully compliant with applicable regulations regarding magnetic fields and electromagnetic radiation. Scalable Design Enables Solutions from milliwatts to Kilowatts WiTricity systems is being designed to handle a broad range of power levels. The benefits of highly efficient energy transfer over distance can be achieved at power levels ranging from milliwatts to several kilowatts. This enables WiTricity technology to be used in applications as diverse as powering a wireless mouse or keyboard (milliwatts) to recharging an electric passenger vehicle (kilowatts). WiTricity technology operates in a load following mode, transferring only as much energy as the powered device requires. WiTricity technology is being designed so that it can be easily embedded into a wide variety of products and systems. The physics of resonant magnetic coupling enables WiTricity
engineers to design power sources and devices of varying shapes and sizes, to match both the packaging requirements and the power transfer requirements in a given OEM application. WiTricity has designed power capture devices compact enough to fit into a cell phone.
WiTricity technology will make our products:
More Convenient: No manual recharging or changing batteries. Eliminate unsightly, unwieldy and costly power cords.
More Reliable: Never run out of battery power. Reduce product failure rates by fixing the weakest link’: flexing wiring and mechanical interconnects.
More Environmentally Friendly: Reduce use of disposable batteries. Use efficient electric grid power’ directly instead of inefficient battery charging.
5.2 Applications of Witricity
Direct Wireless Power: when all the power a device needs is provided wirelessly, and no batteries are required. This mode is for a device that is always used within range of its WiTricity power source.
Automatic Wireless Charging: when a device with rechargeable batteries charges itself while still in use or at rest, without requiring a power cord or battery replacement. This mode is for a mobile device that may be used both in and out of range of its WiTricity power source.
5.2.1 consumer Electronics: Automatic wireless charging of mobile electronics (phones, laptops, game controllers, etc.)
in home, car, office, Wi-Fi hotspots while devices are in use and mobile.
Direct wireless powering of stationary devices: (fiat screen TV’s, digital picture frames, home theater accessories, wireless loud speakers, etc.)eliminating expensive custom wiring, unsightly cables and wall-wart power supplies.
Direct wireless powering of desktop PC peripherals: wireless mouse, key-board, printer, speakers, display, etc eliminating disposable batteries and awkward cabling.
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Fig.5.1: wireless charging of mobile electronics.
5.2.2 Industrial:
Direct wireless power and communication interconnections across rotating and moving joints (robots, packaging machinery, assembly machinery machine tools)eliminating costly and failure-prone wiring.
Direct wireless power and communication interconnections at points of use in harsh
environments (drilling, mining, underwater, etc.) where it is impractical or impossible to run wires.
Direct wireless power for wireless sensors and actuators, eliminating the need for expensive power wiring or battery replacement and disposal.
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6. Conclusion
From these researches and discoveries it can be said that wireless power transmission is going to be a major field of interest for scientists and for people. The facts that the power can be transmitted from space to earth will revolutionize the field of satellites. Since the uses of wireless power transmission are many, from easy installation, neatness, easy maintenance to multi-equipment working are amazing, the area for researchers on this field seems very interesting. Rather concentrating on the false beliefs, the concentration should be put on advantages of witricity for further increasing the efficiency of wireless power transmission with more safety measures. It is a rocking technology provided the researches continue to move in same speeding direction.
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