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Wireless Power Transmission

Written By: 

Srinvas Arun Tej P

The technology for wireless power transmission or 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. A Representational Image of  Wireless Power Trasnmission

Fig. 1: A Representational image of Wireless Power Trasnmission

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 Wireless Power Transmission
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 wireless power transmission 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 Wireless Power Transfer works

How Does Wireless Power Transfer 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. 
 

Pictorial Presentation of Wireless Power Transfer Process Via Microwaves

Fig 2: Pictorial Presentation of Wireless Power Transfer Process Via Microwaves
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. 

Cross Sectional Diagram of Magnetron that Converts DC Power to Microwaves

Fig. 3: Cross Sectional Diagram of Magnetron that Converts DC Power to Microwaves

 
 
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.
 
 

Wireless Power Transfer Today

 
Wireless Power Transfer 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. 
 

Picture of a Cyclotron Wave Converter used to convert RF energy in DC

Fig 3:  Picture of a Cyclotron Wave Converter used to Convert RF Energy in DC
 
Using magnetic induction to make the conversion, the cyclotron was tested to have greater than 74% efficiency.  The Russians also made there own contribution to the field of solar powered satellites. In a demonstration, Russian scientists developed a large solar reflector to illuminate a small spot on the earth's surface, from space.  This was done to test the deployment of such large surfaces in space.  Indeed this is still an important experiment to undertake because solar sails would be the means of collecting radiant energy from the sun if a solar cell WPT approach was implemented. 
 
Being concerned about global warming, Japan has created an energy plan that states that 30% of the world’s electrical needs will come from space by the year 2040.  The equipment is not being developed but the plan is to have a series of solar powered satellites that would provide 1 GW of power each to the main power grid.  Another experiment titled ISY-METS is one of the most successful among recent years.  From a distance of 250 km, 823 watts of power was transmitted from a solid state phased array straight upwards to a spacecraft containing two rectenna paddles.  Although one paddle was made of two circular microstrip antennas and the other a six-element film, both rectennas absorbed RF energy.  This demonstration proved to be one of the most important steps toward developing a solid WPT system.
 

Sustainability

Sustainability
Sustainability is an attempt to provide the best outcomes for the human and natural environments both now and in the future. When looking at WPT, the standard application of charging small electrical devices has no harmful affects on the environment. WPT on a large scale has been projected to produce only twenty grams of carbon dioxide- the worst cause of global warming- per kilowatt hour. This is significantly lower than oil which emits eight hundred forty six grams of carbon dioxide per kilowatt hour.  Nuclear energy emits about the same rate as WPT, but WPT has no radioactive waste that is harmful to the environment.
 
Wireless power transfer also has an application in the medical field.  A  Pacemaker, (electrical device designed to stimulate regular beating of the heart, using electrodes implanted in the body) today has an average lifespan of five to eight years. If the pacemaker is wirelessly powered, there is no need to have open heart surgery every five to eight years.  This can be used for any electronic medical device placed in a human.  A new idea that uses wireless power transfer is a device that automatically distributes medicine to the body.  It works by placing an apparatus, filled with a medicine of some kind, under the skin.  Then by sending power to the apparatus, it releases the correct amount of previously specified medicine.
 
The most futuristic idea that uses wireless power transfer is space travel.  If scientists and engineers can find a way to send out energy waves to limitless distances, space travel would change significantly.  First, a space ship would be built that could take in the energy waves and use the waves to thrust the vehicle and run the other system within it.  Once in space, the vehicle would be able to travel as far as the RF waves would allow it. This will help the world learn more about our galaxy and beyond.
 

Major Concerns

Major Concerns With Wireless Power Tranfer
Wireless power transfer is possible, but when trying to sustain a constant power level, some problems can occur with the efficiency of the transmission.  This occurs most noticeably in the electromagnetic wave system.  The problem with radio waves is that they scatter the energy in different directions through free space propagation.  This causes the efficiency to be much lower than if they could be transmitted directly to the receiving antennas.  If a world-wide electromagnetic wave WPT system was used, then free space propagation would cause numerous problems.   The free space energy would either go unused or would be received by antennae that the transmission was not intended for.  This would pose the most direct problem to consumers within the WPT grid.  Individual consumers would either not be receiving their required energy, or would be receiving too much and paying a much high electricity bill. These are some problems that are trying to be corrected through multiple antennae arrays, but high efficiencies have yet to be accomplished.
 
Another concern consumers have is the safety of free flowing energy and its affect on the human body.  Microwave beams are the main concern of wireless power transfer. The safety issues are closely related to those that involve cell phones, radar, and wireless internet. Unlike what many consumers think, cancer is not the main concern.  Typically other problems such as severe headaches, sleep disturbances, memory loss, learning disabilities, attention deficit disorder, and infertility affect a person before cancer. So far there has not been any reported major health issues related directly to microwaves but any extended period of EM exposure can cause serious health risks.
 
Another concern consumers may have is the fact that energy is around them all the time.  The air filled with energy is not a danger to humans.  The way the system works is that the transmitted energy only reacts with an object resonating at the same frequency as itself.  This means that the energy would only interact with something if it is vibrating at the same rate as the energies wavelength, or, is in resonance.  If an electronic device would have a system in itself that operate at the same wavelength as the energy being transmitted, it could fry the electronic device and blow the circuit.  Some things that could still be affected by wireless power include RADAR, x-rays, radar guns, and radio.  These are major things that will have to be dealt with if wireless power transfer becomes widely used.
 
The topic with the most unsolved concerns is the outer space transmission system.  This system uses massive solar panels to collect the Sun’s rays and send that energy back to Earth through microwaves.  The concerns are somewhat the same as any wireless power transfer system, but scaled to a much higher level.  Environmentalists are concerned about the affects on the ozone.  With beaming the amount of microwaves that would be generated from massive solar panels in space to Earth, there is valid reason to be concerned.  Even though concern is high, little to no research has been done on fixing this problem.  This will be more of a concern once wireless power transfer can reach that level.
 
Problems that may arise in the future if wireless power transfer replaces our modern ways of power transfer include transition to WPT, sustainability, and the affect on today’s electronics.  The transition to wireless power transfer would be same at first, using it to charge household items and electronics.  Then it could be used to charge electric cars and entire houses. Finally, it gets to the point where the entire world would be a completely wirelessly powered planet.   So what would this all mean for people who do not use electricity? Nothing, as long as there are no health risks.  What would the cost to consumers be to create this worldwide system?  That is a question that has yet to been answered.  Once the price is paid, the system would be very efficient.  The one thing that would need to be figured out is how big the solar panels would have to be to collect enough energy for the entire Earth. 
 
Also, the disposal or modification of the wired systems is a major issue. Another unsolved concern that worries consumers and scientists alike.  Researchers of wireless power are trying to invent a chip that can be placed into existing electronics to convert them to wireless.  This is still an ongoing process and has not yet been completed.
 

Applications

Applications of Wireless Power Transfer
Wireless power transfer has the ability to change the world with all the different applications it has to offer.  As simple as charging a cell phone to supplying the Earth with all the energy it needs.  The first applications that consumers are most likely to see would be a charging station that will range from about one to five meters. This is a small box-like object that will be able to charge compatible electronics within the range of the system.  Wireless power transfer charging systems are proven to have efficiencies near that of conventional charging devices.  For example, a household would need about one transmitter per room and allow the house be completely wireless.  This will strictly be a convenience to consumers and not serve any other service.  Electronic companies may not like this new concept due to the fact that they will not be able to make a different charger for everything they make and force consumers to pay for the different chargers. The biggest advantage, especially with personal devices incorporating wireless chargers is that one doesnot have to consciously ‘charge’ the devices, they charge themselves everytime they come within the range of  a power transmitter.
 
Once wireless power transfer becomes more advanced, the scale of applications can begin to grow.  The range of the electromagnetic waves will begin to increase.  Once the range reaches around twenty meters, entire homes will be able to be charged by a single transmitter located close to the home.  As the range expands entire blocks and streets will be powered by a single transmitter. Once this point emerges, consumers can notice cars converting to completely electric with the capability of being wireless charged.  Roads will adapt to have wireless chargers spaced so far apart so a car will be able to run endlessly.
 

Graph Showing Comparison between DC or AC Charging Process And Wireless Microwave Charging

Fig. 4: Graph Showing Comparison between DC or AC Charging Process And Wireless Microwave Charging

 

Fig 4:  This graph compares the charging capacity of a conventional AC or DC charging with that of a wireless microwave charging process.  The results conclude that charging wirelessly has little effect on total capacity of the charged device
 
The final step in this phase involves outer space solar panels. Using the outer space solar panels to collect the Sun’s energy, solar powered satellites will beam it back to Earth. This will severely reduce our dependence on conventional fuels. This will be a never ending supply of energy. At this stage of wireless power transfer, consumers will notice changes in the stock market including petroleum, natural gas, and coal.  Homes and cars will no longer need to be heated or powered by these resources.  The two major problems in the world are oil dependence and global warming.  This system will help treat both problems.  This will take many years to complete but the world should start to look at this as a permanent solution.  Wireless power transfer will solve the energy crisis.
 

Leading Companies Involved with WPT

Leading Companies Involved With WPT
There are currently few companies that are having marketable success with WPT. In the earlier technology of partly wireless power systems like Splashpower, one had to place the devices on an inductive charging pad or base.  

A Splashpad Wireless Charging Pad

Fig. 5: A "Splashpad" Wireless Charging Pad

Fig 5: The “Splashpad”, a brand of wireless charging pads based on mutual induction technology. 
    
Another company, Powercast, out of Ligonier PA, is on the cutting edge of WPT for small devices. Their transmitter, entitled PowercasterTM, uses patented algorithms that double the effective transmitting range without increasing the average power output.  Their receiver, entitled PowerharvesterTM, uses circuits much like those described for Dr. Mickle’s energy harvester to collect the incoming RF energy.   

Image Showing Real Dimensions of Powercast's Transmitting Circuit

Fig. 6: Image Showing Real Dimensions of Powercast's Transmitting Circuit
 
An IC (integrated circuit) is used within the receiver to increase the effective antenna area many times greater than the physical area. This is done by using a tank circuit (or LC circuit) to provide feedback energy to the antenna. The reason energy can be redirected through the circuit and back into the receiving antenna is because LC circuits do not dissipate any energy because of the lack of resistors. Even while moving the Powerharvester is able to capture up to 70% of the maximum power transmitted, as long as it is within the Powercaster’s effective range. Powercast’s chips have been commercially marketable for many reasons. They are proven to be more cost effective per Watt than batteries. It is environmentally friendly and is easily integrated into battery-operated devices. Powercast has also expanded the boundaries of WPT by offering devices that transmit at different voltages, frequency ranges, and environmental conditions.
 
However, larger devices like laptops are too big to be powered by RF and work better with inductive charging, where they need to be closer to the source. Another company dealing in WPT is eCoupled.  eCoupled has taken a different approach than Powercast and went with transmitting power through inductive coupling.  Using a shared magnetic field, power can be transmitted between two identical circuits through inductive coupling.  eCoupled has taken this technology to a new level by allowing their coupled power circuits to dynamically seek resonance to maximize the efficiency of the transmitted load.  Unconventional to normal inductive coupling, eCoupled uses ultra-high frequencies for communicating between the transceivers.  This enables one device to be slightly mobile from the other.  eCoupled has claimed power losses as low as 2% and increases conventional inductive coupling by over 36%. The company’s circuits can be driven off of AC or DC power and can power devices directly or power a rechargeable circuit.  Unique to eCoupled is their circuits’ ability to ‘talk’ to one another.  The transmitter can communicate with the object it is recharging and allow data to travel back and forth within the inductive coupling range.  The circuits also transmit at frequencies above the audible range so surrounding circuits will not receive signal attenuation from broadcasted energy .
 

 Conclusion

Conclusion
Wireless power transfer has the potential to change this planet on so many different levels.  Whether it is charging a handheld device, to changing the effect of global warming on this planet, wireless power transfer has an answer.  The most commercially viable application arising to counter the effects of global warming and the increasing demand for electricity is WPT through microwave transmission from space.  This application will supply limitless power to earth and also open up many new opportunities for space exploration.  With WPT through resonance and inductive coupling, emerging tech companies are able to broaden the capabilities of most small electronics including cell phones, PDAs, and mp3 players.
 
With global warming having significant impact on this earth by 2050, alternatives to high polluting fossil fuel plants need to be created.  Earth’s future could be dependent entirely on wireless power transfer.  Even though the actual process does not produce any energy, the improvements it can make to this earth are staggering. Only a few examples include pollutant free electricity, reduction of fossil fuel demand, a solution to the oil crisis, increased safety formedical procedures, and the mobility of having power anywhere at anytime. Currently wireless power transfer is the most marketable and sustainable alternative to fossil fuel power plants. With advancements in the field happening all the time, a worldwide wireless power transfer system is a possibility in the near future.

 

Comments

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Lighthouse DEV based in Maryland as delivered an “eye-safe” based laser WPT demo system to the U.S. Army that can transmit power for several miles very much like the system shown in this BBC video

wwwDOTbbcDOTcoDOTuk/programmes/p00yjt99

nice i liked it

 

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