Introduction:
Wireless power transfer is a concept that dates back nearly two centuries. The thought behind it is to be able to transfer power across significant distances without the use of cables or wires. In order to understand how this is possible we must take a look back to how the idea was first introduced. The foundation for the idea first came about when André-Marie Ampère discovered what it now known as Ampère’s Circuital Law in 1826. Simply put, this law shows that there is an induced magnetic field when there is an electrical current present in a closed loop. The second half to the principle of wireless power transfer was described by Michael Faraday in 1831 when he published his Law of Induction. This law describes how current can be induced in a conductor when there is a changing magnetic field. While both of these laws were known for quite some time and could be seen in many experiments, there was little theory relating the two. Many thought the results to simply be phenomena. This was until James Clerk Maxwell developed a theory that unified these previous theories of electricity and magnetism. This theory correctly predicted that electromagnetic energy was carried by electromagnetic waves. With this theory, Maxwell derived four equations describing electromagnetism that are now known as Maxwell’s Equations. Then in 1884, a scientist named John Henry Poynting came up with a theorem that used the conservation of energy to describe the flow of power across an area. Poynting’s theory gave us the correct analysis of wireless power transfer we base our calculations off of today. From here, ideas on the uses of wireless power transfer grew exponentially. From here many scientist and inventors came up with numerous ideas ranging from transferring power long distances to using to create motive power. One of these inventors was Nikola Tesla. Beginning around 1890, Tesla experimented with transferring power by means of inductive and capacitive couplings. With these couplings, the use of radio frequency transformers was used to generate high voltage outputs. With this, he attempted to use this near-field wireless power transfer to create a wireless lighting system. His ideas then grew in size. Tesla wanted to create a wireless power system that would help distribute power across the globe. While these ideas never panned out, he gave us a great insight into the possibilities created by wireless power transfer.
Types of Wireless Power Transfer
While there once was only a few methods of wireless power transfer available, today there are a plethora available all with different advantages and disadvantages. Often times it depends on the desired application of the power transfer that determines which method is used. In order to choose which one will most effectively meet the task at hand, one must have an understanding of the different options available. In order to do this, wireless power transfer will be broken down into two categories; non-radiative and radiative power transfer. Non-radiative also known as near-field is power transferred over short distances through means of magnetic or electric fields. Radiative or far-field techniques uses the concept of beaming power over long distances by transmitting electromagnetic energy that has been converted in to forms such as microwaves or laser beams.
Non-Radiative Power Transfer
Near-field power transfer often refers to the power transfer from one conductor to another by means of magnetic field. According to Ampère’s law, by putting an alternating current through one coil, a magnetic field is created around it. As described in Faraday’s Law, when a second coil is introduced into this magnetic field, a current is then induced and can be harnessed as. The system can also be referred to as a two-part transformer. The amount of power that is able to be transferred depends on a number of different factors. Things such the size and shape of the inductors, the distance between the coils, and the coil resistance all affect how efficient the transformer is. These transformers tend to only be efficient at very short distances and drop in efficiency very rapidly as you move them apart. These inductive couplings are often used to do things such as charge devices, such as phones and tablets, wirelessly.
One way scientists have learned to avoid this problem is by using resonance inductive coupling. Tesla first discovered the ability to use resonance in order to increase efficiency, but it was never used extensively. However, recently in the late 2000s, a patent was placed on this idea by a team from the Massachusetts Institute of Technology. Resonance inductive coupling is different in the fact that it uses resonance to increase the amount of power transferred. By providing an alternating current at a certain frequency to one coil and then tuning the receiving coil to receive at the same frequency a much larger amount of power can be transferred over a greater distance. Other advantages to this are that since the resonant wavelengths are often much larger than the resonators themselves, power can be transferred around objects in the vicinity or even through walls. This makes mid-range power transfer possible as well as safe for the environment around it.
Another form of non-radiative power transfer that is used is called capacitive coupling. Capacitive coupling or electric coupling is described as the transfer of power through the use of two electrodes creating a capacitance for which the power is able to be transferred through. In order to do this an electric field is used as source of energy to a cathode and anode making the space in between the dielectric. A dielectric is an electrical insulator that becomes polarized when an electric field is presented. Electrostatic induction is created when an alternating voltage is used. This alternating voltage creates an oscillating electric field, that when it is applied to the transmitter plate, an alternating potential is created on the receiver plate.
Radiated Power Transfer
When looking at radiated power transfer, there a few different ways that it can be done.
The first of these is radio-frequency energy harvesting. Radio-frequency or RF is a non-ionizing form of electromagnetic radiation that is emitted from billions of devices worldwide and continuing to increase especially in urban areas. Devices such as cell phones, Wi-Fi routers, and television and radio broadcast station all transmit some amount of RF energy making it widely available. This RF energy can then be harvested and converted into DC power through harvesting devices. This is done by having an antenna that receives this energy that is essentially free. Then this RF energy is sent to a RF to DC converter where it is converted and then sent through a power output management circuit to control the amount of power sent to the device. In order to save space, sometimes devices call rectennas are used which are the antenna and the rectifying device incorporated into a single device. These harvesting devices are often used in applications such as charging or powering devices very low amounts of power. At close range, this power is able to power devices such as GPS tags, medical sensors and even devices as large e-readers and headsets. It is also able to provide power to things at long distances such as sensors for monitoring air or structural stresses. Often times things that operate on a small amount of power for extended periods of time are potential places that RF energy can be implemented. This method of power also offers unique opportunities due to the fact that it is wireless. These sensors can be stored in places sealed off from environmental hazards such as moisture or in places not accessible by users.
The second form of radiated power transfer being discussed is transfer by means of microwaves. Microwave power transmission was first introduced by an electrical engineer and pioneer in the use of microwaves named William C. Brown. Brown first came up with the idea after first inventing the Amplitron. The Amplitron was a device that was used to generate and then amplify microwave frequencies. Since it is very difficult and inefficient to transfer electricity at low frequencies, the Amplitron opened up new opportunities. In 1964, Brown performed an experiment where he flew a miniature electric powered helicopter powered by microwaves. He was able to keep this helicopter in the air for 10 hours at an altitude of around 60 feet. This was possible due to the ability of microwaves being able to be oriented in certain directions along with its high-power density. The concept behind microwave power transfer is very similar to that of RF energy. The source produces microwaves and is oriented to point at a receiver rectenna. The source rectenna is then used to convert the microwave energy into electricity that can be utilized. Efficiencies have been seen to get as high as 85%. However, this is not an easy feat to accomplish. Many high energy microwave transfers require very large devices. In one study by NASA an output antenna with a diameter of nearly one kilometer was required while the receiver antenna would be an even larger 10 kilometers. This, along with the fact that the antennas must be within line of sight of each other, makes some of the applications unreasonable limiting our actual capabilities. However, many ideas are still proposed on what can be done with microwave power transfer. Concepts such as transmitting energy from orbiting satellites down to earth or beaming power to drones in order to keep them in the skies for extended periods of time are some ideas that have been proposed.
The third and final type of radiation power transfer being discusses is through means of laser beams. Laser beams differs from microwave power transmission as it refers to electromagnetic waves that have wavelengths within or close to the visible spectrum. Devices called photovoltaic cells are used to capture and convert laser energy into useable power. Lasers provide a much higher power density that can be focused into a narrower beam. While they can be relatively small devices, lasers are capable of transferring power over long distances while not interfering with existing RF signals due to lasers being in a different bandwidth. This narrower beam allows for fewer emissions as well as smaller devices used to receive the power. Many different applications have been proposed for laser power transfer and most of them tend to deal with outer space. Some of these are wireless power-driven spacecrafts. These spacecrafts would harness energy being directed at them through lasers in order to propel themselves through space. Another is device referred to as a space elevator. A space elevator that would be used propelled by wireless laser power along a tether connected to an object in space. This would allow for travel into space or orbit without the use of fuel-based engines such as the rockets we currently use. However, there are also a few downsides to laser power transfer. One important one of these is that laser radiation can be dangerous. If low power level lasers are shown into eyes of humans or animals, it can cause temporary or permanent blindness. When high powered lasers are used, potential burning can happen in localized spots due to the high concentrations of energy. The efficiency of converting laser light energy into electricity is also limited due to the photovoltaic cells. The efficiency of these cells tends to be anywhere from 50 to 70 percent depending on the wavelength of the light being used. These lasers also have to combat weather and atmospheric absorption. On cloudy or rainy days, light gets absorbed or scattered from the precipitation causing up to 100% losses. Lastly, similar to microwaves power transfer, there needs to be line of sight connection in order to not have interference.