Research Paper
Wireless Electricity

Imagine the following scenario unfolding. Nick has had a long day at work on Friday and is ready to go home. The whole twenty minute drive home all he can think about is how excited he is to take his family on a weekend trip to the beach. As he pulls into his driveway, he notices that the front light isn’t on like usual, but the fact is quickly lost in his mind as a thousand other thoughts are clamoring for his attention. Just as he steps through the threshold of the door, his phone goes off. It’s his wife. Something’s come up, their daughter isn’t feeling well. She tells Nick that they are at the local hospital and is about to tell him to pick something important up at the pharmacy when his phone dies. Panic sets in. The first thought that leaps into his head is what happened to his daughter. The second is what is he supposed to get to help her. After a couple minutes of searching around his house for a charger, Nick is finally able to call his wife back and discover that he was supposed to pick up the allergy medicine his daughter needs. While this situation may seem a bit more dramatic than a normal situation, dead cellphone batteries, and any battery for that matter, are becoming a familiar headache in a wireless world. But what if this headache had a cure? Imagine that instead of Nick’s phone dying once he got home, it started instantly charging once he entered his house. No cord or cable needed, just wireless electricity feeding directly into his phones battery and charging it up. This fantasy scenario might not be as far away as one might initially think. The following is from the June 8, 2007 edition of InformationWeek, “MIT scientists have been able to wirelessly light a 60-watt light bulb from a source seven feet away, and the experimenters believe it demonstrates -- at least theoretically -- that consumer electronics devices like laptops and cell phones one day could be charged without wires” (MIT). With so many devices being capable of “wireless use” it’s no surprise people are ready to get rid of one last wire, the power cord. With the demonstration by MIT scientists of its potential usefulness, wireless electricity is poised to become the most revolutionary innovation since the lightbulb.

HISTORY Since wireless electricity is not yet a reality, it would be safe to assume that it is a new and groundbreaking technology. That assumption, however, would be incorrect. Impressive demonstrations of wireless electricity have been performed over 100 years ago! In 1899, Serbian engineer Nikola Tesla built a 142-foot-tall, 12-million-volt electric coil in Colorado Springs and transmitted electricity wirelessly across 25 miles, illuminating 200 lamps with the charge. After he flipped the switch, flashes of lightning leaped from the coil, but no one was harmed (Schiffman). As impressive as Tesla’s display was, wireless electricity had been shown to be possible decades before him. 70 years before Tesla, Michael Faraday (picture on left) discovered electromagnetic induction. In electromagnetic induction, an oscillating magnetic field around an electromagnet produces a current in a nearby conductor--in effect, the current jumps the gap. While it is airborne, electric energy exists as a magnetic field (Schiffman). After learning that examples of wireless electricity have been seen since 1831, a question that immediately jumps to mind is, why is wireless electricity not a part of our everyday lives yet? There are several explanations for the apparent lack of progress over the past century, but perhaps the best explanation is that new forms for generating wireless electricity keep being created that are more efficient than the last. The Tesla coil, while producing stunning visual effects, was not the most practical way to transmit power. A Tesla coil is a pulsed air-core resonant transformer capable of creating enormous voltages and huge discharges (The Tesla). The result is the lightning like flow of electricity seen in the picture to the left, which are capable of wirelessly lighting a fluorescent bulb. Although the The Tesla coil can generate wireless electricity, the strong field damages radios, TV, and even pacemakers, hence their limited use. Today, the Tesla coils are mostly used for scientific experiments, x-ray generation, military experiments, lighting, and individual use (Spahiu). The next major breakthrough in wireless energy transfer came more 60 years later and in a completely different method than Tesla’s coils. In 1964 on CBS news, William C. Brown demonstrated a helicopter that was powered wirelessly by 2.45 GHz microwaves. The helicopter was comprised of a propeller attached to a rectena which directly converted the incident microwaves into DC power, keeping the helicopter aloft for 10 hours (William). In 1975, as technical director of a JPL/Raytheon program, Brown beamed power to a rectenna a mile away and converted it to DC power at an efficiency of 54% (William). Microwaves have proved themselves as a legitimate way to move energy wirelessly, and it is a technology still being developed today. The applications of microwaves I will discuss later. Laser beams have been used as a means for transmitting energy wirelessly since 1960. A laser beam system would work in a similar method as a microwave unit would. Laser beams are still being developed as a viable option for wireless electricity. Only recently has been the development of resonant magnetic coupling, first displayed by Massachusetts Institute of Technology professor Marin Soljacic (Wireless). This technology is most easily understood when compared to an opera singer shattering a wine glass with their voice, by matching the correct pitch to the resonant frequency of the wine glass.

DIFFERENT APPLICATIONS

Above was a brief history of some of the more promising forms in wireless power transfer. However, just because one form was invented later than another form does not necessarily mean that it is better. In fact, there is no real reason to argue for a particular form over another, when in reality, not all the forms have the same function. The functions for wireless electricity fall into two main categories, transferring large amounts of power from point A to point B, and providing power to mobile devices wirelessly. Methods such as microwave transmitters and lasers fall into the first, while resonant magnetic coupling (the method used by Witricity) belongs in the second. Despite being created for vastly different purposes, both uses of wireless electricity are becoming ever more important, and may one day revolutionize the way our world operates. Where Witricity focuses on the powering of small devices such as phones and TV’s, microwave transmissions and lasers focus on powering large equipment, namely the Earth. Since the industrial revolution, there has been an ever increasing demand for energy. With warnings of fossil fuel shortages and global warming, searches and funding for alternative energies has increased. One promising source is the sun. Indeed, solar panels are quickly becoming the popular way to both lower a households carbon footprint, and save money on electricity bills. But for all the benefits that solar panels provide, they still have some serious drawbacks. For starters, the sun is only visible for a portion of the day, leading to times when no energy can be collected. Also, the atmosphere protects us from many of the sun rays, cutting down on the total energy reaching the Earth’s surface. Finally, exposure to the elements means repairs are often needed as the wind and rain wear down on expensive parts. There is a better way to get solar power, however. A place where there is exposure to the sun 24/7. A place where rain and the elements do not wear down. A place where sunlight can be eight times stronger than on Earth, and there’s no shortage of real estate to build on (Sample). This magical land of solar energy is known as outer space. The main thing keeping us from tapping into this source right now is the cost of getting a station up, and sending the energy back down (Sample). This is where wireless electricity comes in. Specifically, microwave and laser transmission of energy.

MICROWAVE

The principle of the microwave transmission is simple. First, convert electricity to microwaves, then send those microwaves to a receiver stationed somewhere else, and finally convert those microwaves back into electricity. The trick, however, lies in converting electricity to microwaves and vice versa. Microwaves can be generated by the use of a magnetron. A magnetron resonates much like a flute does, except instead of creating sound waves, it produces electromagnetic waves (Woodford). These waves are short radio waves, generally between 1 and 30 cm (Woodford). A magnetron is constructed of a negatively charged cathode (shown in yellow), surrounded by a positively charged anode (shown in red). When the cathode is heated up, electrons are released from it and they make a path for the anode (line 3). However, there is a powerful magnet located underneath the anode, creating a magnetic field parallel to the cathode. Now there is both an electric field from the cathode to the anode, and a magnetic field between the two. Instead of the electron going straight from the cathode to the anode, it is caught in the magnetic field and speeds around in a circle (blue line on picture). These electrons flying by the cavities creates a resonance that emits microwave radiation. The microwave radiation is then gathered and sent out by an antenna or satellite dish (the explanation for a magnetron was found on explainthatstuff.com). Once the electricity has been converted into microwave radiation and has been sent somewhere, it needs to be converted back into electricity for it to be of any use to anyone. This is where the rectenna comes in. The definition given by wisegeek.com for a rectenna is a rectifying antenna, an antenna used to convert microwaves into DC power. It does this through a series of steps. Once the microwave power is received by the individual antenna, it passes through the high frequency rectifying diodes. The diode is then used to convert high frequency power to DC voltage. The electricity is then sent through a low-pass filter before being delivered as the final product of DC power (Karmakar). Rectennas have been created to have efficiency’s of 90%, meaning a loss of only 10% (wisegeeks). The whole journey of solar energy from space would look something like this. Solar energy is collected by huge solar arrays in space. The solar energy is then converted to microwave radiation and beam it down to a combination rectifier-antenna, called a rectenna, located in an isolated area. The rectenna would convert the microwave energy back to DC (direct current) power (Beam).

Aside from the high costs associated with launching solar panel satellites, another component keeping this idea from fruition is the fear factor that sending microwaves down to Earth has. Many people fear that since an unprotected microwave oven can cause cancer, beaming down microwaves would also do the same thing. Another fear is that anything that crosses through the path of the beam would be fried, be it a hot air balloon or a bird. However, according to Dr. Neville Marzwell, technical manager of the Advanced Concepts & Technology Innovations program at NASA's Jet Propulsion Laboratory, the dangers of being close to the microwave beam would be similar to the dangers of cell phone transmissions, microwave ovens or high-power electrical transmission lines (Beam).

LASERS

An alternate plan to microwave power transmission would be the use of laser beams. The basic plan would remain essentially the same, with power being collected by solar panels in orbit and then sending energy back down to Earth. Lasers provide an advantage over microwaves in that they do not spread out as much with distance, meaning more of the energy sent is received on target. The main difference between microwave transmission and laser transmission is the wavelength. While microwave transmission uses 2.45 GHz or 5.8 GHz (5 cm, 12cm), laser energy transmission takes advantage of the atmospheric transparency window in the visible or near infrared frequency spectrum (Summerer). Laser energy transmission allows much higher energy densities, a narrower focus of the beam and smaller emission and receiver diameters (Summerer). A smaller receiver diameter means that fewer collector antennas would need to be build. The use of lasers for transmitting energy has already been proven quite well. The longest distances between emitting and receiving points achieved so far is in the order to hundred kilometers. The largest amount of energy transmitted so far was during an experiment by the US Jet Propulsion Laboratory in 1975, when 30 kW were transmitted from a 26 m diameter parabolic dish to a 1.54 km distant rectenna with 85% efficiency (Summerer). Besides beaming down energy from the heavens, lasers have other uses. Researchers at NASA's Marshall Space Flight Center, Huntsville, Ala., and Dryden Flight Research Center, Edwards, Calif., and the University of Alabama in Huntsville have flight-demonstrated a small-scale aircraft that flies solely by means of propulsive power from an invisible, ground-based infrared laser (Beamed). The demonstration was a key step toward the capability to beam power to an aircraft, allowing it to stay in flight indefinitely — a concept with potential for the scientific community as well as the remote sensing and telecommunications industries (Beamed). Again, as with microwave transmission of power, there are many fears associated with the transmission of power by lasers. When most people think of a laser beam from space, their first thought is that it’s a weapon. And indeed, lasers do have the potential to be used as weapons. One reason the U.S. has not pursued this option as much as it might seem they should is because they have a treaty with Russia that prohibits high powered lasers from space (Beam). However, the risk of a laser transmitting power accidentally wandering across a city and destroying everything in its wake is unlikely. Even still, it is strongly recommended that power beams to Earth be visible (green, for example) so that the general populace can be aware of their steady location when operating (Dickenson). Should a plane fly through the beam on accident, the biggest concern would be that the passengers eyes might be blinded, not that the plane would be zapped up (Dickenson).

WIRELESS ELECTRICITY AT HOME

Broadcasting electricity for hundreds of miles is just one application of wireless electricity. As the scenario in the introduction showed, another valuable application of wireless electricity would be the charging and powering of everyday household objects, such as phones, computers, TV’s, and small kitchen appliances. Several companies have come up with several different ideas on how to turn this ideal into a reality. Among them are the Powermat, PowerBeam, UBeam, and Witricity. Most of them are taking different approaches to the task of wireless electricity, with varying levels of usefulness. The Powermat (seen at left) is one of the first wireless chargers to hit the market. When a phone or other such device has been coupled with a Powermat, it only needs to set on the mat to start charging, no plugging in of cords is required. The Powermat uses magnetic induction, the same phenomenon discovered by Michael Faraday decades ago. Electricity in the Powermat creates a rapidly changing magnetic field above the mat, which are converted by receivers in the device into electrical power (Powermat). Despite the Powermat transferring electricity wirelessly, the range between the mat and device must be so small that it is not too much more practical than just plugging the device in. PowerBeam takes a much different approach to the task. PowerBeam uses lasers to transfer electricity in the same way NASA hopes to beam electricity from space, albeit on a smaller scale. Electricity is turned into optical energy (laser) and pointed at the receiver to be turned back into electricity (PowerBeam). While the range of the PowerBeam is considerable more than the Powermat (up to 100 meters), it requires line of sight, meaning you cannot use this technology on a mobile device. Several other approaches to the wireless electricity problem have been made. One of them, UBeam, has a story that is rather unorthodox. 22 year old Meredith Perry is a recent college graduate, and new owner of a company she created, based around a product she invented (Noguchi). Not only does Perry have to worry about the success of her product, she also has to worry about every other aspect of running the business. Her approach to creating wireless electricity is just as unorthodox as her story is. "What happens is, the ultrasound, which vibrates the air, vibrates what's called a piezoelectric transducer," she says. "And what happens is the ultrasound will vibrate the piezocrystals, and the crystals will move back and forth, and that will generate an electrical current” (Noguchi). In the current race for creating the most practical form of wireless electricity, there appear to be two groups. The first group focuses on distance, and trying to get as much space between sender and receiver as possible. However, this comes at the cost of power. Products such as PowerBeam lack the power to run much more than a lamp, powering a laptop is still a ways off. The other group focuses on getting enough power to devices, but at the cost of distance. The Powermat and other products can power stronger devices, but their wireless range is limited to a few centimeters. This is where Witricity comes in. CEO Eric Giler’s dream is to combine the best of both world’s, power and distance (Sutter). Not only is Witricity working to power mobile devices, laptops, and TV’s, but Witricity also has plans to wirelessly charge electric cars. To pull of these feats, Witricity is using resonant magnetic coupling. This process creates a magnetic field that causes the receiver to resonate at a certain frequency, causing electricity.

IDEAL

Let’s go back to the scenario in the beginning where Nick’s phone died. But this time, things have changed. Hundred’s of miles above Nick’s head, solar panels are transferring the sun’s power into electricity. This electricity is then converted into microwaves and beamed to the power plant in the city Nick lives in. The power plant then beams this energy to a receiver on Nick’s house. At Nick’s house in a wireless device that creates an area of wireless electricity, almost like a Wi-Fi hotspot. Now when Nick steps inside his house, his phone instantly begins charging wirelessly. In fact, everything he turns on does so wirelessly. When he pulled into his garage, his electric car started to charge without a cord. His TV is attached to the wall with no visible cables. He pulls out his blender from it’s cupboard space and makes himself a milkshake without even thinking about having to plug it in. This totally wireless world may seem like a far off ideal, but it’s premise is feasible, and it is getting closer all the time. As the United States and other developed nations continuing searching for more sustainable sources of energy, the likelihood of these technologies becoming a household staple only increases.

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