Wireless energy transfer

LIFE WITHOUT CORDS

Have you ever thought to yourself, "There are too many cords here," or wished you could put an electronic item somewhere else in a room but couldn't because of how far away the plug for you power cord was? That problem could end in the near future. Some scientists have found an efficient way of transmitting power wirelessly. Though there are a few obstacles to overcome, the idea of wireless power seems like a very distinct possibility in the near future.The man given the most credit for the idea of transferring energy wirelessly is Nikola Tesla. Tesla advanced the science of electricity and energy transfer. He invented such things as the transformer, currently the backbone of modern energy transfer. He made other useful inventions such as circuit breakers and condensers; he also came up with an idea for a wireless telegraph, which paved the way for modern radio (Nikola Tesla Museum).Additionally, Tesla played a big part in creating the modern power system. He had a bitter rivalry with Thomas Edison who supported Direct Current (DC) rather than Alternating Current (AC). Edison used DC for his power plants while Tesla tried to convince him to use AC. AC is more dangerous, but it has the ability, through transformers, to be sent much further with little energy loss. The key problem with using DC is that it does not work well over long distances. It is hard to produce the high voltages needed from a power plant in order to easily transport the electricity over distance. If the electricity is transported at low voltages, the current must be increased to compensate for the low voltage. This is not very efficient as seen in Ohm's law.

V= I*R

This equation shows the voltage drop over a given resistance. With the current higher, the voltage lost due to a certain resistance is increased. Since everything has resistance, even with the best wiring, the longer the electricity travels the more of it will be lost. A solution to this is the transformer. A transformer sets up two coils of wires. The first is small and is attached to the power plant. When a current is passed through it, a magnetic field is formed. The second coil is in close proximity to the first and is affected by the field. The second coil has many more wrappings of wire around it. The amount of times a wire is wrapped around a coil determines the effect the magnetic field will have on it. The next equation demonstrates that the voltage changes in relationship to the number of times the coil is wrapped changes. In the equation, "s" stands for secondary and "p" for primary.

This equation illustrates that the ratio of the secondary voltage over the primary voltage is equal to the ratio of the number of turns around the secondary coil over the number of turns in the primary coil. Also, as the voltage is stepped up, the current is stepped down, as expressed in the following equation.

This creates an ideal situation in which the voltage is high and the current is low, thereby allowing the power plant to send the electricity over large distances without losing much along the way. Once at its destination, the current is stepped back down to a safer voltage (Liebrand 2007). This concept is the basis of power transfer today. Transformers were just one of Tesla's revolutionary ideas, but he had much grander ones than that. Between 1900 and 1902, Tesla received multiple patents for his ideas of wireless energy transfer. He had planned to have a "world-wireless-system" where everyone would receive power without cables. Eventually, he planned to use a very large power plant and magnetic fields to transfer power to the entire world. It is on these ideas that new wireless energy transfer is based. Tesla was unable to make headway on this project because his financial backers pulled out (Grebb 2005). Since he was not able to bring his idea of wireless energy to fruition, he turned his sights to other areas of research. With his efforts thwarted, Tesla spent his later years expanding other areas of engineering. He invented different turbines, pumps, and other water transfer systems that were unique because they did not have paddles in them. Some of these designs are still in use today (Nikola Tesla Museum). But now, about a century later, Tesla's idea for wireless energy transfer may be resurrected; someday a true "world-wireless-system" might actually be created. Transferring energy wirelessly has its complications but is fundamentally simple. The one previous method that scientists have been aware of is using electromagnetic waves. Electromagnetic waves come in many forms such as light and radio waves. These are generally harmless, but there are others such as radiation or microwaves that can be quite harmful. The kind of waves that would be used for energy transfer would be more like those of radiation than ones of radio or light. This is dangerous though because these waves affect most organic and inorganic elements. The design requires the source of power and the object to be in direct line of sight. If something gets between them, the object would stop receiving power, but also whatever got in the way would be affected by the wave. In the case of living beings, this could be very dangerous (Derbyshire 2007). Recently, an idea occurred to some researchers at Massachusetts Institute of Technology (MIT): Rather than using electromagnetic waves, they could use magnetic fields similar to a transformer, but, rather than raising the voltage, the power could be transferred and the receiver could take the voltage as it is or even possible reduce it. This cannot be an ordinary magnetic field though; if it were, it would affect everything in the room. It would not be a pretty sight if a person was to walk into the kitchen and have all the metal bowls and silverware attached to the walls. This is where the ingenious MIT idea fits in. The researchers created coils that oscillate at a specific frequency with the AC. This creates a magnetic field that only affects objects oscillating at that frequency. Thus, most organic and non-organic elements are not affected at all, and the power not absorbed is returned to the circuit. The addition of frequency to the design enhances this technology greatly also because it increases efficiency. If circuits that did not oscillate at certain frequencies were used it would be about one thousand times less efficient (Hadley 2007). Most likely these coils would be placed in the walls of a building. This would allow for the most coverage inside and thereby maximizing efficiently. Wireless energy transfer has been proposed before but so far the research has not yielded practical results. One conceived concept was to convert electricity to electromagnetic waves, as previously mentioned, but in this case, the waves would be beamed from space to the earth. The United States government was investigating the idea, but at some point, it decided that electromagnetic waves could be even more useful as a weapon. Such use is now being evaluated as a possible non-lethal weapon. Similar to the concept of lasers in science fiction, these waves could be set to immobilize, or they could kill (Grebb 2005). Having this kind of technology in homes or offices would have many advantages. One of the best results would be that cell phones, laptops, or other portable devices could stay charged as long as they are in the home or office. This possibility inspired the research at MIT. Marin Soljacic, an assistant professor of physics at MIT, was once awakened by his cell phone at 3 a.m. The phone’s battery was dying, and the warning signal began beeping. He was irritated and wondered why no one had invented a way to have his phone continuously charging while in his house. He started thinking up a solution to this problem, and with some help from his colleagues, it seems that he has succeeded (Castelvecchi 2007). Another advantage is the freedom of space. With wireless power, it would be possible to arrange a room with little concern for placing electronics. Often, in room arrangement, large electronics are the first items placed, but they are limited to a set range away from a power outlet. Also, these items are often placed alongside the wall so that the cords don't get in the way or trip people. If the power cords were removed, this would eliminate that problem and allow for more pleasing arrangement opportunities. Unfortunately, this aspect of the technology has limitations. One is that it does not work over long distances (Castelvecchi 2007). In a regular house or apartment, most rooms are small enough that all electronics would be within range of the power source. Large dining halls or other large rooms could be a bit more complicated if an electronic item were to be placed at the center of the room. In the experiments done at MIT, the researchers discovered that at the approximate distance of six feet the power efficiency was 40% for their coils (Foire 2007). Due to the drop in efficiency over range it could be a problem to have certain objects too far from the wall. If an object is not close enough it might not be able to draw the amount of power it needs to run.

Efficiency vs. Distance

Figure 1: This shows the drop in efficiency over distance (Kurs 2007).