Wireless Power Transmission
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Wireless power transmission is the means to power devices without a built in power source such as a generator or battery. There are multiple needs and uses for such technology. One initial use of such technology is found in powering small devices where much of the size of the device is in the battery itself. By eliminating the battery in a small device it would be possible to compact the device even further. Furthermore, on a larger scale as consumable energy sources on the planet are dwindling in number it remains an important task to look to the future. If it was possible to transmit power wirelessly it would be economical to retrieve power from outer space and simply transmit it back to the planet’s surface as an endless power source. In our initial research we discovered many have looked into the feasibility of wireless power transmission and there are many solutions that all offer promise. Our team chose to research the feasibility of wireless power transmission through inductive coupling. This consists of using a transmission and receiving coils as the coupling antennas. Although the coils do not have to be solenoid they must be in the form of closed loops to both transmit and receive power. To transmit power an alternating current must be passed through a closed loop coil. The alternating current will create a time varying magnetic field. The flux generated by the time varying magnetic field will then induce a voltage on a receiving coil closed loop system. This seemingly simple system outlines the major principle that our research investigated.
This document will detail the need and usefulness of wireless power transmission and furthermore the feasibility of using inductive coupling as the means for wireless power transmission. The subject matter of the report will be directed towards the knowledge level of an electrical engineer. Thus some points about general circuits may not be explicitly stated as they have been taken as common knowledge for the intended audience. However, it is intended that anyone with an interest in electrical circuits and more importantantly transformer theory or electromagnetic fields would be able to understand and follow the subject matter outlined in the following document. The report will outline our teams design process and the logical steps we took in our experimentation and design of the final unit. The first section of the document will explicitly illustrate the problem and what the group intended to accomplish. With the complexity of the problem in mind and what we must accomplish our team then began research on the available means to transmit power without a physical connection. Once the initial background research was accomplished it was necessary to layout the advantages and disadvantages of all the available means for wireless power transmission. Once all the necessary criteria for each system were known we chose the best solution for the problem. After our team had chosen upon using inductive coupling we all began to review the major theories that would determine the constraints of the system and what pieces of hardware must be designed to achieve the transmission of wireless power.
For the completion of this project, we were asked to wirelessly transfer the power of an AC oscillating waveform into a DC voltage on the receiving end which will be used to light an LED to demonstrate the instantaneous power transfer. The frequency of oscillation of the AC signal must not exceed 100MHz. The power transfer needs to be done over a two feet distance or greater. The transferred AC power needs to be converted to DC power and boosted up enough to drive a low power display design, such as an LED in continuous or pulsed mode.
The whole system must be FCC compliant. The detailed specifications are listed in Appendix A.
Nikolai Tesla was the first to develop the designs for wireless power transmission. Tesla was famed for his work in the research and work with alternating current. His wireless research began with his original transformer design and though a series of experiments that separated the primary and the secondary coils of a transformer. Tesla performed many wireless power transmission experiments near Colorado Springs. In Tesla’s experimentation, Tesla was able to light a filament with only a single connection to earth . Tesla’s findings lead him to design the Wardenclyffe plant as a giant mushroom shaped wireless power transmitter. Tesla was never able to complete construction of this project.
Space Satellite System
The concept of wireless power transmission has been an area of research that the U.S. Department of Energy (D.O.E.) and the National Aeronautical Space Administration (NASA) have been working to develop. NASA has been looking into research to develop a collection of satellites with the capability to collect solar energy and transmit the power to earth. The current design for project by NASA and DOE is to use microwaves to transfer power to rectifying antennas on earths .
Microsystem and Microsensor Power Supply
Currently, the use of inductive coupling is in development and research phases. There several different projects that use inductive coupling to create alternatives for batteries. One developed at the Tokyo Institute of Technology is to develop a power supply for a medical sensor while it is left inside the human body. In this system, , power was transmitted by both electromagnetic waves when at close distance to the transmitter an also by magnetic flux when at farther distances. The receiver portion utilizes a cascade voltage booster to charge capacitors within the device to provide the necessary power to the system. Another similar project, , done at Louisiana State University in Baton Rouge, uses inductive coupling in a similar method recharge an internal small battery in a small bio-implanted microsystem.
In our research, as well as practical knowledge, we knew of three possibilities to design a device. There are the use of antennas, inductive coupling, and laser power transfer. In addition, we had to be aware of how antennas and inductive coupling would be affected by the frequency we select.
Antennas are the traditional means of signal transmission and would likely work. In initial research, it appears that system utilizing antennas can receive power gains based upon the shape and design of the antenna. This would allow more power actually being sent and received while also have a small input power. The difficulty comes in the trade off of antenna size versus frequency. In attempting to stay in a lower frequency, one would be require using antennas of very large size.
Inductive coupling does not have the need for large structures transfer power signals. Rather, inductive coupling makes use of inductive coils to transfer the power signals. Due to the use of coils rather than the antenna, the size of the actual transmitter and receiver can be made to fit the situation better. The tradeoff is for the benefit of custom size, there will be a poor gain on the solenoid transmitter and receiver.
Laser Power Transmission
The concept of laser power transmission is addressed in the research of NASA and NASDA solar programs. Lasers would allow for a very concentrated stream of power to be transferred from one point to another. Based upon available research material, it appears that this solution would be more practical for space to upper atmosphere or terrestrial power transmission. This option would not be valid to accomplish our tasks because light wavelengths are higher than the specified allowable operational frequencies.
Very High and Greater Frequency Ranges
High frequency transmissions are common in several devices including cell phones and other wireless communications. Higher frequencies can be made to transmit in very specific directions. In addition, these antennas can be rather small. This set of frequency ranges includes microwave frequency bands. Very High Frequencies to Extremely High frequencies are described as being in the range of 30 MHz to 300 GHz and Microwave frequencies are described as being the range of 3 GHz to 300 GHz. The safety issues of using the high end of the spectrum are not completely known. There is currently research looking into the safety of microwave and higher frequencies. However, many of the devices in this frequency range are not permissible due to the frequency limitations placed on our research.
Very Low to Extremely Low Frequency Ranges
Antennas of these frequencies would need to be of sizes that are very impractical to build and would be better suited for power transmission over wire. Several of these frequencies are specifically used for submarine communication transmission . Extremely low frequencies and possibly other frequencies in the band up to 3 KHz have the uncertain risk of being potentially hazardous the humans and the environment. There is still on going research on the dangers on very low to extremely low range frequencies.
Low, Medium, and High Frequency Ranges
Radio Frequencies in these bands seem to have few hazardous concerns given by the FCC. In addition, these frequencies are commonly used as the primary frequency bands of radio transmission. The high frequency band is typically used in short range communications due to the ease of the reflection of these waves off the ionosphere. This range is described as being from 3 MHz to 30 MHz. In addition, this frequency range includes two experimental frequency bands. The major disadvantage of working in this frequency range is the inability to properly test in the design phase due to effects parasitic capacitance in breadboards . Medium Frequency includes the AM broadcast band. Medium frequencies are described as being from 300 KHz to 3 MHz. This band includes one band used for testing purposes. The Low frequency band is primarily used for aircraft, navigation, information and weather systems . In addition, this frequency includes a band commonly used for testing purposes. The low frequency band is described as being from 30 KHz to 300 KHz.