WIRELESS BATTERY CHARGING SYSTEM USING RADIO FREQUENCY ENERGY HARVESTING
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Joined: Sep 2010
18-10-2010, 09:24 AM
Daniel W. Harrist
It seems these days that everyone has a cellular phone. Whether yours is for business purposes or personal use, you need an efficient way of charging the battery in the phone. But, like most people, you probably don’t like being tethered to the wall. Imagine a system where your cellular phone battery is always charged. No more worrying about forgetting to charge the battery. Sound Impossible?
It is the focus of this thesis to discuss the first step toward realizing this goal. A system will be presented using existing antenna and charge pump technology to charge a cellular phone battery without wires. In this first step, we will use a standard phone, and incorporate the charging technology into a commercially available base station. The base station will contain an antenna tuned to 915MHz and a charge pump. We will discuss the advantages and disadvantages of such a system, and hopefully pave the way for a system incorporated into the phone for charging without the use of a base station.
Cellular telephone technology became commercially available in the 1980’s. Since then, it has been like a snowball rolling downhill, ever increasing in the number of users and the speed at which the technology advances. When the cellular phone was first implemented, it was enormous in size by today’s standards. This reason is two-fold; the battery had to be large, and the circuits themselves were large. The circuits of that time used in electronic devices were made from off the shelf integrated circuits (IC), meaning that usually every part of the circuit had its own package. These packages were also very large. These large circuit boards required large amounts of power, which meant bigger batteries. This reliance on power was a major contributor to the reason these phones were so big.
Through the years, technology has allowed the cellular phone to shrink not only the size of the ICs, but also the batteries. New combinations of materials have made possible the ability to produce batteries that not only are smaller and last longer, but also can be recharged easily. However, as technology has advanced and made our phones smaller and easier to use, we still have one of the original problems: we must plug the phone into the wall in order to recharge the battery. Most people accept this as something that will never change, so they might as well accept it and carry around either extra batteries with them or a charger. Either way, it’s just something extra to weigh a person down. There has been research done in the area of shrinking the charger in order to make it easier to carry with the phone. One study in particular went on to
find the lower limit of charger size . But as small as the charger becomes, it still needs to be plugged in to a wall outlet. How can something be called “wireless” when the object in question is required to be plugged in, even though periodically?
Now, think about this; what if it didn’t have to be that way? Most people don’t realize that there is an abundance of energy all around us at all times. We are being bombarded with energy waves every second of the day. Radio and television towers, satellites orbiting earth, and even the cellular phone antennas are constantly transmitting energy. What if there was a way we could harvest the energy that is being transmitted and use it as a source of power? If it could be possible to gather the energy and store it, we could potentially use it to power other circuits. In the case of the cellular phone, this power could be used to recharge a battery that is constantly being depleted. The potential exists for cellular phones, and even more complicated devices - i.e. pocket organizers, person digital assistants (PDAs), and even notebook computers - to become completely wireless.
Of course, right now this is all theoretical. There are many complications to be dealt with. The first major obstacle is that it is not a trivial problem to capture energy from the air. We will use a concept called energy harvesting. Energy harvesting is the idea of gathering transmitted energy and either using it to power a circuit or storing it for later use. The concept needs an efficient antenna along with a circuit capable of converting alternating-current (AC) voltage to direct-current (DC) voltage. The efficiency of an antenna, as being discussed here, is related to the shape and impedance of the antenna and the impedance of the circuit. If the two impedances aren’t matched then there is reflection of the power back into the antenna meaning that the circuit was unable to receive all the available power. Matching of the impedances means that the 2
impedance of the antenna is the complex conjugate of the impedance of the circuit. The energy harvesting circuit will be discussed in Chapter 3.
Another thing to think about is what would happen when you get away from major metropolitan areas. Since the energy we are trying to harness is being added to the atmosphere from devices that are present mostly in cities and are not as abundant in rural areas, there might not be enough energy for this technology to work. However, for the time being, we will focus on the problem of actually getting a circuit to work.
This thesis is considered to be one of the first steps towards what could become a standard circuit included in every cellular phone, and quite possibly every electronic device made. A way to charge the battery of an electric circuit without plugging it into the wall would change the way people use wireless systems. However, this technology needs to be proven first. It was decided to begin the project and implimentation with a cellular phone because of the relative simplicity of the battery system. Also, after we prove that the technology will work in the manner suggested, cellular phones would most likely be the first devices to have such circuitry implemented on a wide scale. This advancement coupled with a better overall wireless service can be expected to lead to the mainstream use of cell phones as people’s only phones. This thesis is an empirical study of whether or not this idea is feasible. This first step is to get an external wireless circuit to work with an existing phone by transmitting energy to the phone (battery) through they air.
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