Wireless power transfer (WPT) has recently become a hot topic. The idea behind it, however, has been known for more than a century when in 1891 Nikola Tesla lit electric lamps wirelessly in his laboratories in New York City. This experiment was well ahead of its time, as there arguably existed no devices at the time that actually needed wireless power.
Since the beginning of the new century, there has been a wide propagation of devices that need wireless power, such as cell phones, laptops, and electric vehicles, which has set the stage for the second coming of wireless power. In this second wave, the main areas of WPT applications can be categorized as follows :
- Industrial (operation in harsh environment, e.g. mining, next to explosive gases)
- Automotive (battery charging for electric cars)
- Aerospace (transferring energy to airplanes, satellites)
- Consumer electronics (charging a cell phone or a laptop wirelessly)
- Biomedical (inductive interface to power implantable biomedical devices)
1 Induction Physics
Even though terms “wireless power” and “power through air” are relatively modern, the physics behind them is fairly established: Maxwell’s equations were formulated in 1862. In particular, there is Faraday’s law of induction that states in case of harmonic oscillations: “The voltage induced in a closed circuit is proportional to the magnetic flux it encloses”. In practice, it means that if you have two coils, then the magnetic field from the first (transmit) coil passes through the second (receive) coil and induces voltage in it.
This causes voltage to appear in the second coil, which in turn causes current to flow in the second coil. The current then flows through a load connected to the second coil. Then we can say that there is a power transfer between the two coupled coils.
It is fairly straightforward to come up with equations that relate voltage applied to the first coil, input power, power delivered to load, and efficiency of the power transfer. We design our system with a goal to minimize percentage of the power lost in the transmit coil, in the receive coil and by means of radiation.
As we all know, magnetic fields are invisible. However, there are tools that can be used to help visualize the processes taking place during the wireless power transfer. By positioning the coils at four distances (7.5cm, 10cm, 15cm, and 20cm) from each other, one can analyze the efficiency of the power transfer. A video can then be generated to show a magnetic field around the two coils and that demonstrates the input power, delivered power and efficiency for a certain input voltage.
Combined (click here for video)
Magnetic field around each coil is proportional to current that flows in it. The observations that we make from the simulations are:
- The closer the coils are to each other, the more efficient is the transfer between the coils. In other words, higher percentage of input power reaches the load in the receive coil.
- The farther the coils are from each other, the higher the current is in the transmit coil.
- The delivered power first increases with distance (even though the efficiency decreases!), has a maximum and then decreases.
Finally, one can demonstrate the radiation pattern from the system of two coils. The field is plotted in a 60×60 m domain, which is a little over one wavelength at 6.78 MHz. In this particular example, the radiation loss is tiny percentage of input power.
Radiation (click here for video)
2 Challenges Facing Wireless Power
Even though wireless power is a promising technology, there are certain challenges that may limit its practical use.
First, transmit loads that excite currents not only transmit power to the cellphones or computers that we have around the house, but also they excite undesirable eddy currents in nearby objects, which may include ourselves, our children or pets. These eddy currents lead to dissipation of heat inside objects that can be quantified by what’s called the Specific Absorption Rate (SAR). The value of SAR is limited by FDA regulations and it is of the order of 2-3 Watts per kilogram. The value of SAR is highly non-uniform and increases drastically when near the windings of a transmit coil. Care must be taken as to keep the power level under control.
Another challenge facing WPT is the concept of “wireless homes,” or when the handheld devices can be charged anywhere while inside the home. This means that the transmit coils used for charging will have to be pretty sizeable (a few yards) to be able to reach anywhere in the house. During the process of charging, most of the room, along with people inside, will be subjected to high-frequency RF fields. Wireless power in not selective: power generally cannot be “focused” toward a particular device that we want to charge. Do people want to live inside a giant microwave? Clearly, there is a still a long way to go until the idea of “wireless home” becomes widely accepted.
Finally, in any power transfer, wired or wireless, there are heating loss. Efficiency of the wireless power transfer is far from being ideal; ninety percent efficiency is considered to be good, but in many WPT cases, the values are much lower than that.
When it comes to specific applications of wireless power, each use should be carefully considered. There are a few obvious ones that nearly everyone has heard about:
(a) cell phones on the low-power charging pad,
(b) electrical vehicle changed wirelessly in the garage with transmit coil being located in the garage floor,
(c) implant charged inside the human body.
Each one of these applications has its own justifications: (a) the area covered by RF field is small and power level is small; (b) it is difficult for humans to access areas with high RF fields; (c) an absolute necessity as using wireless power to charge the implant battery is still preferable to surgical intervention. Two more examples that we can think of are to wirelessly power a lamp on the ceiling, and to wirelessly power TV on the wall. Both of these examples clearly fall into “difficult to access” category.
As wireless power becomes more and more a part of our lives, it is important to understand what is behind the wireless charging process.
1 A. Abdolkhani, “Fundamentals of Inductively Coupled Wireless Power Transfer Systems,” Wireless Power Transfer - Fundamentals and Technologies, ISBN 978-953-51-2468-9, Print ISBN 978-953-51-2467-2, 138 pages, Publisher: InTech, Chapters published June 29, 2016 under CC BY 3.0 license.