What is Witricity and its Advantages over Inductive Charging Henry Ho∗ Department of Physics and Astronomy, University of British Columbia 6224 Agricultural Road, Vancouver, British Columbia, Canada, V6T 1Z1 (Dated: December 7, 2011) A new form of wireless energy transfer, named Witricity, is compared with traditional inductive charging. Its use of magnetic resonance coupling makes it far more efficient than inductive coupling at varying distances, but only at the resonant frequency. There is also evidence that supports the use of Witricity for handheld devices. I. INTRODUCTION As our modern lifestyle continues to become increas- ingly complex with the emergence of new electronic de- vices, the need to power them also grows in complex- ity. Power cables are becoming a hindrance as they oc- cupy multiple power sockets and require frequent main- tenance. The proposed solution for these problems is wireless charging. Wireless charging allows devices to charge in areas near or under water. This method of charging can extend to the medical field where implants, such as pace makers, can be charged wirelessly [1]. Wire- FIG. 1. Set-up for this experiment less charging is not theoretically difficult, but there are obstacles researchers must face. Efficiency and versatility problems are inherent in inductive charging. In 2007, a addition, their modified inductive coupling system built group of researchers in MIT presented an alternative to specifically for high power energy transfer (in this case, 2 traditional wireless charging through inductive coupling. kW) had an 80 percent efficiency over a distance of 1 cm. They used the principle of magnetic resonance coupling This is impractical because a power cable would be much and branded this method as Witricity. In order to ob- more reasonable and efficient at this distance. In addi- serve the differences between Witricity and traditional tion, efficiency in traditional inductive charging systems inductive charging, one must first understand the basics drop quickly as a function of distance. A higher current of both techniques. will need to run through the source coil to compensate for this drop. However, a higher current will generate more heat, which in turns increases the system’s resistivity. In II. INDUCTIVE CHARGING the end, the problem remains unsolved. Although inductive charging is plagued by obstacles, Inductive charging is a familiar concept; it is essen- practical uses for it surfaced in the medical realm. tially electromagnetic induction, which is governed by Stephenson et al. (1967) demonstrated the use of in- Faraday’s law of induction. ductive coupling to charge implants. Patients will no The inductive charging system is essentially comprised longer have to undergo surgery to have their pacemaker of two coils. One coil acts as the power source, the other recharged. Inductive charging was the leading method for the receiver. A fluctuating electromagnetic field is cre- wireless energy transfer until the emergence of Witricity ated by running an electric current through the power in 2007. coil. The receiver coil interacts with this field and pro- duces its own current in response. Figure 1 illustrates how the system operates. III. WITRICITY Though theoretically simple, application of this method has proven to be somewhat ineffective. Vande- Witricity is a company name that specializes in de- voorde, G., and Puers, R. (2001) observed the effects signing products that transfer energy wirelessly through of high power energy transfer with inductive charging. strongly coupled magnetic resonances. Witricity is simi- Their analysis indicated that traditional inductive cou- lar to traditional inductive coupling, but it utilizes mag- pling designs are not suitable for high power transfers. In netic resonances. The system also involves two coils. One coil acts as the power source, and the other acts as the receiver. An alternating current on the source coil, run- ning at a frequency ω0, induces a fluctuating magnetic ∗ [email protected] field. The frequency, ω0, is the resonant frequency of the 2 coil, which is dependent on the coil’s dimensions. The receiver coil interacts with the magnetic field and then resonates at the same frequency. To resonate, the current must oscillate so that the cur- rent at the coil’s ends is zero. The lowest mode at which s 0 this is possible is denoted by: I = I cos(π l )exp(iωt), where I = current at point s, I0 = maximum current, l is the wire’s length and s is the coordinate along the wire. The current and the charge density then become π/2 out of phase with each other. This means the energy is at times totally dependent on the current, and some- times totally dependent on the charge. Knowing that 1 2 1 2 0 0 U = 2 L|I | = 2C |q | , the resonant frequency of the 1/2 coil equals f0 = 1/[2π(LC) ], where L = inductance and C = capacitance. The coil radiates a magnetic field which the receiver coil receives. Since the two coils are FIG. 2. The theoretical results compared with the actual identical, the receiver coil will also resonate at the same experimental results. frequency. The system’s efficiencty also depends on the coils’ syn- chonicity, definted as their coupling coefficient. The cou- with a capacitor, there was almost no interaction with pling coeffecient is a representation of the fraction of flux extraneous objects. The capacitor confines the electric the receiver coil receives from the transmitting coil. If the field, leaving only the magnetic field to radiate. Since receiver’s flux is not in synch with the transmitter coil’s, every-day objects rarely interact with magnetic fields, κ is zero. The higher the synchronicity, the higher the the energy does not dissipate into extraneous objects. value of κ. The coils’ inductance, frequency, and mutual This was supported in the experiment carried by Soljaˇci´c inductance affect the value of κ with this relationship: et al. (2007). Extraneous objects shifted the resonant 1/2 κ = ωM/[2(LSLR) ], where M = effective mutual in- frequency, but this was easily corrected for with a feed- ductance, LS = inductance from the source coil, LR = back circuit. Confining the electric field also makes it inductance from the receiver coil. Maximization of κ is safer because fewer objects will interact with the field. done by choosing a frequency specific to the coil’s param- High efficiency and long range seems to give Witricity eters. the advantage over traditional inductive charging. Soljaˇci´cet al. (2007) designed an experiment to test the practicality of this theory. The team used two copper wires with a radius of 25 cm and 5.25 loops. The expected IV. WITRICITY VS. INDUCTIVE CHARGING resonant frequencies of these loops were f0 = 10.56 ± 0.3 MHz. They aligned these two coils coaxially and Without empirical proof that Witricity is superior to tested the efficiency at varying distances. Though the traditional inductive charging, it is difficult to convince system outputted a frequency of 9.9 MHz (5 percent off consumers to discard traditional inductive charging and from theoretical), the system yielded results really close adopt Witricity. In 2011, Ho et al. (2011) set out to to the theoretical prediction. The receiver coil, which compare the efficiency between Witricity and traditional was loaded with a calibrated 60 W lightbulb, achieved inductive charging. Due to the high coupling rates as resonance at all distances. Figure 2 compares efficiency a result of resonance, Witricity should be more efficient between the actual results and theoretical prediction. than traditional inductive charging. The researchers de- The shaded area illustrates the maximum efficiency signed an experiment with two identical copper square when given the theoretical and the experimental value coils, varying only the number of spirals for the tradi- for the coupling coefficiecy, κ. The black points are pre- tional inductive charging (the effects of using square coils dictions based on the experimental value of κ, while the are later discussed). When tested for efficiency over a red points are actual experimental data. Though κ de- range of frequencies and distances, the Witricity system creased with distance, the light bulb illuminated even at proved to be more efficient. Figure 3 shows that though distances of more than 2 m. However, this only worked if overall the received power in a Witricity system is less both coils are resonating. In addition to these results, the than that of a traditional inductive charging system, it team also found that external objects have no observable is by far more efficient at the resonant frequency at 4.8 interaction with the radiating field unless placed a few MHz. Figure 4 shows the vast difference in efficiency be- centimetres away. tween Witricity and traditional inductive charging from The experiment also tested the theoretical predictions 0 to 20 cm. outlined in a paper by Soljaˇci´cet al. (2008). The paper This comparative study demonstrated that Witricity is discussed the possible effects of extraneous objects on far superior to traditional inductive charging in efficiency, the system. When a loop of conducting wire was loaded range and also practicability. 3 The researchers presented flat rectangular coils to re- place traditional circular coils. The coils had a maximum length of 208 mm. Geometry is not a problem if the transmitting coil’s frequency is a resonant mode of the receiver coil. Experimental results indicated that geom- etry did not influence the resonant nature of this Witric- ity system. At a range of 5 cm, the system’s efficiency was 50-80 percent based on simulations and experimen- tal results. Although this is much smaller than the rates produced in Soljaˇci´cet al.’s (2007) experiment, it is still far more efficient than inductive coupling. A traditional inductive charging system will require a separation dis- FIG.