Most people are familiar with energy transfer by electricity since it already powers lights and a tremendous share of consumer devices. In all cases, there is a voltage and current associated with energy transfer by electricity. Maxwell’s laws tell us that anytime there is a current, it also induces a magnetic field in the space surrounding the flow of the current and vice versa such that anytime a magnetic field moves it induces a current in a nearby wire. Magnetic induction is the principle upon which WPT systems are built.
The figure below illustrates how induction can be used to transfer power wirelessly. First, a power source such as the electrical grid is transformed to have certain desirable characteristics of current, voltage, and fluctuation speed (frequency). This power travels through wires just as in most electrical applications. Next, the wire moves through a special geometry and surrounding made of particular materials. The flow of current in the wire induces a specially shaped and fluctuating magnetic field that permeates the WPT hardware and the air, water, road materials, etc. Most materials are not susceptible to this magnetic field and are not damaged or altered by them. However, if another set of wire and field susceptible materials are brought into the field, it now induces a current in them. That current carries energy from the original power source and can be transformed and manipulated into forms useful for many applications (such as battery charging in electric vehicles).
The basics of WPT have been understood by scientists for at least a century, but until recently, the required materials for wire and magnetic field shaping were unavailable. Further, the desirable characteristics of current, voltage, and frequency were unattainable with electronic circuitry. Over the last 15 years, these barriers have been eliminated, and researchers around the world have learned how to apply the principles of magnetic induction to ever more capable systems.
Two of the key materials that were previously unavailable are ferrite and litz wire. In a WPT system, the purpose of ferrite is to shape, locate, and encourage the presence of magnetic fields. Ferrite needs to be highly susceptible to magnetic fields (unlike air or common materials) and must pass and shape the magnetic fields without losing energy in the form of heat. Current WPT systems use special ferrites that are usually made via powder metallurgy processes where one of several compositions of elements are made into powders, mixed, and put into a mold. The mold is subjected to heat and pressure so that the powder particles conglomerate and form a uniform material. The process is well understood and can produce high-quality ferrite materials of good purity and performance. However, it is not necessarily the most cost effective process for large scale production because of intensive labor. Ferrites cost about $1.50 per pound in a useful form. WAVE has identified that a more continuous process for ferrite production is possible and has the potential to bring ferrite costs under $1.00 per pound in high quantity.
A second key material is litz wire, made from typical copper strands but with many small strands woven in particular ways. With WPT, it is generally helpful to use materials that allow high frequencies without detrimental electrical losses. In medium quantities, litz wire costs WPT researchers about $2.40 per foot.
WAVE does not feel that easy advances are possible in the wire drawing and weaving processes, but there are other ways to potentially decrease the cost of the current-carrying wire. WAVE will investigate the applicability of new wire-cladding techniques and high-temperature superconductor strategies. Photo-lithography techniques may allow similar performance without the need for wire drawing.
In current EV WPT system applications, there are power conversion electronics between the grid-based power and the litz wire, and between the vehicle-side power reception hardware and the vehicle battery and charger. These electronics involve components standard in the electronics industry, but they are particularly power dense and can be physically large depending on the power being transferred. We believe our continuing research into control, and circuits for high power WPT systems will increase the efficiency of WPT systems and provide opportunities for reducing electronics size. Reductions in the size of metal housings, printed circuit boards, connectors, etc. will help reduce costs. It is encouraging that electronics applications have historically shown good per-unit cost reductions with increases in scale; WAVE believes our applications will realize the same benefits.