Wireless Power

By: Peter Woods

Supervising Professor: Dr. Chiao July 16th 2013

Overview

• Brief history

• Basic theory• Inductive coupling• Important terms

• Applications• Currently available• Possible future applications

• Safety

History of Wireless Power

• Nikola Tesla• 1893 Demonstrated wireless power by illuminating light bulbs at the World’s Columbian Exposition in Chicago • 1917 The Wardenclyffe tower is destroyed

• Modern day• 2007 A research group at MIT power a 60W light bulb with 40% efficiency at 2 meters

Theory (Part 1)

• Faraday’s Law• Any change in the magnetic environment of a coil of wire will cause a voltage to be induced in the coil

• Lenz’s Law • When a voltage is generated by a change in flux, there will be a current whose polarity opposes the change which it produces

Theory (Part 2)

• Efficiency• Efficiency is the ratio of output to input power• How much of the magnetic field is captured and is useful (not lost to resistance in coils and circuits)

• Coupling factor• When the transmitter coil and receiver coil are separated, only a portion of the magnetic field penetrates the receiver coil and causes a voltage • Determined by distance, shape, and angle of coils

Theory (Part 3)• Quality factor of coils

• The ratio of inductance to the resistance of a coil multiplied by the angular frequency.

• The higher the quality factor of the coils, the higher the efficiency will be

• Resonant magnetic coupling• According to WiTricity, “Magnetic coupling occurs when two objects

exchange energy through their varying or oscillating magnetic fields. Resonant coupling occurs when the natural frequencies of the two objects are approximately the same.”

Theory (Part 4)• Litz Wire

• Used to carry alternating current at high frequencies, consists of many strands of wire wire to reduce the skin effect

Current Application of Wireless Power (Part 1)• Medical implants

• Heart pump• GERD (Gastroesophageal Reflux Disease) stimulator

• Inductive recharging• Toothbrushes• Laptops• Cellphones

• Through wall application• Outside power from solar panel beamed inside to power a light bulb

Current Applications (Part 2)• Transportation

• Wireless powered bus system for public transportation• Wirelessly powered cars

• Wireless recharging purse • Purse that can recharge cell phones

Future Applications of Wireless Power • Elimination of cords and plugs for charging

• Creating a standard for charging stations• Wireless Power Consortium VS. WiTricity• ensure that mobile • devices from different vendors can charge anywhere in a common

wireless ecosystem. A

• Charging electric vehicles• Parking lots• Traffic lights

• Clothing that charges phones

Safety (Part 1)• Tissue heating

• IEEE (Institute of Electrical and Electronic Engineers), and ICNIRP (International Commission on Non-Ionizing Radiation Protection)

• According to an article by WiTricity, “there is no established evidence showing that human exposure to radio frequency (RF) electromagnetic fields causes caner, but that there is established evidence showing that RF electromagnetic fields may increase a person’s body temperature or may heat body tissues and may stimulate nerve and muscle tissues.”

• SAR (specific absorption rate), is the measure of the amount of electromagnetic energy absorbed by the human body and turned into heat.

• For the U.S. the FCC (Federal Communications Commision) has adopted a SAR limit of 1.6 W/kg averaged over 1 g of tissue

Safety (Part 2)

Summary• Wireless power is not a new technology

• Internal combustion engine invented in 1898

• How it works• Two coils, a transmitter and a receiver. An alternating current in the

transmitter generates a magnetic field which induces a voltage in the receiver coil

• Available products• Wireless chargers, medical implants, transportation

• Safety• Difficult to measure how safe, but current experiments show that

wireless power is safe

Questions