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| Fig. 1: Nikola Tesla holding a light bulb that is unconnected to any power source, yet still illuminated; an early example of wireless power transfer. (Source: Wikimedia Commons) |
Wireless power transfer (WPT; also known as wireless power transmission, wireless energy transfer, and electromagnetic power transfer) is the method by which devices can be charged without a physical connection like a cord plugged into an outlet to the power grid. Within WPT, there are two main subdivisions, short-range WPT and long-range WPT. The dominant technique for short-range WPT is inductive coupling, while long-range WPT has been accomplished via microwaves, radio waves, lasers, and even ultrasound.
WPT is not anything new. Nikola Tesla's original patent on the matter was issued over 100 years ago. [1] Tesla also conducted successful experiments in Colorado Springs, Colorado in transmitting power wirelessly, successfully lighting wireless light bulbs using the technology (see Fig. 1). On the whole, Tesla was a believer is a global wireless power system, though his experiments were said to have been halted by the potential danger of the transmissions to humans and machinery in the area, as well as funding problems.
More recently, we have verifiable examples of both short- and long-range WPT. For example, Oral-B electric toothbrushes have used inductive charging since the early 1990s, and smartphones have increasingly had the capability since at least 2012 when the Droid DNA was released with Qi compatibility, a certification by the Wireless Power Authority signifying that the device can be wirelessly charged up to 4 cm away from a Qi power source.
On the long-range front, William C. Brown demonstrated the ability to use microwaves to beam 30 kilowatts of energy across one mile with efficiency of 54 percent, noting that efficiency upwards of 70 percent should be possible with greater refinement of several parts of the system involved. [2] Another impressive feat was completed in 1987 by the Canadian Communications Research Centre, which managed to fly an unmanned, 4.5-meter wingspan aircraft called Stationary High Altitude Relay Platform (SHARP) in a 2km-diameter circle at a height of 13 miles by beaming 500 kW of energy at a frequency of 5.8 GHz, on the very-low end of the microwave radiation spectrum. [3] SHARP was capable of staying airborne for months at a time.
Although short-range WPT has already been commercialized, it has a more limited applicability. The race to commercialize long-range WPT is underway. In Silicon Valley, many startups have recently secured funding to develop the technology. One is uBeam, which utilizes ultrasound, effectively vibrating the air in the direction of the receiver. The receiver then converts the vibrations back into electrical energy and charges the device. Another startup is Reach Labs, which uses radio waves to transmit power to receivers, much in the same way that Wifi is transmitted. Fortunately, the interest of the private sector should only accelerate the development of long-range WPT.
We can only start to imagine some of the applications and consequences of successful long-range WPT. Batteries in electronics could be smaller since there would be a constant charging source available without a need to plug in, reducing the need to make environmentally-harmful power cells. Solar panels could be put into space and the electricity beamed down to Earth, creating an effectively limitless, renewable supply of energy. Or electricity could be beamed electricity to drones or electric automobiles so they can fly or drive uninhibited by the need to stop and recharge. Taking that a step further, we can see that electric planes might become the norm. All together, long-range WPT has the ability to reduce our carbon footprint towards zero: solar energy coming from space to power all transportation and machinery.
© Jamie MacFarlane. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] N. Tesla, "System of Transmission of Electrical Energy," U.S. Patent 645,576, 20 Mar 1900.
[2] W. C. Brown, "The History of Wireless Power Transmission," Sol. Energy 56, 3 (1996).
[3] A. Alden and T. Ohno, "Single Foreplane High Power Rectenna," Electron. Lett. 28, 1072 (1992).
