NTT and Mitsubishi Heavy Industries announced on September 17 that they had conducted an optical wireless power transmission experiment using laser beams to wirelessly transmit energy one kilometer away. They successfully radiated a laser beam with an optical power of approximately 1 kilowatt (1035 watts) onto a silicon photoelectric panel one kilometer away and obtained 152 watts of electric power on the receiving side.
At a press conference, Natsuha Ochiai of NTT Space Environment and Energy Laboratories (Zero Environmental Impact Research Project) explained that they had achieved an efficiency of 15% in converting laser beams to electric power, which represents the world's highest efficiency demonstration for optical wireless power transmission using silicon photoelectric conversion elements in an environment with strong atmospheric turbulence.
This achievement enables wireless power supply to remote locations where power cables are difficult to use and is expected to enable applications such as on-demand power transmission to remote islands and disaster-stricken areas in the future, as well as to satellites in space and flying drones.
Photo by courtesy of NTT
As smartphones, wearable devices, drones, and electric vehicles become more widespread, there is a growing interest in wireless power transmission technology that supplies power without cables. Existing wireless power transmission methods include those that use microwaves and laser beams, and microwave wireless power transmission has already been put into practical use.
Optical wireless power transmission using laser beams has not yet been put into practical use. However, it is expected to become a technology that can realize long-distance wireless power transmission on the order of kilometers with compact devices by taking advantage of the high directionality of laser beams.
However, laser wireless power transmission technology has poor power conversion efficiency, which is a challenge for practical implementation. One of the reasons for this is that when laser beams propagate over long distances, especially through the atmosphere, the intensity distribution becomes non-uniform, and the efficiency of converting laser beams to electric power in the photoelectric conversion elements on the receiving side is low.
Therefore, for this experiment, the researchers combined NTT's beam shaping technology and Mitsubishi Heavy Industries' light receiving technology to conduct a demonstration experiment and achieved high efficiency in laser wireless power transmission.
On the transmitting side, they used NTT's "long-distance flat beam shaping technology," which makes the intensity of laser beams uniform at a distance of one kilometer. On the receiving side, they used Mitsubishi Heavy Industries' "output current leveling technology," which suppresses the effects of atmospheric turbulence using a beam homogenizer (a device that makes light intensity distribution uniform) and a leveling circuit. They constructed these setting to conduct long-distance optical wireless power transmission experiments in an outdoor environment.
The experiment was conducted from January to February of this year using the old runway at Nanki Shirahama Airport in Shirahama Town, Nishimuro District, Wakayama Prefecture. A transmission booth containing optical components for transmitting laser beams was installed at one end of the runway, and a receiving booth containing the light-receiving panel was installed one kilometer away.
During transmission, the optical axis was set at a low height of approximately one meter from the ground. Because the optical axis was aligned horizontally, parallel to the ground, it was strongly affected by ground heat and wind, making this an experiment under conditions with particularly strong atmospheric turbulence.
In the transmission booth, the researchers generated a laser beam with an output of 1035 watts and shaped the beam using a diffractive optical element so that the intensity distribution would become flat at a distance of one kilometer (long-distance flat beam shaping technology). Furthermore, to accurately irradiate the receiving panel, they adjusted the beam direction with a direction control mirror.
Having been shaped in this way, the beam was emitted from the aperture of the transmission booth, propagated through one kilometer of space, and reached the receiving booth. Intensity spots caused by atmospheric turbulence during propagation were diffused by the beam homogenizer in the receiving booth, and a uniform beam was irradiated onto the receiving panel so that the laser beam was converted to electric power with high efficiency (output current leveling technology).
A silicon photoelectric conversion element was adopted for the receiving panel, in consideration of cost and availability. In this experiment, the average power extracted from the receiving panel was 152 watts, and the researchers succeeded in optical wireless power transmission with an efficiency (ratio of received power to transmitted power) of 15%. Furthermore, in the experiment, they also succeeded in continuous power transmission for 30 minutes, confirming that long-duration power transmission is possible with this technology.
Technical points
Beam uniformity and output stabilization made possible
[Long-distance flat beam shaping technology]
Technology that makes the outer part of the beam into a ring-shaped pattern using the effect of an axicon lens (conical lens), and modulates the phase so that the central part of the beam expands through the effect of a concave lens. After propagation, these ring-shaped beams and diffused beams overlap, making the light intensity uniform. In the experiment, the design was optimized so that the desired intensity distribution would be achieved at a distance of one kilometer, and beam shaping was performed using a diffractive optical element.
[Output current leveling technology]
With the aforementioned flat beam shaping technology, the intensity distribution of the beam can be made uniform to some extent. When atmospheric turbulence is strong, high-intensity spots occur.
Therefore, a beam homogenizer was installed in front of the light-receiving panel to diffuse high-intensity spots so that the beam would be uniformly irradiated onto the receiving panel. By connecting leveling circuits to each photoelectric conversion element of the receiving panel, fluctuations in current due to atmospheric turbulence were suppressed and the output was stabilized.
With these two technologies, beam uniformity in transmission on the order of several kilometers, which was difficult with conventional beam shaping technology, and output stabilization in outdoor environments have become possible. Stable power supply to remote locations such as isolated islands and disaster-stricken areas can be expected.
The researchers also stated that "in this experiment, silicon was used for the photoelectric conversion elements, but if photoelectric conversion elements (compound semiconductors) designed to match the wavelength of the laser beams are used, more efficient power transmission can be expected. Furthermore, if a laser beam source with greater power is used, large-scale power supply is also possible."
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.