Power supply by light

It is not always possible to supply sensors and actuators with power through copper cable. In these cases, Power-by-Light systems could fill the gap.

In traffic, hospitals, industry, or in the energy sector: Networked systems with decentralized sensors and actuators are gaining ground. However, in some cases it is not feasible to supply distributed electronics with electricity through copper cables. For example, where there is a risk of electromagnetic interference, where copper adds too much weight on board airplanes or cars, or where the installation space does not permit other cabling in addition to the required data lines. Alternatives are also needed where heat generation in the power cables distorts measurements or where sparks could cause an explosion. Photonics has the answer in the form of Power-by-Light systems.

Light waves transmit electricity

In these systems light transmits the electricity. Lasers or LEDs couple it in optical fibers that deliver it to highly efficient photovoltaic power converters. As opposed to roof-based photovoltaic cells that convert a broad wavelength spectrum in to electricity, these semiconductor-based laser power cells are optimized to convert monochromatic light. At a wavelength of 860 nanometers (nm), they are already able to reach an opto-electric efficiency level of almost 70 percent. This is achieved by closely coordinating the band gap of the respective semiconductor materials with the wavelength of the light. This enables the laser power cells to convert light into electricity with minimum thermal losses. Scientists at the Fraunhofer Institute for Solar Energy Systems ISE have achieved an efficiency level of 67.3 percent using a gallium arsenide (GaAs)-based laser power cell, where a laser emits 860-nm light waves with irradiance of 9.6 watts per square centimeter. The scientists in Freiburg are also working with other compound semiconductors in the III-V group, with wavelengths from 660 nm (Ga0.51In0.49P) to 1,700 nm (GaSb). Using multi-segment cells connected in series, their systems achieve voltage ranges of up to 12 volts.

Fiber-based systems cover large distances

The SUNLAB at the University of Ottawa in Canada is also developing optical energy transmission solutions. The focus here is on Power-over-Fiber systems. Optical fibers transmit the light waves efficiently, are lighter and less expensive than copper cables, corrosion-resistant and, last but not least, they do not emit any electromagnetic interference. They also have the advantage that, in addition to supplying electricity, they can handle bidirectional data transport.

To optimize these applications and bridge distances of several kilometers, the Canadian scientists use wavelengths that are typical in the telecom sector. They have a cooperation agreement with the U.S. semiconductor and network engineering company Broadcom, which markets the technology.

As well as GaAs semiconductors with 850 nm wavelength for short distances, the Canadian team also uses indium gallium arsenide absorbers (InAlGaAs) on indium phosphide (InP) substrates. The converters convert infrared wavelengths of 1310 nm into electricity very efficiently. One key advantage is that at 1310 nm wavelength the power losses fall to 0.3 dB per kilometer, which corresponds to seven percent, compared to 3 dB/km at 850 nm wavelength—where the power is halved every kilometer.

The Canadian scientists are also working with a new cell architecture. Instead of connecting several cells in series next to each other, they are experimenting with a vertical arrangement of the absorber layers. According to the scientists, this has the benefit of reliable high voltage. With horizontal serial connection, the voltage level of the entire system drops as soon as one cell segment receives less light. With the vertical layer structure the scientists vary the thickness of the layers so that the bottom layers also convert sufficient light into electricity. The layers are thinner the closer they are to the surface. In this way the team ensures that the bottom InAlGaAs layers convert light into electricity at the same voltage level as the top ones. These active layers are separated by transparent tunnel diodes. At 0.63 volts per layer, with the sandwich converters the scientists plan to achieve more than 5 volts at almost 60 percent efficiency. Other GaAs converters that they developed with the U.S. company reach voltage levels of 23 volts.

Power-by-Light for 5G networks and retina implants

With their new system architecture and at 1310 nm wavelength, the team of scientists says that 30 percent of the input energy arrives at decentralized devices even over transmission distances of ten kilometers. This opens up new supply channels for decentralized 5G networks in the Industrial Internet of Things (IIoT). Other target markets include automotive and aviation, energy plant monitoring, and magnetic resonance imaging. ISE scientists in Freiburg also see a wide range of potential applications for Power-by-Light technology. These include structural monitoring of wind turbines, optical power and data transfer to fuel sensors in airplanes and automobile sensors as well as supplying power to smart implants. For this purpose, the ISE is currently working together with partners from industry on the government-funded research project “LightBridge”. The aim is to develop a wireless power supply for retina implants for sight-impaired people. Currently, a cable is fed through the eye wall to power these implants, which often leads to complications. Instead of this, the researchers have developed an optical transmission path that can be used for non-contact supply of the implant with infrared light. This also provides benefits with regard to reliability, energy consumption, and compatibility. If it is possible to integrate laser power cells into the human body without problems, this would be the starting signal for countless medical innovations.

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