Scientists from Tianjin Institute of Power Sources and CETC company created highly efficient laser photo converters and demonstrated a fully-featured system of energy transfer by laser at a distance exceeding 50 meters. This technology opens the way to a new format of supplying energy to space vehicles, lunar rovers and unmanned aerial vehicles in the situation, when it is impossible to lay the cable and solar panels do not operate reliably.
Since long ago there were attempts to transfer energy with a luminous beam, but in practice the efficiency of such systems remained at a low level: losses were too high at every stage – when generating the laser, laser beam propagation and re-conversion of light to electricity. Chinese researchers focused on two bottlenecks previously impeding the progress. The first one was temperature sensitivity of photo converters: standard lab samples worked well only within a narrow range of conditions and quickly failed when exposed to heating or cooling. The second one was the shape of the beam: the majority of powerful lasers produce the so-called Gaussian beam, bright in the center and weak on the edges, which leads to efficiency failures in big arrays.
A vertical multistage cell based on gallium arsenide (GaAs) was created to solve the first problem. It included six ultra-thin sub-cells each of them absorbing its own portion of light. The main technological break-through was in selecting the thickness of each layer for three representative temperatures: –140 °C, +25 °C and +150 °C. The optic behavior of GaAs changes drastically with the change of temperature, and if the layer is too thick or too thin, the absorption balance gets upset decreasing the current output. Eventually, three optimized versions of the converter were made -–each for specific thermal mode. The low-temperature version LPC₋₁₄₀ demonstrated the highest productivity. Under –140 °C and given the area of almost 14 cm² the record-high efficiency of 70.2% was achieved, i.e., more than two thirds of the laser energy was converted into electricity.
But one big element does not yet constitute a fully-featured system. To collect more energy, photo cells are combined into arrays, and this where the nonuniformity of the Gaussian beam is a problem. In simple rectangular arrays, the central elements receive too much light, and the ones on the edges have the shortage of it, so the entire chain operates at the level of the “weakest” cell. To exclude this effect, the scientists offered an unusual circular configuration. A circular laser spot was divided into ten equal segments, and ten photo converters were made according to the shape of such segments. Thanks to that, each of them receives an approximately equal stream of light despite the redistribution of illumination inside the laser spot. Such array of 78.5 cm² demonstrated 50.6% efficiency at room temperature – the best result among big laser arrays.
A full-scale demonstration of the technology was the culminating point. A transmission unit with 350 W semiconductor laser and a receiving station with a new circular array were deployed in the territory of the institute. At 50.6 m distance the system was stably transmitting energy achieving 16.9% maximum end-to-end efficiency “electricity-electricity”. And that is without accounting for energy consumption for cooling the laser – an important step towards practical application scenarios.
Further on the researchers plan to scale-up the technology and adjust it to real tasks of space missions.
