MANUFACTURING


as the scaling up of hydrogen production and the scaling up of distribution and retail, but improving the manufacture of bipolar plates could have the biggest impact on cost reduction, as it currently accounts for 28% of the cost of fuel cells,” said Roi Yaacov, business development manager from Israeli laser manufacturer Civan Lasers. Each plate is around the size of an A4 sheet of paper and is not much thicker than a human hair. Each of the several hundred plates must be welded together, requiring hundreds of thin seams to be made, each anywhere from three to five metres in length and being barely visible to the naked eye. At a weld speed of 1m/s this would result in a single stack taking up to around 40 minutes to weld, with a medium size car requiring one fuel cell stack and heavier vehicles requiring more. Most importantly, each weld seam needs to be absolutely gas-tight and perfect. “Even the smallest error would be fatal,” Dr Christian Schmitz, CEO of Trumpf’s Laser Technology. “This is because hydrogen is the smallest molecule in the world, much smaller than natural gas, and it slips through every crack and pore. A single leaky bipolar plate can render a complete fuel cell stack unusable. The welding


process must be of the highest quality, and this is best achieved with a laser.” The welds must be made over complex


geometries over a very large area with tremendous accuracy. In addition, stack manufacturers must keep the heat entering the workpiece to a minimum in order to prevent the metal sheets from warping. This stringent list of requirements rules out almost all the available joining processes –


“The welds must be made over complex geometries over a very large area with tremendous accuracy”


apart from the laser. In recent years the laser industry has mobilised to research and develop new techniques and solutions for perfecting and conducting the complex task of bipolar plate welding. Civan Lasers, for example, recently commercialised coherent beam combining laser technology to overcome ‘humping’, the physical limit of welding at high speeds. “The laser combines 10s of different lasers, while being able to control the phases of each of the 10s of beams,” said Yaacov.


FRONTIERS PHOTONICS


“What this technology permits us to have, besides different levels of power, is a dynamic beam that enables us to produce any arbitrary shape that we’d like to have within the beam. If we’d like to have, for example, a spiral beam, a circle, a doughnut, or whatever beam that you think of, we can, and all without any moving parts. We can change the different parameters, so we can control the melt pool better, and eliminate the phenomena of humping.” While Scanlab, together with its sister companies Blackbird Robotersysteme and Holo/Or, is developing a novel scan set-up that showed the potential to nearly double the productivity of welding bipolar plates for hydrogen fuel cells. In March, Blackbird Robotersysteme set


up a test rig integrating the 2D scan head intelliSCAN from Scanlab and Holo/Or’s Flexishaper, an adjustable beam shaper. The necessary beam shape was determined based on welding process simulations. The layout of the beam shaper is the


result of a combined optical design, integrating both diffractive optical elements (DOE) and scan system. The processing tests demonstrated to shift the speed limit of failure-free welding speed from 45m/min up to 70m/min. l


SOLAR CELL PRODUCTION


New method cuts photovoltaic production times in half


A


project led by the Fraunhofer Institute for Solar Energy Systems


(ISE) has developed a proof of concept for a silicon solar cell production line with a throughput of 15,000 to 20,000 wafers per hour. This represents double the


usual throughput and is due to improvements to several individual process steps. The concept was presented


at the World Conference on Photovoltaic Energy Conversion in Milan in September 2022. The consortium investigated


every stage of the production of high-efficiency silicon solar cells to optimise the entire process. Several process steps required new developments. “For some processes, established production workflows needed to be accelerated, other processes needed to be reinvented from


scratch,” explains Dr Florian Clement, project manager at Fraunhofer ISE. One of the new developments


saw the researchers implement new on-the-fly laser equipment that continually processes the wafers as they move at high speed under the laser scanner. For the metallisation of solar cells, the consortium introduced rotary screen printing instead of the current standard process, flatbed screen printing. Solar cells require differently doped sections, for example where silicon layer and metal contacts meet. The Fraunhofer team integrated the diffusion process used in this context and the thermal oxidation of the wafers into one process step. Wafers are no longer placed individually but stacked on top of each other to be processed in the furnace. As a result, the oxidation process creates


Experimental wafer stack design for diffusion in special quartz boats


the final doping profile and achieves surface passivation at the same time increasing the throughput of the process by a factor of 2.4. Following the electrode imprint on the solar cells, the contact of the electrodes to the silicon solar cell is formed on both sides in inline furnaces. Standard furnaces would have required a significantly larger heating chamber to increase throughput at this stage. Instead, the project installed a three times faster belt speed in the furnace and compared the quality of the sintered solar


cells with today’s standard. For the characterisation


of the complete solar cells, the consortium devised two concepts. A contactless method and a method using sliding contacts were implemented to enable future production lines to test cells faster. This makes it possible to keep up a continuous speed of 1.9 metres per second while measuring the cells, with the team demonstrating great measurement accuracy for both concepts. A patent has been filed for the contactless method.l


Photonics Frontiers 2023 35


© Fraunhofer ISE


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