H 2 STORAGE The powertrain in Toyota’s hydrogen-powered Hilux uses core elements from the Toyota Mirai hydrogen fuel cell electric sedan to create a structurally sound shell, with one end of the tank fitted with a valve, incorporating various sensors and a thermal pressure relief device (TPRD). The Toyota Mirai and Hyundai Nexo, currently on sale, use Type 4 tanks, as do the vast majority of FCEVs in the prototype or testing stage. Type 4 tanks allow for storage pressures up to 700 bar, representing the current state of the art, and are subject to extensive R&D to improve their performance and manufacturing efficiency. Elisabeth Gleis is a research associate at TU Munich’s Chair of Carbon Composites, which is heavily involved in the development of H₂ pressure vessels. She explains that much of the Chair’s work centers around developing more cost-effective manufacturing processes to reduce the mass of these tanks, as well as research into novel solutions that move away from traditional cylindrical tank construction. 110m/min Gleis and her team in Munich have been working on two approaches as an alternative to wet winding; the use of towpreg material and robot placement of thermoplastic materials. Looking at towpreg first, there are several advantages over wet THE PROCESSING SPEED OF TU MUNICH'S TOWPREG MACHINE PROCESS EFFICIENCY Most commercially available Type 4 tanks are produced using a ‘wet’ filament winding process, where the carbon fiber filaments are passed through a bath of resin before being wound onto a mandrel to create the form of the tank. This is a well-proven technique but there is considerable scope to improve production and mass efficiency. “You have limitations regarding process speed,” highlights Gleis. “For example, if you spin the vessel Overcoming hydrogen packaging problems too fast, the liquid resin is forced to the outside [of the Though cylindrical tanks might represent the current state of the art for automotive layup]. Also, if the filament applications, their form factor still presents a packaging challenge. There is considerable is pulled through the research into the development of so-called ‘conformal’ tanks, which promise greater impregnation baths too quickly, volumetric efficiency. At TU Munich, two projects are currently underway. the quantity of resin on the One is looking at creating a flat tank that uses endless-fiber-reinforced thermoplastic fiber is reduced. Let’s not forget that the process relies on the tension struts in the interior of the tank. These form a structure that is then filament handling of resin, which can wound to cover the ends of the struts, which are bent over to tie them into the outer casing. present a hazard to workers.” “Vessels that have a flat geometry rather than a cylindrical geometry need reinforcement Using a wet filament also structures inside to prevent it expanding outward under pressure,” explains Elisabeth places constraints on location Gleis. The aim of the project is to ready the technology for the automotive industry. It is and orientation of the fibers. being undertaken with partners that have expertise in areas such as design of the internal “The fibers tend to slip so you membrane and optimization of winding techniques for non-cylindrical form factors. have less freedom with the angles the fibers can be laid Concurrently, another project is looking to create a conformal tank for an H 2 -powered down at,” notes Gleis. “If you drone, which is under development by another department within the university. This choose a path that's not so takes a different approach of joining together multiple, small, flattened cylinders. secure, regarding the pitch of Globally, a variety of other efforts are underway to develop conformable tanks and the fibers, they have a tendency tanks that can also replace structural elements within a vehicle chassis. to slip, the result being that you have quite high material consumption with wet winding.” The shape of things to come 46 www.automotivepowertraintechnologyinternational.com / March 2024