24 stack of thin, multi-layer membrane electrode assemblies (MEAs), individually separated by bipolar plates (BPPs). Each MEA generally consists of a central proton-exchange membrane (typically made of Nafion or a similarly semi-permeable material), sandwiched on either side by a catalyst layer – often platinum, carbon or a mixture of the two – and outside of the catalyst layers are gas diffusion electrode layers: one an anode layer, the other of cathode. H2 and O2 (the latter typically drawn from ambient air using a fan) are pumped into the stack, with the H2 fuel flowing into the anode sides of the MEAs, and the O2 reactant flowing into the cathode sides, with the BPPs constructed and shaped to optimise the flow channels of either gas. The catalyst layers in the anode side induce hydrogen ions to split from the H2 molecules, pass through the PEM and bond with the O2 molecules to form water vapour, which passes out of the MEA stack with unused air as exhaust (why PEM fuel cells are not zeroemission, but zero-carbon, technically). When the hydrogen ions split and pass through the MEA, each releases an electron, which travels along and is (ideally) conducted out of the MEA as direct current, producing power through the cable running from the stack. The energy efficiency of modern PEM fuel cells – the proportion of electrons drawn from the H2 as electricity, as opposed to being released as heat – tends to hover around 50%. While material and architectural specifics inside the H2D2’s stack are confidential, the fuel-cell system in the snow groomer (comprising the MEA stack, its control electronics and power electronics) measures about 1 m tall, 1 m wide and 1.5 m long, while weighing 400 kg and producing 175 kW net power output. A liquid-cooling circuit runs about the stack, given not only the heat dissipation requirements but also the potential for redistributing the fuel cell’s heat around the snow groomer to prevent excessively cold conditions along the powertrain and elsewhere. MEAs inherently produce current at a lower voltage than is ideal for recharging HV batteries. To that end, a DC/DC is installed at the stack’s outlet, which steps up its voltage output from 370 V to 500 V, compatible with the 800 V battery. A dedicated fuel-cell ECU monitors a variety of temperature, electrical, gas flow and pressure sensors to track the health and performance of the stack – that tracking is also available via the driver’s display. “Going forwards, we will be focusing on and studying how well the cooling system performs, as that is critical for the health and thermal efficiency of the fuel cell. That’s the main piece of a larger action plan to optimise and mature the fuel cell over time, with testing plans aimed generally at running it as much as we can to uncover issues that need fixing,” Aussibal notes. Energy storage and recuperation Fuel cells typically produce power and voltage at varying rates, so a DC/DC as well as a small battery are typically installed downstream to stabilise the power output. The consortium realised the battery pack in the snow groomer would need to fit in a space that is 1.4 m wide, 0.5 m long and 80 cm tall, and it has opted for one that fits these dimensions and weighs about 950 kg. “It has a capacity of 144 Ah, and while the original project target was 84 kW/h of battery energy onboard, we provide just under 90 kW/h of useable energy storage. Technically, the pack’s actual maximum energy storage is higher, but we recommend against using all this as it’s bad for battery lifespans to repeatedly run SoC down to 0%,” says GCK Battery project manager Herve Santiano. One may ask: if the weight of the energy system can be minimised by using hydrogen gas as a low gravimetric energy density storage system, why has the consortium designed the snow groomer with this (seemingly) unnecessarily heavy battery? Two reasons come from the fact that traditional snow groomers are expected to be able to handle very steep gradients due to their low CoG and large contact area. Having a large and heavy battery pack under the cabin helps keep that low CoG and a wide, snug contact with the ground. While some snow groomers use cables and winches to tether themselves to the upper parts of slopes to prevent free-falling, the electrified May/June 2024 | E-Mobility Engineering The battery pack stores just under 90 kWh and has a 144 Ah capacity (Image courtesy of Groupe GCK)
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