The perspective of hybrid electric hydrogen propulsion systems

Abstract

Electric cars are becoming increasingly popular, with the European Union and the United States administrations committed to making their share overwhelming by the end of this decade. In analogy, electric powerboats and aircraft are also being proposed. Here we show as hybrid electric vehicles (HEVs) have advantages vs. battery-only electric vehicles (BEVs) for all these applications. Adopting a hybrid propulsion system based on storing most of the energy in hydrogen, and producing electricity on board through a fuel cell (FC) stack, or an internal combustion engine (ICE) plus a generator, gives a much better techno-economic perspective, and reduced life cycle analysis CO2 emissions, than the use of huge lithium-ion batteries. An HEV having plug-in capability and a relatively small battery permits pure electric operation on short travel, without the burden of huge batteries to deliver range. The battery of the HEV buffers the upstream hybrid generation system, which may recharge the battery and/or power directly the electric motor, and store the braking energy in ground vehicles. HEVs fuel energy storage and onboard electricity generation on demand permit reduced total mass of the propulsion system for the same design range as huge batteries. HEVs with ICE working on hydrogen offers the minimum total mass of the propulsion system, then HEVs with FC. This assessment is correct for the current liquid electrolyte batteries and the near-future solid-state electrolyte batteries, based on their energy density per unit mass, and the indirect emissions of batteries.