A hybrid without a combustion engine. The idea is back in vogue with Renault, which has unveiled a preview of the Scenic E-Tech ahead of its expected arrival in 2024. Although the production model will be equipped with an all-electric battery-powered propulsion, the Vision Concept was born with the H2-Tech powertrain, designed for a mixed battery-hydrogen fuel cell power supply. The proposal invites us to dwell on the potential benefits that a mixed propulsion disconnected from an internal combustion engine could guarantee.
Powertrains powered by combustion cells exploit the recombination of hydrogen with oxygen to produce electricity with which to power one or more engines on board. The architecture is similar to that of a 100% electric car, with the difference that instead of the accumulator there are fuel cells and hydrogen tanks. However, this is a simplistic comparison, as even cars powered by combustion cells have a battery, albeit of modest capacity. Purpose of its presence is the absorption of power transients, ie allowing the fuel-cell to work at constant power in the condition of maximum efficiency, delivering the additional power to meet the peak demand from the driver to the pedal. It is no coincidence that these accumulators are developed to optimize power density rather than energy density, which instead constitutes the design objective on full-electric cars, to the point of often resorting to super-capacitors to replace lithium cells. At the same time, the presence of a battery, albeit of modest size, allows fuel-cell vehicles to regenerate braking energy from electric motors, storing it in the accumulator and extending their range. It can therefore be said that in hydrogen cars the battery supports the combustion cell. In Renault’s H2-Tech powertrain this vision is reversed, as it is the fuel cell that supports the battery in the propulsion of the vehicle.
The limitations of fully battery-powered cars are well known: long recharging times when compared to refueling; high vehicle weight; higher carbon footprint of hybrid cars considering both the production cycle and energy supply. The predominant solution in recent years has been the downsizing of the batteries through the use of a mixed electric-endothermic propulsion, also born as a need to overcome the technological limitations of electric while awaiting progress in the sector. The most advanced hybridization is embodied by plug-in hybrids, equipped with generously sized rechargeable batteries, although still smaller than those of 100% electric cars, so as to cover short journeys entirely with electric propulsion. The heat engine, on the other hand, supports the conditions in which higher performance or greater autonomy is required for long-distance journeys. With the Renault Scenic Vision Concept, the fuel-cell takes the place of the thermal engine, while keeping intact the basic idea of cooperation between two powertrains. The fuel cell maintains a limited power of 16 kW, around 22 horsepower, thus placing itself more like a range-extender for the production of energy to be fed into the battery for subsequent delivery to the engines, very similar to the setting of a series hybrid.
The most suitable comparisons are those with the other models in the Renault range, in particular with Megane E-Tech whose curb weight is around 1700 kg to which the new Scenic is also credited. In the mixed electric-hydrogen H2-Tech configuration, Scenic features a 40 kWh battery. This is a similar capacity to that of the electric Megane, whose accumulator is available in both the 40 and 60 kWh versions. Much smaller is the Megane plug-in hybrid battery, with a capacity of 9.8 kWh.
In the form in which it was proposed, the H2-Tech powertrain therefore still sees a strong dependence on the battery, but the fact remains that if developed the idea could lead in the future to different hybridization rates, with a downsizing of the accumulator. However valid, such an architecture still carries with it the limits of hydrogen power supply, above all the difficulty of using equipped refueling stations. Precisely for this reason, however, an electric-hydrogen hybrid could constitute an alternative or a transition solution waiting for the conditions to become ripe for a large-scale diffusion of fuel-cell propulsion.
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