Specs at a glance: Riversimple Rasa Body type 2-seat, 3-door hatchback Power source 8.5kW Hydrogenics hydrogen fuel cell Transmission Four wheel-mounted electric motors Power 16kW continuous (55kW peak) Torque 4x 60Nm continuous (170Nm peak) Chassis Carbon composite monocoque with aluminium crash structure Bodywork Self-coloured thermoplastic panels Steering Unassisted rack and pinion Suspension Double wishbone (front)

Semi-trailing arm (rear) Tyres Michelin 115/80R15 Top speed 60mph (97km/h) 0-60mph Under 10 seconds Fuel tank capacity 1.5kg (hydrogen) Extra power storage 1.9MJ (lithium-ion hybrid capacitors) Rated max range 300 miles (485km) Weight 580kg (1278lbs) Wheelbase 2272mm (89.4in) Dimensions 3673mm (144.6in) x 1630mm (64.1in) x 1332 (52.4in) (LWH) Base price TBA

An industrial estate on the outskirts of a sleepy spa town in deepest Powys, Wales, may not strike you as the obvious place to find an ambitious little hydrogen vehicle maker with plans to revolutionise the way we power, drive, and own our cars. But it shouldn’t come as that much of a surprise that it's there.

Why not? Well, if you drive seventy-five miles to the south-west from Riversimple’s HQ in Llandrindod Wells, you end up in Swansea, once the home of William Robert Grove who in 1842 pretty much invented the hydrogen fuel cell. And it’s a hydrogen fuel cell that part-powers the Rasa, Riversimple’s funky little two-seater prototype.

"Part-powers?" I hear you ask. While the majority of electric and hydrogen cars currently on the market are essentially conventional designs with battery or fuel-cell-and-battery power sources, the Rasa—the name comes from tabula rasa, the Latin for blank or clean slate—is the result of altogether more clever thinking. I’m inclined to use a word I usually avoid like the plague—holistic—to describe Riversimple’s view of automotive design.

The first clue to what’s afoot here is the fuel cell itself. The unit in the Rasa is an off-the-shelf component with a piffling 8.5kW output that's best known for powering forklift trucks in Walmart warehouses. Compare that to the 100kW fuel cell in the Hyundai ix35 Fuel Cell or the 114kW stack in Toyota’s Mirai. Despite that limp output, the prototype Rasa can hit 60mph (which is also its top speed) in under 10 seconds.

The Rasa uses a number of clever techniques to pull off this low-power but decent performance trick. To start with the vehicle has a low drag coefficient of 0.248. I’ve passed wind with more drag than that. Next is its weight: at 580 kilos (1278lbs) all-in, the Rasa is only 130kg heavier than Renault’s Twizy. And that’s as much scooter as car. Lastly but most importantly is the unusually efficient regenerative braking system that harvests the kinetic energy usually wasted under retardation from the four in-wheel traction motors.

The clever part in all this is that the fuel cell only solely powers the Rasa when it is cruising. An average family car uses about twenty percent of the engine's output when wafting along at a steady 70mph. For acceleration or climbing hills, the Rasa summons extra power from four banks of lithium-ion hybrid capacitors that store the energy recuperated from the braking system.

The advantage of super capacitors over batteries is that you can dump a large amount of energy into them quickly without anything going “bang” and without doing long-term damage. The downside is limited storage capacity.

The lithium-ion hybrid capacitors used in the Rasa consist of 120 cells with a capacitance of 3300F each and a total energy storage capacity of 1.9MJ or 0.53kWh. They offer a best-of-both-worlds compromise: only slightly less robust energy capture than a supercapacitor but higher energy storage capacity.

What all this means is that the power sources for cruise and acceleration are decoupled. This is the reason Riversimple calls the Rasa a “network electric car." The electric drive system uses power from varying sources as and when needed.

Alun Taylor

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Granted the top speed may seem a little on the low side, but it’s worth clarifying that Riversimple envisages the Rasa as a fundamentally local car for running around—not one for long-distance motorway cruising. A higher top speed would necessitate a more powerful fuel cell and that goes against the grain of what Riversimple is trying to achieve.

Most of you reading this will have an idea of how a hydrogen fuel cell works. But for those who don’t: the Hydrogenics fuel cell in the Rasa has two electrodes—a negatively charged anode and a positively charged cathode. The anode is supplied with hydrogen and the cathode with oxygen. The hydrogen molecules are attracted to the cathode through a membrane that is designed to allow only protons (H+) to pass through, not electrons.

The H+ ions pass through the membrane to combine with the oxygen, creating the only byproduct of the process: water. The electrons, meanwhile, are blocked at the anode level and can only move into an external circuit, thus generating an electric current. As a method of power generation it’s roughly twice as efficient as a petrol engine, as well as cleaner, more compact, and more reliable.

Locomotion meanwhile comes courtesy of four radial, flux-wheel, hub-mounted motors developed specially for the Rasa by Printed Motor Works of Alton in Hampshire. The total power output of the drivetrain is 16kW continuous and 55kW peak. Each motor generates 60Nm continuous and 170Nm peak torque.

Thanks to those four motors, the Rasa can recover more than 50 percent of the available kinetic energy when braking. The cunning part of the Rasa’s efficient recuperative braking is due to something Riversimple calls phased braking. In a nutshell: above 5mph braking is entirely down to the electric motors. The wasteful hydraulic disc brakes play no part except in emergency stops or at speeds below 5mph.

Listing image by Alun Taylor