Hydrogen headway: Riversimple’s fuel-cell car is taking shape

16 December 2015

Riversimple hydrogen car prototype

It’s more than six years since UK startup Riversimple unveiled its hydrogen car project with great fanfare, at a midsummer reception held a stone’s throw from an actual river – the Thames – in central London. You might be forgiven for thinking that the project then quietly dried up, gone the way of so many other eco-car upstarts like Aptera and Mia – both of which floundered in the difficult transition from interesting idea to profitable product.

Happily, however, Riversimple is still burbling along, even if it is still a long way from generating any torrents of cash. Its plan is to produce a clean-sheet car that prioritises “simplicity, efficiency, lightness, strength, affordability, safety and sustainability”. If everything goes according to plan, you won’t ever be able to buy a Riversimple vehicle – they will be leased for a monthly fee and periodically revamped, refurbished and updated, with each chassis designed for a 15-year working life across multiple contracts.

Riversimple hydrogen car early prototype

The tiny early prototype, which resembled a beach ball balanced on a rollerskate, fashioned in carbon fibre and graced with dramatic scissor doors, has made way for a bigger, better proportioned alpha test car with a longer wheelbase. There is still a carbon passenger cell in the centre, but the roof now looks designed for gull-wing doors and there are extra chassis sections front and rear, which presumably provide crash protection as well as the wheelbase needed to package all the bits of technology the car will have to call upon.

For starters, hydrogen is not the most obliging of things to carry around. You need a big, strong tank to contain even relatively modest amounts of the gas in high-pressure storage. Riversimple has put its tank directly above the rear axle, taking up what might normally be used as luggage space. The aim is to provide a boot big enough for “some crates of wine”, presumably above the hydrogen tank under a streamlined rear hatch or bootlid. Bodywork, penned by former Nissan, Fiat and Alfa designer Chris Reitz, is due to be unveiled in January – so watch this space.

The Riversimple hydrogen tank will be pressurised to a relatively modest 350 bar – half the 700 bar of the fuel-cell Hyundai ix35, so it will not need to be quite as strong or heavy as the Hyundai’s main tank. It’s also physically smaller, holding only 1.5kg of hydrogen gas at full pressure, which Riversimple says will be sufficient for about 300 miles of travel. At 200 miles per kilo of H2, Riversimple is shooting for about three times the fuel efficiency of Hyundai’s admittedly much larger and heavier hydrogen car.

Riversimple hydrogen car cutaway

Between Riversimple’s tank and passenger cell is the heart of the machine – the fuel-cell stack that can combine the on-board hydrogen gas with oxygen from the air to make electricity, with only heat and water as byproducts. It produces up to 8.5kW (the equivalent of about 11.5bhp), which isn’t a heck of a lot.

Drive comes courtesy of four permanent-magnet hub motors, one at each corner, encircled by skinny Michelin tyres. Riversimple says each motor will provide up to 160Nm of torque, meaning there should be as much thrust as the four tyres can take from a standstill. Data at the motor maker’s website suggests a peak output in the region of 23kW (31bhp) for each wheel, totting up to a robust 92kW (124bhp) for the car as a whole. Note that official power figures have not yet been published by Riversimple.

The motors also provide regenerative braking. Rather than trying to turn the recaptured energy back into hydrogen, which is possible but not sensible, the Riversimple design employs a bank of lithium-ion supercapacitors for short-term electricity parking. Recaptured power is fed back into the motors during the next bout of acceleration.

Riversimple’s supercapacitors act much like the battery in a hybrid car like a Toyota Prius. Batteries and capacitors each have their pros and cons when it comes to sucking in and spitting out electrons. Capacitors tend to be very efficient at moving energy in and out, but are not so hot at storing energy in bulk amounts. Fully charged, the bank of 120 capacitors in Riversimple’s car can hold just over 0.5kWh, or about half the capacity of a Mk3 Prius’s nickel-metal-hydride battery.

Riversimple hydrogen car being tested

The supercapacitors can, however, provide big slugs of energy in short bursts – up to 80% of the power needed to accelerate. Brake regeneration, plus the steady trickle of energy from the fuel cell, then tops up the capacitors during those times when the throttle isn’t pinned to the floor.

This design has the benefit of smoothing out the power demands made of the fuel cell, which means that it only needs to be big and powerful enough to support the car’s average needs, rather than the relatively rare moments when full throttle is called for. As keen readers will have noticed, an 8.5kW fuel cell cannot feed four 23kW motors unless it has this kind of help.

Happily, spreading the load allows the fuel cell to be smaller, lighter and, crucially, cheaper. Which helps to keep everything else bolted to the car smaller, lighter and cheaper too.

The drawback of a small fuel cell becomes apparent when it comes to sustained power – keeping the car going at a fast cruise. Here, the word “fast” has been used loosely. When it comes to acceleration, Riversimple quotes a 0-55mph time of 9 seconds, which offers a broad hint about the car’s top speed. Somewhere between 55 and 60 is likely to represent flat out.

Riversimple hydrogen car cockpit

Physics is to blame. The power required to keep a car at a steady speed increases exponentially as you go faster. In 2008, Tesla published data that showed 10kW was sufficient to keep its 2-seater Roadster travelling at about 48mph, whereas 60mph (a 25% increase in speed) required 15kW (a 50% increase in power). Staying at 90mph required about 38kW.

Riversimple’s design – skinny tyres, no transmission and lighter powertrain – means its car ought to be more efficient than the Tesla Roadster, but with only 8.5kW to call upon it will never be an autobahn blaster.

Clean aerodynamics will help. When the bodywork is unveiled next month, some resemblance to previous wind-cheating champions like the original Honda Insight and Volkswagen XL1 seems likely.

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