While the momentum towards next generation cars has started to favor Battery EV s at the expense of the Hydrogen Fuel Cell concepts that have been a favourite of major auto manufacturers, the recently launched Riversimple EV concept car is still very interesting.
The Riversimple vehicle is conceived within an innovative new architecture for cars. It embodies various key features - hydrogen fuel cells, four electric motors, one on each wheel, regeneration of electricity by these motors when the car is braking – known as regenerative braking - and a body made of lightweight composites. However these do not on their own explain the breakthrough in energy efficiency achieved.
Although novel in the industry, they are not unique and none make economic or performance sense on their own. The breakthrough really comes through the synthesis of them all – the benefits derive from their interactions, requiring that we combine them all. Here is an illustration of how Riversimple has rearranged the components in what we call a Network Electric Vehicle.
The two principles that really are new (conceived by Amory Lovins and the Rocky Mountain Institute) and that make this synthesis so powerful are decoupling acceleration and cruise demands and mass decompounding
Decoupling acceleration and cruise means that the fuel cell needs only to be large enough to meet the maximum steady demand when cruising, which is usually only about 20% of the maximum power required when accelerating.
In a conventional car, acceleration also has to be provided by the engine; but as a car is only accelerating for about 5% of the time, and the power needed then is five times what it is when cruising, it means that for 95% of the time the car is carrying around an engine and transmission that is five times larger than necessary.
In our network electric vehicle, almost all braking is done by the electric motors, capturing the energy of the car in motion, rather than using conventional brakes that just waste the energy as heat. This energy is then stored in a bank of ultracapacitors which can provide 80% of the power required for acceleration. This allows us to have a fuel cell a fifth the power than would be required in a conventional car.
Mass decompounding is an emergent property of whole system design – designing the car as a whole system rather than attempting to squeeze a fuel cell into a car architecture that is designed for a combustion engine. The reduced size of the fuel cell and elimination of a gearbox and driveshafts, results in a weight reduction. This leads directly to a lighter chassis, as this is usually designed to hold on to a heavy engine and gearbox in accidents. This in turn means less power is needed, which means lighter components, which means a lighter chassis, meaning less power and so on, and this effect is magnified by using lighter materials, composites, for the chassis as well. These weight reductions make power-assisted systems for brakes and steering redundant, leading to further mass decompounding and improvements in efficiency.
The hydrogen fuel cell is about 50% efficient, double that of a petrol engine. The combination of this efficiency and the new, and much more efficient, architecture allows for a vehicle which is more sustainable because it needs far less hydrogen energy. Further fuel and emissions savings are gained because, unlike an internal combustion engine, the electric motor is not running when the car is stationary in traffic.
The Riversimple urban car uses a 6kW fuel cell. (For comparison, Honda is using a 100kW fuel cell in its new Clarity model currently being trialled in California - admittedly in a four seater car, not a two seater). In the Riversimple car, less hydrogen needs to be carried (and less held at the filling stations) and the costs of the fuel cell drop dramatically.
They have been working with Horizon Fuel Cell Technologies of Singapore, who have developed a highly efficient yet simple fuel cell system for their urban car. As the output required of the fuel cell is less than in most fuel cell prototypes, Horizon has been able to prioritize cost reduction in the development of the system. This has been the focus of their research since they were founded and so there is a natural synergy between their technology and our approach to commercializing hydrogen fuel cell vehicles. Together they are pursuing a combined development programme on the next generation for their production prototype vehicle.
The ultracapacitors which store electricity can also be smaller: Riversimple urban cars have just 21 kg of ultracapacitors, capable of absorbing over 30kW of power from regenerative braking, and of delivering 15 kW for bursts of acceleration of up to ten seconds, enough time to reach maximum cruising speed.
The result is a car with an expected fuel consumption equivalent to 300 miles per gallon, a range in excess of 200 miles, a top speed of 50mph, and greenhouse gas emissions at 30 gms per km (well to wheel), less than a quarter of the most efficient petrol-engined cars currently available. Note: The Polo Blue Motion is currently the lowest emission car available at 99g/km but this is only “tank to wheel” – when extraction, refining and distribution of the petrol is included this rises to 121 g/km “well to wheel”.
While no technical details at all are provided about the Wheel Motors, based on their Ultracapacitor specs and the fact they state the caps can provide up to 80% of the power needed to accelerate we estimate the wheel motors output at 5 kw peak each at most. With a total vehicle weight of 350 kg that's only 77hp per ton, about average for a mini car.
Only time will tell if the Riversimple open source business model is too radical to reach critical mass but we're sure the huge energy efficiency improvements resulting from the use of wheel motors on all four wheels that allows the use of such a small energy source will also be applied to hybrid and battery EV vehicles.
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