Powering a car with batteries is basically a question of numbers. The more you have, the further you can expect it to go. However, larger batteries do not necessarily bring more mileage. Their sheer weight and size can cancel out power gains and limit performance; a battery in a 1200 kg Tesla Roadster weighs in at a hefty 450 kg.
© iStockphoto/Stefan Schulze
This is encouraging the automotive industry to look for other ways to reduce weight and improve performance.
One such approach, which is being explored as part of a European Commission funded project called STORAGE, would be to use lightweight carbon composite body parts that also store electricity.
Carbon composites are already used in products such as sports equipment, aircraft and some high-per formance spor ts cars to provide strength and reduce weight in the products. They tend not to be employed in mass-produced vehicles yet because of current high costs. According to Emile Greenhalgh of Imperial College, London and the project's coordinator, this could change if the composites can also be used to provide power to the vehicle. "It could mean that we can get rid of the batteries altogether and power an electric car just from its body work", he says.
The technology relies on carbon composites acting as super-capacitors to deliver power. The strength of the material and the fact that it can be moulded into any shape could make it ideal for car body parts. Both batteries and super-capacitors store energy. However, that is just about where the similarity ends. Batteries store energy chemically which is then converted to electrical energy. This is a relatively slow process but it means lots of power can be delivered over a sustained period.
Super-capacitors meanwhile store electrical charge in a layer of ions absorbed on a carbon sur face. As there is no chemical reaction, charging can be very quick and recent developments mean that charge can be delivered quickly and stored for much longer. The €3.4 million EC supported project combines the skills of a number of academic and industrial partners, including Volvo. Since 2010, when the project started, the partners have focused mainly on improving energy density and the structural properties of the material.
The material consists of layers of carbon fibres, separated by a rigid glass-based insulating material, which are then bound together with a glue-like resin rich in lithium ions. Each layer of the sandwich then acts as an electrode between which the ions can flow when they are placed in a circuit. By chemically treating the carbon fibres prior to applying the resin, the surface area can be increased considerably, which translates into increased capacity. The sandwich is then covered in insulating material to ensure it is safe.
"The challenge has been to find new ways of reinforcing the materials, while maintaining the amount of energy stored in them. We now know how to store the energy. The next steps are about power (i.e. speed of delivering the energy) and improving the mechanical properties of the material", says Greenhalgh.
Reducing weight is a key factor in determining the final performance of any electric vehicle, and the project has already shown quite how much weight might be saved using this material. On a prototype test vehicle they replaced the plastic plenum (a cover that distributes air to the engine), with one made of the carbon composite. The original weighed over 6 kg. The replacement weighed 2.5 kg and has the added benefit of acting as a power source for the vehicle.
According to Greenhalgh, that scale of saving made on much larger body parts could bring performance improvements if it becomes possible to eventually remove the batteries altogether and power the car completely through the composite material. Interest in the project has indeed been considerable, particularly following the widespread media coverage it has received.
Numerous issues with electric cars, including their cost, range, power and charging times, mean they have not been a popular choice with drivers in Europe in the past. Weight reduction and improved performance will be crucial for electric cars becoming widely adopted.