Researchers from Imperial College London and several European partners, including Volvo, aim to develop a composite material that can not only store and discharge electrical energy but which is strong and lightweight enough to form part of the car’s structure.
In 10 years, they expect that the carbon-fibre-resin composite could be used in hybrid petrol/electric vehicles to make them lighter, more compact and more energy efficient, enabling drivers to travel for longer distances before recharging them.
The project’s co-ordinator, Dr Emile Greenhalgh, a reader in composite materials at the Department of Aeronautics at Imperial College London, believes that cars of the future could eventually be drawing power from their roofs, bonnets or even their doors.
Initially, the engineers are planning to enhance the properties of a prototype material they have developed so it can be used to create a composite component that will replace the metal flooring in the well of a car boot where the spare wheel is kept.
Volvo is investigating the possibility of fitting this wheel well into a prototype electric hybrid car for testing purposes, and the Imperial researchers believe that doing so could enable the company to reduce the number of batteries needed to power the electrical systems in the vehicle, including the motor, the lights and auxiliary equipment such as the GPS and radio.
Developing the initial prototype of the composite so that it could both store charge and also carry mechanical loads, however, presented the Imperial College researchers with two conflicting requirements.
’In a structural composite, you need to use a rigid polymer resin to support the fibres, but for a composite that stores electrical energy, the polymers must be soft to allow ions to flow through the material. The way that we have achieved both is to nanostructure the resin material we use such that it can deliver both the electrical and mechanical properties that we require,’ said Greenhalgh.
The carbon-fibre resin composite is expected to make hybrid vehicles lighter and more energy efficient
Unlike a battery that uses a chemical process to store charge, the composite material functions as a supercapacitor, and as such should be quicker to recharge than a conventional battery. Furthermore, the recharging process will cause little degradation in the composite material, because it does not involve a chemical reaction, unlike conventional batteries that degrade over time as they are recharged.
The research, however, is still at an early stage. Dr Greenhalgh admits that, at present, the material only stores 0.005Whr/kg - several orders of magnitude less than a conventional supercapacitor that stores about 3.29Whr/kg. Greenhalgh said there are several ways in which the charge storage could be increased.
Since the energy stored by the material varies as the square of the voltage applied to it, doubling the voltage applied to the material would enable it to store four times the charge. At the moment, however, that has been difficult to achieve with the prototype composite laminates, because they are not produced in a moisture-free environment, and the moisture absorbed by the laminate limits the voltage that can be applied to it.
’In the future,’ Greenhalgh said, ’the laminates will be made in a dry environment, as is done with conventional batteries, and by doing that, we will be able to increase the voltage that we can apply to the material and hence the charge it stores. At higher voltages of 3V or more, we expect to see a tenfold improvement. And we anticipate, with development of the resin and fibres, we should be able to get to more than 0.3Whr/kg in the next couple of years. Longer term, in 10 years or so, we envisage reaching 20Whr/kg, which is akin to a conventional battery.’
The team will also improve the material’s capacity to store more energy by growing carbon nanotubes on the surface of the carbon fibres used in the composite, which should also improve its mechanical properties.
In terms of its present mechanical properties, Greenhalgh said that the stiffness of the material is acceptable for use in an automotive application - the longitudinal modulus of the laminates is around 50GPa - but the strength of the composite is still quite poor.
The basic constituents of any composite - the fibre and the matrix - have different strengths. Where the property of the Imperial composite is dominated by the fibres, the strength is akin to that of a conventional composite - greater than 1,000MPa.
But across the fibres, where its strength is dominated by the matrix, Greenhalgh was reluctant to disclose what its strength might be, aside from saying that it was low. This, he says, is due to the poor wetting of the fibres by the composite matrix, leading to poor delamination resistance in the material that is limiting its compressive strength.
But, again, he believes that it is only a matter of time before that challenge is solved too. ’The wetting at the moment is only a few per cent when it should be approaching 60-70 per cent,’ said Greenhalgh, ’but over the next few years, we expect to be able to address that issue as well.’ This will be achieved through optimising the chemistry for wetting the fibres; modifying the surface groups on the fibres to enhance compatibility with the matrix, as well as improving the processing of the composites by eliminating dry regions.
The researchers are also planning to investigate the most effective method for manufacturing the composite material on an industrial scale. This is where engineers at a key partner - Advanced Composites Group - will play an important role in the project. They will examine Imperial’s lab-scale process and consider ways to scale it up for volume manufacturing.
Initially, the team will be using conventional composite manufacturing techniques to make the material, such as liquid resin infusion, where the carbon fibres are first laid up in a laminate, after which the resin is injected. But they also plan to consider using pre-preg manufacturing techniques, where the resin will be introduced to the unidirectional fibres in a film form to produce a unidirectional sheet that can then be stacked up into a laminate.
“With long-term development of the resin and fibres, we envisage reaching 20Whr/kg, which is akin to a conventional battery”
DR EMILE GREENHALGH, IMPERIAL COLLEGE LONDON
Secondary manufacturing processes, however, may have to be reviewed due to the nature of the new material. ’With a conventional carbon-fibre composite, a laminate is first manufactured and is then trimmed to the required size. But the problem with the new material is that since it has two carbon-fibre layers that act as electrodes, using a conventional machining process would create a short circuit across them,’ said Greenhalgh.
For that reason, he believes that new methods of manufacturing may need to be considered to enable structures to be built from the new material - it will be necessary for the researchers to consider how to make the components without making use of any secondary processing routes such as cutting, trimming or drilling.
Dr Greenhalgh also said that a number of other factors will need to be examined by the researchers before the new material can be deployed in earnest in a vehicle - notably how environmental effects such as moisture and temperature might affect its performance.
Nevertheless, he hopes that by the end of the three-year EU-funded project, the technology will be far enough advanced such that the wheel-well component will be ready to be deployed in Volvo’s prototype car, where its use will highlight significant weight and power savings.
In the meantime, under a separate MoD contract, the Imperial researchers also hope to demonstrate the usefulness of the new material by developing a small panel that could be used as a structural element in an unmanned aerial vehicle (UAV) to help power its electrical system.
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