Electron microscopic picture of the single fibre pull-out
Most batteries are bulky and heavy. So why not use an existing case such as, for example, the car body as energy storage? European researchers came up with this idea and organised the StorAGE EU project to develop flexible power storage from fibre composites. The project has now been completed after three and a half years and now provides the first results. The carmaker Volvo has equipped a test vehicle with the new composite material which is placed in the engine bonnet and boot lid and provides an extended car battery. The BAM Federal Institute for Materials Research and Testing was involved in the project which was headed by the Imperial College London with nine research institutes and industrial partners also participating. BAM carried out the mechanical tests on the energy storage devices and participated in the development of a conductive epoxy resin for fibre-reinforced composites.
So how does the new energy storage operate? There are supercapacitors in the engine bonnet or boot lid made of carbon fibres. Supercapacitors are a further development of conventional capacitors, which for example have been used in photo flash equipment for some time. They are very quick to recharge, are durable and are capable of working at minus 40 degrees Celsius.
The carbon fibre and CNT composite is enclosed by a polymer resin. Glass fibre mats are used as an insulation layer between the energy-storing layers. BAM coordinated various investigations on self-discharge, electrolyte decomposition and material stability. The single fibre “pull-out test" developed at BAM was used to determine the adhesion of individual fibres to the surrounding material. Fibre composites contain many fibre bundles, each comprising several thousand individual fibres. To produce stable materials from this tissue, they are impregnated with a curable plastic, for example, epoxy resin.
The fibres studied in the single fibre pull-out test may be very thin. For instance, a carbon fibre is only about five microns thick, or about 20 times thinner than a human hair and almost invisible to the naked eye. The fibre is embedded in a polymer matrix droplet and fixed by an adhesive on the other side. An optical microscope allows the tracking of a crack process during measurement. The magnitude of force transmitted between the plastic and the fibre is then a measure of the coupling strength. For the project, it was important to ensure that the novel conductive resin systems have good adhesion to the fibres – thus the composite is stable enough to be installed in an automobile.
"In general, the energy storage works," says Gerhard Kalinka, a BAM engineer. It is promising, but one is still at the beginning of the development because not all questions have been completely clarified. For example, how safe is the system when it comes to damage? In the Volvo prototype, the car battery can be replaced to some extent, however research is still needed until a genuine electric car can be supplied with energy in this way. Nevertheless, Kalinka sees promising applications for computers and mobile phones as well. There, the traditional battery may disappear and the housing can deliver the necessary power over the long term. In addition to weight saving, a further advantage of super capacitors would be a fast recharge within minutes. GD