Surface charge: making electric cars lighter with body parts that double as batteries
Materials that turn car body parts into batteries could help lighten the load on electric vehicles - and a host of other technologies.
Batteries represent one of electric vehicles’ biggest problems. They’re heavy, take ages to recharge, have a limited lifetime and create extra safety challenges. But what if a car’s body itself could store energy, reducing or possibly even eliminating the need for a big block of chemicals sat in the middle of the chassis?
Structural energy storage has been an idea of both automotive and defence manufacturers for more than a decade, and now several groups of British engineers and scientists are helping to make it a reality. If the technology can be refined, it could help to create not just better electric cars but also lighter equipment for soldiers and consumer electronics - and perhaps even aircraft.
While cars that hold hours’ worth of electricity in their roofs and wall panels might be some years off, 2012 has seen the unveiling of one of the first prototype vehicles to put the technology to use: the Lola-Drayson B12/69EV electric racing car demonstrator, which comes complete with a panel of structural energy storage carbon-fibre composite developed by BAE Systems.
’To get the quantity of batteries we need in the car while also retaining the structural integrity and the crash protection is quite a challenge,’ said Angus Lyon, chief engineer of electric drivetrainsat Drayson Racing. ’Anything that allows you to integrate energy storage, which is the biggest single lump in any electric car, into the structure is always going to be a big step forward.’
Only around 30-60 per cent of the mass of conventional batteries is used for power storage, with the rest dedicated to cooling systems and safety casing. Replacing vehicle batteries and other components with structural energy material could yield weight savings of more than 50 per cent. And once the technology is sufficiently developed, it should provide a way for the battery to add to the car’s strength instead of creating extra weight.
The battery that sits at the rear of the Lola-Drayson car is very much a first step, testing the material’s ability to power low-voltage lighting systems on the car and dealing with aerodynamic loads rather than supporting the car’s weight - an indication of how far the technology has yet to go. But even at this early stage, structural energy storage isn’t limited to the experimental racing sector: commercial manufacturers including Volvo and Audi are keen to exploit the technology and are researching how it could create components for mainstream electric vehicles - and possibly even bring some costs down by using a material that does two jobs instead of one.
Replacing vehicle batteries with structural energy material could yield weight savings of more than 50 per cent
’We are not sure what the cost will be yet but, because we can integrate [structural and battery technology] and eliminate some components, it may be cheaper than today,’ said PerIvar Sellergren, senior research and development engineer at Volvo.
The Swedish car manufacturer, which is working on a €3.3m (£2.8m) EU-funded research project led by Imperial College London, also sees structural storage as a way to introduce ultra-strong and -light carbon-fibre materials into commercial vehicles. ’Using carbon fibre alone is quite expensive, but if you combine it with a battery then we suddenly have a good economic situation,’ explained Sellergren. ’All the car industry wants to use carbon fibres, but it’s too expensive. This is a way to go around that.’
To turn carbon fibre into an energy carrier requires the right mixture and structure of polymer resin and metallic elements. The Lola-Drayson component uses a nickel-based chemistry and Volvo has opted for lithium-ion batteries, while Imperial has created a supercapacitor that transfers a static charge from one layer of composite to another via an electrolyte in between.
Details of how the battery technologies work are closely guarded, but the idea is that the carbon and metallic elements of the material form two electrodes between which a chemical reaction occurs to release electricity.
’You can say each carbon fibre is a battery in itself,’ said Sellergren. ’In the size of human hair, there are more than 100 fibres, so the battery is on the nanoscale. Every fibre has a voltage of about 3.7V but [with] very low amperes, so we have to connect them in parallel to reach higher energy levels.’
Source: Cpl Rupert Frere
The design of the Lola-Drayson battery harks back to its origins with defence company BAE Systems, which has been working on multifunctional materials for at least five years - initially as a way of reducing the weight of soldiers’ combat gear. ’Lithium and bullets is not a happy combination,’ said Stewart Penney, commercialisation manager at BAE’s Advanced Technology Centre. ’And lithium-ion batteries have a fairly limited servicelife. It’s measured in years, whereas the service life of a [defence] platform is measured in decades.’
Instead, BAE used a relatively benign nickel-based chemistry already common in defence application batteries, developing prototype components with an energy density of 10Wh/kg. Given that traditional lead-acid batteries produce around 30Wh/kg (a figure BAE’s material has reached in lab tests) and lithium-ion batteries much more, it’s understandable that the Lola-Drayson car isn’t yet using the structural battery in its propulsion system.
But having demonstrated the material in a handheld torch and a micro unmanned aerial vehicle (UAV), BAE wanted a platform to further trial and develop the technology in a tougher environment and to show that another party could manufacture it on a larger scale. The Lola-Drayson team was keen to assess its capabilities so took the material and manufactured a panel for the tail of the car.
’The motivation wasn’t to get it on the car as a critical power supply as much as just to start understanding the reliability and performance of it - before we start becoming more critically dependent upon it,’ said Lyon.
Once Lola-Drayson sees how the material fares against the high forces of motor racing and the rigours of repeated charging, the team can think about how it could create a battery that is tailored to electric vehicles - instead of inserting technology somewhat better suited to laptops and mobile phones. ’I think the next step would be to get batteries built into the structure of the car and therefore get weight and mass lower down and in areas where currently you need the structure so you can’t put the battery,’ said Lyon.
The academics at Imperial began from a similar starting point to BAE Systems when, in 2006, they began exploring the feasibility of multifunctional materials using funding from the Ministry of Defence (MoD). They eventually joined Volvo and several other European companies, including Leamington-based Advanced Composites Group, in the STORAGE research programme.
But Volvo’s interest in structural energy goes back further, to 1995, when the company began developing thin-film batteries that could be integrated into body panels. From there, the goal became fully integrated structural power and, unlike BAE, Volvo was happy to pursue lithium-ion technology - which provides one of the highest energy densities and is typically lighter than other battery types.
However, as a first step, the company is using Imperial’s structural supercapacitor design to develop prototype components -including a spare-wheel well for the car boot - that also capture energy from the brakes and power the stop-start function of a hybrid engine. ’[The wheel well] is a big, flat surface and it’s simple to replace and doesn’t have a complicated design,’ said Sellergren. ’We hope we will have this for demonstration by the end of next year.’
The major challenge in producing structural energy material is optimising its balance of electrical and mechanical properties. First, that means understanding what that balance should be for each different application, according to Dr Emile Greenhalgh, a reader in composite materials and the academic lead for the Imperial project. ’We’ve recognised we’re not producing one material - it’s actually a spectrum of materials - and we now have an understanding of what we need to do to the constituents of the material to get that balance.
For example, for the resins we develop, the ratio of their constituents dictates the composite’s properties. It gets a little more complicated because it turns out that mixing these two constituents into the resin has a big effect on how you can actually process this material.’
Turning the material into a working component is a whole other challenge, and one of the reasons BAE wanted to work with Lola was to exploit the company’s composite manufacturing expertise. ’For the practical proposition of putting it on a car, how do you design the structure to take into account the fact that you’ve got to have these battery components in there?’ said Penney. ’You’ve got to be able to connect the power; you’ve got to do the venting and charge the battery in some way. There’s also a real safety imperative to it and you have to ask how you build the structure so someone doesn’t get an electric shock. You don’t create coatings; you use the way that you build up a structure to give you that level of protection.’
The next challenges for the Lola-Drayson team will emerge as the car and its battery are tested for the first time over the coming months. This will feed back into the development process, as BAE seeks to tackle issues such as manufacturing and how to structure the material to adjust the power and energy output of a component.
“There’s a safety imperative to it and you have to ask how to build the structure so that someone doesn’t get an electic shock”
STEWART PENNEY, BAE SYSTEMS
Volvo, meanwhile, has set its sights firmly on getting structural batteries into production vehicles - possibly within the next five to 10 years, as an addition to a traditional battery unit. ’[Eventually] we want to replace the hood or the roof or the walls with this storage material, but I think there will be a conventional battery somewhere in the car because it will take some years to get into production,’ said Sellergren. ’It’s also the time for the car industry to accept this. It’s a hugely new way of designing a car.’
It’s possible we may see the technology used in personal equipment such as laptops before we see it in commercial vehicles. BAE aims to develop materials for lighter electronic systems and says it may work with a partner that wants to develop the technology for non-defence applications with lithium-ion batteries.
And if structural batteries could one day store enough energy to power a car’s motor, then perhaps they could also drive systems on boats or even aircraft, said Sellergren. ’Using it in hybrid aeroplanes is a very good idea as you get lighter material with high power, which you need in take-off. Then when you land you could use the propellers to get back the energy.’
The researchers at Imperial and BAE are about to start on an MoD-funded project looking at applications for the technology that will probably focus on UAVs. The dangers of lithium appear to preclude it from being the main energy source in military vehicles, said Penney. ’I can’t see a future fast jet being powered by structural batteries, although never say never. If there was somebody who wanted to develop the capability for non-defence applications, absolutely it could become a prime mover.’
Supercapacitors present fewer hurdles for the team and may reduce the time taken to produce a prototype
One of the key components to emerge from the Volvo and Imperial project will be a supercapacitor that is also a load-bearing part of the structure of a car’s boot.
Starting with a structural supercapacitor rather than a battery should enable the team to produce a prototype more quickly, said university project co-ordinator Dr Emile Greenhalgh.
’Supercapacitors are the low-hanging fruit. They’re still challenging, but there are fewer fundamental hurdles. For instance, a supercapacitor uses a purely physical process - there’s no chemistry involved - while a battery uses a chemical reaction.’
But there’s also motivation to develop hybrid storage devices that combine the rapid discharging function of a supercapacitor with the high energy density of a battery, he added. ’We’re making our supercapacitors have a battery component, for instance adding something into one of the electrodes to give a redox reaction so we get some battery performance as well.’
As well as providing superior performance, these devices could last longer as the supercapacitor acts as a buffer for the energy coming out of the battery, smoothing the discharge process and reducing damage to the battery’s structure that can shorten its life.
Some companies are looking at incorporating the batteries of electric cars into the chassis structureDespite advances in structural energy storage, central battery units are likely to be a feature of electric vehicles for some years to come. As a result, some companies are looking to use conventional batteries in a more structural way.
Alongside its experimental material, the Lola-Drayson electric car features a primary battery that is integrated into the carbon chassis of the car rather than sitting in a casing that is bolted on separately - simplifying the design and halving the weight of the battery support.
Green utility company Ecotricity has also produced an electric racing car called Nemesis (pictured) that incorporates its own version of structural energy storage, packaging a 400V lithium-cobalt battery system into part of the chassis structure.
The structure, developed by Norfolk-based Router Automotive, features an aluminium, carbon-fibre and Kevlar outer enclosure bonded and mechanically bolted to the chassis. A resin and glass-fibre inner enclosure complete with a fan-assisted labyrinth air flow system provides electrical and thermal insulation and manages the battery’s temperature.
’[It will] minimise the risk to the battery cells and substantially increase the torsional stiffness of the chassis for this sports car and thereby maintain or even improve the vehicle handling dynamics,’ said Trevor Saunders Ecotricity’s head of special projects.