Structural form, electrical function

  • 07-Nov-2010 03:43 EST
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An EU-funded project led by Imperial College London that includes Volvo and eight other European companies and institutes hopes to develop carbon composite structural materials that can also store energy to power an electric motor and ancillary systems.

Designers of electric vehicles (EVs) face stark engineering realities that have so far yielded less-than-ideal driving range. Electrochemical batteries are large and heavy and, perforce, place a considerable drag on maximum EV range. Enabling motorists to travel farther before plugging in today’s batteries requires EV builders to employ innovative ways to shed overall weight and free up space.

To help lightweight their distance-challenged designs, EV development teams have turned increasingly toward car structures made of carbon fiber-reinforced polymer (CFRP) materials, which provide excellent strength-to-weight properties. Carbon composites are 50% lighter than steel for a given component while still providing superior strength and rigidity.

Some exotic supercars, such as the forthcoming McLaren MP4-12C, incorporate this advanced material (supplied in this case by Austria’s Carbo Tech), but automakers generally consider it too expensive to apply to mass-produced vehicles. Although the cost of carbon fibers has in the last decade dropped significantly as production volumes rose, it remains high compared to metals. Meanwhile, composite fabrication methods have mostly resisted attempts to automate it.

Nevertheless, big changes seem imminent as the automotive, chemical, and composites industries are expending considerable effort to improve design, process, and materials technologies with the aim of reducing structures costs and, thus, moving composites closer to the automotive mainstream.

BMW, for example, is collaborating with the SGL Group to expand CFRP production in Germany and has announced plans to build a $100 million factory in Moses Lake, WA, to supply carbon composite materials for a battery-powered vehicle for metropolitan regions as part of the automaker’s 2013 MegaCity EV project.

Meanwhile, Volkswagen and Renault in Europe, and Toyota and Honda in Asia are all reportedly working on readying CFRP composites for vehicles, as is Daimler, which has announced it will introduce in 2013 Mercedes models that incorporate advanced composites developed with Japan’s Toray Industries.

Going multifunctional

Some researchers have pursued a more radical approach to the automobile’s weight problem. These investigators think that part of the solution is to go multifunctional—to build car components of carbon-based composites that can double up as its battery. The dual-function material, simultaneously mechanical structure and energy source, could make battery EVs and hybrid vehicles lighter, more compact, and more energy efficient.

“Structural power technology combines electrical energy storage capabilities and high specific weight properties,” said Emile Greenhalgh, a physicist and composites specialist at the Composites Center at Imperial College London (ICL). “We’re trying to have our cake and eat it too, with what some call a ‘mass-less’ energy storage scheme,” he said.

Batteries are structurally parasitic, so going multifunctional yields a significant mass benefit.

Greenhalgh heads a five-member interdisciplinary research team at ICL that, with U.K. Ministry of Defence (MoD), has worked for the past five years on incorporating electrical storage materials—supercapacitors, in ICL’s case—into the bodywork of automobiles, military hardware, and other products including portable electronic devices. The MoD is interested in extending the operating range of unpiloted aerial drones or UAVs.

Similar work, he noted, has been conducted in recent years by researchers at the U.S. Army Research Laboratory (ARL) at Aberdeen Proving Ground, MD, who have concentrated on creating electrocapacitive structures for reactive armor applications as well as investigations of structural fuel-cell and battery technology.

Combination floor/battery

Greenhalgh leads a wider, three-year European Union project called STORAGE, which has been allocated $4.7 million to create the new materials to serve as batteries, capacitors, supercapacitors, or combinations thereof. Besides ICL, the team includes Volvo and eight other industrial participants.

“Volvo, who we’ve spoken to about this issue for a long time, says that structural power technology will be key to the EVs that they’re developing,” he said.

One of the goals of STORAGE is to test within three years a demonstrator product—a wheel-well panel (spare wheel compartment flooring in the trunk)—in a prototype EV. “We’re expecting a 15% weight savings compared to the standard supercapacitor/battery combination in a conventional structure,” said Per-Ivar Sellergren, a Volvo developmental and testing engineer at the Volvo Cars Materials Center in Gothenburg, Sweden. He said that the company has been preparing for the research project since 2008.

“The floor panel is a comparatively large structure that is easy to replace,” Sellergren noted. “And even though it is not large enough to power the entire car, it would be enough to, for instance, switch the engine off and on when the car is stopped at a traffic light.”

The Volvo engineer speculated that if the composite battery structures could hold as much energy as current lithium-ion batteries, it would require only the car roof, hood, and trunk lid alone to supply an EV with a 130-km (80-mi) driving range. The new, distributed power technology could also lessen some of the extensive wiring now used in cars.

“The big problem is the weight of cars, a factor that affects all other attributes of the vehicle—fuel economy, crash safety, braking distances,” Sellergren said. “The trick is to design in a way that sets off a positive, so-called ‘virtuous cycle’ that drives you toward improved environmental (and operational) performance.”

Multifunctional design strategies such as structural power technology is an example of this approach.

Road to structural power

“The idea of structural power arose from work we did with the MoD five or six years ago,” Greenhalgh recalled. The combination was a natural one, he said, as carbon fibers are commonly used as both electrodes and structural reinforcements.

The ICL group focused on supercapacitors, performing a feasibility study and building a simple prototype that proved the concept. Patents soon followed, but funding remained sparse until 18 months ago, when the MoD funded a multidiscipline effort to create a structural battery material for use in UAVs. The EU support broadened the project further.

“Squeezing mechanical and electrical performance out of a single, integrated material is a technically challenging task, one that requires the full commitment of all the disciplines,” he noted. On the one hand, “you want the material to be rigid and stiff, but you also want the electrical ions to flow easily.”

Current experimental systems at ICL employ polyethylene glycol diglycidyl ether (PEGDGE) epoxy resins, with lithium salts as the electronic component.

The lab’s demonstration devices, typically sandwich configurations, must be carefully designed at the ply, fiber, and tow scale levels, Greenhalgh explained. The ICL supercapacitor concept involves a sandwich of an insulating glass fiber mat between two electrodes of activated woven carbon fiber mat, with “a nanostructured bicontinuous polymer electrolyte” (laced with positively charged lithium ions) infusing all. The electrolyte is two-phase polymer that spontaneously forms a bicontinuous (dual, tree-like) nanostructure. “One provides a conduit for charged particles, the other structural rigidity that holds the material together,” he said.

“We think that an interpenetrating (or intermingled) nanoscale network of two phases is the key,” said the ICL researcher. Mechanical defects that propagate and cause failure occur at the mesoscale (or micron-size scale), so the smaller nanoscale structure of the resin could avoid creating deleterious flaws. But problems with chemical resegregation, or the effect of contact with the carbon fibers, could disrupt the key nanostructure of an advanced resin.

“In parallel we worked to improve the fibers,” Greenhalgh continued, noting that they have “learned how to use alkali treatments to massively increase surface area of the carbon fibers—with multiple tiny pits or pores on their surfaces—for better mechanical and electrical interaction with the resin. But when we activate the fibers for electrochemical duties, we are careful not to compromise the mechanical properties, elastic modulus, tensile strength, and toughness.”

Another STORAGE team member, a European division of Japan’s Nanocyl (which is working to maximize the fraction of mesopores), is also investigating growing carbon nanotubes (CNTs) on the fiber surfaces to create so-called “hairy fibers” that would interface more fully with the surrounding matrix to improve both mechanical and electrical performance.

Unfortunately, CNT grafting is “ultrasensitive” to the particular processing conditions—the gases, the temperatures, and so forth, Greenhalgh said. The treatment procedure’s chamber-borne, batch-type nature makes it difficult to convert into a cost-effective continuous-line process.

Structural batteries

A sister effort in the EU STORAGE project, conducted by Swerea Sicomp, the Swedish Institute of Composites, focuses on structural batteries, which are “technologically immature as yet but could be crucial in a few years,” Greenhalgh said.

Conceptually, the plan is to deposit onto each carbon fiber concentric electrode and solid electrolyte coatings, such that each fiber would run as a battery, he explained. In theory, the device’s total energy storage could be in excess of that of a conventional battery.

Researchers at the U.K. carbon fiber manufacturer Advanced Composites Group are simultaneously working to optimize the composite processing route needed to produce the electrostructures, a liquid resin infusion fabrication method.

Other project participants include Germany’s Federal Institute for Materials Research and Testing (BAM), which will characterize the new material’s performance; Sweden’s Chalmers University of Technology; ETC Battery and Fuel Cells Sweden AB; and Greece’s Integrated Aerospace Sciences Corp. (INASCO Hellas).

Greenhalgh warned that “we’ve only really been working on this technology full time for 18 months. It’ll take from 5 to 10 years before there’ll be road tests in EVs.”

Any composite battery component would need to undergo considerable qualification procedures, not least because of its more complex, dual function and the fact that it would interface both with electronic and structural subsystems of a car. Still, having your cake and eating it too sounds like a good thing for future EVs.

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