Researchers Look to Supercapacitors for Future Mild Hybrids

Supercapacitors may help bring fuel economy to mass production in France.

Valeo SA and PSA Peugeot Citroen are using them to add propulsion power to a cheap mild-hybrid system that would double the power of Valeo’s Stop & Start technology and allow it to accept braking energy and provide torque boost through the alternator belt.

Supercapacitors long have been discussed as a way to capture braking energy, because they can endure perhaps millions of charge-recharge cycles, and because they accept and discharge electricity faster than a battery.

In the MHYGALE project partially funded by the French environmental agency ADEME, the French companies are trying to extend the utility of Valeo’s start-stop system based on a belt-driven (and belt driving) motor-alternator.

Valeo already has plans to deliver a supercapacitor mild-hybrid system to another European auto maker in 2011, but the PSA project is targeted at reducing costs so the system could be produced at high volumes.

PSA was the first auto maker to use Valeo’s start-stop technology with the Citroen C3 in 2004, later extending it to other models. Beginning in 2010, a second generation will be standard in dozens of PSA cars, with a goal of 1 million vehicles in 2011.

This third generation under study would change that micro-hybrid to a mild-hybrid by adding regenerative braking and acceleration.

Capturing such brake energy “would give an electric boost that is too much for the lead-acid battery,” says Bernard Sahut, PSA’s innovation sub-domain manager for production, storage and distribution of electrical energy of the Automotive Research and Innovation Div. “You would need a 25-kg (55-lb.) battery, and that would be too big.”

MHYGALE is a collection of initials from the phrase “Mild Hybrid Generalisable,” or mild hybrid for everyone.

“The objective of this project is to develop an affordable mild-hybrid solution that will significantly reduce (carbon-dioxide) emission levels on a global scale,” Valeo says. “This system is cheaper and less intrusive than other solutions.”

Valeo and PSA are joined in the project by five public research labs, electronics supplier Freescale Semiconductor Inc. and small specialist companies Alter France and CEITEC (Excellence Center for Advanced Electronics Technology).

Already in 2008, Valeo partnered with Freescale, Alter and CEITEC on a project funded by another public program to develop power electronics for the system, a MOS (metal-oxide semiconductor) transistor for a 14-volt system that could handle the temperature and current needed by the automobile for 60 million cycles.

The MHYGALE project aims at supplying 10-15 kW of power to the propulsion system, compared with the 5-6 kW in the second-generation stop-start system that Valeo is preparing for PSA and other clients.

PSA also has been working on supercapacitors on its own.

“They have little energy, but lots of power, and they can handle many more cycles than a battery,” says PSA’s Sahut. “Energy and power are different requirements, and lead-acid batteries do both now.”

The auto maker developed a demonstration hybrid version of a Citroen C4 with a trunk full of supercapacitors able to handle 20 kW that captured enough energy during braking over 328 ft. (100 m) to drive off on electric power for 984 ft.-1,312 ft. (300 m-400 m).

The volume was important – “like 100 Coke cans,” says Sahut – but the mass of the supercapacitors was about one-third less than that of a battery with the same performance.

A second French company, the Bollore group, is using its own supercapacitors in an electric car it is developing.

“Bollore is using supercapacitors to extend the life of the battery by handling the power requirements and letting the battery handle the energy,” Sahut says. The supercapacitor will act as a sort of shock absorber, accepting and discharging energy from the regenerative braking program to avoid having the lithium-ion battery take on that task.

Heat must be managed with supercapacitors, Sahut says, and the question of lower cost will be answered by high volumes.

Sahut also is charged with monitoring lithium-ion battery developments. PSA has not chosen a supplier of batteries yet for its future electric-car projects, because the group has not yet decided on details. But PSA is likely to launch what amounts to a plug-in hybrid between 2013 and 2015 that would have an internal combustion engine for weekends, but a daily urban range on electricity alone of 31-43 miles (50-70 km).

“We are looking for the best technical and economic solution,” Sahut says. “Today, there are many solutions being offered, and we must evaluate each one that emerges.”

For the second-generation Stop & Start micro-hybrid, PSA will use a nickel-metal-hydride battery, the technology Toyota Motor Corp. uses in its Prius hybrid.