New technology could help increase electric vehicle range

UK scientists claim to have developed a way to help electric vehicles (EVs) travel further on a single battery charge by simplifying their power electronics.

NPL claims its new capacitor material holds much more energy than conventional devices.

Researchers led by the National Physical Laboratory (NPL) have created a new material they say acts as a much more effective capacitor than those currently used in EVs, which require complex cooling systems that add weight to the car.

The NPL-led team have built a prototype capacitor using the ceramic material and claim it can function at much higher temperatures than conventional devices made with liquid electrolytes or polymers and hold more energy.

‘Electric vehicle operating temperatures can go up to 140ºC and even higher in some cases,’ NPL’s lead researcher on the project, Tatiana Correia, told The Engineer, double the temperature conventional capacitors can withstand.

‘To compensate for that, the power electronics are covered by a complex cooling system that brings extra weight and that translates to the energy consumption and mileage range of electric vehicles as well.’

Electric vehicle capacitors store energy during the process of converting DC power from the battery to AC power for the motor.

This produces a large amount of heat, even in highly efficient power electronics systems. Liquid electrolytes chemically decompose above a certain temperature so a cooling system is essential to prevent potential safety issues.

The researchers say the new capacitor, dubbed HITECA, can operate at close to normal efficiency at temperatures over 200ºC and can be made in a conventional multi-layer design from relatively cheap raw materials using standard fabrication methods.

‘[It has] significantly higher energy density compared to other ceramic-based capacitors,’ said Correia, adding that the capacitor also didn’t include toxic materials such as lead that could be banned under future guidelines.

The capacitor is made from a ceramic paste with a granular structure, comprising a bismuth ferrite (BiFeO3) compound doped with strontium-titanate (SrTiO3).

NPL’s Tatiana Correia

Correia said only one scientific paper had been published on a version of this material (in the 1960s) and the researchers, after trialling a range of other substances, began experimenting with its composition to tailor it for use in capacitors.

‘It’s very difficult to increase energy density in these kind of ceramic materials. So we have to tailor the composition, we need to change the structure, the lattice and the doping.

‘The main difficulty of these materials is that they can store quite a lot [of energy] but they can’t release it. So we introduced some strontium-titanate and other dopants to reduce the remnant energy.’

Billy Wu, a PhD student at Imperial College London’s Energy Futures Lab who is unconnected with the NPL research, said cooling was a major problem for electric vehicles because of the amount of current passing through their components.

‘Cooling systems add extra mass and extra power losses because you need to power pumps. So anything you can do to minimise the heat that you generate adds to the lifetime and range of electric vehicles.’

He added: ‘A lot of manufacturers are looking at getting rid of active cooling – pumping water around – and instead just blowing air over it. But there’s only a certain amount of heat you can remove through convection in the air.’

The NPL-group is now looking to integrate the capacitor technology into an EV power electronics system for further testing.

They believe the new capacitor could be used for any systems that involve power conversion under extreme conditions, for example in photovoltaic solar, space or oil and gas technology.

The research was funded by the Technology Strategy Board (TSB) and partners included Queens University of Belfast, Queen Mary University, capacitor firm Syfer and French automotive component manufacturer Valeo.