New Capacitor Technology May Improve Hybrid Vehicle Performance

Ed Stiles
March 12, 2007


Hybrid vehicles — those that run on of both rechargeable batteries and gasoline — benefit greatly from being able to rapidly convert mechanical energy to electricity and vice versa.

Hybrids need to do this during short periods of heavy acceleration or braking, both of which require nearly instant access to large energy pulses.

Unfortunately, batteries aren't very good at these tasks. Batteries have high storage capacity, but they have difficulty quickly charging or discharging energy to meet pulse power requirements.

Digitized Energy Storage Devices
Researchers at The University of Arizona are developing a technology based on DESDs (Digitated Energy Storage Devices). that could solve this problem. DESDs quickly store and discharge large amounts of power using capacitors built on nano-scale structures.

Capacitors have been around for a long time. In their simplest form, they consist of two conducting plates separated by an insulating material. If they're connected to a battery, a charge appears nearly instantly on their plates and they can discharge all this energy in a fraction of a second.

But conventional capacitors aren't suitable for hybrid vehicles because they would have to be huge to handle the energy needs of automobiles.

The DESD breakthrough was made by Professor Olgierd Palusinski, his former graduate student Ken Bartley, Research Engineer Jaeheon Lee, and others on their team in Electrical and Computer Engineering.

DESDs have a very high capacitance-to-volume ratio that's more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. This makes for a device with large capacitance in a small package.

The UA researchers construct DESD capacitors by using commercially available porous membranes as template platforms. The membranes have a pore diameter ranging from 15 nanometers to 1 micron and a hole density of 10 million to 100 trillion pores per square centimeter.

One micron is 1/1000th of a millimeter. A nanometer is 1/1000th of a micron. To get an idea of how small this is, consider a pore that's 15 nanometers wide. You could line up 66,000 of them in a space that's one millimeter wide.

To form the capacitors, the membrane pores are filled with copper to create a large copper surface area in a small space.

This is important because the ability to store electric charge is proportional to the surface area of a capacitor's plates. The honeycomb of conductors formed in the nano-meter-sized membrane pores has a much larger surface area and ability to store electricity than a conductor with just the surface area of the membrane alone.

Environmental Advantages
In addition to making hybrid vehicles more efficient, DESDs also could make them more environmentally friendly because DESDs don't wear out like batteries and would last for the life of the vehicle and beyond.

"The limiting factor right now is the low voltage (less than 5 volts) that can be imposed on the DESDs," Palusinski said. The voltage limit is caused by the small space between conductors in the membrane. At higher voltages, electricity will spark between the conductors, causing loss of charge in the same way that the static charge on your body will discharge to a doorknob during dry weather.

This voltage limitation can be bypassed by connecting the DESDs in series, with the voltage capacity increasing in direct proportion to their number. Unfortunately, connecting them in series lowers the overall capacitance of the array, lowering the amount of electricity it can store. "But this reduction in capacitance can be compensated by connecting several DESD arrays in parallel," Palusinski explained. The capacitance of devices adds when they are connected in parallel.

"We are looking to both industry and NSF for additional funding to pursue this research," Palusinski added. "We are getting close to the commercial development stage, but still need to do additional studies."

Other Applications
In addition to hybrid vehicles, DESDs also have application to microchip technology, portable electronic devices and sensor networks.

Bartley, who is now working at the U.S. Patent Office, is a finalist for the WAGS/UMI Innovation in Technology Award for his work in developing DESDs while he was a master's student at UA. WAGS is the Western Association of Graduate Schools and UMI is University Film International.

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Olgierd Palusinski
Professor
Electrical and Computer Engineering
palusinkski@ece.arizona.edu