Controlling Heat Key to Hybrid Performance

Feb 23, 2009 - NREL

From Tokyo to Detroit, this year's buzz on the auto show circuit is all about really new cars — plug-in hybrids, gas-electric hybrids, electric cars and more.

Chrysler has two new concepts. Honda has four. Mercedes Benz has four, including one that would be powered by a fuel cell. Chevy will begin production of its plug-in electric Volt in 2010, and is working with cities now to prepare charging stations.

Together they suggest a very different transportation lineup in the next decade.

At NREL, engineers are exploring several ways to improve hybrid performance to such a convincing level that millions of commuters will make the switch.

To the lab, that means increasing the vehicles' range to 100 mpg or more, improving reliability and reducing costs — all while operating with drastically reduced tailpipe emissions.

"The real answer isn't Detroit making the right car but the consumer deciding they want the right car," said Rob Farrington, manager of NREL's advanced vehicles program.

In the Advanced Electronics Laboratory, research engineers are exploring key components of the electric drive systems.

Their ground-floor windowless research unit is about the size of a large suburban garage. There isn't an intact car in sight, however.

Instead, researchers are testing key power components at bench stations, including motor controllers, AC to DC converters, and inverters that condition the electrical signal between the power generation unit (a fuel cell or battery) and the electric motor to provide power to various components.

Greater than the Sum

No single improvement will make the difference. But combined, the results can help automakers overcome technical barriers can delay the successful commercialization of advanced vehicles.

"About one-third of the incremental cost of hybrids is in its power electronics," said senior engineer Ken Kelly, task leader for the advanced electronics lab. "We need to process that electricity in ways that are reliable and extend the range of the vehicle. That's what this lab is all about."

On one bench, engineers are testing the performance of a Toyota hybrid system drive. It already works at 90 percent efficiency. But heat is an enemy; the vehicle's efficiency dips as the coolant temperature increases.

If it could run cooler, it would run longer. And it could be manufactured using less expensive materials.

Kelly's group is experimenting with heat exchangers made of different layers, including graphite and indium.

"One third of your radiator's capacity is used to cool the vehicle's electronics," Kelly said. "People want to package things smaller and powerfully. That means there is more heat in a small space. So the challenge is growing to get heat from the device to the coolant."

All in the Grease

In a separate heat-related experiment, senior engineer Sreekant Narumanchi is exploring advanced materials for the interface between power electronics components.

Silicon chips in power electronics typically rest on a metal base plate that conducts heat away from the chip. Coolant flows underneath the plate to carry away the heat. The heat transfer is aided by a very thin layer of "grease" spread between the parts.

"Even though the layers fit together, there are little gaps that cause resistance to the transfer of heat," Narumanchi explained. "The grease is used to close those gaps. It's a much better conductive pathway than air."

Automakers don't use conventional grease containing animal fats; typically it's a silicon gel containing aluminum particles and other inorganics. The gels' performance eventually suffers under the heat and pressure generated inside the engine in harsh conditions such as summertime rush hour traffic

In extended tests, Narumanchi is precisely testing new gels that include different metals, graphite and even advanced ingredients such as carbon nanotubes.

The bench-scale equipment simulates years of high-temperature conditions and temperature cycling over weeks.

"Our target is a material that will perform for 15 years," he said.

— Joseph B. Verrengia