Encouraging signs on diesel solid-oxide fuel cells

Diesel Fuel News, April 1, 2002 by Jack Peckham

Tempe, Ariz. - Indications are that a solid-oxide fuel cell (SOFC) should be able to function properly even with diesel fuels containing some sulfur - although the less sulfur, the better.

In research & development testing so far, Delphi Automotive Systems has been using a <30-ppm sulfur ultra-low-sulfur diesel (ULSD) and has "yet to see any problems," explains Delphi's fuel cell development manager Chris DeMinco.

However, a long-term testing program starting this summer aims to find out whether the SOFCs can tolerate at least some sulfur over the many thousands of hours of operating time as will be required in the commercial world.

So far, "we don't believe ULSD will give us problems, based on our tests with Swedish Class 1 [<10 ppm sulfur] diesel," DeMinco told us. This looks promising because similar ULSD fuels will become widely available in North America, Europe, Japan and some other countries later this decade.

"But there are higher-sulfur fuels in other parts of world, and we have to think about a truck leaving the U.S. and refueling 500 miles inside of Mexico," DeMinco said. This also will be an issue for military SOFC users tapping high-sulfur distillate fuels (JP-8 for example) in various parts of the world, perhaps for many years to come.

SOFCs run on a combination of easier-to-make [H.sub.2]/CO reformed from hydrocarbon fuels, including diesel fuel. This is in contrast to proton exchange membrane (PEM) fuel cells that run exclusively on pure hydrogen, require costly precious metal loadings, and are poisoned by even slight amounts of sulfur or carbon monoxide (CO).

"We don't look at fuel cells to eliminate diesel engines," but rather as a

supplementary power source, DeMinco told the Clean Heavy Duty Vehicles Conference here.

A "second-generation" system that Delphi is working on now greatly reduces weight and volume over the first-generation system, he said. The new system improves total thermal management and better integrates energy recovery for the fuel reformer.

Such a system pre-heats fuel and thus avoids reformer sooting, a typical problem with heavier fuels such as diesel or jet fuel. Waste heat not only heats fuel and the cathode area, but also can be channeled for truck cabin heating, he said.

Even so, a practical SOFC commercial application is still "a few years off' as system costs still must be reduced, thermal cycling improved, start-up time cut, and power density boosted.

* Pony-Pack Target

For a real-world truck auxiliary power unit (APU) application, for example, the competitive target is to outperform the roughly $5,000 cost of auxiliary-engine "Pony-Pack" type systems by delivering heat, air conditioning and other "hotel" truck power loads.

Delphi hopes to beat the cost target by a wide margin, while offering users a wider menu of benefits (not just heat as with many conventional APUs).

More SOFC advantages: It won't emit criteria pollutants (unlike a diesel auxiliary engine), it runs quiet, has twice the thermal efficiency (about 40%) and uses one-quarter the fuel of a conventional engine, he said. It also has very few moving parts, probably meaning low maintenance.

Prices of aerospace-type materials for advanced SOFC systems are dropping, and power density is improving, he said. Fuel efficiency is climbing over 20% today with diesel anode tail gas recycle (including water-gas shift), and this can be improved further.

Start-up time still needs more improvement, but given that the APU is a steady-state auxiliary application rather than the primary motive power application, SOFC doesn't need to achieve the near-instantaneous start-up of a PEM, for instance.

As for robustness/durability, researchers believe that "we've got a pretty good plan" to achieve the required commercial targets, he said. But this will take time, as reformer testing can't be accelerated by merely copying techniques now employed by automakers for catalytic converter accelerated-aging tests (even though the materials can be similar).

So far, over 35 original equipment makers (OEMs) are talking to Delphi about possible SOFC test programs.

Delphi hopes to have a diesel-using demonstration unit running by early 2003.

Separately, Delphi reports in a paper (SAE 2002-01-0411) that SOFCs can neatly fit with the emerging dual-voltage 42v/14v automotive electric systems. The SOFC also looks promising for work trucks, recreational vehicles, fire-rescue vehicles, military vehicles, ships, stationary power, distributed power, residential power and even aircraft.

Delphi is working with European refiner TotalFinaElf and the U.S. Los Alamos National Laboratory on "robust diesel reforming technology," the paper notes. It's also working with U.S. Department of Energy in the Solid State Energy Conversion Alliance, aiming to develop ways to mass-produce 5kW SOFC modules for both transport and residential power.

Separately, a new study by Arthur D. Little (see SAE 2002-01-0414) points to the wisdom of Delphi's SOFC business case, by concluding "APU markets appear to be more easily met by fuel cells than those in the propulsion markets, and the fuel cells provide tangible benefits to the vehicle users."