The first vehicle with a fuel cell to roll down the road is likely to be a Class 8 tractor-trailer truck, with a Delphi solid-oxide fuel cell (SOFC) mounted behind the fairings but turned off. When the big rig pulls in for the typical eight-hour rest stop, the SOFC will start up to power lighting and other accessories as well as heat for sleeper cab warmth in winter and electricity for sleeper cab air-conditioning in warm weather. The truck's diesel engine will not need to run for those purposes.
There are restrictions on engine idling in 25 states, and unless an over-the-road truck operator can pick spots for its drivers to sleep, it needs an alternative. Even in states where there are no laws regulating idle, operators must consider the cost: the price of up to 2000 gal (7600 L) of diesel fuel—about 1.0 gallon/h (3.8 L/h) for about 2000 h/year. It's a high price to pay and an inefficient way to provide climate control, lighting, and other comfort/convenience features for the drivers during off hours.
As a result, many big rigs have auxiliary power units (APUs) powered by small diesels that operate heat pumps for heating and cooling and a generator for lighting and electrical accessories. With a tightening of diesel emissions regulations this year, however, some of the diesel APUs are not available, and like the truck diesels, others have had to be equipped with particulate and NOx traps, increasing their cost. This creates an opportunity to make the SOFC a competitive entry, and Delphi says it will have a 5-kW APU on the market in 2012.
The Delphi SOFC, which produces no emissions, has a fuel consumption of 0.13 gal/h at no load and about 0.2 gal/h in typical operation. That's at least 15% lower than the small two-cylinder diesels that normally power the APUs and is about 80% lower than idling the truck engine. The Delphi SOFC uses, as an electrolyte that is called yttria-stabilized zirconia, a zirconium-oxide-based ceramic. The material, which is stable at room temperature, is hard and chemically inert, so it also is used in dental crowns.
The Delphi fuel-cell stack is designed to run on an assortment of fuels, but the primary choices are natural gas and low-sulfur diesel fuel, with the latter obviously the major market. The SOFC often is thought of as the type of fuel cell most tolerant of nonhydrogen fuel components, including carbon monoxide and sulfur. But it still needs reforming and it needs to have a lot of the sulfur removed, even from low-sulfur fuel. Both functions are performed in a sophisticated pretreat system.
When the fuel cell is first started, the diesel fuel goes into a reformer to change it to a hydrogen-rich gas. The Delphi reformer operates on either of two processes, one called partial catalytic oxidation, the other steam reforming. Although steam reforming is more efficient, it does use water, and Delphi has been committed to a self-contained system that would not require a stored water supply. So for openers, the partial catalytic oxidation process is used, and the treated fuel then enters a desulfurizer, which is a bed of absorbing material that pulls hydrogen sulfide out of the reformate stream, lowering sulfur content to a level safe for the fuel cell. The bed requires only a single pass, and although it eventually must be replaced, the service interval ranges from six months to a year, depending on fuel-cell usage.
Once the fuel cell is operating, water is produced by the anode. That water (called anode tailgas) is routed to the reformer, which switches to steam reforming.
The SOFC has an operating efficiency of 30% on diesel fuel and 40% on natural gas, according to Steven Shaffer, Delphi Chief Engineer for fuel cell development. It lends itself to mass production, and because it contains no precious metals, it is cost-effective in the APU application. Delphi said it would compete against "midrange" APUs—i.e., in the $8000-$9000 installed price range (some diesel APUs are as much as 50% higher).
The operating temperature of over 700°C (1300°F) results in long warm-up times. This would be an issue for a prime mover but not for the APU application. In fact, the high operating temperature means that exhaust heat normally should be sufficient to heat the sleeper compartment at close to no-load idle under favorable operating conditions, perhaps even the entire passenger cabin if that is used for a rest break. The Delphi APU will use exhaust ducting and a heat exchanger for the heating function. Air-conditioning, lighting, and sleeper compartment convenience accessories will be electrically powered, Shaffer said, with the voltage output dependent on the vehicle electrical system and the OE-provided output. Delphi is working with OEs, Shaffer noted, so the APU would include whatever is requested, from a simple 12-V output to something beyond.
Although the SOFC operates at high temperature, the Delphi unit is well-shielded, and Shaffer said the exterior would be safe to touch.
The Delphi APU (and diesel APUs) will have a lower-priced electric competitor but with lower capability: the battery-operated air-conditioning system, available in systems that typically produce approximately 3000 to a little over 6000 Btu (6330 kJ), will use deep-cycle lead-acid batteries. It's a well-proved design that can maintain comfort in a cabin or sleeper compartment already cooled by the vehicle's engine-powered A/C. However, at 20-40% the performance of an APU, generally it is not capable of providing meaningful initial cool-down. And if used also in areas where winter heating is needed, a diesel-fuel-fired heater is a necessary additional cost.