|Projected system efficiency of the Delphi prototype SOFC APU compared to that of a conventional diesel APU. Click to enlarge. Source: Delphi|
Two US DOE (Department of Energy)-funded projects to develop a SOFC (solid oxide fuel cell)-based APU (auxiliary power unit) system for heavy-duty trucks reported on their progress this week during the DOE’s Hydrogen Program annual merit review in Washington, DC.
One team is led by Cummins Power Generation and includes Protonex LLC and International Truck and Engine. The other is led by Delphi—which is leveraging its SOFC work in the Solid State Energy Conversion Alliance (SECA) (earlier post)—and includes OEMs PACCAR Incorporated (producer of Kenworth, Peterbilt, DAF and Foden Trucks) and Volvo Trucks North America.
|Diagram of a solid oxide fuel cell. Click to enlarge.|
The goal of each project is to build and demonstrate a diesel-fueled SOFC truck APU. APUs are increasingly being turned to as a solution to reduce the fuel consumption drain of idling and hotel loads on heavy-duty long-haul diesels. While current APUs can use a small diesel-fueled genset, a diesel-fueled SOFC APU would ideally deliver improved efficiency and thus lower fuel consumption.
SOFCs use a hard, ceramic compound of metal oxides as an electrolyte, rather than the thin, permeable polymer electrolyte sheet in a PEM. In a PEM fuel cell, hydrogen ions cross the membrane; in an SOFC, oxygen ions cross the electrolyte. SOFCs operate well on hydrogen and mixtures of hydrogen and carbon monoxide, among other fuels.
Benefits of a SOFC APU for a truck application include:
Hydrocarbon fuel reformation requirements for SOFCs are greatly simplified (they are thermally matched, the CO output is a fuel constituent, and there is some sulfur tolerance);
There is no internal water management issue;
SOFCs are a lower cost fuel cell option with no or low requirements for precious metals;
No external cooling is required; and
There is a high quality, high temperature single waste heat stream that can be used in the fuel reforming process as well as for vehicle heating.
On the other hand, there are thermal management issues, as well as issues with startup time, stack degradation, and the OEM requirement for zero net water diesel fuel reforming.
|The Cummins SOFC APU system. Click to enlarge. Source: Cummins|
Cummins Power Generation. The Cummins program began in 2004, but was placed on hold due to budget issues, before being restarted in 2007 (the Delphi project has a similar history.) DOE’s share of the project is $3,225,611, with the industry partners providing $1,732,938.
The Cummins system delivers 2 kW of steady state power from the Protonex SOFC stacks, and about 3 kW of intermittent power from a battery bank. A bi-directional inverter and control (which Cummins leveraged from its work in the recreational vehicle industry) provides up to 5 kW of intermittent power or 2 kW of continuous power to the truck systems.
The stack consists of 4 modules, each comprising 66 tubular fuel cells, which are currently delivering 12.8 W each. The initial target was 10 W each. Each module will include a CPOX (catalytic partial oxidation) reformer, fuel cells, recuperator, tail-gas combustor, and insulation. Cummins says that the SOFC system fuel to electric efficiency is 21% gross, 17% net.
The partners have demonstrated both atomization and vaporization of the fuel. They will use vaporization for the initial units, but will move, in the longer-term, to the atomizer, which requires less start-up energy and has extended maintenance intervals.
Protonex has screened four catalysts and selected one that is capable of more than 93% carbon and H2 selectivity. The team has demonstrated steady-state operation of the tubes for more than 500 hours with the selected catalyst, without carbon formation.
Later this year, the team will build single modules and 4-module sets, and begin testing. Performance optimization is targeting for Q4 2008, along with fuel feed system improvements.
Road testing of the units is due to begin in 2009.
Delphi. DOE is contributing $3,000,000 to the Delphi project—which is now 50% complete—while Delphi is contributing $1,750,000. Delphi’s system has a rated net power output of 3.5 kW, with a target fuel to electric efficiency at rated power of 25%.
The Delphi system is designed with a cycle life—i.e., to go from ambient temperature to the operating temperature of around 750°C and back down to ambient—of 250 cycles. Delphi anticipate one cycle/week, 50 weeks per year, for a 5 year lifetime. (In other words, Delphi sees the system powering up at the beginning of the truck work week and staying on until the end of the week.)
Life the Cummins system, the Delphi system is packaged into the form factor of an existing diesel APU unit.
Delphi is developing reforming technology for diesel/JP-8 SOFC applications by modifying its existing natural gas reformer. Two main designs are being developed:
A CPOX reformer; and
A recycle based (endothermic) reformer.
The CPOX reformer offers moderate efficiency and simplicity of design. The endothermic reformer offers higher efficiencies through anode tail-gas recycling. With an SOFC, water is created on the anode side of the fuel cell. Delphi is looking at taking that water and bringing it back into the reforming process to accommodate autothermal or steam reforming capability.
The current version of the platform only uses CPOX; the next generation will use the endothermic capability.
Through the rest of this year, the Delphi team will complete the SOFC APU hardware design and build and begin subsystem testing and development iterations. The following year will see complete system module testing and the beginning of full SOFC APU testing, with road tests scheduled for 2010.
Delphi sees the mobile SOFC market expanding beyond heavy-duty diesel trucks to recreational vehicles (diesel and LPG), truck and trailer refrigeration (diesel), and military (JP-8) applications.