Washington State/Boeing SOFC shows promise for aviation and automotive applications
17 June 2014
|MoO2-based SOFC using a fuel mixture consisting of n-dodecane, CO2 and air. Kwon 2013. Click to enlarge.|
Researchers at Washington State University, with colleagues at Kyung Hee University and Boeing Commercial Airplanes, have been developing liquid hydrocarbon/oxygenated hydrocarbon-fueled solid oxide fuel cells (SOFCs) for aviation (the “more electric” airplane) and other transportation applications, such as in cars. These fuel cells first internally—i.e., no external reformer—reform a complex liquid hydrocarbon fuel into carbon fragments and hydrogen, which are then electrochemically oxidized to produce electrical energy without external fuel processors. The SOFCs feature a MoO2 (molybdenum dioxide) anode with an interconnecting network of pores that exhibit excellent ion- and electron-transfer properties.
In a new paper in the journal Energy Technology, the team reports that this novel fuel cell, when directly fueled with a jet-A fuel surrogate (an n-dodecane fuel mixture), generated an initial maximum power density of 3 W cm-2 at 750 °C and maintained this high initial activity over 24 h with no coking. The addition of 500 ppm of sulfur into the fuel stream did not deactivate the cell.
The ability of the MoO2-based SOFC to operate with the direct input of complex liquid fuels makes it a promising energy converter to meet the increasing electrical power demand of future airplane designs.—Kwon et al.
For aviation applications, the researchers envision integrating their fuel cell with a battery to power auxiliary power units. These units are currently powered by gas turbines and operate lights, navigation systems and various other electrical systems. The two technologies—battery and fuel cell—complement each other’s weaknesses, says WSU Professor Su Ha.
The results of this research are a key step in the integration of fuel cell technology in aviation and the development of the more electric airplane.—Joe Breit, associate technical fellow at Boeing and co-author
The team also has a paper in press in the Journal of Power Sources on the gasoline-fueled performance of the SOFC. Fueled with premium gasoline, the SOFC demonstrated a power density of greater than 3.0 W cm-2 at 0.6 V. Over a 24 h period of operation, the open cell voltage remained stable at ∼0.9 V. At the cell voltage of 0.6 V, current density dropped over the first 7 h to a value of ∼3.0 A cm-2, where it stayed for the remaining 17 h of the test with a minor fluctuation. Power density of ∼2.0 W cm-2 at 0.6 V was still measured after 24 h on stream with a continuous feed of gasoline.
Scanning electron microscopy (SEM) examination of the anode surface pre- and post-testing showed no evidence of coking, which hints at the reason for the observed stability under the harsh cell operating conditions. The team suggested in the paper that the results of this preliminary study indicated that an SOFC using a MoO2-based anode has potential for generating electrical power from gasoline for future hybrid electric vehicles.
The work began on developing a solid-oxide fuel cell to provide electrical power on commercial airplanes began about 10 years ago. Fuel cells offer a clean and highly efficient way to convert the chemical energy in fuels into electrical energy. In addition to increasing fuel efficiency and reducing emissions of harmful pollutants, fuel cells are quiet and would be particularly helpful when a plane is at a gate and the main jet engines are turned off.
The process could be approximately four times more efficient than a combustion engine because it is based on an electrochemical reaction. The solid-oxide fuel cell is different from other fuels cells in that it is made of solid materials, and the electricity is created by oxygen ions traveling through the fuel cell.
Using jet fuel and gasoline to power their fuel cell proved tricky. To avoid the added weight of a reformer that would convert the complex fuel into syngas, the researchers wanted to be able to directly feed the liquid fuel into the fuel cell. Furthermore, they had to overcome the problems of sulfur poisoning and coking, a process in which a solid product is created from imperfect combustion. Sulfur is present in all fossil-based fuels and can quickly deactivate fuel cells.
The MoO2-based anode is fabricated on to an yttria-stabilized zirconia (YSZ) electrolyte via combined electrostatic spray deposition (ESD) and direct painting methods.
Byeong W. Kwon, Shuozhen Hu, Oscar Marin-Flores, M. Grant Norton, Jinsoo Kim, Louis Scudiero, Joe Breit and Su Ha (2014) “High-Performance Molybdenum Dioxide-Based Anode for Dodecane-Fueled Solid-Oxide Fuel Cells (SOFCs),” Energy Technol. doi: 10.1002/ente.201490009
Xiaoxue Hou, Oscar Marin-Flores, Byeong Wan Kwon, Jinsoo Kim, M. Grant Norton, Su Ha (2014) “Gasoline-Fueled Solid Oxide Fuel Cell with High Power Density, Journal of Power Sources doi: 10.1016/j.jpowsour.2014.06.038
Byeong Wan Kwon, Caleb Ellefson, Joe Breit, Jinsoo Kim, M. Grant Norton, Su Ha (2013) “Molybdenum dioxide-based anode for solid oxide fuel cell applications,” Journal of Power Sources, Volume 243, Pages 203-210 doi: 10.1016/j.jpowsour.2013.05.133
It's impossible to be 4 times as efficient as an internal combustion engine unless you are talking averages, including idling. SOFCs are around 60% efficient. The Prius powerplant hits about 38%, so the improvement at peak efficiency is closer to 50%.
That said, this cell has great possibilities. The high operating temperature of 750°C is considerably hotter than gas-turbine compressor exhaust air, allowing that air to be used for SOFC air supply and cooling. That in turn allows the SOFC to function as a topping cycle for the gas turbine, with the SOFC waste heat doing work a second time in the GT. Operating under pressure, the reactant concentrations in the SOFC will be marginally higher and so will the operating voltage; this may increase efficiency slightly.
If we assume that an SOFC unit and afterburner replaces the combustor on a GE LMS100 gas turbine, and that the turbine requires approximately 50 MW(th) to idle (projecting its operating curve to zero power suggests 30-odd MW, but that is probably optimistic), the SOFC operating at 60% efficiency would output 75MW(e) and 50 MW(th). Adding methane to the SOFC exhaust would add thermal power. Bringing the LMS100 up to its full 217.4 MW(th) heat input would add 167.4 MW(th) of fuel and add 100 MW(e) of output. The net efficiency at full power would be 175 MW(e)/292.4 MW(th) = 59.9%. Not too shabby.
Posted by: Engineer-Poet | 17 June 2014 at 04:27 AM
The cell efficiency may not be 4 times the best ICE but it is already higher than the best ICE will ever reach.
Using cleaner bio-fuels could reduce cell contamination rate and extend operation hours between major refurbishing and/or cleaning. Self-cleaning cells may be a future possibility?
Some of the excess heat could be recovered to heat the passenger cabin and/or to generate more e-energy?
Larger units could be used for cleaner running, e-buses, e-trucks, e-locomotives, e-ships, e-tanks etc?
Lighter units could be used for future long range e-drones?
Posted by: HarveyD | 17 June 2014 at 07:29 AM
There are other ways to increase the system efficiency than using just waste heat in a combined cycle. By combusting the tail gas (the SOFC only needs the h2) a hybrid SOFC turbine or hybrid SOFC IC engine can achieve greater than 70% efficiency. Check out this Stanford reference (http://gcep.stanford.edu/events/symposium2013/docs/Poster2013_RebeccaPass.pdf).
General Electric is also investigating using SOFC and their Gas Engines (see http://spectrum.ieee.org/energywise/energy/fossil-fuels/ge-to-muscle-into-fuel-cells-with-hybrid-system, also patent US 20120251899, SOFC high-efficiency reform-and-recirculate system).
Posted by: Account Deleted | 17 June 2014 at 09:20 AM
Franklin Fuel Cells had a copper-ceria design that would take diesel directly more than 5 years ago, this is not a totally new breakthrough.
It seems obvious that you need to have something that runs 24/7 to make this really useful, they still have not cured the temperature cycling seal cracking as far as I know. So plans of putting one of these in a car may be a bit off.
Harvey is right, if we start to use more synthetic fuels we get much less sulfur and the resulting "coking" that happens. If we proceed with bio synthetic we can be more CO2 neutral as well.
Posted by: SJC | 17 June 2014 at 09:41 AM
gryf, SOFCs can run on pure carbon monoxide. They are oxygen-ion transport cells, and any fuel that scavenges oxygen at the anode will do.
Posted by: Engineer-Poet | 17 June 2014 at 09:51 AM
Any way you slice it, SOFC is not practical for automotive use if it costs any more than $0.10/W , and $0.03-0.05/W is a lot more realistic.
For aviation use, call the GE90 turbofan 90MW for $20MM, or ~$0.22/W, so maybe $0.15-$0.20/W is acceptable in that application.
Posted by: Otis | 17 June 2014 at 12:41 PM
What difference would it make if end users had to pay for CO2 emitted @ $40/tonne to $100/tonne?
Posted by: HarveyD | 17 June 2014 at 02:23 PM
Using Fuel Cells to replace APUs in aircraft is something that is a great fit and will happen once they work out the technology issues. Especially if they can use regular jet fuel!
Boeing needs to figure out their battery situation though - the 787 still uses the Li-Cobalt batteries that are much more temperamental and subject to thermal runaway than other Li-Ion batteries available. Boeing really didn't fix the battery problem, they just put the batteries in a big iron box that will contain the molten battery and vent the smoke. The root cause wasn't fixed. They need a high power battery that can change output quickly to match the energy demand of the aircraft (which is why they need batteries to buffer the output of the APU in the first place). Its just a matter of when they can validate batteries that will match their needs.
Posted by: Anthony F | 17 June 2014 at 02:30 PM
A practical point is that the relatively pure oxygen of the stratosphere would benefit the SOFC, and the waste heat would be invaluable for deicing in flight and before takeoff, if the battery system were distributed across the wings and a fluid based heating system were manageable. There is still a great need to cool the jets, even from runway heat, so coolant management will always be there. The big issue is the energy cost and benefit of compressing the air at high altitudes to run the SOFC. What pressure range does the model here operate at? Cabin air pressure and power to maintain it are ongoing issues. So is CO2 and particulate scrubbing, for which the basic tech here is a help.
Posted by: kalendjay | 17 June 2014 at 02:42 PM
The stratosphere has the same oxygen/nitrogen ratio as the troposphere.
Posted by: Engineer-Poet | 17 June 2014 at 08:06 PM
Engineer-Poet is correct, when hydrogen is derived by “reforming” hydrocarbons such as gasoline or dodecane in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Since the reforming process does not convert 100% of the fuel or have a 100% H2 yield, the combustibles content of the tail gas can vary widely.
Posted by: Account Deleted | 17 June 2014 at 08:43 PM
SOFC may have a future for Auxiliary Power Units (APU) provided costs are reasonable as Otis pointed out. The “more electric” airplane could use this APU as an "Electric Green Taxiing System" (EGTS) with an electric motor in the nose wheel. Taxi operations represent up to 6% of fleet fuel consumption. Honeywell and Safran are developing the EGTS and think that up to 2/3 of the taxi operations fuel could be saved (or 4% of total fuel costs) by using the EGTS with power from the aircraft's APU. There would be even greater savings with an SOFC APU since it is much more efficient than a gas turbine APU.
Posted by: Account Deleted | 17 June 2014 at 10:02 PM
gryf, you're mistaken about SOFCs in general. The only fuel cells that require hydrogen are PEM (Proton Exchange Membrane) FC's. Hydrogen supplies the protons which carry charge across the electrolyte. SOFCs use oxygen ions, which are supplied from the air. They need no hydrogen at all.
SOFCs generally have combustible tail gas because it's not efficient to completely consume the fuel; as the fuel goes to complete consumption the oxygen content in the gas rises and the cell voltage drops. Leaving enough un-consumed fuel to burn with some excess air cleans up the emissions.
There's potential for an SOFC APU, but the real advantage would come from a full integration of SOFCs with the main engines. If the total engine efficiency could be boosted from 40% to 60%, fuel consumption would drop by a third; electrification of much of the propulsion system would allow further improvements from e.g. boundary-layer control. That is an immense advantage.
Posted by: Engineer-Poet | 18 June 2014 at 08:32 AM
If the total engine efficiency could be boosted from 40% to 60%, fuel consumption would drop by a third; electrification of much of the propulsion system would allow further improvements from e.g. boundary-layer control.
Would efficiency be improvable even further by adding a stage to take advantage of exhaust heat, then the cooled unburned fuel could be burned in a final stage?
Intake -> Electric fans -> Hot SOFC exhaust air/fuel stage -> cool air/fuel ignited stage -> exhaust
So you'd have an electric stage, an "external combustion" stage and an "internal combustion" stage. I tend to think insulated EGR would be a better idea with the SOFC, especially since there shouldn't be any particulates.
Posted by: Otis | 18 June 2014 at 03:02 PM
The hybrid FC/turbine engine would use the FC as an air pre-heater for the gas turbine. Two bites at the energy apple.
Posted by: Engineer-Poet | 18 June 2014 at 05:19 PM
SOFC can only hit 60% efficiency at very low load. 40% is more realistic and a diesel can do the same at lower weight and considerably lower cost.
Posted by: SimonDM | 19 June 2014 at 04:22 AM
Good point, Simon. However, a diesel is not an option for jetliner. Diesel can do about 45% peak thermal efficiency, while a more recent turbofan core engine with pressure ratio of 40 or greater can deliver 45-50% thermal efficiency at cruise, depending on size and pressure ratio and turbine inlet temperature. It is very hard to improve on this kind of efficiency and still keep the core engine light and compact enough for a jetliner.
Posted by: Roger Pham | 19 June 2014 at 12:10 PM
The reason that the SOFC is good for APU purpose is that small gas turbines for APU are very inefficient, at around 15% peak thermal efficiency, while a SOFC can be 50%-efficient no matter what size. A SOFC produces electricity that can be used for taxiing the aircraft on the ground and for cabin cooling, thus saving a lot of jet fuels that is used very inefficiently when the large turbofans are at idle. Otherwise, SOFC's are expensive per kW basis and are quite bulky.
Posted by: Roger Pham | 19 June 2014 at 12:14 PM
It's hard to use a diesel as a topping cycle for a gas turbine.
If we assume a 500°C fuel-cell air inlet temperature and 800°C output temperature, the air can carry away about 302 kJ/kg of heat. A 40%-efficient SOFC would dump that much heat while generating 201 kJ(e)/kg of air. I'm not sure how much heat-dissipation capacity to allocate to the fuel, but using the FC to pre-heat and reform the fuel would allow more heat dissipation in the FC and increase the amount of fuel energy recoverable as electricity.
IIRC, the best modern gas turbines have turbine inlet temperature limits on the order of 1380°C, a temperature rise of about 880°C over the compressor outlet. The FC would produce about a third of this ΔT. The actual fuel energy input to the FC would be 5/9 of the total turbine heat input, with FC electric output of 2/9 of the turbine heat input. If the GT efficiency remains at 40% of heat input, the net efficiency becomes:
2/9 / 11/9 = 2/11 of fuel energy recovered in the FC (18.2%)
9/9 / 11/9 * .40 = 32.7% recovered in the GT
Net efficiency 50.9%, up more than 25% over the bare GT.
Posted by: Engineer-Poet | 19 June 2014 at 08:14 PM