PNNL small solid oxide fuel cell achieves record efficiency; microchannels, external steam reforming and recycling
|Power system configuration diagram. Powell et al. Click to enlarge.|
Researchers at the Pacific Northwest National Laboratory report on a highly efficient, small-scale solid oxide fuel cell system featuring PNNL-developed microchannel technology in combination with adiabatic, external steam reforming and anode gas recirculation. The heat and water required for the endothermic reforming reaction are provided by the recirculated anode gas emerging from the SOFC stack. They refer to this as adiabatic steam reforming because external heat sources, such as a combustor or an electric-resistance heater, are not necessary to support the reaction.
The new fuel cell system achieves up to 57% efficiency—significantly higher than the 30 to 50% efficiencies previously reported for other solid oxide fuel cell systems of its size—according to a study published in this month’s issue of the Journal of Power Sources. The pilot system generates about 2 kW of electricity; the PNNL team designed it to be scaleable to produce between 100 and 250 kW.
Solid oxide fuels cells are a promising technology for providing clean, efficient energy. But, until now, most people have focused on larger systems that produce 1 megawatt of power or more and can replace traditional power plants. However, this research shows that smaller solid oxide fuel cells that generate between 1 and 100 kilowatts of power are a viable option for highly efficient, localized power generation.—Vincent Sprenkle, a co-author on the paper and chief engineer of PNNL’s solid oxide fuel cell development program
PNNL’s system includes fuel cell stacks developed earlier with the support of DOE’s Solid State Energy Conversion Alliance and uses methane as its foundation fuel. The PNNL uses external steam reforming to convert the methane to a syngas (CO and H2) which then react with oxygen at the fuel cell’s anode, generating electricity as well as the byproducts steam and carbon dioxide.
|“A critical distinction between fuel cell technologies and other energy conversion devices, such as internal combustion engines, is that fuel cell efficiency is not Carnot-limited and fuel cells can achieve relatively high conversion efficiencies at smaller scale operation. Of the available fuel cell technologies, solid oxide fuel cell (SOFC) offers the highest electrical conversion efficiencies.” |
—Powell et al.
Steam reforming has been used with fuel cells before, but the approach requires heat that, when directly exposed to the fuel cell, causes uneven temperatures on the ceramic layers that can potentially weaken and break the fuel cell. So the PNNL team opted for external steam reforming, which completes the initial reactions between steam and the fuel outside of the fuel cell.
The external steam reforming process requires a heat exchanger. On one side of the wall is the hot exhaust that is expelled as a byproduct of the reaction inside the fuel cell. On the other side is a cooler gas that is heading toward the fuel cell. Heat moves from the hot gas, through the wall and into the cool incoming gas, warming it to the temperatures needed for the reaction to take place inside the fuel cell.
|Microchannel heat exchanger and photochemically etched shim. Source: PNNL. Click to enlarge.|
The key to the efficiency of this small SOFC system is the use of a PNNL-developed microchannel technology in the system’s multiple heat exchangers. Instead of having just one wall that separates the two gases, PNNL’s microchannel heat exchangers have multiple walls created by a series of tiny looping channels that are narrower than a paper clip. This increases the surface area, allowing more heat to be transferred and making the system more efficient. PNNL’s microchannel heat exchanger was designed so that very little additional pressure is needed to move the gas through the turns and curves of the looping channels.
The second unique aspect of the system is that it recycles. Specifically, the system uses the exhaust, made up of steam and heat byproducts, coming from the anode to maintain the steam reforming process—i.e., the system doesn’t need an electric device that heats water to create steam. Reusing the steam, which is mixed with fuel, also means the system is able to use up some of the leftover fuel it wasn’t able to consume when the fuel first moved through the fuel cell.
The combination of external steam reforming and steam recycling with the PNNL-developed microchannel heat exchangers made the team’s small SOFC system extremely efficient. Together, these characteristics help the system use as little energy as possible and allows more net electricity to be produced in the end. Lab tests showed the system’s net efficiency ranged from 48.2% at 2.2 kW to a high of 56.6% at 1.7 kW. Although the single-pass fuel utilization is only about 55%, because of the anode gas recirculation the overall fuel utilization is up to 93%.
The team calculates they could raise the system’s efficiency to 60% with a few more adjustments.
The PNNL team would like to see their research translated into an SOFC power system that’s used by individual homeowners or utilities. The research was supported by DOE’s Office of Fossil Energy.
M Powell, K Meinhardt, V Sprenkle, L Chick and G McVay (2012) Demonstration of a highly efficient solid oxide fuel cell power system using adiabatic steam reforming and anode gas recirculation. Journal of Power Sources, Volume 205, Pages 377-384 doi: 10.1016/j.jpowsour.2012.01.098