A new, small-scale solid oxide fuel cell (SOFC) system developed at the Department of Energy’s Pacific Northwest National Laboratory (DoE PNNL) could be used for household and neighborhood power generation. Fueled by methane, the system achieves an efficiency of up to 57 percent, improving on the 30 to 50 percent efficiencies seen previously in SOFC systems of similar size. The PNNL researchers say the pilot system they have built generates enough electricity to power the average American home, and can be scaled up to provide power for 50 to 250 homes.
Solid Oxide Fuel CellsLike batteries, fuel cells use anodes, cathodes and electrolytes to produce electricity. But unlike most batteries, fuel cells can continuously produce electricity if provided with a constant fuel supply. Fuel cells are characterized by their electrolyte material, which in the case of SOFCs is a solid oxide or ceramic. Ceramic materials also form the anode and cathode which, along with the electrolyte, form three layers.
Air is pumped up against the cathode, which forms the outer layer, with oxygen from the air becoming a negatively charged ion where the cathode and the inner electrolyte layer meet. The negatively charged oxygen ion then moves through the electrolyte to reach the final anode layer where it reacts with a fuel to create electricity, as well as steam and carbon dioxide byproducts. SOFCs can run on different fuels, including natural gas, biogas, hydrogen, but the PNNL team chose methane - the primary component of natural gas - to fuel its new SOFC.
Because they are more efficient than other methods of electricity generation, including coal power plants, SOFCs consume less fuel and create less pollution to generate the same amount of electricity. Small-scale SOFCs also have the advantage of being able to be placed closer to where the electricity generated is consumed, reducing the amount of power that is lost when sent through transmission lines.
"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," said Vincent Sprenkle, chief engineer of PNNL's solid oxide fuel cell development program. "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."
With the aim of designing a small system that was more than 50 percent efficient and could also be scaled up to produce electricity for neighborhoods, the PNNL team combined external steam reforming and fuel recycling with microchannel technology.
Steam reformingSteam reforming involves mixing steam with the fuel so that they react to create carbon monoxide and hydrogen, which in turn reacts with oxygen at the fuel cell’s anode. Because this process requires heat that can cause uneven temperatures on the ceramic layers and lead to weakening and breakage of the fuel cell, the PNNL team used a heat exchanger to allow the initial reactions between steam and the fuel to be completed outside of the fuel cell in what is known as external steam reforming.
Heat exchangers consists of a wall made of a conductive material that separates the two gases. The hot exhaust that is expelled as a byproduct of the reaction inside the fuel cell is located on one side, while a cooler gas that is heading toward the fuel cell is located on the other. Heat from the hot gas moves through the wall to warm the incoming gas to temperatures needed for the reaction to take place inside the fuel cell.
Microchannel heat exchangersBut instead of having just one wall separating the two gases, the PNNL researchers created multiple walls using a series of tiny looping channels, narrower than a paperclip. These microchannel heat exchangers increase the surface area to allow more heat to be transferred, thereby increasing the efficiency of the system. The microchannel heat exchanger was also designed so that the gas moves through the looping channels with very little additional pressure.
Steam RecyclingThe PNNL system also recycles the exhaust coming from the anode, consisting of steam and heat byproducts, to maintain the steam reforming process. Not only does this recycling negate the need for an electrical device to heat water and create steam, it also means that the system is able to use up some of leftover fuel that wasn’t consumed the first time around.
The combination of external steam reforming and steam recycling and use of microchannel heat exchangers allow the system to use as little energy as possible with the end result being more net electricity production. In lab tests, the team say net efficiencies ranging from 48.2 percent at 2.2 kW, up to 56.6 percent at 1.7 kW. With a few more adjustments, the team believes they can raise the system’s efficiency to 60 percent.
With the average American home consuming roughly 2 kW or electricity, the pilot system could be used for household power generation. However, they also designed it so it could be scaled up to produce between 100 and 150 kW, which could provide enough electricity to power 50 to 100 homes. The PNNL team hope to see their research translate into just such a system that could be used by individual households or utilities.
The PNNL team’s small-scale SOFC is detailed in a paper published in the Journal of Power Sources.