The smaller system, developed at the Department of Energy’s PacificNorthwest National Laboratory, uses methane, the primary componentof natural gas, as its fuel. The entire system was streamlined tomake it more efficient and scalable by using PNNL-developedmicrochannel technology in combination with processes calledexternal steam reforming and fuel recycling. PNNL’s system includesfuel cell stacks developed earlier with the support of DOE’s SolidState Energy Conversion Alliance. “Solid oxide fuels cells are a promising technology forproviding clean, efficient energy. But, until now, most people havefocused on larger systems that produce 1 megawatt of power or moreand can replace traditional power plants,” said VincentSprenkle, a co-author on the paper and chief engineer of PNNL’ssolid oxide fuel cell development program.
“However, thisresearch shows that smaller solid oxide fuel cells that generatebetween 1 and 100 kilowatts of power are a viable option for highlyefficient, localized power generation.” Sprenkle and his co-authors had community-sized power generation inmind when they started working on their solid oxide fuel cell, alsoknown as a SOFC. The pilot system they built generates about 2 kWof electricity, or how much power a typical American home consumes.The PNNL team designed its system so it can be scaled up to producebetween 100 and 250 kW, which could provide power for about 50 to100 American homes. Goal: Small and efficient Knowing the advantages of smaller SOFC systems (see the “Whatis an SOFC?” sidebar below for more information), the PNNLteam wanted to design a small system that could be both more than50 percent efficient and easily scaled up for distributedgeneration. To do this, the team first used a process calledexternal steam reforming. In general, steam reforming mixes steamwith the fuel, leading the two to react and create intermediateproducts.
The intermediates, carbon monoxide and hydrogen, thenreact with oxygen at the fuel cell’s anode. Just as described inthe below sidebar, this reaction generates electricity, as well asthe byproducts steam and carbon dioxide. Steam reforming has been used with fuel cells before, but theapproach requires heat that, when directly exposed to the fuelcell, causes uneven temperatures on the ceramic layers that canpotentially weaken and break the fuel cell. So the PNNL team optedfor external steam reforming, which completes the initial reactions betweensteam and the fuel outside of the fuel cell. Water Playground Equipment
The external steam reforming process requires a device called aheat exchanger, where a wall made of a conductive material likemetal separates two gases. On one side of the wall is the hotexhaust that is expelled as a byproduct of the reaction inside thefuel cell. On the other side is a cooler gas that is heading towardthe fuel cell. Heat moves from the hot gas, through the wall andinto the cool incoming gas, warming it to the temperatures neededfor the reaction to take place inside the fuel cell. China Aquatic Play Structures
Efficiency with micro technology The key to the efficiency of this small SOFC system is the use of aPNNL-developed microchannel technology in the system’s multipleheat exchangers. Instead of having just one wall that separates thetwo gases, PNNL’s microchannel heat exchangers have multiple wallscreated by a series of tiny looping channels that are narrower thana paper clip. This increases the surface area, allowing more heatto be transferred and making the system more efficient. PNNL’smicrochannel heat exchanger was designed so that very littleadditional pressure is needed to move the gas through the turns andcurves of the looping channels. Water Sprayground
The second unique aspect of the system is that it recycles.Specifically, the system uses the exhaust, made up of steam andheat byproducts, coming from the anode to maintain the steamreforming process. This recycling means the system doesn’t need anelectric device that heats water to create steam. Reusing thesteam, which is mixed with fuel, also means the system is able touse up some of the leftover fuel it wasn’t able to consume when thefuel first moved through the fuel cell. The combination of external steam reforming and steam recyclingwith the PNNL-developed microchannel heat exchangers made theteam’s small SOFC system extremely efficient.
Together, thesecharacteristics help the system use as little energy as possibleand allows more net electricity to be produced in the end. Labtests showed the system’s net efficiency ranged from 48.2 percentat 2.2 kW to a high of 56.6 percent at 1.7 kW. The team calculatesthey could raise the system’s efficiency to 60 percent with a fewmore adjustments. The PNNL team would like to see their research translated into anSOFC power system that’s used by individual homeowners orutilities.
“There still are significant efforts required to reduce theoverall cost to a point where it is economical for distributedgeneration applications,” Sprenkle explained. “However,this demonstration does provide an excellent blueprint on how tobuild a system that could increase electricity generation whilereducing carbon emissions.” The research was supported by DOE’s Office of Fossil Energy. What is an SOFC? Fuel cells are a lot like batteries in that they use anodes,cathodes and electrolytes to produce electricity. But unlike mostbatteries, which stop working when they use up their reactivematerials, fuel cells can continuously make electricity if theyhave a constant fuel supply.
SOFCs are one type of fuel cell that operate at higher temperatures– between about 1100 and 1800 degrees Fahrenheit — and can run ona wide variety of fuels, including natural gas, biogas, hydrogenand liquid fuels such as diesel and gasoline that have beenreformed and cleaned. Each SOFC is made of ceramic materials, whichform three layers: the anode, the cathode and the electrolyte. Airis pumped up against an outer layer, the cathode. Oxygen from theair becomes a negatively charged ion, O 2- , where the cathode and the inner electrolyte layer meet.
The ionmoves through the electrolyte to reach the final layer, the anode.There, the oxygen ion reacts with a fuel. This reaction createselectricity, as well as the byproducts steam and carbon dioxide.That electricity can be used to power homes, neighborhoods, citiesand more. The big advantage to fuel cells is that they’re more efficient thantraditional power generation. For example, the combustion enginesof portable generators only convert about 18 percent of thechemical energy in fuel into electricity. In contrast, some SOFCscan achieve up to 60 percent efficiency.
Being more efficient meansthat SOFCs consume less fuel and create less pollution for theamount of electricity produced than traditional power generation,including coal power plants.