Pacific Northwest National Laboratory (PNNL) using methane, the primary component of natural gas for fuel has designed a new, small-scale solid oxide fuel cell system that achieves up to 57 percent efficiency with room to improve.

PNNL Fuel Cell Process Graph and Components. Click image for the largest view.

Fifty seven percent for a solid oxide fuel cell system at this size is significantly higher than the 30 to 50% efficiencies that’ve been reported for other solid oxide fuel cell (SOFC) systems.

Both PNNL and Blacklight have realized a potential.  Over time hundreds of thousands or perhaps millions distributed generation installations at about 15 amps, the practical home electrical demand baseload, would equal a huge share of the electrical demand.  At the installation mass production would drive down costs and reduce billings and the grid would carry much less total current allowing more economic growth when meeting peak demands, a condition that could also reduce rates and billings to ratepayers.

The PNNL SOFC system has been streamlined to make it more efficient and scalable by using PNNL-developed microchannel technology in combination with processes called external steam reforming and fuel recycling. PNNL’s system includes fuel cell stacks developed earlier with the support of Department of Energy’s Solid State Energy Conversion Alliance.

PNNL Fuel Cell Microchannel Heat Exchanger. Click image for the largest view.

Vincent Sprenkle, chief engineer of PNNL’s solid oxide fuel cell development program explains the situation in the research field with, “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.”

The group’s paper has been published in this month’s issue of Journal of Power Sources.

Sprenkle and his co-authors had community-sized power generation in mind when they started working on their solid oxide fuel cell. The pilot system they built generates about 2 kW of electricity, or how much power a typical American home consumes. The PNNL team designed its system so it can be scaled up to produce between 100 and 250 kW, which could provide power for about 50 to 100 American homes.

The PNNL team’s goal became design a small system that could be both more than 50% efficient and easily scaled up for distributed generation. To do this, the team first used a process called external steam reforming.  Generally steam reforming mixes steam with the fuel leading the two to react and create intermediate products. The intermediates, carbon monoxide and hydrogen, then react with oxygen at the fuel cell’s anode and the reaction generates electricity, as well as the byproducts steam and carbon dioxide.

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’s body.

The external steam reforming process requires a device called a heat exchanger, where a wall made of a heat conductive material like metal separates the two operating gases. 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.

The big numbers for 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.

Even more interesting is the second unique aspect of the system – it recycles the heat. Specifically, the system uses the exhaust, made up of steam and heat byproducts, coming from the anode to maintain the steam reforming process. This recycling means 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.

When compared to a hydrogen fuel cell that can exceed 90% efficiency the energy cost to come up with the free hydrogen is usually overlooked.

Combining external steam reforming and steam recycling with the PNNL-developed microchannel heat exchangers make the team’s small SOFC system extremely efficient. Added together the characteristics help the system use as little energy as possible and allow more net electricity to be produced at the end. Lab tests showed the system’s net efficiency ranged from 48.2 percent at 2.2 kW to a high of 56.6 percent at 1.7 kW.

The team calculates they could raise the system’s efficiency to 60 percent 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.  There will come a price point that will be very compelling for both homeowners and utilities.

On the other hand Sprenkle explains, “There still are significant efforts required to reduce the overall cost to a point where it is economical for distributed generation applications. However, this demonstration does provide an excellent blueprint on how to build a system that could increase electricity generation while reducing carbon emissions.”

That matter is the membrane inside the fuel cell.  Currently platinum is still the long life leader – a very expensive solution.  Other ideas are making progress, but are not commercial yet.

A quick refresher on SOFCs.  They are one type of fuel cell that operate at higher temperatures – between about 1100 and 1800 degrees Fahrenheit – and can run on a wide variety of fuels, including natural gas, biogas, hydrogen and liquid fuels such as diesel and gasoline that have been reformed and cleaned. Each SOFC is made of ceramic materials, which form three layers: the anode, the cathode and the electrolyte. Air is pumped up against an outer layer, the cathode.  Oxygen from the air becomes a negatively charged ion, O2- , where the cathode and the inner electrolyte layer meet. The ion moves through the electrolyte to reach the final layer, the anode. There, the oxygen ion reacts with a fuel. This reaction creates electricity, as well as the byproducts steam and carbon dioxide.

PNNL is close enough, at 60% efficiency with a common fuel like methane compared to the net efficiency of a more pure hydrogen fuel cell.  The problem for everyone is the catalyst at the membrane.  That’s the breakthrough we’re all looking for.


2 Comments so far

  1. jp straley on June 1, 2012 9:58 AM

    Cost ya forty bucks to read the paper!

    Landfills, wastewater treatment plants, biomass, municipal yard waste…let’s make syngas thence electricity!

    What are the poisons? Can it withstand sulfur, etc.

    If robust, it’s a big advance.

    JP Straley

  2. Jagdish on June 2, 2012 4:55 AM

    If it really works, it could replace a lot of diesel generators.

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