As the quality and lifespan of Solid Oxide Fuel Cells (SOFC’s) are getting better, methods to measure the “state-of-health” of operating stacks are becoming of increasing interest to help the technology close the gap to a commercial break through.
PhD student Rasmus Rode Mosbæk at DTU Energy (formerly known as DTU Energy Conversion) has installed new equipment and developed new methods to measure impedance, ie. Frequency dependent electrical resistance of fuel cells, making it possible to measure both the individual fuel cells and the entire fuel cell stack all at once and get significantly better data on top.
"Previously we could only measure one cell at a time and it took 13 hours to measure the 16 cells and the whole stack this way.. With the new equipment we can measure 16 cells and the entire stack in half an hour"
Rasmus Rode Mosbæk, PhD Student
Previously researchers were only able to measure one cell at a time, and the cell should preferably not be too large with measurements being done at 16 cm2 cells and preferable 1 cm2 cells. Rasmus Rode Mosbæk has now increased the span of detailed measurements to include 90 cm2 cells.
In collaboration with colleagues he has also developed computer programs able to handle the huge amounts of data, which has suddenly become available to the researchers.
"It's basically better equipment and changed measurement geometry," explains Rasmus Rode Mosbæk.
Modified geometry minimize magnetic noise
Researchers have for years been able to collect good data on single cells with an area of 16 cm2, typically having a resistance of 12 mΩ (milliohms). In stacks the cell area is 90 cm2 with and has a resistance around 3 mΩ, making it very difficult to make reliable measurements due to the spectral noise of the surrounding magnetic fields.
“Magnetic fields generated by the alternating current can influence measurements at very small voltages, but I have now changed the measurement geometry to minimize the noise”, says Rasmus Rode Mosbæk.
Size matters and the smaller the cell the higher resistance, gives better measurements. The resistance dramatically decreases when going to larger cell areas. The improved equipment changes this.
"We have gone from being able to measure at 16 cm2 cells to 90 cm2 at present; also we are able to record parallel impedance data, allowing us to do simultaneous measurements on all 16 fuel cells plus the entire stack, i.e. 17 measurements in total all at once."
This enables researchers to measure what happens in the electrodes, what happens to the gas distribution within the individual fuel cell and within the stack and what happens to the contact loss at the same time.
"Using very detailed impedance we are now able to make what is called Distribution of Relaxation Times, DRT, and from this and using some mathematics we get the gas conversion, the gas diffusion and the reactions inside the fuel electrode and the oxygen electrode. This way I can as an example calculate, that the variation of fuels inside some cells varies plus/minus 7%, inside the stack, as well as which individual cells receive too little gas and those who get plenty."
This allows for optimizing the gas distribution plates called interconnects and adjusting other stack components to achieve greater efficiency.
From 13 hours of measurement to half an hour
Optimizing the measurement method has also led to reduced time needed to do the measurements.
"Previously we could only measure one cell at a time and it took 13 hours to measure the 16 cells and the whole stack this way, leading to most people only measuring three-four cells to minimize the usage of time. With the new equipment we can measure 16 cells and the entire stack in half an hour," says Rasmus Rode Mosbæk.
Besides the obvious advantages in terms of resources, the significantly lower time spent on measuring leads to more accurate data as temperature and amount of fuel varies less in half an hour than 13 hours.
"I have also fully automated the measurements with the test rig, making it possible to measure at night. I don’t need to come down and push the button each time I want to measure. Now the test rig sends out a short text that the equipment is ready to measure and it does so automatically. Ideally, we can, in theory, just press start and go home, because when it finishes a measurement, it goes on to the next step in the test plan."
Better measurements of both stacks and fuel cells in less time quickly spawned a new problem; a vastly increased amount of data.
New scripts for new data
"Doing over 1200 impedance measurements on a 2000 hour test produces huge amounts of data. With the modern computers, this is no big problem in terms of computer-power, but is quite a challenge to analyze this huge amount of data efficiently," says Rasmus Rode Mosbæk.
With better equipment giving more data, Rasmus Rode Mosbæk and his colleagues had to deal with this problem as well. Researcher Chris Graves, a colleague of Rasmus, has developed a piece of software called Rapid and Versatile Data Analysis/Visualization, RAVDAV, an in-house analysis program at DTU Energy that enables the user to analyze and plot data faster. Parallel to this Rasmus Rode Mosbæk and senior researcher Johan Hjelm made a series of programs that can read every single folder with test series, read all the impedance measurements and take out the values in separate text files.
Previously, researchers had to search and plot all the data in manually, spending tremendously amounts of time when plotting 1200 impedance measurements. Now the programs collect, analyze the data and stores the results in separate text files ready to be used directly in statistical programs.
At least so it is in theory, as some programs are still a few bugs from being finished.
"We're not quite finished with the script, but we have shown that it can be done. I can actually get these four values about gas conversion, gas diffusion, fuel and the oxygen electrode resistances out of my 1200 impedance measurements. After which you can analyze how the four values change over time. "
This opens for creating stacks of different kind of cells with variations in the type of the oxygen electrode or the fuel electrode, allowing the researchers to develop and test the new and/or different cells under the exact same conditions. This will give a better insight to enhancing the durability of the cells.
The equipment has created a stir in the SOFC community
It took Rasmus Rode Mosbæk over three years and several experimental setups to develop and install the test equipment, which so far only exists in a single test rig at DTU Energy. But it is mobile and relatively easy to reproduce if the financing falls in place and funding seems not to become a problem.
Although each test rig cost a six-figure sum in Danish Crowns, Rasmus Rode Mosbæk created quite a stir when in July he presented and PhD project at the fuel cell industry conference, European SOFC and SOE Forum in Lucerne, Switzerland. His research and project was praised by several stack manufacturers afterwards.
"Research Centre Jülich is one of the really big SOFC research centers in Europe, and they praised me on stage. Afterwards SOFC Power and Ceres Power, which is among the leading SOFC manufacturers, came over and said that it was good work. So did the people from Karlsruhe Institute of Technology and Topsoe Fuel Cell A/S. It was a little overwhelming," says the PhD student from DTU Energy.
Rasmus Rode Mosbæk defends his PhD thesis on the new impedance measurement equipment the 11th December 2014 at 1:00 PM in Niels Bohr Auditorium at DTU Risø Campus.