Wink of Knowledge: Improved methanol/water concentration model for fuel cells
Why this test?
The methanol fuel cell is an important technology for the energy transition. Although today’s methanol production is still heavily dependent on fossil sources, renewable raw materials such as biogas, sewage sludge or even atmospheric CO2 are becoming increasingly important. A fuel cell such as the direct methanol fuel cell (DMFC) then generates electricity from this methanol in a similar way to a conventional generator. It is important for the efficient and safe operation of such fuel cells to feed in a methanol/water mixture with a constant concentration – the optimum concentration depends on the type of fuel cell. Process control is made more difficult by the incomplete conversion of the methanol. A varying proportion of the mixture exits the fuel cell unused and should be continuously recycled. Figure 1 shows the process schematically:
Figure 1: Schematic of a direct methanol fuel cell with various possible measuring points.
Providing a controlled mixture of the recyclate and a methanol stock solution is therefore a challenge. This is precisely where the concentration measurement using the DLO-M2 density sensor comes into play (orange measuring point 2 in Figure 1).
What is a Wink of Knowledge?
Do you need to quickly measure, draw or do/build something? The speed with which the result may be achieved counts more than the perfect (scientific) approach. For this reason, we have introduced the Wink of Knowledge. Science in the wink of an eye, so to speak. We don’t want to prove anything scientifically. We simply want to quickly demonstrate something pragmatically. If you are interested, we would be happy to discuss these results in more detail with you and your project.
Results
Existing data on aqueous methanol solutions were combined with our own measurements using a DSA 5000 M laboratory density meter (Anton Paar). The own measurements mainly included typical operating conditions of fuel cells such as concentrations <10% at temperatures >40°C.
The collected data was processed into a concentration model for our DLO-M2 density sensor. As a result, this can now calculate and output the methanol concentration of a solution directly from the measured density value with an accuracy of ±0.2%w/w:
Figure 2: Accuracy analysis of the new concentration model methanol in water according to %w/w
Of course, in addition to the model accuracy, the measuring accuracy of the sensor is also crucial. In the case of the VLO-M2, this is approx. ±0.2 kg/m3 for the mixtures under consideration (the DLO-M2 achieves a comparable performance after adjustment). For the entire concentration range and an example temperature of 25°C, the following picture emerges:
Figure 3: Overall accuracy of the measurement including the measuring accuracy of the DLO-M2 density sensor
In the complete measurement range of 0-100%w/w, the overall accuracy remains very good and is around ±0.3 %w/w (shaded gray in Figure 3). Thanks to the excellent measurement accuracy of the TrueDyne MEMS sensor technology, the error due to the density measurement affects the overall error in this case even less than the pure model accuracy (shown by the orange line at ± 0.2%w/w).
Which sensors were used?
density sensor DLO-M2
- Click here to learn more about our sensor
viscosity sensor VLO-M2
- Click here to learn more about our sensor
Conclusion
A new, much more accurate methanol/water concentration model has been integrated into the DML product family (DLO-M2 / VLO-M2). The background for this update is the increasing use of methanol as an energy source, for example for power supply via fuel cells. In combination with the high-precision density measurement of our sensor technology, the model enables real-time concentration monitoring of the methanol/water mixture and thus efficient and safe operation of the fuel cell. This enables optimum efficiency to be achieved with maximum service life of the fuel cell.
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