Voltage monitoring capabilities increase for higher power fuel cell testing for heavy-duty applications in automotive and aerospace
A new generation of test benches is being developed in Germany that covers full stacks of fuel cells up to 200 kW and uses high-quality electrochemical measurement methods. As part of the project, funded as part of the Baden-Württemberg BWPLUS programme, Smart Testsolutions is responsible for cell voltage monitoring as well as developing a multi-channel impedance spectroscopy measuring device.
Currently, fuel cell test benches can accommodate up to 150 kW of power, which is no longer sufficient for heavier duty applications, which require fuel cells with an output of up to 200 kW. In addition to Smart Testsolution’s involvement, Fraunhofer ISE will also develop a corresponding test environment.
Smart Testsolutions will increase the performance of its CVM (Cell Voltage Monitoring) systems for fuel cells from a maximum of 500 to a maximum of 600 measuring channels with 1000 measured values per second per measuring channel. In addition, the company will develop multi-channel impedance spectroscopy technology to be able to determine specific ageing parameters of the fuel cells.
Current impedance spectroscopy devices are limited to a few channels, so that time-synchronous measurements are not possible for fuel cell stacks with several hundred cells. The greater the level of parallelisation, the more precise statements can be made about the electro-chemical condition of the stack. In addition, the corresponding measurements are accelerated and costs are saved.
“With this project, we want to contribute to shortening the development times of fuel cells and systems,” says Wolfgang Neu, Managing Director of Smart Testsolutions.
Cell Voltage Monitoring Modules
CVM “IntelliProbe” modules are very compact and robust and can easily be plugged together to form a multi-cell monitoring stack. Up to 60 modules allow the measurement of 600 single-cell voltages. However, it is also possible to measure double cell voltages, which means that correspondingly more cells can be monitored. This can be done with high measuring accuracy and time-synchronous data processing.
The modules can be used to monitor the cell voltages of electrolysers and thus ensure that the voltages remain within the permissible range during the electrolysis process. They are housed in an ATEX-certified enclosure, so that the explosion protection requirement, which is important in electrolysis, is fulfilled.
Additionally, IntelliProbe U02 measuring modules also make it possible to record the current curve in a fuel cell stack synchronously with the individual cell voltages.
In order to measure not only the individual cell voltages but also the current flow in a fuel cell or an electrolyser, a two-channel U02 module is added to the classic 10-channel CVM modules. This works by simply plugging it in.
According to Neu, thanks to the new U02 module, the two most important measured variables on a fuel cell stack can be recorded by one measuring technique and absolutely synchronously in time.
“This means that cause and effect can be made plausible in the case of observed effects and conclusions can be drawn about the internal resistance in the stack,” he says.
Another possible application of the measuring module is the signal acquisition of other sensors. Thus, all physical or electrical variables can be integrated into the CVM system and recorded via sensors with a maximum output voltage of 5 volts. These could be pressures or temperatures, for example. Here, too, the measured value acquisition is time-synchronous with all other channels of the measurement technology. The two input channels of the U02 module are galvanically decoupled in all directions, so they can be used completely independently of each other. The maximum sampling rate per channel is 10 kilohertz.
The IntelliProbe U02 module is not limited to use in a measurement module stack for voltage measurement on fuel cells. Due to its high dielectric strength, the module is ideal for use wherever higher system voltages are used, for example in the battery sector.
Monitoring to Reduce Cost
According to Dr Markus Schuster, Business Development Manager of New Energy Electronics at Smart Testsolutions, one of the disadvantages of fuel cells in their existing form is that although they offer advantages in terms of range and energy supply, the technology is too expensive for widespread use. He believes that the use of less expensive materials would be of benefit in terms of uptake but would then require extensive testing and continuous monitoring. The new test bench will go a long way towards gathering the data required to prove the technology.
The heart of a fuel cell vehicle is the PEM fuel cell stack, in which hydrogen and oxygen are converted directly into water, producing electrical energy in the process. Alternative materials to the current platinum catalyst in the electrodes make the cell vulnerable to irreversible corrosion processes in certain conditions, for example freezing.
According to Schuster, such unwanted processes can only be avoided by monitoring the fuel cell voltages. These provide detailed information on the condition of the stack at all times and so enable users to react promptly to critical operating states.
Cell monitoring advantages
Cell Voltage Monitoring (CVM) is therefore an important component of the test benches employed in car manufacturers’ development departments. The standard procedure is to monitor the fuel cell stack as a complete system. This does however have certain disadvantages, as it does not permit the precise localisation of any faults occurring. Only with individual cell monitoring such as that provided by the MCM IntelliProbe system is it possible to gain an in-depth insight into the situation inside the stack. Critical operating states are not just detected, they can also be accurately pinpointed.
A CVM system has to satisfy many requirements. Stationary test bench applications demand a high level of measurement accuracy in combination with a high scanning rate. This permits detailed examination of how the systems react to changes of state, for example changes in load. Certain designers also expect an extended measuring range to allow the simulation of critical operating states which cause the cell voltages to drop into the negative range.
A further important aspect in the development of fuel cell vehicles is the investigation of how the systems are affected by ambient conditions such as temperature and moisture. The monitoring technology employed must therefore be capable of supplying perfect results even at temperatures down to as low as -40°C and the electronics must be protected against the ingress of moisture.