Jülich— Researchers at Forschungszentrum Jülich have commissioned a highly efficient fuel cell system that achieves an electrical efficiency in hydrogen operation of over 60 percent. Such a high value has not been reported by any other research team worldwide. And the plant has yet another special feature: The newly developed reversible high-temperature fuel cells can not only generate electricity, but can also be used for the production of hydrogen by electrolysis.

Reversible fuel cells, or “reversible solid oxide cells” in English, or rSOC for short, combine virtually two devices in one. The cell type is therefore particularly suitable for the construction of plants that can store electricity in the form of hydrogen and these can back-flow at a later date again. Such storage technology could play an important role in the energy transition. It is needed to compensate for fluctuations in renewable energies and to counteract the divergence between supply and demand. In addition, the use of remote stations on islands and mountains, in order to ensure a self-sufficient energy supply.

The exceptional property of reversibility is exhibited only by high-temperature fuel cells, SOFC for short, which operate at around 800 degrees Celsius. Due to the high temperature, less noble and less expensive materials than for low temperature fuel cells may be used for this type of fuel cell. At the same time, high-temperature fuel cells work extremely efficiently. Unlike low-temperature systems, whose efficiency with hydrogen is limited to around 50 percent, high-temperature fuel cells can also achieve significantly higher efficiency.

Researchers at Forschungszentrum Jülich have now succeeded in further increasing their efficiency and achieving a value of more than 60 percent for the first time. For their system, the researchers determined in the test operation an electrical efficiency of 62 percent. “This was made possible by an improved stack design in conjunction with an optimized and highly integrated system technology, which electrochemically converts more than 97 percent of the supplied hydrogen,” explains Prof. Ludger Blum from the Jülich Institute for Energy and Climate Research (IEK-3).

One of these improvements lies in the dimensioning of the converter unit (“stack”). “Our stack comes to a power of 5 kilowatts, which could be about the power consumption of two households covered. So far, you have always had to combine several units in kilowatts to achieve comparable performance, “explains Ludger Blum. The researcher hopes that this can also reduce the manufacturing costs, since fewer units are needed for the construction of high-performance equipment.

In electrolysis mode, when the system produces hydrogen, the Jülich system can even drive at a much higher output. With a current consumption of the stack of 14.9 kilowatts, it then generates 4.75 cubic meters (Nm3 / h) of hydrogen per hour, which corresponds to a system efficiency of 70 percent. As a result, the pilot plant is already operating more efficiently than alkaline and polymer electrolyte electrolysers, which account for 60 to 65 percent and are now standard.

“The electrolysis works quite well for the beginning, but here we still see a potential for improvement,” says Ludger Blum. High-temperature systems from other developers, which have been specially optimized for electrolysis, today achieve efficiencies of over 80 percent. In fuel cell mode, however, they will not work as efficiently as the new Jülich system.

The Jülich researchers have already considered further optimizations with which they want to further increase the so-called “round-trip” efficiency. The figure describes the efficiency that remains in the recuperation, ie after the production of hydrogen and reconversion. The scientists want to improve the value from the current 43 percent to more than 50 percent.

For a hydrogen storage, this value would be sensational, even if the technology can not keep up with battery storage in this regard, which sometimes comes to over 90 percent. For fuel cell systems offer other benefits. Since the energy converter, the fuel cell, and the energy carrier hydrogen are clearly separated from each other, again and again can be supplied or derived hydrogen. The size of the storable amount of energy are so limited.