The addition of hydrogen is considered a transitional technology for making gas turbines for power generation as well as large engines for ships and vehicles "greener" without having to completely replace the equipment. The goal of many research projects and companies is to operate combustion engines such as motors and turbines entirely on hydrogen. The demand for gas turbines for power generation is increasing because, with the expansion of renewable energies, gas-fired power plants are in demand to stabilize the power grid in the event of fluctuations in energy production, as they allow for flexible operation.
As the hydrogen concentration within the combustion process increases, CO2 savings grow, as do the challenges for the materials that come into contact with the highly diffusive hydrogen. Consequently, requirements for safety and reliability assessments are increasing. The key is the design regarding so-called thermomechanical fatigue (TMF), which directly affects service life. The current consequences are higher safety margins in component design, the use of expensive materials, and costly component testing. What is needed are sound bases for decision-making that take into account the complexity of the stresses involved.
Testing materials and components in the laboratory and qualifying them for use in contact with hydrogen is obvious. However, the practical conditions must be represented on a laboratory scale, and the significance of the material properties determined must be transferable to industrial practice in order to ensure service life and optimize component design.
Combine hydrogen diffusion, temperature changes, and tensile-compressive stress independently of each other
The option of testing high-temperature materials in a hydrogen pressure chamber is ruled out because the high and varying operating temperatures cannot be combined with the pressurized hydrogen inside. Scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg therefore use so-called hollow specimens, in which hydrogen flows through a borehole in the interior of the specimen and the specimen material is subjected to the thermomechanical operating conditions from the outside. This allows the hydrogen pressure, temperature changes, and tensile and compressive stress cycles to be varied experimentally independently of each other, enabling conclusions to be drawn about materials suitability for a wide range of operating conditions.
Recent tests (and publicly funded projects) have shown that hollow specimens are suitable for determining the effects of hydrogen gas on material properties at constant temperatures (isothermal test conditions) and at varying temperatures (anisothermal test conditions).
Gradual implementation of the test results in practice
The test results (stress, strain, service life) from the TMF tests initially provide a basis for deciding on suitable materials. When designing components for use in combustion engines, it is important to strike a balance between conservative design and expected service life, i.e., stress cycles until materials fatigue leads to the formation of cracks. Finally, hydrogen-related reductions in service life depending on pressure can and must be evaluated and taken into account. The various interrelationships to be optimized are represented in a materials model and are thus used in component simulations.
This is another milestone on the road to safe and resource-efficient components in contact with hydrogen.
With the experimental combination and variation of material loads in turbines and engines using hydrogen-containing fuels, the effects of hydrogen can now be evaluated quickly and economically. Materials technology decisions for the decarbonization of combustion processes thus become calculable.
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Fraunhofer Institute for Mechanics of Materials IWM
