As part of the renewable energies and because of their low environmental impact, solar thermal power plants enjoy a wide acceptance in the public. In the past years, several projects have been launched to install plants even with a total power output level beyond 200 MW, which require large size steam turbines. Steam turbines of solar power plants face much more start-ups and shutdowns, compared to typical fossil type baseload machines. In order to provide the required lifetime of steam turbine components, i.e. in high- and intermediate-pressure modules, accurate calculation methods of temperatures and heat transfer coefficients are essential for natural cooling and start-up assessment. Beside rotors, also turbine inner casings face high thermal stresses, especially close to the inlet spiral. At these conditions high thermal stress occurs, which prevents the part to meet the technical requirements.

The paper below gives a solution how to avoid this high stress and a calculation method for inner casings. A heat-shield introduced around the inlet spiral separates the active cooling domain of the turbine cavity relative to a narrow domain around the inlet spiral, where the fluid velocity is negligible. A method on how to simplify heat transfer calculations below the heat shield region is investigated and discussed. The results are verified vs. a CFD based sensitivity analysis. Finally a reduction of the peak stress on the configuration with heat-shield is demonstrated relative to the peak stress calculated without heat-shield.

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