Within the European research project SOL2HY2, key components for a solar hybrid sulfur cycle are being developed and demonstrated at pilot scale in a real environment. Regarding the thermal portion, a plant for solar sulfuric acid decomposition is set up and initially operated at the research platform of the DLR Solar Tower in Jüulich, Germany.

One major component is the directly irradiated volumetric receiver, superheating steam and SO3 coming from a tube-type evaporator to above 1000 °C. At the design flow rate of sulfuric acid (50%-wt.) of 1 l/min, a nominal solar power of 57 kW is required at the receiver. With a flat ceramic absorber made from SiC and a flat quartz glass window, the design is based on lab scale reactors successfully demonstrated at the solar furnace of the German Aerospace Centre (DLR) in Cologne, Germany.

A flexible lumped thermodynamic tool representing the receiver, compiled to assess different configurations, is presented in detail. An additional raytracing model has been established to provide the irradiation boundaries and support the design of a conical secondary concentrator with an aperture diameter of 0.6 m. A comparison with first experimental data (up to 65% nominal power), obtained during initial operation, indicates the models to be viable tools for design and operational forecast of such systems. With a provisional method to account for the efficiency of the secondary concentrator, measured fluid outlet temperatures (up to 1000 °C) are predicted with deviations of ±60 °C. Respective absorber front temperatures (up to 1200 °C) are under-predicted by 100–200 °C, with lower deviations at higher mass flows. The measured window temperature (up to 700 °C) mainly depends on the absorber front temperature level, which is well predicted by the model.

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