Design of Advanced Porous Geometry for Open Volumetric Solar Receivers Based on Numerical Predictions

Due to the continuous global increase in energy demand, a possible source of renewable energy is certainly represented by the sun.Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale. In CSP systems, specular surfaces (heliostats) reflect the incoming sunlight, focusing it on a single or multiple focal points.In some of those systems, the Solar Power Tower plants (SPT), the conversion of solar radiation into heat occurs in certain components defined as solar receivers, placed in correspondence of the focus of the reflected sunlight. In a particular type of solar receivers, defined as volumetric, the use of porous materials is foreseen. The core of such receivers is a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers heat to the evolving fluid, which is generally air subject to forced convection. The proper design of this element is essential to achieve high thermal efficiencies, making such structures extremely beneficial for the overall performances in the energy production process.For this purpose, a proper combination of the structure parameters such as porosity, heat exchange surface and optical properties is needed. Thus, a deep preliminary study has been performed to evaluate the consequences of the variation of the effective parameters and properties on the thermal performance of the porous absorber.The knowledge and results gained through this study have been used to define an optimization path in order to improve the absorber microstructure, starting from current in-house state-of-the-art technology till obtaining a brand new advanced geometry. The newly designed structure has been numerically tested, achieving good performance and the presence of the so-called volumetric effect, as the outlet fluid temperature is higher than the solid inlet temperature. Afterwards, a test sample has been produced for laboratory experiments at the DLR solar facility in Cologne, in the form of 3:1 scaled-up demonstrator due to technology limits. Additive manufacturing process technology and a titanium-aluminium alloy have been used for the production by the Fraunhofer IFAM Institute. The outcome of the experimental campaign has been useful for the validation of previously predicted numerical results and, in general, of the overall design procedure. This also confirmed that, as the manufacturing technology will progress and become cheaper in the near future, it will be possible to improve the overall performance of SPT using advanced micro-geometry in open volumetric receivers