Innovative Solar Receiver Micro-Design Based on Numerical Predictions

Kurzfassung

Due to the continuous global increase in energy demand, a possible source of renewable energy is certainly represented by solar energy. Concentrated Solar Power (CSP) represents an excellent alternative, or add-on to existing systems for the production of energy on a large scale, like steam turbine power plants. In CSP systems, specular surfaces (heliostats) reflect the incoming sunlight, focusing it on a single or multiple focal points. In some of these 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. These receivers are characterized by a porous structure called absorber. The latter, hit by the reflected solar radiation, transfers heat to the evolving fluid; generally air subject to natural convection. The proper design of these elements is essential in order to achieve high efficiencies, making such structures extremely beneficial for the overall performances of the energy production process. In order to obtain higher efficiencies, a proper combination of the structure parameters such as porosity, heat exchange surface and optical properties is needed. Thus, a deep preliminary study for the characterization and the optimization of the shape of the absorber has been performed. Furthermore, the conclusions and results obtained by the preliminary study reported above have been used in order to design a porous structure that is able to improve the performance of the current in-house technology of open volumetric air receivers. A parametric study and an optimized characterization of the structure have been performed, taking into account current technology limitations with the use of a self-developed continuum-based numerical model. Here, the porous volume is represented by a homogeneous zone described by means of effective weighted parameters and properties, depending on the type of material and the structure considered, such as the porosity, the heat exchange surface and the extinction coefficient. With this initial numerical work, it was possible to evaluate the consequences of the variation of the effective parameters and properties on the performances of the heat and mass transfer in porous media. As a result of this study, hypothetical structures presenting high porosity in combination with a high exchange surface show the best performances. Furthermore, a proper adaptation to the radiative needs, that means the use of low absorptive zones for the front and high absorptive zones for the inner, is needed in order to achieve even higher efficiency. 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 characterized by extremely thin walls and, since presenting a graded porosity, a radiative behaviour that is well adapted to the different needs during the heat transfer process. This structure has been numerically tested through detailed CFD simulation, showing performance improvements and it will be object of study during an upcoming experimental campaign. The numerical tool used in this other working package, differs from the continuum-based one introduced above since a detailed representation of the porous geometry is herein considered. Thus, with the use of a commercial CFD code, it has been possible to simulate the conjugate heat and mass transfer phenomena. The presented advanced geometry will be manufactured in the following months and a cylindrical sample prototype will be tested with the use of the Xenon-high-flux solar simulator test facility.