Numerical models of advanced ceramic absorbers for volumetric solar receivers

Kurzfassung

Nowadays, due to the pressing global energy demand, a possible source of renewable energy is represented by solar energy. Concentrated solar power (CSP) represents an excellent alternative, or add-on to existing systems for large-scale energy production. In those systems and, particularly, in solar tower plants (STP), mirroring surfaces reflect sunlight, focusing it on the core-component defined as volumetric receiver. Here, a porous structure (absorber) is responsible for converting the solar radiation into heat with the use of evolving air under natural convection. The proper design of this element is essential in order to achieve high efficiencies that can make such facilities extremely advantageous. In this paper, an overview of the different approaches for the numerical modeling developed at the German Aerospace Center (DLR), are presented, based both on detailed and homogeneous representation (discrete and continuum approaches, respectively) of the absorber. The corresponding numerical models were run again and compared where possible, using as reference case pre-exiting experimental evaluation, pointing out shortcomings and differences. Temperature profiles for solid and fluid phase, as well as efficiency value, are used as key parameters in the comparison, showing a substantial gap between numerical and experimental results. This inconsistency can be addressed both to inherent difficulties in the experiments butalso on the lack of a proper characterization of the absorber radiative behavior that directly affects the heat transfer process. This gap is even more pronounced when the use of a continuum approach is considered, due to implicit approximation introduced by the homogeneous representation of the porous volume. For those reasons, a better representation of the radiative heat transfer must be introduced in future conjugate numerical models and the use of a continuum approach can be addressed to a preliminary design procedure, given the possibility to quickly manipulate geometric and thermodynamic parameters of porous structures.