Finite-volume ray tracing using Computational Fluid Dynamics in linear focus CSP applications

The modelling of solar irradiation in concentrated solar power (CSP) applications is traditionally done with ray-tracing methods, e.g. the Monte Carlo method. For the evaluation of CSP receivers, the results from ray-tracing codes are typically used to provide boundary conditions to Computational Fluid Dynamics (CFD) codes for the solution of conjugate heat transfer in the receivers. There are both advantages and disadvantages to using separate software for the irradiation and heat transfer modelling. For traditional ray-tracing methods, advantages are the cost-effectiveness of the Monte Carlo method in modelling reflections from specular surfaces; the ability to statistically assign a sun shape to the rays; the statistical treatment of reflectivity and optical errors (e.g. surface slope errors), to name a few. When considering a complex mirror field and a complex receiver with secondary reflective surfaces, especially with selective coatings to enhance absorption and limit re-radiation losses, standard ray tracers may be limited in specifying emissivity and absorptivity, which are both specular and temperature dependent, and are hence not suitable as radiation analysis tool. This type of scenario can be modelled accurately using CFD, through the finite volume (FV) treatment of the radiative transfer equation (RTE) and a banded spectrum approach at an increased computational cost. This paper evaluates the use of CFD in the form of the commercial CFD code ANSYS Fluent v15 and v16 to model the reflection, transmission and absorption of solar irradiation from diffuse and specular surfaces found in linear CSP applications. 2-D CFD solutions were considered, i.e. line concentration. To illustrate and validate the method, two sources were used. The first source was test cases from literature with published solutions and the second a combined modelling approach where solutions were obtained using both FV and ray tracing (with SolTrace). For all the test cases, good agreement was found when suitable modelling settings were used to limit both ray-effect and false scattering errors.