INTRODUCTION

FluxTracer can be used to evaluate the design and optical quality of volumetric receivers. Figure 1 shows a novel hierarchically-layered fractal-like volumetric absorber, designed to be employed in high temperature concentrating solar power applications, as imported in Tonatiuh++ in the focal region of a parabolic dish. The possibility of importing .obj files offers advanced simulation possibilities for complex geometries that cannot represented with analytical functions. To see how to do this in Tonatiuh++ please press here. The novel hierarchically-layered fractal-like volumetric absorber shown in Figure 1 is designed to be employed in high temperature concentrating solar power applications [1]. The Figure shows a piece of the receiver as imported in Tonatiuh++ in the focal region of a parabolic dish. For this example, we will use FluxTracer to investigate the penetration and spread of the light within the receiver cavities. The receiver is located at the focal point of the dish which is at (X ,Y ,Z)=(0 , 0, 4.5)m.

Figure 1: Complex geometry import in Tonatiuh++ as an .obj file format. The actual 3D printed volumetric receiver also shown (The Figure of the receiver is obtained from [1])

Note: Please request all the relevant files mentioned in this tutorial from [email protected]

RAY TRACING

The receiver will be composed of a cluster of several of the above pieces, creating a final receiver geometry as shown in Figure 2. As seen in the Figure below, for this case we will consider that the surfaces of the receiver are specular surfaces with a reflectivity of 1. Instead of placing a virtual surface, the receiver surface is now considered as a VirtualRoof surface (see Figure 1).

Figure 2: The final receiver geometry loaded in Tonatiuh++

After setting up the Tonatiuh++ scene, the entire photon map is saved in a corresponding folder. For this case, 300.000.000 rays have been casted from the sun.

Figure 3: The final receiver geometry loaded in Tonatiuh++

FLUXTRACER

In FluxTracer, we will use the voxel traversal functionality in order to investigate the penetration and spread of the light within the receiver cavities. For this reason, a rectangular bounding box is defined around the entire receiver geometry using 500x500x500 voxels. The bounding box is as seen in Figure 2 in green color around the receiver. This voxel density is quite fine but is needed in order to resolve the small empty regions/cavities that are created by the receiver and to filter out the receiver geometry in high resolution during post processing.

Figure 4: Setting up FluxTracer

The voxel traversal functionality is set as follows in FluxTracer:
<VoxelTraversal cornerMin="-0.1, -0.1, 4.5" cornerMax="0.1, 0.1, 4.52259" dimensions="500, 500, 500" output="VoxelTraversal.vtk"/>

This means that a rectangular box of (X,Y,Z)=(0.2x0.2x0.02259)m will be created around the receiver and will be discretized using 500 voxels in each direction. Running FluxTracer will create a .vtk file which is then post-processed in Paraview.

POST-PROCESSING

The .vtk file can be then processed in Paraview for visualizing the results. The images below illustrate some post-processing results showing how the radiant energy is distributed within the receiver and how the light penetrates within the cavities of the receiver.

References [1] S. Luque, G. Menéndez, M. Roccabruna, J. González-Aguilar, L. Crema, and M. Romero. Exploiting volumetric effects in novel additively manufactured open solar receivers. Solar Energy, 174:342 – 351, 2018. ISSN2580038-092X. doi: https://doi.org/10.1016/j.solener.2018.09.030.