INTRODUCTION

A more sophisticated application of the radiant density analysis functionality is to study the distribution of the radiant energy density inside a cavity receiver. As an example, we have carried out such an analysis for CyI's iStore receiver. This is a 150 kWth receiver designed by CyI as part of the proof of concept of a Concentrating Solar Power and Desalinated Sea Water (CSP-DSW) plant being developed at the CyI. For this application, instead of using a Virtual Roof for the calculations, the cavity shell of the iStore is used as a Rough Surface with Specular Standard Material properties, and then by post processing the results, the radiant density distribution inside the receiver can be obtained. The receiver as imported in Tonatiuh is shown in Figure 1.

Figure 1: Screen shot of the iStore receiver cavity as imported in Tonatiuh

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

DESIGN OF THE iStore RECEIVER and TONATIUH SIMULATIONS

To carry out the analysis, the geometry of the cavity of the iStore receiver, which is relatively complex, was modelled and imported as a 3D CAD (Computer Aided Design) object into Tonatiuh. The optical behaviour of the surfaces of the cavity was modelled as "Specular Rough Standard Material" using a reflectivity of 30% and sigma slope of 45 degrees. Figure 1 shows the iStore cavity receiver and the heliostat field of the CSP-DSW facility being modelled and simulated in Tonatiuh. The instant of time simulated was the summer solstice at noon. Over 6 billion rays where cast from the sun; of these, 314.6 million reached the region of interest. This region was defined as a cube of 2 m × 2 m × 2 m and centred at the focal point of the heliostat field, placed 13.834 m above the ground. This region was voxelized into 125 million cubic voxels (500 × 500 × 500). All the rays intersected by the walls of the cavity, including the secondary rays associated to the reflections in the interior of the cavity, where post processed in FluxTracer.

Figure 2: Screen shot of the ray tracing simulations in Tonatiuh of PROTEAS facility

RUNNING FLUXTRACER

In FluxTracer, a bounding box is defined around the entire receiver geometry using 500x500x500 voxels. This voxel density is quite fine but is needed in order to filter out the receiver geometry in high resolution during post processing. Figure 3 shows the outside shell of the the receiver geometry composed of voxels as obtained from FluxTracer simulations. The shape is obtained after applying a threshold on the number if rays. In addition, Figure 4 shows a clipped views of the receiver illustrating the distribution of the radiant energy within the receiver cavity.

Figure 3: Voxelized iStore receiver surface as obtained from FluxTracer colored by normalized radiant intensity. Reflectivity is set to 0.3. It can be seen that small details such as small round fillets on the receiver are captured efficiently.

Figure 4: Clipped views of the iStore receiver colored by the normalized radiant density. It can be seen that for reflectivity set to 0.3 the distribution is quite symmetric on the X-Y plane because of the symmetry of the heliostat field, but the distribution is not uniform on the Y-Z plane due to the tilt of the receiver.