Optical performance analysis of a porous open volumetric air receiver in   a solar power tower
  An optical model for a solar power tower with an open volumetric air receiver (OVAR) was developed in this paper, and the optical performance and characteristics of the OVAR were studied based on the model. Firstly, the detailed distributions of the non-uniform solar flux (qsr) on the aperture and the solar source (Ssr) in the OVAR were studied. The solar flux was found to be relatively uniform across the aperture for a single OVAR. However the incident angle of the rays varies between 0o and 42o, which indicates that the parallel assumption of the rays is not appropriate at this condition. Furthermore, the Ssr in the absorber decreases from the inlet to the outlet, and the maximum source (Ssr,max) of 2.414X108 Wm-3 appears at the inlet. Moreover, the Ssr,max was found to appear at the region near the wall rather than the center of the receiver as usual for the combined effect of the non-parallel incident rays and the diffuse reflection on the wall. In addition, study on effects of the porous parameters indicates that the solid emissivity influences both the Ssr distribution and the receiver efficiency significantly. However, the pore diameter and the porosity influence Ssr distribution importantly, but have negligible effect on the efficiency. Finally, the optical efficiency of 86.70%, reflection loss of 13.20% and transmission loss of 0.10% were found to be achieved by the OVAR. 
Yu Qiu, Ming-Jia Li, Ya-Ling He, Ze-Dong Cheng
physics.gen-ph