  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.
