Effect of surface material properties and operating conditions on the heat flux and temperature distributions in the cavity receiver of a solar-dish-coupled biomass gasification reactor

Abstract

In this paper, a new 5.8 kW solar biomass gasification reactor is modeled by means of Monte-Carlo ray-tracing method and computational fluid dynamics. The reactor is coupled to the cylindrical cavity receiver of a parabolic-dish solar concentrator, where the incoming solar heat flux is concentrated. The absorbed heat flux distribution (flux density map) at each receiver surface is determined with the ray-tracing software SolTrace®. The sensitivity of the flux distribution to three uncertain variables is evaluated: the receiver solar absorptance, the receiver reflection type and the tracking error. It was found that the tracking error causes a pronounced increase in the flux peak at the cylindrical surface, while reflection scattering greatly reduces the flux peak at the bottom surface. Solar absorptance redirects the flux from every surface to the cylindrical surface, and so, it increases its flux peak. Reflection scattering reduces reflection losses. Two extreme flux concentration cases were found: 0.95 absorptance and 0.5° tracking error produce a 231 kW/m2 peak at the cylindrical surface; 0.25 absorptance, no tracking error and specular reflection produce a 250 kW/m2 peak at the bottom surface. The heat transfer within the reactor is modeled with ANSYS® CFX® with the flux distributions of the extreme cases as boundary conditions. The receiver temperature contours were obtained. Peak temperature locations coincided with the peak flux locations. For a 1200 K gas outlet temperature, peak temperatures of up to 1470 K were found, which exceed the material limit (1273 K). This issue is avoided by selecting suitable surface properties: solar absorptance <0.42 and a Gaussian specularity error >300 mrad.