THE ROLE OF THE OPTICAL-PROPERTIES OF SOLIDS IN SOLAR DIRECT ABSORPTION PROCESS

Solid particles are considered to be good candidates for absorbing solar energy in an endothermic reaction, due to both their ability to absorb solar radiation directly (as opposed to heat transfer mechanism) and their high ratio of absorbed radiation to mass. Only a limited number of studies have been devoted so far to convert solar energy to chemical energy by means of direct absorption. The major advantages of this concept are the inherent high efficiency of the light absorption, and the short time required to reach the desired reaction temperature. In the work described, an attempt to evaluate the effect of the optical properties of the particles and their temperature dependence, upon the solar driven chemical reaction was carried through. The particles investigated were oil shale particles. Their optical properties were measured in the optical lab at the DLR, Stuttgart, whereas the solar gasification experiments of the oil shale, were conducted in the central receiver of the Weizmann Institute. The results show that temperature profiles for the gasification experiments of oil shales can be predicted with good accuracy by assuming isotropic scatter and a particle albedo of 0.5. This is due to the relatively limited role of direct absorption in low expanding fluidized beds. Calculations of a set-up however, where direct absorption is dominant, show resulting temperature profiles to be sensitive to optical properties. Temperature differences arising from calculations based on assumed optical properties compared to those based on measured optical properties are as high as 300-degrees-C.