Inherent Limitations of Volumetric Solar Receivers

Abstract

The flow in volumetric absorbers is investigated using a simple mathematical model. It is found that there are several restrictions and failure mechanisms that are inherent to the volumetric absorber, regardless of the precise structural details, material properties, etc. The heat that the absorber can extract safely is limited by flow-related constraints. Multiple steady solutions exist for certain parameter values: a “fast” solution corresponding to a low exit temperature, a “slow” solution which is unstable, and a “choked” solution for which the absorber is near to stagnation temperature. The existence of multiple solutions may lead to abrupt local “switching” and absorber failure. For a given irradiance applied to the absorber, the existence and the character of the solutions are determined by a single dimensionless parameter, the Blow parameter B. Neglecting the variation of the hydraulic resistivity with temperature may lead to a dangerous overestimate of the receiver’s ability to sustain irradiation. For reasonable efficiencies control of mass flow or outlet temperature of the absorber, rather than pressure control, may be required.