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.

The promise and challenge of solar volumetric absorbers

“…However, in real solar concentrator systems the incident radiation flux, the temperatures, and the local air velocity are likely to have a non-uniform lateral distribution. In particular, the flow into the absorber will not be uniform as well, and in some cases this might lead to thermally induced flow instability (Kribus et al, 1996). The lateral non-uniformity of the temperature can increase the overall emission loss (Pitz-Paal et al, 1997).…”

Experimental evaluation of the performance of solar receivers for compressed air

“…The pressure difference did not mean that the pressure drop was due to the receiver; however, in the experiments, the flow rate in the rectangular receiver was smaller than that in the circular receiver despite similar pressure differences (refer to Table 1). This difference may be related to the flow instability issue in the volumetric solar receiver [20]. The high flow resistance in the rectangular receiver prevented the increase of flow rate and consequently resulted in a high POM range.…”

Heat transfer simulation in a thermochemical solar reactor based on a volumetric porous receiver

“…For a gas phase reaction, the study made by Valdés-Parada et al [17] is another example of optimization of the reactor size by means of this kind of technique. In the field of volumetric solar receivers or catalytic monoliths, the heat transfer to the reactor volume and the temperature homogenization are targeted [18][19][20][21][22][23][24].…”