Optimal parallel flow in solar collectors for nonuniform irradiance

We consider volumetric thermal absorbers exposed to nonuniform irradiance. The fluid flows perpendicular to the absorber surface. The fluid transport from front to rear surface elements is connected in a parallel configuration. We investigate the optimum adjustment of the local mass flux or, equivalently, the local absorber temperature. Three different strategies are compared: (1) strategy of uniform mass flux, (2) strategy of uniform exit temperature, and (3) a strategy with a locally optimized temperature-flow profile. Significant improvement of strategy (2) compared to (1) and of strategy (3) compared to (2) is observed for the case of a black-body absorber which is isothermal in the direction normal to the absorber surface. We find it to be advantageous to include even regions where the stagnation temperature is below the desired exit temperature.