Flux augmentation and temperature enhancement through secondary parabolic concentrator and air recirculation for a central receiver tower system

Flat volumetric receivers for central receiver tower (CRT) systems face two critical shortcomings: light spillage nearby the receiver and low exit air temperature due to suction of air in ambient condition. Here, a thorough design modification is proposed and analyzed to address both the issues by introducing a secondary optical addendum based on the principle of compound parabolic concentrator (CPC). As a case study, existing and modified designs of PS10 plant with flat heliostats are compared. For the same receiver, the modified plant increases the intercept efficiency by 6% whereas the incorporation of CPC augments the concentrated flux by 18%. A glass covered secondary optical element not only reduces operational difficulties due to absorber-soiling, but also provides an opportunity of heated air recirculation. If the heated air is partially recirculated, it can provide a control parameter, i.e. the air return ratio (ARR), by which the exit air temperature can be enhanced. It is demonstrated here that temperature as high as 1775 K is achievable for the modified PS10 plant when sig-nificant fraction of heated air is recirculated. Hence, the present study opens up optimization routes for CRT based plants based on limited area heliostat field and volumetric porous absorber through CPC based secondary optical element and fractional recirculation of heated air.