Forced Convective Heat Loss from Cavities of Multi-Megawatt Scale Solar Receivers

Abstract

Cavity receivers may increase the efficiency of concentrated solar thermal energy (CSTE) systems because they lose less heat via thermal radiation compared to external receivers. For large CSTE cavity receivers on the multi-megawatt scale, the understanding of forced and mixed convection has not advanced enough to predict the heat loss accurately. Hence, this doctoral thesis focuses on investigating (i) the heat transfer in the forced convection limit (Ri << 1) for large cavity receivers (Re >= 10^6); (ii) the heat transfer in the mixed convection regime (Ri ~ 1) for large cavity receivers (Gr >= 10^10, Re >= 10^6); and (iii) possible convective heat loss reduction measures. The forced convective heat loss from 5 geometrical configurations and 3 reduction measures was measured in a high-pressure wind tunnel. All models were scaled and included the relevant part of the tower. The experiment covered a Reynolds number range of between 1.5 x 10^6 and 6 x 10^6, based on the cavity diameter. For the measurements, novel ringlike hot-film sensors were designed and mounted on the inside of the cavity. These sensors were operated with a constant-temperature anemometry (CTA) system. In addition, a numerical model was validated with a selection of the wind tunnel measurement points. The numerical model was then adapted to the original scale for simulations of multi-megawatt cavity receivers in the mixed convection regime. The measurements showed that the forced convection from a cavity without reduction measures strongly varies with a factor of up to 6.1 depending on the wind speed and its direction. With a reduction measure a reduction of more than 50% of the forced convective heat loss may be achieved for specific wind directions. Further, it was observed that the forced convection in the cavity is governed by the external flow characteristics in direct vicinity of the aperture. The simulated cases revealed that the forced convective heat loss contributes substantially to the mixed convective heat loss. The mixed convective heat loss results to be of the same order of magnitude as the radiative heat loss. Finally, it was deduced that the optimization of the design of a multi-megawatt CSTE cavity receiver with respect to convective heat loss is possible when a specific site is given and its meteorological boundary conditions are well known.