Influence of wind on receivers for solar power towers

Kurzfassung

In solar thermal power towers an array of mirrors reflect the sunlight to the top of a tower, where it is absorbed by the so-called receiver heating up a fluid. The heat of the fluid is used afterwards. Since the receiver is exposed to the ambience, part of the energy is lost due to the emitted radiation and the convective losses of the hot surface. In order to increase the efficiency of the system, the losses of the receiver should be minimized. Cavity receivers, based on the concept of a cavity, match this requirement, since they have low thermal and optical losses. The convective losses of this receiver type caused by natural convection have been in the focus of several studies: they can be efficiently reduced by inclining the cavity. The influence of wind on large scale cavity receivers with different inclination angle has not been analyzed so far, which leaves a huge uncertainty about the influence of wind on the efficiency of solar thermal power towers with cavity receivers. The analysis of the convective losses is very challenging due to the size of the receiver leading to typical Rayleigh numbers of Ra>1E10. In order to analyze the influence of wind on cavity receivers with Ra≈2.8E10, a simulation model of a single cavity with isothermal walls in a wind tunnel environment was created. Different inclination angles, wind directions and wind speeds up to a Reynolds number of Re=3.7E5 were simulated using OpenFOAM 2.3.0. Due to the high Reynolds and Rayleigh number a RANS simulation was performed using the k-omega SST turbulence model. For an accurate calculation of the wall heat fluxes inside the cavity the dimensionless wall distance y+ was in the order of one. The meshes for the different inclination angles and wind directions consist of approximately 3.4 mio hexahedral elements each. Since the buoyant flow inside the cavity is slightly unstable, unsteady simulations were carried out using buoyantPimpleFoam. In order to validate the simulation an experiment was performed in the cryogenic wind tunnel in Cologne. Reducing the ambient temperature to -173°C allowed reaching the required Rayleigh number. The simulation results are in good agreement with the experimental data, giving confidence in the simulation results. Hence the simulation results can be used to obtain a deeper understanding of the relevant phenomena insight the cavity.