Open cavity receiver geometry influence on radiative losses

Open cavity receivers can be used to efficiently absorb concentrated solar radiation at high temperatures. Using ray-tracing and a stochastic optimisation method, the geometry of such receivers is optimised looking at radiative losses only. Results confirm the major role of the aperture in cavity losses mitigation and highlight the flux distribution variation on geometries with comparable radiative performances. 1 Open cavity receivers losses in concentrated solar applications. In Concentrated Solar Power (CSP) systems, the receiver, placed at the focus of the light-concentrator, absorbs concentrated solar radiation and transfers this heat to a Heat Carrier (HC). Recent advances in CSP applications target higher temperatures of operation for receivers in order to increase the thermodynamic efficiency of the overall CSP system. In the present study, the geometry of cavity receivers is analysed using a stochastic brute force optimisation technique. Open cavity receivers act as virtual black bodies, trapping light through multiple reflections in order to increase the amount energy absorbed. These receivers are subject to 3 general types of losses that have to be minimized:  Radiative losses due to optical behaviour and thermal emissions.  Convective losses to the surroundings due to difference of air temperatures in the cavity enclosure and the environment.  Losses through the walls of the receiver mostly driven by heat conduction. Convective losses, not considered here, are hard to model with confidence due to the difficulty in validating the results of simulations and correlations used [1]. Losses through the walls by heat conduction, driven by the conditions of concentrated solar radiation on the internal walls of the cavity and insulation material chosen are neglected in this study. The cavity receiver of choice is assumed to be composed of grey-body behaving internal surfaces. Radiative losses, as labelled in this study, are regrouping the optical losses due to non-ideal concentration of the incoming solar radiation as well as thermal emission losses described by the Stefan-Boltzmann law. Although the influence of the aperture on the radiative efficiency of cavity receivers is generally understood, the rest of the cavity geometry is usually not analysed in detail. The overall first-law efficiency of cavity receivers is reported to be fairly independent of the geometrical design chosen, provided that the aperture of the cavity is carefully chosen and the internal area over aperture area ratio is high enough to provoke a significant “cavity effect” [2].