Computational and experimental investigations into cavity receiver heat loss for solar thermal concentrators

Tubular cavity receivers offer excellent prospects as low-cost, high-efficiency heat collection elements for paraboloidal dish solar-thermal concentrators, as well as for central tower systems. Major heat loss mechanisms for these receivers are convection and radiative emission, as well as reflection and spillage (non-capture). Compared to glazed receivers, convection heat loss can be quite a large fraction of the total, though the losses depend on solar elevation angle; at higher angles, and in low-wind conditions, a trapped buoyant region of air inside the receiver helps to suppress the convective losses. Radiative losses can be reduced through selective surface coatings, as well as through optimised cavity geometry, to reduce the view factor from the heated surface to the surroundings. This paper is a progress report for an ANU-led project to increase receiver efficiency by at least 2%. Experimental studies using an electrically-heated air cavity are presented, which are making use of thermal imaging to improve accuracy compared to earlier studies; planned tests will extend this work to several non-cylindrical cavity geometries, to assess whether predicted efficiency improvement can be shown in practise. Air-curtain effects are also under investigation, including through an initial series of tests with an analogous saline-bouyancy 2D experiment presented here. Finally, CFD simulations using OpenFOAM software are presented, which have the goal of first reproducing measured results and then predicting performance of novel cavity designs. An initial comparison of the current SG3 receiver against a proposed reduced-aperture cavity shows the expected benefits that have come from the improved optics of the new SG4 Big Dish at ANU. CFD results will be further used in annual performance simulation to optimise receiver design in the context of the larger system.