A modification of design has been proposed to extend the longevity of cavity-type solar thermal receivers by improving the distribution of radiant solar flux in the cavities. A receiver of this type is used in conjunction with a solar concentrator, which is a mirror that focuses the radiant solar flux into the cavity. A large part of the radiant flux is absorbed on the inner cavity walls and is thereby converted to usable heat. Inevitably, imperfections in the surface of the concentrator give rise to a maldistribution of flux in the cavity — hot spots — that shorten the useful life of the receiver.
The proposed modification consists mainly of the addition of a thermally conductive, thermally emissive radiant shield between the aperture and the walls of the cavity (see figure) to even out the hot spots. Concentrated solar flux would enter the cavity through the aperture in the usual way, but would be initially absorbed on the shield and would thus initially heat the shield instead of the inner cavity walls. The heat would be conducted through the shield from the side facing the aperture to the side facing the inner cavity walls.
Being hot on both sides [typical temperature of 1,550 °F (843 °C)], the shield would reradiate diffusely toward the inner cavity walls, toward the aperture, and toward facing parts of the shield itself. The diffuseness of the reradiation would diffuse the hot spots, so that the temperature [typical temperature 1,400 °F (760 °C)] around the circumference of the inner wall would be more nearly even.
The fraction of reradiated heat lost through the aperture would be greater than it is in a comparable receiver of unmodified design (without a shield). However, the loss could be reduced to a few percent or perhaps eliminated by suitable choice of the size and shape of the shield, the rest of the receiver, and the concentrator. In any event, the economic disadvantage of the small loss of energy-conversion efficiency may be more than offset by economic advantages of (1) the increase in useful life and (2) the decrease in cost of the concentrator occasioned by elimination of the need to carefully tailor the concentrated flux to minimize hot spots.
This work was done by Charles T. Kudija of Rockwell International Corp. forJohnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 175on the TSP Order Card in this issue to receive a copy by mail ($5 charge).