Cavity enhancement by controlled directional scattering.

The interest in efficient solar collection design has stimu­ lated investigation of 2-D (troughlike) cavity structures. In fact, it was recently shown that such cavities can be designed to produce absorption enhancement in the solar collector.' In this Letter we describe a novel principle for designing cavity enclosures. The new method can be applied to advantage to nonimaging concentrator design. Thus, it is by now well known that radiation collimated within an angular divergence ±θ can be concentrated on a receiver by the factor Cmax = sinθ and without transmission loss by the use of reflectors of the CPC type. To meet strictly both requirements (i.e., maximum concentration and no transmission loss) the re­ flector should touch the receiver. In cases where it is not practical for reflector and receiver to touch, a solution is available that maintains flux concentration Cmax but intro­ duces some transmission loss. The present method main­ tains high transmission albeit at the expense of some con­ centration in the presence of a gap between reflector and re­ ceiver. The slight loss of concentration may be partly offset by enhanced absorption of radiation by the receiver, resulting from the cavity effect. The idea is to immerse the receiver in an enclosure with special properties. Small elements of the enclosure wall have the specific property that radiation incident on the element from outside the angular subtense of the receiver is scattered (or redirected) onto the receiver in the manner shown in Fig. 1(a). Assuming that such elements are realizable in practice (this will be shown to be the case later), it is easy to see how to construct the cavity. The enclosure is composed of a chain of such scattering elements. The receiver R is immersed as shown in Fig. 1(b) sufficiently deep so as to block the effective aperture ABCD of the cavity from seeing itself.The con­ centration ratio of the enclosure for diffuse radiation is just the area of the effective aperature divided by the receiver area,