Design and test of an optimized secondary concentrator with potential cost benefits for solar energy conversion

Point-focus solar thermal concentrators usually employ only one concentrating mirror, a large-area paraboloidal dish or similar configuration with approximately the same optical properties. Substantial improvements can be achieved with recently developed two-stage configurations employing a non-imaging Compound Parabolic Concentrator (CPC)-type secondary. Such compound systems have the capability of achieving typically a four-fold increase over the concentration attainable by a single-stage primary of given optical quality. Recent efforts have been to design the secondary to relax the optical quality requirements for the large-area primary as much as possible, thereby potentially reducing system cost considerably. These new designs favor longer focal ratios than those characteristic of typical single-stage configurations. Two features of these designs make the approach particularly promising. First, the long focal length two-stage design always provides a fundamental increase in the tolerance for optical errors. Second, many primary fabrication and design techniques which seek to approximate a paraboloid become much better approximations in the long focal length limit. A conceptual design which illustrates these features and which could achieve a geometrical concentration of 215 × with a CPC secondary and a primary comprising only 60 flat mirror facets is described. To carry out an operational test a secondary concentrator suitable for use with a variety of long focal length primaries was designed and several different test models were fabricated. A series of optical and thermal performance measurements was carried out at the Advanced Components Test Facility (ACTF) operated by the Georgia Tech Research Institute. The results have established that thermal control can be readily achieved with simple passive means for moderate concentration and power ranges. Preliminary analysis yields values of approximately 90 ± 3% for the optical efficiency of the CPC secondaries with silver reflecting surfaces.