2-STAGE CONCENTRATOR PERMITTING CONCENTRATION FACTORS UP TO 300X WITHONE-AXIS TRACKING

Economic operation of high-efficiency concentrator solar cells require s solar concentration ratios which up to now can only be achieved with two-axis tracking. In this paper we present a two-stage concentrator approaching concentration ratios up to 300X while being tracked around only one polar axis. Its principle is as follows: a parabolic trough focuses the direct solar radiation onto a line parallel to the polar t racking axis. The half rim angle of this first concentrating stage is chosen to be equal to the sun's maximum declination of 23.5 degrees. T he second stage consists of a row of dielectric, nonimaging 3-D-concen trators, which couple the concentrated light directly into square sola r cells. In contrast to linear secondaries the 3-D-secondaries make us e of the limited divergence of +/- 23.5 degrees in the NS-direction to achieve additional concentration. The performance of the system depen ds sensitively on how well the angular acceptance characteristic of th e second stage matches with the square-shaped angular irradiance distr ibution in the focal line of the parabolic trough. A new concentrator profile has been found that exhibits an almost ideal square acceptance characteristic with a very sharp cut-off. A prototype two-stage conce ntrator has been constructed with a total geometrical concentration of 214X. In outdoor measurements a total optical efficiency of 77.5% was obtained.