THE OPTICS OF A 2-STAGE SOLAR FURNACE

The combination of a stationary parabolic dish concentrator and a heliostat has come to be known as solar furnace. The advantage of this combination as compared to a parabolic dish directly tracking the sun is that the focus is stationary. Both dish and instrumentation of the focus can be placed indoors. In the usual configuration a heliostat is facing a dish of approximately equal area. We have combined a rectangular glass heliostat of area 52 m2 and focal length 100 m with a parabolic dish of area 5.7 m2 and focal length 1.93 m to a two-stage concentrator. In this arrangement the heliostat produce a primary image of the sun at a distance of 100 m to the South. This image has a diameter of roughly 1 m. The bundle of rays contributing to this image has a cross section equal to the area of the dish at a distance of 70 m south of the heliostat. At this location we placed the dish in order to intercept all rays. From the primary image of the sun situated 30 m behind, the dish produces a secondary image in a plane situated 12 cm behind its focal plane. The secondary image is calculated to have a diameter of 6 cm and a numerical aperture of 0.64 (f-number = 0.78). Thus except for reflectivity losses, the concentration ratio of the furnace is expected to correspond to that of the dish if used alone tracking the sun, while the total power is basically that intercepted by the much larger heliostat. This two-stage concentration has the advantage of requiring a smaller dish which can be at the same time cheaper and of higher optical quality. The disadvantage is that oblique reflection at the heliostat causes an astigmatism which distorts the secondary image to an elipse roughly 15 cm x 6 cm and reduces the concentration ratio accordingly. This effect is larger for larger angles of incidence and for smaller dish dimensions. We show the flux distribution measured in the secondary plane of our heliostat and discuss possibilities to improve the optics by compensating the astigmatism.