High temperature solar collector with optimal concentration: Non-focusing fresnel lens with secondary concentrator

Abstract

A non-evacuated collector consisting of a linear Fresnel lens and a second stage concentrator of the CPC type is described and tested in detail. Use of a Fresnel lens accomplishes two different objectives simultaneously: it allows for the design of a nearly ideal light collector (of the CPC type) of high concentration and height-to-aperture ratio close to 1 and plays the role of a cover, making the collector less sensitive to the environment than one with exposed reflector surface. The geometric concentration is 15.56 and the acceptance half angle is 3°. The optical efficiency measured with an Active Cavity Radiometer (ACR) is 65.6 per cent and the efficiency at ΔT I of 0.235 is 48 per cent ( ΔT = T avfluid − T amb = 200°C, I ACR = 850 W/m 2). Heat loss measurements for double glazed configurations are reported and the resulting efficiency at ΔT I of 0.3 is predicted to be 48 per cent. These numbers are expected to be raised by 3 percentage points for a next generation of lenses. The collector is mounted with its tracking axis oriented oriented NS since EW tracking axis orientation is impractical for a linear Fresnel lens, but its wide acceptance angle permits tracking by a simple clock mechanism at constant speed. Two different strategies are considered (i) polar mount, (ii) two adjustments of the tracking axis a year (summer and winter); the predicted yearly performance is calculated for four locations and four working fluid temperatures. The projected cost is estimated to be $70.00/m 2 (1976 dollars), possible because the construction of the collector lends itself to the use of inexpensive materials such as plastic and glass.