In search of a sharper line focus: Rotating the ′etendue

Conventional line-focus solar concentrators are limited by the 2D concentration limit, two orders of magnitude lower than the three-dimensional limit. This leads to low concentration ratios and strict manufacturing tolerances. It has been shown that by eliminating the continuous translational symmetry of these systems, it is possible to go beyond the 2D limit while maintaining the linear geometry of a line focus. We demonstrate that one way to break this symmetry is through ' etendue rotation, and we present two new concentrator configurations based on this insight. The first configuration uses an ' etendue rotating retroreflector array to boost the concentration of a parabolic trough. Ray-tracing simulations show that this configuration can achieve very high geometric concentration ratios or very high acceptance angles (1484 x at a +/- 9mrad acceptance angle, or 25 x at +/- 70mrad). However, this configuration requires two-axis external solar tracking. To get around this, we demonstrate a second configuration that uses an ' etendue rotating lens array with tracking integration. We demonstrate a design that achieves a geometric concentration of 146x at a +/- 9mrad, with a simulated average yearly efficiency of 94.9% when used with conventional horizontal single-axis external tracking at an installation latitude of 30 degrees. The extra constraints of the tracking integration gives this design a more modest concentration ratio, but it is still higher than the 2D concentration limit and more than three times as high as the concentration of a parabolic trough evaluated under the same conditions. We believe that these new configurations show that the design landscape for line-focus solar concentrators can be widened, and that a practical high-concentration line-focus concentrator may be within reach.

Automated summary

Conventional line-focus solar concentrators are limited by the 2D concentration limit. By breaking the symmetry of these systems, it is possible to go beyond the 2D limit and achieve higher concentration ratios. 'Etendue rotation is one way to break this symmetry, and two new concentrator configurations are presented based on this insight.