Minimal movement solar concentrator of segmented heliostats and a Fresnel lens for sun tracking applications

Concentrating solar power (CSP) systems are used on applications that require high temperatures and are often paired to a tracking system, i.e. a mechanical system capable of adjusting the position of the optical concentrating surface according to the position of the sun, which is dependent of both the time of the day and the location of the CSP system. These concentrators are often classified by their focusing principle: line focusing or spot focusing. Linear concentrators require a single axis rotation to compensate the sun's altitudinal component when the orientation of the system is East-West or the azimuthal component when on a North-South orientation. On the other hand, spot concentrators require a two-axis rotation, altitudinal and azimuthal as depicted in the literature review. In both cases, strong movable structures capable of supporting the optical surface while resisting wind loads are required, which translates to increasing CSP system costs. The present work is centered around the objective of proposing a concentrator that requires minimal tracking movement to lower these costs.

The proposed system consists of a two-axis segmented heliostat (SH) tracker, a vertical fixed Fresnel lens (FL) and a spot-type receiver as shown in Figure 1. In order to concentrate the incoming sun rays toward the receiver, the SH must perform a minimal movement, the azimuthal tracking ρ is performed by the synchronized rotation of the SH's facets around their local u-axis, while the altitudinal tracking σ is performed by the rotation around the z-axis of the frame containing the facets' array. Since the FL and the receiver are fixed, their weights are not supported by the tracking system, thus requiring minimal mechanical effort for movement.

The proposed solar concentrator performance was analyzed in terms of two effects caused by the elements' configuration: blocking and screening. Blocking refers to the obstruction of irradiance from one facet to another given the ratio between their distance and width, this causes a reduction of the facet's reflective area, especially at early and late hours of the day. On the other hand, screening refers to the facets’ projection on the FL caused by the distance between facets and the cosine effect. In order to reduce both of these effects, a mathematical model aimed to find the optimal distance and number of facets on the array was developed.