Session: 05-07 CSP System Analysis, Controls, and Standards

Paper Number: 114552

114552 - Model Characterization of Blocking and Shading Losses in a Novel Two-Stage Heliostat 

Small heliostats have some potential advantages over large heliostats including higher optical efficiency, less expensive support structure, and reduced wind loads. However, the main disadvantage of smaller heliostats is that the mirror area per drive pair is small, leading to higher drive and cabling costs for the same mirror size.

Conventional heliostat designs use single-stage heliostats in which the process of collecting and then concentrating (or focusing) the energy from the sun onto a receiver surface is carried out by a single reflective surface (hence a single stage). The two-stage heliostat design considered here splits these processes into two stages. The first stage (the tracking stage) faces south (in the northern hemisphere) and is composed of mirrors that move to track the sun, reflecting energy towards the second stage. The second stage (the concentrating stage) is positioned to the south of the first stage and is composed of stationary mirrors that receive the energy and focus it on the receiver. The first and second stage together form mirror pairs, and a single heliostat unit is composed of multiple pairs that will run east to west.

The uncoupling of the tracking and concentrating functions allow the tracking mirrors in a single unit to share a common orientation and rotations. This allows for a large number of small inexpensive mirrors to be controlled by a single set of drives. This can be thought of as controlling several small heliostats simultaneously and allows the two-stage heliostat to combine the economical advantages of both large and small heliostats.

The addition of the second stage and the tracking structure introduce elements that may lead to an increased level of self-shading within a heliostat unit. The focus of this work is to specifically investigate the effects of self-shading within a single, two-stage heliostat unit. Self-shading includes the loss of energy both before and after reflection by the tracking stage.  To estimate the self-shading loss, SolTrace is used to construct a detailed geometric model that is subjected to Monte-Carlo ray tracing simulations. The model captures only the effects of self-shading present for a single heliostat-unit. Loss mechanisms such as spillage and absorption are intentionally neglected and are considered by a separate field level model.

The self-shading model shows that when the heliostat unit is placed relatively close to the tower, the effects of self-shading reduce the optical efficiency by less than 10% for a reasonable design. As the heliostat unit is placed further away from the tower, the effects of self-shading increase. The model allows the mechanisms that contribute to self-shading to be isolated and show that this reduction in efficiency is primarily due to loss mechanisms that occur in the second stage and are independent of solar position. To address the increased self-shading further from the tower, the geometry of the heliostat-unit can be altered to increase the spacing between adjacent mirrors and stages. When the tracking and concentrating mirrors are directly across from one another (i.e., north and south), high efficiency bands start to develop within the field due to gaps between tracking mirrors that line up with the receiver and through which the second stage can aim. Based on this observation, the two stages may be offset from one another to intentionally put the gaps in line with the receiver.  This approach mimics the radial staggered layout that is used by more traditional heliostat fields.

Presenting Author: Ty Glisczinski University Wisconsin Madison

Presenting Author Biography: Ty Glisczinski is a mechanical engineering graduate student at the University of Wisconsin-Madison. He is advised by Michael Wagner and Gregory Nellis. Ty completed his undergraduate studies in the spring of 2022 with a degree in Applied Mathematics, Engineering, and Physics.