MODEL CHARACTERIZATION OF BLOCKING AND SHADING LOSSES IN A NOVEL TWO-STAGE HELIOSTAT

Abstract

In 2011 the United States Department of Energy launched the SunShot initiative with the goal of maturing concentrating solar power technologies and reducing the levelized cost of energy from $0.21 per kWh to $0.05 per kWh. Solar power tower systems, a configuration of concentrating solar power, consists of a receiver mounted to a tower and a heliostat field. The heliostat field is the most expensive component of a solar power tower system, recognizing this SunShot has set a target of reducing the installed cost of the heliostat field to $50/m2. Advanced heliostat technologies have reached approximately $96/m2. This work is part of a more comprehensive project to investigate the feasibility of a novel two-stage heliostat. Typically, the process of collecting and concentrating sunlight is achieved in a single stage; however, the two-stage heliostat splits this process between a set of stages. The first stage, the tracking stage, rotates to track the sun reflecting the sunlight towards the second stage, the concentrating stage. The second stage consists of stationary mirrors that reflect sunlight towards the receiver. The two stage concept enables the tracking mirrors to share a common orientation; hence, multiple tracking mirrors can be controlled be a dedicated set of drives. The addition of the second stage and structure in the first stage introduce blocking and shading loss mechanisms termed self-shading. The methodology widely used for estimating blocking and shading in conventional heliostat fields is not well suited for estimating self-shading. The work presented in this thesis develops the methodology needed to estimate and characterize self-shading by utilizing ray-tracing. SolTrace is used to perform the ray- tracing. The SolTrace model uses a series of three stages and a mix of real and virtual elements. The real and virtual elements are used as a book keeping tool so that individual rays can be classified. This approach allows all self-shading loss mechanisms to be

individually resolved.

The findings in this thesis characterize the self-shading experienced by the two-stage heliostat at different locations within the field and for a variety of geometric parameters. The majority of self-shading losses occur when sunlight is reflected towards the receiver and the two-stage heliostat is far from the tower. In addition to characterizing the self- shading impact the findings are used to enable full field ray-trace simulations to utilize a simplified approach.