Session: 18-01 HelioCon Metrology

Paper Number: 130652

130652 - Optical Effects of Temperature Change for Heliostat Mirrors 

Abstract: 

Heliostat solar fields can produce very high temperature (>1000 °C) and also very high power (>100 MW_th), but only when the heliostats have high optical accuracy in both shape and pointing.  Optical tolerances are demanding; slope error tolerance can be ±0.5 mrad.  Meanwhile, heliostats operate under harsh environmental conditions.  Desert environments typical of heliostat field installations can experience temperatures as low as -26 °C and as high as 49 °C, with wide temperature swings occurring within a single day.  Nonetheless, optical tolerances must be maintained across these temperature variations.

Heliostats have been produced with a variety of mirror designs.  One common approach used at multiple sites includes a relatively thin glass mirror (e.g., 3 mm thick) adhered to a stiff structural backing made of stamped steel.  The coefficients of thermal expansion (CTE) for these materials vary with specific composition; examples values are 9 × 10^-6 °C^-1 and 12 × 10^-6 °C^-1, respectively.  When bonded together, this difference in CTE would be expected to induce curvature as a function of temperature change, much like a bimetallic thermostat sensor.  To study this effect, we interfaced the SOFAST deflectometry system with an environmental test chamber to produce an apparatus capable of measuring high-resolution optical slope maps as a function of temperature.  We then used this apparatus to measure the optical change in mirror slope for multiple heliostat mirror types, over temperatures varying from 0 °C to 50 °C.  (Lower temperatures were possible, but condensation interfered with measurement.)  Change in slope varied depending on the mirror design.  For the commonly used glass/steel mirror design, slope over the temperature range [-0.4 °C, 50.7 °C] varied from [-3 mrad, +4 mrad] relative to room temperature nominal.  This variation is substantially larger than target tolerances.

In this paper we will discuss the fundamental mechanism behind this optical change, present details of the apparatus, review detailed experimental results, discuss implications for heliostat field performance, and discuss design aspects that make some mirrors more susceptible to temperature change than others.  The apparatus is available as a resource for heliostat designers to assess their mirrors’ response to temperature changes.

Presenting Author: Randy Brost Sandia National Laboratories

Presenting Author Biography: Randy Brost is trained as a computer scientist, and throughout his career he has developed computational tools supporting robotics, precision manufacturing, optical metrology, and concentrating solar optics. He now leads optical metrology research at Sandia's Concentrating Solar Technologies group, with a focus on developing improvements to both indoor and outdoor heliostat optical metrology.

Authors:

Randy Brost Sandia National Laboratories
Felicia Brimigion Sandia National Laboratories
Braden Smith Sandia National Laboratories
Anthony Evans Sandia National Laboratories