Session: 18-04 HelioCon Heliostat Designs

Paper Number: 142196

142196 - Performance and Durability Testing of Advanced Composite Mirror Facets 

Abstract: 

In an effort to reduce the cost of concentrating solar power (CSP), this work seeks to increase the performance of heliostat facets from 4mm solar mirrors with ~93% reflectivity to using 1mm solar mirrors with ~96% reflectivity.  Samples of 1 mm mirror facets will be constructed by laminating a stack that consists of the 1 mm ultra clear solar mirror, a plastic adhesion layer, a lightweight honeycomb structure, a plastic adhesion layer, and a final thin sheet of aluminum.  Variants of this basic structure are included such as different types of plastic laminates, and different thicknesses of the honeycomb and aluminum layers.   The goal is to produce a structural facet that can avoid the use of the steel supports that typically support 4 mm mirrors while minimizing weight, establishing the necessary stiffness and optical performance under operational wind speeds, providing robustness to hail and long term durability, and maintain costs inline with ~$24/m²  (the cost estimate for the mirrors and adhesives and the mirror support structure as given by the NREL 2022 report, “Cost Update: Commercial and Advanced Heliostat Collectors”).   The solar mirrors as manufactured can withstand temperatures of 120 ℃ for short duration.  Multiple Polyolefin’s will be testing for the adhesive layer as there are wide range of these plastics with varying properties.  Generally, polyolefins that are polar cure and can provide the stability and mechanical strength for mirror facets but melting temperature and costs vary.  The final selection will be subset of polar polyolefins that melt between 90 – 120 ℃ . Also considered is Ethelene Vinyl Acetate (EVA), which is commonly used for photovoltaic panels as cost is low and melting temperatures also range from 90 – 120 ℃.  While EVA can be cost effective, the degradation over time and production of acetic acid is potentially problematic.  The thickness of both the honeycomb layer and the final aluminum back sheet are varied as they will impact mechanical strength, weight, cost, and shipping volume.   The facets will be subjected to performance testing including:  hemispherical reflectance, specular reflectance, and surface uniformity.  The performance testing will occur both before and after the facets are subjected to accelerated environmental and mechanical strength testing.  Environmental testing will include: damp heat, temperature cycling, humidity freeze, salt spray, and extended UV exposure.   The before and after performance test results will be reported for each stack variant along with weight differences and projections for cost variation.

Presenting Author: Matthew Muller NREL

Presenting Author Biography: Since 2008 Matthew Muller has been a research engineer within NREL's PV Performance and Reliability group. The focus of his work has covered topics such as PV soiling, PV surface coating durability, PV and concentration PV (CPV) module and system performance, thermal modeling, spectral performance modeling, methods for on-sun CPV cell temperature evaluation, solar trackers, IEC standards development, test design, prototyping and design of instrumentation, data acquisition systems, programming and data analysis . He is a member of ANSI has served as a technical expert within multiple IEC working groups. Currently he serves as the co-lead for the Heliostat Consortium's work on heliostat components and controls.

Authors:

Tucker Farrell NREL
Matthew Muller NREL