Effects of Outdoor Exposure on the Solar Reflectance Properties of Silvered Glass Mirrors.

摘要

The specular reflectance loss of silvered glass heliostat mirrors has been studied over a nine-month period during which the mirrors were exposed to outdoor weather conditions. A bidirectional reflectometer was used to measure the reflected beam profile of approximately 9cm square samples exposed to the environment on outdoor racks. It was determined from these experiments that elevation differences and variations in sample mounting angles up to 60exp 0had little influence on dust accumulation rates. Weather conditions such as wind and rain can decrease as well as increase the reflectance of a mirror. The largest decrease in reflectance for a two-day period was 0.06reflectance units,while the largest increase for a two-day period was 0.083reflectance units. A snow weather condition can restore an exposed mirror's specular reflectance to within 0.01reflectance units of the initial value obtained after the laboratory ultrasonic cleaning. Because of varying weather conditions,it is very difficult to take a short-term cleaning cycle result and predict the reflectance loss for a longer period of time. Long-term exposure tests,where the mirrors were never cleaned,showed a gradually decreasing specular reflectance trend together with occasional sudden increases which corresponded to snow or rain weather conditions. Because New Mexico has very localized weather conditions,compared to many other parts of the country,reflectance loss due to these weather conditions is dependent on mirror sample location. (ERA citation 04:013020)