Solar Reflector Soiling Pattern Distributions and Reflectance Measurement Requirements

Short-term specular reflectance losses from optical surfaces used in the collection or concentration of solar energy results in significant reduction of these systems' output. Losses range from 0.1% to 1.0% per day, approaching asymptotes of 25% to 60% for periods greater than one year, depending on site and season. To appropriately assess the value of a particular location for the production ofpower, consideration of the rates of soiling and strategies to minimize losses resulting from soiling must be considered. Strategies for measuring the optical performance of reflector materials to a specified degree of accuracy have been developed, according to the types of soiling patterns observed. It was found that most soiling occurs with the accumulation of particulates in spots ofdifferent sizes, and the spot sizes follow a log normal distribution. For most practical situations, it was determined that 10 measurements with a 1-cm-diameter beam are enough to place the average value within 3% of the true value, with a confidence level of 95%.