Durability testing of a newly developed hydrophilic anti-soiling coating for solar reflectors

Anti-soiling coatings for solar reflectors are one of the most useful technical tools to reduce the amount of soiling accumulated on the reflector surfaces, contributing to reduce the water consumption and to increase efficiency in concentrating solar fields. A new anti-soiling coating formulation based on the hydrophilic effect has been recently developed by Tekniker and Rioglass. Reflector samples with this innovative coating were assessed through an accelerated aging test campaign as well as an outdoor exposition at the PSA by CIEMAT and DLR. According to the results obtained, the coating exhibited an appropriate behavior, which implies that the new product represents a promising solution to reduce water consumption

T-TraCS – An automated method to measure soiling losses at parabolic trough receiver tubes

Soiling of the envelope tubes of parabolic trough collectors can significantly reduce their transmittance and hence the overall collector efficiency. There are only a few methods to quantify soiling losses at absorber tubes of parabolic trough collectors. The existing methods are either laboratory based and cannot be applied automatically or they are personnel intense because they can only be used manually inside of operational solar fields. In this work we present a novel device called T-TraCS capable of automatically measuring the transmission of a sample glass during outdoor exposure with the current solar spectrum and imitating the movement of operational parabolic trough collectors. It can be used in resource assessment campaigns in order to better estimate future soiling losses at the tube level or it can be set up inside a solar field in order to measure the tube soiling losses in real time for CSP plant operation. Scattering simulations are presented that correct the measurement raw values of the T-TraCS and a spectrophotometer for their differences to the optics of a receiver tube. The validation with these final measurements shows good agreement with the reference spectrophotometer with a R2 of 0.996. The T-TraCS is therefore capable of automatically determining the soiling induced transmission losses with high accuracy

Acceptance criteria for accelerated aging testing of silvered-glass mirrors for concentrated solar power technologies

Solar reflectors for Concentrating Solar Power (CSP) technologies are required to maintain their optical properties in demanding environments for more than 20 years of service-life. The durability of the commonly used silvered-glass reflectors is typically qualified by means of accelerated aging. Recently, the Spanish standardization committee UNE has published the first specific standard for this topic, which defines a set of accelerated aging tests for CSP reflectors. However, the standard does not contain pass/fail criteria. This paper proposes useful acceptance criteria for the accelerated tests defined by UNE, helping to interpret the obtained Degradation results. The criteria have been determined by analyzing the collected accelerated aging data over the past 5 years in the OPAC laboratory, a joint research group of DLR and CIEMAT. Data from six different 4 mm silvered-glass manufacturers is presented, covering nearly the entire market of commercially available silvered-glass mirrors, and going way beyond the recommended testing times of the UNE standard. The data may be used to benchmark initial reflective properties (before aging) and the performance during accelerated durability testing. In addition, recommendations for improvements of the standard are given and an estimate of the acceleration factor of the Copper Accelerate Salt Spray (CASS) test with respect to a highly corrosive outdoor environment is presented

Uncertainty Study of Reflectance Measurements for Concentrating Solar Reflectors

The solar reflector is one of the main components of concentrated solar thermal systems. Therefore, accurate knowledge of its solar-weighted, near-specular reflectance is highly important. Currently, this parameter cannot be properly measured with a single commercial instrument. There is a great interest in having a suitable procedure that can guarantee the accuracy of reflector quality analysis, which already led to the publication of an international measurement guideline (title “Parameters and method to evaluate reflectance properties of reflector materials for concentrating solar power technology”). Still, more research work is needed to improve the state of the art. At present, both the specular reflectance and the spectral hemispherical reflectance are measured by using commercial portable reflectometers and spectrophotometers, respectively, to gain enough information. This article concentrates on the evaluation and calculation of the type-B (nonstatistical) uncertainties associated with these employed instruments and, therefore, leads to a more accurate definition of the measurement uncertainty. Considering type-B uncertainty, the expanded uncertainties of measurements for most of the reflector types are UB,ref = 0.006 for monochromatic specular reflectance and UB,spec = 0.016 for solar-weighted hemispherical reflectance

Influence of gaseous pollutants and their synergistic effects on the aging of reflector materials for concentrating solar thermal technologies

Concentrating solar thermal technologies have experienced an important boost in the last few years. Besides the production of electricity, they are particularly useful for the supply of industrial process heat. The industrial atmospheres affecting these solar plants typically contain gaseous pollutants that are likely to promote corrosion on the components of the solar facility, specifically solar reflectors, thereby compromising their optimal performance and the overall system efficiency. Seven accelerated aging tests were designed to study the effects of three air pollutants (H2S, SO2 and NO2) on the durability of two commercially available reflector types (silveredglass and aluminum), both in single-gas tests and in multicomponent gas mixtures. Additionally, the same material types were exposed outdoors at five representative polluted sites, including industrial, urban and coastal environments. Reflectance and optical microscope monitoring corroborated which degree of corrosion was developed on a specific type of reflector in the different tests with gaseous pollutants, as well as the synergistic effects of gas combinations. For example, tests with sulfur were harmful for silvered-glass reflectors (up to a total of 16 corrosion spots), whereas aluminum was particularly affected by tests with NO2 (numerous micro spots of around 50 μm size). Moreover, comparisons of the corrosion patterns found in accelerated-aging and outdoor exposures revealed which laboratory test reproduced the different real polluted atmospheres in the most realistic way, which is the main goal of this work. For instance, the degradation found at Site 2 was reproduced by Test NO2+SO2, with an acceleration factor of 27

Degradation types of reflector materials used in concentrating solar thermal systems

Reflector materials for concentrating solar thermal (CST) technologies must preserve high quality properties (in terms of solar specular reflectance) over the complete lifetime of the solar system. However, these solar components are exposed during their operation to a combination of ambient agents that provokes a number of different defects that might severely curtail their quality. To avoid significant failures, durability studies are being conducted both in outdoor and accelerated aging conditions. The identification of the possible degradation of solar reflectors is thus a key issue for the feasibility of a CST system. This review paper presents a survey of the durability tests most commonly used (mechanical abrasion/erosion, humidity, temperature, thermal cycling, UV radiation and chemical agents), as well as the main degradation types reported in the literature for different reflectors materials (erosion, abrasion, stains on the glass, metal corrosion, tarnishing, cracking, delamination, decohesion, blistering, yellowing, discoloration, haze, agglomeration, diffusion, chalking, etc.). In addition, some defects recently identified are revealed in this publication for the first time. The results shown comprise a useful tool for the interpretation of further durability studies

Simplified analysis of solar-weighted specular reflectance for mirrors with high specularity

The most relevant parameter to properly characterize solar mirrors is the solar-weighted near-specular reflectance. As this parameter cannot be directly measured with off-the-shelf instruments, a simplified procedure to be applied for highly specular solar mirrors is proposed in this paper. The approach, based on two criteria, was experimentally employed to check a wide variety of solar reflector materials. Only those mirrors with known high specularity passed the criteria, indicating that the proposed method is suitable