Modern solar energy harnessing technology demands high grade energy to achieve efficient power generation with compact plant size and least payback period. But readily available solar energy is low grade energy. Concentrated Solar Power (CSP) technology is capable to cater the demand. Reflector or mirror in CSP contributes 50% of total cost of installation, hence huge initial investment and high life cycle costs are the major challenges associated with such plants. Moreover due to adverse service conditions and environmental impacts, it gets degraded early, causing substantial drop in efficiency and consequently reduction in life cycle. Fresnel lens as solar concentrator in Photovoltaic/Thermal (PV/T) applications may prove to be a promising alternative due to its potential to overcome techno-commercial constraints associated with conventional reflector based CSP. A critical review covering global CSP deployment, operational requirement and failure mechanism in mirrors/reflectors is being presented. As an alternative, design considerations of Fresnel lens and its effect on various efficiencies, reflectance, transmittance and associated losses are discussed. Learning from early research work, innovative and emerging trends, economics, challenges and advantages are also presented.

Fresnel lens: A promising alternative of reflectors in concentrated solar power

Concentrating solar power (CSP) technologies use large mirrors to collect sunlight to convert thermal energy to electricity. The viability of CSP systems requires the development of advanced reflector materials that are low in cost and maintain high specular reflectance for extended lifetimes under severe outdoor environments. The long-standing goals for a solar reflector are specular reflectance above 90% into a 4 mrad half-cone angle for at least 10 years outdoors with a cost of less than $13.8/m 2 (the 1992 $10.8/m 2 goal corrected for inflation to 2002 dollars) when manufactured in large volumes. Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the CSP Program at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Test results to date for several candidate solar reflector materials will be presented. These include the optical durability of thin glass, thick glass, aluminized reflectors, front-surface mirrors, and silvered polymer mirrors. The development, performance, and durability of these materials will be discussed. Based on accelerated exposure testing the glass, silvered polymer, and front-surface mirrors may meet the 10 year lifetime goals, but at this time because of significant process changes none of the commercially available solar reflectors and advanced solar reflectors have demonstrated the 10 year or more aggressive 20 year lifetime goal.

Optical Durability of Candidate Solar Reflectors

https://csegroups.case.edu/vuv/files/1105solmat-optprop-cpvmatls.pdf

Optical properties of polymeric materials for concentrator photovoltaic systems

Reflectors are a vital part of a concentrating solar thermal power plant. One of their most important characteristics is their durability, which entails the maintenance of their optical properties throughout their service lifetime, aimed at 10–30 years or more. The assessment of their optical durability involves the design of two types of aging tests, outdoor exposure testing under real ambient conditions and accelerated exposure testing in weathering chambers under simulated conditions. After exposure to different stress factors for certain periods of time, the optical performance of reflectors is evaluated mainly in terms of reflectance, but also regarding qualitative parameters such as their visual appearance and degradation patterns. The ultimate goal of a durability study is to conceive meaningful accelerated testing procedures that simulate real outdoor degradation in a short time and provide service lifetime estimates for a certain type of reflector at a specific site. To achieve this, more research on service lifetime prediction should be conducted and the standardization of accelerated testing procedures and reflectance evaluation methods should become widespread, to obtain comparable representative results. In this article, the most significant durability studies performed on the three main types of solar reflectors (glass-based, aluminum and silvered-polymer) and prospective approaches for improving future endeavors are discussed.

Durability studies of solar reflectors: A review

A better understanding of the degradation modes and rates for photovoltaic (PV) modules is necessary to optimize and extend the lifetime of these modules. Lifetime and degradation science (L&DS) is used to understand degradation modes, mechanisms and rates of materials, components and systems to predict lifetime of PV modules. A PV module lifetime and degradation science (PVM L&DS) model is an essential component to predict lifetime and mitigate degradation of PV modules using reproducible open data science. Previously published accelerated testing data from Underwriter Laboratories on PV modules with fluorinated polyester backsheets, which included eight modules that were exposed up to 4000 hrs of damp heat (85% relative humidity at 85°C) and eight exposed up to 4000 hrs of ultraviolet light (80 W/m2 of 280-400 nm wavelengths at 60°C) (UV preconditioning) were used to determine statistically significant relationships between the applied stresses and measured responses. There were 15 different variables tracking aspects of system performance, degradation mechanisms, component metrics and time. Modules were analyzed for three system performance metrics (fill factor, peak power, and wet insulation). The results were statistically analyzed to identify variable transformations, statistically significant relationships (SSRs) and to develop the PVM L&DS model informed by a generalization of structural equation modeling techniques. The SSRs and significant model coefficients, combined with domain analytics, incorporating materials science, chemistry, and physics expertise, produced a pathway diagram ranking the variables' impact on the system performance, which were iteratively examined using sound statistical analysis and diagnostics. The SSRs determined from the damp heat exposure for the system response of Pmax corresponded to the degradation pathway of polyester terephthalate (PET) and ethylene vinyl acetate (EVA) hydrolysis. A linear change point for the damp heat exposure with the system response of Pmax was determined to be 1890 hrs. The UV preconditioning exposure did not induce sufficient degradation shown by the quality of the R2 values for many of the best fitting models. This exemplifies the development of a methodology to determine rank ordered lifetime and degradation pathways present in modules and their effects on module performance over lifetime.

Statistical and Domain Analytics Applied to PV Module Lifetime and Degradation Science

Durability testing of a variety of candidate solar reflector materials at outdoor test sites and in laboratory accelerated weathering chambers is the main activity within the Advanced Materials task of the Concentrated Solar Power (CSP) Program. Outdoor exposure testing (OET) at up to eight outdoor, worldwide exposure sites has been underway for several years. This includes collaboration under the auspices of the International Energy Agency (IEA) Solar Power and Chemical Energy Systems (SolarPACES) agreement. Outdoor sites are fully instrumented in terms of monitoring meteorological conditions and solar irradiance. Candidate materials are optically characterized prior to being subjected to exposure in real and simulated weathering environments. Optical durability is quantified by periodically re-measuring hemispherical and specular reflectance as a function of exposure time. By closely monitoring the site- and time-dependent environmental stress conditions experienced by the material samples, site-dependent loss of performance may be quantified. In addition, accelerated exposure testing (AET) of these materials in parallel under laboratory-controlled conditions may permit correlating the outdoor results with AET, and subsequently predicting service lifetimes. Test results to date for a large number of candidate solar reflector materials are presented in this report. Acronyms are defined. Based upon OET and AET results to date, conclusions can be drawn about the optical durability of the candidate reflector materials. The optical durability of thin glass, thick glass, and two metallized polymers can be characterized as excellent. The all-polymeric construction, several of the aluminized reflectors, and a metallized polymer can be characterized as having intermediate durability and require further improvement, testing and evaluation, or both.

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Optical Durability Testing of Candidate Solar Mirrors

/pdf/measurements-of-backsheet-moisture-permeation-and-5eth00tjzv.pdf

Measurements of Backsheet Moisture Permeation and Encapsulant-Substrate Adhesion

Presented at the 2001 NCPV Program Review Meeting: Measurements of backsheet moisture permeation and encapsulant-substrate adhesion. At the March 2001 NCPV workshop on ''Moisture Ingress and High-Voltage Isolation'', industry participants identified several properties associated with PV module durability that are critical for commercial success. These include interface conductivity, adhesion of encapsulants to substrate materials as a function of in-service exposure conditions, and moisture permeation through backsheet materials as a function of temperature. Electrical data is discussed in a companion paper; adhesion and water vapor transmission rate (WVTR) measurements are presented herein.

/pdf/measurements-of-backsheet-moisture-permeation-and-13libyiff3.pdf

C. Kennedy

Papers

Optical Durability Testing of Candidate Solar Mirrors

Measurements of Backsheet Moisture Permeation and Encapsulant-Substrate Adhesion

Measurements of Backsheet Moisture Permeation and Encapsulant-Substrate Adhesion: Preprint