• Energy Research

AbstractBy means of large mirrors, CSP (Concentrated Solar Power) technologies concentrate the solar energy on an absorber where it is collected as thermal energy. The decrease of the kWh cost and the insurance of a 25 years lifetime minimum are the key points to make these technologies cost competitive and ensure their large deployment. The solar field is the first expense item (20 to 30% of the investment) and is composed by the reflectors which concentrate the solar flux on the absorber. The durability of the mirrors is therefore of principal interest and has to be studied since the conception of the power plant. Actually, the majority of this technology uses glass mirrors made with silver and copper layers for a high reflectance. The protection of the silver layer from oxidation is ensured both by the copper layer and backside paint. In service conditions, the mirrors are submitted to arid climate where temperature, humidity and solar irradiation are extreme. Accelerated climatic tests in controlled environment are used to create the same degradations than the ones happening on site. Specular reflectance being the primary function of the mirrors, this is the parameter usually characterized to evaluate the aging of a mirror. Depending on the aging test and specifically for outdoor exposure, it might take a long time before a loss of reflectance occurs. We present here a new methodology based on the monitoring of the protective back layer degradation that can be used to anticipate the reflectance loss. Based on the activated energy calculated via an Arrhenius law, the lifetime prediction in normal condition can be established.

AbstractThe degradation of solar glass mirrors is analyzed as a function of various constraints such as temperature, humidity, rain and solar irradiation. In order to understand the degradation mechanisms, two kinds of tests have been performed: simple tests to identify the effect of each constraint and complex tests to highlight the effects of synergy and to reproduce the natural ageing. The study of the paints coat degradation evidences that UV exposure is one of the most aggressive constraints for paint binder. But results show also a strong effect of liquid water on paint degradation and particularly on pigments loss and blistering phenomenon. Whereas temperature in dry environment is not a very degrading factor for coats, presence of liquid water is a strong accelerating constraint at high temperature. Agreement between outdoor ageing and indoor ageing with a new procedure involving UV, temperature and rain also shows that liquid water is the critical constraint responsible to edge corrosion of silver layer and paint delamination. This work shows also that relation between paint degradation and corrosion of silver layer is not obvious. Homogeneous corrosion by pitting was observed without any deep chemical deterioration of the paint coat system.

AbstractThe durability of solar mirrors is a critical factor for the deployment of concentrating solar power plants. Accelerated aging test models currently applied in the polymer, electronic, and photovoltaic fields have recently been reviewed, and the issues of their application to solar mirrors have been discussed. This article first reports the results of temperature, humidity, and light irradiance accelerated aging tests performed to assess the dependent parameters of selected models from the literature. These parameters include the apparent activation energy for the Arrhenius temperature law, the Peck and Eyring coefficients for humidity models and the Schwarzschild coefficient for the irradiance law. The experimental values were then assessed for specular reflectance loss of solar mirrors. Finally, using these parameters, acceleration factors were calculated for solar mirrors. An effective temperature considering the Arrhenius degradation law was used rather than the commonly used mean temperature. This question is also addressed for light irradiance by using the dose instead of the mean value.

CSP (Concentrated Solar Power) plants technologies use the concentration of solar energy on a receiver to produce heat and then electricity by a thermodynamical process. A solar absorber material is used to convert the energy carried by light into heat. This type of material works at high temperatures (up to 1000 °C) under a highly concentrated solar flux (up to x1000 or more). Optical properties determine the performance of absorbers and it is thus necessary to measure their spectral absorptance and emittance. Solar absorptance is directly linked to the capacity of the absorber material to convert the solar flux into heat. Emittance drives the radiative thermal losses for the heated absorber and depends on the absorber temperature. The characterization of a material in operational conditions at high temperatures requires advanced apparatuses, and different measurement methods exist for the characterization of these two quantities of relevance regarding an absorber. A Round Robin Test (RRT) was conducted with the objective of comparing different new optical apparatuses and methods for measuring the emittance or luminance of various solar absorbers in air. Measurements were carried out directly at temperatures up to 560 °C while heating the samples, and also indirectly by hemispherical reflectance measurements at room temperature. In this paper, the Round Robin Test procedure to compare apparatuses is described, as well as the corresponding reflectance and emittance results on four types of materials. In addition, a discussion of some factors of influence over high temperature measurements in air and of the observed discrepancies among results from the evaluators is presented. The reliability of reflectance/emittance measurements is also demonstrated and statistics of deviations from the mean value are analysed. These allow us to infer information about measurement reproducibility. The reflectance spectra of all samples after high temperature measurements in air (up to 500 °C) do not show any significant changes.

The thermal runaway model used is a semi-empirical model based on a kinetic equation, parametrised by three parameters (m,n,p). It is believed that this kinetic equation can describe any reaction. The choice of parameters is often made without justifications. We assumed it necessary to develop a method to select the parameters. The method proposed is based on data coming from an accelerating rate calorimeter (ARC) test. The method has been applied on data obtained for cells aged on different conditions. Thanks to a post-mortem analysis and simulations carried out using the parameters obtained, we have shown that the ageing mechanisms have a major impact on the parameters.

The Concentrated Solar Thermal Energy (STE) system requires components with a lifetime of more than 25 years to be profitable. Optical materials performances, such as mirrors and absorber coatings, are essential to the efficiency of the system. Any degradation of their optical properties leads to a reduced efficiency. In the frame of STAGE-STE project we are studying the durability of solar absorber coatings on metallic substrate for non-vacuum applications (in air). The definition of the test conditions was based on a critical review of the stress factors affecting the material such as temperature, humidity and saline atmosphere. The optical performances were measured over time for each sample to follow the variation of solar absorbance and emittance. For each test, a comparison of the performance criterion (PC=Δα-0.5e < 0.05) variation gives a relative resistance of each absorber to the corresponding stress factor. In this paper we will present the results of the accelerated tests campaign on the three solar absorbers samples. Materials characterization was used to analyse the degradation process occurring on each sample. A lifetime prediction based on the calculation of the activation energy has been presented for the thermal test. Finally, the relevance of each test has been discussed.

Abstract Solar mirrors for concentrated solar power (CSP) plants are expected to last at least 30 years. As this delay is far too long to obtain useful information regarding in-service degradation, accelerated approaches to weathering testing are performed by manufacturers and research laboratories in order to quickly assess the lifetime of commercial or new technologies. However, most published studies that have been performed in the CSP field are based on phenomenological approaches. The characterization of the degradation, which mostly considers reflectance loss, has rarely been linked to physical or chemical processes that are responsible for the degradation of properties. Furthermore, the general laws that can be established from these data to establish material behaviour are empirical. Ageing tests have been used for many years in other fields, particularly in the domain of polymeric materials. The impacts on the material properties of stress factors such as temperature, irradiation and humidity have been extensively studied, and models have been proposed for different kinds of materials, even though most are based on empirical observations. One of the goals of this article is to determine how these models could be applied to the weathering of solar mirrors, and as such, the goal of this paper is to provide a critical review of the various models that are most used and accepted by the scientific community. All of these models include material-dependent parameters, and the values that have been determined in these studies are reported here to list their order of magnitude.