K Möller

Qualification test procedure for solar absorber surface durability

A general test procedure for the qualification of solar absorber surface durability has been developed based on the results of a comprehensive case study performed within the framework of the IEA Solar Heating and Cooling Programme Task X. It was assumed, in the development of the qualification procedure, that the intended use of the absorber surface to be qualified, was in single-glazed flat plate solar collectors for domestic hot water production. The absorber surface should be considered qualified if it met the requirement of a design service life of 25 years with maximum loss in the optical performance of the absorber surface corresponding to a 5% relative reduction in the performance of a solar domestic hot water system. The testing procedure, consisting of three kinds of constant load-accelerated life-time tests, was limited to simulation of the following three kinds of absorber surface degradation processes: (a) high-temperature degradation, e.g. oxidation, (b) degradation by the action of moisture or condensed water on the absorber surface, e.g. hydration or hydrolysis, and (c) degradation caused by high humidity air containing a small concentration of sulphur dioxide as an airborne pollutant, e.g. atmospheric corrosion. To quantify expected environmental stress on the absorber surface related to the environmental factors of interest, microclimate data, representing typical service conditions for absorbers in single-glazed flat plate collectors for domestic hot water production were used.

Accelerated Life Testing of Solar Absorber Coatings –Testing Procedure and Results

Abstract A procedure for accelerated life testing of solar absorber surfaces was developed within the framework of the working group MSTC (Materials in Solar Thermal Collectors) of the IEA-SHCP (International Energy Agency-Solar Heating and Cooling Programme). The intensive material and micro-climatic investigations on solar thermal collectors and solar systems within the preceding IEA Task X (1985–1991) as well as further studies in this field by the group itself formed a basis for this work. The procedure was formulated as a standard, and submitted to ISO at the beginning of 1997 as a ‘Draft Proposal’. It carries the designation ISO/CD 12592,2 ‘Solar Energy — Materials for flat-plate collectors — Qualification test procedures for solar surface durability’ . The proposed standard describes in detail a procedure for the examination of the long-term stability of solar absorber coatings used in flat-plate collectors for domestic hot water systems. The minimum lifetime of the absorber surface is estimated to be 25 years. Possible degradation caused by the thermal load, by condensation and high humidity are considered, as well as by air pollutants (sulphur dioxide). In order to examine the feasibility and reliability of the standard procedure, a ‘Round Robin’ test was performed in a further project of the working group MSTC. Within this ‘Round Robin’ the durability of five different absorber coatings was tested in accordance with the proposed standard by three independent laboratories. The outcome of this test is that all of the laboratories obtained the same results for each of the coatings. As the sulphur dioxide test described in the standard procedure was performed by only one laboratory, a comparison of the results cannot be made. Therefore, this article will be limited to the accelerated ageing tests regarding the resistance to high temperature and to high humidity and condensation. The limitation is made for both the description of the testing method and the presentation of the results of the ‘Round Robin’ test.

Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems

Comparison between predicted and actually observed in-service degradation of a nickel pigmented anodized aluminium absorber coating for solar DHW systems

Qualification of polymeric components for use in PV modules

The aim of these investigations was to develop, implement and evaluate an appropriate procedure in order to qualify new polymeric components for the use in PV modules and to predict the lifetime of materials and modules. A test program concerning five accelerated artificial aging tests (varying UV, temperature and humidity levels) was set up and the degradation behavior of six polymers was investigated. Visual transmittance and strain-at-break were identified as most significant degradation indicators. A suitable material specific material degradation model was obtained, where changes of the material properties are accurately related to the applied environmental stresses. For the modeling of service life time the micro-climate, to which the encapsulation materials are exposed in a PV-module, was considered and reasonable end-of-life criteria were defined. Accelerated aging tests showed that effects of heat and humidity are dominating in optical properties, whereas changes in mechanical properties are equally effected by UV and humidity. An exact description of the micro-climate was found to be most critical for the accuracy of the material degradation model. For first qualification of the principal applicability of new materials both, a standard damp heat test and a UV test have to be done in order to validate all degradation effects properly.

Case Study on Polymeric Glazings

Publisher Summary This chapter focuses on two specific glazings and demonstrates how the optical performance of these materials can be accurately predicted during in-service use. A summary is given for data obtained by outdoor exposure and indoor testing of different polyvinyl chloride (PVC) and polycarbonate (PC) materials. An initial risk analysis is given for the two materials. Screening tests and analyses for service lifetime prediction are discussed. A methodology that provides a way to derive correlations between degradation experienced by materials exposed to controlled accelerated laboratory exposure conditions and materials exposed to in-service conditions is given. A validation is presented for the durability methodology based upon durability test results for PVC and PC.

Innovative Measurement Methodology in Quality Assessment of Coatings

In the European project MANIAC-Innovative Measurement Methodology in Quality Assessment of Coating, a methodology was developed to assess the long term durability of coatings. The project was focused on environmentally friendly waterborne automotive multi-layer coating systems. Although developed for waterborne coatings, the methodology is very generic and can be applied to all kinds of coatings. The first step in the evaluation of long-term performance of automotive coatings involves exposures to weathering (natural as well as artificial). Traditionally, the durability of coatings is evaluated by examining the changes in performance properties like gloss, color, and scratch resistance. In the MANIAC methodology, the focus is on changes in structures on molecular levels. Microtomed slices of the coatings were examined by FTIR spectroscopy for monitoring oxidation. UV spectroscopy and GC-MS were used to study the migration of additives like HALS and UVA. In the methodology, the environmental variables (outdoors as well as indoors), e.g., temperature, relative humidity, UV irradiation, were carefully monitored and included in the modeling scheme. The results from accelerated tests are modeled and, together with outdoor climatic data, the different models are used to predict degradation during outdoor exposure.

Accelerated indoor testing

Publisher Summary This chapter elucidates the role of accelerated indoor testing in identifying marginal products with screening testing, accelerating the degradation rates of materials and components for durability testing, and making a service lifetime prediction for a product or the solar system. Aging processes can be accelerated either by increasing the concentration of reactive components or by increasing the degradative stress intensities, such as ultraviolet light, temperature, relative humidity, and cycle frequencies. To illustrate the application of some of the aging processes, the degradation of absorber coatings in solar thermal collectors was chosen. Elevated temperatures, higher humidities, and increased concentrations of reactive pollutants were chosen as the accelerating parameters. The experimental design used for studying degradation of absorber coatings is presented. A brief description is given of the accelerated testing equipment used. The effect of temperature fluctuations on the results was analyzed using the Arrhenius relation. Measurements that are useful for monitoring the changes in materials and their performance at various stages of degradation are briefly summarized. The change in the performance criterion of a selective solar absorber with time is given for 255 and 285°C, different relative humidities, and a pollutant concentration of 1 ppm, which corresponds to 20 to 500 times the concentrations found outdoors.

Measurements of Environmental Stress Conditions and Evaluation for Service Life Prediction

Publisher Summary This chapter describes some measurement techniques and methods that can be employed in the characterization of environmental stress for the durability assessment of components and materials. For service life prediction the knowledge of the environmental stress conditions is of essential importance. In recent years, a lot of research on durability assessment of absorber coatings for solar thermal collectors was made within the framework of the Solar Heating and Cooling Program (SHCP) of the International Energy Agency (IEA). In the case of solar absorbers, the environmental factors that may result in loss in optical performance are of principal interest. These are identified as high temperature, high humidity and moisture, and airborne pollutants. Measurement techniques adapted to these factors will be discussed in detail. The form of data acquisition is not restricted to time-dependent data only. Sometimes the acquisition of extreme values or dose functions is much more appropriate. Even well-defined standard specimens may be used as a sensor.

Development of Accelerated Life Testing Procedures for Solar Absorber Coatings

The oil crisis in the early seventies showed drastically the energy dependencies of countries with too few fossil energy resources. The exploration of alternatives yielded three options mainly: nuclear, renewable energy and energy saving techniques. Energy saving and renewables also have the advantage of sustainability and low environmental impact. The disadvantages of renewables are their fluctuations, requiring storage media, and their low power density (1 kW/m2 in maximum), hence their incompatibility with the traditional power grids. But their exploitation offers the possibility of a really sustainable energy supply in the future. This paper is dedicated to some aspects of solar-thermal energy conversion for low temperature heat used in domestic hot water, heating or cooling systems. Because of the low power density of solar irradiation, the energy has to be collected from an appropriate area by the so-called solar collector, shown in the schematic in figure 1. This device looks very simple at a first glance: The solar irradiation is absorbed and converted into heat by the absorber coating on top of a flat heat exchanger, where the heat is transferred to a fluid for transportation to thermal storage. The absorber is thermally isolated in order to reduce thermal losses. The glazing, acting as a convection barrier, is used for the isolation of the front side. Solar collectors in moderate climates need a high performance to become competitive with conventional heat sources.