Solar absorber layers for collectors on the test stand
Since the beginning of 2008 the Thermal materials group has provided the ISFH together with its collector testing station certified by the DIN CERTCO GmbH, with the ability to test the quality of selective absorber layers in respect of their durability during ageing.
Selective layers absorb about 95% of the energy in the sun’s spectral range up to 2500 nm. In the wavelength range above 2500 nm, however, they largely reflect irradiance (Fig. 49). This characteristic leads to the absorber only suffering slight losses during heating due to thermal irradiation. Solar absorption should be as high as possible, thermal emissions as low as possible and they should preferably remain unchanged for the collector’s lifetime of 25 years. Optical parameters calculated from the reflection spectrum therefore form the evaluation criteria for all reliability tests for solar absorbers. These are also supplemented by tests of the bonding strength of these layers.
In 1994 three European research establishments developed accelerated ageing tests for single-glazed flat collectors in the ‘Task 10, Solar Heating and Cooling’ programme. In 2004 they modified the temperature endurance test in the Task-10-procedure because the advent of high-quality absorber layers created with PE-CVD and PVD techniques as well as the use of anti-reflection coatings for collector glass covers had led to higher temperatures and therefore greater stress on the materials. The ISFH provides not only the so-called Task 10 test in its latest version, but also makes use of chemical reliability tests for newly-developed products in order to ensure their suitability for particular environmental conditions.
In addition to this minimum requirement for a practically applicable absorber for flat collectors, the question of the continued reliability of the coating with age again arises for newly-developed collectors or – in an age of global marketing – for use in areas with particular climatic and environmental problems. Collectors in coastal locations are prone to greater problems with salts, while those near conurbations may be prone to increased levels of nitrogen oxides and sulphur oxides. For unglazed façade collectors, these demands on the materials can be particularly high due to humidity and spray. The development of double-glazed collectors as well as the trend towards solar-assisted heating and therefore larger-scale solar installations will further increase the demands on materials, especially as stagnation occurs more frequently and more severely than the current Task 10 test could take into account. Stagnation is the name given to the situation frequently occurring in summer when the solar energy generated by intense irradiance is not used and therefore the absorber area is subjected to temperatures of up to 230°C following the evaporation of the cooling fluid.More info at SOLAR SERDAR.
In order to be able to provide suitable tests for selective coatings and new collector concepts, chemical endurance tests have been introduced at ISFH going beyond the conventional temperature and condensation endurance test (Task 10). Our aim was both to be able in the testing procedure to depict chemical pressures of various types and to be able to meet the industry’s justified demand for an economical testing procedure which could be converted into a norm. We therefore followed the normative testing procedure used in the construction industry for corrosion prevention and testing coated glass and modified this in such a way as to be able economically to carry out the salt spray test in accordance with DIN 50021, the acid spray test in accordance with DIN 50021SS and SO2 and NOx endurance tests with condensing humidity – in a form equivalent to EN 1096-2 – in one piece of equipment (Fig. 50 and Fig. 51).
The ability to perform SO2 and NOx endurance tests in a humid atmosphere using a spray chamber is based on the fact that these gases dissolve very quickly in water as sulfurous or nitrous acids and are oxidized with the ambient oxygen to form sulfuric or nitric acid. Instead of using climate simulators with costly gas injection and safety equipment, we spray the acids at a concentration equivalent to the quantities of gases in the norm.
The test methods have been used for a project supported by the Arbeitsgemeinschaft industrieller Forschung (AiF), in which the Fraunhofer Institutes for Electron Beam and Plasma Technology (FEP), for Surface Engineering and Thin Films (IST) and BlueTec GmbH & Co KG have developed corrosion protection layers for blue absorbers. Extensive tests have proved that the protective layers markedly improve the corrosion resistance of blue absorber layers even in extremely adverse conditions (Fig. 52 and Fig. 53). However, the classic Task 10 test is passed irrespective of the protective coating and, in contrast to our corrosion test, it cannot highlight the difference in quality.