Michael Koehl

Universität Stuttgart, Institute for system theory and automatic control, Alumnus

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<title>Application of advanced methods for durability assessment of new absorber coatings</title>

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIV, 1995

Accelerated life testing of solar energy materials has been one of the main topics within the for... more Accelerated life testing of solar energy materials has been one of the main topics within the former Task X 'Solar Materials Research and Development' of the International Energy Agency (IEA) Solar Heating and Cooling Programme. In the case study of some selective solar absorber coating materials for domestic hot water systems procedures are presented for service time assessment, applicable for example to absorber coating materials. As a continuation of the finalized Task X a working group, 'Materials in Solar Thermal Systems' also in the framework of the International Energy Agency (IEA) 'Solar Heating and Cooling Programme' was initiated. One of the first projects is described here. Its aim is to investigate the conformity of the durability assessment, applied on solar absorber coatings tested by different laboratories. Each lab is working independently, this means there is no information exchange between the labs until the results are on paper. Test sample...

<title>Characterization of a-C:H/W and a-C:H/Cr solar selective absorber coatings</title>

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII, 1994

Tungsten and chromium containing hydrogenated amorphous carbon films were deposited in a process ... more Tungsten and chromium containing hydrogenated amorphous carbon films were deposited in a process which combines plasma activated chemical vapor deposition of methane and r.f.- sputtering of a metallic target. The metal content of the deposits can be adjusted by the ratio of gas flow between argon and methane and was determined by X-ray Photoelectron Spectroscopy (XPS). For the a-C:H/W composites the XPS data are presented in detail and supply information about the chemical state of carbon and tungsten. The presence of W in carbidic state could be proven. Furthermore the optical constants n and k of a-C:H/W were obtained in the wavelength range between 0.4 and 2.6micrometers and for W concentrations of up to 33 at.%. The morphology of the deposits, determined by atomic force microscopy, ranges from very porous to compact and can be controlled by the substrate bias voltage. Accelerated ageing investigations were performed in air in order to characterize the deterioration mechanisms of...

<title>Durability testing of antireflection coatings for solar applications</title>

by Michael Koehl and Arne Roos

Solar Optical Materials XVI, 1999

Antireflection (AR) coatings can be incorporated into highly transmitting glazings that, dependin... more Antireflection (AR) coatings can be incorporated into highly transmitting glazings that, depending upon their cost, performance, and durability of optical properties, can be economically viable in solar collectors, agricultural greenhouses, and PV systems. A number of AR-coated glazings have been prepared under the auspices of the International Energy Agency (IEA) Working Group on Durability of Materials for Solar Thermal Collectors. The AR coatings are of two types, including 1) various sol-gels applied to glass and 2) an embossed treatment of sheet acrylic. Typically, for unweathered glazings, a 4-5% increase in solar-weighted transmittance has been achieved. For AR-coated glass, reflectance values as low as 0.5%-0.7% at selected wavelengths (680-720 nm) were obtained. To determine the durability of the hemispherical transmittance, several collaborating countries are testing these materials both outdoors and in accelerated weathering chambers. All materials exposed outdoors are affixed to mini-collector boxes to simulate flat-plate collector conditions. Results for candidate AR coatings weathered at geographically disperse outdoor test sites exhibit changes in spectral transmittance primarily in the high visible range (600-700 nm). Accelerated testing at measured levels of simulated solar irradiance and at different constant levels of temperature and relative humidity have been performed in different countries. Parallel testing with different levels of laboratory-controlled relevant stress factors permits the time-dependent performance of these materials to be compared with measured results from in-service outdoor exposure conditions. Coating adhesion and performance loss resulting from dirt and dust retention are also discussed.

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