Polymeric Thermotropic Materials for Overheating Protection of Solar Collectors

Research output: ThesisDoctoral Thesis

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The overall objective of this dissertation was to provide basic knowledge on the development and characterization of thermotropic layers for overheating protection purposes in all-polymeric solar collectors. Specific focus was given to the establishment of a fundamental understanding of relationships between the material formulation and structural parameters and the light-shielding properties of thermotropic layers, based on sound principles of polymer physics and materials science as well as solar physics. For that purpose an evaluation and strength-weakness analysis of existing thermotropic layers produced from polymeric materials concerning their applicability in solar collectors was carried out, thermotropic prototype-layers were developed and designed for solar collector applications considering aspects of polymer physics, thermotropic prototype-layers were characterized as to relevant morphological parameters and performance properties, structure-property relationships between the performance properties and the inner material structure and formulation parameters of thermotropic prototype-layers were established, and the effect of thermotropic prototype-layers on the efficiency of an all-polymeric flat-plate collector was investigated and modeled. A review on various thermotropic glazing materials showed that thermotropic systems with fixed domains (TSFD) possess the highest development potential for overheating protection applications in solar collectors. In TSFD scattering particles (also referred to as additives), which exhibit a sudden change of refractive index upon reaching a threshold temperature, are statically embedded in a matrix material (in general in a thermoset resin). In the clear state the produced TSFD prototype-layers exhibited a hemispheric solar transmittance between 76 and 87%. Switching from the clear to the scattering state occurred at temperatures between 40 and 80°C. Transition temperature ranges between 10 and 25 K were recorded. Above the switching threshold the hemispheric solar transmittance changed to values ranging between 62 and 85%. The light-shielding properties of the thermotropic layers and thus the opacity in the opaque state are enhanced by a high cross-linking density and low chain segment mobility of the matrix, differences in refractive index between matrix and additive >0.04, a uniform additive distribution across the film thickness and disk-like scattering particles with a thickness below 400 nm. Thermotropic layers were found to reduce the maximum absorber temperatures to values between 129 and 146°C.
Translated title of the contributionThermotrope Polymermaterialien als Überhitzungsschutz für Sonnenkollektoren
Original languageEnglish
  • Wittwer, Volker, Assessor B (external), External person
  • Lang, Reinhold, Assessor A (internal)
Publication statusPublished - 2008

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