Thermochromie III – Tehrmochromic coasts for overheating protection of solar thermal collectors

• Forschungsstelle: BFE
• Projektnummer: SI/500739

Kurzbeschreibung (Englisch)

Overheating and the resulting stagnation of solar thermal collectors is a common problem even in central European latitudes. The high temperatures occurring during stagnation lead to water evaporation, glycol degradation and stresses in the collector with increasing vapour pressure. Special precautions are necessary to release this pressure; only mechanical solutions exist nowadays. Additionally, the elevated temperatures lead to degradation of the materials that compose the collector, such as sealing, thermal insulation and the selective absorber coating. The goal of this project is to find a new way of protecting solar thermal systems without any mechanical device (e.g. for shading or for pressure release).

Publikationen / Ergebnisse (Englisch)

Novel thermochromic coatings are developed, which exhibit a change in optical properties at a critical temperature Tc.
Undoped, tungsten and aluminum doped samples of inorganic coatings showing thermochromic switching behaviour are produced by magnetron sputtering. A dynamical switching of the thermal emittance ε can be achieved by thermochromic transition metal oxides.
The effects of doping thermochromic oxide films by tungsten or aluminium, respectively, have been studied in great detail. The structural and optical properties of these films have been characterized in by methods such as X-ray diffraction (XRD) for phase identification, and Rutherford backscattering spectrometry (RBS) for determining the doping concentration.
The performance of a solar collector with an absorber based on a thermochromic film has been simulated in order to determine the stagnation temperature and the produced energy as a function of the thermochromic transition temperature.
In parallel, the possibility of using the switch in emissivity of the absorber coating in order to trigger the transition of a thermochromic or thermotropic coating on the glazing of the solar collector has been studied. In a first step, an analytical approach yielded the required transition temperature of such a switching glazing. Subsequently, the availability of thermotropic glass and other kind of smart glass have been studied. Samples of identified commercial products were characterised to know their optical properties and define if they would be suitable for our application.
The fascinating optical properties of these switchable films elucidate the way towards novel "intelligent" thermal solar collector materials. 

Zugehörige Dokumente

• Thermochromic coatings for overheating protection of solar collectors - temperature matching and triggering (Thermochromie III) - Rapport annuel [PDF] 1'196 kB

Schlussbericht (Englisch)

Overheating and the resulting stagnation of solar thermal collectors is a common problem even in central European latitudes. The high temperatures occurring during stagnation lead to water evaporation, glycol degradation and stresses in the collector with increasing vapour pressure. Special precautions are necessary to release this pressure; only mechanical solutions exist nowadays. Additionally, the elevated temperatures lead to degradation of the materials that compose the collector, such as sealing, thermal insulation and the selective absorber coating. The goal of this project is to find a new way of protecting solar thermal systems without any mechanical device (e.g. for shading or for pressure release). Novel thermochromic coatings are developed, which exhibit a change in optical properties at a critical temperature Tc. Undoped, tungsten, aluminium, D1 and D2 doped samples of inorganic coatings showing thermochromic switching behaviour are produced by magnetron sputtering. A dynamical switching of the thermal emittance ε can be achieved by thermochromic transition metal oxides. The effects of doping thermochromic oxide films by tungsten, aluminium, D1 and D2 have been studied in great detail. The structural and optical properties of these films have been characterized by methods such as X-ray diffraction (XRD) for phase identification, Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectrometry (XPS) for quantitatively determining the doping concentration and Energy-dispersive X-ray spectroscopy (EDX) for determining the chemical composition. The performance of a solar collector with an absorber based on a thermochromic film has been simulated in order to determine the stagnation temperature and the produced energy as a function of the thermochromic transition temperature. In parallel, the possibility of using the switch in emissivity of the absorber coating in order to trigger the transition of a thermochromic or thermotropic coating on the glazing of the solar collector has been studied. In a first step, an analytical approach yielded the required transition temperature of such a switching glazing. Subsequently, the availability of thermotropic glass and other kind of smart glass have been studied. Samples of identified commercial products were characterised to know their optical properties and define if they would be suitable for our application. The fascinating optical properties of these switchable films elucidate the way towards novel “intelligent” thermal solar collector materials.

Auftragnehmer/Contractant/Contraente/Contractor:
EPFL LESO-PB

Autorschaft/Auteurs/Autori/Authors:
Paone,Antonio

Zugehörige Dokumente

• Thermochromic coatings for overheating protection of solar collectors – temperature matching and triggering (Thermochromie III) - Rapport final [PDF] 4'371 kB