Design of Nanostructured Selective Surfaces for Solar to Thermal Energy Conversion
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Design of Nanostructured Selective Surfaces for Solar to Thermal Energy Conversion Daniel L. Gau1, Ricardo E. Marotti1, Federico Davoine1, Lucía I. Amy1, Enrique A. Dalchiele1, Rocío Romero2, José R. Ramos-Barrado2, Dietmar Leinen2, Francisco Martín2, Santiago Botasini3, Eduardo Méndez3, Gonzalo Abal1 1
Instituto de Física & CINQUIFIMA, Facultad de Ingeniería, Universidad de la República, Julio H. Reissig 565, CC 30, CP 11000, Montevideo, Uruguay. 2
Lab. de Materiales y Superficies (Unidad Asociada al CSIC), Dptos. de Física Aplicada & Ingeniería Química, Universidad de Málaga, Campus de Teatinos s/n, E29071 Málaga, Spain. 3
Lab. de Biomateriales, Instituto de Química Biológica, Facultad de Ciencias, Universidad de la República, Iguá 4225, CP 11400, Montevideo, Uruguay. ABSTRACT The solar absorptance s of nanostructured selective surface (NSS) for solar thermal energy is improved. The NSS are prepared by AC electrochemical impregnation of metal inclusions (MI) into porous anodized aluminum oxide (AAO). The dependence of the NSS performance with composition depth profile and MI is studied by numeric simulations based in a gradient index model and effective medium theory. The results are compared with experimental NSS prepared varying three control parameters and MI (Ni, Cu, Ag). The s is improved to > 85% (keeping thermal emittance T relatively low) for Ni MI, mainly by increasing MI content. Increasing AAO thickness or MI molecular weight (for a given experimental composition profile) also improves the performance. For Ag the s was further improved to 90%. INTRODUCTION Selective surfaces (SS) for solar to thermal energy conversion require low optical reflectance R() in the visible and near IR (high solar absorptance s) and a high R() in the IR beyond 2.5m (low thermal emittance T) [1,2]. This can be achieved with the incorporation of metal inclusions (MI) into a dielectric matrix [1,3-5]. Nanostructured SS (NSS) can be prepared by the electrochemical impregnation of MI (e. g. Ni) into the nanoporous alumina (np-Al2O3) obtained from aluminium anodization [6,7]. Typical values for s and T of 82% and 7%, respectively, are easily obtained [8]. The low T values in these NSS are due to their metallic nature. Higher s values are possible in other kind of materials, but they usually also has higher T [9]. In present work numerical simulations that predicts R() from the SS atomic composition depth profiles (CDP) [10] allow improving their performance (increasing s while keeping low T). The MI are also modified. The results are used to increase the s of experimental samples. EXPERIMENT The NSS preparation has three major steps: Preliminary Treatments, Anodization and Impregnation. In the former step commercial Al alloy sheets (25x50x0.3mm3) were first thermally treated at 300ºC during 1h, for diminishing internal stress. All further treatments and steps were conducted at room temperature. After that, the sheets were degreased by successive
ultrasonic cleaning processes in acetone, isopropanol and distil
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