Study of abrasion tests for antireflective and antisoiling/antireflective coatings on glass solar tubes

The abrasion resistance of the antireflective coatings (ARC) applied on both sides of the parabolic-trough glass tubes is a key point to maintain throughout time the enhanced efficiency which produces the use of this film. The cleaning processes as well as the erosion produced by dust or sand in desert environments can damage the ARC, even removing it, affecting the system efficiency. The use of an antisoiling (AS) treatment on the antireflective (AR) coated solar glass envelopes are reaching increasing interest to reduce soiling and the water consumption associated to cleaning. RIOGLASS AR coated tubular samples were coated with a commercial hydrophobic treatment and the abrasion resistance of the AR + AS coatings combination against sole ARC were studied by carrying out two abrasion tests. Initial average solar transmittance values of 96.9 % and initial average static contact angle of 107 degrees were obtained for the AR + AS samples. The results from the two abrasion tests have shown that the AR + AS coatings combination present a higher resistance to the abrasion. It has also been observed that two different abrasive media in the sand oscillating test produce different degradation mechanisms, (reflection losses versus absorptance losses). Additionally, the results have shown that solar transmittance values are not enough to characterize the abrasion degradation of AR coatings, being also important the haze value. Similar solar transmittance values (93.9 % and 94 %) but with different haze values (1.2 % against 3.2 %) and very different transmittance spectra indicating distinct degradation mechanism were obtained for samples abraded with two different sands.