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De La Salle University SPECTRALLY SELECTIVE VISIBLE AND SOLAR TRANSMITTING HEAT BARRIER COATING FOR FLAT-PLATE COLLECTOR A Thesis Presented to The Faculty of the Engineering Graduate School College of Engineering De La Salle University - Manila In Partial Fulfillment Of the Requirements for the Degree of Master of Science in Mechanical Engineering By Reynaldo C. Muli April 19, 2002De La Salle University ABSTRACT A spectrally selective antireflection optical thin-film coating to reduce the reflectance of glass cover of a solar flat-plate collector was designed and fabricated. The antireflection coating was applied on the incident surface of a 3 mm thick clear glass, commonly used in flat-plate collectors. The transmittance of solar radiation of ~89% of clear glass increased to =92% on the entire visible light range of the spectrum. Consequently, the collector efficiency of the flat-plate collector also increased. The design focused on the optical properties of glass and the interference of light waves to alter the optical characteristics of glass to a preferred value on a selected spectrum. Fabrication was done using the high-vacuum system technology. IVrm: De La Salle University Modifying the optical characteristics of glass covers of atplate collectors can increase its performance, both to increase its transparency to visible radiation, and to prevent heat escaping lhreugh it. This paper tends to present one of the possible approaches to improve glazing performance through the use of spectrally selective optical thin-film coatings on glass covers of flat-plate collectors. 1.2 Statement of the Problem Solar energy is one of the promising alternative energy sources for the future. The chalienge is how to utilize it properly and efficiently. Presently available flat-plate collectors use uncoated glass covers that reflects a considerable amount of light energy that should have been transmitted and converted into heat energy. Spectraliy selective coatings on glass covers oftlatplate collectors can reduce the reected light energy and prevent the escape of infrared energy, thus increasing the collection efficiency ofthe solar flat-plate collector. De La Salle University 1.3 Objectives of the Study The general purpose of this study is to design and produce an antireflection optical thin-film coating to reduce the reflectance of the visible spectrum of sunlight on uncoated glass cover of solar flat-plate collectors. More specifically, the objectives of the study are: (i) To determine the effects of individual thin-film coating materials on the reflectance and transmittance of glass. (ii) To design and produce an antireflection thin-film coating on glass. (iii) To test, verify and compare the transmittance and reflectance characteristics of glass, coated and uncoated, using the Macleod optical coating design software and a spectrometer. 1.4 Significance of the Study The reduction of the reflectance of light on the visible region of the solar spectrum will increase the available energy for absorption, thus increasing the collection efficiency of a flat plate collector. This study will also be beneficial in future studies regarding the optimization of solar energy in photovoltaic applications. Alternatively, this 34.\De La Salle University (vii) Economic feasibility or viability of the study shall not be considered. (viii) Normal incidence of sunlight on the coated glass surface. 5De La Salle University Chapter 6 CONCLUSIONS AND RECOMMENDATIONS 6.1 . Conclusions The Design and fabrication of the spectrally selective antireflection coating Solar2 successfully reduced the reflectance of the flat-plate collector glass cover and increased its transmittance by nearly 3% on the visible band of the spectrum. This will mean an increase of 3% of solar energy added to the capacity of a solar flat-plate collector. Low reflection at a single reference wavelength can easily be accomplished with one layer of antireflection coating. However, for a greater bandwidth or a selective spectrum, multiple layers with refined thicknesses of each layer would be required. The 93% transmittance of solar energy of the coated Solar2 is equivalent to a 12.96% increase in collector efficiency if applied on the thermosyphon solar water heater designed and fabricated by Culaba, 1998. 39,' De La Salle University 6.2 Recommendations The study showed that the number of layers, thickness and quality of each layer of coating is very important in antireflection thin lm coating. The following recommendations are necessary: 1. Further study on the optical effects ofcoating the bottom surface of the glass. 2. Future studies on all optical coating materials and their processes should be undertaken. 3. The effects of antireflection coating are based only on the glass; study should be made on the overall effects on a small scale at plate collector application such as solar water heating system. 40 231