Les revêtements sélectifs et leur rôle dans l'amélioration des performances des collecteurs solaires

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In the future the recovery of solar energy will become more and more indispensable. The use of absorbing selective surfaces allows the yield of plane or concentrated solar collectors to be increased significantly. Such surfaces have a coefficient of absorption close to unity in the solar spectrum and a coefficient of thermal emissivity close to zero. We recall that this combination of properties can be achieved because of the weak superposition of wavelength regions of the solar spectrum and of the radiation emitted by the surface of the absorber at a temperature T. A selective surface is usually composed of a thin film that absorbs in the solar spectrum and is transparent in the thermal infrared and a metallic support of weak emissivity. We review the various physical principles which allow this selectivity to be achieved. We then point out the principal characteristics of practical selective surfaces: optimal selectivity in the required temperature region; thermal stability; resistance to humidity and to corrosion; limited cost (500–1000 Fr (Belgian) per square metre). We summarize the main achievements in a general table, specifying the method of preparation, the values of the optical parameters, the selectivity and the prime cost. We conclude by giving some more detailed information on black nickel, black chrome and selective paints. Black chrome, with its excellent performance and its exceptional resistance when it is associated with a nickel undercoat, appears to be the most promising coating in low and medium temperature collectors.