A solar spectrum almost ranges between 0.3 and 2.5μm, while a radiation spectrum of black body heated at several hundreds [K] is contained in infrared region. If we apply on ITO film with plasma wavelength of 2.0μm to a window of solar collector, we can efficiently shut up the solar thermal energy inside the window, because the solar radiation transmits through the film but the radiation of heated body is reflected by the film. We can obtain the dielectric function by Drude theory, and then reflectance and transmittance of the material. we calculate the efficiency of energy confinement of solar collector consisting of the film and the black body absorber. Next, we made prototype of solar water heaters with the window of ITO glass and with the normal glass as a reference.
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