Passive radiative cooling using optical thin film coatings
Keçebaş, Muhammed Ali (2016) Passive radiative cooling using optical thin film coatings. [Thesis]
Radiative cooling is a passive way of cooling by which a body loses heat by emitting energy. When a body is exposed to sky, heat transfer between the body and sky occurs depending on transparency of the atmosphere through radiation. During nighttime, due to extremely low incident solar irradiation cooling can be achieved. However, during daylight nearly 940 W/m2 energy is present in Istanbul, due to sun and since emission by the object is not as high as this energy, cooling cannot be achieved. So, in order to achieve radiative cooling during daylight, incident solar energy has to be reflected strongly which prevents heating of the object. Also, by maximizing emission in the atmospheric transparency window in 8‐13 μm range, in which very low amount of solar energy is carried, radiative cooling can be achieved. In this study, design studies about thin film filters are carried out whose focus is to achieve high reflection in the visible and near‐infrared spectrums in which high amount of solar energy is present and maximize absorption/emissivity in 8‐13 μm spectrum where atmospheric transparency window is present. For these purposes, different design methods are examined, e.g. quarter wavelength stacks for high reflection and an impedance matching technique, Chebyshev transform, is used to increase emission in 8‐13 μm spectrum. For the performance evaluations, radiative heat transfer dynamics are examined and cooling powers are compared with a design results given in the literature. It is observed that significant performance improvement can be observed by proposed design methods.
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