Preparation and optical characterisation of antireflection coatingsand reflector materials for solar energy systems
Abstract: An angle-resolved scatterometer and an integrating sphere for reflectance measurements at oblique angles of incidence have been designed and evaluated. The integrating sphere has a centre-mounted sample holder and the detector sits at the end of the sample holder and therefore always faces the same sphere wall area. The sphere geometry plays an important role for the modelling of detected signals and the reflected intensity has to be divided into a specular and a diffuse component. These components must be treated separately in the modelling. These two instruments, as well as traditional spectrophotometers, have been used for the evaluation of solar energy materials. Scattering as well as non-scattering surfaces have been studied, requiring different measurement techniques.By using angle-resolved scatterometry it has been demonstrated that a solar reflector does not need to be perfectly specular provided the concentration factor is low. Thus it is possible to use inexpensive aluminium foil as the reflector material. The possibility of increasing the reflectance of aluminium with thin dielectric films of silicon and titanium oxides for pv-cell and solar thermal collector application has been investigated. Particular attention has been paid to the angular optical properties since thin films strongly affect them owing to interference effects. In an under-glazing application for pv-cells, the use of aluminium coated with titania and silica is recommended. The long-term stability of several reflector materials has been studied and anodised aluminium protected by a UV-stabilised polymer coating is recommended for solar collector reflectors.Antireflective films consisting of porous silicon oxide for solar collector cover glazings have been studied. The films were prepared by a dip-coating process using a suspension of nano-sized silicon oxide particles. This treatment increased the solar transmittance by 5.5 percentage points. It has also been shown that it is possible to temper antireflection treated glazings without seriously affecting the optical performance. The tempering also strongly improves the mechanical stability of the film.Some of the measurements presented in this thesis were used as input data to simulation programs, which calculate the collected annual energy as a function of the optical properties of the different components. It was found that spectrophotometric laboratory measurements agree well with outdoor collector testings.
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