Luminescent solar concentrators for fibre optic daylighting
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Daylight is an abundant source of energy that, if used effectively for lighting in buildings, can improve quality of life and decrease the demand for electric lighting and air-conditioning, reducing energy consumption and costs. Various daylighting systems are available for daylighting the perimeter zones of buildings. However, it is more difficult to transport daylight to remote rooms. The few systems available for remote room daylighting are expensive or disruptive to the building design and rely heavily on direct sunlight. Luminescent Solar Concentrators (LSC’s) contain fluorescent dyes that absorb both direct and diffuse sunlight without tracking the sun, causing fluorescent emission in a specific wavelength range. LSC’s can potentially be used for daylighting, but the only previously demonstrated system is rather bulky and architecturally intrusive, and its output is yellow-green and difficult to control. A stack of three coloured LSC’s is proposed in this thesis, which produces a good white output of over 1,000 lumens under solar illumination of 100,000 lux. This output is transported to a remote room in narrow flexible polymer light guides. A theoretical model of the three-colour LSC stack was developed, which uses the absorption and emission spectra of the dyes to predict the LSC’s output spectrum and lumens. Studies with this model revealed the importance of highly accurate absorption tails data for good prediction of the stack performance. The model was used to determine the optimum size and dye concentration for each LSC. A simple experimental method was devised for characterising the optical performance of a fixed size LSC. Half of the emitted light is trapped at the end of the light guides, so a ‘light extractor’ is required to enable this light to escape. Ray tracing simulations were carried out for various light extractor designs, from which the optimal light extractor size, shape and configuration were determined for maximum optical gain. With a good light extractor design, a gain of 1.7-1.8 should be achievable, but with the current prototypes, the gain is limited to 1.2-1.3, limiting the output to around 1,100 lumens. The violet collector sheet in the LSC stack was also found to be problematic, so an alternative blue light source is proposed. Hence there is room for improvement in future prototypes.
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