Light generation, transport, mixing and extraction in luminescent solar collectors
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The difficulty of directing daylight deep into the heart of buildings means that much artificial lighting is required during the day, which substantially increases energy costs for lighting and air conditioning. This thesis explores the feasibility of daylighting with luminescent solar collectors. An LSC is a stack of thin sheets of polymer doped with fluorescent dyes. Sunlight entering the sheets is absorbed and emitted isotropically at longer wavelengths. 75% of this emission is trapped by total internal reflection and propagates towards the sheets’ edges. A special coupler channels some of this light into a flexible optical fibre that guides it to a remote luminaire. High quality white light with zero excess heat is produced by appropriate dye use. LSC’s collect both diffuse and specular sunlight, so their luminous output is only weakly affected by light clouds. The best previous LSC’s for daylighting gave an outdoor-to-indoor lumens-to-lumens efficiency of only 0.2%. This project achieved an efficiency of 5%. The basic tool for optical design was étendue analysis. Key results are: i) the system’s cross sectional area must not decrease along the optical path, ii) the collector sheets need a high aspect ratio, and iii) an often neglected requirement for a solid optical system with no air gaps. Other optical design problems solved include high-efficiency flat-collector-sheet to cylindrical-optical-fibre couplers and high-efficiency light extractors (which boost output by approximately 50%). Major advances in mechanical design resulted in several new practical solutions including: strong, enduring optical joints; mass produced collector-sheet to optical-fibre couplers using injection moulding with demonstrated efficiencies of 96%; affordable flexible light guides; high-performance cover materials; roof and façade mounting; and reduced mass. Required system performance is impossible without high quality LSC sheets. Maximising fluorescence yield involves detailed understanding of the roles of: dye quantum efficiency, Stokes shift, long wavelength absorption “tails”, dye dispersion, light transport inside a sheet and long term sheet stability. A substantial improvement in the performance of collector sheets was achieved. Solutions to all the key problems for daylighting with practical LSC systems have been demonstrated using outdoor mounted collectors channeling light to indoor spaces, with one key exception: the increase in absorption tails over the long term. Techniques were developed for measuring this weak tails absorption, which significantly reduces light output from the required long collector sheets. Suggestions are made as to its cause, and possible methods of its reduction.
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