| Summary: | Global concerns over climate change and ever-increasing energy demand have led to a growing interest in developing renewable energy technologies. Building Integrated Photovoltaic (BIPV) window, which is conventionally designed by incorporating a semi-transparent thin film solar cell or evenly spaced crystalline-silicon (c-Si) solar cells between two layers of glass, is a promising technology to generate electricity and reduce cooling demands in buildings. In this thesis, an innovative BIPV smart window system where an optically switchable thermotropic membrane is laminated with a c-Si solar cell layer and glass covers has been proposed. The thermotropic membrane layer can switch between a transparent and light-scattering state as its temperature changes; in the meanwhile, a proportion of the scattered solar radiation is trapped in the window and redirected onto the solar cell surfaces for electricity generation. Compared with conventional BIPV windows, this smart window has the potential to offer better control of the daylight transmitted into building spaces as well as higher electrical power outputs. The concept is new, and findings regarding the window performance have not been reported in the literature. To prove this concept, in this thesis, a comprehensive research including prototype design, development and characterisations has been carried out:
(1) The system was preliminarily designed and fabricated with the aid of a simplified optical model where the thermotropic membrane layer was assumed as a Lambertian reflector with no angular dependence. The thermotropic membrane was made of a thermo-sensitive natural polymer at relatively low cost, named Hydroxypropyl Cellulose (HPC), and a gelling agent with good thermal stability, named Gellan Gum type F (GGF).
(2) The thermotropic membrane was further characterised in terms of angular scattering distribution by using an optical modelling technique, which combines the Inverse Adding-Doubling (IAD) method, Double-Integrating-Sphere (DIS) spectral measurement and Monte-Carlo (MC) ray-tracing method. This IAD-MC optical model is firstly reported and can be applied to the parametric design and optimisation of smart windows based on anisotropic scattering materials.
(3) The thermotropic membrane was optimised to have a transition temperature of 31°C and a solar transmittance modulation of 76%. The optimised smart window was experimentally characterised under both controlled laboratory conditions and dynamic outdoor environmental conditions.
(4) The smart window when applied in buildings was evaluated by using EnergyPlus, a validated whole-building energy simulation program. It was found that applying the smart window could potentially reduce the annual energy consumption by 39.0% and improve the luminous environment of a cellular office under the UK climatic condition, as compared with using an ordinary BIPV window.
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