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Title: Identification and development of novel optics for concentrator photovoltaic applications
Author: Shanks, Katie May Agnes
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2017
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Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. CPV systems are however still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based photovoltaic (PV) systems. This work outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. Comparisons on materials, optical efficiency and optical tolerance (acceptance angle) are made in the literature review as well as during theoretical and experimental investigations. The subject of surface structure and its implications on concentrator optics has been discussed in detail while highlighting the need for enhanced considerations towards material and hence the surface quality of optics. All of the findings presented contribute to the development of higher performance CPV technologies. Specifically high and ultrahigh concentrator designs and the accompanied need for high accuracy high quality optics has been supported. A simulation method has been presented which gives attention to surface scattering which can decrease the optical efficiency by 10-40% (absolute value) depending on the material and manufacturing method. New plastic optics and support structures have been proposed and experimentally tested including the use of a conjugate refractive-reflective homogeniser (CRRH). The CRRH uses a reflective outer casing to capture any light rays which have failed total internal reflection (TIR) due to non-ideal surface topography. The CRRH was theoretically simulated and found to improve the optical efficiency of a cassegrain concentrator by a maximum of 7.75%. A prototype was built and tested where the power output increase when utilising the CRRH was a promising 4.5%. The 3D printed support structure incorporated for the CRRH however melted under focused light, which reached temperatures of 226.3°C, when tested at the Indian Institute of Technology Madras in Chennai India. The need for further research into prototyping methods and materials for novel optics was also demonstrated as well as the advantages of broadening CPV technology into the fields of biomimicry. The cabbage white butterfly was proven to concentrate light onto its thorax using its highly reflective and lightweight wings in a basking V-shape not unlike V-trough concentrators. These wings were measured to have a unique structure consisting of ellipsoidal pterin beads aligned in ladder like structures on each wing scale which itself is then tiled in a roof like pattern on the wing. Such structures of a reflective material may be the answer to lightweight materials capable of increasing the power to weight ratio of CPV technology greatly. Experimental testing of the large cabbage white wings with a silicon solar cell confirmed a 17x greater power to weight ratio in comparison to the same set up with reflective film instead of the wings. An ultrahigh design was proposed taking into account manufacturing considerations and material options. The geometrical design was of 5800x of which an optical efficiency of either ~75% with state of the art optics should produce and effective concentration of ~4300x. Relatively standard quality optics on the other hand should give an optical efficiency of ~55% and concentration ratio ~3000x. A prototype of the system is hypothesised to fall between these two predictions. Ultrahigh designs can be realised if the design process is as comprehensive as possible, considering materials, surface structure, component combinations, anti-reflective coatings, manufacturing processes and alignment methods. Most of which have been addressed in this work and the accompanied articles. Higher concentration designs have been shown to have greater advantages in terms of the environmental impact, efficiency and cost effectiveness. But these benefits can only be realised if designs take into account the aforementioned factors. Most importantly surface structure plays a big role in the performance of ultrahigh concentrator photovoltaics. One of the breakthroughs for solar concentrator technology was the discovery of PMMA and its application for Fresnel lenses. It is hence not an unusual notion that further breakthroughs in the optics for concentrator photovoltaic applications will be largely due to the development of new materials for its purpose. In order to make the necessary leaps in solar concentrator optics to efficient cost effective PV technologies, future novel designs should consider not only novel geometries but also the effect of different materials and surface structures. There is still a vast potential for what materials and hence surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light.
Supervisor: Mallick, Tapas Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solar Energy ; Photovoltaic ; Optics ; solar concentrator ; concentrator photovoltaic ; biomimicry ; material