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Concentrating photovoltaic technology harnesses solar energy by increasing the solar density upon solar cells using optical concentrators. Ongoing research on concentrating photovoltaic systems aim to improve the achievable energy harnessing and utilisation potential. Increasing the concentration ratio for high energy generation raises many advances and limitations in the concentrating photovoltaic design. However, the field of concentrating photovoltaic research is still in progress where new configurations, methods and materials are fabricated to reach a competitive cost by enhancing the efficiencies of the system to standard silicon photovoltaic systems.
The work presented in this thesis focuses on developing and demonstrating an ultrahigh concentrated photovoltaic system beyond 3000×. This system is based on a Silicon-on-Glass Fresnel lens resulting in a geometrical design of 5831×. The Fresnel lens as a primary optical interface was investigated theoretically, numerically, and experimentally to understand the operating limits in terms of power output, optical performance (optical efficiency and concentration ratio), and working temperature. The discrepancy between a Fresnel lens's theoretical and experimental optical characterisation results was studied. All the equations were elaborated for single- and multi-junction solar cells, emphasising the performance when the focal spot area is larger or lesser than the solar cell area. The prediction approach of optical characterisation has shown a strong agreement between the theoretical and experimental results of the multi-junction solar cells with a discrepancy of 2% at 7.7 W (77 suns) and 6% on the average cross a solar irradiance on the cell from 3.1 W – 7.7 W corresponding to 31 suns – 77 suns in concentration ratio. The numerical model using COMSOL Multiphysics software was established to study the Fresnel lens optically and thermally. The developed optical model was validated theoretically and experimentally to show a firm agreement with a discrepancy of ≤1%. Also, the developed thermal model was validated experimentally to show a difference of only 2.18%. Further, optical and electrical characterisations of the flawed glass have been conducted. The optical characterisation has shown a drop of 3.2% in optical efficiency. I-V and power curves of cracked and non-cracked Fresnel lenses were also compared to show a drop of 3.2% in short circuit current and power.
A theoretical analysis of the optical performance for a ¼ of the ultrahigh concentrated photovoltaic system design grouping three optical interfaces is performed to estimate the optical loss and its influence on the optical efficiency and concentration ratio. Also, a numerical model was established using COMSOL Multiphysics software to simultaneously evaluate the thermal and optical performance of a ¼ of the ultrahigh concentrated photovoltaic system. The system was analysed under direct normal irradiance ranging from 400 W/m^2 to 1000 W/m^2 in an interval of 100 W/m^2 , showing a simulative optical efficiency of ~93% and a simulative concentration ratio of 1361 suns at 1000 W/m^2 . The thermal model was interlinked with the optical model to generate the results accordingly. The final stage receiver shows a maximum temperature ranging between 157.4 ℃ and 78.5 ℃.
Moving toward a ultrahigh concentrated photovoltaic design raises the importance of a cooling management system due to thermal excitation. Although the thermal performance and thermal management for the ultrahigh concentrated photovoltaic system are beyond this thesis's scope, the cooling mechanism arrangement based on either pre- or post-illumination techniques was explored. The post-cooling mechanism study was established using COMSOL Multiphysics software for numerical analysis. A flat-plate and micro fin heatsink studied the effect of concentration ratio up to 2000 suns to determine their limits as a passive cooling system and establish when an active cooling system is needed based on the recommended operating temperature of the solar cell of 80 °C. On the other hand, Graphene was experimentally exploited as a pre-illumination cooling technique for a solar cell with different graphene coating thicknesses. The concept of utilising graphene as a neutral density filter for focal spot concentrating photovoltaic (Fresnel lens primary optic) reduces the solar cell temperature significantly and maintains the cell temperature for a more extended period. The graphene coating orientation further influenced the temperature gradient behaviour of the focal spot and incident temperature.
The Fresnel lens working parameters (focal length and the focal spot) were defined to establish the mechanical structural design accordingly. The system was mechanically designed based on three optical interfaces, built in-house, and incorporated with a sun tracker. Different aspects were examined initially before the outdoor testing, the sun tracker alignment accuracy and payload capacity, windage load, and counterbalance weight and moments effects using SOLIDWORKS software. The ultrahigh concentrated photovoltaic system was tested outdoor with three types of secondary mirrors, resulting in an effective concentration ratio of 984 suns, 1220 suns, and 1291 suns and an average optical efficiency of 18.5%, 20.25%, and 22% for Aluminium reflective film, Pilkington Optimirror, and ReflecTech® Polymer secondary optic types, respectively. The fabricated ultrahigh concentrated photovoltaic system and tested experimentally outdoor is the highest in both geometrical and effective concentration ratios so far.
It would not be possible to design and perform the ultrahigh concentrated photovoltaic system without fully characterising its primary optic, which helps set the performance basis and associated losses. Although the experimented system showed the highest value in terms of both geometrical and effective concentration ratios, the subsequent optics to the Fresnel lens were standard optics. The attained outcomes are practical in progressing concentrating photovoltaic technologies to a higher concentration ratio. |
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