Holographic Spectrum-Splitting Design for Micro-Concentrating Photovoltaic Applications

Abstract

Solar energy can be a competitive renewable energy source when it provides high optical-to-electrical power conversion efficiency and energy yield at low cost. During the past few years the spectrum-splitting technology has been recognized as an important method to improve the overall power conversion efficiency and energy yield of solar photovoltaic systems. It can reach high efficiency by spatially assigning photons to PV cells with different bandgaps using optical filtering methods. This dissertation begins with a review of the background of holographic spectrum-splitting PV systems which includes models of solar illumination, the theory and models of PV cells, and the design of spectrum-splitting volume holographic gratings. This is followed by the formulation of a set of metrics that can be used to analyze and design an efficient multi-junction photovoltaic system. Ideal spectrum-splitting systems and tandem multi-junction PV systems are then analyzed and compared using the metrics. The main body of this dissertation demonstrates a design of holographic spectrum-splitting micro concentrating photovoltaic (HSSMCPV) systems. The second part of this work summarizes a method of designing, optimizing and fabricating high quality dichromated gelatin (DCG) holographic filters for HSSMCPV systems. To achieve a broad spectral bandwidth with sharp cut-off wavelengths, cascaded structure of multiple thick holograms is investigated and compared with single layer holographic elements. In addition, fabrication process of making two-layer cascaded DCG holographic filter is demonstrated for three-junction HSSMCPV systems. The physical properties and the optical performance of a prototyped filter are measured and used to confirm the model results. The dissertation concludes with an analysis and design of two- and three-junction holographic spectrum-splitting micro concentrating photovoltaic (HSSMCPV) systems for non-ideal PV cells. HSSMCPV systems combines the ideas of spectrum-splitting and micro concentrating array, and are expected to obtain a high system power conversion efficiency and energy yield in a compact module. The methodology for designing and optimizing the geometrical configuration, holographic filters, and lens array are described in detail for both systems. Diffraction efficiency across each holographic filters is simulated using Rigorous Coupled Wave Analysis (RCWA). A full system model is developed that allows non-sequential ray tracing through the system. Spectral Conversion Efficiency of each PV cell is found with published data and the quantum concentration ratio for different illumination conditions. High optical performance, quantum concentration ratios and power conversion efficiency are then obtained for the proposed systems with different spectral and spatial solar illumination conditions. In addition, the analysis on diffuse power conversion efficiency and annual energy yield are performed for the three-junction design.