太陽能發電系統之電控調變液晶聚光器

Abstract

光伏效應是一種能將光能轉換成電能輸出的物理現象。此效應最早於1839年被提出，至今已根據此效應實現了許多不同種類的太陽能電池。然而，目前全世界的太陽能發電系統仍不普及，這主要是因為太陽能發電系統造價昂貴且效率過低，因此，如何提高發電效率是目前太陽能發電領域的主要課題。液晶材料常用於電控式的光調制器，但至今尚未有人將液晶元件應用在太陽能系統中的聚光器或追日器等，用以增加發電效率的部分。在本篇論文中，我們提出一種將電控液晶原件用於太陽能發電系統以提高發電效率的機制，並以實驗驗證。任何種類的太陽能電池基板，皆有一個特定的入射光強度範圍能使其工作在最佳效率：當入射光強度過弱時，入射光能量過低而無法有效激發光伏效應，使得產生的光電流較低；當入射光強度過強時，多餘的光子無法被半導體層吸收，將造成串連電阻效應，使得光電轉換效率下降，甚至造成基板溫度過高而損壞。一般的聚光型太陽能系統常使用菲涅爾透鏡作為聚光器，並搭配機械式追日模組以維持太陽光直射。但由於菲涅爾透鏡的聚光倍率為固定值，而太陽光強度會隨著時間及天氣等因素變化，因此太陽能系統的輸出效率及電功率無法維持穩定，造成能量損失。本篇論文中我們利用液晶實現了可調式的聚光器，此聚光器具有可調變的聚光倍率：當入射光強度較弱時，我們提高聚光器的聚光倍率，使光電流提高；當入射光強度較強時，我們降低聚光器的聚光倍率，以防止發生串連電阻損耗。如此可將被聚光後而入射至太陽能基板的光強度維持在固定值，使得太陽能系統的輸出電功率及效率不隨太陽的位置或強度而變化，長時間維持在最大值，並得到較高的總發電量。我們由液晶的光調制原理出發，提出利用液晶的相位調制、振幅調制、及表面能調制等方法提升太陽能系統的發電效率，並分別以液晶透鏡、分散型液晶聚合物、及液晶聚合物薄膜等實驗驗證。這些裝置的理論亦可應用在其他領域，例如光學防手震系統、電控眼鏡與生物檢測器。在太陽能發展陷入瓶頸的今日，我們期望藉由此論文促進液晶元件或者其他可調變式元件在太陽能系統中的應用，以達到跨領域的合作並為科技產業找到新的出路。

The photovoltaic effect is the process of converting light energy into electrical energy. The photovoltaic effect was first experimentally demonstrated in 1839. Multiple solar electricity generation systems have been realized based on the photovoltaic effect. Currently, however, because solar electricity generation systems are expensive to fabricate and have low efficiencies, they are still not widely used. Therefore, the current goal is to enhance the efficiency of solar electricity systems. Liquid crystal materials are typically used in various types of light modulators, but have not been used in concentrators or sun trackers in a solar electricity system for enhancing efficiency. In this dissertation, we propose applying electrically tunable liquid crystal devices in solar electricity generation systems to enhance the efficiency, and we also present the experiments. Each type of solar cell has a specific incident irradiance range in which it functions at its highest efficiency. When the incident irradiance is too weak, the photovoltaic effect cannot be excited by the light energy, resulting in a low photocurrent; when the incident irradiance is too strong, the unnecessary photons cannot be absorbed by the semiconductor, inducing series resistance, decreasing the efficiency, and even breaking the panels. Conventional concentrating photovoltaic systems use Fresnel lenses as the concentrators and use mechanical rotating planes as the sun tracker to preserve the normal incidence of sunlight. However, the concentration ratio of a Fresnel lens is fixed, whereas the irradiance of sunlight changes with time and weather, resulting in an unstable system that has low output power. In this study we realized concentrators with tunable concentration ratios using liquid crystals: When the incident irradiance was too weak, we increased the concentration ratio to enhance the photocurrent; and when the incident irradiance was too strong, we decreased the concentration ratio to prevent series resistance. The irradiance of the concentrated light that hit the surface of the solar cell was then maintained at a fixed value, resulting in a system with high and stable output power and efficiency regardless of time and weather. Based on the light modulation of LC, we proposed the improvement of PV system using LC phase modulation, amplitude modulation, and surface free energy modulation, and also experimentally proved by the LC lens, surface-pinning effect of polymer-dispersed liquid crystals, and liquid crystal and polymer composite film. The operating principles of these devices can also be used in other applications, such as optical image stabilization, electrically tunable glasses, and biosensing. We believe our findings can be used to facilitate the cooperation between the solar cell and the liquid crystal industries, and the development of new solutions for improving solar electricity generation systems.