Thermo-optics of Luminescent Solar Concentrators
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Thermo-optics of Luminescent Solar Concentrators Ahmadreza Hajiaboli1 and Mark P. Andrews1 1 Department of Chemistry, McGill University, Montreal, Canada, 801 Sherbrooke Street West. H3A 0B8, Quebec. [email protected], [email protected]
ABSTRACT We present a numerical study on effect of temperature on the performance of a waveguide luminescent solar concentrator (LSC). The purpose is to determine how changes in temperature of the ambient environment of an LSC affect device performance. The thermo-optical coefficient of the polymer waveguide is modeled using the well known Prod'homme formulation and applied in a forward Monte Carlo ray-tracing simulation. We show that the number of collected photons decreases almost linearly as the ambient temperature increases from -50 ºC to +50ºC. This behavior is associated with several competing loss mechanisms in the waveguide. For example, increases in optical confinement due to increased refractive index at low temperature are opposed by increases in cone loss (escape loss) of photons. Other competing mechanisms that exhibit temperature dependence are explained in terms of a detailed balance treatment of the LSC as a function of temperature. INTRODUCTION Luminescence solar concentrators (LSC) are optical waveguides doped with fluorescent (dye) molecules that are used to harvest sunlight and guide photons to the edge facets populated by photovoltaic (PV) cells. LSCs are much studied for their potential to reduce the cost of solar energy conversion by reducing the areal density of PV cells. Moreover, they have attracted attention as low energy consumption alternate light sources in day-light solar illuminators, i.e. conducting natural light to the dark areas of a building. Although LSCs have been around since the early 1970s, there has been renewed interest in them for solar energy conversion. This interest has spawned a number of papers focusing on simulating LSCs to explore various material and design parameters that can affect efficiency. Despite the fact that LSCs are intended for use largely in outdoor environments, little numerical work has been done on exploring their performance (efficiency) as a function of ambient temperature fluctuations. While most research has concentrated on achieving a higher concentration efficiency to make LSCs more competitive with state-of-the-art technologies, it is important to note that the average performance of these devices on daily, seasonal and annual time scales is an important parameter in making LSCs commercially competitive. The performance of LSCs depends on the detailed balance between several gain and loss mechanisms. The major gain processes are those that depend on the quantum yield of the guest dye, on total internal reflection and on penetration of the optical field through the top surface. The major loss mechanisms comprise absorption of photons by matrix, escape-cone loss due to refraction of emitted photons from the top surface, re-absorption of photons due to consumption of emitted photons by dyes, and
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