Structured luminescent solar energy concentrators : a new route towards inexpensive photovoltaic energy

Abstract

The solar energy market has grown considerably over the last decade due to increasing global awareness of environmental issues, the effects of greenhouse gases and fossil fuel shortages. More and more areas are now perceived as potential markets for solar energy conversion devices with the ultimate goal of replacing traditional power generating methods. However, in order for electricity generated by a photovoltaic module to be viable for industrial electricity production, the cost should be about $ 0.06/kWh according to the US department of Energy. The lowest cost of electricity generated by standard silicone photovoltaic modules (US $0.25/kWh) today is still a factor of two to three higher than conventional grid-connected electricity. An attractive alternative to the expensive photovoltaic panels and concentrated photovoltaic systems is the simple, colorful and cost efficient luminescent solar concentrator (LSC). LSCs consist of a transparent substrate embedded with fluorescent dyes or with the dyes as a thin film coating on top of the substrate. Generally, polymeric plates that are commercially available in large quantities for a low price are used as the transparent substrates. The fluorescent dyes in the LSC act as a light converter that absorbs sunlight and emits light at longer wavelengths. A fraction of the emitted light is directed towards the edges of the plastic plate where photovoltaic cells are attached to convert light energy into usable electricity. The simplicity and low material prices of LSCs potentially result in low energy costs (~US $0.30/Wp). However, the luminescent solar concentrator exhibits several potential light loss mechanisms that limit its efficiency and practicality for many potential applications. In this work, the possibility of producing a more efficient luminescent solar concentrator is explored by reducing the intrinsic losses of the system using inexpensive and straightforward methods. First, the behavior of standard dye-embedded and thin film coated LSCs was studied as a function of absorbance and dye concentration. Both theoretical and experimental results demonstrated that increases in dye concentration above a critical point leads to a negligible increase in edge emission and a decrease in emitted photon to absorbed photon efficiency. The decrease in efficiency is mainly a result of internal re-absorption losses, which also increases the external surface losses. The former refers to light emitted from dye molecules that is re-absorbed by neighboring dyes, and the latter refers to light lost through the top and bottom surfaces of the fluorescent substrate. To broaden the absorption range of the LSCs, stacked LSC systems consisting of two substrates doped with different fluorescent dyes was simulated theoretically and compared to experimental results. The total edge emission of the stacked LSCs was found to be more than double that of the single substrate devices. To reduce the probability of re-absorption in the luminescent solar concentrators, the thin film dye coating was patterned to create line and square dye arrays. The surface area coverage of the dye coating was reduced through patterning, which in turn decreases the probability of emitted light encountering other dye molecules, leading to a decrease in re-absorption losses in the LSC system. This concept was confirmed experimentally and theoretically as the emitted photon to absorbed photon efficiency of the patterned LSC systems increased with decreasing dye coverage. However, a significant fraction of light was lost through the clear regions of the patterned waveguide and the edge emission (in absolute light intensities) decreased as dye coverage is reduced. A possible solution for recovering light lost through the clear regions of the patterned waveguide is to use a lens array to focus incident light on the patterned dye areas. The objective of the lens array design was to maximize the acceptance angle while simultaneously minimizing the focal spot size, which dictates the size of the dye areas required. The resulting lens, designed using ray-tracing techniques, was aspherical in shape and exhibits an acceptance angle of 30?. Subsequently, the lens array was combined with a line patterned LSC to form an integrated system. Experimental results demonstrated that the addition of a lens array resulted in better performing systems where the edge emission exceeded a fully covered standard luminescent solar concentrator by more than 20%. Introducing a lens array to the patterned LSCs induces preferred emission at two opposite edges of the substrate, and this effect was further enhanced by aligning the dye molecules using liquid crystals in a guest-host system. Three different dye alignments, isotopic, homeotropic and planar, were studied using patterned LSCs. The planar aligned systems demonstrated higher emission-to-absorption power efficiency than both isotropic and homeotropic aligned systems. In addition, emission was enhanced from two edges in the planar aligned dye system, which exceeded the edge emission of isotropic systems by 20%. These planar aligned dye systems are potentially advantageous for reducing the material cost of LSC solar modules as the preferred emission allows photovoltaic cells to be attached to two edges of the waveguide instead of four. A large fraction of emitted photons is lost through the top and bottom surfaces of the waveguide in the patterned luminescent solar concentrator systems with and without a lens array. To limit the surface losses and to further enhance the edge emission of the patterned and lens array integrated system, wavelength-selective chiral nematic (cholesteric) liquid crystal reflectors were employed. The addition of the cholesteric reflectors generally increased the edge emission of the patterned LSC system, both with and without the lens array, when the position of the reflection band of the cholesteric was chosen correctly. The increase in edge emission was higher at low dye coverage, which suggests that the cholesteric reflectors are more suitable for LSC systems with little re-absorption losses. The LSC studied in this work primarily use inexpensive materials and fabrication methods, and may be suitable for applications in energy generating rooftop installations in both urban and remote areas. The relatively low cost of energy generated by LSCs compared to standard silicon PV panels makes them an attractive alternative for large-area installations. For example, the large area of industrial building rooftops allows a vast number of LSC systems to be installed to produce the required amount of energy. In addition, the integrated patterned and lens system are relatively flat compared to standard concentrating photovoltaic systems, which makes them more visually appealing in urban environments. The ability of LSCs to alter the solar spectrum, as well as the flexibility in material choices, open new potential applications in areas such as modern agriculture where LSC plates can be used to built energy generating greenhouses.