Fluorescent fibers coupled to monolithic photovoltaic arrays for sunlight conversion
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Fluorescent fibers coupled to monolithic photovoltaic arrays for sunlight conversion Oleg V. Sulima, Jeffrey A. Cox, Paul E. Sims, Michael G. Mauk AstroPower Inc., Solar Park, Newark, DE 19716, U.S.A. ABSTRACT Thin fluorescent organic fibers are used to collect short wavelength sunlight, convert it to longer wavelengths and illuminate photovoltaic (PV) arrays. Using this approach, miniature and lightweight PV arrays have been fabricated. The incoming light is collected by many fibers and mixed before reaching the PV array. Thus homogenous illumination is provided even with partially shadowed fibers, which can cover a large area and can be arranged in a form of a fabric. The PV array itself can be located in any (even dark) place. In this work we developed and tested high-performance miniature 21 mm2 AlGaAs/GaAs monolithic arrays consisting of 6 cells connected in series. Output electric power density of 75 mW/cm2 and an open-circuit voltage about 6 V was measured outdoors for the arrays integrated with a bundle of 0.25 – 1.00 mm thick polystyrene fibers. The influence of PV array output parameters on the fluorescent fiber diameter, length, as well as on bandgap and design of solar cells is discussed.
INTRODUCTION Recently, AstroPower proposed the concept of combining photovoltaic/solar power arrays with fluorescent fibers [1,2]. One of the possible applications of this approach is recharging batteries. The main problem with standard solar cells is that they generate a relatively high current but a low voltage (usually less than 1 V). This voltage is too low to charge standard batteries. To increase the voltage, a series connection of several solar cells is necessary. However, for many practical applications when the homogeneous illumination of the cell array is not possible, this simple approach is not efficient. A local shadowing of the array drastically decreases its output power. Moreover, shadowed cells are driven into reverse voltage operation and may be damaged if bypass diodes are not used. A uniform illumination of the array can be provided by fluorescent fibers that absorb the short wavelength part of the sunlight at their surface and /or in the bulk, re-emit it at longer wavelength, and couple it to a PV array. The difference of refractive indices between the fiber and the air sets the conditions for total internal reflection and gives the fiber its wave guiding properties. Therefore, a portion of light can propagate in the fiber towards the PV array situated at its end. A bundle of thin (0.25-1 mm in diameter) fibers can be arranged and coupled with the PV array in such a way that shadowing of some fibers does not affect the homogeneity of the array illumination. An additional advantage of such an approach is that the light delivered by the fibers can exceed one sun illumination. Therefore, if the sunlight is concentrated, a smaller array is necessary to produce a unit power. Moreover, it is easier to integrate a smaller device in a shock resistant design than a larger one. Thus, miniature solar
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