Loss Mechanisms in Photovoltaic Fluorescent Collectors
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Loss Mechanisms in Photovoltaic Fluorescent Collectors Liv Prönneke1, and Uwe Rau2 1 Institut für Physikalische Elektronik, Universität Stuttgart, Stuttgart, 70569, Germany 2 IEF5-Photovoltaik, Forschungzentrum Jülich, Jülich, 52425, Germany ABSTRACT Monte-Carlo simulations calculate the photon collection of fluorescent collectors in photovoltaic systems. We focus on two collector geometries: solar cells mounted at the collector sides and solar cells covering the back side of the collector. A mirror covers the bare back sides of both systems. On top lies optionally a photonic structure, which acts as an energy selective filter. Ideal systems in their radiative limits are compared to systems where loss mechanisms in the dye, at the mirror, or the filter are included. The examination of loss mechanisms in photovoltaic systems with fluorescent collectors enables us to estimate quality limitations of the used materials and components. INTRODUCTION Photovoltaic fluorescent collectors (FCs) act as solar light traps and concentrators. A dielectric material surrounds fluorescent dye molecules, which absorb incoming photons with energy E1. The dye then emits light due to Stokes Shift with energy E2 < E1 and randomized direction. Total internal reflection traps part of the light and guides it to the collector sides. The solar light is converted to electrical energy either by solar cells applied to the collector sides [1] or its back side [2]. Goetzberger and Wittwer describe the technological potential of FCs in photovoltaic systems [3]. For the analysis of FCs Yablonovitch [4] and Smestad et al. [5] developed profound theoretical tools. However, for the estimation of theoretical limits of photovoltaic systems with fluorescent collectors highly idealized assumptions have been made [2,6]. In realistic photovoltaic set-ups loss mechanisms occur, which need to be considered. The present paper uses Monte-Carlo simulations to compare such loss mechanisms in two photovoltaic FC systems where the solar cells are either side-mounted or bottom-mounted to the collector. Maximum photon collection is achieved in the radiative limit with a perfect mirror at the back side and an energy selective photonic band stop (PBS) filter [2]. Starting from such ideal systems, we examine the influences of non-radiative recombination in the fluorescent dye, of non-perfect reflection at the mirrors, and of non-perfect reflection conditions at the PBS filter. The results point out, that reflection losses at the back surface cause a higher decrease than losses due to non-radiative recombination in the dye. Compared to a system without applied PBS nonradiative losses induce higher decreases of photon collection in the PBS covered system. Finally, we find that the side-mounted system performs better than a bottom-mounted system at larger collector sizes.
SIMULATION This section describes the two photovoltaic systems discussed in this paper. An explanation of the functionality of a PBS filter follows. We model an FC consisting of an acrylic p
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