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Helmers H.,Fraunhofer Institute for Solar Energy Systems | Bett A.W.,Fraunhofer Institute for Solar Energy Systems | Parisi J.,Carl von Ossietzky University | Agert C.,Research Center for Energy Technology
Progress in Photovoltaics: Research and Applications | Year: 2014

An energy balance model for concentrating photovoltaic and thermal (CPVT) systems is presented. In the model, the CPVT system and its environment are represented using a set of input parameters. The main outputs of the model are the system's electrical and thermal efficiencies. The model accounts for optical losses. Thermal losses are derived from a thermal network model of the hybrid receiver. The solar cell performance is modeled as a function of the temperature and the irradiance. The robustness of the model is demonstrated by a sensitivity analysis of all input parameters. The influence of the operating temperature on the electrical and thermal performances and the overall efficiency of the CPVT system are discussed. The limiting cases of maximum electrical and thermal power outputs are presented. Further, the influence of the concentration ratio on the electrical and thermal performance and on the partitioning of these two power outputs is analyzed in detail. It is shown that high concentration reduces the thermal losses considerably and increases the electrical efficiency. At concentration ratios above 300, the system operates with an overall efficiency of 75% at temperatures up to 160 °C. Copyright © 2012 John Wiley & Sons, Ltd. An energy balance model for concentrating photovoltaic and thermal systems is presented and applied to investigate the influence of operating temperature and concentration ratio on the hybrid performance, that is, the electrical and thermal efficiencies. It is shown that high concentration offers several advantages to a hybrid photovoltaic and thermal system. As the concentration increases, the electrical efficiency also increases and, at the same time, thermal losses are reduced significantly, enabling overall conversion efficiencies of 75% at operating temperatures up to 160 °C. © 2014 John Wiley & Sons, Ltd.


Derendorf K.,Fraunhofer Institute for Solar Energy Systems | Derendorf K.,Research Center for Energy Technology | Essig S.,Fraunhofer Institute for Solar Energy Systems | Oliva E.,Fraunhofer Institute for Solar Energy Systems | And 9 more authors.
IEEE Journal of Photovoltaics | Year: 2013

GaInP/GaAs//Si solar cells with three active p-n junctions were fabricated by surface activated direct wafer bonding between GaAs and Si. The direct wafer bond is performed at room temperature and leads to a conductive and transparent interface. This allows the fabrication of high-efficiency monolithic tandem solar cells with active junctions in both Si and the III-V materials. This technology overcomes earlier challenges of III-V and Si integration caused by the large difference in lattice constant and thermal expansion. Transmission electron microscopy revealed a 5-nm thin amorphous interface layer formed by the argon fast atom beam treatment before bonding. No further defects or voids are detected in the photoactive layers. First triple-junction solar cell devices on Si reached an efficiency of 23.6% under concentrated illumination. © 2011-2012 IEEE.


Das P.R.,Research Center for Energy Technology | Komsiyska L.,Research Center for Energy Technology | Osters O.,Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Synthetic Metals | Year: 2016

Rheological measurements were used to study the flow behavior of composite slurries consisting of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) polyelectrolyte complex as binder and LiFePO4 as a model active material for the preparation of positive electrodes for lithium ion batteries. Various aqueous slurries containing 92% LiFePO4 and 8% PEDOT:PSS with different solid loadings were prepared. All slurries showed solid-like behavior due to formation of a network structure of LiFePO4 bridged by PEDOT:PSS chains. However, the solid loading of the slurries influences the distribution of the agglomerates and the binder affecting also the thickness, the adhesion and the electrical conductivity of the coatings casted from the different slurries under the same conditions. Hence, the electrochemical performance of single cells prepared with these coating changed with the solid loading of the slurry. The optimum electrochemical performance is achieved with slurries containing 40% solid loading. © 2016 Elsevier B.V. All rights reserved.


Bohn P.,Audi AG | Barragan S.A.G.,Research Center for Energy Technology | Komsiyska L.,Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
Journal of the Electrochemical Society | Year: 2015

A new laboratory lithium ion cell in two electrode arrangement has been developed in order to apply in-situ short-term thermal stress tests to both electrodes. Cells were made from commercially available LiCoO2 cathodes, graphite anodes and electrolyte. A 60 s thermal stress was applied with different temperatures ranging from 100 to 250°C at the anode side after the cell formation and capacity tests. By comparison of the charge-discharge behavior of the cells before and after the thermal stress, capacity losses, increasing overvoltages and self-discharge have been observed as a function of the stress temperature. For detection of changes in the anode properties scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and computed tomography (CT) characterizations were used, and changes in the morphology and composition of the solid electrolyte interfaces (SEI) layer were observed. © 2015 The Author(s). All rights reserved.


Bohn P.,Audi AG | Garnica Barragan S.A.,Research Center for Energy Technology | Komsiyska L.,Research Center for Energy Technology | Wittstock G.,Carl von Ossietzky University
ECS Transactions | Year: 2014

A new laboratory lithium ion cell in two electrode arrangement has been developed in order to apply in-situ short-term thermal stress tests on both electrodes. Cells were made from commercially available LiCoO2 cathodes, graphite anodes and LP30 electrolyte. A 60 seconds thermal stress with different temperatures ranging from 100 to 250 °C has been performed at the anode side after the cell formation and capacity tests. By comparison of the charge-discharge behavior of the cells before and after the thermal stress, capacity losses, increasing overvoltages and self-discharge have been observed as a function of the stress temperature. For detection of changes in the anode properties SEM, TGA and CT characterizations were used, and changes in the morphology and composition of the SEI layer were observed. © The Electrochemical Society.

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