05 Fakultät Informatik, Elektrotechnik und Informationstechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6

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Institute: search.filters.UBSInstitut.Institut für PhotovoltaikStart date: 2010End date: 2019Subject: search.filters.subject.621.3

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Now showing 1 - 4 of 4
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    Pressure characteristics and chemical potentials of constrained LiFePO4/C6 cells
    (2018) Singer, Jan Patrick; Kropp, Timo; Kuehnemund, Martin; Birke, Kai Peter
    Constraining lithium-ion cells increases the cyclic lifetime. However, depending on an expected volume expansion during charge and discharge cycling, defining the optimal constraining pressure range is not straightforward. In this study, we investigate a lithium iron phosphate/graphite pouch cell at four initial constraining pressure levels. As a function of C-Rate, the thermodynamic principle of the non-monotonic pressure curve during full charge and discharge cycles is evaluated. Using the rubber balloon model to calculate the chemical potential of lithium iron phosphate and discussing the relationship between the chemical potential and pressure, we illustrate the pressure curve qualitatively. By applying differential pressure analysis, we evaluate the resulting pressure curves of a single graphite stage. Approaching a fundamental understanding of reduced cycling lifetime of full cells with unknown material composition, we allocate the stages and stage transitions of graphite as well as the phase transition of lithium iron phosphate. Local extreme values in the differential pressure analysis indicate phase and stage transitions. These values can identify critical operating conditions that should be considered for defining the optimum initial constraining pressure range.
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    Quantifizierende Elektrolumineszenz für Silizium-Solarzellen und -module
    (2019) Kropp, Timo; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)
    Diese Arbeit präsentiert zwei neue Messmethoden auf Basis der Elektrolumineszenz zur Charakterisierung von Solarzellen und -modulen. Beide Methoden nutzen die Strominjektion, um ein Lumineszenzbild zu quantifizieren. Der Unterschied zwischen den Methoden besteht in der zeitlichen Variation der Strominjektion bzw. Stromextraktion. Bei der gepulsten Strominjektion sowie -extraktion hängt der zeitliche Verlauf der resultierenden Elektrolumineszenz von der effektiven Ladungsträgerlebensdauer in der untersuchten Solarzelle ab. Die eingeführte analytische Beschreibung der normierten periodischen Intensitätsdifferenz zwischen zwei unterschiedlich strommodulierten Lumineszenzbildern ist unabhängig von der Belichtungszeit der Bildaufnahme. Bei der zeitlich konstanten Strominjektion ist die Amplitude der Lumineszenzintensität zusätzlich durch den lokalen Serienwiderstand bzw. Parallelwiderstand einer Solarzelle bestimmt. Die zweite entwickelte Methode dieser Arbeit ist in der Lage, Leistungsverluste von Photovoltaikmodulen durch mechanische Defekte sowie potentialinduzierte Degradation anhand eines einzelnen Lumineszenzbildes quantitativ zu bewerten. Der durch einen Defekt hervorgerufene Leistungsverlust gegenüber der ursprünglich nach dem Datenblatt verfügbaren Leistung wird präzise vorhergesagt.
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    Measuring test bench with adjustable thermal connection of cells to their neighbors and a new model approach for parallel-connected cells
    (2019) Fill, Alexander; Mader, Tobias; Schmidt, Tobias; Llorente, Raphael; Birke, Kai Peter
    This article presents a test bench with variable temperature control of the individual cells connected in parallel. This allows to reconstruct arising temperature gradients in a battery module and to investigate their effects on the current distribution. The influence of additional contact resistances induced by the test bench is determined and minimized. The contact resistances are reduced from 𝑅Tab+=81.18 μΩ to 𝑅Tab+=55.15 μΩ at the positive respectively from 𝑅Tab-=35.59 μΩ to 𝑅Tab-=28.2 μΩ at the negative tab by mechanical and chemical treating. An increase of the contact resistance at the positive tab is prevented by air seal of the contact. The resistance of the load cable must not be arbitrarily small, as the cable is used as a shunt for current measurement. In order to investigate their impacts, measurements with two parallel-connected cells and different load cables with a resistance of 𝑅Cab+=0.3 mΩ, 𝑅Cab+=1.6 mΩ and 𝑅Cab+=4.35 mΩ are conducted. A shift to lower current differences with decreasing cable resistance but qualitatively the same dynamic of the current distribution is found. An extended dual polarization model is introduced, considering the current distribution within the cells as well as the additional resistances induced by the test bench. The model shows a high correspondence to measurements with two parallel-connected cells, with a Root Mean Square Deviation (RMSD) of 𝜉RMSD=0.083 A.
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    Fluorescent materials for silicon solar cells
    (2012) Prönneke, Liv; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)
    Photovoltaic systems with fluorescent collectors use the conversion and concentration of solar photons to increase solar cell efficiencies. Fluorescent dye in a dielectric plate absorbs incoming rays and emits spatially randomized photons with a lower energy range. The acrylic plate then guides part of the emitted spectrum to the collector side surfaces due to total internal reflection. Conventional research therefore applies solar cells to the side surfaces. This work analyzes the efficiency enhancement due to fluorescent collectors on top of solar cells which promises an easier technological handling. The first part of this work uses a Monte-Carlo simulation to model photovoltaic systems with fluorescent collectors and photonic structures. The results allow the comparison between side- and bottom-mounted solar cells. Examining the systems in the radiative limit achieves maximum theoretical limits. In each system, the photon collection probability depends strongly on the scaling of cell size and distance. The side-mounted solar cells perform better for larger scales, but for small scales bottom-mounted solar cells achieve equally high efficiencies. Consideration of non-radiative loss mechanisms and the application of a photonic structure also leads to the result that the application of solar cells to the collector back side needs careful scaling but performs as good as side-mounted solar cells. The second part presents the results of five experiments which analyze basic mechanisms in the fluorescent collector. Additionally, the experiments explore the benefits of fluorescent material in photovoltaic modules. i) The reabsorption experiment directs photons from an LED with wavelength 406 nm onto the collector top surface. A camera under the collector photographs photons which leave the back side. These photons are reabsorbed at least once. An analytical description extracts the reabsorption coefficient a = 0.021 1/mm from the camera picture. ii) Light beam induced current (LBIC) measurements on an amorphous silicon solar cell show that a fluorescent collector on top increases the collected current by 7%. The additional application of a photonic structure enhances the current by 95%. An analytical description of the absorption and emission processes in the collector using the reabsorption coefficient determined in the first experiment predicts the line-scans gained in the LBIC measurements. Therefore, the reabsorption measurement is sufficient enough to predict the collection performance of photovoltaic systems with fluorescent collectors without performing long LBIC-measurements. iii) Outdoor experiments compare mono crystalline silicon (c-Si) solar cells in acrylic troughs with and without fluorescent collectors on top. Fluorescent distribution added to the geometrical concentration decreases the current gain if limited to the trough aperture. A five times larger fluorescent collecting plate leads to a current gain enhancement by at least 50% compared to the limited aperture. This shows the advantage of fluorescent concentration. Achieving an increased current gain with geometrical concentration requires a new trough and more solar cell material. The experiments also show another advantage: Fluorescent collectors concentrate photons independent of their angle. Thus, photovoltaic systems using fluorescent concentration perform best even without tracking. iv) Two parallel connected c-Si solar cells under a fluorescent plate achieve an electrical output power P = 189 mW. The same set-up with an undoped acrylic plate on top gains P = 125 mW. By varying the cell distance this experiment additionally points out that the activation of surrounding photovoltaic inactive area is crucial to compensate losses directly above the solar cell. v) The last experiment avoids unfavorable losses by applying fluorescent dye to only the optical inactive cell connectors of an industrial c-Si solar cell encapsulated under glass. The fluorescent dye covering the white painted connector distributes incoming photons at all angles. The glass-air surface guides distributed photons onto the solar cell via total internal reflection. Derived with LBIC and Quantum Efficiency measurements, the efficiency of the solar cell increases from 16.0% to 16.2%. In conclusion, this work not only finds a new characterization method for the fluorescent concentration. Additionally, it presents that applying fluorescent dye on top of photovoltaic solar modules increase efficiencies under careful consideration of the scaling.