Calculation of the Shunt Resistance across the Absorber Layer of Hydrogenated Amorphous Silicon Photovoltaic Cells

In contrast to the existing conventional models, that describe the shunt resistance of hydrogenated amorphous silicon as a uniform resistance a cross the absorber layer (i-layer), our paper calculates the shunt resistance of a-Si:H PV cells as a function of the location across the i-layer resulting in a more detailed description of the shunt resistance. The photo-generation of the electron-hole pairs depend the photons' wavelength values and the potential across the PV cell. The shunt resistance exists because of the current leakage between the front-and back-contact layers within the i-layer. The electric current of the electrons and holes is calculated, in this paper, by solving the Poisson, continuity, and transport equations at each location within the i-layer for wide range potential values. In this article, the contribution of electrons and holes on the shunt leakage is calculated for each carrier independently by separating the current density/voltage (J/V) curves of the electrons and of the holes at each location within the i-layer. This work proves that the effective value of the location-dependent shunt resistances due to the electrons and holes equals the effective shunt resistance of the PV cell calculated from the total J/

Impact of distributed PV generation on relay coordination and power quality

Distributed generation (DG) has gained popularity among electricity end users who are determined to contribute to a cleaner environment by conforming to green and sustainable energy sources for various daily needs. The power system impact of such trends (e.g. roof-top solar-PV) need thorough investigation, such as impact on fault current levels on the distribution network. Varying fault current levels could adversely affect the operation of protection relays, which may lead to localized blackouts. Therefore, it is imperative to avoid localised blackouts due to mal-operation of protective relays under high penetration of DGs in distribution network. The focus of this research is to study the importance and implications of protective relays and over-current protection in the presence of distributed generation; where the impact of distributed generation on distribution network is identified. Relay coordination is observed to determine their operation characteristics to avoid mal-operation with the presence of DGs (e.g. solar-PV). This paper uses the UK generic distribution network model to simulate different scenarios in DIgSILENT Power Factory. The resulting power quality measures, such as voltage levels, short-circuit current levels and frequency are presented and discussed in the paper. The research highlights that small-scale DG penetration allows for existing protection infrastructure to continue operation and expensive upgrades for overall network are not required as fault levels remain the same

Improving the Efficiency of Partially Shaded Photovoltaic Modules without Bypass Diodes

Photovoltaic (PV) modules comprise bypass diodes to limit hotspot formation. However, they suffer from performance reduction in the presence of partial shading. This paper proposes external circuitry to control the connection type (series/parallel) of the PV cells through a pair of on/off switches resulting in three different operation modes. Mode 1 represents the typical 36 series-connected cells, while mode 2 represents two parallel-connected strings, and mode 3 maximizes the output current where the four strings are connected in parallel. The added values of the approach are that (1) the output current of the PV module can be increased without the need for a buck-boost converter and (2) the partial shading has less impact on the output power than the adoption of bypass diodes. This work shows that simulating three monocrystalline PV modules (120 W, 200 W, and 241 W), consisting of 36, 60, and 72 series-connected cells, lose about 74% when one cell has 80% shading in the absence of bypass diodes. The application of a bypass diode for each pair of strings in the PV module improves this decrease to 61.89%, 40.66%, and 39.47%, respectively. According to our proposed approach, this power loss can be significantly decreased to 19.59%, 50%, and 50.01% for the three PV modules, respectively, representing more than a 42% improvement compared to bypass diodes

Series connected photovoltaic cells-modelling and analysis

As solar energy costs continue to drop, the number of large-scale deployment projects increases, and the need for different analysis models for photovoltaic (PV) modules in both academia and industry rises. This paper proposes a modified equivalent-circuit model for PV modules. A PV module comprises several series-connected PV cells, to generate more electrical power, where each PV cell has an internal shunt resistance. Our proposed model simplifies the standard one-diode equivalent-circuit (SEC) model by removing the shunt resistance and including its effect on the diode part of the circuit, while retaining the original model accuracy. Our proposed equivalent circuit, called here a modified SEC (MSEC), has less number of circuit elements. All of the PV cells are assumed operating under the same ambient conditions where they share the same electric voltage and current values. To ensure the simplification did not come at a reduction in the accuracy of the SEC model, we validate our MSEC model by simulating both under the same conditions, calculate, and compare their current/voltage (I/V) characteristics. Our results validate the accuracy of our model with the difference between the two models falling below 1%. Therefore, the proposed model can be adopted as an alternative representation of the equivalent circuit for PV cells and modules

Improving the Efficiency of Partially Shaded Photovoltaic Modules without Bypass Diodes

Photovoltaic (PV) modules comprise bypass diodes to limit hotspot formation. However, they suffer from performance reduction in the presence of partial shading. This paper proposes external circuitry to control the connection type (series/parallel) of the PV cells through a pair of on/off switches resulting in three different operation modes. Mode 1 represents the typical 36 series-connected cells, while mode 2 represents two parallel-connected strings, and mode 3 maximizes the output current where the four strings are connected in parallel. The added values of the approach are that (1) the output current of the PV module can be increased without the need for a buck-boost converter and (2) the partial shading has less impact on the output power than the adoption of bypass diodes. This work shows that simulating three monocrystalline PV modules (120 W, 200 W, and 241 W), consisting of 36, 60, and 72 series-connected cells, lose about 74% when one cell has 80% shading in the absence of bypass diodes. The application of a bypass diode for each pair of strings in the PV module improves this decrease to 61.89%, 40.66%, and 39.47%, respectively. According to our proposed approach, this power loss can be significantly decreased to 19.59%, 50%, and 50.01% for the three PV modules, respectively, representing more than a 42% improvement compared to bypass diodes

Solarzellen auf der Basis von amorphem Silizium

This thesis focuses on the deposition of hydrogenated amorphous silicon (a-Si:H) films bymeans of plasma enhanced chemical vapour deposition (PECVD). This technique allows the growth of device quality a-Si:H at relatively low deposition temperatures, below 140 °C and, therefore, enables the use of low-cost substrates, e.g. plastic foils. The maximum efficiencies of a-Si:H solar cells in this work are η= 6.8 % at a deposition temperature Tdep = 180 °C and η = 4.9 % at a deposition temperature Tdep = 135 °C. Decreasing the deposition temperature deteriorates the structural and electronic quality of a-Si:H films. Therefore, the deposition conditions are carefully optimized at low temperatures. The mismatch in the mechanical properties of the plastic foils and the inorganic semiconductor layers have less effect on the a-Si:H films at low deposition temperatures. As a result, the deposition temperatures should be decreased to minimize mechanical deterioration of the films but without losing too much of the electronic properties of the films. A novel analytical description of the current density/voltage (J/V) characteristics of p-i-n solar cells well represents experimental J/V curves of a-Si:H solar cells. The extended model solves the continuity and transport equations for electrons and holes, and fully accounts for the contributions of the drift and the diffusion currents. Many analytical models neglect the contribution of the diffusion current in describing the a-Si:H solar cells. Other existing models assume the diffusion lengths of electrons and holes to be equal, resulting in a symmetric distribution of carrier concentrations around the center of the intrinsic layer of the p-i-n solar cells. Both restrictions strongly limit the ability of these analytical models to accurately reproduce the J/V-characteristics of real solar cells. In contrast to existing analytical models, the new analytical description solves the continuity and transport equations of carriers at each location within the i-layer for the whole range of applied voltages. The peculiar extension of this model over previous ones enables a more realistic description of solar cells. My novel analytical model implements i) different values of the diffusion lengths, or mobility-lifetime products, of electrons and holes, and ii) realistic wavelength and depth dependencies of the photogeneration rate of charge carriers. The results of the model demonstrate that the location of the main recombination path of the photogenerated carriers inside the i-layer is voltage dependent, rather than being fixed at the middle of the i-layer as existing models assume. For a realistic description of the solar cell optics in calculating the J/V-characteristics, I fully account for the reflection of photons at the back contact. The model proves that the performance of a-Si:H solar cells which are illuminated through the p-layer is better than the one of cells illuminated through the n-layer. Testing corresponding J/V-characteristics from this model against experimental data of bifacial a-Si:H solar cells with transparent front and backside contacts, reveals that this extended analytical model well describes the output characteristics of real a-Si:H p-i-n solar cells. The model proves that the current collection of bifacial p-i-n solar cells is larger if the light enters through the p-layer because the mobility μn of electrons is larger than the mobility μp of holes. This thesis also investigates the dependence of the electrical and optical properties of a-Si:H films on the deposition conditions, and how those properties are enhanced by optimizing the deposition conditions. I apply the optimized layers to solar cells deposited on glass and on polyethylene terephtalate (PET) substrates. The incorporation of a buffer layer or a microcrystalline layer enhances the performance of the cells.Die vorliegende Arbeit untersucht die Herstellung von Solarzellen aus wasserstoffhaltigem amorphen Silizium (a-Si:H) durch Plasma Enhanced Chemical Vapor Deposition (PECVD) bei Temperaturen unter 140 °C. Eine niedrige Herstellungstemperatur ermöglicht die Verwendung flexibler Kunststofffolien als Substrat, verschlechtert aber die elektronischen Eigenschaften der a-Si:H Schichten und Solarzellen. Daher müssen die Abscheideparameter bei niedrigen Temperaturen besonders sorgfältig optimiert werden. Bei niedrigen Herstellungstemperaturen wirken sich die Unterschiede zwischen den mechanischen Eigenschaften der Kunststoffsubstrate und denjenigen der anorganischen Halbleiterschichten weniger stark aus als bei hohen Substrattemperaturen. Insgesamt sucht die vorliegende Arbeit, einen Kompromiss zwischen der Absenkung der Abscheidetemperatur und der elektronischen Qualität der Schichten und Solarzellen zu erreichen. Ein wichtiger Teil dieser Arbeit stellt in Kapitel 3.1 eine neue analytische Beschreibung der Strom/Spannungs-Kennlinie von a-Si:H p-i-n Solarzellen vor. Dieses erstmals veröffentlichte Modell löst die Kontinuitäts- und Transportgleichungen für Elektronen und Löcher und berücksichtigt Feld- und Diffusionströme. Bisher bekannte analytische Modelle für p-i-n Solarzellen vernachlässigen den Beitrag der Diffusionströme, oder sie gehen von der Annahme aus, dass die Diffusionslängen von Elektronen und Löchern gleich sind, was zu einer symmetrischen Verteilung der Ladungsträgerkonzentrationen um die geometrische Mitte der intrinsischen Absorberschicht der p-i-n Dioden führt. Beide Einschränkungen beschreiben die Situation in p-i-nStrukturen, beispielsweise aus a-Si:H, nur unzureichend und grenzen daher die Fähigkeit der vorhandenen analytischen Modelle stark ein, J/V Kennlinien realer Solarzellen genau zu reproduzieren oder vorher zu sagen. Die wesentliche Erweiterung gegenüber den existierenden analytischen Modellen besteht darin, die Kontinuitäts- und Transportgleichungen der Träger für jeden Ort innerhalb der i- Schicht über den gesamten Bereich der angelegten Spannungen zu lösen. Diese Erweiterung ermöglicht eine realistische Beschreibung tatsächlicher Solarzellen, wie die Vergleiche mit experimentellen Daten in den Kapite-ln 3.1.3 belegen. Realistische Annahmen zur Anwendung der analytischen Modellierung berücksichtigen zum einen unterschiedliche Werte der Diffusionslängen von Elektronen und Löchern, und zum anderen die Energie- und Ortsabhängigkeit der Photogenerationsrate innerhalb der i- Schicht entlang der Einstrahlungsrichtung des einfallenden Lichtes. Die Ergebnisse der erweiterten analytischen Beschreibung demonstrieren, dass der Ort maximaler Ladungsträgerrekombination innerhalb der i- Schicht spannungsabhängig ist, anstatt einer Fixierung in der Mitte der i- Schicht wie in bisherigen Modellen. Um die optischen Eigenschaften der p-i-n Solarzellen aus a-Si:H realistisch zu beschreiben, berücksichtigt das neue Modell auch den Beitrag der Reflexion niederenergetischer Photonen am rückseitigen Kontakt. Die Modellierung bestätigt, dass die Leistung von amorphen Solarzellen, die durch die p-Schicht beleuchtet werden, besser ist als diejenige von Zellen, die durch die n-Schicht beleuchtet werden. Entsprechend modellierte J/V Kennlinien stimmen gut mit experimentellen Daten von a-Si:H Solarzellen mit durchsichtigem Front- und Rückkontakt überein. Der experimentelle Teil der Arbeit untersucht die Abhängigkeit der elektronischen und optischen Eigenschften von a-Si:H Filmen von den Abscheidebedingungen, und optimiert die Eigenschaften durch die Variation der Abscheideparameter. Die optimierten Schichten kommen in Solarzellen auf Glas sowie auf Polymerfolien aus Polyethylenterephtalat (PET) und Polyethylennaphtalene (PEN) zum Einsatz. Die Leistung der a-Si:H Solarzellen verbessert sich durch Einfügen einer Pufferschicht oder einer mikrokristallinen Dotierschicht und erreicht einen maximalen Wirkungsgrad η = 6.8 % bei einer Abscheidetemperatur Tdep = 180 °C und η = 5.9 % bei einer Abscheidetemperatur Tdep = 135 °C. Bei der übertragung von Abscheideparametern, die bei niedriger Temperatur auf starren Glassubstraten optimiert wurden, auf flexible Foliensubstrate müssen die Unterschiede in den mechanischen Eigenschaften der anorganischen Halbleiterfilme und der Polymerfolien berücksichtigt und untersucht werden. Die Wechselwirkung zwischen den amorphen Filmen und den flexiblen Folien erzeugt mechanische Spannungen und ein Aufrollen der Substraten. Je höher die Fehlanpassung zwischen den abgeschiedenen Filmen und dem Substrat ist, desto stärker rollt sich die flexible Solarzelle ein. Um dieses Aufrollen zu quantifizieren und zu reduzieren, variiert die Arbeit das Verhältnis der Dicken der abgeschiedenen a-Si:H Filme und der Substratfolien

Towards Contactless Learning Activities during Pandemics Using Autonomous Service Robots

The COVID-19 pandemic has had a significant impact worldwide, impacting schools, undergraduate, and graduate university education. More than half a million lives have been lost due to COVID-19. Moving towards contactless learning activities has become a research area due to the rapid advancement of technology, particularly in artificial intelligence and robotics. This paper proposes an autonomous service robot for handling multiple teaching assistant duties in the educational field to move towards contactless learning activities during pandemics. We use SLAM to map and navigate the environment to proctor an exam. We also propose a human–robot voice interaction and an academic content personalization algorithm. Our results show that our robot can navigate the environment to proctor students avoiding any static or dynamic obstacles. Our cheating detection system obtained a testing accuracy of 86.85%. Our image-based exam paper scanning system can scan, extract, and process exams with high accuracy

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Tag der mündlichen Prüfung: 29.01.200