• Energy Research
• DE close
• AT close
• PL close
• RWTH Aachen University close

Abstract The lifetime and safety of lithium-ion batteries are key requirements for successful market introduction of electro mobility. Especially charging at low temperature and fast charging, known to provoke lithium plating, is an important issue for automotive engineers. Lithium plating, leading both to ageing as well as safety risks, is known to play a crucial role in system design of the application. To gain knowledge of different influence factors on lithium plating, low-temperature ageing tests are performed in this work. Commercial lithium-ion batteries of various types are tested under various operational conditions such as temperature, current, state of charge, charging strategy as well as state of health. To analyse the ageing behaviour, capacity fade and resistance increase are tracked over lifetime. The results of this large experimental survey on lithium plating provide support for the design of operation strategies for the implementation in battery management systems. To further investigate the underlying degradation mechanisms, differential voltage curves and impedance spectra are analysed and a post-mortem analysis of anode degradation is performed for a selected technology. The results confirm the deposition of metallic lithium or lithium compounds in the porous structure and suggest a strongly inhomogeneous deposition over the electrode thickness with a dense deposition layer close to the separator for the considered cell. It is shown that this inhomogeneous deposition can even lead to loss of active material. The plurality of the investigated technologies demonstrates large differences between different technologies concerning low-temperature behaviour and gives insight to the impact of cell properties. For the sample of cells considered in this work, cells rated to provide high power are found to be subject to faster degradation at low temperatures compared to high-energy cells, probably due to little self-heating. For application this result shows that cells designed for high current rates are not necessarily providing a good low-temperature performance.

Abstract The lifetime and safety of lithium-ion batteries are key requirements for successful market introduction of electro mobility. Especially charging at low temperature and fast charging, known to provoke lithium plating, is an important issue for automotive engineers. Lithium plating, leading both to ageing as well as safety risks, is known to play a crucial role in system design of the application. To gain knowledge of different influence factors on lithium plating, low-temperature ageing tests are performed in this work. Commercial lithium-ion batteries of various types are tested under various operational conditions such as temperature, current, state of charge, charging strategy as well as state of health. To analyse the ageing behaviour, capacity fade and resistance increase are tracked over lifetime. The results of this large experimental survey on lithium plating provide support for the design of operation strategies for the implementation in battery management systems. To further investigate the underlying degradation mechanisms, differential voltage curves and impedance spectra are analysed and a post-mortem analysis of anode degradation is performed for a selected technology. The results confirm the deposition of metallic lithium or lithium compounds in the porous structure and suggest a strongly inhomogeneous deposition over the electrode thickness with a dense deposition layer close to the separator for the considered cell. It is shown that this inhomogeneous deposition can even lead to loss of active material. The plurality of the investigated technologies demonstrates large differences between different technologies concerning low-temperature behaviour and gives insight to the impact of cell properties. For the sample of cells considered in this work, cells rated to provide high power are found to be subject to faster degradation at low temperatures compared to high-energy cells, probably due to little self-heating. For application this result shows that cells designed for high current rates are not necessarily providing a good low-temperature performance.

Isolating the faults in low-voltage DC grids remains a major technical obstacle in designing the protection system for LVDC grids [1]. Conventional mechanical circuit breakers need to be connected in series to make them compatible for DC grids and they are still not fast enough to disconnect the DC faults with fast rising DC short-circuit currents. In this paper, performance of a hybrid DC circuit breaker topology using a counter-current injection method for producing current-zero switching [2] is analyzed. The topology allows use of a mechanical circuit breaker assisted with power electronic based auxiliary circuit for the switching of DC currents at minimal arcing. A thyristor is used instead of mechanical circuit breaker to enforce switching at current-zero crossing. The circuit breaker topology is simulated in Matlab/Simulink and implemented in hardware for 380Vdc networks. The study analyzes the effect of different circuit elements on the maximum interruption capability and turn-off time of the circuit breaker.

Isolating the faults in low-voltage DC grids remains a major technical obstacle in designing the protection system for LVDC grids [1]. Conventional mechanical circuit breakers need to be connected in series to make them compatible for DC grids and they are still not fast enough to disconnect the DC faults with fast rising DC short-circuit currents. In this paper, performance of a hybrid DC circuit breaker topology using a counter-current injection method for producing current-zero switching [2] is analyzed. The topology allows use of a mechanical circuit breaker assisted with power electronic based auxiliary circuit for the switching of DC currents at minimal arcing. A thyristor is used instead of mechanical circuit breaker to enforce switching at current-zero crossing. The circuit breaker topology is simulated in Matlab/Simulink and implemented in hardware for 380Vdc networks. The study analyzes the effect of different circuit elements on the maximum interruption capability and turn-off time of the circuit breaker.

Engineering and technology-based solutions can address the global challenges associated with sustainable development. In this context, engineers have a substantial responsibility in achieving the Sustainable Development Goals (SDGs). Meeting the challenges of all SDGs influences economic, political and social aspects of human life. However, engineering students’ understanding of sustainability is often limited to its ecological and economic dimension, not taking into account or even neglecting social issues. Therefore, teaching approaches in engineering education should address the different dimensions of sustainability and the responsibility of technological development concerning society.

Engineering and technology-based solutions can address the global challenges associated with sustainable development. In this context, engineers have a substantial responsibility in achieving the Sustainable Development Goals (SDGs). Meeting the challenges of all SDGs influences economic, political and social aspects of human life. However, engineering students’ understanding of sustainability is often limited to its ecological and economic dimension, not taking into account or even neglecting social issues. Therefore, teaching approaches in engineering education should address the different dimensions of sustainability and the responsibility of technological development concerning society.

Solar energy has the potential to provide clean and sustainable energy to all of humankind and during the past years the amount of operating solar power plants constantly increased. The increasing installation and production volumes call for powerful measuring techniques for the process control of solar cell production lines, but also for the monitoring of modules operating in solar power plants. Luminescence imaging of solar cells and modules is such a measuring technique. By observing the radiative recombination that is emitted by a solar cell, just like the light emission of LEDs, luminescence imaging provides spatially resolved information about a solar cells electrical and optical properties. Therefore, luminescence imaging has the ability to locate and rate defects and inhomogeneities in solar cells. This thesis focuses on the use of luminescence imaging for quantitative evaluations. Hence, it is not only looked at the ability of luminescence imaging to locate abnormalities in solar cells or modules but also on possibilities to use luminescence imaging as a way to quantify the strength of a defect or to estimate the influence of a defect on the photovoltaic performance of a whole device. During this work it was made use of two model solar cell technologies. Crystalline silicon solar cells are currently the most successful solar cell technology for which luminescence imaging is already a well established tool. This technology is primarily used in this thesis to test newly developed imaging methods. However, the focus of this thesis lies on the analysis of the so called CIGS solar cells and modules, which belong to the thin-film technologies. Although the market share of the thin-film technologies was only 5 % of the produced solar cells in 2016 their production volumes are constantly increasing. To allow for a quantitative evaluation of luminescence images of CIGS solar cells it is first essential to understand the influence of metastable effects in this technology. Metastable effects alter the properties of CIGS solar cells, including the luminescence signal, during the exciation with illumination or the application of bias. An in depth analysis of the metastable effects at different applied currents and temperatures showed that the metastable effects lead in most cases to a reduction of the series resistance and dark recombination current in CIGS solar cells. The changes vary in magnitude for the different conditions and may happen within a matter of seconds. However, a stabilization of the solar cells can only be reached after a constant excitation of several hours. This knowledge is essential for an accurate quantitative evaluation of luminescence images. In this work, an influence of metastable effects on the results were avoided by automation and combining image data only with electrical data measured simultaneously. In the following a quantitative luminescence method is analyzed in detail that allows to determine the current/junction voltage characteristic of individual cells already connected within a module. The characteristics allow in turn the quantification of defects, like shunts, in the individual cells. The current/junction voltage characteristics obtained by imaging unexpectedly vary depending upon the illumination conditions at which they are measured. The difference is explainable by an error resulting from an averaging procedure of measured local junction voltages. This procedure does not yield an accurate measure for the voltage over a cell. As a second quantitative evaluation method, the so called photocurrent collection efficiency imaging method is analyzed. The method yields spatially resolved information about the ability of a solar cells to collect photocurrent and was experimentally demonstrated and verified on a crystalline silicon solar cell. The influence of parameters, like series resistances and shunts, on the photocurrent collection efficiency is extensively discussed and demonstrated with the help of simulations. Furthermore, the original method, which only yielded differential information is extended to allow a determination of the total amount of photocurrent collected from a certain area of a solar cell. This total photocurrent collection efficiency is closer related to the actual influence a certain area has on the performance of a solar cell. The new method was also verified experimentally with a crystalline silicon solar cell. On solar modules the photocurrent collection efficiency behaves different to single cells due to the series connection of cells. For example, defected cells may show a larger photocurrent collection efficency. The effects are demonstrated with CIGS mini-modules and explained using simulations. At low voltages the photocurrent collection efficiency imaging could not be verified for CIGS solar cells. It is shown that CIGS solar cells exhibit an injection dependent series resistance that increases at low voltages. This injection dependent series resistance could also be reproduced in device simulations and is resulting from the minority charge carrier transport through the CIGS absorber. An equivalent circuit model including an injection dependent series resistance is in the following used to explain the observed deviations in the photocurrent collection efficiency imaging of CIGS solar cells. Finally, it is discussed how this model could also be interesting to describe voltage dependent photocurrent that is often observed in many kinds of new solar cells technologies.

Solar energy has the potential to provide clean and sustainable energy to all of humankind and during the past years the amount of operating solar power plants constantly increased. The increasing installation and production volumes call for powerful measuring techniques for the process control of solar cell production lines, but also for the monitoring of modules operating in solar power plants. Luminescence imaging of solar cells and modules is such a measuring technique. By observing the radiative recombination that is emitted by a solar cell, just like the light emission of LEDs, luminescence imaging provides spatially resolved information about a solar cells electrical and optical properties. Therefore, luminescence imaging has the ability to locate and rate defects and inhomogeneities in solar cells. This thesis focuses on the use of luminescence imaging for quantitative evaluations. Hence, it is not only looked at the ability of luminescence imaging to locate abnormalities in solar cells or modules but also on possibilities to use luminescence imaging as a way to quantify the strength of a defect or to estimate the influence of a defect on the photovoltaic performance of a whole device. During this work it was made use of two model solar cell technologies. Crystalline silicon solar cells are currently the most successful solar cell technology for which luminescence imaging is already a well established tool. This technology is primarily used in this thesis to test newly developed imaging methods. However, the focus of this thesis lies on the analysis of the so called CIGS solar cells and modules, which belong to the thin-film technologies. Although the market share of the thin-film technologies was only 5 % of the produced solar cells in 2016 their production volumes are constantly increasing. To allow for a quantitative evaluation of luminescence images of CIGS solar cells it is first essential to understand the influence of metastable effects in this technology. Metastable effects alter the properties of CIGS solar cells, including the luminescence signal, during the exciation with illumination or the application of bias. An in depth analysis of the metastable effects at different applied currents and temperatures showed that the metastable effects lead in most cases to a reduction of the series resistance and dark recombination current in CIGS solar cells. The changes vary in magnitude for the different conditions and may happen within a matter of seconds. However, a stabilization of the solar cells can only be reached after a constant excitation of several hours. This knowledge is essential for an accurate quantitative evaluation of luminescence images. In this work, an influence of metastable effects on the results were avoided by automation and combining image data only with electrical data measured simultaneously. In the following a quantitative luminescence method is analyzed in detail that allows to determine the current/junction voltage characteristic of individual cells already connected within a module. The characteristics allow in turn the quantification of defects, like shunts, in the individual cells. The current/junction voltage characteristics obtained by imaging unexpectedly vary depending upon the illumination conditions at which they are measured. The difference is explainable by an error resulting from an averaging procedure of measured local junction voltages. This procedure does not yield an accurate measure for the voltage over a cell. As a second quantitative evaluation method, the so called photocurrent collection efficiency imaging method is analyzed. The method yields spatially resolved information about the ability of a solar cells to collect photocurrent and was experimentally demonstrated and verified on a crystalline silicon solar cell. The influence of parameters, like series resistances and shunts, on the photocurrent collection efficiency is extensively discussed and demonstrated with the help of simulations. Furthermore, the original method, which only yielded differential information is extended to allow a determination of the total amount of photocurrent collected from a certain area of a solar cell. This total photocurrent collection efficiency is closer related to the actual influence a certain area has on the performance of a solar cell. The new method was also verified experimentally with a crystalline silicon solar cell. On solar modules the photocurrent collection efficiency behaves different to single cells due to the series connection of cells. For example, defected cells may show a larger photocurrent collection efficency. The effects are demonstrated with CIGS mini-modules and explained using simulations. At low voltages the photocurrent collection efficiency imaging could not be verified for CIGS solar cells. It is shown that CIGS solar cells exhibit an injection dependent series resistance that increases at low voltages. This injection dependent series resistance could also be reproduced in device simulations and is resulting from the minority charge carrier transport through the CIGS absorber. An equivalent circuit model including an injection dependent series resistance is in the following used to explain the observed deviations in the photocurrent collection efficiency imaging of CIGS solar cells. Finally, it is discussed how this model could also be interesting to describe voltage dependent photocurrent that is often observed in many kinds of new solar cells technologies.

Abstract Greenhouse gas emissions from the transportation sector could be reduced by using biofuels. To avoid competition with the food chain, second generation biofuels produced from lignocellulosic biomass are of major interest. The interaction of second generation biofuels and water needs to be studied since water is known to have major impacts for first generation biofuels. In this work, we explore the water solubility and phase stability for second generation biofuels from catalytic conversion of biomass. Tetrahydrofurans are potential biofuels for compression ignition engines. We find that the water solubility in tetrahydrofurans is increased by a factor 200 compared to Diesel. In furans, suitable as fuels for spark ignition engines, the water solubility is 4 orders of magnitude larger than in gasoline. In blends of biofuels, water solubility can either be increased or decreased depending on the blend component. Water can strongly influence the miscibility of biofuel-blends: adding small amounts of water to a γ-valerolactone – di- n -butyl ether blend leads to phase separation resulting in two organic phases with different combustion behavior. At the same time, the biofuels studied dissolve much better in water, which is relevant for potential environmental impacts. The phase behavior with water is thus shown to be an important key performance indicator for the development of biofuels.