Medium Voltage DC-DC Converter for Power Collection Links

Nowadays electrical energy generation is an important topic discussed in the society. Since technologies like wind or solar energy generation are most of the time placed far away from the city centres one point to focus on is the transmission of the energy opening opportunities for DC-distribution within the energy distribution. Focussing on solar plants as application it became obvious that to achieve high power levels a large surface area is used. Covering such a large area, where the energy is generated, means that the power must be collected over long distances, which leads to a low efficiency if the transmission is done under low voltage conditions. A better efficiency could be expected by increasing the voltage to a medium voltage level using a DC-DC converter. The introduced converter is based on the current diverter topology, which is used to equalize the voltage between different cells. To use this topology as a step-up converter a boost stage was added as input stage forming the Multistage Stacked Boost Architecture. To ensure that this topology would be a possible solution for the before mentioned challenges calculations were done to verify a satisfying efficiency of the converter. In addition an Energetic Macroscopic Representation was done and a control was designed. This control uses two cascaded controllers in each stage with feedback of the load current to generate the PWM. Since that control relies on the measurement of nine state variables, it was considered important to introduce a second control. The first step was to introduce a State-Space control, which is based on the description of system with n first-order differential equations. Then the research continued into a control strategy using an observer. The observer is basically a copy of the state space system; it has the same input and almost the same differential equations. An extra term compares the actual measured output to the estimated output; this will cause the estimated states to approach the values of the actual states. To verify simulation results and the working of the proposed controls a low voltage prototype was built. The measurements that were conducted all included normal steady-state operation as well as test to verify the correct functioning during perturbations such as variations of the load. All the measurement showed that the controls feasible. In addition, there were only very small resonant phenomena that are of no consequence for the correct functioning of the converter. To conclude, the work that has been done for this thesis included considering applications for medium voltage DC-DC converters. In addition, the problems of the approach of having huge solar plants use a low voltage distribution inside were shown. A converter that could be used to elevate said voltage to a medium voltage level was introduced - The Multistage Stacked Boost Architecture. This topology was then investigated in detail and calculations for the efficiency of the topology were conducted. Three different dedicated controls were investigated. A control based on the feed forwarding of the output current, a state-space control and an observer control. All control schemes were first tested with simulations and then verified on a low voltage prototype. The measurement results obtained with the different controls proved satisfactory, showing that the Multistage Stacked Boost Architecture is a reliable and interesting topology.

Assessment of Solar Thermal Power Technologies

James Spelling

In the present climate of growing awareness about the dangers of global climate change, the development of new renewable energy technologies is primordial in the efforts to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign fuel sources. Amid the plethora of option available, one of the more promising solutions is the increased use of solar power. Two main contrasting technologies exist for the conversion of the Sun’s energy into useful work, namely photovoltaic cells and solar thermal systems. The development of photovoltaic power has shown massive growth in recent years despite high costs, due mainly to the modular nature of the cells which allow their installation by private citizens, coupled with a number of government incentives. Development of solar thermal power generation has however been reasonably limited until recently, with the only continually operated commercial plants being the Luz SEGS I-IX plants at Kramer Junction in the United Sates. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, notably with the construction of the PS10 central receiver plant in Spain. The Spanish government plans to bring the total solar generation capacity to a total of 300 MW by 2013. In other countries around the world, interest is growing in solar thermal power, with increased government and private spending fuelling research and innovation. Most solar thermal conversion technologies are still at the experimental stage, and no single technology has yet to impose its dominance. Different countries and research groups hold different solutions dear, but if solar thermal power is to become a fully mature electricity generating technology, the best of these diverse solutions must be combined in order to achieve optimal performance of the power plants.

2008

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

As our awareness of the dangers of global climate change grows, the development of new renewable energy technologies is of primary importance in the effort to reduce emissions of carbon dioxide and other greenhouse gases. Rapid increases in the price of oil and worries about the stability and security of the extraction of fossil fuels have lead to renewed interest in the development of local energy sources, thereby reducing dependence on foreign sources. Amid the plethora of option available, one of the more promising solutions is the increased use of concentrating solar thermal power. Solar thermal power generation shows great potential for supplying the base electricity needs of numerous countries in the sun-belt regions, stretching from the tropics to the Mediterranean. This has lead to a recent renewal in the development of such systems, most notably with the construction of the PS10 and PS20 central receiver power plants in Spain. However, all currently existing solar thermal power plants have been based around the use of Rankine or steam turbine cycles, which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. Recent developments in the field of high temperature solar receivers [13] have made it possible to imagine the use of solar thermal energy to drive gas turbine units, potentially allowing the use of combined cycle setups in solar thermal power plants. In order to evaluate the potential improvement in efficiency that can be achieved by using combined cycles in solar thermal power plants, a detailed simulation of such a setup will be performed. By taking into account the requirements of thermal energy storage and the design of the necessary heat exchangers, an objective evaluation of the performance of the combined cycle can be obtained. As a key element in the energy conversion process, particular attention will be given to the modelling and simulation of the high temperature receiver. Due to the highly variable nature of the solar flux, the performance of the receiver will be evaluated over a range of operating points. In order to obtain a detailed breakdown of the sources of losses and irreversibilities within the system, a complete exergy balance will be performed on the power plant. Combined with the standard energy balance, this analysis will allow identification of the key areas for improvement in the design of solar thermal power plants. Finally, in order to determine whether such a setup is economically viable, the construction, operation and maintenance costs will be evaluated in order to predict the levelised energy cost associated with electricity production in a combined cycle plant.

2008

Life Cycle Analysis and Thermo-Environomic Optimization of Concentrated Solar Thermal Power Plants

Aurélie Kuenlin

Confronted to resources depletion and the global warming, the humanity has to reduce its fossil fuels dependence. The renewable energy is an interesting option to reach this aim. The most powerful one is the solar energy since in few hours, the Earth receives more energy than the humanity consumes in one year. In order to convert this energy to electricity, the concentrated solar power plants (CSP) are more advantageous than photovoltaics (PV) because of the possibility to store the energy. Indeed, they concentrate with numerous mirrors the sunlight to heat up a ﬂuid which can be stored in a big insulated tank. Then, the heat energy is transferred to a steam which drives a turbine and a generator to produce electricity. Nowadays, this type of power plants has a real potential. The aim of this work is to test and conﬁrm that the CSP engender less environmental impacts than other types of power plants over their entire life cycle including production, operation and end of life. Furthermore, four different technologies of CSP (tower, parabolic trough, Fresnel and dish) are compared in order to ﬁnd the most efﬁcient one for the environment and identify their strong and weak points. Finally, multi-objectives optimizations, which consider economic and thermodynamic aspects, are performed to ﬁnd the optimal conﬁguration for each power plant. In this work, the environmental impacts are calculated by carrying out a life cycle analysis (LCA) with the Impact 2002+ method. It takes into account the damages on resources, climate change, human health and ecosystem quality. According to this study, the results indicate that CSP produce lightly less impacts than PV but much less impacts than fossil fuels energy. The comparisons between the different CSP reveal that the dish technology engenders less damages than other due to its high efﬁciency and less facilities. However, its investment costs are too important to be competitive. From the other hand, Fresnel and parabolic technologies produce too much environmental impacts for lower costs. The weak point of Fresnel is its very low efﬁciency and the disadvantage of parabolic is the use of a speciﬁc synthetic oil which is very harmful for the environment, without speaking of the possible spills. Moreover, the hybridation with natural gas of both technologies reduces their costs but increases considerably the impacts on climate change and resources categories. Finally, the tower technology is the most advantageous CSP regarding the impacts, the electricity production and costs. It does not need hybridation to have low costs and have an important storage capacity to produce electricity during nights and cloudy days. However, the investment is still too important to be more attractive than fossil fuel power plants. Therefore, there are two possibilities: to implement a CO2 tax of around 60$/MWh or to improve CSP technology. For the last option, the steel of the mirrors structure, which is the most harmful material according to LCA results, can be reduced and thus, the impacts and the costs will be lower. It can also be possible to use recycled steel or another material. Another hint is to build CSP in a very sunny regions with a DNI of at least 1750 kWh/m2 /yr with short transmission lines. Finally, it is important to increase as much as possible the efﬁciency of CSP to produce more energy and decrease the environmental impacts too.

2011

Assessment of Solar Thermal Power Technologies

James Spelling

2008

Evaluation of a Combined Cycle Setup for Solar Tower Power Plants

James Spelling

2008

Life Cycle Analysis and Thermo-Environomic Optimization of Concentrated Solar Thermal Power Plants

Aurélie Kuenlin

2011