AbstractsPhysics

As fossil fuel reserves are rapidly being depleted, sustainable alternatives have to be found to fulfil the world's energy demand. Numerous concepts have been proposed to generate electricity by harnessing renewable energy sources such as solar or wind power. One of these concepts is the the so-called solar updraft tower (SUT). The SUT consists of three elements: a solar air collector, wind turbines and a chimney. The taller the chimney, the larger the stack effect and thus the more energy which can be generated by the turbines. The proposed concepts for this chimney schematise it as a reinforced concrete cylindrical shell, with the bottom half shaped like a hyperboloid and the top half as a flared cylinder, outfitted with ten stiffening rings evenly distributed over the height. Chimneys as tall as 1500m have been proposed, and, previous research shows that these tall structures have very low eigenfrequencies which come very close to the peak of the wind power spectrum. This makes them extremely vulnerable to resonance induced by storm actions. Two types of resonance can be distinguished in these structures; along-wind resonance, and across-wind resonance. Along-wind resonance is caused by turbulence in along-wind gusts. The second type, across-wind resonance, is caused by the alternating shedding of vortices. This leads to pulsating excitation forces in the across-wind direction, and, if the frequency of the vortex shedding is the same as one of the eigenfrequencies of the chimney, resonance will occur. In this thesis, a finite element model is created based on a pre-existing design. This so-called base model is then analysed to determine which key problem areas could benefit from improvement. The analyses show that especially the first two eigenfrequencies are critical for along-wind resonance as well as across-wind resonance. These eigenfrequencies are seen as two individual problem areas as improvements to one eigenfrequency not necessarily guarantee improvements to the second eigenfrequency. Furthermore, tension on the windward side leads to cracks in the stiffening rings which negatively influence the eigenfrequencies and thus the dynamic response. The last area which could benefit from optimisation is the cost of the chimney; an optimal solution does not use more material than necessary. A design tool called SUMAT (Solar Updraft Modal Analysis Tool) is created which enables the user to analyse multiple chimney configurations at once, subsequently being able to compare their results. Various sensitivity analyses are carried out to determine the influence of geometric and material parameters on the four key problem areas of the chimney. A multi-objective optimisation process is followed to optimise each of the key problem areas, ie. objective functions, by hand. The first step in optimising the structure is to subdivide the parameters which were researched into four categories, depending on their usefulness. The second step consists of gradually introducing these parameter changes into the base model. The…