Optimization of central receiver concentrated solar thermal : site selection, heliostat layout & canting

In this thesis, two new models are introduced for the purposes of (i) locating sites in hillside terrain suitable for central receiver solar thermal plants and (ii) optimization of heliostat field layouts for any terrain. Additionally, optimization of heliostat canting, is presented as an application of the heliostat layout optimization model. Using the site selection model, suitable sites are located based on heliostat field efficiency and average annual insolation. By iteratively defining the receiver location and evaluating the corresponding site efficiency, by sampling elevation data points from within the defined heliostat field boundary, efficiency can be mapped as a function of the receiver location. The case studies presented illustrate the use of the tool for two field configurations, both with ground-level receivers. The heliostat layout optimization model includes a detailed calculation of the annual average optical efficiency accounting for projection losses, shading & blocking, aberration and atmospheric attenuation. The model is based on a discretization of the heliostats and can be viewed as ray tracing with a carefully selected distribution of rays. The prototype implementation is sufficiently fast to allow for field optimization. In addition, inspired by the spirals of the phyllotaxis disc pattern, a new biomimetic placement heuristic is described and evaluated which generates layouts of both higher efficiency and better ground coverage than radially staggered designs. Case studies demonstrate that the new pattern achieves a better trade-off between land area usage and efficiency, i.e., it can reduce the area requirement significantly for any desired efficiency. Finally, heliostat canting is considered. Traditionally, canting has been parabolic, in which the focal point of the heliostat lies on the axis of symmetry. Two alternative off-axis canting methods are compared in this thesis, fixed facet (static) canting in which the facet alignment is optimized for a single design day and time and then rigidly mounted to the frame and dynamic canting in which the facets are actively controlled such that the center of each facet is always perfectly focusing. For both methods, two case studies are considered, a power tower with planar heliostat field and a hillside heliostat field which directs light down to a ground-level salt pond. Thesis Supervisor: Alexander Mitsos Title: Rockwell International Assistant Professor

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