Fundamental investigation into the design and operation of a high flux sodium receiver

Abstract

This thesis aims to answer the question: What fundamental information is required to allow successful design and operation of a high flux sodium billboard receiver? The thesis also presents a design and associated analysis for a high flux billboard receiver that is a possibility for being a cost effective alternative to the state-of-the-art CSP power tower technologies based on molten salt heat transfer fluids. The motivation behind the work is to help achieve a solar thermal generation system that can be competitive with conventional generation technologies. Concentrated solar thermal power plants using heliostat-tower technology are expected to provide a path for achieving large-scale deployment of electricity generators that use a renewable resource. However, as yet, current state of the art heliostat-tower systems, have not achieved cost levels that make them competitive with conventional generation technologies. In a heliostat-tower system, the costs largely derive from the heliostat field. Thus, system design must ensure that the heliostats can be a low cost design and that the system efficiency is maximised through better integration of the components. External tubular planar receivers (billboard receivers) using liquid metals provide a possibility for achieving these goals whilst maintaining design simplicity and a low risk design. To assist with the design and operation of billboard receivers this thesis investigates liquid sodium as a suitable heat transfer fluid, and compares its use as a heat transfer fluid to the current state of the art; a nitrate salt, commonly known as Solar Salt. The results show that high heat transfer rates achievable with liquid sodium can greatly simplify receiver geometry and allow for high flux intensities, increasing receiver efficiency. As the melting temperature of sodium is still above ambient temperatures, the risk of solidification of the sodium in the receiver remains an issue. To assist with receiver operation, this has been addressed through a validation of a methodology for the simulation of the melting and solidification of sodium in solar thermal components. To assist with design of billboard receivers a study of the receiver surface temperatures that result from an incident flux from the heliostat field has been undertaken for four different receiver concepts. From this work, a new concept is presented that best mitigates the issues of a non-uniform flux. The studies presented in this thesis provide insights into the design and operation of high flux sodium receivers, with the hope that the contributions will assist in the successful deployment of heliostat-tower technologies.