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Description
Power tower concentrated solar power (CSP) plants are capable of producing extremely high temperatures, as they have the ability to oversize their solar field and achieve a greater concentration ratio. This theoretically allows power towers to use more efficient, higher temperature cycles including air Brayton and supercritical Rankine cycles, as well as experimental cycles such as the supercritical CO2 cycle. As part of this thesis, the heat demand of each cycle, as well as the cycle diagram is examined for its suitability for use with CSP plants with thermal energy storage (TES). This will help develop criteria to determine if these cycles could be coupled with a higher temperature storage system. After the general cycle overview, this thesis describes the development, validation, and results of a TES system targeted for use with an air Brayton power cycle. This research analyzes the use of metal alloys as phase change storage materials and makes a case for their use with CSP plants. The numerical model developed here is intended to analyze the performance of high temperature systems suitable for thermodynamic cycles that do not have a commercially available storage system. The storage system essentially combines a heat exchanger and storage system to help realize potential savings over molten salt two-tank storage systems currently employed by industry. FLUENT computational fluid dynamics software is used to model the storage tank. After initial modeling of a hypereutectic binary alloy system it was discovered that FLUENT does not properly account for an individual material phase diagram. So, the thermal properties were altered in FLUENT in order to accurately reflect the thermal performance of the primary storage material chosen for study, AlSi. The model was also analyzed with pure metals and eutectic binary alloys in an effort to achieve stable air temperatures that could support an air Brayton turbine. These included both cascaded and non-cascaded geometries as well as reversed and non-reversed flow. The results of the cascaded models show that the heat exchanger/thermal storage concept is able to buffer the air entering the turbine, and produces relatively stable outlet air temperatures. It was found that using a single, pure metal as the storage material produced the highest, most stable air temperatures of the systems studied. With further optimization and research, this system could become a viable storage option for CSP plants in support of higher temperature cycles.
