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Description
Growing concerns about global warming has invoked a push for renewable energy technology. Power generation technology has progressed rapidly in the last few decades, specifically concentrating solar energy. To combat intermittency and facilitate wide adoption of concentrating solar power plants, thermal energy storage must be integrated into the system. In this thesis, a metal alloy thermal energy storage system (MATESS) was proposed. During this research, extensive numerical modeling was conducted to simulate the melting and freezing behavior of four aluminum-silicon alloys: one eutectic, and three offeutectic. The computational software used was Ansys Fluent. The lever rule and effective specific heat method were employed to model the latent heat in the off-eutectic alloys. The results produced by the software were validated through a series of benchmarks by comparison to the respective analytical solutions. The MATESS design and simulation was the main focus of this thesis. The full-scale system design consists of a cylindrical tank with concentric borehole pipes penetrating the tank lid. The pipes are surrounded with aluminum-silicon alloy, which acts as the latent heat storage medium. Supercritical carbon dioxide (sCO2) flows through the pipes to melt and freeze the alloy, enabling system coupling with a concentrating solar power plant and sCO2 Brayton cycle. The system was designed to store 200 MWh of thermal energy and deliver sCO2 at a constant temperature over a 10-hour period. Once the geometry was defined, a single borehole pipe was modeled in Fluent with each of the four alloys. The discharge behavior of each alloy and the mean sCO2 outlet temperature were observed. The offeutectic alloys delivered successively higher outlet temperatures as the Si content was increased, though they were initially not constant due to the mushy-zone latent heat release. The eutectic alloy was found to deliver fluid at a constant temperature for 19 hours and up to 37.9 hours for the off-eutectics. The off-eutectic alloys have more potential based on the higher achievable fluid outlet temperatures and more compact design. The geometry can be optimized for each alloy to reduce the tank dimensions, and consequently reduce the heat losses to the environment.
