Modelling High-Temperature Thermal Storage Systems for Concentrated Solar Power Plants
Thermal energy storage (TES) is an important aspect of concentrated solar power (CSP) systems addressing the mismatch between electrical demand and supply profiles and intermittency in the solar resource. The design of TES components should be optimised from a system-level perspective to minimise the cost of electricity generation and maximise energy yield of a CSP system. This thesis presents models of two different TES concepts for high-temperature sodium-based CSP systems in the \textit{OpenModelica-SolarTherm} system modelling framework. The first TES concept modelled was a novel integration of PCM storage component with a sodium tubular boiler CSP receiver at a 1 MWe scale. The goal of the study was to determine the techno-economic feasibility of this novel concept, which combines the benefits of a high-temperature power cycle and a low-cost NaCl PCM with the drawbacks of a high-temperature receiver and increased cost of temperature-resistant infrastructure. A multi-objective optimisation of heliostat field, receiver and detailed storage geometry was performed to minimise the levelised cost of electricity (LCOE) and maximise capacity factor of the sodium-boiler system and a reference, conventional 100 MWe two-tank molten salt system. For a polar field with billboard sodium receiver configuration, an optimised LCOE of 128.3 USD/MWhe was obtained, which was higher than the 107.1 USD/MWhe obtained for the reference system. The sodium-boiler concept could still be promising for small scale, fringe-of-grid applications. The system-level analysis identified areas for improvement, namely receiver flux limits, receiver thermal losses and the high cost of temperature-resistant storage materials. The second TES concept modelled was a packed-bed storage tank with sodium fluid and MgO filler operated as a thermocline, this validated against experimental data. A detailed system model which integrates a dynamic thermocline TES model into a full annual simulation was developed with additional bypass and blending modes compared to prior literature. This allowed for the impact of different component-level design parameters and multi-tank control strategies to be quantified in terms of system-level LCOE of a CSP plant. The novelty of this work lies in the integration of a sodium-based thermocline storage array into a full annual system model along with the investigation of different multi-tank operating strategies. For a MgO sphere diameter of 10 cm, the energy utilisation of a 3-Tank thermocline storage array could be improved from 55.5 % to 85.0 % via the implementation of a mixed-outlet blending strategy. After the implementation of a reduced-order simplification method to reduce the computational time of annual simulations, a LCOE of 58.71 USD/MWhe was estimated for a 100 MWe CSP system. Further work includes implementing more aggressive heliostat aiming strategies and optimising more detailed component parameters to reduce the LCOE of sodium-based thermocline CSP systems to the 2030 SunShot target of 50 USD/MWhe.
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