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Abstract
The two-tank molten salt thermal energy storage (TES) system in CSP plants is limited by cost, environmental, and operational issues. The literature on TES systems suggests a few promising alternatives, but several key questions remain unanswered. Thus, the present work aims to investigate what is the ‘best’ way forward for high temperature TES materials and systems? Attempting to answer this, however, opens up several other important questions: How can complicated underlying physical phenomena (phase change and natural convection) be incorporated into the design process? How do these systems techno-economically compare against each other? Answering these questions requires detailed annual integrated analysis. As a first step, it was found that alternative TES systems require alternative operational modes and control strategies, which motivated the author to develop appropriate off-design models for the receiver and the Rankine cycle. Additionally, it was noticed that full fundamental mathematical models for high temperature shell-and-tube TES systems were not available, so, this work expanded the literature by: (i) Determining the circumstances under which natural convection must be considered through proposing error correlations and (ii) Developing natural convection Nusselt correlations that can be used via the effective thermal conductivity method. After developing and validating these mathematical models, along with pulling other salient models from the literature, an integrated system model was developed for comparative analysis amongst all alternatives (including various sensible and phase change systems). It was shown that pushing the CSP plant to regions far from its design point, an ideal performance of two-tank systems with alternatives TES systems can be achieved with a minimal difference in terms of annual electricity generation. Economically, a maximum reduction of 60% compared to two-tank systems was obtained with a thermocline system with concrete filler in terms of the total TES specific cost, followed by ~50% reduction with pipeless shell-and-tube systems with concrete. Shifting from solid fillers to compact low cost PCMs can lead to ~60% reduction in shell-and-tube system with embedded pipes, but only a 40% reduction was achieved for a thermocline system. The least desirable system for future development was a single medium thermocline system with ~20% reduction capacity.