Experimental performance characterization of a thermal energy storage system using gallium
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[EMBARGOED UNTIL 12/1/2023] Experimental study of a noble thermal energy storage (TES) method is performed to provide insight towards the practical implementation of such a method. The method includes the use of gallium as phase change material (PCM) for latent TES, and the alteration of the temperature condition at the interface between the heat transfer fluid (HTF) and PCM in a concentric double-pipe heat exchanger. Gallium, stored in the outer pipe of the heat exchanger, is a low melting-temperature metal and a good candidate for use as PCM due to its thermophysical properties, but its molecular structure and anomalous phase change behavior have delayed its widespread application. Initially expected to provide a uniform temperature condition at the HTF-PCM interface, periodically reversing the direction of HTF flow in the center pipe of the heat exchanger has highlighted the effects of the phenomena associated with gallium, which would have otherwise been unnoticed with fixed unidirectional flow. Analysis of heat transfer rate and total stored energy indicates a bias in the TES performance induced by the gallium. The bias is towards a particular flow direction of HTF, therefore affecting the performance of the TES system and creating inconsistent results. By gaining some understanding of how the bias in the gallium changes during the freezing process, a procedure was developed to achieve repeatable results with the bias, and a parametric study was performed to characterize the melting performance of the TES system with the bias. The effects of HTF inlet velocity, HTF inlet temperature, and flow reciprocation period on the TES performance were analyzed for both unidirectional and reciprocating flow. The TES system performance with unidirectional flow is largely dependent on flow direction, therefore highlighting the advantage of reciprocating flow which provides more consistent results with gallium as PCM.