Thermal conductivity measurement techniques for characterizing thermal energy storage materials – A review

Researchers have sought for standards, methodologies and procedures to properly measure the thermal properties of Thermal Energy Storage (TES) materials. Among them, thermal conductivity plays a key role in the TES system design as it dictates the charging/discharging dynamics of a TES system. The lack of thermal conductivity measurement's standards led to a large extent of discrepancies in terms of measurement method, measurement equipment, and sample preparation as reported in this review. Such discrepancies create uncertainties in the thermal conductivity values, which results in misleading interpretation that will be later used by other researchers for comparison purposes. This is particularly important when thermal properties are strongly linked to complex nano and micro scale materials structure. This review paper was motivated by the need of filling the gap in the literature when assessing the thermal conductivity measurements used for TES media. To that end, steady-state methods and transient methods have been reviewed by its device (commercial or own-developed), sample size, accuracy, repetitions, measurements time, etc. The outcomes of this study are threefold: regarding TES media, not all three TES technologies have been studied at the same extent; just few papers have been reported on thermochemical storage, being phase change materials and sensible the most studied; Regarding thermal conductivity techniques, transient plane source, laser flash apparatus and transient hot wire are the most used, whereas modulated DSC and 3ω the least; And lastly, regarding the measurement procedure, the data is widely spread especially for accuracy, sample size and repetitions within the same material/technique. Overall, TES community have many challenges to face in the future to converge on a common agreement aiming comparable results among studies. That means optimized sample preparation and method procedure across the different techniques achieving high accuracies for the different TES materials.