Thickening and gelling agents for formulation of thermal energy storage materials – a critical review

Thermal energy storage (TES) provides an effective approach for alleviating energy supply and energy demand mismatches, and utilizing renewable energy sources, excess off-peak electricity, and industrial waste energy. Thickening and gelling agents are additives for addressing the stability and shape stabilisation of TES materials, which have been and remain one of main challenges in TES technology deployment. This paper represents the first comprehensive review on thickening and gelling agents for TES applications. An insight is provided first regarding recent progress in this area, covering the classification and mechanisms of the thickening and gelling agents. The impact of the thickening and gelling agents on the physical properties of different fluids is then discussed to further address the role of thickening and gelling agents in the field of TES. A standard preparation methodology and associated evaluation process are then proposed for the establishment of a thickening and gelling agent database and the provision of a guidance for the screening and formulation of composite TES materials. Even though the thickening and gelling agents have a promising role in enhancing the TES system performance, further investigations are clearly needed to tackle the issues that limit their wide-spread applications, particularly cost and standardisation.