Experimental characterisation of heat transfer and energy storage performance in agitated microencapsulated phase change slurries

Thermal energy storage is a key step towards energy transition. To increase the energy density of thermal storage, the use of phase change materials is a very promising solution. Organic phase change materials, such as paraffin waxes, have poor thermal conductivity, resulting in low heat transfer rates or the need for special heat exchangers with complex geometry. Alternatively, the phase change material can be encapsulated in small particles dispersed in water (namely, microencapsulated phase change slurries). This approach allows the storage medium to retain its fluidity throughout all stages, enabling heat transfer by convection and the option to be mixed with a mechanical stirrer if required. The slurries can be integrated into conventional geometry storage tanks with helical coil immersed heat exchangers. However, the experimental data available in the literature that quantify the heat transfer capabilities in such configurations is very limited. We experimentally investigated the heat transfer and energy storage performance of a laboratory-scale tank using a microencapsulated phase change slurry as a medium, under natural convection and in the presence of mechanical agitation (mixed and forced convection). We showed that this storage solution offers a good compromise between energy storage and heat transfer, particularly when using agitation, which increased heat transfer coefficient up to 10 times with negligible mechanical power.