Short-chained ZnO nanostructures intensify heat transfer in d-mannitol based thermal energy storage systems

Abstract

Nanostructures as additives can enhance the performance of thermal energy storage systems that employ organic phase change materials. A composite (ZnO-Mannitol) containing short-chained zinc oxide nanostructures dispersed in d-Mannitol has been prepared with the concentration of dispersed phase ranging between 0.5 and 5 wt%. The composites were characterized for morphology, dispersed phase-Mannitol interactions and the latent heat of phase transition. The performance of the composites was compared with that of d-Mannitol during the discharge cycle by measuring the overall heat transfer coefficient for latent heat and solid phase sensible heat release under identical test conditions. With the parameters of planetary milling employed for the preparation of composites, those containing ZnO nanostructures at 0.5 wt% and 1 wt% reduced the degree of supercooling by 6.3 °C and 6.5 °C, respectively. The presence of short-chained nanostructures as clustered networks, heterogeneous nucleation and possible elevation in thermal conductivity enhanced the discharge rate for all the compositions of the composites investigated. This work clearly demonstrates 88.4 % and 98 % relative increase in the overall heat transfer coefficient for 1 wt% and 2 wt% ZnO-Mannitol composite, when discharged through a vertical cylindrical surface, in contact with a well-stirred liquid maintained at a constant temperature.