Heat transfer enhancement of latent heat thermal energy storage with nanoparticle by bubble-driven flow

This study investigated the heat transfer enhancement of phase change material (PCM) with nanoparticles by bubble-driven flow. The addition of nanoparticles can improve conduction heat transfer, while bubble-driven flow can improve convection heat transfer. The 5 wt% nanoparticles increased the thermal conductivity by 6.4 % and viscosity 205 % of phase change material, while decreased the convective flow accordingly. However, bubble-driven flow strengthened the flow velocity by 8.3 times. Since nanoparticles tend to sediment over time in PCM, this issue of sedimentation should be overcome before they are applied in practice. The particle concentration in the vertical direction was analyzed by laser transmission experiment, and as expected, the concentration was the highest at the bottom. The bubble-driven flow uniformized the particle concentration in the vertical direction, which could solve the sedimentation problem. The combined application of nanoparticles and bubble-driven flow to phase change material both improved conductive and convective heat transfer, and provided uniform distribution of particle concentration. A latent heat thermal energy storage (LHTES) unit was designed for combined application study, and a charging performance experiment was conducted. Nanoparticle only improved the average charging power by 1.09 times, while bubble-driven flow only improved it by 1.27 times. Bubble-driven flow and nanoparticles applied altogether increased the average charging power by 1.41 times. Therefore, it was confirmed that the combined application of nanoparticles and bubble-driven flow can effectively improve the heat transfer performance of the latent heat thermal energy storage. Thus, this method is expected to be applied to a wide range of industries.