Nano-PCMs for enhanced energy storage applications

Phase Change Materials (PCMs) represent an interesting possibility to improve the sensible energy storage exploiting their high latent heat. The solid-liquid phase change process can be used to adsorb and then release heat loads, therefore, PCMs are considered potential solutions for energy saving. Amongst the available PCMs, paraffin waxes have been found to exhibit many desirable characteristics, such as high latent heat, low vapour pressure in the melt, they are chemically inert and stable, and non-toxic. Moreover, they can be chosen according to their phase change temperature, to best fit the application needs. However, they also have a very low thermal conductivity and a high volume change during the melting process. Thus, heat transfer enhancement techniques are required for their possible implementation in PCM energy storage applications, especially in case of intermittent operations. The present work aims at investigating a new challenging use of aluminium oxide (Al2O3) nanoparticles to enhance the thermal properties of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by dispersing different amounts of oxide nanoparticles in two paraffin waxes having melting temperatures of 60 and 70 °C. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Finally, a numerical comparison between the use of pure paraffin waxes and the nano-PCMs obtained in a typical water energy storage for sanitary application was developed and implemented. The modelled heat storage is a typical 70 L water tank, where a certain number of pipes filled up with PCMs are located to improve its heat storage capabilities; the results are presented in terms of loading and unloading time, and total amount of energy stored. A numerical model is accomplished to simulate the heat transfer inside the cavity with PCM and nano-PCM. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures and stream function were reported. Moreover, a comparison with experimental results was also performed.