Nanoenhanced materials for cold storage applicationsan insight into design, thermophysical profile and rheological behaviour

Resumo

The acute threat of the global warming establishes a new energetic scenario which leads to new challenges to face that must be addressed with new technological solutions particularly, the general deployment of renewable sources for energy production. One of the main challenges of such energy sources is the intermittence of production, which mismatch with the demand and consumption. Also, the continuous rise of the worldwide energy consumption has not stopped and in some sectors, the climate change even boosts this growth, such as cooling production. Furthermore, the recent COVID-19 pandemic has led to a scenario with temporary reduced GHG emissions, evidencing that actions on climate effectively mitigate the global warming. Thermal energy storage (TES) has been considered a prospective technology to facilitate the reduction of peak demand from high consumption periods, or peak hours, to low consumption periods, or off-peak hours. In such systems, phase change materials (PCMs) are becoming popular due to the ability of absorb and release larger amounts of heat than conventional materials and at a nearly constant temperature. The solid-liquid transition of these materials is the most commonly used. Among the existing variety of PCMs, organics have been considered an interesting option for TES systems since they show little or no sub-cooling, especially under low heating or cooling rates, low vapour pressure, and they are recyclable. The low thermal conductivity of such materials leads to long charging and discharging times, constituting the main drawback for their implementation in TES systems. In the last decade, well-dispersed nanosized materials (one dimension lower than 100 nm) within PCMs have proved to enhance the thermal conductivity and reduce the sub-cooling phenomenon, such colloidal dispersions being called as nanoenhanced phase change materials (NePCMs). In this PhD Thesis, new NePCMs are designed and proposed for cold storage applications based on stability criteria, phase change characteristics, thermophysical profile, and rheological behaviour, also with the aim to reach a new understanding on transport properties of nanostructured fluids for heat transfer purposes. Accordingly, three organic PCMs whose solid-transition lies on the temperature range for cooling storage (from 277 K to 285 K) were selected as base fluids, namely PureTemp8 (PT8), isopropyl palmitate (IPP), and n-tetradecane (n-C14). Four different nanoadditives were studied, spherical MgO nanoparticles and three type of graphene nanoplatelets (GnP, GnP7, and GnP40) with diverse chemical state and lateral size. In order to analyse the effect of different morphologies, chemical natures, and sizes on the NePCMs, the base fluids and the nanomaterials were characterized by means of transmission electron microscopy, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Five different nanofluid sets have been carefully designed and elaborated, viz. MgO/PT8, GnP/PT8, GnP7/IPP, GnP40/IPP, and MgO/n-C14. Thus, especial attention was focused on monitoring and control the stability of the NePCMs, through the determination of the size distributions in static and stirred conditions by using dynamic light scattering. For each nanofluid set, a surfactant was rationally selected based on the criterion of maximum colloidal stability, optimizing also the surfactant:nanomaterial mass ratio. The thermophysical profile of the proposed NePCMs was analysed from the experimental determination of the thermal conductivity, density, isobaric heat capacity, and thermal diffusivity. All the employed techniques have been detailed, establishing methodologies that allows reliable data. The study of the rheological behaviour of the different nanofluids allows the description of their viscoelastic state under a wide variety of operating conditions, as well as to obtain the temperature dependence of the shear viscosity of the NePCMs. Finally, a new relationship on transport properties of nanofluids was presented as a significant contribution to the understanding of the heat transfer performance of those wide and differentiated soft materials. This proposal suggests a novel understanding of the enhancement mechanisms based on previous studies on transport properties of liquids by Andrade, Osida, and Mohanty. Thus, an interesting tool for the design of nanofluids for heat transfer purposes and in particular for cold storage application, is presented. The reliability of this proposed relationship was assessed by experimental data for magnesium oxide within n-tetradecane here reported, along with available literature data for titanium oxide within water, silver within poly(ethylene glycol), and aluminium oxide within (1-ethyl-3-methylimidazolium methanesulfonate + water).