Performance Assessment of Nano-enhanced Phase Change Material for Thermal Storage

The use of nano phase change material in thermal energy storage applications appears promising, but the often-poor performance and the lack of understanding of the heat transfer mechanisms interconnectedness remains a challenge and hinders their widespread integration. The existing numerical work has unveiled numerous impediments in predicting the actual melting behaviour. They rarely combine the effects of conduction enhancement, convection degradation, and latent heat reduction, due to inaccurate characterization of the thermophysical properties and the limitations of their model assumptions. In the present study, an enhanced numerical approach was developed to investigate the melting performance of xGnP-octadecane filled in a vertical cylindrical enclosure at different weight concentrations. The model results for the pure phase change material were compared and validated against the experimental data. The progression of the melting front, temperature probes, energy storage capacity and heat transfer rate of the nano phase change material were thoroughly evaluated. The current numerical observations demonstrate that the addition of nanoparticles improves, up to a critical concentration of 0.5wt%, the melting rate. The results showed that by adding 0.5wt% of xGnP in the base phase change material (octadecane), the melting rate decreases by 9.7% and the heat storage rate increases by 12.6%. However, at higher loadings, the heat transfer rate is deteriorated due to worsening of other thermophysical properties provoking the prevalence of viscous forces over natural convection and latent capacity. The system overall efficacy was found to be dependent on the net effects of relative changes of all thermophysical properties with nanoparticle concentration and temperature in the solid, so called mushy, and liquid zones. Finally, when characterizing nano phase change material, the thermal conductivity cannot be considered alone as a criterion for nano phase change material selection. A high thermal conductivity is needed for maximum heat absorption in thermal transport applications. Nevertheless, low viscosity, high latent heat and specific heat capacities are also essential to ensure a better thermal energy storage efficiency in terms of capacity and heat extraction/release rate.</p

NanoPCM Based Thermal Energy Storage System for a Residential Building

Implementation of thermal energy storage (TES) systems into a building and facilities improves the performance of the heating/cooling system by reducing energy waste. Thermal performance of a TES relies on the thermophysical properties of the thermal storage medium (TSM). In the present study, a novel two-step selection model has been implemented to choose the best TSM to improve TES system performance. Various types of thermal storage media are investigated considering phase change materials combined with nanoparticles. Thermal storage capacity, heat storage rate, and thermal storage efficiency have been considered as the main selection parameters in a hierarchy method. A significant contribution of this work is the development of a modelling methodology which can be used as a material selection process or tool. It enables the selection of the most efficient TES on a case-by-case basis. This TSM selection technique adds new understanding of selection tools, and new modelling capabilities to this field. It works with a variety of building cooling/heating loads, and helps minimize the environmental impact of extracting/releasing heat to the ground in geothermal applications. The TSM includes PCM and various PCMs have been considered with a melting range of 5–11℃. A material database of 90 different nano phase change materials (nanoPCMs) has been generated by considering ten types of TSMs and nine types of nanoparticles. First, a 2D numerical model has been used to investigate the heat transfer characteristics of the TSM filled in a cylindrical enclosure with a height of 5 cm and a diameter of 1.2 cm. Fourteen different nanoPCMs were selected to further study based on their thermal storage capacity, heat storage rate, and improvement coefficient and implemented into a second numerical model (3D) to calculate the thermal storage efficiency of the designed underground TES system with a height of 20 m and a diameter of 1.5 m. Finally, a building heating/cooling load has been implemented in the numerical model to evaluate the performance of the final designed system on the ground temperature throughout five years of operation. The ground temperature and its variation have an effect on the performance of the ground source heat pump. After five years of operation simulation of the no-PCM system, the ground temperature has increased by 1.78℃ to 9.78℃. However, by adding PCM and nanoPCM, the average temperature reduced to 8.95℃ and 8.72℃, respectively.</p