Evaluation of Thermal Energy Storage Capacity of Heat Transfer Fluid Based on Phase Change Materials Nanocapsouls

Phase change materials (PCMs) are used for thermal energy storage and other thermal management applications. This applications are due to PCM crystalline structure and high latent heat of fusion. In this work, polyethylene glycol with high molecular weight and high crystallinity was developed in the synthesis of phase change systems of heat transfer fluids. In this research, the nanocapsule of the phase change material with the average size of the core diameter and shell thickness of about 90 and 80 nm, respectively, which was synthesized in the polymer engineering department of Tarbiat Modares University, was used to prepare the heat transfer fluid. In this research group, by controlling the solubility parameter of core and shell with temperature changes in the solvent, the phase change nanocapsule system was synthesized by sequential sedimentation method. This nanocapsule consists of a polyethylene glycol core with a molecular weight of 2000 as a phase change material and a thermal conductive polystyrene/active carbon shell. This molecular weight polyethylene glycol was selected for use at an operating temperature of 55℃. The results of time-temperature history evaluations of the heat transfer fluid containing only one weight percent of nanocapsules showed that at 55°C, the amount of heat energy absorption in the heating/cooling cycle is about 17% higher than pure water. In addition, the comparison of the thermal profile of thermal energy absorption in the control sample (pure water) and the heat transfer fluid showed that in the condition that 20 ml of water becomes the same temperature with the hot environment in 6 minutes, to reach the same temperature value in the heat transfer fluid it takes 12 minutes. This time delay in heat transfer fluid compared to pure water indicates a 100% increase in heat energy absorption efficiency. One weight percent of phase change nanocapsule system in distilled water improves the special properties of this system as a heat transfer fluid. So that compared to pure water, the prepared heat transfer fluid improves the specific heat capacity, heat transfer coefficient, and thermal energy absorption rate by about 35, 3.3, and 17 percent, respectively. The absorption of thermal energy during the phase change of the core reduces the thermal diffusion coefficient in the system from 12×10-7 for water to 2.17×10-7 m2.s-1 for the heat transfer fluid. The 82% reduction in thermal diffusivity is due to energy absorption of latent heat of melting of the phase change material. One weight percent of phase change nanocapsules in water increases the specific heat capacity of water from 4200 to 5800 J.kg-1.K-1. According to the mentioned properties, thermal conductive polyethylene glycol/polystyrene nano-sol is suggested for making heat transfer fluid in cooling or thermal energy transfer applications.