Thermal energy storage using paraffin wax and stability study of the phase change material containing nanoparticles

Saydam, Vahit (2018) Thermal energy storage using paraffin wax and stability study of the phase change material containing nanoparticles. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis has two main parts. In the first part, the performance of a helical coil heat exchanger was investigated with paraffin wax as the phase change material (PCM) for a latent heat thermal energy storage system (LHTESS). The effects of heat transfer fluid (HTF) inlet temperature, HTF flow rate and flow direction were experimentally examined by measuring PCM temperature changes in the charging and discharging processes. The experimental results showed that HTF inlet temperature has the greatest influence on the charging/discharging processes. The flow direction of HTF had only an insignificant effect on discharging time. Higher heat recovery efficiency was achieved at high flow rates during discharging. Overall, it was seen that the low thermal conductivity of paraffin wax led to poor heat transfer performance, specifically causing much longer discharging times compared to charging times. In the second part of the thesis, nanoparticle-enhanced phase change materials (NEPCMs) were proposed as a heat transfer enhancement method. Various highly conductive nanoparticles were dispersed into paraffin wax to improve the thermal conductivity of the PCM. Multi-walled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs) and Aluminum oxide (Al₂O₃) nanoparticles were selected as enhancers. Nanoparticles were dispersed into paraffin wax using mechanical dispersion methods (sonication, stirring) with and without surfactants at varying mass fractions (0.1, 0.5, 1 and, 2 wt.%). The stability of nanoparticles was investigated after consecutive melting/solidification cycles were performed in an environmental chamber. Significant deposition and coagulation were seen over thermal cycles regardless of the nanoparticle type, nanoparticle content and dispersion method. The presence of nanoparticles did not lead to the desired thermal conductivity enhancement due to particle deposition and stability issues. The highest thermal conductivity enhancement was achieved by 13% for a 2 wt.% MWCNT-wax sample at 35°C. Differential Scanning Calorimetry (DSC) measurements also showed an insignificant change in latent heat capacity. In conclusion, NEPCMs could be an alternative storage material for LHTESS to improve overall heat transfer performance only if the issues associated with particle stability are resolved. Therefore, further study regarding the stability of NEPCMs with a multidisciplinary approach is needed to solve this problem.

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