A two-dimensional, transient simulation of charging and discharging in a shell-and-tube type thermal-energy storage device are performed with ANSYS-Fluent. In this, the phase-change is modelled using the enthalpy-porosity formulation. Nanoparticle of aluminium oxide (Al₂O₃) in a paraffin wax PCM is used with water as heat transfer fluid (HTF). The fins material is copper. To model effective thermal conductivity of PCM enhanced with nanoparticle, various empirical correlations are used. Other properties, are considered to be constant. Similarly, properties of HTF and fin material are considered constant The ANSYS-FLUENT model is validated with the available numerical and experimental data for rectangular annular fins. The model matches reasonably well within acceptable limits. With validated model, the effect of nano-particle enhanced-PCM, triangular annular fins, fin pitch, and HTF flow reversal on time taken for the completion of melting and solidification is performed. A sensitivity analysis of the correlations for effective thermal conductivity performed and the results are found to be independent of the correlation used. From charging and discharging simulations, the contribution of nanoparticle, fin pitch and HTF flow direction to the overall heat transfer performance is respectively found to be 8.8-10.66%, 29.38-30.83% and 1.57-1.77%.