This paper discusses how the arrangement of fins can affect the charging process. Mixture of paraffin and copper oxide nanomaterial has been utilized. Several configurations of fins with various position, thickness and length were discussed. Volume fraction of NEPCM in domain is constant, thus reducing melting time is criterion for good efficiency. Outputs reveal that with incorporating fins in both sides, melting process becomes more uniform. Lower melting time with using longer fins is a result of more effective buoyancy forces by dividing the whole domain into smaller blocks. Using fins over both inner and outer pipes shorten the melting time. The main purpose of current modeling is finding the optimized style of fins to gain the highest performance. Nice accommodation of outputs with empirical previous data indicates the verification of code. (C) 2020 Elsevier B.V. All rights reserved.