Developing cost-effective and efficient energy harvesting technologies is critical with the rising demand for clean and sustainable energy. In this research paper we presents a CFD-based study on a PDMS-Graphene Triboelectric Nanogenerator (PG-TENG) for blue energy harvesting. Our study focuses on the vertical (contact-separation) mode of the PG-TENG and investigates the effect of TENG’s varying size on its electrical output performance. To optimize the properties and performance of the PG-TENG system, we used COMSOL Multiphysics for mathematical-modeling and simulations. The outcomes show that the varying size of the PG-TENG significantly impacts its electrical output capabilities, with larger PG-TENGs exhibiting higher current and voltage outputs. Moreover, we compared our outcomes to other studies on graphene-based TENGs and highlighted the advantages of our PG-TENG in terms of durability, performance, and mechanical stability. Our research contributes to the field of blue energy generation by providing insights into the design and optimization of PG-TENGs for low-cost, effective, and efficient energy harvester devices.