We introduce a reusable energy harvesting system that can recover discarded thermal energy by utilizing temperature variation of the environment. The system consists of two different phase change materials (PCMs); poly ethylene glycol (PEG) and 1-tetradecanol (1-TD), and a cell of N and P type semiconductor. Since a large amount of heat energy is absorbed during phase transition, PCMS are generally utilized to maintain isothermal temperature on the opposite sides of the N and P type semiconductor as long as possible. The resulting temperature difference between two sides of semiconductor induces an electric current during the heating and cooling process. It is important for the phase change material (PCM) to possess a good shape stability without significant loss of latent heat. A 3D porous graphene aerogel is selected for the pure phase change material to infiltrate into the aerogel and to increase the shape stability and thermal conductivity of the PCM composite. The thermal conductivity of PEG and 1-TD composites is significantly increased to 0.4268 W/m.K, and 0.3408 W/mK, respectively. Thermal and electrical analyses are performed to predict the energy harvested by the device. The electrical energy is generated in the harvesting system due to the Seebeck effect. The maximum value of the electric current reaches 10 mA in both heating and cooling processes and an LED lamp was turned on successfully. Additionally, the energy harvesting system is modelled by using the finite element method (FEM) and the numerical prediction is in good agreement with the experimental results.