The potential for harvesting energy from salinity gradients using nanoporous membranes has attracted significant attention in the blue energy field. To maximize the harvesting efficiency, it is necessary to both enlarge the effective area of the nanopores and control the nanopore density in such a way as to suppress the ion concentration polarization (ICP) effect. However, this is not easily achieved in large-scale manufacturing due to technology limitations. Accordingly, this study proposes a method for minimizing the ICP in conventional high-nanopore-density membranes by imposing a temperature gradient across the low- and high-concentration salt reservoirs. The feasibility of the proposed method is demonstrated experimentally by increasing the temperature of the low salt concentration reservoir by 25 K compared to that of the high concentration reservoir. It is shown that the application of an asymmetric heating effect increases the power generation by around 64% compared to the isothermal case. The experimental results are validated by means of COMSOL multiphysics simulations based on the Poisson-Nernst-Planck, Navier-Stokes and heat transfer equations. The simulation results indicate that the higher power generation obtained under asymmetric thermal heating is the result of a lower ICP at the nanopore-reservoir interface, which enhances the ion transport through the membrane.