To improve the efficiency of a Savonius vertical axis wind turbine, this investigation proposes the use of expandable blades instead of rigid blades. The expandable blades have the ability to change their form during the turbine rotation. The expansion of the advancing blade increases the positive torque, and the contraction of the returning blade decreases the negative torque, which boosts the turbine efficiency. Two-dimensional numerical simulations have been carried out using the commercial code ansys fluent 18.0 with a deformable mesh to fit the changing shape of the blades during the rotation cycle. The paper involves the effect of the expansion amplitude as well as the effect of the blades gap and overlap ratios on the turbine overall performance. The numerical model is validated by comparison of predictions with experimental results. Results show that the torque coefficient is improved by about 24% for the lowest expansion amplitude and by about 90.6% for the highest expansion amplitude. A further improvement of 7% is recorded for an expandable turbine with a gap ratio of 1/20 the turbine diameter.