Three-dimensional unsteady numerical analysis has been performed in order to analyze the aerodynamic characteristics of an H-Darrieus vertical axis wind turbine with two straight blades. The reliability of the numerical models has been demonstrated through good agreement between the calculated and measured efficiency of an H-Darrieus. Flow characteristics are closely investigated according to tip speed ratios and solidities. A comparison of aerodynamic characteristics at various operational conditions, including the maximum power point is performed. The direction of the free stream approaching the blade is considerably bent following the interaction between blade-to-blade and blade-to-free stream. Even though, the peak value of a torque increases as solidity increases, the blockage and interaction also increase, and thus, increasing the solidity alone does not improve the performance of the H-Darrieus. On the other hand, decreasing the solidity can reduce the effect of blockage and interaction, but the self-starting features via the negative torque at the low tip speed ratio becomes lost. Therefore, a theoretical model such as the DMST (double multiple stream tube) is not suitable for predicting the performance of H-Darrieus with a high solidity. The blockage by blades in the upwind revolution and the interaction between blades significantly change the magnitude of an incidence velocity, and the angle of attack. Thus, the tip speed ratio of the operation point (i.e. the highest power coefficient point) is found to be lower than it is expected. (C) 2015 Elsevier Ltd. All rights reserved.