Wells turbine is a promising self-rectifying device in the field of ocean wave energy conversion. This study, firstly offers a second law analysis of the isothermal flow through a Wells turbine. Numerical computations is carried out in OpenFOAM by solving steady, incompressible and three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations with Spalart-Allmaras turbulence model in a non inertial reference frame rotating with the turbine rotor. Then local entropy generation rates due to the viscous dissipation around the rotor blades are added to the numerical code and accumulated with other irreversibilities to calculate the exergy efficiency. The results showed that separation and boundary layer interaction have a direct effect on the entropy generation and thus efficiency. The blade entropy generation increases from hub to tip at the leading edge and decreases from the leading edge to the trailing edge. Also, the point of maximum efficiencies coincides with the point of stall. The results proved that distribution of viscous entropy generation provides useful information about the causes of the flow irreversibilities for designers. Secondly, the equations of plasma actuator have been added to the numerical code and its effect on aerodynamics characteristics and first and second law efficiencies has been discussed. The results showed that by applying plasma, the average increase in torque coefficient is about 39.36% and average decrease of lift coefficient is about 30.53%. However, the second law efficiency increases about 39.16% without considering viscous dissipation term and decreases about 64.63% with considering viscous dissipation term. In order to reduce the negative effect of plasma on second law efficiency, it is suggested to apply it in pulsative manner. Also investigation of applying plasma on the leading edge can be useful. (C) 2017 Elsevier Ltd. All rights reserved.