Awareness of efficiency limitations of turbines helps us be able to optimize the design of wind and water power generation fields. Exact assessment of theoretical generating power of the turbine in free flow of the fluid is of great importance. This is because the growing tendency to increase production of power from wind and from water sources with zero altitude is of ever-increasing attention. These include great kinetic energy source from oceans movement and tidal energy and from rivers with no dams built. In this article, a forthright and solvable mathematical model for estimation of maximum efficiency of turbine in free flow is given. These results can be utilized for electricity generating hydro/water turbines, where building dams is impossible. This model is associated with a finite two dimensional plane with fairly low permeability in an incompressible flow which is desirable for two dimensional blades but is less suitable for turbines with transverse three dimensional flows. The most interesting finding of our analysis is that for free flow highest efficiency of flat blade is 30%. This result is in great opposition with the 60% value given by Betz, which has been adopted presently and for the last few decades. It has been shown that Betz’s above assessment is the consequence of ignoring the curvature of the fluid flow. Also, it has been proved that the three dimensional spiral turbine is much more efficient than two dimensional blade, at least for hydro applications.