Thanks so much for this link! I've been saying that the Betz is not properly applied to VAWT's since about 2006. I'm not really confident in their numeric simulation in this paper, which is the "proof" they have that VAWT's may have a higher limit. I've talked with experts in CFD flow simulation, at UVIC, and 2 years ago with one of the research heads at Canada's largest wind tunnel, they don't believe that CFD is capable of good modelling of a VAWT with 3D flows, yet. That's not really relevant to the question of should Betz be applied to VAWT's when the calculations are based on the conventional HAWT. I'm not sure the limit is higher, or lower, just that Betz is a useful tool that doesn't fit in this application. There's a paper by Gorlov which numerically "proves" a limit of 35% (by conicidence the exact number Gorlov claimed to reach with his helical darius). Since then Gorlov's work has been shown to be problematic. I worked with an independent group who licensed the Gorlov design and bought test turbines from Gorlov. The turbines did not survive testing, too fragile and never reached more than 20% (Cp 0.20). A straight blade Darius was tested in it's place which reached 25%, using the same blade profiles.