High hydraulic performance in horizontal waterwheels

Slow rotating waterwheels have been recently proposed as affordable sources of renewable energy in rural areas. In terms of hydraulic efficiency, classical horizontal waterwheels reach values on the order of 50%, being well below those obtained from overshot (71%), waterfall (66%) and, even, undershot (65%) vertical ones. Based on the study of an horizontal waterwheel built prior to 1940s that includes features from both elementary turbines and antique waterwheels, we conclude that horizontal waterwheels may actually reach hydraulic efficiencies as high as 81%. These quantitative results are obtained by analytical approximations and, more important, by numerical integrations through accurate computational fluid dynamics (CFD) simulations. In addition, we show that such a high hydraulic efficiency does not substantially vary when changing the net available head. We suggest that this relevant progress made on improving the efficiency of classical horizontal waterwheels may have implications in future designs of nano/micro hydropower devices.