A new nozzle design methodology for high efficiency crossflow hydro turbines

Small-scale hydropower systems are mainly used in remote locations for generating electricity where reasonable hydropower resources are available. Many small-scale systems employ crossflow turbines due to their simplicity in design and manufacture, low cost, sturdy construction and longer life-span. However, compared to most advanced and efficient designs, such as Pelton and Francis turbines, they suffer from a lower maximum efficiency. In a crossflow turbine, the nozzle increases the velocity of the flow and directs it at a suitable angle to the runner whose axis is tangential to the flow. The runner extracts the angular momentum of the flow. Therefore, the runner entry flow is critical for the turbine efficiency. However, it is not yet known clearly how the entry flow affects the runner performance and how the best nozzle can be designed. This study presents a new nozzle design method so that high efficiency crossflow turbines can be designed. An analytical model is formulated to convert the head into kinetic energy at the entry and obtain a suitable flow angle. Three-dimensional Reynolds-Averaged Navier-Stokes simulations were conducted on a 7 kW turbine with a measured maximum efficiency of 69% and a 0.53 kW turbine with a maximum efficiency of 88%. The predictive capability of the computational model was assessed by comparing the computational and experimental results for the power over a range of operating conditions on both turbines. By redesigning only the nozzle of the 7 kW turbine by the new method, the maximum efficiency increased from 69% to 87%. Thus the nozzle design has a significant influence on turbine performance, and we conclude that the conversion of head into kinetic energy and matching of the nozzle flow with the runner design are fundamental in turbine design.