Hydroelectric power generation is generally regarded as clean power as it does not emit greenhouse gases during the generation process. With the projected increase in its installed capacity in the next decade, future hydroelectric power generation will require large-scale constructions that pose a great threat to the environment. Thus, expanding installed capacity requires the active use of unutilized hydropower.
Hydraulic turbines used in generating hydroelectric power can be classified into two main groups: those installed directly in open channels, mostly rivers and waterways, like overshot water wheels and those installed in pipes like Francis turbines. Whereas open channel hydraulic turbines have a small hydraulic head and a large hydraulic turbine body, hydraulic turbines for pipes have relatively smaller body as they use the head. To this end, small hydraulic turbines for pipes that can be applied with a low head are increasingly being studied for small-scale distributed hydraulic turbines.
Different design approaches for ultra-small axial flow hydraulic turbines applicable with small head in open channels and existing pipes have been proposed in the previous studies. For example, a feasible design method involving uniformizing the axial flow velocity incorporating conventional one-dimensional (1D) design and three-dimensional (3D) flow analysis exhibited improved efficiency and performance. Unfortunately, the efficiency of this turbine tends to decrease with time due to the small Reynolds number induced by the ultra-small size of the turbine. Therefore, a new design method independent of the conventional theories is required to further improve the turbine efficiency.
To this note, Prof. Yasuyuki Nishi, Ms. Nozomi Mori, Mr. Naoki Yamada and Prof. Terumi Inagaki from Ibaraki University developed an optimization method for the axial flow runner to improve the efficiency and overall performance of the turbine. This design method combined 1D design method with a design of experiments, a response surface method and an optimization method. The mechanism of performance enhancement and loss of vortex structure was numerically and experimentally investigated by examining the performance and internal flow of the runner to validate the effectiveness of this method. The work is currently published in the journal, Renewable Energy.
The authors demonstrated the applicability of the proposed design approach to the runner of palm-sized ultra-small axial flow hydraulic turbine applicable to low head open channels and existing pipelines. By creating the initial form using the 1D design approach, the design space was narrowed down to achieve the desired degree. Regarding the design variables, the blade angle significantly affected the turbine efficiency. The sensitivity of the turbine efficiency to the design variables was relatively high, especially in the order of tip and mean blade angle.
Compared to the original runner designed using only the 1D design scheme, the optimized runner presented in this study produced an effective head and output closer to the recommendable design values. At the same rotational speed, the optimized runner recorded a 9.1% improvement in the experimental value of the turbine efficiency. This was because optimization resulted in a remarkable reduction in the loss rate by 5.3% and 74.3% at the runner and the outlet channels, respectively.
In summary, an optimization design method combining the optimal objective functions and design variables was presented to improve the efficiency and performance of axial flow hydraulic turbines. The performance improvement was due to the dramatic decrease in the loss by tip leakage vortex, vortex near the runner outlet hub and vortex due to swirling downstream flow. The findings indicated the effectiveness of the optimization design approach. In a statement to Advances in Engineering, the authors said the new presented design would contribute to large-scale hydroelectric power generation for future energy demand.
Nishi, Y., Mori, N., Yamada, N., & Inagaki, T. (2022). Study on the design method for axial flow runner that combines design of experiments, response surface method, and optimization method to one-dimensional design method. Renewable Energy, 185, 96-110.