Development of high-efficiency and low-cost unshrouded turbine runner for small scale hydropower plant

Facing a growing demand for low initial cost and high-efficiency small scale hydropower plants, a new type of Francis turbine was developed for Kazunogawa small hydropower plant with cooperation from industrial, governmental, and academic sectors. To reduce the cost of the turbine while increasing its efficiency, a prototype turbine with an unshrouded runner and a cylindrical casing was developed. The unshrouded runner was designed by Waseda University. Typically, the turbine runner for small scale hydropower is manufactured via a casting method; however, the unshrouded runner can be milled with a 5 axis CNC machine, thereby reducing cost and manufacturing lead time. A cylindrical casing has advantages of low fabrication cost and compact footprint. However, it had lower efficiency compared to a spiral casing with conventional design. Therefore, in this study, topology optimization based on CFD analysis was conducted on the casing and double circular cascade to improve efficiency. In addition, the axial thrust was estimated from CFD analysis to design the bearings and to determine the runner gap. Finally, a performance test was conducted on-site and peak efficiency of 91% was confirmed. This paper also reports the follow-up observation of the prototype turbine after commercial operation started.