Entropy Production Analysis of Energy-Efficiency Optimization for Variable-Speed Operation in a Francis Turbine

In this paper, the influence of the speed changes on a low-head Francis turbine has been discussed, aiming at efficiency optimization. For forty two different operating scenarios with and without considering the speed variations, energy losses of the turbine are evaluated with the help of entropy generation method. Comparative analyses are carried out in terms of overall energy harvesting performance, entropy production distribution, and detailed distribution of local entropy production rate (LEPR). It is found turbine efficiency is up by 10% under off-design conditions since the variable-speed operation took effect. For deep part load condition, while providing the same power output, it can save up to 14% of discharge thanks to the increased efficiency. At design point, as turbine is already operating at its maximum efficiency, it is unlikely to generate any benefits. Meanwhile, model hill-chart is sufficient for a first inspection of variable-speed operation, and preliminarily predicts the efficiency improvement of prototype turbine. The results are encouraging and prove that variable-speed technology can work well for Francis turbines and is especially applicable to the turbine of low-head