What is the key difference between Kaplan and Francis turbines, and when should each be used?

The key difference between Kaplan and Francis turbines lies in their runner design and their ability to maintain high efficiency over a range of flow rates. A Kaplan turbine has adjustable runner blades, similar to a propeller, while a Francis turbine has fixed runner blades with a more complex, three-dimensional shape. Kaplan turbines are used for low-head, high-flow applications, typically with heads below 50 meters. The adjustable runner blades allow Kaplan turbines to maintain high efficiency even when the flow rate changes significantly. Francis turbines are used for medium-head, medium-flow applications, typically with heads ranging from 50 to 500 meters. Because their runner blades are fixed, Francis turbines are most efficient at a specific flow rate. Their efficiency drops off more sharply than Kaplan turbines when the flow rate deviates from the design point. Kaplan turbines are well-suited for situations where the water flow varies seasonally or due to changing water demand. The adjustable blades allow them to adapt to these changes and maintain efficient power generation. Francis turbines are better suited for situations where the water flow is relatively constant, allowing them to operate near their peak efficiency point. For example, a Kaplan turbine would be a good choice for a run-of-river hydroelectric plant with significant seasonal flow variations, while a Francis turbine would be a better choice for a dam with a consistent water release schedule.