Kaplan and Francis turbines - Kaplan suits low-head high-flow sites; Francis fits medium-head applications. Both see continuous engineering improvements for efficiency.

Kaplan and Francis turbines are the two most common types of reaction turbines used globally and within the U.S. hydropower industry. Reaction turbines generate power not only from the velocity (kinetic energy) of the water but also from the pressure differential across the runner blades, enabling them to harness power from water heads across a wide range. These two designs are crucial for the vast majority of medium and large-scale conventional and pumped-storage hydropower projects.

Francis Turbines: The Workhorse of Hydropower
The Francis turbine, invented by British-American engineer James Francis in 1849, is the most frequently used type of turbine in hydroelectric power plants worldwide.

Design and Principle: It is an inward-flow reaction turbine. Water enters the runner (wheel) in a radial direction (from the outside toward the center) and exits in an axial direction (downward). Its runner has a fixed set of blades (often nine or more) and is encased in a spiral casing (volute) that ensures the water is evenly distributed around the runner. Adjustable guide vanes control the flow of water into the runner, adjusting the power output.

Application Range: Francis turbines are ideally suited for medium to high head (the vertical distance the water falls) and medium flow conditions. They can operate effectively in falls ranging from as low as 2 meters to as high as 300 meters (or even up to 2,000 feet in some high-head applications). This wide operating envelope is why they are so prevalent, dominating the installed base in the U.S. As of recent estimates, more than half of the turbines installed at existing U.S. hydropower facilities are Francis turbines.

Efficiency and Advantages: Francis turbines boast very high peak efficiencies, often exceeding 90% and reaching up to 95%. Their design is compact, robust, and durable, making them ideal for the heavy-duty, long-lifespan requirements of major hydropower facilities, including the massive powerhouses at the Grand Coulee Dam.

Kaplan Turbines: Optimized for Low Head
The Kaplan turbine, developed by Austrian inventor Viktor Kaplan in 1919, is an evolution of the Francis design, specifically engineered to handle large volumes of water at lower heads.

Design and Principle: The Kaplan turbine is an axial-flow reaction turbine, meaning the water flows through the runner blades in an axial direction (parallel to the axis of rotation). Its defining feature is the adjustable blades on the runner, resembling a ship's propeller. Crucially, the Kaplan turbine has both adjustable runner blades and adjustable guide vanes (wicket gates).

Application Range: This adjustability is what gives the Kaplan turbine its unique advantage: it maintains high efficiency over a much wider range of flow rates and low head conditions. They are preferred for heads from about 2 to 70 meters (6 to 230 feet) where the flow rate is very high. This makes them ideal for run-of-river projects or power plants on large, slow-moving rivers.

Efficiency and Versatility: While its peak efficiency is comparable to the Francis turbine, the Kaplan's ability to adjust its blade pitch allows it to maintain a high efficiency across variable river flows, which is critical for maximizing generation in fluctuating hydrological conditions. A variation, the Bulb turbine, is often used in very low-head situations and is a close cousin to the Kaplan.

Market Dynamics in the U.S.
The U.S. hydro turbine market is projected to see a continued dominance of the reaction turbine segment (which includes both Francis and Kaplan). This dominance is driven by the need to upgrade and retrofit aging U.S. infrastructure and the development of new pumped-storage projects, which primarily use reversible Francis turbines.

Retrofitting and Modernization: A major market driver involves replacing older turbines with new, more efficient Kaplan and Francis designs. Modernizing a turbine can yield significant efficiency increases (e.g., a 3% increase per unit has been noted), which translates to hundreds of millions of additional kilowatt-hours of annual generation from the existing water resource.

Pumped Storage: The push for new PSH capacity (up to 50 GW potential) heavily relies on reversible Francis turbines that can operate in both generating mode (as a turbine) and pumping mode (as a pump). This makes the Francis design central to the U.S. energy storage future.

In essence, while the Francis turbine serves as the robust workhorse for established, medium-to-high-head facilities, the Kaplan turbine provides the necessary technological flexibility to exploit the high-flow, low-head opportunities presented by the U.S.'s many large rivers and a growing number of small-scale hydro applications.

Kaplan and Francis Turbines

Q1: What are Kaplan and Francis turbines?
Widely used hydro turbines suitable for different water flows and head levels.

Q2: Where are they applied?
Kaplan turbines for low-head sites; Francis turbines for medium-head projects.

Q3: Why are they popular?
They provide efficiency, reliability, and adaptability in diverse hydropower setups.