Hydroelectric power, or hydropower, is a type of power that uses the force of the flowing water to generate electricity. While there are a multitude of different hydroelectric facilities, they all use turbines/generators to convert the kinetic energy of flowing water as it moves downstream into electricity for homes, businesses, and industry. There are different types of turbines used in hydroelectric power plants, and each type has its own advantages and disadvantages.
Turbines used in hydroelectric power plants fall into two categories: Impulse and Reaction based. Impulse based turbines include Pelton and cross-flow turbines. Reaction based turbines include three subtypes: Francis turbine, propeller turbine (bulb turbine, Straflo turbine, Kaplan turbine types), and kinetic turbine.
As different types of turbines have their pros and cons, we recommend starting your research by answering a few questions below:
- What is the “head”? (Head is the vertical height change in water levels between the hydro intake and hydro discharge points.)
- What is the volume (flow rate) of the water?
- What are the efficiency requirements?
- What is the budget?
Types of Turbines: Impulse Turbine vs. Reaction Turbine
The two main differences between impulse and reaction turbines are the number of stages in a turbine and the maximum steam velocity. Impulse turbines capture energy at a single point where the water jet is hitting it; reaction turbines capture energy across the whole wheel at once, which makes them more efficient in producing energy. Therefore, sites with high head and low flow should always consider the impulse turbines over reaction turbines.
Impulse Turbine Types
In a Pelton turbine pressurized streams of water via nozzles are directed towards a series of spoon-shaped impulse blades, also called buckets, at splitters which divide the water jet into two streams. These streams then flow along the inner curve of the buckets and then leave in the opposite direction they came in.
This creates an impulse on the blades and generates torque and rotation in the turbine. Pelton turbines are generally used at sites with heads greater than 985 feet and have a reservoir of water above.
The cross-flow turbine also called an Ossberger turbine, has a shape of a drum. Its structure is similar to that of a water wheel. When the water enters the turbine at the edge, it moves inward. Then it flows through the runner and exits from the inside back out.
As the water passes the turbine blades twice, it provides additional efficiency in creating power as well as self-cleaning itself of small particles and debris. Due to its shape, the cross-flow turbine is a low-speed machine and is best used for locations with a low head and a high flow.
Reaction Type Turbines
Inward vs. Outward Reaction Turbines
All reaction turbines fall into the inward and outward category based on the water flow direction. If water enters the runner from outwards to inwards, we call the turbine an inward reaction turbine. On the contrary, outward reaction turbines use the water that flows through the runner from inwards to outwards.
The Francis turbine is the most popular type of turbine used in hydropower plants and sites with high head applications (130 to 2,000 feet). Moreover, this type of turbine works equally well in both horizontal and vertical orientations. Water enters the Francis Turbine radially and flows inwards toward the center. Once the water has flown through the turbine, it exits axially parallel to the rotational axis. Due to a wide head range and high efficiency, the Francis turbine type became the industry leader in the hydropower world.
High-pressure water enters the Francis turbine through a snail-shell casing called a volute. This lowers the pressure of the water but maintains its speed prior to encountering guide vanes. Consequently, the guide vanes direct the water flow toward the fixed blades of the runner at an optimum angle, causing the water to deflect slightly sideways and axially. Finally, the water exits out to the draft tube and into the tail race. As a result, water contacting the runner blades and being deflected results in a force that pushes the blades in the opposite direction thereby rotating the runner and transferring power from the water to the turbine shaft for electric generation.
The propeller turbine is a variation of inward flow reaction turbine with a runner shaped like a propeller that you see in ships and submarines. It comes with either fixed or adjustable blades. The water flow in the propeller turbine is controlled by adjustable guide vanes (wicket gates). The vanes push the water through the runner and react with the blades. Propeller turbines are commonly used in sites with high flow rates. There are a few types of propeller turbines available on the market, including:
The bulb turbine is a compact propeller turbine with an upstream watertight casing that contains a generator located on a horizontal axis. The main pro of the bulb turbine is its space-saving design since the turbine and generator form one sealed unit. However, this makes the bulb turbine hard to access for service. It also requires a certain temperature and air circulation conditions.
The Straflo turbine is a compact propeller turbine with a generator built into the rim of the turbine runner. Thanks to this design, Straflo turbines allow the unit to operate in low-head conditions while keeping most of the generator components out of the water.
The Kaplan turbine is a propeller turbine with adjustable blades and automatically adjusted wicket gates. It offers users a wide range of head/flow levels. Kaplan turbine is also efficient in the low head/high flow applications which makes it stand out compared to the Francis turbine.
The kinetic turbine is a free-flow turbine that generates electricity from the kinetic energy of the flowing water as a combination of an efficient axial flow propeller and advanced controls. It uses the natural water stream pathway and does not require any diversion of water through pipes, human made channels, or riverbeds, making it compact in size and easy to install.
Now that you have discovered how different types of turbines used in hydroelectric power plants operate, you may be considering which one to choose for a particular project. Since there are various turbine types to choose from and multiple factors to consider, Sorensen Systems is here to help you. While we do not supply turbines, we partner with turbine manufacturers on hydroelectric installation and refurbishment projects. Our cost-effective turbine-generators and turbine control systems provide reduced maintenance requirements and savings to small hydro sites around the world.