Types of hydroelectric turbine

Types of hydroelectric turbine

The hydroelectric turbine is the heart of your system. It is the component that captures the kinetic energy of your watercourse and spins the generator to generate electricity. There are many different designs of the hydroelectric turbine, ranging from the huge, heavy and old fashioned water wheels rotating at 20 revolutions per minute (rpm) and originally used for milling, to high-speed turbines spinning at over 1,600rpn1 that you can hold in one hand.

Efficiency is key when selecting a hydroelectric turbine. Turbines are designed for specific heads and flows, and each turbine type can be adapted to meet a wide variety of requirements. Minor design differences, such as the curvature of the blades, the materials used to make the Turbine and the design of the turbine housing can all significantly change the characteristics of a turbine. Turbines are designed around the head and flow of the system and are critical to the design of your hydro-electric system.

To choose the right turbine for you, consult with the experts. Talk to hydro-electric designers and turbine manufacturers to identify the best solution. This book can guide you in the types of turbines that are available and give general guidance as to what kind of Turbine is best suited to different kinds of system, but it is no substitute to talking to an expert about your specific application.

There are three very distinct categories of turbines. Reaction turbines and impulse turbines are the main ones, while the traditional waterwheel is also used, typically for converting old water-powered flourmill into a hydro-electric power generator.

what is the impulse hydroelectric turbine?

An impulse turbine works with a jet of water aimed at the blades of the Turbine that make them spin around. The blades of the Turbine are usually bucket-shaped so that they catch the water and direct the water off at an angle. The flow of the water is diverted by moving the Turbine.

impulse turbines diagram

The diagram above demonstrates how an impulse turbine works: the jet of water is directed at the blades of the Turbine (1). The water then bounces back, imparting its kinetic energy on the Turbine. This force then turns the Turbine, in this case in a clockwise direction. The water, once its force has been spent, is then returned to the watercourse.

Some impulse turbines have more than one jet of water operating them, thereby increasing the power a_nd enabling the Turbine to spin faster. Impulse turbines HIE‘: typically used for medium and high-head applications where the head of water enables a high pressure for a comparatively small amount of water. The water is often fired through a nozzle to produce a high-velocity jet of water.

Common impulse hydroelectric turbine designs

Pelton wheel turbine

Designed for high-head systems, the Pelton wheel turbine works by directing powerful jets of water against a rotary series of bucket-shaped blades. The force spins the wheel while the water runs out of the buckets and exits the turbine housing.

Pelton wheels are designed to run extremely fast, often as fast as 1,6o0rpm, and achieve around 90% efficiency. The benefit of running at such a high speed is that there is often no need for a gearing system between the Turbine and the generator, thereby improving efficiency.

Pelton wheels are usually used for systems with a head of at least 25 meters (80 feet) and a flow rate of between 10-250 liters per second (2-65 US gallons per second).

Turgo turbine

Turgo turbines are a development of the Pelton wheel, using a different shaped bucket. The water enters one side of the bucket and exits the other, making for a more efficient water flow when compared to the Pelton wheel. They can handle a higher flow rate to a similarly sized Pelton wheel.

Crossflow turbine

Crossflow turbines are Widely used for micro-hydro sites, particularly in the United Kingdom. They can work with both low-head and l1igh-head systems.

In a crossflow turbine, the water runs across the Turbine. Shaped blades spin the Turbine as the water enters the wheel. As the water exits the wheel, the force of the water pushes the wheel again. Going through the blades of the Turbine twice provides additional efficiency and provides a self- cleaning mechanism: if small debris gets caught up in the Turbine, the crossflow method will usually flush it through very quickly.

Crossflow turbines run much slower tl1an other impulse turbine designs, typically at a_round 800 rpm. They require gearing to increase this speed to power the generator.

Crossflow turbines have been successfully deployed at sites with a head of under 2 metres (61/2 feet), although they are generally regarded as more suitable for sites with a head of 15 meters (50 feet) or more with a flow rate between 4o-5,oo0 liters per second (8-1,320 US gallons per second). ‘While on paper, they do not seem as efficient as some other turbine designs, they have a relatively flat efficiency curve. This means they can perform well with run-of-the-river systems where the flow rate can vary throughout the year, often yielding greater performance over a year than other turbines.

Crossflow turbine diagram

A simplified diagram crossflow Turbine, showing the water flowing through the Turbine. Real systems tend to have two or even four nozzles which are arranged so that theflows do not interfere with each other.

Reaction hydroelectric turbine

A reaction turbine is immersed in water, and the water pushes through it to make it rotate. Unlike an impulse turbine, where the water flow radically changes direction, the water flows through the Turbine to continue on its way.

Reaction turbines often look like propellers and in fact, are doing a similar job: a propeller on a boat turns at speed and pushes the water backward through the propeller, driving the boat forward. Here, the water is pushing through the propeller, forcing it to turn and driving a generator.

Reaction turbine diagram

In the diagram above, the water (1) pushes through the Turbine, making it turn clockwise (2). As the water loses its force and is cut by the Turbine, it becomes turbulent (3). This water is then returned to the watercourse.

Common reaction hydroelectric turbine designs

Kaplan turbine

A Kaplan Turbine is a propeller with adjustable blades inside a tube. It is designed for low to medium head systems, typically between 11/2-10 meters (5-30 feet) with flows of between 100-3o,000 liters per second (25-8,000 US gallons per second).

Kaplan turbine diagram

Water flows through two inlets to drive the blades of the propeller

The Kaplan Turbine is extremely flexible due to its adjustable pitch blades. A flat profile works best for low-flow systems while a steep pitch profile provides the best power for higher water flows. In micro-hydro systems, they typically rotate at between 300-500rpm.

The diagram shown on page 107 shows a simplified example of a hydro-electric system powered with a Kaplan turbine.

Archimedean screw hydroelectric turbine

Designed for low head systems from 11/2-5 meters [5-161/2 feet) with flows of between 1,000- 20,000 liters per second (260-5,300 US gallons per second), the Archimedean screw works using the same principle of the Archimedean screw pump used for pumping water uphill, but running in reverse. They are a slow turning turbine, typically rotating at around 20-30rpm.

Archimedean screw turbines graphe

The force of the water turns the screw turbine, which in turn powers a generator via a gearbox.

The Archimedean screw has become a popular choice for weirs on run of the river hydro-electric projects. There are many reasons for this: they work across a wide flow of rivers, handling different seasons well; they are fish-friendly, fish can swim through the screw safely; they do not easily clog up with debris, and they have proved to be extremely reliable.

Larger systems often have multiple Archimedean screws. On larger rivers, such as the River Thames in the United Kingdom, it is not uncommon to see small hydro-electric systems with two or three Archimedean screws running side by side.

Propeller turbines

The propeller turbine is similar to the Kaplan turbine, except they do not have adjustable blades. They are sometimes used for pico or nano hydro projects, collecting between 50-500w, for where the requirement is a simple system for generating sufficient power to power a small amount of on-site equipment.

Propeller turbines are designed to operate completely underwater. Many designs, such as the Ampair UW10o shown above, combine the propeller, the gearbox and the generator in a single unit. One or more turbines can then be frame mounted and installed entirely underwater.

Some of these turbines are marketed as suitable for zero-head systems as they are based purely on water flow. Efficiency is low, but if your requirement is modest, then these systems may be worth considering.

Waterwheels turbines

Waterwheels have been in use for thousands of years as a way of powering machinery for milling flour. Today, waterwheels are not commonly used for new hydro-power projects as their efficiency is not a match for newer designs. However, there are hundreds of old watermills still in existence, many with old and decaying waterwheels. Restoring and converting the waterwheels to generate electricity is a beautiful way of bringing these historic buildings back into use.

Waterwheels work by using the sheer weight and force of the water to turn an immense waterwheel. Rotating at between 6-15rpm, the wheels generate a huge amount of torque, which in turn can power a generator through a gearbox.

The amount of electricity generated from a waterwheel system tends to be fairly small typically in the region of 15-30kW is common and the engineering costs means that this is not a project that can be done on economic grounds. However, far too many watermills have fallen into disrepair over the past century, and using hydro-power is excellent of bringing these magnificent buildings back to life.

Many hydro-electric professionals steer clear of watermills due to the complexity of the installations. However, if you have a waterwheel and wish to convert it to generate electricity, there are several specialist companies available.

Drive systems and gearing

The drive system connects the Turbine to the generator. It enables the Turbine to rotate at its optimum speed, then delivers the power to the generator at the optimum speed for power generation.

In the case of the Pelton and Turgo turbines, the speed of the Turbine is usually the same speed that the alternators run at. In this case, the drive system is called a direct drive. Direct drive is the simplest, most efficient, and most reliable drive system available. For other turbines, the drive system needs to incorporate gearing to allow both the Turbine and the generator to run at their optimum speeds. There are different types of gearing, using gears, chains, or belts. All of these systems have a small amount of inefficiency-typically in the region of 2-5%.

For micro-hydro systems, belt drive systems tend to be popular, due to their lower purchase price, their reliability and their length of service. Belt drive systems are typically 96-97% efficient.

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