Types Of Water Turbines, Matched To Your Stream

Six Shapes, Six Jobs

Hoover Dam, one of the best known power plants in America, does not run on one kind of turbine. Its main generators use one design, while two smaller units of a totally different design make the electricity for the dam itself.

There are several types of water turbines because no single shape works on every stream. A turbine that loves a steep mountain creek will barely move on a slow, wide stream, and the other way around.

Everything comes down to two numbers. Head is how far the water falls, and flow rate is how much water passes each second. More of either means more power. Here are the six types you will meet in home hydroelectric systems, and which stream each one wants.

How Does A Water Turbine Work?

A water turbine catches the energy of moving or falling water and makes a shaft rotate. That shaft drives a generator, and the generator makes electricity.

Scientists call the energy of moving water "kinetic energy". It is just a fancy name for the push something has because it is moving, like a rolling ball. The turbine converts that push into mechanical energy (the spinning), and the generator converts mechanical energy into electricity. The faster the water moves, the more kinetic energy it carries.

Engineers sort turbines into two families:

• Impulse turbines squeeze the water through a nozzle into a fast jet that slams into the blades out in the open air. Think of a water hose pointed at a pinwheel, where the pressure of the water turns into speed. They are typically used on high head sites, where the water falls a long way.
• Reaction turbines sit inside a closed casing completely full of water. The water pressure changes as it flows past the blades and changes direction, pushing them around. They suit lower drops with a lot more water.

Jet Turbines: Pelton And Its Cousin

The first one looks like a metal bicycle rim covered in little cups. Each cup catches the jet and flips the water back in the other direction, and the wheel rotates on so the next cup takes a hit.

Lester Pelton worked it out in the 1870s in California mining country, and the first one went to work at the Mayflower Mine in 1878. In micro hydro it can work from about 20 meters of drop, and even small units can reach about 90% efficiency, so nine-tenths of the water's power ends up turning the shaft.

The Turgo is its cousin, designed in 1919 as a tweak of the same idea. The jet hits the runner on one side at a slant and leaves from the other side, so the outgoing water never gets in the way of the incoming jet. That allows it to swallow more flow than a cup runner of the same size. For the same power the runner can be about half as wide, but its vanes are more fragile than the cups.

Cross-Flow: The Micro Hydro Turbine Used In Backyards

The cross-flow (or Banki) turbine looks like an old paddle wheel from a river boat, turned into a drum. Water enters one side, crosses through the middle of the rotor, and hits the blades a second time on the way out. That double pass is where the name comes from.

Its advantages are simple. The water leaving the drum washes away small leaves and dirt, so it partly cleans itself. It has only two bearings and three spinning parts, so a local mechanic can often fix it.

Its best efficiency is a bit lower than the other designs, but it keeps working well when the water flow drops in summer. That matters more at home than a top number on a test bench.

Reaction Turbines: Francis, Kaplan And Propeller

The first of these is the most widely used water turbine in the world. James Francis designed it in 1848 in Lowell, Massachusetts. Water spirals in through a snail-shaped casing, makes the curved runner rotate, and drops out through the middle.

It covers heads from about 40 to 600 meters, and big ones pass 90% efficiency. That is why you find them in giant hydroelectric power plants like Grand Coulee and the Three Gorges Dam in China. Outside big power plants it is usually too heavy and too precise to make sense.

The Kaplan looks like a ship's propeller inside a pipe, and it is built for a slow river with lots of water and a low head. Viktor Kaplan designed it in 1913. Both the blades and the gates that guide the water can change angle, so it keeps making electricity as the water flow rises and falls. A propeller turbine is the same idea with fixed blades, which is cheaper and fine where the flow hardly changes. The axial-flow turbine and generator set in the photo above is that kind of low-drop machine.

Archimedes Screw: The Oldest Design Still Spinning

The screw is a giant corkscrew in a tilted trough, and the earliest record of it is from between 250 and 220 BC. People cranked it to lift water into fields and out of ships.

Run it backwards and it becomes a turbine. Water pours in at the top, its weight makes the screw rotate on the way down, and a generator on the shaft turns that into electricity. It works on drops from 0.1 to 10 meters, where most other types of turbines struggle.

It turns slowly, so fish get through with far less harm, and that helped it win approval under strict rules in Europe. One on the River Thames even makes electricity for Windsor Castle.

Which Type Of Turbine Fits Your Stream?

Start with your own two numbers: measure your head and flow first, then check whether your stream carries enough water.

Impulse turbines are mostly a poor fit for low-head sites, while reaction turbines need lots of water. Real ranges overlap, and turbine makers say proper selection requires real expertise. Give a supplier your head and flow and they can estimate how efficient each design will be on your site.

Want to see the parts? Here is what sits inside a water turbine, and here is what a home hydro system costs. For the big picture, go back to the hydro overview.

Sources

• US Department of Energy, Types of Hydropower Turbines
• US Department of Energy, How Hydropower Works
• Wikipedia, Water turbine
• Wikipedia, Hoover Dam
• Wikipedia, Pelton wheel
• Renewables First, Pelton and Turgo turbines
• Wikipedia, Turgo turbine
• energypedia, Micro Hydro Power Plant: Turbine Types
• Wikipedia, Cross-flow turbine
• Wikipedia, Francis turbine
• Wikipedia, Kaplan turbine
• Wikipedia, Archimedes' screw
• Canyon Hydro, Turbines and Efficiency