water turbines

Water turbines are basically fairly simple systems.  They consist of the following components:

• intake shaft  - a tube that connects to the piping or penstock which brings the water into the turbine
• water nozzle - a nozzle which shoots a jet of water (impulse type of turbines only)
• runner - a wheel which catches the water as it flows in causing the wheel to turn
• generator shaft - a steel shaft that connects the runner to the generator
• generator - a small electric generator that creates the electricity
• exit valve - a tube or shute that returns the water to the stream it came fro
• powerhouse - a small shed or enclosure to protect the water turbine and generator from the elements

Impulse vs. Reaction Turbines

Water turbines are also often classified as being either impulse turbines or reaction turbines.  In a reaction turbine the runners are fully immersed in water and are enclosed in a pressure casing.  The runner blades are angled so that pressure differences across them create lift forces, like those on aircraft wings, and the lift forces cause the runner to rotate.

In an impulse turbine the runner operates in air, and is turned by one or multiple jets of water which make contract with the blade.  A nozzle converts the pressurized low velocity water into a high speed jet much like you might use with a garden hose nozzle. The nozzle is aligned so that it provides maximum force on the blades. 

Types of Turbines

There are many kinds of micro hydro turbine designs. Typical microhydro generators have outputs of 10 kilowatts (kW) or less and can generate either DC or AC current depending upon the design. You will often hear water turbines referred to as either Pelton or Turgo turbines.  These terms have to do with the structure of the water wheel inside the turbine. 

Turgo Turbines

Pictured here is a Turgo style wheel. A Turgo turbine is an impulse type of turbine in which a jet of water strikes the turbine blades.  The structure of a Turgo wheel is much like that of airplane turbine in which the hub is surrounded by a series of curved vanes. These vanes catch the water as it flows through the turbine causing the hub and  shaft to turn.  Turgo turbines are designed for higher speeds than Pelton turbines and usually have smaller diameters.

Pelton Turbines

A Pelton turbine is also an impulse turbine but in this type of turbine the hub is surrounded by a series of cups or buckets which catch the water. The buckets are split into two halves so that the central area does not act as a dead spot incapable of deflecting water away from the oncoming jet.  The cutaway on the lower lips allows the following bucket to move further before cutting off the jet propelling the bucket ahead of it.  This also permits a smoother entrance of the bucket into the water jet. 

Cross-Flow Turbines

A cross-flow turbine, also sometimes called a Michell-Banki turbine (from the name of the manufacturer) is a turbine that uses a drum shaped runner much like the wheel on an old paddle wheel steamboat.  A vertical rectangular nozzle is used with this type of turbine to drive a jet of water along the full length of the runner. One advantage of this type of turbine is that it can be used in situations where you have significant flow but not enough head pressure to use a high head turbine. 

Francis Turbine

The Francis type of turbine is a reaction type of turbine in which the entire wheel assembly is immersed in water and surrounded by a pressure casing.  In a Francis turbine the pressure casing is spiral shaped and is tapered to distribute water uniformly around the entire perimeter of the runner.  It uses guide vanes to ensure that water is fed into the runners at the correct angle.

Propeller Turbine

A propeller turbine is just what its name implies.  It uses a runner shaped just like a boat propeller to turn the generator.  The propeller usually has six vanes.  A variation of the propeller turbine is the Kaplan turbine in which the pitch of the propeller blades is adjustable.  This type of turbine is often used in large hydroelectric plants.  An advantage of propeller type of turbines is that they can be used in very low head conditions provided there is enough flow.

Selecting the Best Type of Turbine

Which type of water turbine is best for a particular situation often depends on the amount of head (water pressure) you will have in your location and whether you want to suspend the turbine in the water  (reaction) or whether you want to use jets of water (impulse).  By looking at these factors together you can get some indication of what type of turbine design will work best: