microhydro systems

Hydro electricity is created from the energy of water moving under the force of gravity.  Microhydro systems are those that leverage this energy through the use of small water turbines.  Such systems are becoming increasingly popular both because they have become more sophisticated and because they are among the most efficient ways of producing electricity.   Water is a much denser form of matter than air and because of this even a relatively small water turbine is capable of providing all of the energy needs for a single home.

In talking about microhydro systems two terms that will be used are head and flow.  The head pressure is determined by the vertical distance the water falls and is usually measured either in feet or in units of pressure such as pounds per square inch (PSI). Formally a unit of 1 PSI is define as the pressure exerted by a vertical column of water 2.31 feet high.  Flow is the quantity of water flowing past a given point in a given period of time.  This is usually defined in terms of gallons of water per minute (GPM).  The greater the head and the flow, the more power the water turbine can generate.

Water turbines are fairly straightforward devices and based on the same principals of water movement as the water mills that used to drive so much of our industry in the northeast.  To begin with a water intake is created either in a river, or a dammed section which is fed by a river or stream. The water from the intake is then fed to the turbine via a pipe or flume.  The water enters the turbine initially through a set of guide vanes which insure that the water hits the runner blades at the most effective angle.  When the water strikes the runner blades (think of the blades of a boat propeller) this force is turned into a circular motion by the blades causing the turbines rotor to rotate.  This rotor is connected to a generator shaft which then drives the electrical generator.  The water then simply exits via an opening at the bottom of the  turbine called a tailrace to then flow downstream.  At this point it is referred to as tailwater. This process is illustrated below:

Most small water turbines are divided into two types; high head turbines and low head turbines.  A high head turbine, as the name suggests, is one in which there is high head pressure created by a significant vertical drop in height.  This type of turbine is ideal for circumstances where a stream or river takes a significant drop such as in a hilly region or mountain.  In high head turbines there does not need to be a significant volume of water captured since the head pressure is very high and exerts a great deal of force even with a low volume of water.

Low head turbines tend to be used in scenarios like a slow moving river where the change in altitude is fairly minor over any given distance.  In order to get sufficient energy out of a low head turbine, much greater flow must be captured.  Because more flow is required for a low head turbine they tend to be placed in bigger casings and the flow is routed through much bigger pipes.  The runner vanes are also bigger in order to capture the most energy from the slower moving water.