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
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.