Introduction hydroelectric Power what is it?

How Power is Computed
Before a hydroelectric power site is developed, engineers compute how much power can be 
produced when the facility is complete. The actual output of energy at a dam is determined by 
the volume of water released (discharge) and the vertical distance the water falls (head). So, a 
given amount of water falling a given distance will produce a certain amount of energy. The 
head and the discharge at the power site and the desired rotational speed of the generator 
determine the type of turbine to be used. 
The head produces a pressure (water pressure), and the greater the head, the greater the pressure 
to drive turbines. This pressure is measured in pounds of force (pounds per square inch). More 
head or faster flowing water means more power. 

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To find the theoretical horsepower (the measure of mechanical energy) from a specific site, this 
formula is used: 
THP = (Q x H)/8.8 
where: THP = theoretical horsepower 
Q = flow rate in cubic feet per second (cfs) 
H = head in feet 
8.8 = a constant 
A more complicated formula is used to refine the calculations of this available power. The 
formula takes into account losses in the amount of head due to friction in the penstock and other 
variations due to the efficiency levels of mechanical devices used to harness the power. 
To find how much electrical power we can expect, we must convert the mechanical measure 
(horsepower) into electrical terms (watts). One horsepower is equal to 746 watts (U.S. measure). 
Turbines
While there are only two basic types of 
turbines (impulse and reaction), there are 
many variations. The specific type of 
turbine to be used in a powerplant is not 
selected until all operational studies and 
cost estimates are complete. The turbine 
selected depends largely on the site 
conditions.
A reaction turbine is a horizontal or vertical 
wheel that operates with the wheel 
completely submerged, a feature which 
reduces turbulence. In theory, the reaction 
turbine works like a rotating lawn sprinkler 
where water at a central point is under 
pressure and escapes from the ends of the 
blades, causing rotation. Reaction turbines 
are the type most widely used. 

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An impulse turbine is a horizontal or vertical wheel that uses the kinetic energy of water striking 
its buckets or blades to cause rotation. The wheel is covered by a housing and the buckets or 
blades are shaped so they turn the flow of water about 170 degrees inside the housing. After 
turning the blades or buckets, the water falls to the bottom of the wheel housing and flows out.