Hydropower - Environmental Impacts of Renewable Energy - page 126

DID YOU KNOW?
An alternative to calculating the Hazen-Williams formula, called an align-
ment chart , has become quite popular for fieldwork. The alignment chart can
be used with reasonable accuracy.
Slope
Slope is defined as the head loss per foot. In open channels, where the water flows by
gravity, slope is the amount of incline of the pipe and is calculated as feet of drop per
foot of pipe length (ft/ft). Slope is designed to be just enough to overcome frictional
losses, so the velocity remains constant, the water keeps flowing, and solids will not
settle in the conduit. In piped systems, where pressure loss for every foot of pipe is
experienced, slope is not provided by slanting the pipe but instead by pressure added
to overcome friction.
Minor Head Loss
In addition to the head loss caused by friction between the fluid and the pipe wall,
losses also are caused by turbulence created by obstructions (i.e., valves and fittings
of all types) in the line, changes in direction, and changes in flow area.
r eservoIr s tored e nergy
A major component of a hydroelectric dam is the area behind the dam, its reser-
voir. The water temporarily stored there is called gravitational potential energy . The
water is stored above the rest of the dam facility so gravity can carry the water down
to the turbines. Because this higher altitude is not where the water would naturally
be, the water is considered to be at an altered equilibrium. The result is stored energy
of position, or gravitational potential energy. The water has the potential to do work
because of the position it is in. As shown in Figure 4.4 , gravity will force the water
to fall to a lower position through the intake and the control gate. When the control
gate is opened, the water from the reservoir moves through the intake and becomes
translational kinetic energy as it falls through the next main part of the system, the
penstock. Translational kinetic energy is the energy due to motion from one location
to another. The water moves (falls) from the reservoir through the long shaft of the
penstock toward the turbines, where the kinetic energy becomes mechanical energy.
The force of the water is used to turn the turbines, which turn the generator shaft.
The generators convert the energy of water into electricity and then step-up trans-
formers increase the voltage produced to higher voltage levels.
DID YOU KNOW?
In practice, if minor head loss is less than 5% of the total head loss, it is usu-
ally ignored.