Remember from the "Power Equation" that the two most important factors are flowrate (Q) and head (h). Also, remember that head (h) is measured in feet and it is the vertical distance (elevation difference) between the lake elevation and the elevtion of the discharge of the power plant. Also, remember that head (h) is a surrogate for pressure. In fact, for each foot of head (elevation difference) there is an increase in pressure of 0.433 pounds per square inch (psi). It is this pressure along with flowrate (Q) that provides energy to the turbine which ultimately spins the generator.
Focus on Terrora (Rabun) and Tallulah Falls in the above table. Note that the operating head (h) is much greater than the dam height. How is this? Well, the designers took advantage of the topography (slope of the land) and put the power plant down hill from the lake and feed water to the power plant through large pipes. The extreme of these two (Terrora and Tallulah Falls) is Tallulah Falls. In fact the power plant is over a mile downstream and over 450 feet of elevation lower. By the way, 450 feet lower increases the pressure by about 195 psi. This additional 450 feet plus the height of the dam produces a pressure of about 250 psi at the Tallulah Falls power plant.
Now focus on Tallulah Falls as compared to the other projects in the above table. Also, be aware there are two products from a hydropower project that are of value. First is capability itself, that is to say merely the power (P). Even if the project is not being operated at a specific moment, that available power is of value (assuming it could be operated if you chose to). The second product of value is energy. So these two products are the second and sixth (final) columns. Note that Tallulah Falls has a power capability (Generators) of 72 MW (six at 12). Also, note that the average annual energy is 170.8 GWHr (gigaWatt hours). So, Tallulah Falls produces over 10 times as much energy as Nacoochee and has 15 times as much power capability (Generator column above) as Nacoochee. A portion of this difference is because the flowrate (hydraulic capacity) is greater but the vast, vast majority of the difference is because of this additional pressure (operating head above). And that additional operating head is produced by placing the power plant down hill from the lake.
So, why do we not just locate all hydropower plants significantly down hill from the lakes and use penstocks to feed the water to the plant? Topography dictates that decision. The "lay of the land" Remember ...
Two major factors determine the desireability of a particular site for hydropower. These are:
- Water Availability
- Topography of the land
Tallulah Falls Lake with Water Intake, Gorge and Powerhouse (over one mile downstream from lake)
