Environmental Engineering Reference
In-Depth Information
• This design should have direct action pump shutdown.
• This system should be redundant.
• This system should have mechanisms for testing and calibration.
Lower Reservoirs
Lower reservoirs or afterbays may be found on existing reservoirs or in
stream and river valleys. The area of the afterbay should be large enough to
accommodate the spillage needs of the forebay. Alternative site designs for
an afterbay may include oceans, large lakes, various underground configu-
rations, water treatment ponds, and agricultural water storage reservoirs.
Waterways
The elements of the waterways needed for a PHES system are the headworks,
penstock, tailrace, and one or more surge tanks that allow the water to be
moved between the forebay and the afterbay.
Headworks
The headworks connect the forebay with the penstock and serve as the
entrance in the generation mode and the exit in the pumping mode. This
dual directional requirement of a PHES facility means that the flow must
avoid vortices in both directions to maximize efficiency of the facility. The
head works should also include trash racks to remove debris and prevent it
from entering the system.
Penstocks
A penstock or main water conduit between the forebay and the turbo machin-
ery is an important design component of a PHES facility. A PHES may have
single or multiple penstocks located above or below ground.
A major siting consideration for a PHES facility is minimizing the ratio of
total water conductor length to the head. Ideally this ratio is 1:1 where the
total conductor length is equal to the head; thus the positioning of the fore-
bay would be directly overhead of the turbo machinery (motor or generator).
See Figure 3.7.
When sizing a penstock for a Francis or propeller-type turbine more than
5 feet in diameter one can start with the empirical formula developed by
Sarkaria.
8
With an approximate penstock diameter and a desired power rat-
ing, one can approximate water velocities.
D = 4.44 (P
0.43
/H
0.65
)
(3.3)
where D = economical diameter of penstock (feet), P = rated horsepower (hp)
of turbine, and H= rated head of turbine (feet).
