Hydropower - Environmental Impacts of Renewable Energy - page 112

• Upstream reach is the segment of the river above the diversion forebay or

reservoir.

• Bypass reach is the segment of the river or stream between the diversion

structure or dam and the powerhouse. Dam facilities do not have bypass
reaches.

• Downstream reach is the segment of the river or stream below the power-

house discharge.

HYDROPOWER BASIC CONCEPTS *

Air pressure (at sea level) = 14.7 pounds per square inch (psi)

The relationship shown above is important because our study of hydropower basics
begins with air. A blanket of air many miles thick surrounds the Earth. The weight
of this blanket on a given square inch of the Earth's surface will vary according to
the thickness of the atmospheric blanket above that point. As shown above, at sea
level the pressure exerted is 14.7 pounds per square inch (psi). On a mountaintop, air
pressure decreases because the blanket is not as thick.

1 cubic foot (ft 3 ) of water = 62.4 lb

The relationship shown above is also important; note that both cubic feet and
pounds are used to describe a volume of water. A defined relationship exists between
these two methods of measurement. The specific weight of water is defined relative
to a cubic foot. One cubic foot of water weighs 62.4 lb. This relationship is true only
at a temperature of 4°C and at a pressure of 1 atmosphere, conditions referred to as
standard temperature and pressure (STP). One atmosphere equals 14.7 psi at sea
level, and 1 ft 3 of water contains 7.48 gal. The weight varies so little that, for practical
purposes, this weight is used for temperatures ranging from 0 to 100°C. One cubic
inch of water weighs 0.0362 lb. Water 1 ft deep will exert a pressure of 0.43 psi on
the bottom area (12 in. × 0.0362 lb/in. 3 ). A column of water 2 ft high exerts 0.86 psi
(2 ft × 0.43 psi/ft), one 10 ft high exerts 4.3 psi (10 ft × 0.43 psi/ft), and one 55 ft high
exerts 23.65 psi (55 ft × 0.43 psi/ft). A column of water 2.31 ft high will exert 1.0 psi.
To produce a pressure of 50 psi requires a 115.5-ft water column (50 psi × 2.31 ft/psi).
The two important points being made here are

1. 1 ft 3 of water = 62.4 lb (see Figure 4.2 ).

2. A column of water 2.31 ft high will exert 1.0 psi.

Another relationship is also important. As noted above, at standard temperature and
pressure, 1 ft 3 of water contains 7.48 gal and weighs 62.4 lb. Thus, we can determine
the weight of 1 gal of water:

Weight of 1 gal water = 62.4 lb/ft 3 ÷ 7.48 gal/ft 3 = 8.34 lb/gal

* Adapted from Spellman, F.R., The Science of Water , CRC Press, Boca Raton, FL, 2008.