Energy and Society, 2nd Edition

223

15

Electricity from

Falling Water

THE ROTARY GENERATOR

Michael Faraday’s discovery of the concept of continuous generation of electric-

ity was the rst crucial step on the way to large-scale availability of electricity.

Converting Faraday’s discovery to a practical working device is credited to several

engineers, among them Zénobe Gramme. The generator is, in essence, as shown

in Figure 15.1. In this simple generator, the conductor moves through the magnetic

eld because we turn it. In other words, we must do  on (or supply 

to) the generator, for which we therefore have the “reverse energy diagram” shown

in Figure 15.2. (Two points to keep in mind regarding these “reverse energy dia-

grams” are, rst, as the name implies, it is “backward” from the energy diagrams

introduced earlier, and, second, both arrows in the diagram have to change direction

from one type of energy diagram to the other.) The entire essence of large-scale

electric energy generation hinges on nding ways to spin the generator reliably and

economically.

THE WATER TURBINE

Turning the generator reliably and, ideally, at low cost requires a source of 

that itself is reliable and inexpensive. One source of cheap, reliable rotary motion

is the waterwheel. In principle,  provided by the waterwheel could be used to

operate a generator, as shown in Figure 15.3. Two changes occur in this . In

the waterwheel, water ows spontaneously from high to low gravitational potential

energy. When this happens, some  becomes available. The waterwheel is

the device used to capture or extract a portion of this energy as useful . The

generator causes a nonspontaneous movement of electrical charge from low to high

electrical potential energy—from low voltage to high voltage. This nonspontaneous

change will happen only if we supply  to the generator. The  being sup-

plied is that provided by the waterwheel.

Waterwheels used in medieval times were not very efcient devices for convert-

ing kinetic energy into , because the water still possesses signicant kinetic

energy even after it leaves the wheel. The quest for more efcient waterwheels

began around the time of the blossoming of the Renaissance. Leonardo da Vinci

recognized that water owing through curved blades bears against those blades

with a force or impulse, illustrated schematically in Figure 15.4. The device that

Leonardo invented was at rst called an “impulse wheel.” Nowadays it is generally

224 Energy and Society

known as a turbine. A turbine is a device that converts the kinetic energy of a

moving uid into rotating kinetic energy. The uid used in the turbine is called the

working uid. For the kinds of turbine discussed in this chapter, the working uid

is water. Later we will meet an extremely important family of turbines in which the

working uid is steam.

In a turbine operated by water, the water ows through pipes or ducts over curved

vanes. To provide the impulse to the vanes, the turbine has to be enclosed in a casing.

The higher the pressure of water supplied to the turbine (essentially, the higher the

(a) (b)

(c)

(d)

FIGURE 15.1 (a) The eld magnet and (b) drum armature of a direct current generator.

(c) Armature and (d) eld magnet of an alternating current generator.

225Electricity from Falling Water

column of water, or its “head”), the faster the turbine will spin. The rotational speed of

a waterwheel is limited by the speed of the stream. This is not so with turbines, where

it is easy to get rotational speeds as high as 30,000 revolutions per minute (rpm).

The development of the turbine during the mid-nineteenth century was a major

engineering advance in providing a source of  for the electrical generator.

The combination of a turbine and a generator can be represented as shown in

High gravitational

potential energy

Low gravitational

potential energy

Generator

Waterwheel

High electrical

potential energy

Low electrical

potential energy

FIGURE 15.3 Hydroelectricity is based on the relationship between gravitational and elec-

trical  as shown in this pair of energy diagrams.

Water

supply

FIGURE 15.4 A basic water-operated turbine relies on a stream of water directed against

the curved blades of the turbine.

High potential

Low potential

Work supplied to system

FIGURE 15.2 The generic reverse energy diagram, which is applicable to all .