Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

12.3 Direct Current and Alternating Current 349

grouped together in many satellites to create an array that is used to generate electric power. Pho-

tovoltaic solar farms are also becoming common. A solar farm consists of a vast area where a

great number of solar arrays are put together to convert the sun’s radiation into electricity.

Power Plants

Electricity that is consumed at homes, schools, malls, and by various industries is generated in a

power plant. Water is used in all steam power-generating plants to produce electricity. A simple

schematic of a power plant is shown in Figure 12.3. Fuel is burned in a boiler to generate heat,

which in turn is added to liquid water to change its phase to steam; steam passes through turbine

blades, turning the blades, which in effect runs the generator connected to the turbine, creating

electricity. The electricity is generated by turning a coil of wire inside a magnetic field. A conductor

placed in a changing magnetic field will have a current induced in it. Magnetism is the most com-

mon method for generating electricity. The low-pressure steam leaving the turbine liquefies in a

condenser and is pumped through the boiler again, closing a cycle, as shown in Figure 12.3(a).

U.S. electricity generation by fuel type for the year 2005 is shown in Figure 12.3(b).

12.3 Direct Current and Alternating Current

Direct current (dc)is the flow of electric charge that occurs in one direction, as shown in

Figure 12.4(a). Direct current is typically produced by batteries and direct current generators.

In the late 19th century, given the limited understanding of fundamentals and technology and

for economic reasons, direct current could not be transmitted over long distances. Therefore,

it was succeeded by alternating current (ac). Direct current was not economically feasible

Turbine

Generator

Cool water

Steam

Gas

Bottom ash

hopper

Cleaned Boiler

plant gas

Air quality

control

system Pump

Water

River

Condenser

Coal mill

Coal silo

Coal

Chimney

Disposal to storage

pond or landfill

Transmission

line

Transformer

■Figure 12.3(a) A schematic of a steam power plant.

Source:Courtesy of Xcel Energy.

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