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

13.5 Efficiency 387

The efficiency of today’s power plants where a fossil fuel (oil, gas, coal) is burned in the boiler

is near 40%, and for nuclear power plants the overall efficiency is nearly 34%.

Electricity is also generated by liquid water stored behind dams. The water is guided into

water turbines located in hydroelectric power plants housed within the dam to generate electric-

ity. The potential energy of the water stored behind the dam is converted to kinetic energy as the

water flows through the turbine and consequently spins the turbine, which turns the generator.

Example 13.10 In Example 13.6, we determined the power required to move 30 people, with an average mass

of 61 kg (135 lbm) per person, between two floors of a building: a vertical distance of 5 m

(16 ft) in 2 s. The energy and the power requirements were 90,000 J and 45,000 W, respectively.

Moreover, we estimated savings in energy if 1 million people decided to walk up a floor instead

of taking the elevator on a daily basis. The minimum amount of energy saved during a year,

based on an estimate of 220 working days in a year, would be 660 GJ. Let us now estimate the

amount of fuel, such as coal, that can be saved in a power plant, assuming a 38% overall

efficiency for the power plant and a heating value of approximately 7.5 MJ/ kg for coal.

amount of coal required

1.74 10

12

J

7.5 10

6

J

kg

232,000 kg 1 511,472 lbm 2

0.38

660 GJ

energy input from fuel

1 energy input from fuel1.74 10

12

J1.74 TJ

power plant efficiency

energy generated

energy input from fuel

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 13.10 A schematic diagram of a steam water plant.

Source:CourtesyXcel Energy.

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