Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 2 | 85

Standard Motor

60 hp

h = 89.0%

High-Efficiency Motor

60 hp

h = 93.2%

FIGURE 2–61

Schematic for Example 2–17.

Then the rate at which mechanical energy is supplied to the turbine by the
fluid and the overall efficiency become

(b) Knowing the overall and generator efficiencies, the mechanical efficiency
of the turbine is determined from

(c) The shaft power output is determined from the definition of mechanical
efficiency,

Discussion Note that the lake supplies 2455 kW of mechanical energy to
the turbine, which converts 1964 kW of it to shaft work that drives the gen-
erator, which generates 1862 kW of electric power. There are losses associ-
ated with each component.

EXAMPLE 2–17 Cost Savings Associated with High-Efficiency
Motors

A 60-hp electric motor (a motor that delivers 60 hp of shaft power at full
load) that has an efficiency of 89.0 percent is worn out and is to be
replaced by a 93.2 percent efficient high-efficiency motor (Fig. 2–61). The
motor operates 3500 hours a year at full load. Taking the unit cost of elec-
tricity to be $0.08/kWh, determine the amount of energy and money saved
as a result of installing the high-efficiency motor instead of the standard
motor. Also, determine the simple payback period if the purchase prices of
the standard and high-efficiency motors are $4520 and $5160, respectively.

Solution A worn-out standard motor is to be replaced by a high-efficiency
one. The amount of electrical energy and money saved as well as the simple
payback period are to be determined.
Assumptions The load factor of the motor remains constant at 1 (full load)
when operating.
Analysis The electric power drawn by each motor and their difference can
be expressed as

 1 Rated power 21 Load factor 211 >hst 1 >heff 2

Power savings
W

#

electric in,standardW

#

electric in,efficient

W

#

electric in,efficient
W

#

shaft>heff
^1 Rated power^21 Load factor2>heff

W

#

electric in,standard
W

#

shaft>hst
^1 Rated power^21 Load factor2>hst

W

#

shaft,out
hturbine^0 ¢E

#

mech,fluid^0 
^1 0.80^21 2455 kW^2 
1964 kW

hturbinegen
hturbinehgeneratorShturbine

hturbinegen

hgenerator

0.76

0.95

0.80

hoverall
hturbinegen

W

#

elect,out

0 ¢E

#

mech,fluid^0

1862 kW

2455 kW

0.76

0 ¢E

#

mech,fluid^0 
m

#

1 emech,inemech,out 2 
 1 5000 kg>s 21 0.491 kJ>kg 2 
2455 kW