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

Chapter 6 | 315

Solution The lightbulb of a refrigerator malfunctions and remains on. The

increases in the electricity consumption and cost are to be determined.

Assumptions The life of the lightbulb is more than 1 year.

Analysis The lightbulb consumes 40 W of power when it is on, and thus

adds 40 W to the heat load of the refrigerator. Noting that the COP of the

refrigerator is 1.3, the power consumed by the refrigerator to remove the

heat generated by the lightbulb is

Therefore, the total additional power consumed by the refrigerator is

The total number of hours in a year is

Assuming the refrigerator is opened 20 times a day for an average of 30 s,

the light would normally be on for

Then the additional hours the light remains on as a result of the malfunction

becomes

Therefore, the additional electric power consumption and its cost per year are

and

Discussion Note that not repairing the switch will cost the homeowner

about $50 a year. This is alarming when we consider that at $0.08/kWh, a

typical refrigerator consumes about $70 worth of electricity a year.

 1 616 kWh>yr 21 $0.08>kWh 2 $49.3/yr

Additional power cost 1 Additional power consumption 21 Unit cost 2

 1 0.0708 kW 21 8699 h>yr 2 616 kWh/yr

Additional power consumptionW

#

total,additional
^1 Additional operating hours^2

 8760  61 8699 h>yr

Additional operating hoursAnnual hoursNormal operating hours

61 h>yr

Normal operating hours 1 20 times>day 21 30 s>time 21 1 h>3600 s 21 365 days>yr 2

Annual hours 1 365 days>yr 21 24 h>day 2 8760 h>yr

W

#

total,additionalW

#

lightW

#

refrig^40 30.870.8 W

W

#

refrig

Q

#

refrig

COPR



40 W

1.3

30.8 W

SUMMARY

The second law of thermodynamicsstates that processes
occur in a certain direction, not in any direction. A process
does not occur unless it satisfies both the first and the second
laws of thermodynamics. Bodies that can absorb or reject
finite amounts of heat isothermally are called thermal energy
reservoirsor heat reservoirs.

Work can be converted to heat directly, but heat can be

converted to work only by some devices called heat engines.

The thermal efficiencyof a heat engine is defined as

hth

Wnet,out

QH

 1 

QL

QH