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

Chapter 6 | 311

Solution A heat pump maintains a house at a constant temperature. The

required minimum power input to the heat pump is to be determined.

Assumptions Steady operating conditions exist.

Analysis The heat pump must supply heat to the house at a rate of QH


135,000 kJ/h 
37.5 kW. The power requirements are minimum when a

reversible heat pump is used to do the job. The COP of a reversible heat

pump operating between the house and the outside air is

Then the required power input to this reversible heat pump becomes

Discussion This reversible heat pump can meet the heating requirements of

this house by consuming electric power at a rate of 3.32 kW only. If this

house were to be heated by electric resistance heaters instead, the power

consumption would jump up 11.3 times to 37.5 kW. This is because in

resistance heaters the electric energy is converted to heat at a one-to-one

ratio. With a heat pump, however, energy is absorbed from the outside and

carried to the inside using a refrigeration cycle that consumes only 3.32 kW.

Notice that the heat pump does not create energy. It merely transports it

from one medium (the cold outdoors) to another (the warm indoors).

W

#

net,in

QH

COPHP

37.5 kW

11.3

3.32 kW

COPHP,rev

1

1 TL>TH

1

1  1  5 273 K2> 1 21 273 K 2

11.3

Refrigerators to preserve perishable foods have long been one of the essen-

tial appliances in a household. They have proven to be highly durable and

reliable, providing satisfactory service for over 15 years. A typical household

refrigerator is actually a combination refrigerator-freezer since it has a

freezer compartment to make ice and to store frozen food.

Today’s refrigerators use much less energy as a result of using smallerand

higher-efficiencymotors and compressors,better insulation materials, larger

coil surface areas, and better door seals(Fig. 6–54). At an average electric-

ity rate of 8.3 cents per kWh, an average refrigerator costs about $72 a year

to run, which is half the annual operating cost of a refrigerator 25 years ago.

Replacing a 25-year-old, 18-ft^3 refrigerator with a new energy-efficient

model will save over 1000 kWh of electricity per year. For the environment,

this means a reduction of over 1 ton of CO 2 , which causes global climate

change, and over 10 kg of SO 2 , which causes acid rain.

Despite the improvements made in several areas during the past 100 years

in household refrigerators, the basic vapor-compression refrigeration cycle

has remained unchanged. The alternative absorption refrigeration and

thermoelectric refrigerationsystems are currently more expensive and less

TOPIC OF SPECIAL INTEREST* Household Refrigerators

*This section can be skipped without a loss in continuity.

Refrigerator

More efficient motors

and compressors

Better door

seals

Better insulation

materials

FIGURE 6–54

Today’s refrigerators are much more

efficient because of the improvements

in technology and manufacturing.

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