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

For a heat engine,the exergy supplied is the decrease in the exergy of the

heat transferred to the engine, which is the difference between the exergy of

the heat supplied and the exergy of the heat rejected. (The exergy of the

heat rejected at the temperature of the surroundings is zero.) The net work

output is the recovered exergy.

For a refrigeratoror heat pump,the exergy supplied is the work input

since the work supplied to a cyclic device is entirely available. The recov-

ered exergy is the exergy of the heat transferred to the high-temperature

medium (which is the reversible work) for a heat pump, and the exergy of

the heat transferred from the low-temperature medium for a refrigerator.

For a heat exchanger with two unmixed fluid streams, normally the

exergy supplied is the decrease in the exergy of the higher-temperature fluid

stream, and the exergy recovered is the increase in the exergy of the lower-

temperature fluid stream. This is discussed further in Sec. 8–8.

434 | Thermodynamics

EXAMPLE 8–6 Second-Law Efficiency of Resistance Heaters

A dealer advertises that he has just received a shipment of electric resis-

tance heaters for residential buildings that have an efficiency of 100 percent

(Fig. 8–19). Assuming an indoor temperature of 21°C and outdoor tempera-

ture of 10°C, determine the second-law efficiency of these heaters.

Solution Electric resistance heaters are being considered for residential

buildings. The second-law efficiency of these heaters is to be determined.

Analysis Obviously the efficiency that the dealer is referring to is the first-

law efficiency, meaning that for each unit of electric energy (work) con-

sumed, the heater will supply the house with 1 unit of energy (heat). That is,

the advertised heater has a COP of 1.

At the specified conditions, a reversible heat pump would have a coeffi-

cient of the performance of

That is, it would supply the house with 26.7 units of heat (extracted from

the cold outside air) for each unit of electric energy it consumes.

The second-law efficiency of this resistance heater is

which does not look so impressive. The dealer will not be happy to see this

value. Considering the high price of electricity, a consumer will probably be

better off with a “less” efficient gas heater.

hII

COP

COPrev

1.0

26.7

0.037 or 3.7%

COPHP,rev

1

1 TL>TH

1

1  1 283 K2> 1294 K 2

26.7

8–4 ■ EXERGY CHANGE OF A SYSTEM

The property exergyis the work potential of a system in a specified environ-

ment and represents the maximum amount of useful work that can be

obtained as the system is brought to equilibrium with the environment.

21 °C

Resistance

heater^10 °C

FIGURE 8–19

Schematic for Example 8–6.

SEE TUTORIAL CH. 8, SEC. 4 ON THE DVD.

INTERACTIVE

TUTORIAL