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

Chapter 8 | 431

The irreversibility for this process is determined from its definition,

Discussion Notice that the reversible work and irreversibility (the wasted
work potential) are the same for this case since the entire work potential is
wasted. The source of irreversibility in this process is the heat transfer
through a finite temperature difference.

I
WrevWu
 8191  0 
8191 kJ

EXAMPLE 8–5 Heating Potential of a Hot Iron Block

The iron block discussed in Example 8–4 is to be used to maintain a house
at 27°C when the outdoor temperature is 5°C. Determine the maximum
amount of heat that can be supplied to the house as the iron cools to 27°C.

Solution The iron block is now reconsidered for heating a house. The max-
imum amount of heating this block can provide is to be determined.
Analysis Probably the first thought that comes to mind to make the most
use of the energy stored in the iron block is to take it inside and let it cool
in the house, as shown in Fig. 8–14, transferring its sensible energy as
heat to the indoors air (provided that it meets the approval of the house-
hold, of course). The iron block can keep “losing” heat until its tempera-
ture drops to the indoor temperature of 27°C, transferring a total of
38,925 kJ of heat. Since we utilized the entire energy of the iron block
available for heating without wasting a single kilojoule, it seems like we
have a 100-percent-efficient operation, and nothing can beat this, right?
Well, not quite.
In Example 8–4 we determined that this process has an irreversibility of
8191 kJ, which implies that things are not as “perfect” as they seem.
A “perfect” process is one that involves “zero” irreversibility. The irreversibil-
ity in this process is associated with the heat transfer through a finite tem-
perature difference that can be eliminated by running a reversible heat
engine between the iron block and the indoor air. This heat engine produces
(as determined in Example 8–4) 8191 kJ of work and reject the remaining
38,925  8191
30,734 kJ of heat to the house. Now we managed to
eliminate the irreversibility and ended up with 8191 kJ of work. What can
we do with this work? Well, at worst we can convert it to heat by running a
paddle wheel, for example, creating an equal amount of irreversibility. Or we
can supply this work to a heat pump that transports heat from the outdoors
at 5°C to the indoors at 27°C. Such a heat pump, if reversible, has a coeffi-
cient of performance of

That is, this heat pump can supply the house with 13.6 times the energy it
consumes as work. In our case, it will consume the 8191 kJ of work and
deliver 8191 13.6
111,398 kJ of heat to the house. Therefore, the hot
iron block has the potential to supply

1 30,734  111,398 2 kJ
142,132 kJ142 MJ

COPHP

1

1 TL >TH

1

1  1 278 K2> 1300 K 2

13.6

5 °C

27 °C

IronIron

200200 °C

Heat

FIGURE 8–14

Schematic for Example 8–5.