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

hot NH 3 H 2 O solution, which is weak in NH 3 , then passes through a
regenerator, where it transfers some heat to the rich solution leaving the
pump, and is throttled to the absorber pressure.
Compared with vapor-compression systems, absorption refrigeration sys-
tems have one major advantage: A liquid is compressed instead of a vapor.
The steady-flow work is proportional to the specific volume, and thus the
work input for absorption refrigeration systems is very small (on the order
of one percent of the heat supplied to the generator) and often neglected in
the cycle analysis. The operation of these systems is based on heat transfer
from an external source. Therefore, absorption refrigeration systems are
often classified as heat-driven systems.
The absorption refrigeration systems are much more expensive than the
vapor-compression refrigeration systems. They are more complex and
occupy more space, they are much less efficient thus requiring much larger
cooling towers to reject the waste heat, and they are more difficult to service
since they are less common. Therefore, absorption refrigeration systems
should be considered only when the unit cost of thermal energy is low and
is projected to remain low relative to electricity. Absorption refrigeration
systems are primarily used in large commercial and industrial installations.
The COP of absorption refrigeration systems is defined as

(11–12)

The maximum COP of an absorption refrigeration system is determined by
assuming that the entire cycle is totally reversible (i.e., the cycle involves no
irreversibilities and any heat transfer is through a differential temperature dif-
ference). The refrigeration system would be reversible if the heat from the
source (Qgen) were transferred to a Carnot heat engine, and the work output
of this heat engine (Whth,revQgen) is supplied to a Carnot refrigerator to
remove heat from the refrigerated space. Note that QLWCOPR,rev
hth,revQgenCOPR,rev. Then the overall COP of an absorption refrigeration sys-
tem under reversible conditions becomes (Fig. 11–22)

(11–13)

where TL,T 0 , and Tsare the thermodynamic temperatures of the refrigerated
space, the environment, and the heat source, respectively. Any absorption
refrigeration system that receives heat from a source at Tsand removes heat
from the refrigerated space at TLwhile operating in an environment at T 0
has a lower COP than the one determined from Eq. 11–13. For example,
when the source is at 120°C, the refrigerated space is at 10°C, and the
environment is at 25°C, the maximum COP that an absorption refrigeration
system can have is 1.8. The COP of actual absorption refrigeration systems
is usually less than 1.
Air-conditioning systems based on absorption refrigeration, called
absorption chillers,perform best when the heat source can supply heat at a
high temperature with little temperature drop. The absorption chillers are
typically rated at an input temperature of 116°C (240°F ). The chillers per-
form at lower temperatures, but their cooling capacity decreases sharply with

COPrev,absorption

QL

Qgen

hth,revCOPR,reva 1 

T 0

Ts

ba

TL

T 0 TL

b

COPabsorption

Desired output

Required input



QL

QgenWpump,in

QL

Qgen

Chapter 11 | 633

Source

Ts

T 0

environment

Reversible

heat

engine

Qgen

W = hrev Qgen

QL = COPR,rev × W

Environment

T 0

TL

Refrigerated

space

Reversible

refrigerator

W = hrev Qgen = (1 – )Qgen

QL = COPR,revW = ( (^) )W
COPrev,absorption = = (1 – )( (^) )
T 0
Ts
TL
T 0 – TL
Qgen
QL T 0
Ts
TL
T 0 – TL
FIGURE 11–22
Determining the maximum COP of an
absorption refrigeration system.