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Solar Energy

Vol. 22 , pp . 149 .-154

0038--092X/7910201-0149l 02.0010

Pergam on Pre ss L td . , 1979 . P r in ted in Gre at Britain

D E S I G N N D O P T I M IS T IO N O F N B S O R P T IO N

R E F R IG E R T IO N S Y S T E M O P E R T E D BY SO L R E N E R G Y

S. ALIZADEH,F. BAHARand F. GEOOLA

M aterials and Energy R esearch Centre, Aryam ehr Un iversity of Tech. P.O . Box 41-2927, Tehran Iran.

(Received

10

M a rch

1978;

revision accepted

14

August

1978)

A b s t r a c t A

general theoretical stu dy on des ign and optimisationof the w ater-l i thium bromide and the amm onia-

water ab sorption refrigeration cycles has been un dertaken. The results of th is study show tha t in gen eral for fixed

initial conditions and g iven system refrigeration capacity higher generator tem perature causes h igher cooling ratio

with smaller heat exchange surfaces and consequently lower cost . A comparison of the two cycles also indicate

that the water-lithium brom ide system is sim pler than the amm onia-w ater system and o perates at a higher cooling

ratio and sm aller heat exchan ge surfaces for the sam e conditions.

I . IN T R O D U C T I O N

Theore t i c a l s t ud i e s o f t he pe r fo rmance o f abso rp t i on

re f r i ge ra t i on cyc l e s i nc lud ing t hose us ing wa te r - l i t h ium

b r o m i d e a n d a m m o n i a - w a t e r a s r e f r i g e r a n t - a b s o r b e n t

combina t i ons have a l r eady been r epor t ed by va r ious

au thor s [ l -3 ] . In t h i s pape r fu r the r t heore t i c a l s t ud i e s

have been pe r fo rm ed cons ide r ing i n pa r t icu l a r the de s ign

and op t imi sa t i on o f an absorp t i on r e f r i ge ra t i on cyc l e

ope ra t ed b y so l a r ene rgy . The wa te r - l i t h ium bromide and

a m m o n i a - w a t e r r e f r i g e r a n t - a b s o r b e n t c o m b i n a t i o n s

were chosen fo r t h i s s t udy because o f t he i r ex t ens ive use

in abso rp t i on r e f r i ge ra t i on sys t ems e spec i a l l y t hose

ope ra t ed b y so l a r ene rgy .

2 . C Y C L E

N LYSIS

The des ign and op t im i sa t i on o f t he pe r fo rm ance o f an

absorp t i on r e f r i ge ra t i on cyc l e depends mos t ly on t he

ex i s t ing i n i ti a l cond i t i ons . Fo r exa mple , t he t empe ra tu re

o f t he evapora to r i s f i xed and a s a r e su l t t he l ow-s ide

pre ssu re o f t he cyc l e i s f i xed and cou ld be spec i f i ed . Two

othe r f i xed pa rame te r s a re t he condense r and absorbe r

t empe ra tu re s . I t ha s been show n th a t [ l ] t he coo l ing r a ti o ,

de f ined a s t he r a t i o o f t he ene rgy r emoved f rom the

su r round ings dur ing t he r e f r i ge ra t i on phase t o t ha t

supp l i ed t o t he gene ra to r du r ing t he r egene ra t i on phase ,

i n c r e a s e s a s t h e c o n d e n s e r a n d a b s o r b e r t e m p e r a t u r e s

dec rea se . In sys t em s us ing a wa te r coo l ed cond ense r and

absorbe r t he se two t empe ra tu re s depend on t he

t empe ra tu re o f t he ava i l ab l e coo l ing wa te r . Af t e r t he

t empe ra tu re s o f t he evapora to r , t he condense r and t he

absorb e r have been i den t if i ed t he gene ra to r t empe ra tu re

has t o be de t e rmined . Th i s i s t he l a s t and t he mos t

impor t an t pa ram e te r wh ich m us t be spec i f i ed , because

un l ike t he o the r t h ree t empe ra tu re s o f t he cyc l e t h i s

depend s on o the r f a c to r s . I f t he coo l ing r a t i o o f t he

sys t em i s cons ide red aga ins t o the r va r i ab l e s o f t he cyc l e

the fo l l owing approx ima te r e l a t i ons can be de r ived fo r

the two cyc l e s see F igs . 1 and 2 ) .

F o r t h e w a t e r - l it h i u m b r o m i d e c y c l e

h / , I )

7/= h7 + R - l )h4 -

Rh3

F o r t h e a m m o n i a - w a t e r c y c le

= l h ~ - h w ) + h w + R - l )h , -R h 7

Yr

in which R is the c i rcula t ion fac tor def ined as

2 )

R = yr - y, 3)

y~ - y,

whe re y r is t he w e igh t concen t ra t ion o f l i th ium bromide

or am monia i n t he r e f r i ge ran t vapour l e av ing the gene ra -

to r fo r wa te r - l i th ium brom ide sys t em y , = 0 ) , y , i s t he

we igh t concen t ra t i on o f t he weak so lu t i on weak i n

re f r i ge ran t ) l e av ing t he gene ra to r and ys i s t he we igh t

concen t ra t i on o f t he s t rong so lu t i on l e av ing the ab sorbe r .

The subsc r ip t s in eqns I ) and 2 ) r e fe r t o the s t a t e s

rep re sen t ed i n F igs . 1 and 2 , r e spec t i ve ly . Equa t ions 1 )

and 2 ) have been eva lua t ed fo r a number o f gene ra to r

t empe ra tu re s wi th evapora to r t empe ra tu re o f 1 .7C ,

conde nse r t empe ra tu re o f 35C and abso rbe r t empe ra tu re

of 21C.

3. EFF ECTS OF USING SENSIBLE HEAT EXCHANGERS

I f sens ib l e hea t exchange r s a re used be tween t he ab -

so rbe r and t he gene ra to r fo r bo th t he wa te r - l i t h ium

b r o m i d e a n d t h e a m m o n i a - w a t e r s y s t e m s s o t h a t t h e

s t rong so lu t i on l e av ing t he absorbe r i s hea t ed by t he

weak so lu t i on l eav ing the g ene ra to r , t he t emp e ra tu re r i se

o f t he s t rong so lu t i on A Ts, i s g iven by t he fo l l owing

equa t ion

A T ~ = F ( x , k ) b T , ,

4)

i n w h ic h A T m i s t h e m a x i m u m t e m p e r a t u r e d i ff e re n c e i n

the hea t exchange r and

F ( x , k ) i s

t he hea t exchange r

e f fec t i veness de f ined a s

b e X l / k ) - - l

F(x , k ) = ~ eX~ , l k~_ ,_ k

5)

whe re x i s t he hea t exchange r pa rame te r g iven by t he

149

SE

V o l . 2 2, N o . 2 - - 0

8/10/2019 Design and Optimisation of an Absorption

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150 S. ALIZADEH t al

C o n d e n s e r t

( 8 )

E x p .

E v a p o r a t o r

f

, 6 1

R e f r ig . 7 ) j

G e n e r a t o r

v a p o u r ]

S o l a r e n e r g y )

G

4 )

S t r o n g

s o l u t i o n

W e a k

s o l u t i o n . I

G ) ]

[ A b = b

o .

Fig. 1. Schematic of the water-l i thium bromide system.

( 3 )

( 2 )

S o l u t i o n h e a t

e x c h a n g e r

5

[