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40 J Sr inivasan~ m ix e d z on e

~ w n ; n [ x e : ~ : n : n e

z 2

0 3 0 6 0 9 0 ~ 0 .9 1 0 1 .1 1 2t e m p e m t u r e d e n s i t yFigure 1. Different zones in a solar pond.

g'Tcc

p o n d s ' . T h e r e a r e o t h e r w a y s to p r e v e n t m i x i n g b e tw e e n t h e u p p e r a n d l o w e r la y e r s.O n e o f t h e m is th e u s e o f a t r a n s p a r e n t h o n e y c o m b s t r u c tu r e w h i c h t r a p s s t a g n a n ta i r a n d h e n c e p r o v id e s g o o d t r a n s p a r e n c y t o s o l a r r a d i a t i o n w h i le c u t ti n g d o w n h e a tl o ss f r o m t h e p o n d . T h e h o n e y c o m b s t r u c t u r e is m a d e o f t r a n s p a r e n t p l a s ti c m a t e r ia l .O r t a b a s i & D y k s t e r h u i s ( 19 85 ) h a v e d i s c u s s e d i n d e t a il t h e p e r f o r m a n c e o f ah o n e y c o m b - s t a b i l i z e d p o n d . O n e c a n a l s o u s e a t r a n s p a r e n t p o l y m e r g e l a s a m e a n so f a l l o w i n g s o l a r r a d i a t i o n t o e n t e r t h e p o n d b u t c u t t i n g d o w n t h e l o s s e s f r o m t h epond t o t he am b i en t . W i l k i n s & L ee (1987 ) have d i s cus sed t he pe r fo rm ance o f a ge l( c ro s s - li n k e d p o l y a c r y l a m i d e ) p o n d .I n t h i s r e v ie w w e d i s c u ss s a l in i ty g r a d i e n t s o l a r p o n d s a s t h i s t e c h n o l o g y h a s m a d et r em end ou s p rog re s s i n the l a s t f i ft e en yea r s . T yp i ca l tem pe ra t u re and dens i t y p ro f i le si n a l a rge s a l i n i t y g rad i en t so l a r pond a re show n i n f i gu re 1 . W e f i nd t ha t t he re a r et h r ee d i s t in c t z o n e s in a s o l a r p o n d . T h e l o w e r m i x e d z o n e h a s t h e h i g h e s t te m p e r a t u r ea n d d e n s i t y a n d i s t h e r e g i o n w h e r e s o l a r r a d i a t i o n is a b s o r b e d a n d s t o re d . T h e u p p e rm i x e d z o n e h a s t h e l o w e s t t e m p e r a t u r e a n d d e n s i t y . T h i s z o n e i s m i x e d b y s u r f a c ew i n d s , e v a p o r a t i o n a n d n o c t u r n a l c o o l i n g . T h e i n t e r m e d i a t e z o n e i s c a l l e d t h e n o n -c o n v e c t iv e z o n e ( o r th e g r a d i e n t z o n e ) b e c au s e n o c o n v e c t i o n o c c u r s h e r e . T e m p e r a -t u r e a n d d e n s i t y d e c r e a s e f r o m t h e b o t t o m t o t h e t o p i n t h i s l a y e r , a n d i t a c t s a s at r a n s p a r e n t i n s u l a t o r. I t p e r m i t s s o la r r a d i a t i o n t o p a s s t h r o u g h b u t r e d u c e s t h e h e a tl o s s f r o m t h e h o t l o w e r c o n v e c t i v e z o n e t o t h e c o l d u p p e r c o n v e c t i v e z o n e . H e a tt r a n s f e r t h r o u g h t h is z o n e is b y c o n d u c t i o n o n l y . T h e t h ic k n e s s e s o f t h e u p p e r m i x e dl a y e r , t h e n o n - c o n v e c t i v e l a y e r a n d t h e l o w e r m i x e d l a y e r a r e u s u a l l y a r o u n d 0 " 5 ,1 m and 1 m , r e spec t i ve ly .

3. Therm al performanceT h e t h e r m a l p e r f o r m a n c e o f a s o l a r p o n d c a n b e r e p r e s e n t e d i n a f o r m s i m i l a r t ot h a t u s e d f o r c o n v e n t i o n a l f l at p la t e c o l le c t or s . A s s u m i n g a s t e a d y s t a t e c o n d i t io n ,

Q , = Q , - Q e ,w here Q , = u se fu l hea t ex t r ac t ed , Q a = so l a r ene rg y abso rbed , Q e = h ea t l o s se s.T h e t h e r m a l e f fi ci en c y o f a s o l a r p o n d c a n b e d e f i n e d as r / = ( Q , / I ) where I i s t hes o l a r e n e r g y i n c id e n t o n t h e p o n d . T h e r m a l e ff c ie nc y c a n b e w r i t t e n a s r / = % - Qe/1,whe re % i s ca l l ed the o pt i ca l e f f ic i ency of the po nd (Qa/I). W e exp re ss Qe = U o (Ts - T, ) ,w h e r e T~ is th e p o n d s t o r a g e - z o n e t e m p e r a t u r e , T a is th e a m b i e n t t e m p e r a t u r e a n dU o is t he ove ra l l hea t - l o s s coe f fi c ien t . I f w e neg l ec t hea t l o s se s f rom t he bo t t om ands id e s o f t h e p o n d a n d a s s u m e t h a t t h e t e m p e r a t u r e o f t h e u p p e r m i x e d l a y e r is th e

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Solar pond technology 41%90

A - f l a t p l a t e ; 1 g l a s s c o v e r,f l a t b l a c k c o a t in g80B- f l a t p la t e ~ 2 g lass cove rs ,black chrome s elect ive coat ing

70 C- so la r pond non-con vec t i vezone 0.8 mD so lar pond non-convec t ive60 zone 1.8m

- ! 5 0

~ 4ot. .

30

20 D

10Figure 2. Var ia t ion of thermalefficiency with A T / l , where ATis the temperature differencebetween the storage zone (orabsorber) and the ambient, andI is the intensity of solari r radia t ion.

0 0-1 0-2 0.3 0.4 05 C m 2 /W& T / I

s a m e a s t h e a m b i e n t , t h e n U o = K w / b w h e r e K . . is t h e t h e r m a l c o n d u c t i v i t y o f w a t e ra n d b is t h e t h i c k n e ss o f t h e g r a d i e n t z o n e .

K o o i ( 1 9 79 ) h a s c o m p a r e d t h e e f fi ci e n cy o f a s o la r p o n d w i t h t h o s e o f c o n v e n t i o n a lf l a t p l a t e c o l l e c t o r s as s ho w n i n f igu r e 2 . W e f i nd t ha t t he t h e r m a l e f f i c ie nc y o f as o l a r p o n d is h i g h e r t h a n t h a t o f a f la t p l a t e c o l l e c t o r w h e n t h e o p e r a t i n g t e m p e r a t u r e sa r e h i g h e r , a n d is in t h e r a n g e o f 2 0 t o 3 0 % w h e n t h e t e m p e r a t u r e d i f f e re n c e i s a r o u n d6 0 C . T h e t h e r m a l e f f ic ie n c y is s t ro n g l y d e p e n d e n t u p o n t h e t r a n s p a r e n c y o f t h e p o n dw hi c h i s i n f l ue nc e d by t he p r e s e nc e o f a l ga e a nd dus t . E ve n i f t he s g l a r p on d is fr e eo f d u s t a n d a l g ae , t h e a b s o r p t i o n p r o p e r t i e s o f p u r e w a t e r i n f lu e n c e th e t r a n s m i s s i o no f s o l a r r a d i a t i o n i n t h e p o n d . T h e t r a n s m i s s i v i t y o f s o la r r a d i a t i o n i n p u r e d i s ti ll e dw a t e r i s s h o w n i n fi g u re 3 . W e o b s e r v e t h a t a b o u t h a l f t h e s o l a r r a d i a t i o n is a b s o r b e di n th e f ir st 5 0 c m o f w a t e r. T h i s is o n a c c o u n t o f s t r o n g i n f r a r e d a b s o r p t i o n b a n d si n w a te r . A t a d e p t h o f 2 m e t r e s t h e t r a n s m i s s i o n i s a b o u t 4 0 % . T h i s s e ts t h e u p p e rl im i t o n t h e t h e r m a l e f f ic i en c y o f a s o l a r p o n d . T h e t h i c k n e s s o f t h e g r a d i e n t z o n em u s t b e c h o s e n d e p e n d i n g o n t h e t e m p e r a t u r e a t w h i c h t h e r m a l e n e r g y is n e e d e d . Ift h e t h i c k n e ss o f t h e g r a d i e n t z o n e i s t o o h i g h t h e t r a n s m i s s i o n o f s o l a r r a d i a t i o n i sr e d u c e d w h i l e if i t i s t o o s m a l l i t c a u s e s h i g h h e a t l o ss e s fr o m t h e b o t t o m t o t h e t o po f t h e p o n d . T h e o p t i m u m v a l ue o f th e t h ic k n e ss d e p e n d s o n t h e t e m p e r a t u r e o f t h es t o r a g e z o n e o f t h e p o n d . N i e l s e n (1 9 8 0 ) h a s p r o v i d e d a s t e a d y s t a t e a n a l y s i s o f as o l a r p o n d a n d h a s i n c l u d e d t h e e ff e ct o f s o l a r r a d i a t i o n a b s o r p t i o n i n t h e g r a d i e n tz o n e o n t h e t e m p e r a t u r e p r o f il e. I n t h e s t e a d y s t a te , t h e e n e r g y e q u a t i o n b e c o m e s

K ( d 2 T / d Z 2 ) = I ( d z /d Z ) , (1 )

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42 J S r i n i v a s a n1.0

0.~

0. 6g

0.4

0-2

F i g u r e 3 .

0 -S10

- \ \ \ ~\ \

. . . . . . . . . . . . " \ \ x ~ , x - -1pu re wa te r ( ac tua l AM 1 ~,~,N

- 4 - 3 - 2 - 1 1 210 10 10 10 1.0 10 1 0 ml i g h t p a t h

Variation of transmission of solar radiation w ith path length.

w he r e K is t he t he r ma l conduc t i v i t y o f w a t e r a nd T i s t he f r ac t ion o f so l a r r ad i a t i onI r e ach i ng a dep t h Z .Th i s equa t i on can be i n t eg r a t ed t o ge t

(d T / d Z ) = { I / K } {T ( Z ) - T ( Z ~ ) + (d T / d Z ) z ~} , ( 2 )whe re Z2 i s the in te r face be tw een the grad ien t zone a nd s torag e zone . I f r/ i s thef r ac ti on o f t he i nc i den t s o l a r ene r gy w h i ch i s ex t r ac t ed f r om t he s y s t em a s h ea t( inc luding gro un d losses ), then an en ergy ba lanc e of the s torage zo ne g ives

( d T / d Z ) t z ~ = { I / K } {z(Z 2) -- r/}. ( 3 )W e can com bi ne ( 2) and (3 ) t o ob t a i n t he t emp e r a t u r e p r o f il e in t he g r ad i en t zo ne a s

( d T / d Z ) = { I / K } {(z(Z) - ~/)}.The t emp e r a t u r e p r o fi le in t he g r ad i en t zo ne f o r va r i ous va l ue s o f r / i s s how n i n

f igure 4 for I = 200 W /m 2 . S ince T i s p ro po r t ion a l to I , the a bo ve f igure can a l so beus ed fo r o t he r va l ue s o f I b y mu l t i p ly i ng by an app r o p r i a t e cons t an t .

The e f fect o f g r oun d - hea t l o s se s on t he pe r f o r m ance o f a s o l a r p ond ha s b eenana l y s ed by H u l l e t a l (1984) . They have shown tha t the ground hea t - loss coef f ic ien tcan be exp r e s s ed a s

U g = K ( 1 / D + b P / A ) ,w her e K i s t he g r ound conduc t i v i t y . D i s t he dep t h o f t he w a t e r t ab l e , P and A a r et he pond pe r i me t e r and s u r f ace a r ea and b i s a cons t an t w hos e va l ue i s a r ound 0 ' 9(depending up on the s ide s lope). The therm al e f f ic iency o f a s teady s ta te so la r po nd

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S o l a r p o n d t e c h n o l o g yt ~

0

LO

0.Q

o 2

I

/ 036

30 20

Figure 4.

solar input 1:20 0 W/ m 2

4 0 6 0 8 0 1 0 0 Ct e m p e r a t u r e d i f f e r e n c e

Variation of efficiencywith depth and temperature difference.

43

can now be written as,(Z)dZZ 2 - Z 1 , I '

where AT is the temperature difference between the storage zone and the upper mixedlayer.

Note that the optical efficiency of the solar pond is dependent upon the meantransmittance in the gradient zone. This is because the radiation absorbed in thegradient zone is helpful in reducing the heat losses from the storage zone.The steady-state analysis of a solar pond is useful in the sizing of the pond for aspecific application. There will, however, be strong seasonal variation in the per-formance of the pond on account of seasonal variations in solar insolation, windand temperature. Srinivasan (1990) has proposed a simple two-zone model for thesimulation ofthe storage zone temperature of the pond. How does this simple two-zonemodel predict the observed features of the seasonal variation of storage zone tempera-ture in the Bangalore solar pond? The observed values of solar radiation, heatextraction and gradient zone thickness in the Bangalore solar pond were used in thesimulation. The predictions of the storage zone temperature are compared with theobservations (figure 5). The predicted storage zone temperatures are in good agreementwith observation. Predictions based on climatological variation of solar radiationshow higher deviation. This is because solar radiation in September 1986 was muchlower than predicted by climatology.

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44 J Srinivasan

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e 68 / ,

E 64 / / /L ~N~60 , I~ l 'L ::sd r;t::inO bo .se dne o s u r e d - ~ / / " " ' / . . " ,i; " \56 t "adiation

53 J I I IF M A M J1986

Figure 5. Var iat ion of s torage zone tempe rature wi th t ime in Bangalore solarpond.4 . P o n d c o n s t ru c t io nT h e s it e s e le c te d f o r t h e c o n s t r u c t i o n o f a s o l a r p o n d s h o u l d h a v e t h e f o l lo w i n ga t t r i bu t e s ;( a) be c lo s e t o t he po i n t w he r e t he r m a l ene r gy f r om t he po nd w i ll be u t i li z ed ;( b) be c l o s e t o a s ou r ce o f w a t e r f o r f l u s h i ng the s u r f ace m i xed - l aye r o f t he pon d ;(c ) t h e t h e r m a l c o n d u c t i v i t y o f t h e s o i l s h o u l d n o t b e t o o h i g h;( d ) t he w a t e r t ab l e s hou l d no t be t oo c l o s e t o t he s u r f ace .

A n e s t i m a t e o f th e a r e a r e q u i r e d f o r a s o l a r p o n d ( in t h e t r o p ic s ) c a n b e o b t a i n e df r o m f ig u r e 6 ( a d a p t e d f r o m F y n n & S h o r t 1 9 8 3) . T o m i n i m i z e h e a t l o s s es a n d l in e rcos t s , t he pon d s ho u l d be c i r cu l a r . S ince a c i r cu l a r pon d i s d i f fi cu l t t o co ns t r uc t , as q u a r e p o n d i s n o r m a l l y p re f e rr e d . In s o m e c a s e s, s u c h a s t h e B a n g a l o r e s o l a r p o n d ,t h e s it e c o n s t r a i n t s m a y f o r ce o n e t o c o n s t r u c t a r e c t a n g u l a r p o n d w i t h l a r g e a s p e c tr a t i o . Fo r l a r ge s o l a r ponds ( a r ea > 10 , 000m 2) , t he s hape w i l l no t have a s t r ongi n fl u e n ce o n c o s t o r h e a t l os se s . T h e d e p t h o f t h e s o l a r p o n d m u s t b e d e t e r m i n e dd e p e n d i n g o n t h e s p ec if ic a p p l ic a t i o n . T h e u s u a l t h i c k n e ss e s o f t h e s u rf a ce , g r a d i e n tan d s t o r age z one o f t he p on d a r e 0 .5 , 1 an d 1 m , r e s pec ti ve l y . I f a p a r t i cu l a r s i t e hasl o w w i n d s , o n e c a n r e d u c e t h e t h i c k n e s s o f t h e s u r fa c e l a y e r to 3 0 c m . I f t h e t e m p e r a t u r er e q u i r e d f o r p r o c e s s h e a t a p p l i c a t io n s is a r o u n d 4 0 C ( s u ch a s h a t c h e r ie s ) t h e n t h et h i c k n e ss o f th e g r a d i e n t z o n e c a n b e r e d u c e d t o 0 "5 m . S t o r a g e z o n e t h i c k n e s s h i g h e rt h a n 1 m m a y b e r e q u i r e d t o t a k e c a r e o f l o n g p e r i o d s o f c l o u d in e s s .

T h e e x c a v a t i o n f o r a s o l a r p o n d is s i m i la r t o t h a t f o r c o n s t r u c t i o n o f w a t e r r e s e r v o ir s .T h e s id e s l o p e o f th e p o n d c a n v a r y b e t w e e n 1 : 1 t o 1 : 3 d e p e n d i n g u p o n t h e t y p e o fs o i l . A f t e r t he excava t i on and bund i ng i s com pl e t ed , and be f o r e a l i ne r i s l a i d , onem u s t e n s u r e t h a t t h e a r e a is f re e o f s h a r p o b j e c ts w h i c h m a y d a m a g e t h e l in e r w h e ni t i s be ing l a id .

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46 J S r i n i v a s a nf r e s h w a t e ri n je c t io n d e s i r e d w a t e r l e v e l

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O O o o o o o o o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o -o - . . . . .bo t t om o f / ~ g r a d i e n t z o n e

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Figure 7. Establishmen t of ini-tial (a), halfway (b) and final(e) gradient zones in a solarpond. (From Hul l e t a l 1989.)

t h e p o n d . T h e s a l t d i ss o lv e s r a p i d l y if t h e w a t e r i n t h e p o n d i s c i r c u l a te d t h r o u g h ap u m p a n d t h e w a t e r is d ir e c t e d as a j e t i n t o t h e p o n d . T h e c o n c e n t r a t i o n o f t h e s a lta t t he s t o r age zo ne i s be t w e en 200 t o 300 kg / m 3. H e nce t he s a l t i nve n t o r y is be t w ee n1 /3 t o 1 /2 t o n p e r m 2. T h e n o r m a l m e t h o d o f e s t a b li s h in g t h e g r a d i e n t z o n e i s b yi n j e c ti o n o f f re s h w a t e r. T h i s m e t h o d , d e v e l o p e d b y Z a n g r a n d o (1 98 0), i s c o n v e n i e n ta n d h e n c e h a s b e e n a d o p t e d i n m o s t s o l a r p o n d i n s t a l la t i o n s i n th e w o r l d . F r e s hw a t e r i n j ec t i on i s i n i t i a t ed a t t he i n t e r f ace be t w een t he s t o r age zone and t he g r ad i en tzone us i ng a d i f fu s e r (s ee f igu r e 7 ). Th e f r e s h w a t e r r i s es t o t he t o p an d r educes t heden s i t y o f t he l aye r abov e t he i n j ec t i on po i n t . F o r eve r y 1 cm r is e i n w a t e r l eve l, t hed i f fu s e r i s l i ft ed by 2 cm . W he n t he d i f fu s e r i s a t t he s am e l eve l a s t he w a t e r s u r f acet h e e s t a b l is h m e n t o f t h e g r a d i e n t z o n e i s c o m p l e t e d . M o r e f r e s h w a t e r is a d d e d a b o v et he g r ad i en t zone t o c r ea t e an upp e r m i xed l aye r w i t h a t h i cknes s o f 30 t o 50 cm .T h e e v o l u t i o n O f d e n s i t y p r o fi le d u r i n g t h e e s t a b l i s h m e n t o f th e g r a d i e n t z o n e i n t h eM i a m i s b u r g s o l a r p o n d i s s h o w n i n f i g u r e 8 .

A f t e r t h e e s t a b l i s h m e n t o f th e g r a d i e n t z o n e , t h e p o n d b e g i n s t o h e a t u p i f c l e ars k y c o n d i t i o n s p r e v ai l. T h e t e m p e r a t u r e i n t h e s t o r a g e z o n e o f t h e B a n g a l o r e s o l a rp o n d i n c re a s e d b y 3 0 C w i t h i n o n e m o n t h o f t h e e s t a b l i s h m e n t o f t h e g r a d i e n t z o n e .4.1 P o n d s t a b i l i t yA s o l a r p on d w i ll be s t a t i ca l l y s t ab l e i f i ts den s i t y dec r eas es w i t h h e i gh t f r om t heb o t t o m . A s o l a r p o n d is su b j e c te d t o v a r i o u s d i s t u r b a n c e s s u c h a s t h e w i n d b l o w i n ga t t he t op s u r f ace an d h ea t i ng o f t he s i de w a l ls . Th e c r i t e r i on f o r dyn am i c s t ab i l it yo f t h e p o n d i s s o m e w h a t m o r e s t r in g e n t t h a n t h a t f o r s ta t ic s t a b il it y . T h i s c r i t e r io nc a n b e o b t a i n e d b y p e r t u r b a t i o n a n a l y s is o f t h e b a s i c la w s o f c o n s e r v a t i o n o f m a s s ,

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Solar pond technology 4 7m3 .o

' , f i n o t l e v e lx

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f r e s h w a t e r a d d e d d u r in g f i ll i n gII 9

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Figure 8 . Ev olut io n of gradient zone dur ing estab l ishment in the M iam isburgsolar pond.

m o m e n t u m a n d e n e r g y . T h i s h a s b e e n d i s c u s s e d i n g r e a t d e t a i l b y N i e l d ( 1 9 6 7 ) a n dB a i n e s & G i l l ( 1 9 6 9 ) . T h e c r i t e r i o n f o r s t a b i l i t y o b t a i n e d f r o m s u c h a n a n a l y s i s c a nb e w r i t t e n a s

w h e r eB r ~ _ a S F S c + l q

- 1 - [ @ - ] = t h e rm a l e x p a n s i o n c o e ff ic ie n t,D r = ; L a T J/~ s = + - = s a l i n i t y e x p a n s i o n c o e f f i c ie n t .P r = P r a n d t l n u m b e r ,S c = S c h m i d t n u m b e r .

F o r t y p i c a l c o n d i t i o n s e n c o u n t e r e d i n a s o l a r p o n d , t h i s re s u l t c a n b e s i m p l i fi e d t oS- - > 1 .1 9 o T~ Z ~ Z '

w h e r e S i s i n k g / m 3 a n d T i n C .I n o r d e r t o p r e v e n t f o r m a t i o n o f i n t e r n a l c o n v e c t i v e z o n e s w i t h i n t h e g r a d i e n t z o n ei t i s e s s e n t i a l t h a t t h e a b o v e c r i t e r i o n i s s a t i s f i e d a t a l l p o i n t s w i t h i n t h e g r a d i e n tz o n e . H u l l et al ( 1 9 8 9 ) h a v e r e c o m m e n d e d t h a t a s a f e ty m a r g i n o f 2 is d e si r a b le .

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48 J Sr in i vasanSafety margin (SM) is defined as

whereth . . . t ica l J~S L S c -~- 1 J ( ~ Z "

In the Bangalore solar pond, an internal convective zone was formed although thesafety margin was above 2 (figure 9). This is believed to be on account of side wallheating. Akbarzadeh (1984) has performed laboratory experiments to study the effectof side-wall heating on the formation of internal convective zones. These studiesindicate that side-wall heating can result in the formation of internal convectivezones. In the Bangalore solar pond, the distance between the side walls was as lowas 9 m at the bottom of the gradient zone. The free convective boundary layers thatformed on the side walls could have merged at the centre and led to the formationof internal convective zones. In large solar ponds, side-wall heating would not beable to initiate the formation of internal convective zones and hence a safety marginaround 2 should be adequate.

The thickness of the gradient zone (which provides insulation and hence reducesheat losses) can be reduced by the formation of internal convective zones or erosionof the boundaries of the gradient zone. Erosion of the gradient zone-surface zoneinterface occurs primarily on account of wind-induced mixing. The effect of wind-induced mixing can be reduced by using floating plastic nets or pipes. Nielsen (1983)has reported tha t mean-squared wind speeds exceeding 20 m2/s 2 caused the erosionof the gradient zone at the rate of 1 cm/day, while for values below 10mZ/s 2 therewas no gradient erosion.

1 .0 1 18 { 2 3 . 7 ~ . 1 . 2 0 7 2 { 7 1 . t ~ * [ )

1 . 0 0 9 8 1 2 6 . 2 t 1 . 2 0 t ~ 0 1 7 3 . / ~ [ )

6 5 _ _ , I 10-08 1.10 1-12 1-14s p e c i f i c g r a v i t y

- - 2 1 - t , - - 8 6- - . - - 2 t ,. - t ,. _ 8 6

I1,16 I f I I I3 5 7 - 9 11s a f e t y n u m b e r

Figure 9. Formation of internal convective zone in the Bangalore solar pond.

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Solar pond technology 49E r o s i o n o f t h e g r a d i e n t z o n e fr o m b e l o w d e p e n d s u p o n t h e d e n s i t y a n d t e m p e r a t u r e

g r ad i en t s a t t he g r ad i en t zone - s t o r age zone i n t e r f ace . N i e l s en ( 1983) has de t e r m i nede x p e r i m e n t a l l y t h a t t h e g r a d i e n t z o n e - s t o r a g e z o n e i n t e r f a c e r e m a i n s s t a t i o n a r y i ft he s a l i n i t y and t em per a t u r e g r ad i en t s s a t is f y t he f o l l ow i ng r e l a t i ons h i p .

OS/OZ = A [O T/O Z] 0"63,w h e r e A = 28(kg /m 4) (m /K) '63 .

I f t he ac t ua l s a l i n i t y g r ad i en t i s m o r e t h an t ha t g i ven b y t he above c r i te r i on , theg r ad i en t zone w i ll m ove do w n w a r ds , w h i l e if i t is le ss , t he g r a d i en t zone w i ll e r ode .

T o e n s u r e t h a t t h e g r a d i e n t z o n e d o e s n o t e r o d e f r o m a b o v e , t h e d e n s i t y o f t h es u r face l aye r m us t be kep t a s l ow as pos s i b le . The den s i t y o f t he s u r f ace l aye r i nc reas eso n a c c o u n t o f d i ff u s io n o f s a l t f r o m b e l o w a n d b e c a u s e o f e v a p o r a t i o n . H e n c e t h es u r f ace l aye r m us t be f l u s hed r egu l a r l y w i t h f r e s h w a t e r t o keep t he s a l i n i t y be l ow5% (by weight ) .4.2 Salt replenishmentO n a c c o u n t o f t h e g r a d i e n t o f c o n c e n t r a t i o n b e t w e e n t h e s t o r a g e a n d t h e s u r fa c ezones , t he r e is a s t ead y d i ff u s i on o f s a l t t h r o ug h t he g r ad i en t zone . The t r ans po r t o fs a lt t h r o u g h t h e g r a d i e n t z o n e b y d i f f u si o n c a n b e e x p re s s e d a s

Q ,, = [ ( S t - S, )D]/b ,whe re b = thick ness o f grad ient zo ne, D = m ass d i f fus ion coeff icient , an d St , Su = sal ini tyi n l ow er and uppe r m i xed l aye r s , r e s pec t i ve l y .

I f t he s a l i n it y i n t he s t o r age zone is 300 kg / m 3 and i n t he s u r f ace zone i s 20 kg / m 3 ,g r ad i en t zone t h i cknes s i s I m and d i f f u s ion coe ff ic i en t o f s a lt i s 3 x 10 - 9m 2/ s , t hent he r a t e o f t r ans po r t o f s a lt by d i f f u s ion w il l be a bo u t 30 kg / m 2 yea r . I n s m a l ls o l a r p o n d s t h e s a l t t r a n s p o r t c a n b e a s h i g h a s 6 0 k g / m 2 y e a r b e c a u s e o f a d d i t i o n a ls a l t t r ans po r t t h r ough s i de - w a l l hea t i ng .

I f t he s a l t l o s t fr om t he s t o r age zo ne i s no t r ep l en i s hed r egu l a r l y then t he r e m ayb e a n e r o s i o n o f th e g r a d i e n t z o n e f r o m b e l o w o r f o r m a t i o n o f i n te r n a l c o n v e c t iv ez o n e s . T h e n o r m a l m e t h o d o f s a lt r e p l e n is h m e n t i s b y p u m p i n g t h e b r i n e i n th es t o r age zon e t h r o ug h a s a l t bed ( f igu r e 10 ) ; S r i n i vas an (1990) has s ho w n t ha t f o rs m a l l s o l a r pon ds a pas s i ve s a l t r ep l en i s hm e n t t ech n i que is adeq ua t e ( fi gu re 11 ). I nt h e B a n g a l o r e s o l a r p o n d ( 2 4 0 m z b o t t o m a r ea ) a b o u t 1 0 0 k g o f sa l t w a s a d d e d d a i l yt h r o u g h a c h u t e i n t o t h e s t o r a g e z o n e . T h e s a l t t h a t w a s a d d e d d i s s o l v e d w i t h i n a d a y .4.3 Algae controlT h e t h e r m a l e ff ic ie n c y o f a s o l a r p o n d is s t r o n g l y d e p e n d e n t u p o n t h e c l a r it y o f th epon d , w h i ch is r educe d b y t he p r e s ence o f a l gae o r dus t . B i t s o f deb r i s / dus t o r l e avesl i g h t e r t h a n w a t e r f l o a t o n t h e s u r f a c e a n d c a n b e s k i m m e d o f f . D u s t a n d d e b r i sm u c h h e a v i e r t h a n w a t e r w i l l s i n k t o t h e b o t t o m . S r i n i v a s a n & G u h a ( 1 9 8 7 ) h a v er e p o r t e d t h a t t h e d u s t a c c u m u l a t i n g a t th e b o t t o m o f t h e p o n d d o e s n o t a d v e r s e l ya f fe c t t h e a b s o r p t i o n o f s o l a r r a d i a t i o n a t t h e b o t t o m o f t h e p o n d . T h e d u s t f l o a ti n gi n th e g r a d i e n t z o n e c a n b e s e t tl e d b y a d d i n g a l u m . T h e g r o w t h o f a l g a e c a n b ec o n t r o l l e d b y a d d i n g b l e a c h i n g p o w d e r o r c o p p e r s u l p h a t e. I f t h e w a t e r u s e d i n t h epo nd is a lka l i ne , cop pe r s u l ph a t e w i l l no t d i ss o l ve . H u l l ( i 990 ) has p r o v i ded a de t a i l eda c c o u n t o f th e r e la t iv e m e r it s o f v a r io u s m e t h o d s o f a lg a e c o n t r o l .

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50 J Srinivasan

z \ \ 0zoo'k~, gradient zone

~ storage zone

//

Figure 10. Salt replenishment in solar po nd thr oug h an external salt bed (activereplenishment).

4 .4 Heat extractionH e a t c a n b e e x t r a c t e d f ro m t h e p o n d u s i n g a n i n t e r n a l o r a n e x t e r n a l h e a t e x c h a n g e r .T h e i n t e r n a l h e a t e x c h a n g e r i s i m m e r s e d i n t h e s t o r a g e z o n e . S u c h h e a t e x c h a n g e r sa r e m a d e o f c o p p e r o r p l a s ti c t o e l i m i n a t e t h e e f fe c t o f c o r ro s i o n , a n d a r e a p p r o p r i a t ef o r s m a l l s o l a r pond s . I n l a r ge s o l a r pond s ( a r ea > 1000 m 2 ) ex t e r na l he a t exch ange r sm ay be m or e conv en i en t . Thes e a r e m ade o f s t a in l e s s s tee l o r t i t an i um . I n t he Bang a l o r es o l a r p o n d e x p e r i m e n t s w e r e c o n d u c t e d w i t h t h r e e h e a t e x c h a n g e r s : ( 1 ) a t i t a n i u mhea t exchange r ( ex t e r na l ) , ( 2 ) a coppe r hea t exchange r ( i m m er s ed ) and ( 3 ) a p l a s t i ch e a t e x c h a n g e r ( i m m e r s e d ) . T h e i m m e r s e d c o p p e r h e a t e x c h a n g e r w a s f o u n d t o b em o s t r e li a b le i n t h e B a n g a l o r e s o l a r p o n d . H u l l et al ( 1 9 8 I ) h a v e d e m o n s t r a t e d t h a ta p o l y p r o p y ! e n e s u b m e r g e d h e a t e x c h a n g e r c a n b e u s e d i n a s o l a r p o n d a l t h o u g hi ts e ff ec ti veness w i ll no t be a s h i gh a s a copp e r hea t exchan ge r .

sal t~,,/ ,,~\ 254 mm ~ pipe for - - ~~ sa l t " r ep lenishment . . . . . .

sur f'ace zone

r adient zone

sal t not to scaleFigure 11. Passive salt replenishme nt.

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Solar pond technology 514.5 The ef fect of rainfal lRainfall can have beneficial as well as detrimental effects on the operation of a solarpond. If the rainfall is not heavy, it helps to maintain the density of the surface layerat a low value. During the monsoon, in the Bangalore solar pond, there was no needfor flushing the surface layer to maintain its density at a low value. Heavy monsoonrainfall can, however, penetrate to the gradient zone and dilute it. The analysis ofheavy rainfall (greater than 40 mm per hour) episodes in the Bangalore solar pondindicates that raindrops can penetrate to about 50cm from the surface. Hence, itmay be desirable to maintain higher surface zone thickness during the rainy season.

5. Solar pond applicationsIn the last thirty years more than sixty solar ponds have been built all around theworld. The largest solar pond built so far is the 250,000m 2 pond at Bet Ha Aravain Israel. This pond has been used to generate 5 MWe (peak) power using an organicRankine cycle. The heat stored in the solar pond can be used in a variety of applications.

In Argentina, solar ponds are being used commercially for production of sodiumsulphate using solution-refining techniques. The ore (rich in sodium sulphate) minedin the Andes Mountains is dissolved in a 400 m 2 solar pond constructed adjacent tothe mines. The brine is removed and placed in a cooling pond at night where sodiumsulphate crystallizes (for details, see Lesino et al 1982). Solar pond concepts havebeen used to prevent precipitation of magnesium sulphate in the salt works at theGreat Salt Lake in Utah, USA.

A 2000 m 2 solar pond has been constructed to provide hot water for a swimmingpool in Miamisburg, Ohio, USA, A 3355m e solar pond at E1 Paso, Texas, hasdemonstrated the use of a solar pond for food processing, power generation anddesalination. The feasibility of grain drying using a solar pond has been demonstratedat Montreal (Canada), Ohio (USA) and for heating greenhouses at Lisbon (Portugal).A 20,000m 2 solar pond in Italy has been used for desalination of sea water andproducing 120 tonnes of fresh water per day.

The major limitation of solar ponds for industrial applications is the fact that thetemperature in the storage layer cannot go beyond 95C. Many industries requireprocess temperatures above 100~C. Gommed & Grossman (1988) have discussed theuse of an absorption heat pump in conjunction with a solar pond to generate steam.If this approach is successful, it will open up new applications for thermal energyfrom solar ponds.5.1 The Indian experienceThe first solar pond in India was a 1200 m 2 pond built at the Central Salt and MarineChemicals Research Institute in Bhavnagar, Gujarat, in 1970. This solar pond wasbased on bittern, which is a waste product during the production of sodium chloridefrom sea water. The second solar pond was a 100 m 2 circular pond built in Pondicherryin 1980. This pond used sodium chloride and was operational for two years. Patel &Gupta (1981) have discussed the performance of this pond. The LDPE liner used inthis pond developed a leak and hence had to be replaced. Patel & Gupta (1981) haveshown that a new liner could be placed in the pond without too much loss of thermal

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52 J Srinivasanene r gy . The t h i rd so l a r pond was a 1600m 2 so l a r pond bu i l t a t Bhav naga r i n 1980.Th i s pon d was ba sed on b i t t e rn and hence had p r ob l em s wi th t he c la r i ty o f b i t te r n .The f ou r th so l a r pond was a 240 m 2 so l a r pon d com mis s ioned a t t he I nd i an I n s t i tu t eo f Sc ience, Banga lo r e , i n 1984 . Th i s po nd ha s p r ov ided l ong - t e r m da t a on co n t i nuou shea t ex t r ac t i on f r om sma l l so l a r ponds and ha s demons t r a t ed t he t e chn i ca l andeconom ic v i ab il i ty o f sma l l so la r po nds f o r low t emp e r a tu r e p r oces s hea t . S r i n iva san( 1985) ha s a r gued t ha t l ow t empe r a tu r e p r oces s hea t f r om sma l l so l a r ponds can beused in ha tcher ies , hos te l s an d da ir ies . A 400 m 2 so la r po nd has b een co ns t ru c ted a tM a s u r o n t h e W e s t C o a s t o f I n d i a t o s u p p l y h o t w a t e r n ee d s o f a r u r a l c o m m u n i t y .A 300 m 2 so l a r p ond is be ing bu i l t f o r an eng inee r ing co l lege a t H ub l i ( Ka r n a t aka )to supp ly ho t w a t e r f o r s t uden t hos t el s . A 60 00m 2 so l a r pon d ha s been com ple t edin Bhu j ( Gu ja r a t ) and i s expec t ed t o supp ly ho t wa t e r f o r a da i r y . Th i s p r o j ec t i sexpec t ed t o exp lo r e t he p o t en t i a l f o r de sa l i na t i on and pow er gene r a t i on . A 6000 m 2so l a r pond i s unde r cons t r uc t i on a t P ond i ch e r r y f o r gene r a t i on o f 100 kW e ( peak )power . The so l a r ponds a t Masu r and Bhu j expe r i enced s a l t l e akages on accoun t o fthe fai lure of LD PE l iners.I n I n d ia , G u j a r a t a n d T a m i l N a d u a r e th e t w o s t a te s w h e r e c o m m o n s a l t is p r o d u c e don a la rge-sca le . These two s ta tes have a r id reg ions where the land i s unsui tab le forag r i cu l t u r e and have mor e t han 300 c l ea r days a yea r . Gu ja r a t and Tami l Naducons t i t u t e 10% o f I nd i a ' s a r ea . I f 4% o f t he l and i n t he se two s t a t e s we r e u sed a sso l a r pond power p l an t s , t hey can gene r a t e abou t 200 b i l l i on k i l owa t t hou r s o fe l ec t r i c a l ene r gy . Thus , so l a r pond power p l an t s cou ld have me t a l l t he e l ec t r i c a lene r gy needs o f I nd i a i n 1988 . So l a r ponds shou ld t he r e fo r e p l ay an im por t a n t r o l ein mee t i ng t he f u tu r e ene r gy needs o f Ind i a u s ing a l oca ll y ava i lab l e and r enewab leene r gy sou r ce .5.2 EconomicsSo la r ene r gy conve r s ion dev i ce s have no t f ound widesp r ead app l i c a t i on becausethey requi re h igh in i tia l cap i ta l inves tment . The c os t o f a so la r po nd i s m uch lessthan t ha t o f t he conven t i ona l f l a t p l a t e co l l ec to rs . The co s t o f a so l a r p ond is , howeve r ,s t rongly de pen den t up on s ite-speci fic fac tors such as the loca l cos t o f exca va t ion :mdsal t. The therma l pe r form ance of a so la r po nd i s a l so dep end ent o n s i te -spec if ic fac torssuch a s so l a r i r r ad ia t i on , g r o und t he r ma l co nduc t i v i t y and w a t e r - t ab l e dep th . Hence ,t he r e is boun d t o b e l ar ge va r i a t ion i n the cos t o f t he r ma l ene r gy p r o duc ed bysolar ponds at different s i tes .Rao & Ki sho r e ( 1989 ) and Hu l l et al (1989) have provided a de ta i led ana lys i s oft he va r i ous co m pon en t s o f t he cos t o f a so l a r pond . I f t he cos t o f s a l t and i ts r e cyc l ingis exc luded , H ul l et al (1989) es t ima te the cos t o f a la rge so la r po nd (a rea > 100 ,000m 2)to be a r o und U S $1 0 /m 2 ( in 1986 ) and t ha t o f a sma l l so l a r pon d ( a r ea a r oun d1000m 2 ) t o be a r ou nd US $ 50 /m 2. I f the cos t o f s a lt i s $4 0 / t on ne t he cos t o f la r geand sma l l so l a r po nds a r e a r oun d $ 45 /m 2 and $ 85 /m 2, r e spec t ive ly . In t he e s t ima t e sp r o v i d e d b y H u l l et al (1989), the c os t o f sa l t represen ts 50% o f the to ta l cos t o f asma l l so l a r pond and mo r e t han 75% o f t he t o t a l co s t o f a la r ge so l a r pond . H ence ,i t i s essent ia l tha t l a rge so la r ponds be loca ted c lose to s i tes where sa l t i s ava i lab lea t l ow cos t. I n I nd i a , sma l l so l a r ponds can be cons t r uc t ed a t a cos t o f Rs 200 t oRs 400 pe r m 2 ( S r in iva san 1985 ; Ra o & Ki sho r e 1989 ). Ra o & Ki sho r e ( 1989) havep r ov ided t he t o l lowing de t a i led b r ea kd ow n o f t he cos t o f a so l a r pond pe r squa r e

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Solar pond technology 53m e t r e C r

Cp = 2-546 (Cx + C2)"4- 0"675C3 + 1"3C4 + 0"45 6Cs + 0"04 15C 6+ 0"124C7 + 0"021C8 + 0"085C9 + Clo,

w h e r e C 1 = e xc a va t i on c h a r ge s , R s / m a ; C 2 = w a t e r c ha r ge s , R s / m 3 ; C 3 = s a l t c o s t ,R s / t on ne ; C 4 = l ine r c o s t , R s / m 2 ; C 5 = c la y , R s / t on ne ; C 6 = c os t o f b r i c ks , R s / 100 0b r i c ks ; C ~ = c os t o f c e m e n t , R s / ba g ; C a = c os t o f s a nd , R s / m a ; C 9 = c os t o f b r i c kl i n ing , R s / m 3 ; C ~ o = c o s t o f w a ve s upp r e s s e r , R s / m 2 .

R a o & K i s h o r e ( 1 9 89 ) h a v e u s e d t h e n e t p r e s e n t - v a l u e m e t h o d t o e s t i m a t e t h e c o s to f t h e r m a l o r e l e ct ri ca l en e r g y f r o m s o l a r p o n d s . T h e c o s t o f th e r m a l e n e r g y f r o ms o l a r p o n d s c a n b e e s t i m a t e d a s

C t h = [CRF x Cp + CM]/rlpSi,w h e r e C th = c o s t o f t h e r m a l e n e r g y , R s / k W h ; C p = c a p i ta l c o s t o f t h e s o l a r p o n d ,R s / m 2 ; CM = m a i n t e n a n c e c o s t o f s o l a r p o n d , R s / m 2 ; r/p = t h e r m a l e f f ic i en c y o f s o l a rp o n d , S i - - a v e r a g e i n c i d e n t s o l a r e n e r g y , k W h / m 2 y e a r.

T h e v a r i a t io n o f t h e c o s t o f t h e r m a l e n e r g y f r o m t h e p o n d f o r v a r io u s v a lu e s o ft h e c a p i t a l c o s t o f t h e p o n d a n d t h e m a i n t e n a n c e c o s t o f t h e p o n d i s s h o w n i n f ig u r e 12 .W e f in d t h a t s o l a r p o n d s p r o d u c e t h e r m a l e n e r g y a t a c o s t l o w e r t h a n t h a t o b t a i n e df r om bu r n i ng f o s s i l f ue l s o r e l e c t r i c i t y .

R a o & K i s h o r e ( 1 9 89 ) h a v e e s t i m a t e d t h e c o s t o f e l e c t ri c i ty o b t a i n e d f r o m a s o l a rp o n d a s f o l l o w s

C E - Ca[CP + CppGe/N] + CMG , ( 1 - f )

w h e r e C r = c os t o f e l e c tr i c i t y , R s / k W he ; C a = c a p i t a l r e c o ve r y f a c t o r ; C p = c os t o fR s / k W h

1.0

l_

0"5EJ~4,.,"S8

0.5

s o la r e n e rgy 1 50 0 k W h / m 2ca p i ta l recove ry fac to r 12 %

Rs 120m 2 y ea r ~ I ~ - ~ ' - "

Rs 30m-2-yeor

I I I I I I100 200 300 400 500 600 700 R s l m 2pond cap i ta l cost

Figure 12. Cost o f therma l energy as a funct ion of pon d capi ta l cos t.

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54 J Srin ivasans o l a r p o n d , R s / m 2 ; C pp = c o s t o f o r g a n i c R a n k i n e c y c l e p o w e r p l a n t , R s p e r k W e ;G e = g r o s s e l e c tr ic i ty g e n e r a ti o n , k W h e / m 2 ; N = n u m b e r o f h o u r s o f o p e r a t i o n p e ry ea r ; C u = m a i n t en an c e co s t, R s / m Z ; f = f r ac t i o n a l p a ra s i t ic l o sse s .I f w e a s s um e t h a t C R = 0 . 1 2 5 , C p = R s 1 40 /m 2, G e = 2 0 k W h e / m 2, C p p = R s1500/kW e, N = 5000 h , CM = R s 7 / m 2 , an d f = 0 .2 , w e o b t a i n C g = R s 2 / k W h e . W ef in d t h a t t h e c o s t o f e le c t ri c it y o b t a i n e d f r o m a s o l a r - p o n d p o w e r p l a n t i s h ig h e rt h a n t h a t o b t a i n e d f r o m f o s s il f u e l -b a s e d t h e r m a l p o w e r p l a n t s b u t i s c o m p a r a b l e t ot h e c o s t o f el e c tr i ci ty f r o m d i e s el g e n e r a t i o n s e ts . F r o m t h e a b o v e a n a l y s i s w e c a na l so i n fe r t h a t t h e co s t o f e l e c t r ic i t y f ro m a so l a r - p o n d p o w er p l a n t w i ll r ed u c e t oR s 1 / k W h e i f t h e cap i t a l co s t o f t h e so l a r p o n d r ed u ces t o R s 1 2 / m 2. T h i s i s i m p o ss i b l et o ach i ev e u n l e s s t h e re i s a n a t u ra l s i t e ( su ch a s a s a l t l ak e ) w h i ch r eq u i r e s n o s a l t ,d i g g i n g , o r l i n e r . W e can co n c l u d e , t h e re fo re , t h a t e l e c t r i c i t y g en e ra t i o n f ro m so l a rp o n d s i s n o t e c o n o m i c a l ly v i ab l e u n l e ss t h e s i t e c o n d i t i o n s a r e e x t r e m e l y f a v o u r a b l e .

6. Conclusions

S o l a r p o n d t e c h n o l o g y h a s m a d e t r e m e n d o u s p r o g r e s s i n t h e l a s t f if te e n y e a r s . A ne x c e l l e n t m o n o g r a p h i s n o w a v a i l a b l e o n t h e s c i e n c e a n d t e c h n o l o g y o f s a l i n i t yg r a d i e n t s o l a r p o n d s ( H u l l e t a l 1 9 89 ). T h i s t e ch n o l o g y i s co s t e f fec ti v e fo r l o w t em p e ra -t u r e p r o c e s s h e a t n e e d s o f i n d u s t r y . T h e g e n e r a t i o n o f e l e c tr i ci ty u s i n g s o l a r p o n d sis n o t e c o n o m i c a l ly v ia b le a s y e t. H o w e v e r , t h e n e w c o n c e r n s r e g a r d i n g t h e e n v i r o n -m e n t a n d t h e s a f e ty o f n u c l e a r p o w e r p l a n t s a n d n u c l e a r w a s t e d i s p o s a l m a y c h a n g et h e p i c t u re t o t a l l y .

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