fuss 1984 aquacultural-engineering

7/28/2019 Fuss 1984 Aquacultural-Engineering

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Aquacu l t ura l Eng i neer i ng 3 (1984) 31-37

F e a s i b il it y o f S a l t w a t e r G r a d i e n t P o n d s a s a H e a tS u p p l y f or H a t c h e r y R e a r i n g W a t e rJoseph T. Fuss

National Fishery Research and Development Laboratory, US Fish and WildlifeService, RD No. 4, Box 63, WeUsboro, Pennsylvania 16901, USA

A B S T R A C TSal t g rad i en t so l ar co l l ec t or po nds are econ om i ca l l y feas i b l e as a source o fh e a t e n e r g y f o r h a t c h e r y r e ar in g w a t er . A p o n d 3 m d e e p a n d c o v e r in g a na re a o f 5 0 0 0 m 2 w i l l s u p p l y a p p r o x i m a t e l y 2 8 0 0 G J o f e n e r g y o v e r at h r e e - m o n t h p e r i o d - e n o u g h f o r a o n e - y e a r s m o l t p r o g r a m i n a n A t l a n t i cs a lm o n h a t c h e ry . T h e c o s t t o c o n s t r u c t s u c h a p o n d i s a p p r o x i m a t e l y$ l 7 3 ,0 0 0 , o r t h e e q u i v a le n t o f s e v e n y e a r s ' f u e l b ills u s in g c o n v e n t i o n a lh e a t in g e q u i p m e n t . A l t h o u g h t h e te c h n o l o g y i s n e w a n d n o t e n ti r e lyu n d e r s t o o d , e x i s ti n g p o n d s h a v e p r o v e n t o b e r e la t iv e l y p r o b l e m f r e e a n dr e q u i r e o n l y m i n i m u m m a i n t e n a n c e .

INTRODUCTIONThe pr oduc tion of ma ny hatcheries could be increased if their watersupplies were warmer. In particular, the Atlantic salmon hatcheries ofthe northeast USA could produce a one-year smolt (as opposed to thenormal two-year program) and nearly double their produ ction . Usually,the required temperature rise is only 3 or 4C, but the flow rates can bewell in excess of 600 liters min -t. The cost to a facili ty using an oil-firedboiler to meet this demand would be in the range $20,000-$30,000 perseason- an excessive figure that simply cannot be afforded. Variouseffort s have been made to reduce this expense, including recycling apercentage of the water or installing equipment such as a heat pump toreuse the energy (Fuss, 1983). These measures are effective, but thecosts remain prohibitive. A non-conventional approach to the problem

31Aquacu l t ura l Eng i neer i ng 0144-8609/84/$03.00 - Elsevier Applied SciencePublishers L t d , England, 1984. Printed in Great Britain


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32 Joseph T. Fussis t o e m p l o y a r e l a ti v e l y n e w c o n c e p t , t h e s a l t w a t e r g r a d i e n t s o l a rc o l l e c t o r p o n d , a s a s o u r c e o f e n e r g y . S e v e r al f ac i li t ie s h a v e o l d , n ol o n g e r u s e d , p o n d s i te s a v a i l a b le t h a t c o u l d b e c o n v e r t e d i n t o s o la r h e a tc o l l e c t o r s . I t is e c o n o m i c a l l y f e a s i b l e t o b u i l d a s a lt g r a d i e n t p o n d t h a tis c a p a b l e o f c o l l e c t in g a n d s t o r in g e n o u g h e n e r g y fo r a o n e - y e a r sm o l tp r o g r a m .

S A L T G R A D I E N T P O N D SA l t h o u g h v e r y i n e ff i c ie n t , a f re s h w a t e r p o n d is a f o r m o f s o la r c o l l e c to r .T h e s u n ' s r a y s p e n e t r a t e t h e s u r f a ce a n d w a r m th e w a t e r . U n f o r t u n a t e l y ,t h is w a r m i n g e f f e c t a ls o c h a n g e s t h e w a t e r ' s d e n s i t y , r e s u lt i n g int h e r m a l c u r r e n t s a n d a l l o w i n g t h e w a r m w a t e r t o ri se t o t h e s u r f a c e . A tt h e s u r f a c e , t h e e f f e c t s o f w i n d a n d n a t u r a l c o n v e c t i o n c a u s e a l ar gep e r c e n t a g e o f t h e c a p t u r e d h e a t t o e sc a p e t o th e a t m o s p h e r e . T h e s al tg r a d i e n t p o n d is a n a t t e m p t t o p r e v e n t t h e t h e r m a l c u r r e n t s fr o m o c c u r -r in g b y m a n i p u l a t i n g t h e w a t e r ' s d e n s i t y .

A sa l t g r a d i e n t s o l a r p o n d is c o n s t r u c t e d t o m a i n t a i n t h r e e d i s t i n c tz o n e s o r l a y e r s (F i g . 1 ). T h e b o t t o m l a y e r, o f t e n c a ll e d t h e s t o r a g ez o n e , c o n t a i n s t h e g r e a t e s t p e r c e n t a g e o f s al t a n d is o f c o n s t a n t d e n s i t yt h r o u g h o u t . A s s u c h , i t is s a i d t o b e " c o n v e c t i n g " a n d is s i m i l a r t o af r e s h w a t e r p o n d . T h e r m a l c u r r e n t s w o u l d n a t u r a ll y f o r m in t h is l a y e ri f h e a t w e r e a l l o w e d t o e s c a p e f r o m i ts u p p e r s u rf a c e . T h e c u r r e n t s a rep r e v e n t e d f r o m f o r m i n g b y i n su l a t in g t h e b o t t o m z o n e w i t h t h e m i d d l el a y e r , a ls o c a ll e d t h e n o n - c o n v e c t i n g o r g r a d i e n t z o n e . T h e m i d d l e l a y e r

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Diagram of a typical salt gradient solar collector pond. Zone dimensionsvary widely.


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Saltwater gradient ponds as heat supply for hatchery rearing water 33s e rv e s t o i n s u l a t e t h e s t o r a g e z o n e f r o m t h e a t m o s p h e r e . T h i s z o n e c a nb e c o m p a r e d t o a s t a i r w a y , e a c h s t a i r b ei n g a b o u t 3 c m t h i c k a n dh a v i n g a sa l t d e n s i t y s l i g h t l y l e s s t h a n t h e s t a ir b e l o w i t . S i n c e t h ed e n s i t i e s a r e d i f f e r e n t , t h e s t a ir s c a n n o t m i x t o g e t h e r ( i. e. v i s u a l iz e o i lf l o a t i n g o n w a t e r ) . T h e r m a l c u r r e n t s c a n o n l y f o r m w i t h i n e a c h s t ai r,r e s u l t in g in v e r y m i n o r h e a t l o ss e s. I d e a l l y , t h e s t a i r w a y w o u l d c o n -t i n u e t o t h e t o p o f t h e p o n d , b u t a g a in , t h e e f f e c t s o f w i n d . r a in , a n do t h e r f a c t o r s f o r c e t h e u p p e r s t a i rs t o m i x t o g e t h e r r e s u lt i n g in t h e t o pl a y e r o r s u r f a c e z o n e . T h e s u r f a c e z o n e s e rv e s v e r y l it tl e p u r p o s e o t h e rt h a n a c t i n g a s a b u f f e r f o r t h e g r a d i e n t z o n e . A l t h o u g h d i f f i c u l t t oa c c o m p l i s h , it is d e s i r a b l e t o k e e p i ts t h i c k n e s s t o a m i n i m u m . T h e >r e al ly , t h e t o p l a y e r o f a s a lt g r a d i e n t p o n d b e h a v e s a s a f r e s h w a t e r p o n d ,r el ea s in g m o s t o f it s e n e r g y b a c k i n t o t h e a t m o s p h e r e .

A p r o f i l e t h a t t y p i f i e s s a l t g r a d i e n t p o n d s is s h o w n i n F ig . 2 . T h ed a t a is f r o m a la rg e p o n d ( 2 0 2 0 m 2 ) l o c a t e d i n M i a m i s b u r g , O h i o( E t t e r , 1 9 8 3 ). C o n d u c t i v i t y a n d t e m p e r a t u r e a re p l o t t e d a s a f u n c t i o no f h e ig h t a b o v e t h e b o t t o m . T h e t h r ee d i s ti n c t z o n e s a r e v e r y e v i d e n t.T h e s t o ra g e z o n e e x t e n d s t o a b o u t 2 0 0 c m f r o m t h e b o t t o m a n d h a s an e a r l y c o n s t a n t c o n d u c t i v i t y o f 1 92 m m h o s c m -~. T h i s c o n v e r t s t o a b o u t2 0 % s a lt ( s o d i u m c h l o r i d e i n th i s c a s e ) b y w e i g h t . A l t h o u g h t h e t e m -p e r a t u r e o f t h is z o n e is a p p r o x i m a t e l y 3 5 C , u n d e r o p t i m u m c i rc u m -s t a n c e s it c a n b e in e x c e s s o f 6 5 C ( W i t t e n b e r g a n d H a r r is , 1 9 8 1 ) . T h es li gh t t e m p e r a t u r e i n c re a s e n e a r t h e u p p e r p o r t i o n o f t h e s t o r a g e z o n eis a p h e n o m e n o n t h a t o c c u r s w h e n h e a t is b e i ng e x t r a c t e d f r o m t h ep o n d . T h e g r a d i e n t o f t h e m i d d l e l a y e r c a n a l s o b e s e e n in F ig . 2 . I te x t e n d s f ro m 2 0 0 c m u p t o a b o u t 2 6 0 c m . B o t h t h e c o n d u c t i v i t y a n dt h e t e m p e r a t u r e a re g r a d u a l l y r e d u c e d . T h e s u r f a c e z o n e b e g in s a t2 6 0 c m a n d e x t e n d s t o t h e t o p o f th e p o n d a t a b o u t 3 2 0 c m . T h ea c t u a l t h i c k n e s s o f t h is l a y e r v a ri e s f r o m d a y t o d a y a n d i s a f u n c t i o n o fw i n d , r a i n a n d e v a p o r a t i o n . T h e m i x i n g e f f e c t w i t h i n t h i s z o n e r e s u l t sin t h e c o n s t a n t p l o t o f t e m p e r a t u r e a n d c o n d u c t i v i t y .

S I Z I N G A P O N DA p o n d ' s s iz e is d e t e r m i n e d b y ( a ) h o w m u c h h e a t i t h a s t o re t a i n a f t e rt h e w i n t e r : ( b ) t h e a m o u n t o f s o la r e n e r g y i t c an b e e x p e c t e d t o t ra pt h r o u g h o u t t h e h e a t i n g s e a s o n ( 2 0 - 3 0 % o f t h e e n e r g y r e ce i v e d ): ( c) t h er a t e o r d e m a n d o f h e a t e x t r a c t io n ; a n d ( d ) t h e m i n i m u m t e m p e r a t u r e


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Jo seph T . F ussT E M P E R A T U R E . " C

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Tem perature and con ductivity plot versus depth for the Miamisburg,Ohio, pond on 6 July 1983.

t h e p o n d c o u l d b e r e d u c e d t o . I n t h e c as e o f a o n e - y e a r s m o l t p ro g r a m ,h eated wa ter is gen era l l y req u i red f rom m i d -Feb ru ary th rou gh mi d -M a y . W a t e r f l o w r a t e s i n c r e a s e t h r o u g h o u t t h e s e a s o n , a n d b o t h i n c o m -i n g su p p l y an d rearin g wa ter t em p eratu re s r i se con t i n u a l l y . F ore x a m p l e , a t t h e b e g i n n in g o f t h e h e a t i n g se a s o n , f l o w s m a y b e 6 0 0 l it e rsm in -1, ente ring the hatc her y at 2C and m ay on ly require a 2 r i se intem p eratu re . B y mi d -M ay , f l ow s co u l d i n crease to 1000 l it ers m i n -1,w i th a t emp eratu r e r i s e f rom 5 to 12C. T h u s , th e l oad is ever i n creas in g .

A f ter s tu d y i n g b o th c l i ma ct i c d a ta an d ac tu a l sa lt grad i en t p on di n f o r m a t i o n , i t c a n b e c o n c l u d e d t h a t F e b r u a r y is t h e w o r s t p o s s i b let im e t o b e g in w i t h d r a w i n g h e a t. T h r o u g h o u t t h e w i n t e r t h e p o n d s ha v el os t m u ch o f th e i r s tored th erm al en ergy an d th e su n ' s i n so l a t ion is a ta mi n i m u m . H ow ever , th e d eep er p on d s (3 m) s t i ll re ta i n a large q u an -t i t y o f u s e f u l e n e r g y , a n d w i t h t h e d a i l y i n c r e a s e i n t h e a m o u n t o fso l ar rad i a t i on , th ey recover q u i ck l y .


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Sal twater grad ien t ponds as hea t supp ly f or ha t chery rear ing water 35T a k i n g t h e s e f a c t o r s i n t o a c c o u n t , i t a p p e a r s t h a t a p o n d h a v i ng a

s u r fa c e a r e a o f a p p r o x i m a t e l y 5 0 0 0 m 2 a n d a d e p t h o f 3 m w o u l d m e e ta t y p i c a l A t l a n t i c s a l m o n h a t c h e r y ' s d e m a n d ( 2 8 0 0 G J t o t a l p e r s e a s o n )( N i e ls e n , 1 9 8 0 ; W i t t e n b e r g a n d H a r ri s, 1 9 8 1 ) . S u c h a p o n d w o u l dr e q u i r e 2 7 0 0 t o n n e s s al t ( $ 9 0 , 0 0 0 ) , a l in e r ( 5 5 0 , 0 0 0 ) , a h e a t e x c h a n g e r( 5 3 , 5 0 0 ) , e x c a v a t i o n a n d l a b o r c o s ts ( $ 2 5 . 0 0 0 ) , a n d m i s c e l l a n e o u sc o s t s f o r p u m p s a n d p l u m b i n g ( $ 4 , 5 0 0 ) , f o r a t o t a l o f $ 1 7 3 , 0 0 0 . B a s e do n f ig u r e s r e c e i v e d f r o m t h r e e h a t c h e r i e s l o c a t e d i n t h e n o r t h e a s t U S A ,t h e s i n gl e s e a s o n e x p e n s e o f a n o i l- f ir e d b o i l e r p r o d u c i n g a n e q u a la m o u n t o f he a t e ne r g y w o u l d b e a p p r o x i m a t e l y $ 2 6 , 0 0 0 . F o r t h e c o s to f a p p r o x i m a t e l y s e v e n y e a r ' s f u e l b i l l s , a f a c i l i t y c a n b u i l d a s a l tg r a d i e n t s o la r p o n d e q u a l t o th e d e m a n d , w i t h a m i n i m u m l if e e x p e c -t a n c y e s t i m a t e d a t 2 0 y e a rs . It s h o u l d b e p o i n t e d o u t t h a t t h e c o s td e p e n d s m o s t l y o n t h e p r ic e o f s al t, s in c e s u c h a t r e m e n d o u s a m o u n t isn e e d e d . I f t h e f a c i li t y is l o c a t e d n e a r a s o u r c e o f s e a w a t e r , c o s t s c a n b er e d u c e d c o n s i d e r ab l y .

P R O B L E M S A N D M A I N T E N A N C EW h e n d i s c u ss in g t h e p r o b l e m s an d m a i n t e n a n c e o f s al t g r a d i en t p o n d s ,f o u r m a i n a r e a s o f c o n c e r n g e n e r a l l y a r is e . T h e s e a r e ( a ) m a i n t a i n i n gt h e c l a r i ty o f t h e p o n d ; ( b ) l ea k s t h a t m a y o c c u r i n t h e li n er : ( c ) t h ed i f f u s i o n a n d r e p l a c e m e n t o f s a l t ; a n d ( d ) a n a c c i d e n t a l l o ss o f t h eg r ad i en t . W h e n p o n d t e c h n o l o g y w a s n e w , t he s e p r o b l e m s w e r e o f m a j o rc o n c e r n . S i n c e t h e n , m o r e p o n d s h a v e b e e n b u i l t a n d m o s t o p e r a t o r sw i l l a g r e e t h a t t h e y h a v e n o t p r e s e n t e d a n y m a j o r d r a w b a c k s , b a ts h o u l d n o t b e i g n o re d .

T h e c l a r it y o f a p o n d d i r e c t l y a f f e c t s t h e a m o u n t o f su n l ig h t r e a c h in gt h e s t o r a g e l a y e r, a n d t h u s , t h e o v e r a ll e f f i c i e n c y o f t h e p o n d . F l o a t i n gd e b r i s , w i n d b o r n e d i r t a n d l e a v e s , a n d o c c a s i o n a l l y a l g a e , w i l l b l o c k t h el ig h t. O p e r a t o r s p e r i o d i c a l l y n e t f l o a t i n g d e b r i s , a n d l ea v e s t e n d t o e v e n -t u a ll y s in k to t h e b o t t o m . I t is r e c o m m e n d e d t o l o c at e p o n d s a w a yf r o m f ie l d s t h a t c a n a c t a s a s o u r c e o f d u s t a n d p o s s i b l y p e s ti c i d e s .A l g ae ar e c o n t r o l l e d b y m a i n t a in i n g 1 o r 2 p p m c o p p e r s u l f a te .

L e a k s i n p o n d l in e r s c an b e a s e r i o u s p r o b l e m , a l t h o u g h t h e y h a v en o t b e e n i n t h e p a s t . I f a l e a k d e v e l o p s a n d t o o m u c h s a l t is l o s t , t h ep o n d m u s t b e d r ai n e d a n d r e - s t a rt e d . T h i s w o u l d r e q u i r e e i t h e r r e c y cl i n gt h e r e m a i n i n g s a lt w a t e r o r d i s p o s i n g o f it . R e c y c l i n g c a n b e e x p e n s i v e


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36 Joseph T. Fussa n d t im e c o n s u m i n g a n d t h e d i sp o s al o f s ev e ra l t h o u s a n d c u b ic m e t e r s o fs al t w a t e r h a s o b v i o us p r o b l e m s , a l t h o u g h I a m u n a w a r e o f a n y E P Ag u i d e li n e s t h a t c o v e r s u c h m a t t e r s . I t h a s b e e n s h o w n , h o w e v e r , t h a tl ea k s c a n b e d e t e c t e d i n t h e i r e a r l y s ta g e s b y r o u t i n e l y m o n i t o r i n g t h es a l i n it y a n d l o c a t i o n o f t h e v a r io u s l a y e r s ( N i e l s e n a n d K a m a l , 1 9 8 1 ) .L e a k s h a v e a ls o b e e n s u c c e s s f u ll y r e p a i r e d u n d e r w a t e r w i t h o u t d e s t r o y -i ng t h e g r a d i e n t z o n e . T h e b e s t a d v i c e is t o d e a l w i t h a r e p u t a b l es u p p l i e r / i n s t a l l e r a n d , s i n c e n e a r l y a ll l e a k s o c c u r a t s e a m s , k e e p t h e i rn u m b e r s t o a m i n i m u m ( W i t t e n b e rg a n d H a r ri s, 1 9 80 ).

B e c a u s e o f t h e d e n s i t y g r a d i e n t , s a l t n a t u r a l l y d i f f u s e s to t h e s u rf a c e .T h e r a te o f d i f f u s i o n in e x is t in g p o n d s h a s b e e n m e a s u r e d t o b e a b o u t2 % p e r y e a r ( W i t t e n b e r g a n d H a r r is , 1 9 8 0 ) . W h e n t h e s a lt r e a c h e s t h es u r f a c e , it is w a s h e d a w a y w i t h t h e o v e r f l o w w h e n i t r a in s . E v e n t u a l l y ,t h is s a lt m u s t b e r e p l a c e d . T o d a t e , n o e x i s t in g p o n d s h a v e h a d t o d oth is f r o m n e c e s s i ty , a l t h o u g h s e v e r a l m e t h o d s h a v e b e e n i n v e s ti g a te df o r r e - e s t a b l i s h i n g t h e s a l t c o n c e n t r a t i o n .

T h e f in a l m a j o r a r e a o f c o n c e r n is t h e a c c i d e n t a l l os s o f t h e g r a d i e n tl a y e r . T h i s c o u l d o c c u r b y e i t h e r e x c e s s iv e t u r b u l e n c e , a lo s s o f s a lt o re x t r e m e l y h ig h t e m p e r a t u r e s in t h e s t o ra g e l a y e r. E x p e r i e n c e ha s s h o w nt h a t s li g h t d i s t u r b a n c e s s u c h a s ra f t s o r d i v e rs d o n o t c r e a t e e n o u g ht u r b u l e n c e t o a f f e c t t h e g r a d i e n t l a y e r, b u t h e a t e x t r a c t i o n m a y . O n ea c c e p t e d m e t h o d t o e x t r a c t h e a t f r o m a p o n d i s t o sl o w l y d r a w o f f t h eb o t t o m l a y e r, p a ss it t h r o u g h a h e a t e x c h a n g e r , a n d r e t u r n i t to t h ep o n d . C a r e m u s t b e t a k e n t o a v o i d h ig h v e l o c i t ie s t h a t w o u l d c a u s et u r b u l e n c e a n d m i x i n g w i t h i n t h e p o n d . I t h a s a ls o b e e n d i sc o v e r e dt h a t m i x i n g c a n o c c u r n a t u r a l l y i f t h e s alt c o n c e n t r a t i o n o f t h e s t o ra g el a y e r is r e d u c e d b e l o w 1 2% b y w e i g h t ( E t t e r , 1 9 8 3 ). O n c e m i x i n g ha so c c u r r e d , o n e c a n o n l y r e s o r t t o d r a i n i n g t h e p o n d a n d s t a r t i n g a g a i n .

D I S C U S S I O NF r o m a n e c o n o m i c s t a n d p o i n t , s a lt g ra d i e n t p o n d s a r e d e f i n it e l y m o r ef e a si b le t h a n c o n v e n t i o n a l h e a t i n g s y s t e m s w h e n u s e d in a o n e - y e a rs m o l t p r o g r a m . T h e t e c h n o l o g y , h o w e v e r , is s o m e w h a t n e w a n d n o te n t i r e l y p r o v e n . I n r e v i e w i n g th e a v a il a b le l i t e r a t u r e , t h e p o n d s a p p e a rd e l i c a t e a n d c o m p l i c a t e d . W h e n s e v e r a l s i t e s w e r e v i s i t e d a n d t h e i ro p e r a t i o n d i s c u s s e d , t h e y w e r e f o u n d t o b e s i m p l e , r e li a b le , a n d n e a r l ym a i n t e n a n c e f re e. A d a i ly c h e c k o f t h e t e m p e r a t u r e / d e n s i t y g r ad i e n t


7/7

Saltwater gradient ponds as heat supply ]'or hatchery rearing water 37was highly recommended, but the measurements were automated andwere not time-consuming. An additional benefit of the ponds is thattheir peak efficiency is in late summer. Even if a pond is used for aspring heating season, a tremendous amount of energy will be availablelater that year for other purposes.

As a final note, work is also being conducted at the Jet PropulsionLaboratory on a system that may eliminate the problem areas associatedwith salt gradient ponds (Lin, 1983). Under development is a layered.hon eyc omb st ructure made of UV-stabilized pol ypropylene. By floatingthese structures on the surface of a freshwater pond, covering itentirely, the sun's rays are not only collected but also stored in thepond's water. The structure also insulates the pond from the atmo-sphere. The advantages are that the ponds can be of any water quality,of any depth, and no salt or liner is required. Small, prototype systemsare now being studied, and early results show them to be nearly aseffective as the salt gradient ponds.

REFERENCESEtter, D. (I 983). Personal communication.Fuss, J. T. (1983). Evaluation of a heat pump for an aquacultural application. TheProgressive Fish Culturist, 45, 121-3.Lin, I. H. (1983). Outdoor performance of a honeycomb-covered solar pond model.Progress in Solar Energy, American Solar Energy Society, 6, 411-16.Nielsen, C. E. (1980). Design and initial operation of a 400 m'- solar pond. Pro-

ceedings 25th Annual Meeting American Section International Solar EnergySociety, Phoenix, pp. 381-5.

Nielsen, C. E. & Kamal (1981). The 400 m 2 solar pond: one year of operation.Proceedings 26th Annual Meeting American Section International Solar EnergySociety, Philadelphia.

Wittenberg, L. J. & Harris, J. (1980). Management of a large operational solar pond.15th Intersociety Energy Conversion Engineering Conference.Wittenberg, L. J. & Harris, J. (1981). Construction and startup performance of theMiamisburg salt-gradient solar pond. J. Solar Energy Engineering, 103, 11-16.

fuss 1984 aquacultural-engineering

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