SALINE WATER CONVERSION

Design and Operating Principles in Solar Distillation Basins GEORGE O. G . L Ö F

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1960 | doi: 10.1021/ba-1960-0027.ch016

512 Farmers Union Building, Denver 3, Colo.

Among systems for solar distillation of sea water, the horizontal evaporation basin covered by transparent condensing surfaces has most closely approached commercial use. Solar radiation absorbed on the black bottom of a basin of salt water causes evaporation into the air space. Distilled water condenses on sloping air-cooled covers of glass or plastic film and collects in troughs at the low edges of the covers; unevaporated brine overflows to waste. The interrelationship of the processes of radiant heat transmission, thermal conduction and convection, vapor diffusion and convection, and the energy and material balances is complicated. Coupled with solar and weather variability, these factors make the design of equipment and prediction of performance an involved analysis. A procedure for such an analysis has been developed and a solar distillation plant in an illustrative location has been designed. Predicted water production rates throughout a typical year, the distribution of losses, and methods for improving performance are presented.

g r o w i n g s c a r c i t y o f f r e s h w a t e r i n m a n y places i n t h i s c o u n t r y a n d t h e rest o f t h e w o r l d has s t i m u l a t e d t h e d e v e l o p m e n t o f s e v e r a l p o t e n t i a l l y u s e f u l processes f o r s a l i n e w a t e r d e m i n e r a l i z a t i o n . B e c a u s e f r e s h w a t e r i s s u c h a c h e a p c o m m o d i t y , these p r o c esses m u s t d e m o n s t r a t e the m a x i m u m c o n c e i v a b l e e c o n o m y t o c o m p e t e w i t h e v e n t h e m o s t e x p e n s i v e n a t u r a l fresh w a t e r sources. N e a r l y a l l o f these m e t h o d s r e q u i r e c o n s i d e r a b l e e n e r g y , e i t h e r as h e a t o r as e l e c t r i c p o w e r . S i n c e t h i s i s a l a r g e cost i t e m i n these processes, s o l a r d i s t i l l a t i o n offers s u b s t a n t i a l o p e r a t i n g economies, b u t a t t h e e x pense o f l a r g e i n v e s t m e n t r e q u i r e m e n t . M i n i m i z a t i o n o f c o n s t r u c t i o n cost h a s t h e r e f o r e b e e n a p r i m e o b j e c t i v e i n t h e development of solar distillation. P r o b a b l y the most promising method for its accomp l i s h m e n t i s t h e c o m b i n i n g of a l l t h r e e p r i m a r y elements i n a d i s t i l l a t i o n process—i.e., h e a t s u p p l y f a c i l i t y , e v a p o r a t o r , a n d c o n d e n s e r — i n t o a single piece of v e r y s i m p l e e q u i p m e n t . S u c h a u n i t i s t h e b a s i n - t y p e s o l a r d i s t i l l a t i o n p l a n t (4). B u t the s i m p l i c i t y o f t h i s e q u i p m e n t ceases w i t h i t s g e n e r a l f o r m , a n d o v e r - a l l o p e r a t i o n o f so m a n y f u n c t i o n s m a k e s t h e p h y s i c a l processes o f e n e r g y a n d m a s s t r a n s f e r h i g h l y complex. 156

In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.

LOF—SOLAR DISTILLATION BASINS

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T h i s p a p e r describes a n d e x p l a i n s t h e v a r i o u s e n e r g y - a n d w a t e r - t r a n s f e r processes t a k i n g p l a c e i n t h e b a s i n - t y p e s o l a r d i s t i l l e r , shows t h e i r r e l a t i v e significance i n affecting performance, and indicates the factors w h i c h m a y be altered for over-all i m p r o v e ment and economy.


Characteristics of Solar Radiation I n clear, s u m m e r weather, solar r a d i a t i o n is received o n a surface n o r m a l t o t h e s u n ' s r a y s a t a r a t e of a p p r o x i m a t e l y 1.3 t o 1.4 c a l . / s q . c m . , m i n . , o r a b o u t 300 B . t . u . / sq. f t . , h r . O n a h o r i z o n t a l s u r f a c e i n c e n t r a l U n i t e d S t a t e s , t h i s i s e q u i v a l e n t t o a b o u t 2500 B . t . u . / s q . f t . , d a y . C l o u d i n e s s , h i g h e r o r l o w e r l a t i t u d e , a n d seasonal changes cause a v e r a g e a n n u a l v a l u e s t o r a n g e f r o m n e a r 2000 i n t h e S o u t h w e s t d o w n t o a b o u t 1200 i n t h e N o r t h w e s t a n d G r e a t L a k e s A r e a (2). T h e l o w e n e r g y i n t e n s i t y m a y r e a d i l y b e a p p r e c i a t e d i b y c o m p a r i n g h e a t t r a n s f e r r a t e s i n c o n v e n t i o n a l b o i l e r s , as h i g h as 100,000 B . t . u . / s q . ft., h o u r . C o v e r i n g a wide spectral range, solar r a d i a t i o n is d i v i d e d roughly into t w o equal e n e r g y p o r t i o n s — t h e u l t r a v i o l e t a n d v i s i b l e i n t h e 0.25- t o 0 . 7 - m i c r o n w a v e l e n g t h s , a n d t h e i n f r a r e d o u t t o a b o u t 2 o r 2.5 m i c r o n s . O t h e r p r o p e r t i e s o f s o l a r r a d i a t i o n i m p o r t a n t t o i t s uses a r e i t s d i s t r i b u t i o n b e t w e e n d i r e c t a n d diffuse, i t s a b s o r p t i o n , r e f l e c t i o n , a n d t r a n s m i s s i o n b y v a r i o u s o p a q u e a n d t r a n s p a r e n t m a t e r i a l s , a n d i t s c h e m i c a l effects o n t h e m . A s b a s i n d i s t i l l e r s i n v o l v e no focusing of solar r a d i a t i o n , diffusion of solar r a d i a t i o n b y haze a n d clouds i s n o t detrimental except i n t h e reduction of t o t a l incident energy. I n these s y s t e m s , t h e a b i l i t y o f c o m m o n b l a c k surfaces t o a b s o r b a b o u t 9 5 % o f t h e s o l a r r a d i a t i o n , a v e r a g e d o v e r i t s w h o l e s p e c t r u m , i s u t i l i z e d b y use o f s u c h a s u r f a c e i n t h e b o t t o m of t h e b a s i n . G l a s s a n d c e r t a i n c l e a r p l a s t i c films a r e a l m o s t p e r f e c t l y t r a n s p a r e n t t o t h e s o l a r s p e c t r u m , b u t t h e r e a r e a f e w p e r cent s p e c u l a r r e f l e c t i o n f r o m these surfaces ( r a n g i n g u p t o a l a r g e f r a c t i o n a t h i g h angles o f i n c i d e n c e , h o w e v e r ) , a n d some u l t r a v i o l e t a b s o r p t i o n , p a r t i c u l a r l y i n t h e i m p u r i t i e s i n glass. S o m e p l a s t i c films a b s o r b u l t r a v i o l e t r a d i a t i o n also, a n d i f t h e y d o , t h e r e w i l l b e d e g r a d a t i o n o f the film a n d u l t i m a t e f a i l u r e due t o loss o f s t r e n g t h . A f u r t h e r i m p o r t a n t p r o p e r t y o f these t r a n s p a r e n t m a t e r i a l s is t h e i r h i g h o p a c i t y t o l o n g w a v e r a d i a t i o n b e y o n d , s a y , 5 m i c r o n s . G l a s s i n i t s c o m m o n thicknesses i s c o m p l e t e l y o p a q u e t o t h i s t h e r m a l r a d i a t i o n , as e m i t t e d f r o m a s u r f a c e a t t e m p e r a t u r e s b e l o w a f e w h u n d r e d degrees. S o m e p l a s t i c films h a v e t r a n s m i s s i o n b a n d s i n these ranges, b u t t h e y a r e l a r g e l y o p a q u e . T h i s d i a t h e r m a n o u s p r o p e r t y o r s o - c a l l e d " g r e e n house e f f e c t " i s a d v a n t a g e o u s l y u s e d i n s o l a r h e a t s y s t e m s , i n c l u d i n g s o l a r s t i l l s , b y t h e " t r a p p i n g " o f s o l a r r a d i a t i o n i n t h e t r a n s p a r e n t enclosure w h i l e g r e a t l y r e d u c i n g o r e l i m i n a t i n g d i r e c t r a d i a t i o n loss f r o m t h e h e a t e d a b s o r b i n g s u r f a c e .

Basin-Type Solar Distiller T w o forms of the b a s i n - t y p e solar distiller are s h o w n i n F i g u r e s 1 a n d 2. E a c h of these is s u b j e c t t o n u m e r o u s m i n o r d e s i g n v a r i a t i o n s . T h e c o n f i g u r a t i o n o f t h e g l a s s c o v e r e d s t i l l s h o w n i s a n i m p r o v e d f o r m o f a 1-acre b r a c k i s h w a t e r d i s t i l l a t i o n p l a n t b u i l t i n C h i l e i n 1872 (3). T h e b a s i n is f o r m e d b y l a y i n g a s p h a l t o r c o n c r e t e o n s l i g h t l y s l o p i n g g r o u n d , a n d i f n o t s u f f i c i e n t l y b l a c k , s o m e t y p e of b l a c k p a i n t o r o t h e r c o a t i n g i s a p p l i e d t o t h e b a s i n s u r f a c e . L o w p e r i m e t e r a n d p a r t i t i o n w a l l s of c o n c r e t e b l o c k , s a y 18 i n c h e s h i g h , s u b d i v i d e t h e b a s i n i n t o l o n g b a y s s e v e r a l feet w i d e . M i d w a y b e t w e e n p a r t i t i o n s , a p o s t a n d b e a m a r r a n g e m e n t is p r o v i d e d , w h i c h , i n c o n j u n c t i o n w i t h t h e t o p s o f t h e p a r t i t i o n s , s u p p o r t s l a r g e sheets o f w i n d o w glass a t a n angle of 1 0 ° t o 1 5 ° w i t h t h e h o r i z o n t a l . C h a n n e l - s h a p e d n e o p r e n e e x t r u s i o n s serve as p r o t e c t i v e seals a t t h e u p p e r a n d l o w e r glass edges, a n d p r e s s u r e - s e n s i t i v e t a p e s are a p p l i e d t o seal t h e n a r r o w spaces b e t w e e n a d j a c e n t pieces of glass. A sheet m e t a l g u t t e r i s affixed t o t h e p a r t i t i o n b e n e a t h t h e l o w e r glass edges, w i t h a s l i g h t slope t o o n e e n d o f t h e s t r u c t u r e . P i p i n g i s a r r a n g e d for s u p p l y of salt water t o each basin section a n d f o r overflow of unevaporated brine a n d runoff of d i s t i l l a t e f r o m e a c h s e c t i o n .


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N e w designs are u n d e r d e v e l o p m e n t w h i c h i n v o l v e s i m p l e r a n d cheaper c o n s t r u c t i o n w h i l e r e t a i n i n g t h e same g e n e r a l f u n c t i o n s . T h e m o s t s a t i s f a c t o r y f o r m of p l a s t i c d i s t i l l a t i o n u n i t a l r e a d y t e s t e d i s s h o w n s c h e m a t i c a l l y i n F i g u r e 2. A t u b e is f o r m e d b y s e a l i n g t o g e t h e r t w o l o n g s t r i p s of p o l y e s t e r film a t t h e i r edges. [ T e s l a r , a D u P o n t p o l y e s t e r film i n a 0 . 0 0 3 - i n c h ( 3 - m i l ) t h i c k n e s s , h a s b e e n tested.]




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P r i o r t o s e a l i n g , a loosely f e l t e d m a t of b l a c k a c r y l i c fiber i s l a i d o n one of t h e f i l m s a n d s e c u r e d t o i t a t i n t e r v a l s w i t h a p l a s t i c cement. T h i s long, n a r r o w assembly i s then l a i d o n t h e g r o u n d b e t w e e n p a r a l l e l c o n c r e t e c u r b s a b o u t 6 i n c h e s h i g h , a n d t h e edges are a n c h o r e d i n grooves i n t h e c u r b t o p s . T h e c o n t o u r of t h e c u r b t o p s also f o r m s t r o u g h s f o r d i s t i l l a t e c o l l e c t i o n a n d flow t o t h e l o w e r ends of t h e b a s i n . A t t h e b a s i n ends, t h e p l a s t i c film i s g a t h e r e d i n t o s i m i l a r slots, a n d i n l e t a n d o u t l e t p i p i n g t h r o u g h t h e c u r b s a n d p l a s t i c is s e c u r e d w i t h b o l t e d flanges. O n e of these p i p e s is c o n n e c t e d t o a s m a l l b l o w e r w h i c h inflates t h e p l a s t i c t u b e a n d m a i n t a i n s a s l i g h t p o s i t i v e p r e s s u r e i n t h e enclosed space. T h e c o m p l e t e a s s e m b l y i s t h e n a b o t t o m p l a s t i c film d i r e c t l y o n t h e g r o u n d , a b l a c k , r a d i a t i o n - a b s o r b i n g m a t o n t h e film, a s h a l l o w l a y e r (1 o r 2 i n c h e s ) of salt w a t e r w i t h o c c a s i o n a l t r a n v e r s e d a m s t o m a i n t a i n a c o m p l e t e l y w e t t e d s u r f a c e , a n a i r - s u p p o r t e d c o v e r film, a n d c u r b sides w h i c h secure t h e f i l m s i n p l a c e a n d p r o v i d e c h a n n e l s f o r condensate. T h i s d e s i g n i s also b e i n g i m p r o v e d , p r i m a r i l y b y w i d e n i n g t h e t u b e a n d p r o v i d i n g simpler end arrangements.

Distillation Process General Aspects. S u p e r f i c i a l l y , s o l a r d i s t i l l a t i o n i n b a s i n - t y p e stills i s a n e x t r e m e l y s i m p l e process. S o l a r r a d i a t i o n passes t h r o u g h t h e t r a n s p a r e n t c o v e r a n d t h e salt w a t e r i n t h e b a s i n w i t h o n l y s l i g h t i n t e n s i t y r e d u c t i o n . I t i s t h e n p r a c t i c a l l y c o m p l e t e l y a b s o r b e d o n the b l a c k (bottom, t h e e n e r g y b e i n g released as h e a t t o t h e b l a c k s u r face. T h e salt w a t e r is w a r m e d b y c o n t a c t w i t h t h e h e a t e d s u r f a c e , a n d as i t s t e m p e r a t u r e s rises, so does i t s v a p o r p r e s s u r e . Q u i e t v a p o r i z a t i o n i n t o t h e a i r space a b o v e t h e water thus takes place, increasing the h u m i d i t y substantially t o saturation. C o n v e c t i o n c u r r e n t s t h e n c a r r y t h e v a p o r t o t h e v i c i n i t y of t h e cooler t r a n s p a r e n t s u r f a c e , w h e r e c o n d e n s a t i o n a c c o r d i n g l y occurs. T h e a i r - v a p o r m i x t u r e , s t i l l s a t u r a t e d b u t a t a l o w e r temperature, slowly returns t o the b o t t o m of the still, where i t is rehumidified. C o n densate f o r m i n g o n t h e s l o p i n g c o v e r r u n s d o w n i n t o t h e c o l l e c t i n g t r o u g h s , w h i l e i t s l a t e n t heat of c o n d e n s a t i o n i s d i s s i p a t e d t o t h e s u r r o u n d i n g a i r b y r a d i a t i o n a n d c o n v e c t i o n f r o m t h e c o v e r , a c t u a l l y a n a i r - c o o l e d condenser. T o a v o i d salt a c c u m u l a t i o n i n t h e b a s i n , b r i n e i s i n t e r m i t t e n t l y o r c o n t i n u o u s l y w i t h d r a w n , a n d salt w a t e r i s s u p p l i e d t o m a i n t a i n a r e a s o n a b l y c o n s t a n t l e v e l i n t h e d i s t i l l e r b a s i n . B e c a u s e d i s t i l l a t e a n d b r i n e are w a r m e r t h a n t h e feed w a t e r , a h e a t exchanger m a y be used for heat conservation a n d higher y i e l d . I f a c o m p a r a t i v e l y s h a l l o w l a y e r of salt w a t e r i s p r o v i d e d , as i n t h e p l a s t i c s t i l l d e s c r i b e d , t h e r e are l a r g e f l u c t u a t i o n s i n o p e r a t i n g c o n d i t i o n s a n d rates of w a t e r p r o d u c t i o n . O n a t y p i c a l s u n n y m o r n i n g , c o l d salt w a t e r i n the s t i l l w i l l first b e s l o w l y w a r m e d b y the rather low intensity radiation characteristics of t h a t p a r t of the day. A s the w a t e r t e m p e r a t u r e rises, d i s t i l l a t i o n c o m m e n c e s , s a y a t a t y p i c a l 5 0 ° C . ( a b o u t 120° F . ) . T h e r a t e rises r a p i d l y as t h e salt w a t e r t e m p e r a t u r e increases u p t o a m a x i m u m o f a b o u t 7 0 ° C . s h o r t l y a f t e r n o o n . I t t h e n d r o p s off g r a d u a l l y i n t h e a f t e r n o o n , c o n t i n u i n g a few h o u r s a f t e r s u n d o w n u n t i l t h e salt w a t e r has c o o l e d p r a c t i c a l l y t o t h e cover temperature. I n a s o - c a l l e d " d e e p - b a s i n " s t i l l , t h e r e i s less f l u c t u a t i o n because o f t h e r m a l storage i n a b o u t 1 foot o f s a l t w a t e r . S o l a r r a d i a t i o n causes a d a y t i m e t e m p e r a t u r e rise o f o n l y 5 ° o r 1 0 ° C , t h e b a s i n r e a c h i n g m a x i m a of a b o u t 5 0 ° C . D i s t i l l a t i o n p r o c e e d s s l o w l y t h r o u g h o u t d a y t i m e h o u r s , t h e n increases a f t e r s u n d o w n because o f l o w e r e d c o v e r t e m p e r a t u r e due t o a t m o s p h e r i c c o o l i n g i n t h e e v e n i n g . T h e s t o r e d heat causes distillation to continue throughout the night, accompanied b y basin temperature d e crease of s e v e r a l degrees. T h e c o n t i n u o u s a n d r e a s o n a b l y u n i f o r m w a t e r p r o d u c t i o n of the d e e p - b a s i n s t i l l m a k e s c o n t r o l a n d heat exchange c o m p a r a t i v e l y s i m p l e . E n e r g y C o n s i d e r a t i o n s . A s i n a n y d i s t i l l a t i o n process, t h e f u n d a m e n t a l r e q u i r e m e n t s f o r e n e r g y t r a n s f e r i n a s o l a r s t i l l are s u p p l y i n g h e a t t o t h e e v a p o r a t i n g w a t e r a n d r e m o v i n g heat f r o m t h e c o n d e n s i n g v a p o r . T h e s e t w o h e a t r a t e s are e s s e n t i a l l y e q u a l — a b o u t 1040 B . t . u . p e r p o u n d of d i s t i l l e d w a t e r . I n c i d e n t s o l a r r a d i a t i o n m u s t p r o v i d e heat f o r s e v e r a l o t h e r processes, h o w e v e r . T h e s e are s h o w n s c h e m a t i c a l l y i n


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F i g u r e 3. E x c e p t f o r t h e l a t e n t heat o f c o n d e n s a t i o n released a t t h e t r a n s p a r e n t s u r f a c e , t h e y a l l are f o r m s o f e n e r g y loss. O f p r i m a r y significance i n d e s i g n a n d i n e v a l u a t i o n of performance is the energy balance d r a w n a r o u n d the distiller basin. T h i s i n p u t i s seen t o b e t h e i n c i d e n t s o l a r e n e r g y m i n u s r e f l e c t i o n f r o m t h e c o v e r a n d t h e v e r y s m a l l a b s o r p t i o n i n t h e c o v e r . T h e feed w a t e r m i g h t also b e c o n s i d e r e d a sensible h e a t s u p p l y , b u t i t w o u l d u s u a l l y b e cooler t h a n t h e p r o d u c t s t r e a m s , a n d hence a t a c o n v e n i e n t base t e m p e r a t u r e , h a v i n g zero e n e r g y i n p u t . E n t h a l p y leaving the basin comprises t h e latent heat i n recovered water v a p o r , t h e r m a l r a d i a t i o n f r o m salt w a t e r a n d b a s i n b o t t o m t o t h e c o v e r , sensible h e a t t r a n s ferred f r o m salt water surface t o cover v i a the c i r c u l a t i n g a i r i n the enclosure, conduct i o n loss t o t h e g r o u n d o r o t h e r s u r r o u n d i n g s , sensible h e a t i n effluent d i s t i l l a t e a n d b r i n e s t r e a m s , a n d e n t h a l p y i n a n y v a p o r o r l i q u i d s t r e a m s w h i c h m a y escape t h e e n c l o s u r e a n d r e c o v e r y f a c i l i t i e s . O f these losses, the m o s t s i g n i f i c a n t are r a d i a t i o n f r o m t h e b a s i n t o t h e c o v e r a n d t h e sensible heat t r a n s f e r r e d b y a i r . T h e l a t t e r i s , of course, a n u n a v o i d a b l e a c c o m p a n i m e n t o f t h e u s e f u l t r a n s f e r i n t h e w a t e r v a p o r . T h e salt w a t e r t e m p e r a t u r e i s t h e v a r i a b l e p r i m a r i l y affecting t h i s l o s s — t h e h i g h e r i t i s , t h e l a r g e r is t h e w a t e r v a p o r p r e s s u r e , t h e l o w e r t h e a i r - w a t e r v a p o r r a t i o , a n d hence t h e l o w e r t h e c o n v e c t i v e t r a n s f e r loss. T h e r m a l r a d i a t i o n is i n c r e a s e d , h o w e v e r , as b a s i n t e m p e r a t u r e rises, so these t w o p r i n c i p a l losses are i n f l u e n c e d b y o p e r a t i n g t e m p e r a t u r e i n o p p o s i t e ways.

Sensible heat in condensale -Sensible heal ir> sal+ w o k e Sensible K e e f 3r> brine.

Conduction +o ground

Figure 3. Energy flow in solar distiller Solar radiation, substantially below 2 microns Thermal radiation, substantially above 5 microns C o v e r t e m p e r a t u r e i s a n o t h e r v a r i a b l e w h i c h c o n t r o l s d i s t i l l a t i o n r a t e a n d effic i e n c y . A l l o f t h e heat t r a n s f e r r e d t o the u n d e r s i d e o f the c o v e r f r o m t h e b a s i n , p l u s t h e s m a l l solar a b s o r p t i o n i n i t , m u s t be d i s s i p a t e d b y c o n v e c t i o n t o t h e s u r r o u n d i n g air a n d b y r a d i a t i o n t o the s k y . A m b i e n t temperature, w i n d velocity, a n d atmospheric c l a r i t y a l l influence t h e t e m p e r a t u r e d r i v i n g force necessary t o a t t a i n t h e e q u i l i b r i u m heat t r a n s f e r r a t e . C o v e r t e m p e r a t u r e , i n t u r n , affects b a s i n t e m p e r a t u r e , so t h a t a n o v e r - a l l e q u a l i t y i n h e a t flows p r e v a i l s . T h e p r i m a r y v a r i a b l e r e m a i n s , o f course, t h e s o l a r e n e r g y i n p u t r a t e , i t s m o s t i m p o r t a n t effect b e i n g t h e t e m p e r a t u r e l e v e l i n t h e salt water b a s i n . B e c a u s e s u n s h i n e i s i n t e r m i t t e n t , t h e r e i s a l w a y s a t r a n s i e n t effect i n s o l a r s t i l l c o n d i t i o n s , s t e a d y state n e v e r a c t u a l l y b e i n g r e a l i z e d . E n e r g y c o n s i d e r a t i o n s m a y t h e r e f o r e also i n v o l v e sensible h e a t i n v e n t o r y , a n d i t s c h a n g e , i n t h e salt w a t e r l a y e r , t h e


161


b a s i n s t r u c t u r e , a n d e v e n t h e s o i l u n d e r t h e b a s i n . I n s h a l l o w b a s i n s , m a r k e d changes o c c u r , h o u r - t o - h o u r , a n d i n t h e deeper b a s i n s , l a r g e sensible h e a t storage r e s u l t s i n s u b s t a n t i a l e n t h a l p y i n v e n t o r y changes f r o m d a y t o n i g h t a n d f r o m o n e d a y t o t h e next. T h e numerous uncontrollable weather variables, the v a r i e t y of heat transfer p r o c esses o c c u r r i n g , a n d t h e t r a n s i e n t n a t u r e o f t h e o p e r a t i o n m a k e s o l a r d i s t i l l a t i o n a m u c h m o r e c o m p l i c a t e d process t h a n i t first seems, p a r t i c u l a r l y i n so f a r as i t s d e s i g n a n d predicted performance are concerned.


Design Procedure P r e d i c t i o n o f P e r f o r m a n c e . F o r p u r p o s e s of d i s t i l l e r d e s i g n , a n d p a r t i c u l a r l y t h e d e t e r m i n a t i o n of a r e a r e q u i r e m e n t s f o r a specified w a t e r p r o d u c t i o n r a t e , o r f o r p r e d i c t i n g t h e p r o d u c t i v i t y of a u n i t o f specified d e s i g n a n d size, s i m u l t a n e o u s s o l u t i o n of s e v e r a l e n e r g y r a t e e q u a t i o n s is r e q u i r e d . B u t p r i o r t o t h i s c a l c u l a t i o n , s e v e r a l decisions a n d a s s u m p t i o n s m u s t b e m a d e a n d some a p p r o x i m a t i o n s r e l a t i n g t o s o l a r e n e r g y i n p u t n e e d t o be u s e d . F o r i l l u s t r a t i o n , let us assume t h a t a g l a s s - c o v e r e d , d e e p - b a s i n d i s t i l l e r i s b e i n g c o n s i d e r e d as a m e a n s f o r s u p p l y i n g a specified a n n u a l q u a n t i t y o f d i s t i l l e d w a t e r i n a p a r t i c u l a r l o c a l i t y where solar r a d i a t i o n a n d other meteorological d a t a are available. T h e basic p r o b l e m is t h e n t h e d e t e r m i n a t i o n of t h e a n n u a l o u t p u t of a u n i t area of s o l a r d i s t i l l e r b a s i n o p e r a t i n g a t these c o n d i t i o n s . It m a y be further assumed that the t o t a l p r o d u c t i v i t y of the distiller each m o n t h is t h e m o n t h l y a v e r a g e as d e t e r m i n e d b y u s e o f m e a n v a l u e s o f s o l a r r a d i a t i o n a n d a t m o s p h e r i c t e m p e r a t u r e , a n d t h a t t h e r e i s sufficient t h e r m a l storage i n t h e b a s i n t o r e d u c e d a y - t o - n i g h t t e m p e r a t u r e fluctuations e n o u g h f o r a p p l i c a b i l i t y o f m o n t h l y m e a n conditions. T h e heat b a l a n c e o n t h e t r a n s p a r e n t c o v e r p e r u n i t a r e a m a y b e r e p r e s e n t e d b y the e q u a t i o n : (K,

0

+

hr. o)(t

g

= (Κ. i +

~ t) a

i)(t

b

-

t) g

+ E\

(1)

T h i s r e l a t i o n neglects t h e v e r y s m a l l a b s o r p t i o n o f s o l a r e n e r g y i n t h e t r a n s p a r e n t c o v e r , a n d the s m a l l t e m p e r a t u r e d r o p t h r o u g h t h e c o v e r . A n o v e r - a l l e n e r g y b a l a n c e o n t h e d i s t i l l a t i o n p l a n t , a b o v e t h e reference t e m perature, t i s : g}

|f

= (Κ

ο +

hr, )(t 0

~ t ) + E(t

g

a

b

- Q + L

(2)

I t is a s s u m e d f o r t h e a b o v e e q u a t i o n t h a t c o v e r a r e a i s e q u a l t o b a s i n a r e a , t h a t t h e b r i n e effluent r a t e e q u a l s t h e d i s t i l l a t e r a t e , t h a t t h e sea w a t e r s u p p l y i s p r e h e a t e d b y exchange t o d i s t i l l a t e t e m p e r a t u r e , a n d t h a t t h e effective s k y t e m p e r a t u r e f o r r a d i a t i o n is e q u a l t o a t m o s p h e r i c t e m p e r a t u r e . T h e e v a p o r a t i o n r a t e , E, i s a f u n c t i o n o f v a p o r pressure difference ( o r t e m p e r a t u r e difference) b e t w e e n b a s i n a n d c o v e r . I t is also d e p e n d e n t o n t h e r a t e o f a i r c o n v e c t i o n i n the enclosure a n d the a b s o l u t e h u m i d i t y difference a t b a s i n surface a n d c o v e r s u r f a c e . A s s u m i n g saturation a t each surface, Ε

=

he, i(t

b

- t

g

) l ^ /

(H , a

b

- H, ) a

0

(3)

WH O 2

W i t h Ε expressed i n t e r m s of t a n d t , t h i s r e l a t i o n s h i p c a n b e s u b s t i t u t e d i n E q u a t i o n s 1 a n d 2, w h i c h t h e n c a n b e s o l v e d s i m u l t a n e o u s l y f o r these t w o u n k n o w n t e m p e r a t u r e s . Ε c a n t h e n b e f o u n d b y use o f E q u a t i o n 3. I n E q u a t i o n 1, h c a n b e e v a l u a t e d a t some m e a n w i n d v e l o c i t y b y u s e of t h e relation h = a + b V , w h e r e a, b, a n d η a r e c o n s t a n t s . I n a d e s i g n s t u d y b a s e d o n a C a l i f o r n i a c o a s t a l l o c a t i o n , t h i s c o n v e c t i o n coefficient w a s c o m p u t e d a t 3.1 B . t . u . / h r . , s q . f t . , ° F . , f o r a w i n d v e l o c i t y of 6 m i l e s p e r h o u r . T h e r a d i a t i o n t r a n s f e r , b

g

co

co

n



162

Κ,οΨο a) i s e q u a l t o 0.173 X 1 0 - ( T / - T ) X 0.937, w h e r e t h e l a s t t e r m i s t h e e m i s s i v i t y of glass. T h e i n t e r n a l c o n v e c t i o n coefficient, h m a y b e expressed as h = 0.256(t — t ) representing convection transfer between t w o closely spaced h o r i z o n t a l surfaces (6). I n t e r n a l r a d i a t i o n t r a n s f e r h (t — t ) — 0.173 X 1 0 ~ ( T — T *) X 0.9, w h e r e t h e l a s t t e r m i s t h e effective e m i s s i v i t y o f t h e b a s i n b o t t o m a n d t h e salt water surface. T h e v a l u e o f Ε i n E q u a t i o n s 1, 2, a n d 3, as o b t a i n e d f r o m E q u a t i o n 3, depends o n t e r m s a l r e a d y d e s c r i b e d a n d o n o t h e r f a c t o r s . T h e r a t i o w /w i s t h e p o u n d s of dry a i r circulating i n the distiller p e r pound of water distilled. A s s u m i n g t h e a i r is continually saturated, alternately at basin temperature a n d cover temperature, this r a t i o depends o n l y o n v a p o r pressures, w h i c h d e p e n d i n t u r n o n t h e t w o t e m p e r a t u r e s . T h e t e r m (H — H ) m a y b e r e p l a c e d b y C (t — t ), a n d l

4

8

a

c>i

ci

b

g

0

25

rA

b

8

g

b

4

g


da

ajb

ag

p

b

g

Ε = 0.256ft - « ° - ( ^ H o M i a ) C 2 5

U20

2

(4.)

P

A l t h o u g h c o n d u c t i o n losses t o t h e g r o u n d a n d m i s c e l l a n e o u s h e a t losses t h r o u g h m o i s t u r e escape a r e f u n c t i o n s o f b a s i n o r c o v e r t e m p e r a t u r e , t h e y d e p e n d o n o t h e r factors of indeterminate magnitude. Soil conductivity, construction tightness, a n d o t h e r on-site v a r i a b l e s a r e difficult t o p r e d i c t . F o r these reasons, p r a c t i c a l design c a n be a c c o m p l i s h e d b y a s s u m i n g a c o n s t a n t h e a t loss b a s e d o n n o r m a l l y e x p e c t e d c o n d i t i o n s . I n t h e design s t u d y p r e v i o u s l y r e f e r r e d t o , a h e a t loss of 50 B . t . u . / s q . f t . , d a y was assumed. S u b s t i t u t i o n of Ε = 0.256(t — t )°w / 0 . 2 4 w into E q u a t i o n s 1 a n d 2, a n d replacement of other terms b y their equivalents shown above, y i e l d : b

2rt

g

3.1ft, - la) + 0.162 X 1 0 ~ ( 7 y - 7 „ ) = 0 . 2 5 6 f t - U 8

T

da

H 2 O

4

1 2 5

+ 0.156 X l O ^ C A 1.07ft -

Q.H/24

= 3.1ft, -

t) a

+ 0.162 X

10-*(T * g

-

T ') a

+ 1.07(4 -

tg) * 1

4

-

?V)

ϋ°· ΚοΚ)λ

X %

2 5

2

o / % + 50/24

+ (5) (6)

T h e term Q is the net solar radiant energy absorption rate o n the basin b o t t o m . I t is e q u i v a l e n t t o t o t a l r a d i a t i o n i n c i d e n t o n t h e b a s i n c o v e r m i n u s r e f l e c t i o n f r o m t h e c o v e r , t h e w a t e r s u r f a c e , a n d t h e b a s i n b o t t o m , a n d m i n u s loss d u e t o s t r u c t u r a l s h a d o w ing. I t s d e t e r m i n a t i o n f r o m W e a t h e r B u r e a u records o f t o t a l d a i l y r a d i a t i o n o n a h o r i z o n t a l s u r f a c e i s c o m p l i c a t e d b y m a n y f a c t o r s s u c h as v a r i a t i o n i n angle of i n c i dence, a n d r e s u l t i n g t r a n s m i s s i v i t y o f c o v e r , h o u r l y a n d s e a s o n a l l y , i n t e n s i t y change due t o c l o u d i n e s s , a n d different p r o p e r t i e s of d i r e c t a n d diffuse r a d i a t i o n s . D e t a i l e d e x p l a n a t i o n of these m e t e o r o l o g i c a l a n d o p t i c a l c a l c u l a t i o n s i s b e y o n d t h e scope of t h i s p a p e r , b u t m a y b e f o u n d i n t h e l i t e r a t u r e (δ). sh

A f t e r e v a l u a t i o n of t h e n e t s o l a r h e a t i n p u t rates t o t h e salt w a t e r , as m o n t h l y averages ( o r m o r e f r e q u e n t l y i f d e s i r e d ) , t h e y a n d t h e a t m o s p h e r i c t e m p e r a t u r e a v e r ages a r e s u b s t i t u t e d i n E q u a t i o n s 5 a n d 6. T h e t w o e q u a t i o n s c a n t h e n b e s o l v e d b y t r i a l f o r t h e m e a n b a s i n a n d c o v e r t e m p e r a t u r e s each m o n t h . D i s t i l l a t i o n r a t e i s t h e n o b t a i n e d b y s u b s t i t u t i o n o f these v a l u e s i n E q u a t i o n 4. T h e f i r s t a n d second t e r m s o n the r i g h t - h a n d side of E q u a t i o n 5 m a y b e s e p a r a t e l y e v a l u a t e d f o r c o n v e c t i o n a n d r a d i a t i o n losses f r o m b a s i n t o c o v e r . F i g u r e 4 i l l u s t r a t e s t h e r e s u l t s of t h i s t y p e of a n a l y s i s f o r a d e e p - b a s i n s o l a r s t i l l i n t h e S a n D i e g o a r e a . T h e r m a l r a d i a t i o n f r o m b a s i n t o c o v e r i s t h e l a r g e s t loss, followed b y reflection of solar r a d i a t i o n f r o m t h e cover a n d a i r convection inside t h e s t i l l . S o l a r u t i l i z a t i o n efficiency i s t h e h e i g h t o f t h e l o w e s t c u r v e as a f r a c t i o n o f t h e height of t h e t o p c u r v e , r a n g i n g f r o m a b o u t 3 0 % i n J a n u a r y t o 5 0 % t h r o u g h t h e s u m m e r months. F r o m s u c h i n f o r m a t i o n , t h e p e r f o r m a n c e of a p r o p o s e d s o l a r d i s t i l l e r i n s t a l l a t i o n c a n b e a n t i c i p a t e d a n d t h e a r e a r e q u i r e m e n t s f o r a specified w a t e r p r o d u c t i o n

rate

determined. T h e equations a n d procedure described here a p p l y o n l y t o steady-state operation. I f t h e r e i s l a r g e fluctuation i n b a s i n t e m p e r a t u r e , o r i f a m o r e precise d a y - t o - d a y evaluation is desired, another t e r m must be included i n the heat balances. T h i s is t h e


163



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Figure 4. Predicted average performance of deep-basin solar distiller in San Diego area change i n t h e r m a l e n e r g y i n v e n t o r y f r o m t h e s t a r t t o t h e e n d of t h e a n a l y s i s p e r i o d , r e p r e s e n t e d b y t h e p r o d u c t o f t h e m a s s o f salt w a t e r p e r s q u a r e f o o t o f b a s i n , t h e t e m p e r a t u r e change f r o m t h e s t a r t t o t h e e n d of t h e p e r i o d , a n d t h e heat c a p a c i t y o f t h e w a t e r . A n a p p r o x i m a t i o n f o r t h e change i n t h e r m a l storage i n t h e b a s i n b o t t o m a n d w a l l s o u g h t t o b e m a d e also. I n d i s t i l l e r s c a r r y i n g o n l y a s h a l l o w w a t e r l a y e r , d i s t i l l a t i o n ceases a t n i g h t because the b a s i n cools s u b s t a n t i a l l y t o c o v e r t e m p e r a t u r e . P r e d i c t i o n s of p e r f o r m a n c e m u s t t h e r e f o r e b e b a s e d o n h i g h l y v a r y i n g solar a n d t e m p e r a t u r e c o n d i t i o n s . T h e e q u a t i o n s p r e v i o u s l y d e v e l o p e d , i n c l u d i n g a t h e r m a l storage t e r m , c a n b e u s e d i n a n h o u r - b y - h o u r c o m p u t a t i o n o f t e m p e r a t u r e s a n d d i s t i l l a t i o n rates t h r o u g h o u t a n u m b e r o f t y p i c a l days of low, moderate, a n d h i g h solar intensities a t several atmospheric temperatures, for solar positions corresponding t o summer, fall, a n d w i n t e r (spring is equivalent t o f a l l ) . T h e c o m p u t e d d i s t i l l a t i o n rates c a n t h e n b e u s e d i n c o n j u n c t i o n w i t h w e a t h e r d a t a f o r t h e l o c a l i t y i n q u e s t i o n t o e s t i m a t e m o n t h l y , seasonal, a n d a n n u a l d i s t i l l e d w a t e r output per unit area.

Performance Improvement A n a n a l y s i s o f t h e s o l a r d i s t i l l a t i o n process shows t h a t p e r f o r m a n c e i s r e m a r k a b l y insensitive t o a l l variables except solar r a d i a t i o n rate. A s atmospheric temperature changes, b a s i n a n d c o v e r t e m p e r a t u r e s m o v e s i m i l a r l y , so t h a t t h e i r difference r e m a i n s



164


a t a v a l u e w h i c h d i s s i p a t e s t h e a b s o r b e d s o l a r e n e r g y . W i n d v e l o c i t y also a p p e a r s a v a r i a b l e of m i n o r i m p o r t a n c e . E v e n the m a j o r design v a r i a b l e of b a s i n d e p t h has n o t y e t b e e n f o u n d o f m a j o r influence o n d i s t i l l a t i o n r a t e . O f t h e t h r e e m a j o r losses p r e v i o u s l y s h o w n , r e f l e c t i o n f r o m t h e t r a n s p a r e n t c o v e r c o u l d b e m o s t e a s i l y r e d u c e d . B y t h e use of a n i n t e r f e r e n c e l a y e r s o m e w h a t less t h a n 1 m i c r o n i n t h i c k n e s s , t h e r e f l e c t i v i t y of glass a n d p l a s t i c s c a n b e m a t e r i a l l y r e d u c e d . Processes f o r c o a t i n g o r e t c h i n g t h e s u r f a c e h a v e b e e n d e v e l o p e d , b u t unless a p p l i e d o n l a r g e scale, t h e y are e x p e n s i v e . A t least one process f o r c o n t r o l l e d e t c h i n g o f glass t o p r o d u c e a l o w - r e f l e c t i o n s u r f a c e c o u l d b e e x t r e m e l y c h e a p o n t h e scale o f m a n y t h o u s a n d s o f s q u a r e feet. R e d u c t i o n o f t h i s a v e r a g e 2 0 % loss t o p e r h a p s 5 % w o u l d r e s u l t i n a n increase i n the b a s i n - t o - c o v e r t e m p e r a t u r e difference, a n d a w a t e r d i s t i l l a t i o n r a t e increase o f a b o u t 2 0 % . D i s t i l l e r s c o v e r e d w i t h p l a s t i c f i l m r a t h e r t h a n glass m a y have their performance increased even more i f film reflectivity is reduced a n d i f t h e f i l m i s r e n d e r e d w e t t a b l e , so t h a t r e f l e c t i o n f r o m condensed w a t e r d r o p l e t s is eliminated. Coatings of T i 0 a n d other materials show promise f o r m a k i n g perm a n e n t l y w e t t a b l e films (1), a n d l o w r e f l e c t i o n coatings m a y also be w o r k a b l e . 2

R a d i a t i o n f r o m s a l t w a t e r s u r f a c e t o t h e w e t t e d c o v e r i s t h e largest loss, b u t t h e r e seems t o b e n o a s s u r a n c e t h a t i t c a n b e r e d u c e d . T h e r e i s some evidence t h a t m i c r o s c o p i c r o u g h e n i n g o f t h e u n d e r s i d e o f t h e c o v e r m a y r e n d e r t h a t s u r f a c e reflective f o r t h e b u l k o f t h e l o n g w a v e r a d i a t i o n ( p e a k i n g a t a b o u t 8 t o 10 m i c r o n s ) f r o m t h e w a t e r surface, w i t h o u t a p p r e c i a b l y reducing i t s transparency f o r short wave solar r a d i a t i o n . H o w e v e r , e v e n a t h i n f i l m o f condensate o n t h e c o v e r is a n effective a b s o r b e r f o r t h e r m a l r a d i a t i o n , so t h e benefit of a t h e r m a l l y reflective c o v e r m a y n o t b e r e a l i z e d i n o r d i n a r y basin-type stills. A n o t h e r a p p r o a c h t o r a d i a t i o n loss r e d u c t i o n m i g h t b e t h e a l t e r a t i o n o f t h e s a l t w a t e r s u r f a c e i n some m a n n e r t o l o w e r i t s e m i s s i v i t y f o r t h e r m a l r a d i a t i o n . I f a t r a n s p a r e n t t h i n l i q u i d film o r p o r o u s s o l i d film o f l o w t h e r m a l e m i s s i v i t y , p e r m e a b l e t o w a t e r v a p o r , c o u l d b e floated o n t h e salt w a t e r , s o l a r e n e r g y c o u l d c o n t i n u e t o b e a b s o r b e d o n t h e b a s i n b o t t o m , w a t e r w o u l d v a p o r i z e , b u t t h e r m a l r a d i a t i o n loss w o u l d be r e d u c e d . W h e t h e r m a t e r i a l s w i t h these p r o p e r t i e s c a n b e f o u n d a n d s u c c e s s f u l l y u t i l i z e d r e m a i n s t o b e seen. T h e t h i r d i m p o r t a n t loss i s b y a i r c o n v e c t i o n i n s i d e t h e d i s t i l l e r . T h i s a i r c i r c u l a t i o n i s a necessary a c c o m p a n i m e n t of w a t e r d i s t i l l a t i o n . T h e h i g h e r t h e b a s i n t e m p e r a t u r e , t h e h i g h e r i s t h e w a t e r v a p o r pressure a n d t h e l o w e r t h e r a t i o o f a i r t o w a t e r v a p o r i n t h e atmosphere of t h e distillation u n i t . F a c t o r s tending to m a x i m i z e basin t e m p e r a t u r e s w i l l t h e r e f o r e reduce t h i s h e a t loss because o f t h e l o w e r c o n c e n t r a t i o n o f a i r i n t h e a t m o s p h e r e of t h e d i s t i l l e r enclosure. B u t as r a d i a t i o n loss increases w i t h rise i n b a s i n t e m p e r a t u r e , these t w o losses c a n n o t b e s i m u l t a n e o u s l y m i n i m i z e d b y t e m p e r a t u r e c h a n g e . A n y effort t o w a r d r e d u c i n g c o n v e c t i o n i n t h e d i s t i l l e r w o u l d b e u n d e s i r a b l e , because t h i s i s t h e o n l y s i g n i f i c a n t m e c h a n i s m f o r w a t e r d i s t i l l a t i o n . I f t h e e v a p o r a t i n g a n d c o n d e n s i n g surfaces were v e r y close t o g e t h e r , p e r h a p s a n i n c h o r less, s o m e d i f f u s i o n t r a n s f e r o f w a t e r v a p o r w o u l d o c c u r , w i t h o u t p h y s i c a l t r a n s p o r t b y circulating a i r . H o w e v e r , l i m i t e d heat transfer b y conduction f r o m basin t o c o v e r c o u l d also o c c u r . I t i s possible t h a t t h e n e t effect w o u l d b e a m o d e r a t e increase i n t h e w a t e r y i e l d a n d a c o r r e s p o n d i n g decrease i n h e a t loss d u e t o a i r c i r c u l a t i o n . B u t t h e p r a c t i c a l i t y o f close p o s i t i o n i n g o f salt w a t e r s u r f a c e a n d c o v e r i s q u e s t i o n a b l e . I f t h e r e l a t i v e h u m i d i t y of t h e a i r i n t h e d i s t i l l e r enclosure i s c o n s i d e r a b l y less t h a n 1 0 0 % a t t h e s a l t w a t e r s u r f a c e a n d a t t h e c o v e r , m o r e c o n v e c t i v e loss i s a c t u a l l y o c c u r r i n g t h a n e s t i m a t e d . I n c r e a s i n g these a i r m o i s t u r e c o n t e n t s b y b e t t e r c o n t a c t of a i r a n d c o v e r , o r b y o t h e r w i s e a l t e r i n g c i r c u l a t i o n p a t t e r n s b y f a n , baffles, o r enclosure shape c o u l d reduce a i r c i r c u l a t i o n loss a n d increase p r o d u c t i v i t y . W h e t h e r t h e e c o n o m i c s of s u c h steps m i g h t b e a t t r a c t i v e r e m a i n s t o b e d e t e r m i n e d . I n v i e w o f t h e s e v e r a l p o s s i b i l i t i e s f o r r e d u c i n g e n e r g y losses, s o l a r d i s t i l l e r y i e l d s m i g h t b e s u b s t a n t i a l l y i m p r o v e d . D e c r e a s e o f r a d i a t i o n a n d c o n v e c t i o n losses t o h a l f t h e i r p r e s e n t l e v e l s , a l o n g w i t h t h e u s e o f l o w - r e f l e c t i o n c o v e r surfaces, w o u l d r e s u l t i n a b o u t 5 0 % increase i n s u m m e r p r o d u c t i o n a n d r o u g h l y d o u b l e t h e w i n t e r p e r -



165

formance. I n s u n n y c l i m a t e s , y i e l d s of 0.2 g a l l o n of d i s t i l l e d w a t e r p e r s q u a r e foot s h o u l d b e possible o n a s u n n y s u m m e r d a y . A v e r a g e w i n t e r y i e l d s w o u l d a p p r o a c h 0.1 g a l . / s q . f t . , d a y . Y e a r - r o u n d p e r f o r m a n c e s h o u l d exceed 0.15 g a l . / s q . f t . , d a y , i f these i m p r o v e m e n t s c a n b e r e a l i z e d . T h e t e c h n i c a l a c c o m p l i s h m e n t o f these m e a s u r e s as w e l l as t h e i r e c o n o m i c a t t r a c t i v e n e s s i s , of course, n o t a s s u r e d , b u t studies a l o n g s u c h lines a p p e a r of v a l u e .


Acknowledgment T h e s u p p o r t a n d c o o p e r a t i o n of t h e Office o f S a l i n e W a t e r , U . S . D e p a r t m e n t of t h e I n t e r i o r , i n a s o l a r d i s t i l l a t i o n p r o g r a m , a p o r t i o n o f w h i c h i s t h e s u b j e c t of t h i s paper, are gratefully acknowledged.

Nomenclature h, ο c

h, ο h, i r

c

h, i T

C t, Τ P

g

a

t, Τ tb, T Ε λ a

a

b

Η Q

sh

V L w

d a

Wh o 2

= c o n v e c t i o n h e a t t r a n s f e r coefficient f r o m c o v e r t o a t m o s p h e r e , B . t . u . / h r . , s q . ft., ° F . = r a d i a t i o n h e a t t r a n s f e r coefficient f r o m c o v e r t o a t m o s p h e r e , B . t . u . / h r . , s q . f t . , ° F . = c o n v e c t i o n h e a t t r a n s f e r coefficient f r o m s a l t w a t e r surface t o c o v e r of s t i l l , B . t . u . / h r . , sq. ft., ° F . = r a d i a t i o n h e a t t r a n s f e r coefficient f r o m s a l t w a t e r surface t o c o v e r of s t i l l , B.t.u./hr., sq. ft., ° F . = heat capacity of air, B . t . u . / l b . , ° F . = t e m p e r a t u r e of t r a n s p a r e n t cover, a s s u m e d e q u a l t o d i s t i l l a t e t e m p e r a t u r e , . ° F . or ° R . = atmospheric temperature, ° F . or ° R . = t e m p e r a t u r e of s a l t w a t e r i n b a s i n , ° F . o r ° R . = water evaporation (and condensation) rate, l b . / h r . , s q . f t . = l a t e n t h e a t of c o n d e n s a t i o n a t b a s i n t e m p e r a t u r e (plus difference i n sensible h e a t of condensate a t b a s i n a n d c o v e r t e m p e r a t u r e s ) , B . t . u . / l b . (equals a p p r o x i m a t e l y 1050 B . t . u . / l b . ) = e n t h a l p y of air, B . t . u . / l b . = n e t solar r a d i a t i o n r a t e a b s o r b e d o n b a s i n b o t t o m , B . t . u . , s q . f t . , d a y (equals i n c i d e n t r a d i a t i o n m i n u s r e f l e c t i o n f r o m cover, s a l t w a t e r surface, a n d b a s i n bottom) = w i n d velocity, miles/hr. = n e t m i s c e l l a n e o u s h e a t loss r a t e , B . t . u . / h r . , s q . f t . = p o u n d s of d r y a i r c i r c u l a t i n g p e r u n i t t i m e p o u n d s of w a t e r d i s t i l l e d p e r u n i t t i m e

=

Literature Cited (1) F r a n k l i n I n s t i t u t e , " P r o d u c i n g P e r m a n e n t l y H y d r o p h i l i c Surfaces o n P l a s t i c F i l m s f o r Solar S t i l l s , " Office of Saline W a t e r , U. S. D e p t . of I n t e r i o r , Progress R e p t . 29 (1959). (2) H a n d , I. F., Heating and Ventilating 50 (7), 73 ( J u l y 1953). (3) H a r d i n g ,J.,Proc. Inst. Civil Engrs. 73, 284 (1883). (4) Löf, G . O . G., " D e m i n e r a l i z a t i o n of Saline W a t e r w i t h Solar Energy," Office of S a l i n e W a t e r , U. S. D e p t . of I n t e r i o r , Progress R e p t . 4 (1954). (5) L ö f , G. O . G., " D e s i g n a n d E v a l u a t i o n of D e e p - B a s i n , D i r e c t Solar H e a t e d D i s t i l l e r f o r D e m i n e r a l i z a t i o n of Saline W a t e r , " R e p t . t o office of Saline W a t e r , U . S . D e p t o f I n t e r i o r , 1959. (6) W i l k e s , G. Β., P e t e r s o n , C . M. F., Heating, RECEIVED for review July 20, 1960.

Piping,

and Air Conditioning

9, 505 (1937).

A c c e p t e d July 22, 1960.


SALINE WATER CONVERSION

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