Field Evaluation of Solar Sea Water Stills J. W. BLOEMER, R. A. COLLINS, and J . A. EIBLING
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Battelle Memorial Institute, Columbus, Ohio
Construction methods and performance data are presented on solar sea water stills under evaluation at the Florida Solar Distillation Research Station. Three stills representing two basic designs have been constructed and operated there: a 2500-sq.foot glass-covered deep-basin still and 2300- and 500-sq.-foot air-supported plastic stills. Other types of stills are being developed for future construction and field evaluation.
Among the
m e t h o d s o f saline w a t e r c o n v e r s i o n b e i n g i n v e s t i g a t e d b y t h e D e p a r t m e n t of t h e I n t e r i o r ' s Office o f S a l i n e W a t e r is d i r e c t s o l a r d i s t i l l a t i o n . O n e phase o f t h e s t u d y of s o l a r d i s t i l l a t i o n i n c l u d e s t h e field e v a l u a t i o n o f s t i l l s o f p r o m i s i n g d e s i g n , w h i c h i s b e i n g c a r r i e d o u t f o r t h e Office of S a l i n e W a t e r b y B a t t e l l e M e m o r i a l I n s t i t u t e . T h e objectives of the research p r o g r a m a r e : t o o b t a i n realistic engineering a n d economic data o n solar distillation plants a n d t o develop methods of i m p r o v i n g their i n i t i a l a n d o p e r a t i n g costs. F i e l d tests a r e b e i n g c o n d u c t e d a t a r e s e a r c h s t a t i o n i n n o r t h e r n F l o r i d a , where three solar stills have been constructed a n d others are planned t o b e b u i l t . T h e s t i l l s are e x t e n s i v e l y i n s t r u m e n t e d , so t h a t h e a t losses a n d o t h e r p e r formance data m a y be accurately determined.
Solar Distillation Research Station T h e F l o r i d a r e s e a r c h s t a t i o n w a s e s t a b l i s h e d i n 1958 o n t h e P o n c e d e L e o n L i g h t house R e s e r v a t i o n n e a r P o r t O r a n g e o n l a n d m a d e a v a i l a b l e t o t h e D e p a r t m e n t o f t h e I n t e r i o r b y the U . S . C o a s t G u a r d . A p p r o x i m a t e l y / acre i s fenced i n a n d accessible for the construction of solar stills. S e a w a t e r feed f o r t h e s t i l l s i s p u m p e d f r o m t h e H a l i f a x R i v e r , w h i c h a t t h e p o i n t of feed i n t a k e has a s a l i n i t y o f a b o u t 35,000 p . p . m . , t h e same as t h a t o f t h e ocean. M u c h of the research i n s t r u m e n t a t i o n is housed i n a centrally located concrete block shelter, w h i c h was constructed u n d e r g r o u n d p r i n c i p a l l y to prevent its shading the nearby stills. One of the previously existing buildings o n t h e reservation was renovated t o p r o v i d e a n office, a conference r o o m , a n d e q u i p m e n t storage space. F i g u r e 1 is a view of p a r t of the field-evaluation site as seen f r o m t h e t o p o f t h e l i g h t h o u s e . T h e t w o s t i l l s s h o w n i n t h e b a c k g r o u n d are a i r - s u p p o r t e d p l a s t i c u n i t s . A g l a s s - c o v e r e d d e e p - b a s i n s t i l l c a n b e seen i n t h e f o r e g r o u n d . T h e i n s t r u m e n t house i s i m m e d i a t e l y t o t h e l e f t o f t h e s t i l l s . A 2 0 0 0 - g a l l o n t a n k f o r storage o f sea w a t e r i s visible i n the lower right corner of the fenced-in area. 3
4
166
In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.
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BLOEMER ET AL.-=-SOLAR SEA WATER STILLS
Figure I.
167
Solar still evaluation area as viewed from lighthouse
Deep-Basin Still T h e first s t i l l c o n s t r u c t e d a t t h e s t a t i o n is a g l a s s - c o v e r e d u n i t h a v i n g a s o - c a l l e d deep b a s i n t h a t m a i n t a i n s t h e saline w a t e r a t a d e p t h o f 10 t o 12 i n c h e s . D u r i n g o p e r a t i o n , s o l a r r a d i a t i o n , t r a n s m i t t e d t h r o u g h t h e glass c o v e r , h e a t s t h e w a t e r i n t h e b a s i n b e n e a t h t h e c o v e r . S o m e o f t h e w a r m w a t e r e v a p o r a t e s a n d s u b s e q u e n t l y condenses o n t h e u n d e r s i d e o f t h e c o v e r . C o n d e n s a t e flows t o t h e l o w e r edges o f t h e c o v e r a n d i s c o l lected i n suitable gutters. T h e t h e r m a l inertia of the large mass of water i n the basin provides continuous distillation even d u r i n g the n i g h t . T h e design calls for placement of the basin directly o n the g r o u n d , w i t h n o t h e r m a l insulation, o n the premise t h a t heat losses t o t h e d r y e a r t h w i l l b e r e l a t i v e l y s m a l l . C o n s t r u c t i o n . T h e d e e p - b a s i n s t i l l was c o n c e i v e d a n d designed i n i t i a l l y b y G e o r g e 0 . G . L o f , E n g i n e e r i n g C o n s u l t a n t , D e n v e r , C o l o . , a c o n s u l t a n t t o t h e Office o f S a l i n e W a t e r . A s b u i l t a t t h e F l o r i d a s t a t i o n , t h e s t i l l h a s s e r v e d m a i n l y as a r e s e a r c h t o o l r a t h e r t h a n as a d e m o n s t r a t i o n of t h e l o w e s t possible cost o f c o n s t r u c t i o n . C o n s t r u c t i o n w a s s t a r t e d i n t h e s u m m e r o f 1958 a n d c o m p l e t e d i n J a n u a r y 1959. T h e u n i t has b e e n i n o p e r a t i o n a b o u t 7 m o n t h s , o p e r a t i o n h a v i n g been s u s p e n d e d s e v e r a l t i m e s so t h a t v a r i o u s improvements could be made. F i g u r e 2 is a p h o t o g r a p h of t h e d e e p - b a s i n s t i l l . T h e c o n c r e t e c u r b i n g s h o w n a l o n g t h e n e a r edge o f t h e s t i l l i s p a r t of a h e a t - e x c h a n g e r f l u m e i n w h i c h feed w a t e r c a n be p r e h e a t e d . F i g u r e 3 shows a cross s e c t i o n o f one of t h e b a y s of t h e d e e p - b a s i n s t i l l , w i t h e n l a r g e d v i e w s of t h e u p p e r a n d l o w e r glass seals. T h e o v e r - a l l d i m e n s i o n s o f t h e s t i l l a r e 55 X 55 feet, p r o v i d i n g a p p r o x i m a t e l y 2450 s q . feet o f b a s i n a r e a d i s t r i b u t e d a m o n g s i x 8 - f o o t - w i d e b a y s . F e e d w a t e r enters t h e b a s i n f r o m t h e h e a t - e x c h a n g e r flume, a n d b l o w d o w n is r e m o v e d b y o v e r f l o w w e i r s . T h e b l o w d o w n a n d t h e d i s t i l l a t e effluents pass t h r o u g h p l a s t i c p i p e s i m m e r s e d i n t h e sea w a t e r c o n t a i n e d i n t h e h e a t - e x c h a n g e r flume, t h e r e b y p r e h e a t i n g t h e i n c o m i n g feed. T h e glass c o v e r is m a d e of 4 - f o o t - s q u a r e w i n d o w - g l a s s p a n e l s a r r a n g e d i n g a b l e r o o f f a s h i o n w i t h a slope o f 15 degrees. T h e u p p e r edges o f t h e glass p a n e l s are s u p p o r t e d w i t h t i m b e r f r a m i n g m e m b e r s a n d a r e sealed w i t h n e o p r e n e e x t r u s i o n s a n d a l u m i n u m c a p s t r i p s . T h e l o w e r edges o f t h e glass p a n e l s are sealed b y t h e s p e c i a l e x -
In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.
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ADVANCES IN CHEMISTRY SERIES
Figure 2.
Glass-covered deep-basin still
In SALINE WATER CONVERSION; Advances in Chemistry; American Chemical Society: Washington, DC, 1960.
169
BLOEMER ET AL.—SOLAR SEA WATER STILLS
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t r u d e d n e o p r e n e s t r i p s s h o w n i n F i g u r e 3 . S p a c e s b e t w e e n a d j a c e n t glass p a n e l s a r e sealed w i t h p r e s s u r e - s e n s i t i v e t a p e . T h e e x t e r i o r w a l l s , t h e b a s i n d i v i d e r p a r t i t i o n s , a n d t h e h e a t - e x c h a n g e r flume a r e c o n s t r u c t e d o f concrete b l o c k o n p o u r e d c o n c r e t e footers. T h e b a s i n w a s m a d e w a t e r tight b y placing prefabricated asphalt m a t t i n g directly o n t h e soil a n d o n t h e inside surfaces o f t h e w a l l s o f t h e s t i l l . T w o - i n c h - t h i c k F o a m g l a s s i n s u l a t i o n w a s p l a c e d a r o u n d the p e r i m e t e r w a l l s so t h a t t h e edge h e a t losses w o u l d b e c o m p a r a b l e t o those o f m u c h larger stills. Performance. S o f a r , t h e d e e p - b a s i n s t i l l h a s been o p e r a t e d o n l y u n d e r b a t c h t y p e c o n t r o l — t h a t i s , w i t h o u t c o n t i n u o u s b l o w d o w n o r h e a t exchange t o t h e i n c o m i n g sea w a t e r . I n d e t e r m i n i n g t h e p e r f o r m a n c e o f t h e s t i l l , i n c i d e n t s o l a r r a d i a t i o n a n d d i s t i l l a t e p r o d u c t i o n are m e a s u r e d d a i l y . F r o m t h i s i n f o r m a t i o n , t h e specific p r o d u c t i o n i n g a l l o n s p e r s q u a r e f o o t p e r d a y a n d t h e t h e r m a l efficiency c a n b e d e t e r m i n e d . I n a d d i t i o n t o t h e d a i l y c o l l e c t i o n of p e r f o r m a n c e d a t a , h o u r l y c o l l e c t i o n s a r e m a d e d u r i n g periodic energy- and mass-balance runs. F i g u r e s 4 a n d 5 show t h e d a i l y a n d t h e average m o n t h l y p r o d u c t i v i t y of t h e deepbasin still at various solar-radiation intensities. T h e scatter of d a t a points f o r t h e d a i l y
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