7/30/2019 1-s2.0-S0011916498002240-main
4/8
276 S . AI -Ha l la j e t al . / Desa l i na t i on 120 (1998)
273 -28 0A u n i fo rm in i ti a l wa te r d i s tr i bu t ion was
n ecessa ryt o i n s u r e g o o d w a t e r w e t t i n g t o t h
e p a c k i n ge s p e c i a l l y a t l o w w a t e r f l o w r a
t e s . A v a r i a b le s p e e da i r f a n w a s f i x e d a t t
h e t o p o f b o t h u n it s t o p r o v i d ed i f f e r e n t a
i r f l o w r a t e s t h r o u g h t h e u n i t s i n t h ef o r
c e d d r a f t o p e r a t io n .W a t e r a n d a i r t e m p e r
a t u r es w e r e m e a s u r e d a td i f f e r e n t l o c a ti
o n s i n t h e t w o u n i ts u s i n g C h r o m e l -A l u m e l
t h e r m o c o u p l e s c o n n e c t e d t o a m u l t i-c h a n
n e l p r o g r a m m a b l e r e c o r d e r . W a t e r f l o w r
a t ew a s m e a s u r e d u s i n g r o t a m e t e r s , w h i le
a i r v e l o c i t yw a s m e a s u r e d u s i n g a n e l e c t
ri c a l f a n s p e e d o m e t e r( t y p e A i r f l o w 1 00 ).
H o w e v e r , t h e a i r v e lo c i t y w a stoo sm al l to be m
easu red , e spec i a l l y in t he na tu ra ld ra f t ope ra t i
on . I t was dec ided to ca l cu l a t e t he a i rv e l o c i t y
f r o m t h e e n e r g y b a l a n c e o f t h e u n i t [ 1 3] .S
o l a r r a d ia t i o n w a s m e a s u r e d w i t h a K i p p a
n dZ e n o n p y r o m e t e r c o n n e c t e d t o a n i n t e g
r a t o r t og ive t he da i ly an d ho ur ly so l a r rad i a ti
on .
In t he i ndoor s t eady-s t a t e ope ra t i ons , t he un i tw
a s u s u a l l y le f t r u n n i n g f o r 3 - 4 h t o r e a c h
s te a d ys t at e b e f o r e m e a s u r e m e n t s w e r e ta k
e n . C o n d e n s a t ew a s m e a s u r e d a f t e r e a c h h
o u r o f o p e r a ti o n . In t h eo u t d o o r o p e r a t i o
n , c o n d e n s a t e a n d t e m p e r a t u r em e a s u r e m
e n t s w e r e s t a r t e d e a r l y i n t h e m o r n i n gand
s topp ed l a t e in t he even ing .
3 . O v e r a l l e n e r g y b a l a n c e i n t h e d e s a l
in a t i o nu n i t
I n o r d e r t o c h e c k o u t t h e a c c u r a c y o f t h
em e a s u r e m e n t s , a n o v e r a l l e n e r g y b a l a n
c e w a sm a d e o n b o t h u n i t s . T h e h e a t i n p u t b
y t h ee l e c t r ic a l h e a t e r o r t h e u s e f u l e n e r
g y o b t a i n e d f r o mthe so l a r co l l ec to r (Qu) mus t
equa l t o t hes u m m a t i o n o f:
1 . H e a t r e j e c t e d w i t h t h e b r i n e l e a v i n
g t h eh u m i d i f i e r .2 . H e a t r e j e c t e d w i t h t h
e c o n d e n s a t e l e a v i n gt h e c o n d e n s e r ( u s u
a l l y n e g l i gi b l e) .
3 . H e a t l o s s e s f r o m t h e u n i t t o t h e a m b i
e n tt h r o u g h t h e w a l l s o f t h e u n it .4 . H e a t s
t o r e d i n t h e w a l l s o f th e u n i t ( o n l y int h e o
u t d o o r u n s t e a d y - s t a t e o p e r at i o n ).
I f t h e f e e d w a t e r t e m p e r a t u r e , T ~, i s t a
k e n a s are fe rence , t hen the f i r s t two quan t i t i e s
can bec a l c u l a te d e a s i ly . E v a l u a t i o n o f t h e
h e a t l o s se sf r o m t h e u n i t r e q u i r e s a n a c c u
r a t e e s t i m a t e o f t h ehea t l o ss co e f f i c i en t U
loss. Th e lo ss c oe f f i c i en t fo rb o t h u n i t s w a s t
h e o r e t i c a ll y c a lc u l a t e d f r o m t h es u m m a t
i o n o f a l l th e t h e r m a l r e s i s ta n c e s a s u s u a
l l ydone in hea t t rans fe r t ex tbook s [14] .
In o rde r t o me asu re Ulo~s expe r ime n ta l l y , ac o n t
i n u o u s f l o w o f h o t w a t e r w a s i n t r o d u c e d t
o t h ec o n d e n s e r t o h e a t t h e u n i t f o r a p e r i
o d o f 3 - 4 h t or e a c h s t e a d y s ta t e. T h e f e e d w
a t e r w a s t h e n b y -p a s s e d f r o m t h e c o n d e n s
e r o u t o f t h e u n it . T h e h e a tl o s t f r o m t h e w a
t e r a s i t f l o w s t h r o u g h t h ec o n d e n s e r i s e
q u al t o t h e h e a t l o s se s t h r o u g h t h eun i t wa l
l s a s desc r ibed by the fo l l ow ing equa t ion :m w C p w ( r
l - r 2 ) : U , o , , A [ ( r s+ T 6 ) /2 - r ] ( 1 )
A i r w a s a s s u m e d t o t ra n s f e r th e h e a t u n i
f o r m l yf r o m t h e c o n d e n s e r t o a ll t h e u n i t w
a l ls . A i rt e m p e r a t u r e w a s a s s u m e d to v a r y
l i n e a rl y f r o m t h et o p t o t h e b o t t o m o f th e u
n i t , a n d h e n c e t h ea r i t h m e t ic a v e r a g e o f
T5 and T6 wa s used .
T h e m e a s u r e d a n d t h e t h e o r e t i c a l l y c a
l c u l a t e dUlosswe re 1 .0 and 0 .6 W/m2 .K, re spec t ive ly ,
fo r t hep i lo t un i t and 1 .2 and 0 .8 W/m 2.K fo r t he ben
chu n it . T h e d i f f e r e n c e b e t w e e n t h e e x p e r
i m e n t a l a n dthe t heo re t i ca l va lues o f U loss m ay
due to t he n on-l in e a r u n i f o r m t h i c k n e s s o f t h
e i n s u la t io n a n d t h eassum pt ion used in Eq . (1 ) .
T h e h e a t s t o r e d i n t h e u n i t w a l l s w a s c a
l c u l a t e df r o m t h e e s t i m a t e d h e a t c a p a c i
t y o f t h e u n i t w a l l sa n d t h e m e a s u r e d v a r i
a t i o n i n t h e i r t e m p e r a t u r ewi th t ime . In t he
s t eady-s t a t e ope ra t i on th i squan t i t y i s equa l t o
ze ro ; however , i t i s ve rys ign i f i can t i n t he ou tdoor
ope ra t i on due to t het rans i en t na tu re o f t he so l a r
en e rgy . I t i s u sua l lyp o s i ti v e i n t h e m o r n i n g
, a n i n d i c a ti o n o f h e a ts to rage , and neg a t ive i n
t he a f t e rnoon , i nd i ca t inga h e a t r e l ea s e . I n t
h e e a r l y m o r n i n g , m o s t o f t h euse fu l so l a r
ene r gy i s s t o red as a sens ib l e hea t i nthe un i t wa l l
s .
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