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A Reference heat source for solar collector thermal testing
Harrison, S. J.; Bernier, M. A.
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A REFERENCE HEAT SOURCE FOR SOLAR COLLECTOR THERMAL TESTING
by S.J. Harrison and M.A. Bernier
ANALYZED
Presented
at
ASME Winter Annual Meeting
New Orleans, LA, December, 1984
Solar Energy Division
Pamphlet Paper No. 84-WAISol-2,
7p.
DBR Paper No. 1258
Division of Building Research
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A Reference Heat Source for Solar Collector
Thermal Testing
S.
J. HARRISON
and M.
A. BERNIER
Division of Building Research
National Research Council of Canada
Ottawa, Ontario,
K I A
OR6
ABSTRACT
Liquid-based s o l a r c o l l e c t o r s have been t e s t e d u s i n g a d i r e c t - c o m p a r i s o n r e f e r e n c e h e a t s o u r c e (RHS). An e l e c t r i c h e a t e r i s i n s t a l l e d i n s e r i e s w i t h t h e s o l a r c o l l e c t o r u n d e r t e s t . Energy i s added t o t h e h e a t t r a n s f e r f l u i d a t a measured r a t e by t h e RHS, a n d t h e r a t e of s o l a r e n e r g y c o l l e c t i o n i s d e t e r m i n e d a s t h e p r o d u c t of t h i s e n e r g y i n p u t and t h e r a t i o of t h e d i f f e r e n t i a l t e m p e r a t u r e s a c r o s s t h e c o l l e c t o r a n d t h e h e a t e r . The RHS and a s s o c i a t e d t e m p e r a t u r e s e n s o r s c a n b e t h o u g h t o f a s a t h e r m a l mass f l o w m e t e r where, i n e f f e c t , t h e p r o d u c t of mass f l o w and s p e c i f i c h e a t i s measured d i r e c t l y i n t h e t e s t l o o p .
C a l i b r a t i o n t e s t s were performed on two r e f e r e n c e h e a t s o u r c e s a t v a r i o u s f l o w r a t e s a n d a t t h e t e m p e r a t u r e s n o r m a l l y e n c o u n t e r e d i n f l a t p l a t e s o l a r c o l l e c t o r t e s t i n g (-10°C t o "95OC). V a l u e s o f
n,
t h e r a t i o of t h e measured RHS t h e r m a l power o u t p u t t o t h e f l u i d t o t h e RHS e l e c t r i c a l power i n p u t w e r e d e t e r m i n e d f o r e a c h c a s e . R e s u l t s i n d i c a t e t h a t f o r RHSI1,n1
= 99.61% ( s t a n d a r d d e v i a t i o n of 0.22) and f o r RHS#2,n 2
= 99.59% ( s t a n d a r d d e v i a t i o n of 0.28). NOMENCLATURE A = a p e r t u r e a r e a of c o l l e c t o r (m2) C = s p e c i f i c h e a t of h e a t t r a n s f e r f l u i d (J/ (kg .K) ) G' = t o t a l s o l a r i r r a d i a n c e o n t h e p l a n e o f t h e s o l a r c o l l e c t o r (w/m2) & = mass f l o w r a t e o f h e a t t r a n s f e r f l u i d ( k g / s ) PRHS = i n p u t power t o RHS (W) qc = r a t e of e n e r g y c o l l e c t i o n by t h e s o l a r c o l l e c t o r (W)GRHS
= o u t p u t power of t h e RHS (W) Ta = t e m p e r a t u r e o f a m b i e n t a i r s u r r o u n d i n g t h e RHS (OC) Ti = t e m p e r a t u r e of f l u i d a t i n l e t t o h e a t e r i n t h e RHS (OC) To = t e m p e r a t u r e of f l u i d a t o u t l e t o f h e a t e r i n t h e RHS (OC) Tm = mean f l u i d t e m p e r a t u r e i n t h e RHS (OC)n
= s o l a r c o l l e c t o r e f f i c i e n c ynRHS
= t h e r m a l e f f i c i e n c y of t h e RHS Ap = p r e s s u r e d r o p between t e m p e r a t u r e w e l l s i n t h e RHS ( P a ) ATc = f l u i d t e m p e r a t u r e r i s e t h r o u g h s o l a r c o l l e c t o r (K) ATRHS = f l u i d t e m p e r a t u r e r i s e t h r o u g h r e f e r e n c e h e a t s o u r c e (K) T = measured t i m e i n t e r v a l ( s ) M = mass of f l u i d c o l l e c t e d d u r i n g t i m e i n t e r v a l 7 ( k g ) V = volume f l o w r a t e of h e a t t r a n s f e r f l u i d (m3/s) P = d e n s i t y of t h e h e a t t r a n s f e r f l u i d (kg/m3) S u b s c r i p t s c = r e l a t e d t o t h e c o l l e c t o r RHS = r e l a t e d t o t h e r e f e r e n c e h e a t s o u r c e 1. INTRODUCTION The t h e r m a l e f f i c i e n c y ( n ) of a s o l a r c o l l e c t o r , a s d e t e r m i n e d by t e s t i n g , i s g i v e n a s , where, q c = r a t e of e n e r g y c o l l e c t i o n by s o l a r c o l l e c t o r , = (fi*Cp -AT)c ASHRAE S t a n d a r d 93-77[L]
o u t l i n e s a f o r m a l i z e d s o l a r c o l l e c t o r t e s t i n g p r o c e d u r e and s p e c i f i e s t h e a c c u r a c y r e q u i r e m e n t s of a l l t h e measured t e r m s . I n p a r t i c u l a r , f o r c o l l e c t o r s t h a t u s e l i q u i d a s t h e h e a t t r a n s f e r f l u i d , t h e a c c u r a c y of t h e mass f l o w r a t e measurement s h o u l d b e e q u a l t o o r b e t t e r t h a n f l % of t h e measured v a l u e . It i s normal p r o c e d u r e t o measure t h e v o l u m e t r i c f l o w r a t e and d e r i v e mass f l o w r a t e f r o m p u b l i s h e d v a l u e s of f l u i d d e n s i t y , ( i . e . ,6
= V *p). L i q u i d f l o w m e t e r s c a p a b l e of a c h i e v i n g t h e a c c u r a c y r e q u i r e m e n t o f t h e s t a n d a r d a r e a v a i l a b l e[2,2].
T u r b i n e f l o w m e t e r s a r e among t h e most p o p u l a r , b u t a s n o t e d by Reed[i],
t h e i r a c c u r a c y i s s e n s i t i v e t o d i f f e r e n c e s i n f l o w p a t t e r n , g a s e n t r a i n m e n t i n t h eh e a t t r a n s f e r l i q u i d and wear of t h e b e a r i n g s . The a c c u r a t e d e t e r m i n a t i o n of q c by t h i s method a l s o depends on a knowledge of t h e t h e r m a l and p h y s i c a l p r o p e r t i e s of t h e h e a t t r a n s f e r f l u i d , p r i m a r i l y t h e f l u i d d e n s i t y and s p e c i f i c h e a t , over a r a n g e of t e m p e r a t u r e s . The t e m p e r a t u r e d e p e n d e n c i e s o f t h e s e p r o p e r t i e s a r e o f t e n n o t w e l l known f o r t h e commonly u s e d h e a t t r a n s f e r f l u i d s s u c h a s t h e w a t e r g l y c o l m i x t u r e s . Recognizing t h e s e d i f f i c u l t i e s , i t was d e c i d e d t o u s e t h e r e f e r e n c e h e a t s o u r c e (RHS) method t o d e t e r m i n e t h e r a t e of e n e r g y c o l l e c t i o n of t h e t e s t s o l a r c o l l e c t o r i n t h e D i v i s i o n of B u i l d i n g Research S o l a r C o l l e c t o r T e s t F a c i l i t y
[L].
The r e f e r e n c e h e a t s o u r c e method, a l s o c a l l e d t h e c a l o r i f i c r a t i o t e c h n i q u e by Reed and A l l e n[kl,
u t i l i z e s a n e l e c t r i c a l h e a t e r i n s t a l l e d i n s e r i e s w i t h t h e t e s t s o l a r c o l l e c t o r i n t h e c a l o r i m e t r y l o o p , a s shown i n F i g u r e 1. With t h e same mass f l o w r a t e t h r o u g h b o t h t e s t c o l l e c t o r and RHS, e l e c t r i c a l e n e r g y i s i n p u t t o t h e RHS a t a r a t e PRH,
and t h e r e s u l t a n t f l u i d t e m p e r a t u r e r i s e (ATRHS) a n 8 t h e c o r r e s p o n d i n g f l u i d t e m p e r a t u r e r i s e t h r o u g h t h e t e s t c o l l e c t o r (ATc) a r e measured. The r a t e of e n e r g y c o l l e c t i o n by t h e s o l a r c o l l e c t o r ( q c ) i s g i v e n b y , ROOFI
CONTROL VALVE REFERENCE V) HEAT SOURCEI
19
1 I
TEMPERATURF Fig. 1. DBRINRCC s o l a r c o l l e c t o r t e s t l o o p u s i n g t h e r e f e r e n c e h e a t s o u r c e method The t h e r m a l t r a n s f e r e f f i c i e n c y of t h e r e f e r e n c e h e a t s o u r c e (nRHS) i s d e f i n e d : RHS t h e r m a l power o u t p u t - q ~ H s ( 3 )"'
= RHS e l e c t r i c a l power i n p u t PRHS where, qRHS = r a t e of t h e r m a l e n e r g y o u t p u t t o h e a t t r a n s f e r f l u i d i n RHS = ( 6 . ~ ~ - AT)RHs.
I n most s o l a r c o l l e c t o r t e s t s , t h e d i f f e r e n c e i n mean f l u i d t e m p e r a t u r e between t h e RHS and c o l l e c t o r can be k e p t s u f f i c i e n t l y s m a l l s o t h a t t h e d i f f e r e n c e i n f l u i d s p e c i f i c h e a t c a n b e n e g l e c t e d . Good d e s i g n of t h e RHS can a s s u r e t h a twS
a p p r o a c h e s 1. The r a t e of e n e r g y c o l l e c t i o n by t h e s o l a r c o l l e c t o r c a n , t h e r e f o r e , b e approximated by Thus, t h e RHS method p e r m i t s d e t e r m i n a t i o n of t h e e n e r g y c o l l e c t i o n by t h e t e s t c o l l e c t o r w i t h o u t d i r e c t l y measuring t h e f l u i d f l o w r a t e and w i t h o u t knowing t h e t h e r m o p h y s i c a l p r o p e r t i e s of t h e h e a t t r a n s f e r f l u i d . T h i s p a p e r d e s c r i b e s t h e p o t e n t i a l s o u r c e s of e r r o r w i t h t h i s method, and t h e s p e c i f i c d e s i g n and c a l i b r a t i o n of t h e RHS used i n t h e DBRINRCC S o l a r C o l l e c t o r T e s t F a c i l i t y . 2. SOURCES OF ERROR To e n s u r e t h a t a c c u r a t e r e s u l t s a r e o b t a i n e d when u s i n g a r e f e r e n c e h e a t s o u r c e , c o n s i d e r a t i o n must b e g i v e n t o b o t h i t s d e s i g n and i t s o p e r a t i o n . Heat l o s s e s from t h e r e f e r e n c e h e a t s o u r c e r e s u l t i n an o v e r p r e d i c t i o n of t h e RHS power i n p u t t o t h e f l u i d and, a s a r e s u l t , a n o v e r p r e d i c t i o n o f t h e c o l l e c t o r performance. These l o s s e s c a n b e minimized by i n s u l a t i n g t h e RHS a n d by u s i n g a t h e r m a l g u a r d ( d e s c r i b e d i n S e c t i o n 3 ) w i t h i n t h e RHS.Temperature measurement e r r o r s c a n r e s u l t f r o m l o c a l b o i l i n g of t h e h e a t t r a n s f e r f l u i d on t h e RHS h e a t e r element. T h i s problem i s most l i k e l y t o o c c u r a t low d e g r e e s of s u b c o o l i n g * and low f l u i d f l o w r a t e s . Experiments h a v e been c o n d u c t e d
[ I ]
o n s u r f a c e b o i l i n g i n an a n n u l u s f l o w p a s s a g e s i m i l a r t o t h a t i n t h e DBRfNRCC RHS. The r e s u l t s of t h i s s t u d y c a n b e u s e d t o e s t i m a t e t h e minimum f l o w r a t e s and d e g r e e s of s u b c o o l i n g r e q u i r e d t o a v o i d s u r f a c e b o i l i n g . The maximum a l l o w a b l e h e a t e r power d e n s i t y i n c r e a s e s w i t h i n c r e a s i n g d e g r e e s of s u b c o o l i n g and w i t h i n c r e a s i n g h e a t e r - t o - f l u i d h e a t t r a n s f e r c o e f f i c i e n t . Thus t e m p e r a t u r e measurement e r r o r s due t o l o c a l b o i l i n g c a n-
*
d e g r e e s of s u b c o o l i n g a r e r e f e r r e d t o h e r e a s t h e d i f f e r e n c e ( i n K) between t h e f l u i d a v e r a g e t e m p e r a t u r e i n t h e RHS and t h e s a t u r a t i o n t e m p e r a t u r e a t t h e o p e r a t i n g p r e s s u r e . For example i f t h e f l u i d i s w a t e r and t h e a v e r a g e t e m p e r a t u r e and p r e s s u r e i n t h e RHS a r e 90°C and 108.15 kPa r e s p e c t i v e l y (which c o r r e s p o n d s t o a s a t u r a t i o n temp. of 101.85OC) t h e n t h e f l u i d h a s 11.85 K of s u b c o o l i n g .be a v o i d e d by c a r e f u l c o n s i d e r a t i o n of h e a t e r power d e n s i t y (power i n p u t ) , d e g r e e s of s u b c o o l i n g ( o p e r a t i n g t e m p e r a t u r e ) and h e a t t r a n s f e r c o e f f i c i e n t (f l o w r a t e ) .
E f f e c t i v e mixing d e v i c e s l o c a t e d ahead of t h e t e m p e r a t u r e w e l l s a r e a l s o e s s e n t i a l f o r good t e m p e r a t u r e measurement. These m i x e r s must r e m a i n e f f e c t i v e o v e r a w i d e r a n g e of f l o w r a t e s . As w e l l , f l u i d f r i c t i o n c a n i n c r e a s e t h e f l u i d t e m p e r a t u r e and c a u s e a n o v e r p r e d i c t i o n o f ATRHS; c o n s e q u e n t l y , t h e r e f e r e n c e h e a t e r s h o u l d b e d e s i g n e d t o minimize t h e p r e s s u r e d r o p between t h e t e m p e r a t u r e s e n s o r s . A major s o u r c e of e r r o r i n s o l a r c o l l e c t o r t e s t i n g i s non-steady t e s t i n g c o n d i t i o n s . The r e f e r e n c e h e a t s o u r c e i s s i m i l a r t o a c o l l e c t o r i n t h a t i t h a s t h e r m a l mass; t h u s i t c a n s t o r e and r e l e a s e energy d u r i n g ' t r a n s i e n t o p e r a t i o n . T h i s can r e s u l t i n an over- o r
,
u n d e r p r e d i c t i o n of c o l l e c t o r performance. For t h i s r e a s o n , t h e f l o w r a t e , i n l e t t e m p e r a t u r e , h e a t e r power, and s u b s e q u e n t t e m p e r a t u r e r i s e must b e h e l d c o n s t a n t d u r i n g e a c h t e s t p e r i o d . R e f e r e n c e[L]
o u t l i n e s r e q u i r e m e n t s f o r s t e a d y s t a t e t e s t i n g of s o l a r c o l l e c t o r s : t h e s e same r e q u i r e m e n t s a p p l y t o t h e r e f e r e n c e h e a t s o u r c e . I To minimize t h e e r r o r r e s u l t i n g from t h eI
assumption t h a t t h e f l u i d s p e c i f i c h e a t (C ) i s t h eI
same i n t h e h e a t e r and t h e c o l l e c t o r , t h e g e a n f l u i d t e m p e r a t u r e i n t h e RHS must b e k e p t c l o s e t o t h a t i n t h e s o l a r c o l l e c t o r . With w a t e r a s t h e h e a t t r a n s f e r1
f l u i d , t h i s e r r o r c a n u s u a l l y b e k e p t t o l e s s t h a n 0.25% f o r t h e t e m p e r a t u r e s n o r m a l l y e n c o u n t e r e d i n t e s t i n g . With g l y c o l s and o i l s , however, t h e v a l u e o fC i s s t r o n g l y dependent on t e m p e r a t u r e . It i s ,
i
t g e r e f o r e , s u g g e s t e d t h a t w i t h t h e s e h e a t t r a n s f e r f l u i d s , t h e i n l e t t e m p e r a t u r e and t e m p e r a t u r e r i s e f o r b o t h t h e RHS and t h e t e s t c o l l e c t o r b e k e p ta p p r o x i m a t e l y t h e same. T h i s can be accomplished by c o o l i n g t h e f l u i d between t h e RHS and t h e c o l l e c t o r , and by a d j u s t i n g t h e h e a t e r i n p u t t o make t h e
t e m p e r a t u r e r i s e n e a r l y e q u a l a c r o s s b o t h t h e RHS and t h e c o l l e c t o r . As shown by t h e example i n Table 1 , t h e e r r o r r e s u l t i n g from a l a r g e t e m p e r a t u r e d i f f e r e n t i a l between t h e RHS and t h e c o l l e c t o r i s q u i t e s i g n i f i c a n t ( ~ 2 % ) w i t h a g l y c o l / w a t e r m i x t u r e , b u t i n s i g n i f i c a n t ("0.1%) w i t h water. Although t h e example i s f o r a "worse c a s e " s i t u a t i o n , i t d o e s i n d i c a t e t h e need f o r a v o i d i n g l a r g e d i f f e r e n c e s i n t e m p e r a t u r e c o n d i t i o n s between RHS and c o l l e c t o r .
TABLE 1 EXAMPLE OF ERROR I N ASSUMING Cp,, = Cp, RHS C o l l e c t o r type: e v a c u a t e d t u b e , h i g h f l u i d r e s i d e n c e
t i m e
F l u i d : ( 5 0 % by volume e t h y l e n e g l y c o l / w a t e r m i x t u r e )
Theref o r e ,
For w a t e r , t h e c o r r e s p o n d i n g r a t i o would be:
To summarize: t h e RHS s h o u l d b e d e s i g n e d t o a c h i e v e t h e f o l l o w i n g :
1 ) n e g l i g i b l e o r d e f i n a b l e h e a t l o s s ,
2 ) low power d e n s i t y h e a t i n g e l e m e n t , and a h i g h h e a t t r a n s f e r c o e f f i c i e n t between t h e h e a t e r and t h e f l u i d t o a v o i d b o i l i n g , 3 ) good f l u i d m i x i n g u p s t r e a m of e a c h t e m p e r a t u r e measurement p o i n t t o e n s u r e a c c u r a t e f l u i d t e m p e r a t u r e measurements, 4 ) s m a l l p r e s s u r e d r o p between t e m p e r a t u r e w e l l s t o a v o i d f r i c t i o n a l h e a t i n g of t h e f l u i d . The d e s i r e d o p e r a t i n g c o n d i t i o n s f o r t h e RHS a r e : 1 ) s t e a d y o p e r a t i o n , i . e . , a c o n s t a n t f l o w r a t e , i n l e t t e m p e r a t u r e , and h e a t e r power i n p u t , 2 ) s u f f i c i e n t f l o w r a t e , o r low o u t l e t t e m p e r a t u r e , s o t h a t t h e f l u i d i n t h e RHS d o e s n o t b o i l , 3) t e m p e r a t u r e r i s e l a r g e enough t o minimize AT e r r o r s , 4 ) mean f l u i d t e m p e r a t u r e i n t h e RHS c l o s e t o t h a t i n t h e c o l l e c t o r t o minimize C e r r o r s . P
3. DESIGN OF THE DBR/NRCC REFERENCE HEAT SOURCE Because t h e DBRINRCC S o l a r C o l l e c t o r T e s t F a c i l i t y h a s t h e c a p a b i l i t y of t e s t i n g two c o l l e c t o r s
s i m u l t a n e o u s l y , i t h a s two s e p a r a t e c a l o r i m e t e r l o o p s , e a c h w i t h i t s own r e f e r e n c e h e a t s o u r c e . The two ( r e f e r r e d t o a s RHSiIl and RHSiI2) a r e i d e n t i c a l e x c e p t f o r minor d i f f e r e n c e s i n t h e t h e r m o p i l e d e s i g n . The r e f e r e n c e h e a t s o u r c e d e s i g n e d a t NRCC i s shown i n F i g u r e 2 . It c o n s i s t s of a 7 5 0 q a t t e l e c t r i c immersion h e a t e r e l e m e n t e n c l o s e d i n a s e c t i o n of c o p p e r p i p e . T h i s r e p r e s e n t s a maximm power d e n s i t y of a p p r o x i m a t e l y 49 kw/m2.
The RHS was d e s i g n e d t o b e compact t o f a c i l i t a t e t h e i n c o r p o r a t i o n of a t h e r m a l guard r i n g . The t h e r m a l g u a r d i s i n t e n d e d t o l i m i t t h e h e a t l o s s e s ( o r g a i n s ) between t h e i n t e r n a l components and t h e s u r r o u n d i n g s . G l a s s f i b r e i n s u l a t i o n i s u s e d t o i n s u l a t e t h e i n t e r n a l components from e a c h o t h e r and from t h e g u a r d r i n g . The t h e r m a l g u a r d c o n s i s t s of a c o i l e d c o p p e r t u b e s o l d e r e d t o a copper s h e e t . The f l u i d i s c i r c u l a t e d t h r o u g h t h e g u a r d b e f o r e e n t e r i n g t h e h e a t e r s e c t i o n o f t h e RHS. T h i s c o n f i g u r a t i o n s i g n i f i c a n t l y r e d u c e s t h e t e m p e r a t u r e d i f f e r e n c e and r e s u l t a n t h e a t l o s s between t h e h e a t e r ' s i n t e r n a l components and i t s immediate s u r r o u n d i n g s . The g u a r d r i n g t e m p e r a t u r e f o l l o w s t h e i n l e t f l u i d t e m p e r a t u r e w i t h o u t t h e need f o r a c o n t r o l l e r e v e n when t h e h e a t t r a n s f e r f l u i d i s c o o l e r t h a n t h e a i r a d j a c e n t t o t h e RHS. The e x t e r i o r of t h e t h e r m a l g u a r d r i n g i s i n s u l a t e d t o minimize h e a t l o s s e s and l i m i t t h e t e m p e r a t u r e g r a d i e n t a l o n g i t s f l o w l e n g t h . An a d v a n t a g e of t h e f l u i d g u a r d r i n g o v e r a n e l e c t r i c guard r i n g , i n a d d i t i o n t o i t s c a p a b i l i t y t o o p e r a t e below t h e a m b i e n t t e m p e r a t u r e , i s t h a t i t d o e s n o t r e q u i r e a c o n t r o l c i r c u i t which c o u l d i n t r o d u c e " n o i s e " i n t o t h e t e m p e r a t u r e measurement.
A mixing d e v i c e and a t e m p e r a t u r e measurement w e l l a r e l o c a t e d a t b o t h t h e i n l e t and t h e o u t l e t of t h e h e a t e r . To a c h i e v e h i g h r e s o l u t i o n and r e l i a b i l i t y , t y p e "T" ( c o p p e r c o n s t a n t a n ) d i f f e r e n t i a l t h e r m o p i l e s a r e u s e d t o measure t h e f l u i d t e m p e r a t u r e r i s e a c r o s s t h e h e a t e r e l e m e n t s . (RHSdk1 h a s 1 0 j u n c t i o n s and RHSdI2 h a s 8.) The t h e r m o p i l e s were c a l i b r a t e d by t h e D i v i s i o n of P h y s i c s o f t h e NRCC, i n t e r m s o f t h e I n t e r n a t i o n a l P r a c t i c a l Temperature S c a l e of 1968 (IPTS-68). The e l e c t r i c a l power i n p u t (PRHS) i s
measured w i t h a c a l i b r a t e d e l e c t r o n i c power t r a n s d u c e r (20.25% of f u l l s c a l e ) .
The f l u i d p i p e s l e a d i n g t o and f r o m t h e RHS a r e i n s u l a t e d t o l i m i t a x i a l t h e r m a l c o n d u c t i o n , a s a r e t h e w i r e s g o i n g t o and f r o m t h e t h e r m o p i l e and h e a t e r . The
T O P
V I E W
-.
, ,\ ,THERMAL GUARD RING
(COILED COPPER TUBE SOLDERED TO COPPER SHEET)
BLOWN GLASS FIBRE
-<=-'
1 . INSULATIONd
FLOWEXIT
+
t
FLOW ENTRANCETEMPERATURE WELL
THERMAL GUARD RING
HEATER
16 mrn DIA. x 300 mm LG 750 WATT
FLOW FROM GUARD
4.1 T e s t P r o c e d u r e
A s e r i e s of c a l i b r a t i o n t e s t s was ~ e r f o r m e d t o d e p t h of immersion of t h e t e m p e r a t u r e w e l l s i s
a p p r o x i m a t e l y 30 d i a m e t e r s , which e x c e e d s t h e recommendations of t h e ASHRAE S t a n d a r d 41.1-74: S e c t i o n on Temperature Measurements
[i].
A s mentioned e a r l i e r , a s t a b l e i n l e t t e m p e r a t u r e , good f l o w r a t e c o n t r o l and r e g u l a t i o n o f t h e power s u p p l i e d t o t h e RHS a r e a l l e s s e n t i a l t o s t e a d y - s t a t e o p e r a t i o n of t h e RHS. The DBRINRCC s o l a r c o l l e c t o r t e s t l o o p u s e s a p r e s s u r e - s e n s i t i v e c o n t r o l v a l v e t o m a i n t a i n t h e f l o w r a t e t o f0.1% of t h e s e t p o i n t . The s t a b i l i t y of t h e i n l e t t e m p e r a t u r e , u s i n g a w e l l - m i x e d t e m p e r a t u r e b a t h u p s t r e a m of t h e RHS, was 3.1 K d u r i n g a t e s t p e r i o d . H e a t e r power f l u c t u a t i o n s a r e reduced t o -f0.1% w i t h t h e u s e of a v o l t a g e r e g u l a t o r . d e t e r m i n e t h e e f f e c t on
nRHS,
o f ;-
t e m p e r a t u r e r i s e t h r o u g h h e a t e r ,-
t e m p e r a t u r e d i f f e r e n c e between h e a t e r f l u i d a n d s u r r o u n d i n g a i r ,-
f l u i d f l o w r a t e . I n o r d e r t o p e r f o r m t h e c a l i b r a t i o n t e s t s , t h e upper p o r t i o n of t h e c o l l e c t o r t e s t f a c i l i t y was m o d i f i e d a s shown i n F i g u r e 3, w i t h a dump t a n k and a measuring s c a l e i n p l a c e of t h e s o l a r c o l l e c t o r . T h i s p e r m i t t e d a n i n d e p e n d e n t d e t e r m i n a t i o n of a v e r a g e mass f l o w r a t e , by measuring t h e t o t a l mass of f l u i d f l o w i n g t h r o u g h t h e r e f e r e n c e h e a t e r o v e r a p r e c i s e l y measured time i n t e r v a l . For t h e c a l i b r a t i o n t e s t s , t h e v a l u e of f l u i d s p e c i f i c h e a t (C ) was d e t e r m i n e d a t t h e mean PTABLE 2 SAMPLE DATA FROM CALIBRATION TEST
RHS 11 1
Run Number 5 7
Average Temperature Measurements i n RHS
I n l e t f l u i d t e m p e r a t u r e (Ti) 44.70°C
O u t l e t f l u i d t e m p e r a t u r e (To) 53.08OC
-
1Tm --• (Ti
+
To) 48.89OC2
AT,, 8.39 K
Ambient t e m p e r a t u r e (Ta) 27.0°C
Tm- Ta 21.9 K
WELL-MIXED
TEMPERATURE BATH G r a v i m e t r i c Flow R a t e CONTROL
I VALVE
-
-
- - Net weight 9.26 kg-
-
-
- -
-
Time p e r i o d 599.9 s Mass f l o w r a t e - Volume f l o w r a t e 0.016 k g / s 0.98 Llmin VI PUMP REFERENCE 3 aI
1r PowerHEAT SOURCE
&
-
r l h d TEMPERATURE P~~~ 568.1 w a t t s CONTROL LOOP qRHS 566.6 w a t t s a T ~ ~ ~ nRHs 99.7% Fig. 3. C a l i b r a t i o n set-up A t h i g h f l u i d t e m p e r a t u r e s , e v a p o r a t i o n from t h e f l u i d t e m p e r a t u r e i n t h e h e a t e r , u s i n g v a l u e s t a k e n weighing t a n k i n t r o d u c e s a measurement e r r o r ,
from r e f e r e n c e
[?I.
e s p e c i a l l y a t low f l o w r a t e s when t h e weighing p e r i o dA c a l i b r a t i o n t e s t was performed a s f o l l o w s . The i s long. To r e d u c e t h i s e r r o r due t o e v a p o r a t i o n
h e a t t r a n s f e r f l u i d was drawn from a w e l l - m i x e d l o s s e s , t h e f l u i d was cooled b e f o r e e n t e r i n g t h e
t e m p e r a t u r e c o n t r o l l e d b a t h , pumped through t h e h e a t e r , weighing tank and a 'Over was used O n t h e
cooled and t h e n r e t u r n e d t o t h e bath. Flowrate was tank.
m a i n t a i n e d c o n s t a n t w i t h a p r e s s u r e s e n s i t i v e c o n t r o l A s e r i e s of t e s t s were performed t o d e t e r m i n e t h e
v a l v e . The f l u i d t e m p e r a t u r e r i s e through t h e h e a t e r p r e s s u r e drop c h a r a c t e r i s t i c of t h e r e f e r e n c e h e a t
and t h e i n l e t f l u i d t e m p e r a t u r e were c o n t i n o u s l y s o u r c e , a s s i g n i f i c a n t f l u i d f r i c t i o n h e a t i n g c o u l d
monitored on a s t r i p c h a r t r e c o r d e r t o d e t e c t any i n t r o d u c e e r r o r s . The p r e s s u r e d r o p between t h e r m o p i l e
t r a n s i e n t o r abnormal c o n d i t i o n s , and t o i n d i c a t e when w e l l s was measured a t s e v e r a l f l o w r a t e s from 0.008 t o
s t e a d y c o n d i t i o n s were a c h i e v e d ( s e e F i g u r e 4 f o r an 0.125 k g / s .
example of t h e t e m p e r a t u r e c o n d i t i o n s d u r i n g a t e s t To d e t e r m i n e t h e t h e r m a l r e s p o n s e of t h e RHS t o
r u n ) . When s t e a d y c o n d i t i o n s were achieved, i.e. when variations in power the tests were
t h e i n l e t t e m p e r a t u r e d i d n o t v a r y by more t h a n 3 . 1 K performed a t v a r i o u s f l u i d f l o w r a t e s . For e a c h
and t h e t e m p e r a t u r e r i s e a c r o s s t h e RHS, by more t h a n f l o w r a t e t h e RHS was allowed t o s t a b i l i z e and t h e v a l u e
k0.05 K, f o r a p e r i o d g r e a t e r t h a n t h e t e s t t i m e of ATRHS measured. The h e a t e r power i n p u t t o t h e RHS
i n t e r v a l (approx. 3 t o 1 5 min.), t h e f l o w was d i v e r t e d was t h e n s w i t c h e d o f f , w h i l e t h e i n l e t f l u i d
t o a weighing s c a l e and an a v e r a g e mass flow t e m p e r a t u r e and mass f l o w r a t e were m a i n t a i n e d c o n s t a n t .
measurement o b t a i n e d . T a b l e 2 g i v e s t h e r e s u l t s of a The s u b s e q u e n t decay of t h e v a l u e of ATRHS was r e c o r d e d
t y p i c a l t e s t run. u n t i l i t was e f f e c t i v e l y z e r o .
4.2 T e s t R e s u l t s
F i g u r e s 5a and 5b show c a l i b r a t i o n r e s u l t s f o r two
END i d e n t i c a l RHS's w i t h w a t e r a s t h e h e a t t r a n s f e r f l u i d .
The t h e r m a l e f f i c i e n c y (%HS) i s p l o t t e d a g a i n s t t h e
d i f f e r e n c e between t h e mean f l u i d t e m p e r a t u r e i n t h e RHS and t h e s u r r o u n d i n g ambient a i r t e m p e r a t u r e
(Tm
-
Ta) f o r d i f f e r e n t f l o w r a t e s . The magnitude of It h e v e r t i c a l b a r s r e p r e s e n t s t h e u n c e r t a i n t y a s s o c i a t e d
w i t h each d a t a p o i n t (Appendix A). For a f l o w r a t e
between 0.008 k g / s and 0.050 k g / s (0.5 t o 3 L/min.), t h e v a l u e s of measured RHS e f f i c i e n c y a r e s l i g h t l y l e s s t h a n 100% f o r t h e normal r a n g e of t e s t t e m p e r a t u r e s . I f a " l e a s t s q u a r e s " c u r v e f i t i s a p p l i e d t o t h e d a t a , t h e v a l u e of e f f i c i e n c y i s s e e n t o d e c r e a s e v e r y s l i g h t l y w i t h i n c r e a s i n g v a l u e s of (T,
-
T ). START I f t h e dependence of qRHS on (T,-
T,B
i s n e g l e c t e d , t h e v a l u e of t h e r m a l e f f i c i e n c y (nRHS) o b t a i n e d f o r RHStl i s 99.61% w i t h a s t a n d a r d d e v i a t i o nPig. 4. V a r i a t i o n of t h e t e m p e r a t u r e c o n d i t i o n s d u r i n g Of 0'22 and f o r RHS#Zs %HS = 99'59% with a standard
a t y p i c a l t e s t r u n d e v i a t i o n of 0.28. Thus t h e requirement f o r 1%
,
102 I I I I I 1 1 1 I.-..
RHS No. 1 o 1 Umin o 2 Umin+
3 Umin1
Fig. 5. C a l i b r a t i o n r e s u l t s f o r RHSill ( a ) and RHSl2 ( b )
accuracy on t h e measurement of mass f l o w r a t e o v e r t h e t e s t r a n g e a s r e q u i r e d by ASHRAE
[L]
i s surpassed. F i g u r e 6 p r e s e n t s t h e same d a t a ( f o rRHS82
o n l y ) p l o t t e d a g a i n s t t h e t e m p e r a t u r e r i s e through t h e h e a t e r (ATRHs). A c u r v e f i t t o t h e d a t a shows t h a t t h e e f f i c i e n c y i s e s s e n t i a l l y independent of t h e t e m p e r a t u r e r i s e through t h e RHS. Although t h e l o s s e s from t h e RHS s h o u l d b e z e r o when Tm = Ta ( i . e . , qRH = l o o % ) , t h e r e s u l t s i n F i g u r e 5a and 5b show t g a t t h i s i s n o t t h e c a s e . The r e s i d u a l e r r o r shown c o u l d be t h e r e s u l t of a s y s t e m a t i c e r r o r i n h e r e n t i n t h e d e s i g n of t h e RHS. It could a l s o be due t o t h e t a b u l a t e d v a l u e s of C u s e d i n P t h i s c a l i b r a t i o n , which may n o t a d e q u a t e l y a c c o u n t f o r any d i s s o l v e d a i r o r m i n e r a l s p r e s e n t i n t h e t e s t f l u i d . F i g u r e 7 shows t h e p r e s s u r e d r o p c h a r a c t e r i s t i c of t h e RHS w i t h w a t e r a s t h e h e a t t r a n s f e r f l u i d . With a flow of 0.10 k g / s , a f l o w r a t e h i g h e r t h a n t h a t n o r m a l l y encountered i n s o l a r c o l l e c t o r t e s t i n g , t h e p r e s s u r e d r o p between w e l l s was a p p r o x i m a t e l y 350 Pa, which c o r r e s p o n d s t o a p p r o x i m a t e l y 0.035 w a t t s of f r i c t i o n a l h e a t i n g . T h i s amount i s two o r d e r s of magnitude"
100.5 RHS No. 2 7) = 99.62-
7.466 x ( a T R H S ) Z Y U 9 9 . 5 LL Y w.
H E A T E R T E M P E R A T U R E R I S E .AT^^^,
K Fig. 6. Thermal e f f i c i e n c y v s t e m p e r a t u r e r i s e (RHS#Z) F L O W R A T E , k g l s Fig. 7. P r e s s u r e d r o p between t h e r m o p i l e w e l l s i n RHS TIME I N T E R V A L ,r ,
sI
Fig. 8. Thermal r e s p o n s e of RHS t o a s t e p change i n h e a t e r power l e v e l s m a l l e r t h a n t h e a p p a r e n t h e a t l o s s from t h e RHS and i s , t h e r e f o r e , n e g l i g i b l e . The t r a n s i e n t r e s p o n s e o f t h e r e f e r e n c e h e a t s o u r c e was determined by s h u t t i n g t h e h e a t e r o f f d u r i n g s t e a d y - s t a t e o p e r a t i o n and r e c o r d i n g t h e t i m e d e c a y of t h e d i f f e r e n t i a l t e m p e r a t u r e measurement. R e s u l t s a r e shown i n F i g u r e 8 a s v a l u e s of t e m p e r a t u r e r i s e ATRHS a t time i n t e r v a l 7, (normalized t o ATms a t 7 = 0) v s 7. The r e s u l t s i n d i c a t e t h a t t h e r e s p o n s e t i m e of t h e RHS i s dependent on t h e f l u i d f l o w r a t e . The r e s p o n s e of ATRHS t o a s t e p v a r i a t i o n i n h e a t e r power
i s e f f e c t i v e l y e x p o n e n t i a l , preceded by a s m a l l t i m e l a g which i n c r e a s e s a t lower f l o w r a t e s . To avoid e r r o r s i n t r o d u c e d by t h e t h e r m a l i n e r t i a of t h e RHS, t h e d e v i c e s h o u l d be o p e r a t e d under s t e a d y s t a t e c o n d i t i o n s .
5. CONCLUSIONS I
A major advantage of t h e RHS i s i t s c a p a b i l i t y t o measure t h e p r o d u c t of mass f l o w and s p e c i f i c h e a t d i r e c t l y i n t h e t e s t l o o p d u r i n g c o l l e c t o r t e s t i n g . T h i s a v o i d s e r r o r s t h a t c o u l d b e i n t r o d u c e d t h r o u g h t h e u s e of mechanical f l o w m e t e r s , which r e q u i r e an a c c u r a t e knowledge of t h e t e m p e r a t u r e dependence o f t h e t h e r m a l and p h y s i c a l p r o p e r t i e s of t h e h e a t t r a n s f e r f l u i d b e i n g used. To a c h i e v e s a t i s f a c t o r y r e s u l t s w i t h a n RHS, however, c a r e must be e x e r c i s e d i n i t s d e s i g n and s t e a d y - s t a t e o p e r a t i o n i s e s s e n t i a l ; a s t a b l e i n l e t t e m p e r a t u r e , good f l o w r a t e c o n t r o l and r e g u l a t i o n o f t h e power s u p p l i e d t o t h e h e a t e r , a r e r e q u i r e d .
C a l i b r a t i o n t e s t s w e r e performed on t h e NRCC r e f e r e n c e h e a t s o u r c e o v e r a r a n g e of f l o w r a t e s and i n l e t t e m p e r a t u r e s n o r m a l l y e n c o u n t e r e d d u r i n g t h e t e s t i n g of l i q u i d c o o l e d s o l a r c o l l e c t o r s , ( i . e . ,
&
from 0.008 k g / s t o 0.05 k g / s and Ti from t o 95OC). When u s i n g t h e RHS o u t s i d e t h e s e o p e r a t i n g c o n d i t i o n s , c a u t i o n must b e e x e r c i s e d . At low f l o w r a t e s (c0.008 k g / s ) , l o c a l i z e d b o i l i n g may i n t r o d u c e e r r o r s i f t h e h e a t e r power d e n s i t y i s n o t reduced a s w e l l . O p e r a t i o n a t f l o w r a t e s g r e a t e r t h a n 0.05 k g / s r e d u c e s t h e t e m p e r a t u r e r i s e a c r o s s t h e RHS, i n c r e a s i n g t e m p e r a t u r e measurement u n c e r t a i n t y . R e s u l t s i n d i c a t e t h a t t h e p r e s e n t RHS e x c e e d s t h e ASHRAE t e s t method r e q u i r e m e n t[L]
of 1% a c c u r a c y i n t h e d e t e r m i n a t i o n of mass f l o w r a t e . ACKNOWLEDGEMENT The a u t h o r s wish t o e x p r e s s t h e i r a p p r e c i a t i o n t o L.P. Chabot f o r h i s a s s i s t a n c e . T h i s p a p e r i s a c o n t r i b u t i o n from t h e D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r of t h e D i v i s i o n . REFERENCESMethods of T e s t i n g t o Determine t h e Thermal
P e r f o r m a n c e of S o l a r C o l l e c t o r s . American S o c i e t y f o r H e a t i n g , R e f r i g e r a t i n g and Air-Conditioning E n g i n e e r s , ASHRAE S t a n d a r d 93-77, 1977.
H a l l , J . , Choosing a Flow Monitoring Device, I n s t r u m e n t s and C o n t r o l Systems, J u n e 1981, p. 51-59.
P l a c h e , K.O., C o r i o L i s / G y r o s c o p i c Flow Meter, Mechanical E n g i n e e r i n g , March 1979, p. 36-41. Reed, K.A., I n s t r u m e n t a t i o n f o r t h e r m a l performance measurements: s t r i v i n g f o r measurement a s s u r a n c e i n s o l a r c o l l e c t o r t e s t i n g , ASME S o l a r Energy Conference, Albuquerque 1982, p. 337-340. H a r r i s o n , S.J. and S a s a k i , J.R., S t u d i e s on S o l a r C o l l e c t o r Performance a t NRC, Renewable A l t e r n a t i v e s , P r o c e e d i n g s of t h e F o u r t h Annual Conference of t h e S o l a r Energy S o c i e t y of Canada, August 1978, London, O n t a r i o , Canada.
Reed, K.A. and A l l e n , J.W., S o l a r Thermal C o l l e c t o r T e s t i n g : C a l o r i f i c R a t i o Technique, S o l a r
D i v e r s i f i c a t i o n , P r o c e e d i n g s of t h e I n t e r n a t i o n a l Energy S o c i e t y , I n c . , August 1978, Denver, CO, USA.
McAdams, W.H., e t a l . , H e a t T r a n s f e r a t High R a t e s t o Water With S u r f a c e B o i l i n g , I n d u s t r i a l and E n g i n e e r i n g C h e m i s t r y , September 1949, p. 1945. S t a n d a r d Measurement Guide: S e c t i o n on t e m p e r a t u r e measurements, American S o c i e t y f o r Heating,
R e f r i g e r a t i n g and Air-Conditioning E n g i n e e r s , ASHRAE S t a n d a r d 41.1-74, 1974.
Weast, R.C., ed.. Handbook of Chemistry and P h y s i c s , Forty-Ninth E d i t i o n , The Chemical Rubber Co., C l e v e l a n d , OH, 1968.
K l i n e , S.J. and McClintock, F.A., D e s c r i b i n g U n c e r t a i n t i e s i n Single-Sample Experiments, Mechanical E n g i n e e r i n g , J a n u a r y 1953.
APPENDIX A A n a l y s i s o f t h e Random Measurement U n c e r t a i n t y on nRHS
The u n c e r t a i n t y a s s o c i a t e d w i t h nRHS c a n b e e v a l u a t e d u s i n g t h e p r o p a g a t i o n of random u n c e r t a i n t y t e c h n i q u e s
[El.
U s i n g e q u a t i o n ( 3 ) we have:
The mass f l o w r a t e (ni) i s d e t e r m i n e d by w e i g h i n g a mass of f l u i d (M) f l o w i n g t h r o u g h t h e RHS o v e r a p r e c i s e l y measured time i n t e r v a l ( T ) s o Eq. ( 5 ) becomes,
Performing an a n a l y s i s a s d e s c r i b e d i n Ref. 1 0 t h e u n c e r t a i n t y i n t h e d e p e n d e n t v a r i a b l e (W llauc ~ k a n b e e x p r e s s e d i n t e r m s of t h e u n c e r t a i n t i e s in""e"ach of t h e i n d e p e n d e n t v a r i a b l e s (WM, W T , Wcp, VAT, WPRHS). where, WM = i O . O l k g W~ P +0.1% of C P
