- Email: [email protected]
Experimental investigations on single-absorber solar air heaters
Energy Convers. Mgmt Vol. 27, No. 4, pp. 343-349, 1987 Printed in Great Britain. All fights reserved
0196-8904/87 $3.00+0.00 Copyright © 1987 Pergamon Journals Ltd
EXPERIMENTAL INVESTIGATIONS ON SINGLE-ABSORBER SOLAR AIR HEATERS V. K. G O E L , R A M C H A N D R A and B. C. R A Y C H A U D H U R I Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India
(Received 2 May 1986) Almtraet--Flat, corrugated and vee-corrugated solar air heaters having a single absorber are designed, fabricated and tested. The rear plate in these collectors is replaced by rigid synthetic foam. It is found that the vee-corrugated absorbers give the best performance. The efficiency of the commercially corrugated absorber is about 5% lower than that of the vee-corrugated absorber. Correlations for predicting forced convection heat transfer coefficients for these solar air heaters have been obtained. The analytical model for these air heaters has also been developed.
NOMENCLATURE A, A c = Ag = B= Cf= de =
Collector aperture area (m 2) Gross collector area (m 2) Depth of collector (m) Specific heat of fluid (J/kg °C) Equivalent diameter ( = [4 x cross-sectional area]/[wetted perimeter]) E = Emissivity factor f = Friction factor Fc = Effective transmissivity-absorptivity product G = Mass flow rate of fluid per unit area of collector kg/s m 2) hc = Convective heat transfer coeficient between absorbing plate and fluid (W/m 2 °C) h~ = Covective heat transfer coefficient between rear plate and fluid (W/m 2 °C) he,~ = Convective heat transfer coefficient between i and j (W/m 2 °C) hr = Radiative heat transfer coefficient between rear plate and absorbing plate (W/m 2 °C) hr. ~ = Radiative heat transfer coefficient between i and j (W/m 2 °C) I = Solar radiation intensity following on collector (W/m 2) L = Collector length (m) rh = Mass flow rate of fluid (kg/s) Nu = Nusselt number q~ = Useful heat collected per unit area (W/m 2) T = Temperature (°C) Ta = Ambient temperature (°C) Tf = Local or average fluid temperature (°C) T6 = Fluid temperature entering collector (°C) Tfo = Fluid temperature exiting collector (°C) Tp = Absorbing plate temperature (°C) Tr = Rear plate temperature (°C) AT = Temperature difference across collector (°C) U L = Collector heat loss coefficient between absorber plate and ambient (W/m 2 °C) Uo = Overall heat loss coefficient (W/m 2 °C) Ur = Downward heat loss coefficient through insulation (W/m 2 °C) Ut = Heat loss coefficient from cover, or absorber to outside air including radiation to sky 0,V/m 2 °C) Vf = Mean velocity of air flow (m/s) W = Width of collector (m) ct = Absorptivity ~/= Efficiency p = Density (kg/m 3) = Transmissivity 343
Subscripts gi = Glass i a = Ambient fi = Fluid i p = Plate pr = Rear plate s = Sky 1. INTRODUCTION Solar air heaters are being increasingly used in m a n y areas, such as drying o f agricultural products, space heating and, to some extent, in domestic water heating. Several designs o f solar air heaters have been proposed, a n d some of t h e m give satisfactory p e r f o r m a n c e [1, 2]. A survey o f the literature [3-14] shows t h a t all designs o f solar air heaters forwarded so far can basically be classified into two categories. T h e first type has a n o n p o r o u s a b s o r b e r in which the air-stream does n o t flow t h r o u g h the a b s o r b e r plate. Air m a y flow above a n d / o r b e h i n d the a b s o r b e r plate. T h e second type has a p o r o u s a b s o r b e r where air c a n flow t h r o u g h the absorber. ' A l l these designs have been reviewed by Bansal et al. [3]. A l m o s t all n o n p o r o u s types o f solar air heaters m a k e use of two absorbers. In this c o m m u n i c a t i o n , we have designed, fabricated a n d tested singlea b s o r b e r solar air heaters.
2. DESIGN OF THE SOLAR AIR HEATERS T h e types o f collectors a n d their c o n s t r u c t i o n details are s h o w n in Fig. 1, as well as in T a b l e 1. T h e collector dimensions are 1.83 x 1.22 m (6' x 4') with a flow c h a n n e l d e p t h equal to 5.6 cm (2.2"). T h e collectors differ f r o m c o n v e n t i o n a l designs in such a way t h a t air flows in between the a b s o r b e r plate a n d the rigid synthetic foam which also acts as the rear plate, whereas in c o n v e n t i o n a l designs, air flows between two metallic absorbers.
344
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS SoLar radiation
GLass
f , Absorber Air i n -
-2.25" ~ Air out
--
heti Type
m)
Z
SoLar radiation
GLass covers
b
_I_2.25, '
"/'////////'k'/////// = 0.6"2.95.
~--Insulation
Type
(synthetic foam
Tr GLass covers
Solar radiation
where it is conditioned to a specified temperature before entering the secondary measuring device. Air then flows to the heater connected to the collector inlet. Leaving the collector array, the air passes through the collector outlet measuring section and finally to the atmosphere. It is a forced circulation mode of operation. Its suction counterpart is shown in Fig. 3. The blower is of centrifugal type, rating 1 hp, suitable to work under 2.5 KPa (10" H20) gauge pressure and delivering approx. 250/300 cfm. The amount of air delivered by the blower is controlled by an air gate valve. The air velocity in the duct is measured by a photoelectric airflow meter. All the temperatures are measured by an on-line programmable data acquisition system. The solar radiation incident on a collector surface is measured using a pyranometer. The pyranometer is mounted on the adjacent surface, parallel to the collector, in such a way that it does not cast any shadow on the collector. Sufficient care has been taken to mount the pyranometer at the same tilt as the solar collector. The diffuse component of the incident solar radiation is determined for each efficiency test point by shading the pyranometer. Data acquisition
/ ^ - - "1- --A
Vee - g raoved A absorber
^=("
-
/VVVVV
I
,
2.25"
V///////////~
////////
13/4"
Design A : 8 = 6 0 0 , 0 = 1"8~ 1.5" ~ Insulation (synthetic foam) Design B ; 8 = 9 0 *, a = 1,5"
Type'nT Fig. 1
A variety of data are monitored and recorded by the DATAL-220 Data Acquisition System (DAS). All the temperatures and meteorological data leads are fed into an air-conditioned room which houses the DAS. The DATAL-220 employs the latest concept of distributed processing for providing better system availability. The master CPU is in the DATAL-220 and the slave CPU is in the DATAL220. The DATAL-CPU receives scanned data from the DATAL-220 and can analyse the data in real time as per the program written in FORTRAN. The information is also displayed on a 30 cm CRT. The data can be stored on floppy disks. The results can be obtained on an alphanumeric printer.
3. EXPERIMENTA SET UP AND INSTRUMENTATION The test loop for the air-heating collector is shown in Fig. 2. It is an open-loop configuration. The blower delivers air to the air-reconditioning apparatus,
4. THEORY
4.1. Efficiency The instantaneous thermal efficiency of the collector, defined as the ratio of useful thermal energy to
Table 1. Description of solar air collectors tested Collector type
Gross collector area (mz) Absorber
Flat (2)
2.23
Flat (1)
2.23
Corrugated
2.23
Currugated
2.23
Corrugated
2.23
Glazing
Back insulation
shalimar blackboard
Double
Synthetic foam
shalimar blackboard
Single
Synthetic foam
shalimar blackboard
Double
Synthetic foam
Flow perpendicular to corrugation
shalimar blackboard
Single
Synthetic foam
Flow perpendicular to corrugation
shaiimar blackboard
Double
Synthetic foam
Flow parallel to corrugation
GI sheet shalimar blackboard paint
Double
Synthetic foam
Flow parallel to corrugation
GI sheet pamt GI sheet paint GI sheet paint GI sheet paint GI sheet
Miscellaneous
paint
Vee-corrugated
2.23
GOEL et al.:
SINGLE-ABSORBER SOLAR AIR HEATERS
345
Pyranomete~
eoder
~
Air flow meter
~ S
Header
Air flow meter
I
=, ~ ~ I
LI,Y
Thermocoupe ts
I
Them r ocoup e ls Collector pressure drop
Inclined manometer
BLower
~Wet
~
Air in
bulb
re-conditioning apparatus
dry bulb
Fig. 2. Schematic diagram of solar air heater testing (forced circulation).
Pyranomet~
Air flow meter
Heade~_~
Thermocoupkes
Air flow meter
Thermocouples
~
CoLLecotr
Inclined manometer''
•
~
bulb 8~ dry bulb
pressure drop
Air
,m i n
BLower Air ~ ~ out
re - conditioning apparatus
Fig. 3. Schematic diagram of solar air heater test (suction flow).
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS
346
total incident solar energy, averaged over the same time interval, is mathematically expressed as
,,/h
~/ =
35
(Tfo - T~) dt
rhCf
40
o
(1)
/%
°
30
& J, /(t) dt
Mode of operation:
[]
g~
forced circulation
1
The quantities rh and Cf are assumed to be constant during each set of measurements.
25
FLow rote : 0.0369 k g / s o FLat - plate [] Corrugated V e e - corrugated
20'.005-0
I
I
O.Ol
o o°o o
I
o.o15
o ocOo
I
o.o2
I
0.025
I
0.03
0.035
(~-~)/_r(m 2 -°C/W)
40
Fig. 7. Variation of efficiency with (T:- Ta)/I in double glazed collectors (flow perpendicular to corrugation). 3O
Mode of operation : forced circulation
34
Date : 2 - 3 - 1 9 8 4 -FLat-pLate c.--o Corrugated
// 20
30 []0
o@ lC •0 0
]
I
I
I
[
10.00
11.00
12.00
1.00
2.00
I
o~ 2 5
D
3.00
Time of day
Fig. 4. Variation of instantaneous efficiency with time of day in double glazed collectors (flow perpendicular to corrugation).
20
°
o
Mode of operation :
forced circulation
o
FLow rate : 0 . 0 2 3 6 k g / s
Q
ra Corrugated
o I
0.02:
0.025
ra c~ 0.03
Fig. 8. Variation of efficiency with (T~- To)/I in double glazed collectors (flow perpendicular to corrugation).
30
25 20
-
15
o I
( ~-To]/I ( m2 -°C/W )
35
~
o
o FLat - plate
15 •~ V e e - corrugate,d, 14 0.005 0.01 0.015
40
n
Mode o1 operation : forced circulation FLow rate : 0.0236 kg/s Date: 2 3 - 3 - 8 4 -FLat- plate c---oCorrugoted
20-
15 10 9.00
I 10.00
I 11.00
I 12.00
Time
I 1.00
I 2.00
Mode of operation:
[~
suction flow
I 3.00
of day
FLow rate : 0 . 0 2 3 6 kg/s o FLat - plate [3 Corrugated
[3
Fig. 5. Variation of instantaneous efficiency with time of day in double glazed collectors (flow perpendicular to corrugation).
Vee-corrugoted
o~ lO o
~O
ooo
qn o
o o
10 --
-~
FLow rate : 0 . 0 2 3 6 kg/$ Date : 1 9 - 4 - 8 4 FLat-pLate Corrugated
0
[]
I
I
I
I
0.005
0.010
0.015
0.020
( T~j - To ) /I
I 0.025
(rn2-°C/W)
Fig. 9. Variation o f efficiency with Tji-To/I in double glazed collectors (flow parallel to corrugation). 5-
0 9.00
4.2. Thermal modelling of solar air heaters I 10.00
I 11.00
I 12.00
I 1.00
I 2.00
I 3.00
Time of day
Fig. 6. Variation of instantaneous efficiency with time of day in single glazed collectors (flow perpendicular to corrugation)
The energy balance equations to the collector shown in Fig. 1 (Type I) may be written as follows: Cover glass (gl): astl = h¢,slw(Tst - T~) + hr,g,s(Tg I - T,)
+hr, s,s:(Tsl- T v) + h~,gm (Ts] - T.).
(2)
GOEL et al.: 15
SINGLE-ABSORBER SOLAR AIR HEATERS
These equations can be combined to give a single differential equation for Tf, whose solution is given by
Mode of operation : suction flow FLow rate • 0.172 kg/s o FLat-pLate [] Corrugated •~ Vee -corrugated
10
B7 Tf
I
I
I
o o~o
o.o~5
~a6]
Cfrh //"
(8)
The collector outlet temperature can be determined by putting
o~
o.005
347
I~1
X=L.
I
o.o2o
o.025
The constant Bs are given in the Appendix. The two widely used correlations for evaluating forced convective heat transfer coefficients are Charters' correlation [4]:
( 7}fi- ro )/ Z ( mZ -°C / W )
Fig. 10. Variation of efficiency with ( T : - Ta)/I in double glazed collectors (flow parallel to corrugation).
Nu = 0.0158 Re °'8, Glass plate (g2):
and the Niles et al. [5] correlation:
hr, glg2(Tgl - - Tg2) + hc, n82(Tn - Tg2) + TgIOLg2I =
(9)
h ~ . # r 2 ( r # - Tf2)+ h,.a~(T a -
Nu = 0.0293 Re °8.
(3)
L)"
These two correlations give widely different results (see Goel [6]). We have used Charter's correlation and have determined the constant K such that the correlation
Fluid (f2):
ho,#f2(Tg2 - - Tf2) = h~,f2p(T~ - - Tp).
(10)
(4)
Absorber plate (p):
Nu = 0.0158K Re °s
z , ] z # = p I + hr, g2p(Tg2 - Tp) + hc, f'zp(T~ - Tp)
(l 1)
best describes the experimental results.
=h=,pf(Tp- Tr) + h,,~,(T,- To,). (5) 5. NUMERICAL RESULTS AND DISCUSSION
Working fluid (f): h~'pf(Tp- Tf) = mCf dTr +h~,f,,(Tf-Tpr ). W -~
Although a large amount of data has been collected during the course of this investigation, only a few typical results are described below. The hourly variation of instantaneous efficiency is shown in Figs 4-6. The variation of near-normal efficiency with ( T a - 7",)/1 is shown in Figs %10. These results are on the expected lines, i.e. the
(6)
Rear plate (pr): hc, r , , ( r f -
T , , ) + hr.r, ppr(Tp - Tpr) = U,(Tpr- Ta). ( 7 ) e--e
InLet temperature of coLLector
x ~ x SoLar radiation o ~ o Experimental ~ of fLat-pLate 40
--
Theoretical t/ for different K
Experimental curve of f L a t - p L a t e
E 30
~
o,~
#
~
~
_
-
K - 2.125
~-'<-K.2.o
tO :7 I,.: 40
I
/ oI >L~:~"--0~ •
~ - -
x 6 ,o
"X ~
~ ' ~ - K
= 1.25
5
55
1o
50 9.00 9.30
I 10.00
I I 10.30 11.00 11.30 Time
I 12.00
I I 12.30 1.00
l 1.30
I 2.00
K- 1.0 I I 2.30 3.00
of d o y
Fig. 11. Variation of efficiency vs time of day in flat-plate solar air heater.
4~ o oo
348
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS a--=inlet temperature of collector X--XSolor radiation o ~ o E x p e r i m e n t a L ~/ of corrugated plate
45
Theoretical ~ for different K 40
~
35
30
K'l 2.25 ¢¢
E K - 2.00
/
~
25
-60 -50
o O r. 1.75
so
~ . -_ - ~ x ~ ~ X ~ - - - X
X ~X~X~x"'X.~.X~ K - 1 . 5
tx/
7 g x
20
6 15
10
5
-30 -v
9.00 9.30
10.00 10.30 11.00 11.30 12.00 12.30 1.00 Time
of
1.30
2.00
2.30
3.00
"
O 4O
"~ p
OF}
day
Fig. 12. Variation of efficiency vs time of day in corrugated solar air heater.
thermal performance of the corrugated absorber collector is better than that of the flat absorber collectors. The thermal performance of the veecorrugated collectors is found to be only about 5% larger than that of the commercially corrugated collectors. Keeping this in mind, that the veecorrugated sheets are not available commercially, their application in solar air heaters is not an economically viable proposition. It is, therefore, recommended to use only commercially corrugated collectors. Using the correlation given by equation (11), the theoretical efficiencies for different values of K are plotted in Fig. 11 for the flat plate collector and in Fig. 12 for the corrugated collector. These results have been checked for other flow rates also. It is found that, for flat plate collectors, the analytical results matched very well with experimental results for K = 2 . 1 3 . The new correlation then becomes Nu = 0.03213 Re °'8. The factor 0.03213 is about 9% larger than the factor 0.0293 used by Niles et al. [5]. The K value of the corrugated collectors is found to be 2.3.
6. CONCLUSION Flat, corrugated and vee-corrugated single absorber solar air heaters have been designed, fabricated and tested. The thermal performance of veecorrugated solar air heaters is found to be only about 5% larger than that of the commercially corrugated solar air heaters. Analytical models for these collectors have been presented, and the forced
convective heat determined.
transfer coefficients
have been
Acknowledgements~The financial support from DNES is gratefully acknowledged. The authors are also grateful to Professor M. S. Sodha and Dr N. K. Bansal for helpful discussions. REFERENCES
I. M. K. Selcuk, Sol. Energy 13, 165-192 (1971). 2. G. O. G. Lof, Air based solar systems for space heating. In Solar Energy Handbook (Edited by J. F. Kveith). McGraw-Hill, New York (1981). 3. N. K. Bansal, R. Chandra and M. A. S. Malik, Solar Air Heaters, Reviews of Renewable Energy Resources (Edited by M. S. Sodha, S. S. Mathur and M. A. S. Malik), Vol. 2. Wiley Eastern, India (1983). 4. W. W. S. Charters, Sol. Energy 13, 283-288 (1971). 5. P. W. Niles et aL, Sol. Energy 20, 19-23 (1978). 6. V. K. Goel, Ph.D. Thesis, Indian Institute of Technology, Delhi (1985). 7. S. Satcunanathan and S. Deonarine, Sol. Energy 15, 41-49 (1973). 8. N. E. Wijeysundera, L. L. Ah. and L. E. Tjioe, Sol. Energy 20(3), 303-370 (1982). 9. M. A. S. Malik and F. H. Buelow, Heat Transfer Characteristics of a Solar Dryer, Paper No. 25, Paris (1973). 10. J. E. Hill, J. P. Jones and D. W. Jones, Experimental Verification of a Standard Test Procedure for Solar Collectors. MBS Building Science, Series 117 (1979). 11. R. C. Schubert and L. D. Ryan, Fundamentals of Solar Heating. Prentice Hall, Englewood Cliffs, N.J. (1981). 12. G. A. Zerlaut, W. T. Dokus and R. F. Heiskell, Proc. Flat-plate Solar Collectors Conf., Orlando, Fla., 28 February (1977). 13. D. J. Close and M. B. Yusoff Sol. Energy 20, 459-463 (1978). 14. T. A. Reddy and C. L. Gupta, Sol. Energy 25, 527-530 (1980).
GOEL
et al.:
SINGLE-ABSORBER
APPENDIX
At= A2= A3= A4 =
hc, sl w + hr,~ls + hc, slfl/2 + hr, els2 h c glfl/2 + hr8182 ~'8',I + h¢.,,w'/'7"~ + h~, .l, T, hr, sl~. + h¢,~.n/2
A5 = hr, s2p + hc,82f2/2 + A 4 A6 = h¢,g2n/2 + h~.s2 p
A 7 = hr, 82p + hc, pr2/2 A s = A 7 + h¢ -f3 + h~ pp~ A9 = hr, pf3 -t-'~c, off3 * Aio = G C f / d A H = A 4 A 3 / A , + otslzsll
SOLAR
AIR HEATERS
AI2 = A 4 A 3 / A I - A 5 At3 = T g 2 ~ p / - A T A I I / A t 2 AI4 = A T A 6 / A I 2 + A8 & 5 = "49 -
h2,pf3/Ai4
AI6 = hc, pf3Al3/A 4 BI = hr, ppr ~" hc, prf3 Ur B 2 = U r T a + h r -pr/AI4 B 3 = hr, ppr hc, pf3':Al4 -~- hc, pff 3 B4 = hr, ppr hr, ppr/A [4 - BI B 5 = hc, pff3 q" h¢,pt3h~,w/A,4
B6=
&5 +
B3Bs/B4
B7 = (A~6 - B s ) & / B ~ .
349
0196-8904/87 $3.00+0.00 Copyright © 1987 Pergamon Journals Ltd
EXPERIMENTAL INVESTIGATIONS ON SINGLE-ABSORBER SOLAR AIR HEATERS V. K. G O E L , R A M C H A N D R A and B. C. R A Y C H A U D H U R I Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India
(Received 2 May 1986) Almtraet--Flat, corrugated and vee-corrugated solar air heaters having a single absorber are designed, fabricated and tested. The rear plate in these collectors is replaced by rigid synthetic foam. It is found that the vee-corrugated absorbers give the best performance. The efficiency of the commercially corrugated absorber is about 5% lower than that of the vee-corrugated absorber. Correlations for predicting forced convection heat transfer coefficients for these solar air heaters have been obtained. The analytical model for these air heaters has also been developed.
NOMENCLATURE A, A c = Ag = B= Cf= de =
Collector aperture area (m 2) Gross collector area (m 2) Depth of collector (m) Specific heat of fluid (J/kg °C) Equivalent diameter ( = [4 x cross-sectional area]/[wetted perimeter]) E = Emissivity factor f = Friction factor Fc = Effective transmissivity-absorptivity product G = Mass flow rate of fluid per unit area of collector kg/s m 2) hc = Convective heat transfer coeficient between absorbing plate and fluid (W/m 2 °C) h~ = Covective heat transfer coefficient between rear plate and fluid (W/m 2 °C) he,~ = Convective heat transfer coefficient between i and j (W/m 2 °C) hr = Radiative heat transfer coefficient between rear plate and absorbing plate (W/m 2 °C) hr. ~ = Radiative heat transfer coefficient between i and j (W/m 2 °C) I = Solar radiation intensity following on collector (W/m 2) L = Collector length (m) rh = Mass flow rate of fluid (kg/s) Nu = Nusselt number q~ = Useful heat collected per unit area (W/m 2) T = Temperature (°C) Ta = Ambient temperature (°C) Tf = Local or average fluid temperature (°C) T6 = Fluid temperature entering collector (°C) Tfo = Fluid temperature exiting collector (°C) Tp = Absorbing plate temperature (°C) Tr = Rear plate temperature (°C) AT = Temperature difference across collector (°C) U L = Collector heat loss coefficient between absorber plate and ambient (W/m 2 °C) Uo = Overall heat loss coefficient (W/m 2 °C) Ur = Downward heat loss coefficient through insulation (W/m 2 °C) Ut = Heat loss coefficient from cover, or absorber to outside air including radiation to sky 0,V/m 2 °C) Vf = Mean velocity of air flow (m/s) W = Width of collector (m) ct = Absorptivity ~/= Efficiency p = Density (kg/m 3) = Transmissivity 343
Subscripts gi = Glass i a = Ambient fi = Fluid i p = Plate pr = Rear plate s = Sky 1. INTRODUCTION Solar air heaters are being increasingly used in m a n y areas, such as drying o f agricultural products, space heating and, to some extent, in domestic water heating. Several designs o f solar air heaters have been proposed, a n d some of t h e m give satisfactory p e r f o r m a n c e [1, 2]. A survey o f the literature [3-14] shows t h a t all designs o f solar air heaters forwarded so far can basically be classified into two categories. T h e first type has a n o n p o r o u s a b s o r b e r in which the air-stream does n o t flow t h r o u g h the a b s o r b e r plate. Air m a y flow above a n d / o r b e h i n d the a b s o r b e r plate. T h e second type has a p o r o u s a b s o r b e r where air c a n flow t h r o u g h the absorber. ' A l l these designs have been reviewed by Bansal et al. [3]. A l m o s t all n o n p o r o u s types o f solar air heaters m a k e use of two absorbers. In this c o m m u n i c a t i o n , we have designed, fabricated a n d tested singlea b s o r b e r solar air heaters.
2. DESIGN OF THE SOLAR AIR HEATERS T h e types o f collectors a n d their c o n s t r u c t i o n details are s h o w n in Fig. 1, as well as in T a b l e 1. T h e collector dimensions are 1.83 x 1.22 m (6' x 4') with a flow c h a n n e l d e p t h equal to 5.6 cm (2.2"). T h e collectors differ f r o m c o n v e n t i o n a l designs in such a way t h a t air flows in between the a b s o r b e r plate a n d the rigid synthetic foam which also acts as the rear plate, whereas in c o n v e n t i o n a l designs, air flows between two metallic absorbers.
344
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS SoLar radiation
GLass
f , Absorber Air i n -
-2.25" ~ Air out
--
heti Type
m)
Z
SoLar radiation
GLass covers
b
_I_2.25, '
"/'////////'k'/////// = 0.6"2.95.
~--Insulation
Type
(synthetic foam
Tr GLass covers
Solar radiation
where it is conditioned to a specified temperature before entering the secondary measuring device. Air then flows to the heater connected to the collector inlet. Leaving the collector array, the air passes through the collector outlet measuring section and finally to the atmosphere. It is a forced circulation mode of operation. Its suction counterpart is shown in Fig. 3. The blower is of centrifugal type, rating 1 hp, suitable to work under 2.5 KPa (10" H20) gauge pressure and delivering approx. 250/300 cfm. The amount of air delivered by the blower is controlled by an air gate valve. The air velocity in the duct is measured by a photoelectric airflow meter. All the temperatures are measured by an on-line programmable data acquisition system. The solar radiation incident on a collector surface is measured using a pyranometer. The pyranometer is mounted on the adjacent surface, parallel to the collector, in such a way that it does not cast any shadow on the collector. Sufficient care has been taken to mount the pyranometer at the same tilt as the solar collector. The diffuse component of the incident solar radiation is determined for each efficiency test point by shading the pyranometer. Data acquisition
/ ^ - - "1- --A
Vee - g raoved A absorber
^=("
-
/VVVVV
I
,
2.25"
V///////////~
////////
13/4"
Design A : 8 = 6 0 0 , 0 = 1"8~ 1.5" ~ Insulation (synthetic foam) Design B ; 8 = 9 0 *, a = 1,5"
Type'nT Fig. 1
A variety of data are monitored and recorded by the DATAL-220 Data Acquisition System (DAS). All the temperatures and meteorological data leads are fed into an air-conditioned room which houses the DAS. The DATAL-220 employs the latest concept of distributed processing for providing better system availability. The master CPU is in the DATAL-220 and the slave CPU is in the DATAL220. The DATAL-CPU receives scanned data from the DATAL-220 and can analyse the data in real time as per the program written in FORTRAN. The information is also displayed on a 30 cm CRT. The data can be stored on floppy disks. The results can be obtained on an alphanumeric printer.
3. EXPERIMENTA SET UP AND INSTRUMENTATION The test loop for the air-heating collector is shown in Fig. 2. It is an open-loop configuration. The blower delivers air to the air-reconditioning apparatus,
4. THEORY
4.1. Efficiency The instantaneous thermal efficiency of the collector, defined as the ratio of useful thermal energy to
Table 1. Description of solar air collectors tested Collector type
Gross collector area (mz) Absorber
Flat (2)
2.23
Flat (1)
2.23
Corrugated
2.23
Currugated
2.23
Corrugated
2.23
Glazing
Back insulation
shalimar blackboard
Double
Synthetic foam
shalimar blackboard
Single
Synthetic foam
shalimar blackboard
Double
Synthetic foam
Flow perpendicular to corrugation
shalimar blackboard
Single
Synthetic foam
Flow perpendicular to corrugation
shaiimar blackboard
Double
Synthetic foam
Flow parallel to corrugation
GI sheet shalimar blackboard paint
Double
Synthetic foam
Flow parallel to corrugation
GI sheet pamt GI sheet paint GI sheet paint GI sheet paint GI sheet
Miscellaneous
paint
Vee-corrugated
2.23
GOEL et al.:
SINGLE-ABSORBER SOLAR AIR HEATERS
345
Pyranomete~
eoder
~
Air flow meter
~ S
Header
Air flow meter
I
=, ~ ~ I
LI,Y
Thermocoupe ts
I
Them r ocoup e ls Collector pressure drop
Inclined manometer
BLower
~Wet
~
Air in
bulb
re-conditioning apparatus
dry bulb
Fig. 2. Schematic diagram of solar air heater testing (forced circulation).
Pyranomet~
Air flow meter
Heade~_~
Thermocoupkes
Air flow meter
Thermocouples
~
CoLLecotr
Inclined manometer''
•
~
bulb 8~ dry bulb
pressure drop
Air
,m i n
BLower Air ~ ~ out
re - conditioning apparatus
Fig. 3. Schematic diagram of solar air heater test (suction flow).
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS
346
total incident solar energy, averaged over the same time interval, is mathematically expressed as
,,/h
~/ =
35
(Tfo - T~) dt
rhCf
40
o
(1)
/%
°
30
& J, /(t) dt
Mode of operation:
[]
g~
forced circulation
1
The quantities rh and Cf are assumed to be constant during each set of measurements.
25
FLow rote : 0.0369 k g / s o FLat - plate [] Corrugated V e e - corrugated
20'.005-0
I
I
O.Ol
o o°o o
I
o.o15
o ocOo
I
o.o2
I
0.025
I
0.03
0.035
(~-~)/_r(m 2 -°C/W)
40
Fig. 7. Variation of efficiency with (T:- Ta)/I in double glazed collectors (flow perpendicular to corrugation). 3O
Mode of operation : forced circulation
34
Date : 2 - 3 - 1 9 8 4 -FLat-pLate c.--o Corrugated
// 20
30 []0
o@ lC •0 0
]
I
I
I
[
10.00
11.00
12.00
1.00
2.00
I
o~ 2 5
D
3.00
Time of day
Fig. 4. Variation of instantaneous efficiency with time of day in double glazed collectors (flow perpendicular to corrugation).
20
°
o
Mode of operation :
forced circulation
o
FLow rate : 0 . 0 2 3 6 k g / s
Q
ra Corrugated
o I
0.02:
0.025
ra c~ 0.03
Fig. 8. Variation of efficiency with (T~- To)/I in double glazed collectors (flow perpendicular to corrugation).
30
25 20
-
15
o I
( ~-To]/I ( m2 -°C/W )
35
~
o
o FLat - plate
15 •~ V e e - corrugate,d, 14 0.005 0.01 0.015
40
n
Mode o1 operation : forced circulation FLow rate : 0.0236 kg/s Date: 2 3 - 3 - 8 4 -FLat- plate c---oCorrugoted
20-
15 10 9.00
I 10.00
I 11.00
I 12.00
Time
I 1.00
I 2.00
Mode of operation:
[~
suction flow
I 3.00
of day
FLow rate : 0 . 0 2 3 6 kg/s o FLat - plate [3 Corrugated
[3
Fig. 5. Variation of instantaneous efficiency with time of day in double glazed collectors (flow perpendicular to corrugation).
Vee-corrugoted
o~ lO o
~O
ooo
qn o
o o
10 --
-~
FLow rate : 0 . 0 2 3 6 kg/$ Date : 1 9 - 4 - 8 4 FLat-pLate Corrugated
0
[]
I
I
I
I
0.005
0.010
0.015
0.020
( T~j - To ) /I
I 0.025
(rn2-°C/W)
Fig. 9. Variation o f efficiency with Tji-To/I in double glazed collectors (flow parallel to corrugation). 5-
0 9.00
4.2. Thermal modelling of solar air heaters I 10.00
I 11.00
I 12.00
I 1.00
I 2.00
I 3.00
Time of day
Fig. 6. Variation of instantaneous efficiency with time of day in single glazed collectors (flow perpendicular to corrugation)
The energy balance equations to the collector shown in Fig. 1 (Type I) may be written as follows: Cover glass (gl): astl = h¢,slw(Tst - T~) + hr,g,s(Tg I - T,)
+hr, s,s:(Tsl- T v) + h~,gm (Ts] - T.).
(2)
GOEL et al.: 15
SINGLE-ABSORBER SOLAR AIR HEATERS
These equations can be combined to give a single differential equation for Tf, whose solution is given by
Mode of operation : suction flow FLow rate • 0.172 kg/s o FLat-pLate [] Corrugated •~ Vee -corrugated
10
B7 Tf
I
I
I
o o~o
o.o~5
~a6]
Cfrh //"
(8)
The collector outlet temperature can be determined by putting
o~
o.005
347
I~1
X=L.
I
o.o2o
o.025
The constant Bs are given in the Appendix. The two widely used correlations for evaluating forced convective heat transfer coefficients are Charters' correlation [4]:
( 7}fi- ro )/ Z ( mZ -°C / W )
Fig. 10. Variation of efficiency with ( T : - Ta)/I in double glazed collectors (flow parallel to corrugation).
Nu = 0.0158 Re °'8, Glass plate (g2):
and the Niles et al. [5] correlation:
hr, glg2(Tgl - - Tg2) + hc, n82(Tn - Tg2) + TgIOLg2I =
(9)
h ~ . # r 2 ( r # - Tf2)+ h,.a~(T a -
Nu = 0.0293 Re °8.
(3)
L)"
These two correlations give widely different results (see Goel [6]). We have used Charter's correlation and have determined the constant K such that the correlation
Fluid (f2):
ho,#f2(Tg2 - - Tf2) = h~,f2p(T~ - - Tp).
(10)
(4)
Absorber plate (p):
Nu = 0.0158K Re °s
z , ] z # = p I + hr, g2p(Tg2 - Tp) + hc, f'zp(T~ - Tp)
(l 1)
best describes the experimental results.
=h=,pf(Tp- Tr) + h,,~,(T,- To,). (5) 5. NUMERICAL RESULTS AND DISCUSSION
Working fluid (f): h~'pf(Tp- Tf) = mCf dTr +h~,f,,(Tf-Tpr ). W -~
Although a large amount of data has been collected during the course of this investigation, only a few typical results are described below. The hourly variation of instantaneous efficiency is shown in Figs 4-6. The variation of near-normal efficiency with ( T a - 7",)/1 is shown in Figs %10. These results are on the expected lines, i.e. the
(6)
Rear plate (pr): hc, r , , ( r f -
T , , ) + hr.r, ppr(Tp - Tpr) = U,(Tpr- Ta). ( 7 ) e--e
InLet temperature of coLLector
x ~ x SoLar radiation o ~ o Experimental ~ of fLat-pLate 40
--
Theoretical t/ for different K
Experimental curve of f L a t - p L a t e
E 30
~
o,~
#
~
~
_
-
K - 2.125
~-'<-K.2.o
tO :7 I,.: 40
I
/ oI >L~:~"--0~ •
~ - -
x 6 ,o
"X ~
~ ' ~ - K
= 1.25
5
55
1o
50 9.00 9.30
I 10.00
I I 10.30 11.00 11.30 Time
I 12.00
I I 12.30 1.00
l 1.30
I 2.00
K- 1.0 I I 2.30 3.00
of d o y
Fig. 11. Variation of efficiency vs time of day in flat-plate solar air heater.
4~ o oo
348
GOEL et al.: SINGLE-ABSORBER SOLAR AIR HEATERS a--=inlet temperature of collector X--XSolor radiation o ~ o E x p e r i m e n t a L ~/ of corrugated plate
45
Theoretical ~ for different K 40
~
35
30
K'l 2.25 ¢¢
E K - 2.00
/
~
25
-60 -50
o O r. 1.75
so
~ . -_ - ~ x ~ ~ X ~ - - - X
X ~X~X~x"'X.~.X~ K - 1 . 5
tx/
7 g x
20
6 15
10
5
-30 -v
9.00 9.30
10.00 10.30 11.00 11.30 12.00 12.30 1.00 Time
of
1.30
2.00
2.30
3.00
"
O 4O
"~ p
OF}
day
Fig. 12. Variation of efficiency vs time of day in corrugated solar air heater.
thermal performance of the corrugated absorber collector is better than that of the flat absorber collectors. The thermal performance of the veecorrugated collectors is found to be only about 5% larger than that of the commercially corrugated collectors. Keeping this in mind, that the veecorrugated sheets are not available commercially, their application in solar air heaters is not an economically viable proposition. It is, therefore, recommended to use only commercially corrugated collectors. Using the correlation given by equation (11), the theoretical efficiencies for different values of K are plotted in Fig. 11 for the flat plate collector and in Fig. 12 for the corrugated collector. These results have been checked for other flow rates also. It is found that, for flat plate collectors, the analytical results matched very well with experimental results for K = 2 . 1 3 . The new correlation then becomes Nu = 0.03213 Re °'8. The factor 0.03213 is about 9% larger than the factor 0.0293 used by Niles et al. [5]. The K value of the corrugated collectors is found to be 2.3.
6. CONCLUSION Flat, corrugated and vee-corrugated single absorber solar air heaters have been designed, fabricated and tested. The thermal performance of veecorrugated solar air heaters is found to be only about 5% larger than that of the commercially corrugated solar air heaters. Analytical models for these collectors have been presented, and the forced
convective heat determined.
transfer coefficients
have been
Acknowledgements~The financial support from DNES is gratefully acknowledged. The authors are also grateful to Professor M. S. Sodha and Dr N. K. Bansal for helpful discussions. REFERENCES
I. M. K. Selcuk, Sol. Energy 13, 165-192 (1971). 2. G. O. G. Lof, Air based solar systems for space heating. In Solar Energy Handbook (Edited by J. F. Kveith). McGraw-Hill, New York (1981). 3. N. K. Bansal, R. Chandra and M. A. S. Malik, Solar Air Heaters, Reviews of Renewable Energy Resources (Edited by M. S. Sodha, S. S. Mathur and M. A. S. Malik), Vol. 2. Wiley Eastern, India (1983). 4. W. W. S. Charters, Sol. Energy 13, 283-288 (1971). 5. P. W. Niles et aL, Sol. Energy 20, 19-23 (1978). 6. V. K. Goel, Ph.D. Thesis, Indian Institute of Technology, Delhi (1985). 7. S. Satcunanathan and S. Deonarine, Sol. Energy 15, 41-49 (1973). 8. N. E. Wijeysundera, L. L. Ah. and L. E. Tjioe, Sol. Energy 20(3), 303-370 (1982). 9. M. A. S. Malik and F. H. Buelow, Heat Transfer Characteristics of a Solar Dryer, Paper No. 25, Paris (1973). 10. J. E. Hill, J. P. Jones and D. W. Jones, Experimental Verification of a Standard Test Procedure for Solar Collectors. MBS Building Science, Series 117 (1979). 11. R. C. Schubert and L. D. Ryan, Fundamentals of Solar Heating. Prentice Hall, Englewood Cliffs, N.J. (1981). 12. G. A. Zerlaut, W. T. Dokus and R. F. Heiskell, Proc. Flat-plate Solar Collectors Conf., Orlando, Fla., 28 February (1977). 13. D. J. Close and M. B. Yusoff Sol. Energy 20, 459-463 (1978). 14. T. A. Reddy and C. L. Gupta, Sol. Energy 25, 527-530 (1980).
GOEL
et al.:
SINGLE-ABSORBER
APPENDIX
At= A2= A3= A4 =
hc, sl w + hr,~ls + hc, slfl/2 + hr, els2 h c glfl/2 + hr8182 ~'8',I + h¢.,,w'/'7"~ + h~, .l, T, hr, sl~. + h¢,~.n/2
A5 = hr, s2p + hc,82f2/2 + A 4 A6 = h¢,g2n/2 + h~.s2 p
A 7 = hr, 82p + hc, pr2/2 A s = A 7 + h¢ -f3 + h~ pp~ A9 = hr, pf3 -t-'~c, off3 * Aio = G C f / d A H = A 4 A 3 / A , + otslzsll
SOLAR
AIR HEATERS
AI2 = A 4 A 3 / A I - A 5 At3 = T g 2 ~ p / - A T A I I / A t 2 AI4 = A T A 6 / A I 2 + A8 & 5 = "49 -
h2,pf3/Ai4
AI6 = hc, pf3Al3/A 4 BI = hr, ppr ~" hc, prf3 Ur B 2 = U r T a + h r -pr/AI4 B 3 = hr, ppr hc, pf3':Al4 -~- hc, pff 3 B4 = hr, ppr hr, ppr/A [4 - BI B 5 = hc, pff3 q" h¢,pt3h~,w/A,4
B6=
&5 +
B3Bs/B4
B7 = (A~6 - B s ) & / B ~ .
349