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Development and testing of low cost solar energy collectors for heating air
SolarEnergyVol.33,No.2,pp. 197 208,1984
0038M)92X/84 $3.00+ .00 © 1984PergamonPressLtd.
PrintedintheU.S.A.
DEVELOPMENT A N D TESTING OF LOW COST SOLAR ENERGY COLLECTORS FOR HEATING AIR N. K. BANSALtand R. UHLEMANN Program Group for Systems Research and Technological Development, Kernforschungsanlage Juelich, Postfach 1913, 5170 Juelich l, F.R.G. (Received 25 June 1982; revision received 4 August 1983; accepted 12 August 1983)
Abstract--Solar Energy Collector designs utilising plastic films for the absorber and the glazing are developed for air heating applications. The objective is the cost reduction of such collectors with reasonable performance and acceptable life time. The developed collectors were of nonporous as well as porous type, the latter having a special synthetic textile as the absorber. An experimental facility is described to study the performance of the collectors as a function of typical physical parameters and the experimental results are discussed. The collector, with a black porous textile absorber, was found to be much superior to the collector of nonporous type. The thermal performance of the former gets, however, severelyaffected if the porous absorber is not suitably optimized and if the back of the collector is not properly insulated. Considering life time of a collector as a parameter and taking into account the investment cost, interest rate and the useful energy available from the collector, a simple expression has been developed for rating various collectors. A comparison of the numerical data for the present porous collector and commerciallyavailable conventional collector for air heating shows that the present collectors have a better economic potential even if one assigns a short life time to them. 1. INTRODUCTION The potential applications of solar air heaters are the drying of agricultural products, lumber, space heating and to some extent domestic water heating. Several designs of solar air heaters have been proposed and some of them give good performance [ 1, 2]. All designs of solar air heaters forwarded so far can basically be classified into two categories. The first type has a nonporous absorber in which the air stream does not flow through the absorber plate. Air may flow above and/or behind the absorber plate. The second type has a porous absorber that includes slit and expanded metal, transpired honeycomb or overlapped glass plate absorber. The pressure losses across the collector and the thermal losses to the ambient are generally lower for collectors with a porous absorber; an optimised pore size and the matrix thickness are, however, essential for such collectors to yield a good thermal performance. Large scale use of solar energy for various applications is prohibitive because it is usually not found to be cost effective. In order to achieve a better economy of solar installations more plastic materials have been used recently. A review listing various plastic materials suitable for various components of a solar energy collector has been given by Blaga[3], Boeckmann [4] and Bansal[5]. Solar air heating collectors made completely from plastic material include those of [6-13]. In this communication we present the designs of air fGuest Scientist with Alexander von Humboldt Fellowship. Permanent address; Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India. 197
heating solar energy collectors which use plastic films for the absorber and for the glazing. The aim is the cost reduction of such collectors with reasonable performance and acceptable life time. The developed collectors are of nonporous as well as porous type and are capable of handling large volume of air. The details of the fabricated collectors are given in Section 2. An outdoor facility was set up to study the performance of such collectors. Its details are described in Section 3. Relevant performance equations and the results of measurements have been presented in Sections 4 and 5, respectively; the latter includes a comparison of nonporous and porous types of collectors. Along with its thermal performance, the specific cost and the anticipated life time of the collector are equally important. Considering life time as a parameter and taking into account the investment cost, interest rate and the useful energy available from a solar energy collector, a simple expression has been developed for rating the solar energy collectors. Numerical calculations have been performed corresponding to the data of a conventional collector and the presently developed plastic collectors. These results are presented and a comparison of the collectors made in Section 6. 2. DESIGN OF THE COLLECTORS
Schematic diagrams of the collectors are shown in Figs. l(a, b), respectively. The collector corresponding to Fig. l(a) belongs to the category of nonporous type and it has a black PVC foil as an absorber which is covered by double transparent UV stabilized PVC foils. The upper foil contains a valve for filling the air in the space between the two foils (isolating air layer).
198
N. K. BANSALand R. UHLEMANN
cover
rransparen~
~ /
Solar radiation
n
~ A
~
air out
orber
Crossecfionnt view
Side view
Fig. l(a). Schematic of nonporous collector I.
Solar radiation
air,
~
~
PVC
°ir,out ~ Side ~ew
Fig. l(b). Schematic of the porous collectors II-IV.
The collectors corresponding to the Fig. l(b) belong to the category of porous air heaters with a porous black textile of polyester (100 per cent) which acts as an absorber. At the edges it is attached to two transparent sheets of UV stabilized polyvinyl chloride (PVC) covering it from the top and the bottom with no inside foils. The outer transparent PVC films are 0.6 mm thick and have 83 per cent transmittance for solar radiation. The inside foil, which has been provided for collector I only, is 0.3 mm thick and has a solar transmittance of about 94 per cent. In the spectral range 7-20 #m, the measured transmittance of the PVC sheet is on an average only 7 per cent as shown in Fig. 2. Thus, for absorber temperatures in the range of 40-100°C, the PVC sheet acts as an excellent cover. The lower transparent sheet, in the collectors shown in Fig. l(b), helps to double the life time of the collectors; when the absorptance of the top cover becomes too high, the collector is simply inverted so that the bottom PVC sheet acts as a cover.
The porous textile absorber should have maximum absorption of solar radiation but low air flow resistance. Simultaneously the volumetric heat transfer coefficient should be high. Since the air flows across the entire absorber area, the pressure losses along the collector in such constructions are much lower than in the case of nonporous absorbers. To test the effect of the porous absorber, another collector, having different textile absorber but otherwise with exactly identical specifications, was fabricated. A twenty times enlarged surface photograph of the two textile absorbers taken from an electron microscope is given in Figs. 3(a, b), respectively. It is observed that the pore size of the textile corresponding to Fig. 3(a) is bigger than the one corresponding to Fig. 3(b). The pressure drop measured across the collector, made from the former is, therefore, less than that made from the textile of Fig. 3(b). It was, however, also observed that the transmittance of solar radiation through the textile of collector II is less. A bigger pore size does not justify a low transmittance value, but
Development and testing of low cost solar energy collectors for heating air 6O
I
I
I
199
I
LU Z
I--
Z
"
20
0
2.5
I
I
I
i
I
3.33
5
83
10
12.5
25
X/tim Fig. 2. Spectral transmittance of the 0.6 mm thick PVC sheet, used as cover in the present collectors, in the longwave region.
(a)
(b)
Fig. 3(a). Twenty times enlarged electron microcope photograph of the textile absorber used in collector II.
Fig. 3(b). Twenty times enlarged electron microscope photograph of the textile absorber used in collectors III and IV.
the other parameters such as colour, surface reflectance etc. may be responsible for these values of solar transmittance through two different textiles. To test the effect of increasing the absorber area, a fourth collector of 20 m length and 1 m width was fabricated. The specifications, of each of the four collectors are given in Table 1.
The global insolation on the collector was measured by an Eppley pyranometer (Model PSP) mounted at the same inclination as the collector. Temperatures were measured with Pt 100 and Pt 500 resistance thermometers fixed in a copper tube using a four wire technique. At the outlet, temperature stratification is possible. The sensors are, therefore, distributed across the outlet cross section to measure the temperature profile. An effort was also made to mix the air at the outlet by making an arrangement in such a manner so that the upper layers of the air flow to the bottom side and lower layers flow to the upper side. The temperature distribution along the length of the collector was also measured by putting five temperature sensors between the inlet and the outlet at approximately equal distances. The ambient temperature and relative humidity were measured with a dry and a wet bulb thermometer using the well known relations[15]. The density of air changes with its static pressure and temperature while the specific heat is a function of temperature and the amount of water vapour
3. E X P E R I M E N T A L
SET
UP
AND
INSTRUMENTATION
The test facility consists of frames oriented towards south with the possibility of changing the angle of inclination which was kept 45 ° for the present study (latitude of Jfilich is 50°55'33"N). The blower used for forcing the air through the collector has a maximum air flow rate of 2850 m3/hr at the free blowing operation. The flow rate which can be adjusted by controlling the input electrical voltage was measured by a nozzle. The nozzle and the appropriate tubes were fabricated according to the specifications of DIN 19205 (German Industrial Standard)[14].
N. K. BANSALand R. UHLEMANN
200
Table 1. Dimensions and physical parameters of the plastic collectors Dimensions/Parameters
Collector
Collector
Nonporous
Type
I
Collector
II
Porous
Collector
III
Porous
Collector IV Porous
Length
9.10 m
8.85 m
8.80 m
Width
1.10 m
1.O3 m
1.15 m
1.13 m
10.12 m 2
19.89 m 2
Absorbing
Cross-section inlet the
10 m 2
area
9.12 m 2
17.6 m
at the
and o u t l e t
of
O.O61
m2
O.O61
m2
O.O61
m2
O.O61
Material
of the
Thickness
cover
of the
PVC
l
PVC
PVC
PVC
0.6 m m
0.6 ram
0.6 m m
0.6 m m
measured transmittanc( of the t o p PVC c o v e r in the s o l a r s p e c t r u m
0.83
0.83
0.83
0.83
Thickness
O.3
top
m2
air
cover
inside
of the
PVC
n~n
cover
measured transmittance of the i g s i d e P V C c o v e in the s o l a r s p e c t r u m
Absorber
0.94
Black PVC
Thickness
of the
0.6 m m
1OO % P o l y -
1OO % P o l y -
1OO % PoI~
e s t e r shown ~
e s t e r shown in
e s t e r shown
Fig.
Fig.
3a
3b
inFig.
2 mm
2 mm
2
O,O7
0,22
0,22
3b ram
absorber measured transm i t t a n c e of a b s o r b e r for s o l a r r a d i a t i o n p r e s s u r e loss over collector length at 360 k g / h air
I mm H20
960 k g / h flow rate
4,5 m m
present in the air. The formulas used for incorporating the variation of density and the specific heat are[16]
To [I_1 -- 3.88 x 10-4/°p0 P(P, T ) = 1.2923/0. P0 "T
H20
3 mm H20 10,5 m m H 2 0
humidity according to the formula
W(T) = 0.622. r-
Ps(T) & - P,(T)
(3)
P~ being the atmospheric pressure. In small temperature range Ps(T) can be expressed by the following linear relationship
~'650- _rl/4/] { × ~1--0.575(-----~--o J / J
(1)
cp(w, T) = C,d(r) + W(T)C~m(T)
(2)
Ps(T) = R, + R2T.
where Cpd is the specific beat of dry air and Cp. of moisture, which can be taken from standard tables. Humidity ratio W in eqn (2) is related to the relative
(4)
In the temperature range 10-70°C, the values of R 1 and R2, obtained through a least square fit, are given in Table 2 along with the correlation with the exact values. The wind speed was measured at the same
201
Development and testing of low cost solar energy collectors for heating air Table 2. Constants R~ and R2 obtained through a linear least square fit of the saturated pressure of water in a certain temperature range
T *C
R1
10"-30" 30*-50* 50*-70*
R2
atm
Correlation
atm/deg
-4.788
.
10 -3
1.487
. 10 -3
0.9888
-8.316
. 10 - 2
4.014
. 10 -3
0.9920
-3.616
. lO - I
9.484
• 10 -3
0.9930
height as the collector test stand with an electronic anemometer. All the measurement signals from the different sources were processed by the Microprocessor Aided D a t a Acquisition System (MADAS) designed and constructed for the different requirements of meteorological, collector and solar system studies[17]. The pressure drop A P across the nozzle and the static pressure is read with the help of a Betz manometer and fed into the programme through the terminal manually. The programme calculates the air density as a function of the inlet air temperature and the static pressure at the inlet (see eqn 1) and then uses A P and p for evaluating the rate of air flow G according to the equation
efficiency is written as
;1=FR[(O~T)e--UL(Ti7Ta)]
(8)
A plot of r/ against (T~ - T~)/L approximated by a straight line helps to evaluate the collector parameters FR(ctz)e and FR" UL from the intercept and the slope of the straight line, respectively. The rise in the temperature of the air flowing through the collector can also be written in the form ( T o - T i)
AcFR(z~Z)e I mCp
AcFa(z~)e lc mCp
(9)
where G = ~ • e • ( A P . p),/2
(5)
where the flow rate number 6 and the expansion factor e both are functions of the Reynolds Number and the dimensions of the measuring tract. For the aperture supplied to us and for our measuring tract G, the air volume/h, is simply calculated by the formula G = 111.11 .h 1/2
(6)
where h is the pressure drop across the nozzle measured by the manometer in mm H20. From eqn (6) the mass flow rate in kg/h through the collector is evaluated by multiplying G with air density calculated from the formula (1).
Ic
uL(r,- Ta) (~)o
is the threshold level of the incident radiation. If the inlet air temperature is nearly equivalent to the ambient then (To --Ti) vs I points lie nearly on a straight line and then eqn (9) can also be used to determine the parameters F~(z~t)e from the slope of the straight line. If the measurement data of the whole day fits such a straight line then the day long efficiency is calculated from the expression q = FR(za)~ [ 1 - - ~ ] .
(10)
4. THEORY
5. RESULTS AND DISCUSSION
Instantaneous thermal efficiency of the collector defined as the ratio of the useful thermal energy to the total incident solar energy, averaged over the same time interval is mathematically expressed as
The hourly variation of the various parameters namely the inlet and outlet temperatures, A T and the efficiency q, is shown in Figs. 4 and 5 for the collectors I and II, respectively, along with the corresponding variation of the solar radiation and the ambient temperature. Collector I is of nonporous type while collector II is of porous type, the exact dimensions of the two collectors are given in Table 1. The air flow rate, which was kept constant throughout the day, is the same in the two cases. It is evident that, for nearly the same ambient conditions, the thermal performance of the porous type collector (collector II) is much better than the performance of the nonporous type collector (collector I), the former
m" C,I 1/=
'~(To -
q]ll
r,2
T,) dt
Acj,1I(t)dt
(7)
The quantities m and Cp have been taken out of integration in the numerator since they remain essentially constant during one set of measurements. The Hottel-Whillier-Bliss eqn [18] for the thermal
202
N.K. BANSALand R. UHLEMANN 50,
1000 flow rote 7896 k Wire2' date 15.A.82 wind speed
800. 1.2+o2.1 m,r
3~
600.
;3
'~ 2(3
..li-
/
h
~
~" - ' ~ ' \ .
"-~ ---_.
.~0 0_
"\.
.
o
Insolation~
temperature rise
. . . . .
~
2,
inlet
-
200-
~
~"
temperature
\
"a mbient temperature 12 h
I
,
,
600
i
700
, the
I
16 h
i.
800 Time of
,
i
18h/ ,
900 (rains }
day
10
~
1(, h ,
-20
\
.
~,,~.--~---~ 10 h
lC
Nonporous Collector T -~.
Solar
o
c eJ
~g
,
i
1000
/
1100
Fig. 4. Time variation of the solar insolation on the collector's surface, the various temperatures and efficiency on a typical day at Juelich, F.R.G. (latitude 50°55'33").
70. 6050-
1000
*C
C o l l e c t o r 1"[ Windspeed 1.2 to 2.Orals Wire2 22. t.. 82 ~ " ~ " ~ " -'~"" ~ ' ~ _ ~ . ~...~ flow rate . ~ " ~-~"..... / - ~ , ~ "~--<,~._ 800 789.6 kgl h.,' ' ~ ~ ro'd~'i'ot ion ~ "~~, '~._ 30
,g >,~,0- .~ 6o0
efficte
••~
~
~ r lt _~
~ k~ 3020
t.oo
I/ I I
i i I
:
L
~
~
-
,-~p,
20
--P; ",:.~,.__...__
~
~
(0 ¢X
erature Uttet
_ _----
-'---'-
".
10 10
200 66o
760
' Time
of the
day
11oo
(rains)
Fig. 5. Time variation of solar radiation on surface of the collector II, the typical temperatures and efficiency on a typcial day at Juelich, F.R.G. (latitude 50°55'33").
yielding about a maximum of 60 per cent efficiency while the latter reaching only 45 per cent at these flow rates. The rise A T in the temperature of the air flowing through the collector, gets less and less with the increasing air flux. The efficiency, however, gets correspondingly increased due to the decreased thermal losses to the environment. Figure 6 shows the measured variation of AT and the efficiency r/ with different flow rates of air through the collectors I and II. Figure 7 shows the typical efficiency curve using eqn (8) for both collectors I and II. A least square fit of the data shows that all the measured data points fit to a straight line with a 0.7 value for the coefficient of correlation. From the coefficients of the linear fit one obtains the parameters FR(~)e and FRUL. It is observed from the data that the value of FR(z~)e is
much higher for the porous absorber collector II than the nonporous absorber collector I. The effect in the transmittance of the cover system due to second layer in collector I is not able to explain the low value of FR(z~x). For example the effect of the double transparent layer is to decrease the transmittance from a value of 0.83 (for a single cover) to 0.78. This reduction alone is not responsible to explain the low value of FR(z~)e for collector I. The reason for a higher value of Fx('cOt)e for collector II is the inclusion of textile absorber in the collector. For a double glazed transpired air heater the efficiency equation[19] is r / = 0 . 6 - 4 . 9 Ti-- T, I In contrast the performance of the plastic film solar
Development and testing of low cost solar energy collectors for heating air
203 25
0.8
*C E QJ
co
~-
20 z
0.6
LU
o E
/
0./.
~
-
Temperature
~
Rise
15
J - -
Porous
Absorber
OL2
240
Collector IT
Absorber Collector I
------Nonporous
3L0
~0
i
6;0
73o
i
i
8Zo
10
9~0
mass flow rate ( kg/h } Fig. 6. Variation of r/and AT with air flow rate for an incident solar radiation of about 700-750 W/m2.
.7.6-
>,.5 ¢u
G ./.
x
x
K
xx
x x
x
K x
"
IJJ x
TI=A*B,(-~
x
a)
x x
.2" Nonporous flow
.1' . . . .
.0~1
789.6
POROUS
o62
.0~3
0.7/.
oh/.
.obs
- 31.2
oh6
0.7/.
oh7
31.2
o68
oh9
010
(Ti -r a ) / T
Fig. 7. Variation of efficiency r/ with ((T~
for nonporous (collector I) and porous (collector II) collectors.
- Ta)/l)
air heater comes out to be
~-T~
r / = 0.74 - 3 1 . 2 - -
I
It is clearly seen that the plastic film collectors have a higher loss coefficient. But we are trading off the high loss coefficient for a low economic cost and a high intercept (0.74 vs 0.6). Since the inlet air temperature T~ is very nearly equal to the ambient temperature Ta, the temperature rise z~T varies linearly with the incident radiation on the surface of the collector. A typical plot is shown in Fig. 8 for both the collectors I and II, respectively, along with the calculated values FR(~ct)e and the threshold value I~ for the incident radiation. It was observed that the thermal performance of the porous collector with a textile absorber is considerably effected by the insulation at the back of the
collector and by the pore size of the textile absorber. A decrease in the thickness of the PE-Foam from 6 to 3 cm brought out an efficiency loss of about 20 per cent. It was also observed that the heat losses could be substantially decreased by putting an aluminium foil between the backside insulation and the collector. A quantitative comparison of the measured thermal performance of collector II for various back insulations is given in Table 3. The effect of pore size on the performance of a porous absorber collector was evident by the measurements on another collector III, which has the textile of Fig. 3(b) as the absorber. A longer collector IV helps to get a larger AT, the thermal efficiency however gets reduced due to increased thermal losses. Table 4 gives a comparison of collectors II-IV for a back insulation of only 3 cm thick polyethylene. Within the experimental limitations the electrical power used for the three collectors was found to be nearly the same. The point to note
204
N
K
25 oC
x
flow rote kglh A*C 812.~. -39212
Nonporous
....
BANSAL and R. U H L E M A N N
Porous
7896
/ : /
-3037
/'
~20
S 8
~1o *
B(
/
o
..'I"
.
.
.
.
.
.
.
}e I C
FR ( T ~
Porous .0284
200
0
°C
I
0.684
106.9
. 600
400 Solor
800
Wl m 2
1000
rodiotion
Fig. 8. Variation of d T with incident solar radiation for nonporous (collector I) and porous (collector II) collectors.
Table 3. Performance of the porous collector II with various types of back insulation. Figures in bracket indicate the incident solar radiation in W/m 2 of the collector surface S. NO
Temperature kg/h
504
654
774
862
924
960
Rise
°C
Efficiency %
I
II
III
I
II
23.4
23.9
22.8
O.3991
0.4905 0.4781
(900)
(748)
(732)
19.4
18.0
20.4
0.4622
0.5257 0.5447
(836)
(682)
(746)
18.2
18.4
19.4
0.4898
0.5421
(876)
(800)
(743)
17.4
13.9
18.8
0.5029
0.5560 0.6561
(908)
(656)
(752)
15.9
13.9
18.3
0.5233
0.5588 0.6785
0.5319
0.5688 0.7065
(855) (700)
(759)
16.3
12.1
16.6
(896)
(622)
(687)
I
3 cm t h i c k p o l y e t h y l e n e
II
3 cm t h i c k p o l y e t h y l e n e
III
6 cm t h i c k p o l y e t h y l e n e
w i t h A1
III
0.6090
foil t o p
Table 4. Comparison of the thermal performances of the Collectors II-IV with 3 era thick polyethylene as the back insulation
s.N.
wSm2
I
900
2 3
.k~/.d
Ti
OC
II
III
To IV
II
°C III
~T IV
II
%
°C III
IV
II
III
qu IV
II
W III
326
3 6 1 6 1 2 0 3 0 3756
299
3527 2011
2237 379~
245
3453!1992
3553
2009
220
313711789
3261
1665 281
164
3031i1501
2649
1602 234~
112
2 6 3 0 1490 2233
10.1
17.5 4 8 . 0 26.2 22.6 394~
36.9
10.5 18.2 46.6 25.7 22.2 3826 369~
84O
20.4 21.7
19.9 3 7 . 3 1 3 1 . 8
900
792
20.8 22.6
18.7 38.2 33.1
4
900
732
21.2 21.6
19.5 39.4 32.6 38.2
8.2 11.O 18.7 45.1
5
900
624
21.8 21.7
18.7 41.2 33.3 38.8
9.4 11.6 20.1
6
900
492
21 .7 21.7 2 0 . 0 45.9 33.9 40.6 23.4
12.2 20.6 38.9 18.5 15.6 3191
7
900
324
20.1 21.9 21.9 50.6 39.8 4 8 . 0 3 0 . 5
17.9 26.1
7.4
24.821.1
40.9 22.3
33.4
17.8
19.3 3557
3.0 2742
IV
2 3 1 0 4OO1
37.416.9
900
III
2356 408~
2383
20.2 36.6 29.9 3 7 . 0 16.3
ou II
3603 2 0 2 0 3723
16.8 48.3 26.5 22.7 396(
20.3 20.1
W
363
9.8
876
Pel IV 4086
3481
3702
Development and testing of low cost solar energy collectors for heating air is that while the useful energy q~ = mCpdT goes on increasing with the air flow rate, the net useful energy Q, ( = q, -p~z) increases up to a certain flow rate and then decreases. Detail measurements of the air temperature and pressure loss over the entire length of the collectors (Fig. 9) showed that the resistance to air flow by the porous absorber must be carefully optimised for different air flow rates and collector dimensions. The different curves in Fig. 9 show the temperature profile above as well as below the textile absorber in various collectors. In collectors III and IV, the temperature of the air above the textile absorber is seen to be increasing steadily along the length, while in collector II, the temperature of air above the absorber is seen to be nearly equivalent to the ambient air temperature except for the case when the rate of air flow is very small It is, therefore, obvious that the thermal losses in collector II from the top are negligibly small. The reasons for the different air temperatures in collectors II and III are due to the different pore size of the textile absorbers. Figures 3(a, b) show that the absorber of collector III is denser than that of collector II. So all the air in collector III (and in collector IV) is not able to go through the textile absorber and hence it builds a pressure near the outlet end of the respective collectors. This is noted from the variation of air temperatures along the length of the collector below the textile absorber. In collector III, the temperature is seen to be increasing along the length till the outlet end and suddenly dropping down due to the cold air coming from top towards outlet end. In collector IV, the maximum of the air temperature below the textile absorber is seen to be
70
!
I
i
!
205
reached at a distance of 15 m from the inlet and then it decreases. In collector II, the air temperature below the textile absorber shows a fluctuating behavior along the length; the reason being the nonuniform porosity of the absorber as evident from Fig. 3(a). The better thermal performance of collector II shown in Table 3 is due to the better physical properties of the absorber textile such as higher absorptance for solar radiation and smaller resistance to the flowing air. That the absorptance for solar radiation is higher in case of textile of Fig. 3(a) was estimated from measurements of the transmittance of the textile to solar radiation (Table 1), which was only 7 per cent. In Table 1 also the measured values of the total pressure drop along the length of the collector for two different air flow rates are given and show a better performance in case of the textile of collector II, yielding a higher heat transfer coefficient. 6.
PRACTICAL
CONSIDERATIONS
It is usual to rate solar flat plate collectors in terms of their thermal efficiency as a function of specific insolation and mass flow rate of the heat transfer fluid. It is, however, evident that along with its thermal performance, the specific cost and the anticipated lifetime of a solar collector are equally important. Considering lifetime as a parameter and taking into account the investment cost, interest rate and the useful energy available from a solar energy collector a simple expression is developed below. This formula, at least to a first approximation, allows one to make an economic comparison between various collectors. Taking C to be the initial cost of the collector which has a lifetime of l years, the value P ' of the initial
I
!
•
i
I
I
COLLECTOR IV profilebeLow the textil_~__eeabsorber
60
/
\
...-'"
.~................. K"'.~'......... .."" ""'K
I---- ~ - ' ~
"..:
,~_,0 L, 0
0 -....
/ / f
o
......"\
~,~
6.7~9;z,900.,.............~b~:~.':.~" ................... "~tL,,°
Y ~
.... . . ~
-...~""
\
'~
. ..k*-''''° ............
~:~_
,,.
•
0"+
6.[,31h1 ; I-[WIw~)
.~'~
w20 a=
0
\
I
COLLECTORHI/
/
0 (0)
\
COLLECTORII\
profile above the textile absorber I
I
I
I
I,
I
I
I
I
I
1
2
3
/*
S
6
7
8
9
10
(2)
(4)
16)
ml
{1o)
Distance
(Izl ~) ~61 lle) (zo) from the i n l e t l m ------
11 (22)
Fig. 9. Temperature profile inside the porous collectors along the length (figures in brackets valid for 20 m collector IV).
206
N. K. BANSALand R. UHLEMANN
(13)
The quoted prices of the conventional collector in the market of West Germany is about 400 DM/m 2. The quoted price of the collector II is 32 DM/m 2. If one includes the price of the back insulation and the frame then this collector costs a maximum amount of 80 DM/m 2. For the purpose of comparing the techno-economic qualifications of the two collectors, the incident solar insolation on 5th September 1981 at Juelich, was taken as an example. The corresponding useful energy collected by the two collectors (A T ~ 15°C) for various air flow rates is shown in Fig. 10. Using formula (13), the price of the energy, calculated as a function of lifetime, is shown in Fig. 11 for a mass flow rate of 80 m3/hr per m 2 of the collector area. It is clearly seen from the graph that even if one assigns a shorter lifetime to the present collector, its economics is seen to be much better than that of the conventional one. This makes further development of plastic solar energy collectors for air heating interesting and advisable.
For comparing the energy price we consider two collectors belonging to the category of porous absorbers. One is a conventional collector fabricated from blackened metallic scraps as the matrix, glass plate as cover and a metal frame with back isolation and the other is our presently discussed collector II. Thermal data of the conventional collector were taken from the firm.
Long plastic solar energy collectors developed for heating air show a remarkable economic potential taking into account even their small lifetime. A simple analysis leads to a simple expression which helps to give a techno-economic qualification to a collector. Numerical calculations show that even a compara-
investment after l years is given by P'=C
1+]~
(11)
.
The annual cost of the collector then comes out to be Cq t(q -- 1) qt_ 1
Pa
(12)
where
q=l-t
Z 100
if Q. is the average net useful energy collected from a unit area of the collector in one day then the price of the energy can be calculated from the formula
Pa P = 36--------5" Qu •
7. CONCLUSIONS
Conventionol
collector
Present plastic collector with textile absorber --4
:.:.: ,:.:,
2~
!iiil
(:3
:::f
iiii
>:.: ....
iiiii iiil
::5: :~:!:
!ii~
:.:.:
.>:
i:~:i
:.:.
:5:
::::
...
'
0
lb '
....
3b
!:i:i
~
so
Air flow rote per unit areo of the c o l l e c t o r ( m 3 /hxm 2)
70
::::
9o
Fig. 10. Useful thermal energy collected by the air for various flow rates.
Development and testing of low cost solar energy collectors for heating air
207
Conventional Collector
Piustic
;,
6
8
Collector ff
1'o
12
y 1'6
Life in years Fig. 11. Cost of energy derived from the two collectors as a function of their life time.
tively long lifetime of a collector may not make it competitive with the inexpensive collector o f types
presented in this communication. The porous type collectors with a textile absorber are found to perform much better thermally than the nonporous type of collectors having a blackened plastic foil as the absorber, the cost of the two being almost the same. T h e p e r f o r m a n c e of the p o r o u s collectors is, however, considerably reduced if the pore size is n o t properly optimised. Back insulation, if n o t enough, lowers the t h e r m a l p e r f o r m a n c e significantly.
Acknowledgements--Authors are thankful to Prof. A. Boettcher and Dr. M. Melil3 for deep interest in this work and for providing necessary facilities to carry out the project. Stimulating discussions with them and with other colleagues, especially Dr. M. Kleeman and Dr. M. Griiter at various stages of this work are gratefully acknowledged. The authors also like to thank the assistance of Mr. Schulz, Miss Schulze, Mr. Repschl/iger in carrying out this work. One of the authors (NKB) acknowledges the financial help
of Alexander von Humboldt foundation during his stay in F.R.G. NOMENCLATURE A c collector area, m 2 C initial cost of the collector, DM Ca specific heat of atmospheric air, J/kg K Cpa specific heat of dry air, J/kg K C~TR, . specific heat of water vapour, J/kg K heat removal factor, dimensionless h pressure drop across the aperture, mm of water I solar intensity incident over the top surface of the collector, W/m z I c threshold solar intensity, W/m 2 l lifetime of the collector, yr G volume of the air flow per hour, m3/hr m mass of air flow through the collector per unit time, kg/hr p static pressure of air, atm P0 static pressure of air at 273 K, atm p~ atmospheric pressure, Torr P,t electric power consumed by fan, W P, saturated vapour pressure, Torr q, useful thermal power, W
N. K. BANSALand R. UHLEMANN
208 Ou r
t2 - t~
L Ti
To
UL W Z
q AP AT
net useful power, W relative humidity, dimensionless time interval, s atmospheric temperature, °C inlet air temperature, °C outlet air temperature, °C heat loss coefficient, W/m 2 °K humidity ratio, dimensionless interest rate, effective transmittivity absorptivity product, dimensionless thermal efficiency, dimensionless pressure drop across the aperture, mm of H20 temperature rise in the air flowing through the collector, °C density of air, kg/cm3 transmittivity of the top glazing, dimensionless
REFERENCES
1. M. K. Selcuk, Solar Air heaters and their Applications in Solar Energy Engineering (Edited by A. A. M. Sayigh), pp. 155-182. Academic Press, New York (1977). 2. G. O. G. Loef, Solar Air Systems in Solar Energy Conversion (Edited by A. E. Dixon and J. D. Leslie). Pergamon Press, New York (1979). 3. A. Blaga, Use of plastics in solar energy applications. Solar Energy 21, 331-338 (1978). 4. A. F. Boeckmann, Collectors, pipes and heat accumulators made of plastic. 2nd Meeeting on Fundamentals o f Solar Engineering. Fellbach, F.R.G. (1976). 5. N~ K. Bansal, Plastic solar collectors, Reviews of Renewable Energy Sources, Vol. 1 (Edited by M. S. Sodha, S. S. Mathur and M. A. S. Malik). Wiley Eastern, Calcutta (1983). 6. F. L. Weichmann and D. G. Hughes, Direct absorption solar collector. 3rd Conf. o f Solar Energy Society of Canada, p. 15, Paper No. 40. Edmonton, Alberta (1977).
7. R. G. Sarazin and L. D. Olson, U.S. Pat. 4,023,556 to Universal Oil Products Co., p. 4, 17 May 1977, File date 27 May 1975. 8. J. L. Chiou, Polyolefin plastic solar collector and its applications in cold climates, Conf. 760633 on Solar Energy in Cold Climates, p. 172. Detroit, Michigan, U.S.A. (1976). 9. R. A. Erb, Unitary solar collector--a new approach to low-cost flatplate systems, Int. Solar Energy Soc. Meet. Atlanta, GA, U.S.A., SUN II (Edited by K. W. Boer and B. H. Glenn), p. 338. Pergamon Press, New York (1979). 10. E. J. Hojnowski, Solar heat collector panel, U.S. Pat. 3,995,615 (1976). 11. T. Kay, Solar collector panel and refrigeration system operated thereby, U.S. Pat. 4,126,014, p. 6, 21st Nov. (1978). 12. A. Boettcher and E. Deyannis, Development of a solar air collector. Statusbericht Sonnenenergie, Vol. 1, pp. 95-101. Verein Deutscher Ingenieure (1980). 13. H. Schulz, Oberpriifung von Einfach-Luftkollektoren (Luftkollektor Test), Statusbericht Sonnenenergie, Vol. 1, pp. 171-181. Verein Deutscher Ingenieure (1980). 14. Me[3strecken and Fassungsringe, DIN 19205, Beuth Verlag GmbH, Berlin, F.R.G. (1975). 15. F. Moeller, Einfiihrung in die Meteorologie, pp. 127-133. Bi Hochschultaschenbficher Bd. 276, Mannheim (1973). 16. N. K. Bansal, R. Uhlemann and A. Boettcher, Plastic Solar Air Heaters of a Novel Design-Testing and Performance, Jii1-1783. Kernforschungsanlage Jfilich, 5170 Jiilich, West Germany (1982). 17. J. W. Grfiter, S O L A R N E T Ein Datenerfassungs- und Kommunikationssystem fiir Experimente der Solarenergieanwendung der KFA. KFA Interner Bericht, KFA-IKP-STE IB 6-81 (1981). 18. J. A. Duffle and W. A. Beckmann, Solar Energy Thermal Processes. Wiley, New York (1974). 19. H. Buchberg and D. K. Edwards, Transpired solar air heaters. Proc. 3rd Ann. Solar Heating and Cooling Branch Contractors Meeting, pp. 40-43 (1978).
0038M)92X/84 $3.00+ .00 © 1984PergamonPressLtd.
PrintedintheU.S.A.
DEVELOPMENT A N D TESTING OF LOW COST SOLAR ENERGY COLLECTORS FOR HEATING AIR N. K. BANSALtand R. UHLEMANN Program Group for Systems Research and Technological Development, Kernforschungsanlage Juelich, Postfach 1913, 5170 Juelich l, F.R.G. (Received 25 June 1982; revision received 4 August 1983; accepted 12 August 1983)
Abstract--Solar Energy Collector designs utilising plastic films for the absorber and the glazing are developed for air heating applications. The objective is the cost reduction of such collectors with reasonable performance and acceptable life time. The developed collectors were of nonporous as well as porous type, the latter having a special synthetic textile as the absorber. An experimental facility is described to study the performance of the collectors as a function of typical physical parameters and the experimental results are discussed. The collector, with a black porous textile absorber, was found to be much superior to the collector of nonporous type. The thermal performance of the former gets, however, severelyaffected if the porous absorber is not suitably optimized and if the back of the collector is not properly insulated. Considering life time of a collector as a parameter and taking into account the investment cost, interest rate and the useful energy available from the collector, a simple expression has been developed for rating various collectors. A comparison of the numerical data for the present porous collector and commerciallyavailable conventional collector for air heating shows that the present collectors have a better economic potential even if one assigns a short life time to them. 1. INTRODUCTION The potential applications of solar air heaters are the drying of agricultural products, lumber, space heating and to some extent domestic water heating. Several designs of solar air heaters have been proposed and some of them give good performance [ 1, 2]. All designs of solar air heaters forwarded so far can basically be classified into two categories. The first type has a nonporous absorber in which the air stream does not flow through the absorber plate. Air may flow above and/or behind the absorber plate. The second type has a porous absorber that includes slit and expanded metal, transpired honeycomb or overlapped glass plate absorber. The pressure losses across the collector and the thermal losses to the ambient are generally lower for collectors with a porous absorber; an optimised pore size and the matrix thickness are, however, essential for such collectors to yield a good thermal performance. Large scale use of solar energy for various applications is prohibitive because it is usually not found to be cost effective. In order to achieve a better economy of solar installations more plastic materials have been used recently. A review listing various plastic materials suitable for various components of a solar energy collector has been given by Blaga[3], Boeckmann [4] and Bansal[5]. Solar air heating collectors made completely from plastic material include those of [6-13]. In this communication we present the designs of air fGuest Scientist with Alexander von Humboldt Fellowship. Permanent address; Centre of Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India. 197
heating solar energy collectors which use plastic films for the absorber and for the glazing. The aim is the cost reduction of such collectors with reasonable performance and acceptable life time. The developed collectors are of nonporous as well as porous type and are capable of handling large volume of air. The details of the fabricated collectors are given in Section 2. An outdoor facility was set up to study the performance of such collectors. Its details are described in Section 3. Relevant performance equations and the results of measurements have been presented in Sections 4 and 5, respectively; the latter includes a comparison of nonporous and porous types of collectors. Along with its thermal performance, the specific cost and the anticipated life time of the collector are equally important. Considering life time as a parameter and taking into account the investment cost, interest rate and the useful energy available from a solar energy collector, a simple expression has been developed for rating the solar energy collectors. Numerical calculations have been performed corresponding to the data of a conventional collector and the presently developed plastic collectors. These results are presented and a comparison of the collectors made in Section 6. 2. DESIGN OF THE COLLECTORS
Schematic diagrams of the collectors are shown in Figs. l(a, b), respectively. The collector corresponding to Fig. l(a) belongs to the category of nonporous type and it has a black PVC foil as an absorber which is covered by double transparent UV stabilized PVC foils. The upper foil contains a valve for filling the air in the space between the two foils (isolating air layer).
198
N. K. BANSALand R. UHLEMANN
cover
rransparen~
~ /
Solar radiation
n
~ A
~
air out
orber
Crossecfionnt view
Side view
Fig. l(a). Schematic of nonporous collector I.
Solar radiation
air,
~
~
PVC
°ir,out ~ Side ~ew
Fig. l(b). Schematic of the porous collectors II-IV.
The collectors corresponding to the Fig. l(b) belong to the category of porous air heaters with a porous black textile of polyester (100 per cent) which acts as an absorber. At the edges it is attached to two transparent sheets of UV stabilized polyvinyl chloride (PVC) covering it from the top and the bottom with no inside foils. The outer transparent PVC films are 0.6 mm thick and have 83 per cent transmittance for solar radiation. The inside foil, which has been provided for collector I only, is 0.3 mm thick and has a solar transmittance of about 94 per cent. In the spectral range 7-20 #m, the measured transmittance of the PVC sheet is on an average only 7 per cent as shown in Fig. 2. Thus, for absorber temperatures in the range of 40-100°C, the PVC sheet acts as an excellent cover. The lower transparent sheet, in the collectors shown in Fig. l(b), helps to double the life time of the collectors; when the absorptance of the top cover becomes too high, the collector is simply inverted so that the bottom PVC sheet acts as a cover.
The porous textile absorber should have maximum absorption of solar radiation but low air flow resistance. Simultaneously the volumetric heat transfer coefficient should be high. Since the air flows across the entire absorber area, the pressure losses along the collector in such constructions are much lower than in the case of nonporous absorbers. To test the effect of the porous absorber, another collector, having different textile absorber but otherwise with exactly identical specifications, was fabricated. A twenty times enlarged surface photograph of the two textile absorbers taken from an electron microscope is given in Figs. 3(a, b), respectively. It is observed that the pore size of the textile corresponding to Fig. 3(a) is bigger than the one corresponding to Fig. 3(b). The pressure drop measured across the collector, made from the former is, therefore, less than that made from the textile of Fig. 3(b). It was, however, also observed that the transmittance of solar radiation through the textile of collector II is less. A bigger pore size does not justify a low transmittance value, but
Development and testing of low cost solar energy collectors for heating air 6O
I
I
I
199
I
LU Z
I--
Z
"
20
0
2.5
I
I
I
i
I
3.33
5
83
10
12.5
25
X/tim Fig. 2. Spectral transmittance of the 0.6 mm thick PVC sheet, used as cover in the present collectors, in the longwave region.
(a)
(b)
Fig. 3(a). Twenty times enlarged electron microcope photograph of the textile absorber used in collector II.
Fig. 3(b). Twenty times enlarged electron microscope photograph of the textile absorber used in collectors III and IV.
the other parameters such as colour, surface reflectance etc. may be responsible for these values of solar transmittance through two different textiles. To test the effect of increasing the absorber area, a fourth collector of 20 m length and 1 m width was fabricated. The specifications, of each of the four collectors are given in Table 1.
The global insolation on the collector was measured by an Eppley pyranometer (Model PSP) mounted at the same inclination as the collector. Temperatures were measured with Pt 100 and Pt 500 resistance thermometers fixed in a copper tube using a four wire technique. At the outlet, temperature stratification is possible. The sensors are, therefore, distributed across the outlet cross section to measure the temperature profile. An effort was also made to mix the air at the outlet by making an arrangement in such a manner so that the upper layers of the air flow to the bottom side and lower layers flow to the upper side. The temperature distribution along the length of the collector was also measured by putting five temperature sensors between the inlet and the outlet at approximately equal distances. The ambient temperature and relative humidity were measured with a dry and a wet bulb thermometer using the well known relations[15]. The density of air changes with its static pressure and temperature while the specific heat is a function of temperature and the amount of water vapour
3. E X P E R I M E N T A L
SET
UP
AND
INSTRUMENTATION
The test facility consists of frames oriented towards south with the possibility of changing the angle of inclination which was kept 45 ° for the present study (latitude of Jfilich is 50°55'33"N). The blower used for forcing the air through the collector has a maximum air flow rate of 2850 m3/hr at the free blowing operation. The flow rate which can be adjusted by controlling the input electrical voltage was measured by a nozzle. The nozzle and the appropriate tubes were fabricated according to the specifications of DIN 19205 (German Industrial Standard)[14].
N. K. BANSALand R. UHLEMANN
200
Table 1. Dimensions and physical parameters of the plastic collectors Dimensions/Parameters
Collector
Collector
Nonporous
Type
I
Collector
II
Porous
Collector
III
Porous
Collector IV Porous
Length
9.10 m
8.85 m
8.80 m
Width
1.10 m
1.O3 m
1.15 m
1.13 m
10.12 m 2
19.89 m 2
Absorbing
Cross-section inlet the
10 m 2
area
9.12 m 2
17.6 m
at the
and o u t l e t
of
O.O61
m2
O.O61
m2
O.O61
m2
O.O61
Material
of the
Thickness
cover
of the
PVC
l
PVC
PVC
PVC
0.6 m m
0.6 ram
0.6 m m
0.6 m m
measured transmittanc( of the t o p PVC c o v e r in the s o l a r s p e c t r u m
0.83
0.83
0.83
0.83
Thickness
O.3
top
m2
air
cover
inside
of the
PVC
n~n
cover
measured transmittance of the i g s i d e P V C c o v e in the s o l a r s p e c t r u m
Absorber
0.94
Black PVC
Thickness
of the
0.6 m m
1OO % P o l y -
1OO % P o l y -
1OO % PoI~
e s t e r shown ~
e s t e r shown in
e s t e r shown
Fig.
Fig.
3a
3b
inFig.
2 mm
2 mm
2
O,O7
0,22
0,22
3b ram
absorber measured transm i t t a n c e of a b s o r b e r for s o l a r r a d i a t i o n p r e s s u r e loss over collector length at 360 k g / h air
I mm H20
960 k g / h flow rate
4,5 m m
present in the air. The formulas used for incorporating the variation of density and the specific heat are[16]
To [I_1 -- 3.88 x 10-4/°p0 P(P, T ) = 1.2923/0. P0 "T
H20
3 mm H20 10,5 m m H 2 0
humidity according to the formula
W(T) = 0.622. r-
Ps(T) & - P,(T)
(3)
P~ being the atmospheric pressure. In small temperature range Ps(T) can be expressed by the following linear relationship
~'650- _rl/4/] { × ~1--0.575(-----~--o J / J
(1)
cp(w, T) = C,d(r) + W(T)C~m(T)
(2)
Ps(T) = R, + R2T.
where Cpd is the specific beat of dry air and Cp. of moisture, which can be taken from standard tables. Humidity ratio W in eqn (2) is related to the relative
(4)
In the temperature range 10-70°C, the values of R 1 and R2, obtained through a least square fit, are given in Table 2 along with the correlation with the exact values. The wind speed was measured at the same
201
Development and testing of low cost solar energy collectors for heating air Table 2. Constants R~ and R2 obtained through a linear least square fit of the saturated pressure of water in a certain temperature range
T *C
R1
10"-30" 30*-50* 50*-70*
R2
atm
Correlation
atm/deg
-4.788
.
10 -3
1.487
. 10 -3
0.9888
-8.316
. 10 - 2
4.014
. 10 -3
0.9920
-3.616
. lO - I
9.484
• 10 -3
0.9930
height as the collector test stand with an electronic anemometer. All the measurement signals from the different sources were processed by the Microprocessor Aided D a t a Acquisition System (MADAS) designed and constructed for the different requirements of meteorological, collector and solar system studies[17]. The pressure drop A P across the nozzle and the static pressure is read with the help of a Betz manometer and fed into the programme through the terminal manually. The programme calculates the air density as a function of the inlet air temperature and the static pressure at the inlet (see eqn 1) and then uses A P and p for evaluating the rate of air flow G according to the equation
efficiency is written as
;1=FR[(O~T)e--UL(Ti7Ta)]
(8)
A plot of r/ against (T~ - T~)/L approximated by a straight line helps to evaluate the collector parameters FR(ctz)e and FR" UL from the intercept and the slope of the straight line, respectively. The rise in the temperature of the air flowing through the collector can also be written in the form ( T o - T i)
AcFR(z~Z)e I mCp
AcFa(z~)e lc mCp
(9)
where G = ~ • e • ( A P . p),/2
(5)
where the flow rate number 6 and the expansion factor e both are functions of the Reynolds Number and the dimensions of the measuring tract. For the aperture supplied to us and for our measuring tract G, the air volume/h, is simply calculated by the formula G = 111.11 .h 1/2
(6)
where h is the pressure drop across the nozzle measured by the manometer in mm H20. From eqn (6) the mass flow rate in kg/h through the collector is evaluated by multiplying G with air density calculated from the formula (1).
Ic
uL(r,- Ta) (~)o
is the threshold level of the incident radiation. If the inlet air temperature is nearly equivalent to the ambient then (To --Ti) vs I points lie nearly on a straight line and then eqn (9) can also be used to determine the parameters F~(z~t)e from the slope of the straight line. If the measurement data of the whole day fits such a straight line then the day long efficiency is calculated from the expression q = FR(za)~ [ 1 - - ~ ] .
(10)
4. THEORY
5. RESULTS AND DISCUSSION
Instantaneous thermal efficiency of the collector defined as the ratio of the useful thermal energy to the total incident solar energy, averaged over the same time interval is mathematically expressed as
The hourly variation of the various parameters namely the inlet and outlet temperatures, A T and the efficiency q, is shown in Figs. 4 and 5 for the collectors I and II, respectively, along with the corresponding variation of the solar radiation and the ambient temperature. Collector I is of nonporous type while collector II is of porous type, the exact dimensions of the two collectors are given in Table 1. The air flow rate, which was kept constant throughout the day, is the same in the two cases. It is evident that, for nearly the same ambient conditions, the thermal performance of the porous type collector (collector II) is much better than the performance of the nonporous type collector (collector I), the former
m" C,I 1/=
'~(To -
q]ll
r,2
T,) dt
Acj,1I(t)dt
(7)
The quantities m and Cp have been taken out of integration in the numerator since they remain essentially constant during one set of measurements. The Hottel-Whillier-Bliss eqn [18] for the thermal
202
N.K. BANSALand R. UHLEMANN 50,
1000 flow rote 7896 k Wire2' date 15.A.82 wind speed
800. 1.2+o2.1 m,r
3~
600.
;3
'~ 2(3
..li-
/
h
~
~" - ' ~ ' \ .
"-~ ---_.
.~0 0_
"\.
.
o
Insolation~
temperature rise
. . . . .
~
2,
inlet
-
200-
~
~"
temperature
\
"a mbient temperature 12 h
I
,
,
600
i
700
, the
I
16 h
i.
800 Time of
,
i
18h/ ,
900 (rains }
day
10
~
1(, h ,
-20
\
.
~,,~.--~---~ 10 h
lC
Nonporous Collector T -~.
Solar
o
c eJ
~g
,
i
1000
/
1100
Fig. 4. Time variation of the solar insolation on the collector's surface, the various temperatures and efficiency on a typical day at Juelich, F.R.G. (latitude 50°55'33").
70. 6050-
1000
*C
C o l l e c t o r 1"[ Windspeed 1.2 to 2.Orals Wire2 22. t.. 82 ~ " ~ " ~ " -'~"" ~ ' ~ _ ~ . ~...~ flow rate . ~ " ~-~"..... / - ~ , ~ "~--<,~._ 800 789.6 kgl h.,' ' ~ ~ ro'd~'i'ot ion ~ "~~, '~._ 30
,g >,~,0- .~ 6o0
efficte
••~
~
~ r lt _~
~ k~ 3020
t.oo
I/ I I
i i I
:
L
~
~
-
,-~p,
20
--P; ",:.~,.__...__
~
~
(0 ¢X
erature Uttet
_ _----
-'---'-
".
10 10
200 66o
760
' Time
of the
day
11oo
(rains)
Fig. 5. Time variation of solar radiation on surface of the collector II, the typical temperatures and efficiency on a typcial day at Juelich, F.R.G. (latitude 50°55'33").
yielding about a maximum of 60 per cent efficiency while the latter reaching only 45 per cent at these flow rates. The rise A T in the temperature of the air flowing through the collector, gets less and less with the increasing air flux. The efficiency, however, gets correspondingly increased due to the decreased thermal losses to the environment. Figure 6 shows the measured variation of AT and the efficiency r/ with different flow rates of air through the collectors I and II. Figure 7 shows the typical efficiency curve using eqn (8) for both collectors I and II. A least square fit of the data shows that all the measured data points fit to a straight line with a 0.7 value for the coefficient of correlation. From the coefficients of the linear fit one obtains the parameters FR(~)e and FRUL. It is observed from the data that the value of FR(z~)e is
much higher for the porous absorber collector II than the nonporous absorber collector I. The effect in the transmittance of the cover system due to second layer in collector I is not able to explain the low value of FR(z~x). For example the effect of the double transparent layer is to decrease the transmittance from a value of 0.83 (for a single cover) to 0.78. This reduction alone is not responsible to explain the low value of FR(z~)e for collector I. The reason for a higher value of Fx('cOt)e for collector II is the inclusion of textile absorber in the collector. For a double glazed transpired air heater the efficiency equation[19] is r / = 0 . 6 - 4 . 9 Ti-- T, I In contrast the performance of the plastic film solar
Development and testing of low cost solar energy collectors for heating air
203 25
0.8
*C E QJ
co
~-
20 z
0.6
LU
o E
/
0./.
~
-
Temperature
~
Rise
15
J - -
Porous
Absorber
OL2
240
Collector IT
Absorber Collector I
------Nonporous
3L0
~0
i
6;0
73o
i
i
8Zo
10
9~0
mass flow rate ( kg/h } Fig. 6. Variation of r/and AT with air flow rate for an incident solar radiation of about 700-750 W/m2.
.7.6-
>,.5 ¢u
G ./.
x
x
K
xx
x x
x
K x
"
IJJ x
TI=A*B,(-~
x
a)
x x
.2" Nonporous flow
.1' . . . .
.0~1
789.6
POROUS
o62
.0~3
0.7/.
oh/.
.obs
- 31.2
oh6
0.7/.
oh7
31.2
o68
oh9
010
(Ti -r a ) / T
Fig. 7. Variation of efficiency r/ with ((T~
for nonporous (collector I) and porous (collector II) collectors.
- Ta)/l)
air heater comes out to be
~-T~
r / = 0.74 - 3 1 . 2 - -
I
It is clearly seen that the plastic film collectors have a higher loss coefficient. But we are trading off the high loss coefficient for a low economic cost and a high intercept (0.74 vs 0.6). Since the inlet air temperature T~ is very nearly equal to the ambient temperature Ta, the temperature rise z~T varies linearly with the incident radiation on the surface of the collector. A typical plot is shown in Fig. 8 for both the collectors I and II, respectively, along with the calculated values FR(~ct)e and the threshold value I~ for the incident radiation. It was observed that the thermal performance of the porous collector with a textile absorber is considerably effected by the insulation at the back of the
collector and by the pore size of the textile absorber. A decrease in the thickness of the PE-Foam from 6 to 3 cm brought out an efficiency loss of about 20 per cent. It was also observed that the heat losses could be substantially decreased by putting an aluminium foil between the backside insulation and the collector. A quantitative comparison of the measured thermal performance of collector II for various back insulations is given in Table 3. The effect of pore size on the performance of a porous absorber collector was evident by the measurements on another collector III, which has the textile of Fig. 3(b) as the absorber. A longer collector IV helps to get a larger AT, the thermal efficiency however gets reduced due to increased thermal losses. Table 4 gives a comparison of collectors II-IV for a back insulation of only 3 cm thick polyethylene. Within the experimental limitations the electrical power used for the three collectors was found to be nearly the same. The point to note
204
N
K
25 oC
x
flow rote kglh A*C 812.~. -39212
Nonporous
....
BANSAL and R. U H L E M A N N
Porous
7896
/ : /
-3037
/'
~20
S 8
~1o *
B(
/
o
..'I"
.
.
.
.
.
.
.
}e I C
FR ( T ~
Porous .0284
200
0
°C
I
0.684
106.9
. 600
400 Solor
800
Wl m 2
1000
rodiotion
Fig. 8. Variation of d T with incident solar radiation for nonporous (collector I) and porous (collector II) collectors.
Table 3. Performance of the porous collector II with various types of back insulation. Figures in bracket indicate the incident solar radiation in W/m 2 of the collector surface S. NO
Temperature kg/h
504
654
774
862
924
960
Rise
°C
Efficiency %
I
II
III
I
II
23.4
23.9
22.8
O.3991
0.4905 0.4781
(900)
(748)
(732)
19.4
18.0
20.4
0.4622
0.5257 0.5447
(836)
(682)
(746)
18.2
18.4
19.4
0.4898
0.5421
(876)
(800)
(743)
17.4
13.9
18.8
0.5029
0.5560 0.6561
(908)
(656)
(752)
15.9
13.9
18.3
0.5233
0.5588 0.6785
0.5319
0.5688 0.7065
(855) (700)
(759)
16.3
12.1
16.6
(896)
(622)
(687)
I
3 cm t h i c k p o l y e t h y l e n e
II
3 cm t h i c k p o l y e t h y l e n e
III
6 cm t h i c k p o l y e t h y l e n e
w i t h A1
III
0.6090
foil t o p
Table 4. Comparison of the thermal performances of the Collectors II-IV with 3 era thick polyethylene as the back insulation
s.N.
wSm2
I
900
2 3
.k~/.d
Ti
OC
II
III
To IV
II
°C III
~T IV
II
%
°C III
IV
II
III
qu IV
II
W III
326
3 6 1 6 1 2 0 3 0 3756
299
3527 2011
2237 379~
245
3453!1992
3553
2009
220
313711789
3261
1665 281
164
3031i1501
2649
1602 234~
112
2 6 3 0 1490 2233
10.1
17.5 4 8 . 0 26.2 22.6 394~
36.9
10.5 18.2 46.6 25.7 22.2 3826 369~
84O
20.4 21.7
19.9 3 7 . 3 1 3 1 . 8
900
792
20.8 22.6
18.7 38.2 33.1
4
900
732
21.2 21.6
19.5 39.4 32.6 38.2
8.2 11.O 18.7 45.1
5
900
624
21.8 21.7
18.7 41.2 33.3 38.8
9.4 11.6 20.1
6
900
492
21 .7 21.7 2 0 . 0 45.9 33.9 40.6 23.4
12.2 20.6 38.9 18.5 15.6 3191
7
900
324
20.1 21.9 21.9 50.6 39.8 4 8 . 0 3 0 . 5
17.9 26.1
7.4
24.821.1
40.9 22.3
33.4
17.8
19.3 3557
3.0 2742
IV
2 3 1 0 4OO1
37.416.9
900
III
2356 408~
2383
20.2 36.6 29.9 3 7 . 0 16.3
ou II
3603 2 0 2 0 3723
16.8 48.3 26.5 22.7 396(
20.3 20.1
W
363
9.8
876
Pel IV 4086
3481
3702
Development and testing of low cost solar energy collectors for heating air is that while the useful energy q~ = mCpdT goes on increasing with the air flow rate, the net useful energy Q, ( = q, -p~z) increases up to a certain flow rate and then decreases. Detail measurements of the air temperature and pressure loss over the entire length of the collectors (Fig. 9) showed that the resistance to air flow by the porous absorber must be carefully optimised for different air flow rates and collector dimensions. The different curves in Fig. 9 show the temperature profile above as well as below the textile absorber in various collectors. In collectors III and IV, the temperature of the air above the textile absorber is seen to be increasing steadily along the length, while in collector II, the temperature of air above the absorber is seen to be nearly equivalent to the ambient air temperature except for the case when the rate of air flow is very small It is, therefore, obvious that the thermal losses in collector II from the top are negligibly small. The reasons for the different air temperatures in collectors II and III are due to the different pore size of the textile absorbers. Figures 3(a, b) show that the absorber of collector III is denser than that of collector II. So all the air in collector III (and in collector IV) is not able to go through the textile absorber and hence it builds a pressure near the outlet end of the respective collectors. This is noted from the variation of air temperatures along the length of the collector below the textile absorber. In collector III, the temperature is seen to be increasing along the length till the outlet end and suddenly dropping down due to the cold air coming from top towards outlet end. In collector IV, the maximum of the air temperature below the textile absorber is seen to be
70
!
I
i
!
205
reached at a distance of 15 m from the inlet and then it decreases. In collector II, the air temperature below the textile absorber shows a fluctuating behavior along the length; the reason being the nonuniform porosity of the absorber as evident from Fig. 3(a). The better thermal performance of collector II shown in Table 3 is due to the better physical properties of the absorber textile such as higher absorptance for solar radiation and smaller resistance to the flowing air. That the absorptance for solar radiation is higher in case of textile of Fig. 3(a) was estimated from measurements of the transmittance of the textile to solar radiation (Table 1), which was only 7 per cent. In Table 1 also the measured values of the total pressure drop along the length of the collector for two different air flow rates are given and show a better performance in case of the textile of collector II, yielding a higher heat transfer coefficient. 6.
PRACTICAL
CONSIDERATIONS
It is usual to rate solar flat plate collectors in terms of their thermal efficiency as a function of specific insolation and mass flow rate of the heat transfer fluid. It is, however, evident that along with its thermal performance, the specific cost and the anticipated lifetime of a solar collector are equally important. Considering lifetime as a parameter and taking into account the investment cost, interest rate and the useful energy available from a solar energy collector a simple expression is developed below. This formula, at least to a first approximation, allows one to make an economic comparison between various collectors. Taking C to be the initial cost of the collector which has a lifetime of l years, the value P ' of the initial
I
!
•
i
I
I
COLLECTOR IV profilebeLow the textil_~__eeabsorber
60
/
\
...-'"
.~................. K"'.~'......... .."" ""'K
I---- ~ - ' ~
"..:
,~_,0 L, 0
0 -....
/ / f
o
......"\
~,~
6.7~9;z,900.,.............~b~:~.':.~" ................... "~tL,,°
Y ~
.... . . ~
-...~""
\
'~
. ..k*-''''° ............
~:~_
,,.
•
0"+
6.[,31h1 ; I-[WIw~)
.~'~
w20 a=
0
\
I
COLLECTORHI/
/
0 (0)
\
COLLECTORII\
profile above the textile absorber I
I
I
I
I,
I
I
I
I
I
1
2
3
/*
S
6
7
8
9
10
(2)
(4)
16)
ml
{1o)
Distance
(Izl ~) ~61 lle) (zo) from the i n l e t l m ------
11 (22)
Fig. 9. Temperature profile inside the porous collectors along the length (figures in brackets valid for 20 m collector IV).
206
N. K. BANSALand R. UHLEMANN
(13)
The quoted prices of the conventional collector in the market of West Germany is about 400 DM/m 2. The quoted price of the collector II is 32 DM/m 2. If one includes the price of the back insulation and the frame then this collector costs a maximum amount of 80 DM/m 2. For the purpose of comparing the techno-economic qualifications of the two collectors, the incident solar insolation on 5th September 1981 at Juelich, was taken as an example. The corresponding useful energy collected by the two collectors (A T ~ 15°C) for various air flow rates is shown in Fig. 10. Using formula (13), the price of the energy, calculated as a function of lifetime, is shown in Fig. 11 for a mass flow rate of 80 m3/hr per m 2 of the collector area. It is clearly seen from the graph that even if one assigns a shorter lifetime to the present collector, its economics is seen to be much better than that of the conventional one. This makes further development of plastic solar energy collectors for air heating interesting and advisable.
For comparing the energy price we consider two collectors belonging to the category of porous absorbers. One is a conventional collector fabricated from blackened metallic scraps as the matrix, glass plate as cover and a metal frame with back isolation and the other is our presently discussed collector II. Thermal data of the conventional collector were taken from the firm.
Long plastic solar energy collectors developed for heating air show a remarkable economic potential taking into account even their small lifetime. A simple analysis leads to a simple expression which helps to give a techno-economic qualification to a collector. Numerical calculations show that even a compara-
investment after l years is given by P'=C
1+]~
(11)
.
The annual cost of the collector then comes out to be Cq t(q -- 1) qt_ 1
Pa
(12)
where
q=l-t
Z 100
if Q. is the average net useful energy collected from a unit area of the collector in one day then the price of the energy can be calculated from the formula
Pa P = 36--------5" Qu •
7. CONCLUSIONS
Conventionol
collector
Present plastic collector with textile absorber --4
:.:.: ,:.:,
2~
!iiil
(:3
:::f
iiii
>:.: ....
iiiii iiil
::5: :~:!:
!ii~
:.:.:
.>:
i:~:i
:.:.
:5:
::::
...
'
0
lb '
....
3b
!:i:i
~
so
Air flow rote per unit areo of the c o l l e c t o r ( m 3 /hxm 2)
70
::::
9o
Fig. 10. Useful thermal energy collected by the air for various flow rates.
Development and testing of low cost solar energy collectors for heating air
207
Conventional Collector
Piustic
;,
6
8
Collector ff
1'o
12
y 1'6
Life in years Fig. 11. Cost of energy derived from the two collectors as a function of their life time.
tively long lifetime of a collector may not make it competitive with the inexpensive collector o f types
presented in this communication. The porous type collectors with a textile absorber are found to perform much better thermally than the nonporous type of collectors having a blackened plastic foil as the absorber, the cost of the two being almost the same. T h e p e r f o r m a n c e of the p o r o u s collectors is, however, considerably reduced if the pore size is n o t properly optimised. Back insulation, if n o t enough, lowers the t h e r m a l p e r f o r m a n c e significantly.
Acknowledgements--Authors are thankful to Prof. A. Boettcher and Dr. M. Melil3 for deep interest in this work and for providing necessary facilities to carry out the project. Stimulating discussions with them and with other colleagues, especially Dr. M. Kleeman and Dr. M. Griiter at various stages of this work are gratefully acknowledged. The authors also like to thank the assistance of Mr. Schulz, Miss Schulze, Mr. Repschl/iger in carrying out this work. One of the authors (NKB) acknowledges the financial help
of Alexander von Humboldt foundation during his stay in F.R.G. NOMENCLATURE A c collector area, m 2 C initial cost of the collector, DM Ca specific heat of atmospheric air, J/kg K Cpa specific heat of dry air, J/kg K C~TR, . specific heat of water vapour, J/kg K heat removal factor, dimensionless h pressure drop across the aperture, mm of water I solar intensity incident over the top surface of the collector, W/m z I c threshold solar intensity, W/m 2 l lifetime of the collector, yr G volume of the air flow per hour, m3/hr m mass of air flow through the collector per unit time, kg/hr p static pressure of air, atm P0 static pressure of air at 273 K, atm p~ atmospheric pressure, Torr P,t electric power consumed by fan, W P, saturated vapour pressure, Torr q, useful thermal power, W
N. K. BANSALand R. UHLEMANN
208 Ou r
t2 - t~
L Ti
To
UL W Z
q AP AT
net useful power, W relative humidity, dimensionless time interval, s atmospheric temperature, °C inlet air temperature, °C outlet air temperature, °C heat loss coefficient, W/m 2 °K humidity ratio, dimensionless interest rate, effective transmittivity absorptivity product, dimensionless thermal efficiency, dimensionless pressure drop across the aperture, mm of H20 temperature rise in the air flowing through the collector, °C density of air, kg/cm3 transmittivity of the top glazing, dimensionless
REFERENCES
1. M. K. Selcuk, Solar Air heaters and their Applications in Solar Energy Engineering (Edited by A. A. M. Sayigh), pp. 155-182. Academic Press, New York (1977). 2. G. O. G. Loef, Solar Air Systems in Solar Energy Conversion (Edited by A. E. Dixon and J. D. Leslie). Pergamon Press, New York (1979). 3. A. Blaga, Use of plastics in solar energy applications. Solar Energy 21, 331-338 (1978). 4. A. F. Boeckmann, Collectors, pipes and heat accumulators made of plastic. 2nd Meeeting on Fundamentals o f Solar Engineering. Fellbach, F.R.G. (1976). 5. N~ K. Bansal, Plastic solar collectors, Reviews of Renewable Energy Sources, Vol. 1 (Edited by M. S. Sodha, S. S. Mathur and M. A. S. Malik). Wiley Eastern, Calcutta (1983). 6. F. L. Weichmann and D. G. Hughes, Direct absorption solar collector. 3rd Conf. o f Solar Energy Society of Canada, p. 15, Paper No. 40. Edmonton, Alberta (1977).
7. R. G. Sarazin and L. D. Olson, U.S. Pat. 4,023,556 to Universal Oil Products Co., p. 4, 17 May 1977, File date 27 May 1975. 8. J. L. Chiou, Polyolefin plastic solar collector and its applications in cold climates, Conf. 760633 on Solar Energy in Cold Climates, p. 172. Detroit, Michigan, U.S.A. (1976). 9. R. A. Erb, Unitary solar collector--a new approach to low-cost flatplate systems, Int. Solar Energy Soc. Meet. Atlanta, GA, U.S.A., SUN II (Edited by K. W. Boer and B. H. Glenn), p. 338. Pergamon Press, New York (1979). 10. E. J. Hojnowski, Solar heat collector panel, U.S. Pat. 3,995,615 (1976). 11. T. Kay, Solar collector panel and refrigeration system operated thereby, U.S. Pat. 4,126,014, p. 6, 21st Nov. (1978). 12. A. Boettcher and E. Deyannis, Development of a solar air collector. Statusbericht Sonnenenergie, Vol. 1, pp. 95-101. Verein Deutscher Ingenieure (1980). 13. H. Schulz, Oberpriifung von Einfach-Luftkollektoren (Luftkollektor Test), Statusbericht Sonnenenergie, Vol. 1, pp. 171-181. Verein Deutscher Ingenieure (1980). 14. Me[3strecken and Fassungsringe, DIN 19205, Beuth Verlag GmbH, Berlin, F.R.G. (1975). 15. F. Moeller, Einfiihrung in die Meteorologie, pp. 127-133. Bi Hochschultaschenbficher Bd. 276, Mannheim (1973). 16. N. K. Bansal, R. Uhlemann and A. Boettcher, Plastic Solar Air Heaters of a Novel Design-Testing and Performance, Jii1-1783. Kernforschungsanlage Jfilich, 5170 Jiilich, West Germany (1982). 17. J. W. Grfiter, S O L A R N E T Ein Datenerfassungs- und Kommunikationssystem fiir Experimente der Solarenergieanwendung der KFA. KFA Interner Bericht, KFA-IKP-STE IB 6-81 (1981). 18. J. A. Duffle and W. A. Beckmann, Solar Energy Thermal Processes. Wiley, New York (1974). 19. H. Buchberg and D. K. Edwards, Transpired solar air heaters. Proc. 3rd Ann. Solar Heating and Cooling Branch Contractors Meeting, pp. 40-43 (1978).