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The water-trickle ceramic solar collector
Solar & Wind Technology Vol. 6, No. 5, pp. 517-522, 1989 Printed in Great Britain.
0741-983X/89 $3.00+ .00 Pergamon Press pie
THE WATER-TRICKLE CERAMIC SOLAR COLLECTOR ALl A. BADRAN Mechanical Engineering Department, Faculty of Engineering and Islamic Architecture, Umm A1 Qura University, Makkah, Saudi Arabia
(Received 29 September 1988 ; accepted 16 November 1988) Abstract--A new application for ceramic is investigated. The idea is to use the ceramic as an absorbing surface for fiat plate collectors. In this type of collector, water flows atop the absorbing surface and is exposed to sunrays. The water film is thin (1-2 ram) and it does not touch the glazing. Water is trickled from a header at the top of the collector down the ceramic surface. Solar radiation penetrates the water layer, gets absorbed by the ceramic and the energy is given back to water. Ceramic is selected as a material for the absorbing surface because it is mainly cheap, locally available in developing countries, and easy to manufacture in such countries. Besides, the ceramic surface presents a good solution for several problems practised with solar collectors : (i) It has a surface of granulated nature, which results in good spreading of the water film atop the absorber. (ii) It is superior in resisting the corrosion caused by the moisture inside the collector. (iii) It provides an ample space for the flow to proceed even if some precipitation of minerals carried by the water occurred on the absorbing surface. The collector is tested against a conventional flat-plate-type collector (with tubes atop a steel plate) and the performance of the two is compared. Experimental findings agree with the theoretical analysis of the idea. The instantaneous efficiency of the ceramic collector is about 5--17% higher than that of a certain conventional collector of the fine-tube-type.
1. INTRODUCTION One of the major problems facing flat-plate collectors is the high cost that results from using the fin-tube configuration for the absorber plate. Another problem which is inherent in such a configuration is the loss in the collector efficiency. Many design ideas are used to minimize the loss in fin efficiency. The reduction of spacing between tubes, increasing the tube diameter, improving bond conductance, and using highly conductive absorber plates do increase the fin efficiency but also increase the cost. One of the methods used to reduce the cost is the so-called "water trickle method" [1]. In this method water flows atop the absorber plate and is exposed to the glazing. The plate is corrugated such that the water is trickled down the grooves and the part of the plate that is between the grooves remains dry. The cost is reduced by this method because no tubes are used, but, unfortunately, the efficiency is not improved because :
face in a dropwise form. Dropwise condensation reduces the solar radiation reaching the absorber plate because the drops form a barrier of"lenses" attached to the glazing. The sun rays suffer multiple reflections through these liquid lenses in the off-noon hours. An improvement in the design [2] circumvented the first problem by eliminating the grooves completely. This resulted in a simple design, where the liquid flows atop a flat plate. To keep the uniformity of the flow, a wire mesh is attached to the plate, thereby utilizing surface tension force to spread the flow. The material used is galvanized steel for both the absorber plate and the wire mesh. The new improvement in this research is mainly in the material. The absorber plate is made of ceramic and the wire mesh is cancelled. Ceramic is selected for this rather unusual service because it has many useful characteristics : (a) Water flows uniformly on ceramic surface without the use of a wire mesh to spread it. (b) Ceramic is superior in resisting corrosion and that makes it a superb material for solar energy applications.
(i) the fin-tube configuration is essentially kept in the design, with the only difference being that the liquid is exposed directly to solar radiation and (ii) the liquid vapor is condensed on the glazing sur517
518
ALIA. BADRAN
Aluminium frame /Glazing /Ceramictiles Rubberseal / / /Mortar ,.I.~haa,h,r / / / /cov.,plato. ~ heaoer
Concretebase' / / Insulation// Envelope/ Fig. 1. Longitudinal cross-section in ceramic collector. (c) In areas of hard-water conventional solar collectors of the fin-tube type suffer from the precipitation of salts inside the tubes. (For instance, this has been practised in Irbid, Jordan where this research is carried out.) This problem occurs in conventional collectors mainly because they are subjected to a higher surface temperature. When precipitation occurs, it occurs inside the tubes, which are of limited area. But in water-trickle collectors, the problem of precipitation is not critical because water has enough space to flow through. In fact, with the use of ceramic, the problem is eliminated simply because the surface temperature of ceramic is less than that of steel. (d) Ceramic is a locally available material in almost every country. This feature is important for developing countries because it means less dependence on imported steel. Furthermore, it can be manufactured by non-technical communities from material already used for pottery. (e) Ceramic is an example of a building material that may be used for the construction of roofs. The ceramic collector can be considered, in fact, as a module which is part of a large scale system. A large part of the roof can be covered with glazing to form a large collector. This concept helps in utilizing already built roofs that have slopes facing south. All that is needed is to install the piping work and cover the necessary area with glazing. The ceramic collector is tested against a conventional flat plate type collector of the same area, insulation, glazing and envelope. Results show that this collector is favorably compared with conventional ones.
2. APPARATUS The collector consists basically of a concrete platform upon which ceramic tiles are fitted using cement
mortar to form the absorber part (Fig. 1). The absorber is placed inside a galvanized steel case with a 5 cm thick layer of glass wool insulation in between. A single glass cover, 4 mm thick, fitted to an aluminium frame is placed on the top. Inlet and outlet headers are placed at the top and the bottom made of 2.5 cm dia. galvanized steel pipe but the inlet has 20 holes, each of 2.5 mm dia. and the outlet is slotted. Figure 2 shows a photograph of the collector. It should be noted here that, in the beginning of this work, a great effort was spent in trying to make the absorber completely of ceramic. But large cracks, as wide as 2 cm developed in the clay after it was dried. Attempts were made to fill those cracks with clay but that was not successful either. The problem was finally solved by replacing part of the clay with a reinforced concrete casting and covering it with several pieces of ceramic (32 x 21 × 1 cm) that are made locally. In order to compare the performance of the collector with that of conventional ones, a fin-tube type collector is installed beside it. The conventional collector is chosen arbitrarily from those manufactured locally (Rum Co.). It is made according to the specifications of the Royal Scientific Society, Jordan. Therefore, this collector will be termed the RSS collector. The details of the specifications may be found in Ref. [3]. The test facility, shown in Fig. 3 follows in general what is recommended by A S H R A E 93-77 with minor modifications. Mainly, the direction of flow is reversed to suit the ceramic collector where the flow is from the top to the bottom. 3. PERFORMANCE ANALYSIS
The thermal performance of the solar collector is determined by obtaining: (i) the instantaneous efficiency under steady state conditions and (ii) the time response characteristics under transient conditions. The instantaneous efficiency of flat plate collectors is given [4] by the well-known equation q = Fr(t~)e - F r U u ( t r , - ta)/l. (1) This equation is valid for both collectors under consideration--the conventional, termed the RSS collector and the ceramic one. Since similar glazing, absorbing paint and insulation are used in both, the major difference between the two is in the expression of FR. Another important factor in the equation is Ut.. It is expected, however, that this factor is much less for the case of ceramic collector due to its low mean temperature.
Water-trickle ceramic solar collector
519
Fig. 2. A photograph of the collector.
Pyr'snomGRer \ a.
FR = [GCp/UL][1-exp(-FUL/GCp)]
(2)
and, for the ceramic collector, it can be shown [2] that
Therm~ Hot water I
tank I
I I
FR = [GCp/UL][1 --exp (-- UL/GCp).
By-pau
Flow mater
exchangor
Fig. 3. A sketch of the collector's testing arrangement.
(3)
It is obvious that the difference between the two expressions is that the efficiency factor F ' for the ceramic collector equals 1. Since F ' is always less than or equal to 1, then keeping it at the maximum is an improvement in the collector's efficiency. In the conventional RSS design, the tube spacing is about 10 cm and the diameter is 12 ram. F o r a steel plate of 1 mm thickness, the typical value of F ' ranges from 0.9 to 0.98, depending on the overall heat transfer coefficient. Therefore, it is expected that the ceramic collector shall have up to 10% increase in efficiency, provided that other factors remain the same.
520
ALl A. BADRAN
The time response analysis of the ceramic collector follows exactly those of the conventional ones. The ASHRAE 93-77 standard [5] gives the governing equation as
(CA/Ac) dtf/dT = FRI('c~)e-- FRUL(tfi -- ta)
be greater than 790 W/m2.) Inlet, outlet, and ambient temperatures are recorded, then the incident solar energy is abruptly reduced to zero by shielding the collector from the sun. The inlet and outlet temperatures are plotted as a function of time until the value of the expression
- (rhCp/Ac)(tfo- tr,) (4)
rhCp
FRUL(tf, i--ta) + --Ac (/r,e,T--/f,i) which results in the following expression for the exit temperature of the transfer fluid as a function of time :
FRI('~oOeFR UL(tfi --/a) - - ?~lfP/Ac)(tf,e,T -- tfi) FRI(z~)~ -- FR UL(tn -- ta) -- rhCp/Ac) (tf, e,i - tn) =
exp-
. . . . . . . . . . . . . . . .
FRUL(tr'i--ta) H'- Ac (tfx'initial--lf'i) where
FRI(r~)~ = zero during the test
rmc l (5)
where K = [rnCp/FRULAcl[(F/FR)-- 1].
(7)
~< 0.3
rhCp
then eq. (7) is plotted against time and the time constant is calculated. This is the time required for the above equation to equal l/e = 0,368.
(6)
The quantity KCA/rhC P is known as the time constant and is the time required for the quantity e -~cP/KcA to change from 1.0 to (I/e) = 0.368. It is expected that the time constant of the ceramic collector is one order of magnitude larger than that of the conventional one. The reason is that the heat capacity CA of the collector is much larger than that of the RSS collector. 4. TESTING PROCEDURE AND CALCULATIONS (a) Collector thermal efficiency According to the ASHRAE standard, the testing shall be conducted under steady state conditions. The two collectors are tilted at 47 '~ then the system is primed by the working fluid and then the inlet fluid temperature (t~) is adjusted and held constant. The system is operated with a flow rate of 0.02 kg/m 2 s at the working inlet temperature for 1.5-2 h before taking any data in order to reach a steady state. This period is necessary for the ceramic collector due to its large heat capacity. Then t~, tf,~, t,, and I are recorded each 15 min increment. Data are taken half an hour before and after noon and the average values of tf¢, ta and I are calculated for the same day. This procedure is repeated for different values of the inlet fluid temperature. Then the thermal efficiency and ( t n - ta)/I are calculated every day and plotted. A straight line is obtained and the equation of this line is found by the least squares method.
5. RESULTS The performance tests of the ceramic collector are performed first before the ceramic tiles are painted black. At the same time the RSS collector is tested and the results of both tests are plotted on the same chart (Fig. 4). After that the ceramic collector is painted black and test results are shown on the same chart (Fig. 4). In all cases, a straight line results and the equation of this line is found by the least squares method. It is found that, for the ceramic collector : q = 0.513-5.20 (tf, i-t~)/L before painting the absorber black r/= 0.592-6.22 ( t f j - t , ) / L after painting, and for the RSS collector : q = 0.439- 8.54 (t~,~- t~)/l.
~SO- 80 - Ceramic collector, painted black _ / Ceramiccollector, without painting 70 L I ~ / Conventional collector (RSS) 80
~ 2o
~ 10--~
(b) Collector time constant The test is done according to ASHRAE standard. The system is primed to reach steady state conditions at some inlet fluid temperature. (Solar insolation must
I
1
[
2
I
3
L
4
I
5
I
6
I
7
I
8
I
9
I
10
I
11
(tf. i-ta)/I X 10 - 3 , ~C mzNV
Fig. 4. Instantaneous efficiencycurves for ceramic collector vs conventional collector.
Water-trickle ceramic solar collector
contamination seems likely to take place in the collector. The poor performance observed for the RSS collector may be related to manufacturing defects. The published literature [3] about the original design of this collector reports a much better performance. This performance is reported in the form of the efficiency equation :
°L__ 1.0
0.6
.~ Io
O
•
0.4
0.2 o.o
"e~.Conventional
I 2o
I 4o
collector
I
eo
521
I
so
r/= 0.657-5.79 (Tf,i - Ta)/L I
loo
I
12e
1~
T, min
The selection of the RSS collector for comparison was merely for availability reasons. Most of the collectors in the area follow the same design.
Fig. 5. Time constant of ceramic collector vs conventional collector. 7. CONCLUSION The time constant experimental results are shown in Fig. 5 for both collectors. From the figure, it is found that the time constant is 73 min for the ceramic collector and 13.8 min for the RSS collector.
6. DISCUSSION As expected, the efficiency of the ceramic collector is higher than that of the conventional one, even before the ceramic surface is painted black. Naturally the efficiency is improved when the ceramic surface is painted black such that it became about 17% higher than that of the conventional design. The efficiency could not reach higher values due to the condensation of water droplets on the inner glass surface. This decreases the transmittance of the glass in the off-noon hours and increases the heat losses to the outside. The water flow was uniform on the ceramic surface and the problem of salt precipitation was solved in this type of collector by eliminating the tubes. Even after possible precipitation is assumed, the efficiency is still higher by about 5%. This figure is obtained by assuming that the absorptance of the surface when fully covered by salts is equal to that of the original ceramic surface. The time constant of the ceramic collector is high due to the large heat capacity of ceramic and concrete, which means low response. This is an additional feature for the collector, where the ceramic is utilized for energy storage. During the period of testing, which lasted for about 3 months, no algae growth was observed. It could be suggested that this period was not enough for the anticipated growth, but it is doubtful that the high temperature, which might reach up to 60°C inside the collector will permit such a growth. Therefore, no
The idea of using ceramic as an absorber surface for solar energy is found to be successful. The performance of the collector is superior to conventional ones such that a 17% increase in efficiency is obtained. Practically, ceramic did withstand the tough environment of moisture, heat and salts inside the collector. Economically, the fact that only one model is built makes it hard to conclude that it is really less in cost. There are, however, some problems faced with the ceramic collector. Mainly, the heavy weight (about 250 kg) of the collector made it difficult to handle. This weight, however, mainly belongs to the concrete base. The base does not have to be concrete; wood, sheet metal or any other light weight roofing material could be used. Another problem is that the ceramic surface, when painted, loses a part of its ability to spread the flow. A solution to this could be the use of a dye with the clay itself to make the ceramic black without the need for painting. One more important restriction on this type of collector is that it cannot be thermosyphonic. Therefore, forced circulation is necessary for such a system.
8. RECOMMENDATIONS Other material may be tried using the concept of water trickle collectors. Cement, as well as any other roofing material could be used as an absorber, as long as the surface properties enhance uniform spreading of the flow. A module of glazing, headers, proper frame and sealing medium is recommended to be built. This module could be installed on any sloping type roof to test its potential for solar energy collection. In future testing, it is recommended that the collectors' daily performance characteristics and long term performance characteristics under varying ambient conditions are obtained.
522
ALl A. BADRAN
Acknowledgements---The work of Khalaf A1-Sirhan, who built the collector and Tayseer Qunnies, who tested it is acknowledged. Last but not least, the support of Yarmouk University, Irbid, Jordan which made this research possible is deeply appreciated.
NOMENCLATURE Ac CA Cp F' FR G I k rh m ta /re tfi /f.e.i
collector area, m 2 collector heat capacity, J/°C specific heat, J/°C kg collector efficiency factor collector heat removal factor mass flow rate per unit area, kg/m 2 s solar insolation, W/m 2 defined by eq. (6) mass flow rate, kg/s mass of collector, kg ambient temperature, °C outlet fluid temperature, °C inlet fluid temperature, °C initial outlet fluid temperature, °C
lf.e,T
outlet fluid temperature at time T, °C UL overall heat transfer loss factor (zct)~ effective transmittance-absorptance product r/ solar collector thermal efficiency
REFERENCES
1. J. T. Beard, Engineering analysis and testing of watertrickle solar collector. Report No. 2, ERDA ORO/492776, June 1976. 2. A. A. Badran, Utilization of solar energy by using black liquid collectors. S.M. Thesis, Massachusetts Institute of Technology, January 1979. 3. H. E1-Mulki, A. Jaradat, M. Qashou and R. Ta'ani, Solar and wind energy activities in Jordan. Proc. First Arab Int. Solar Energy Conf. Kuwait, 2-8 December 1983 (Edited by H. Alawi et al.), pp. 427-433. 4. J. A. Duffle and W. A. Beckman, Solar Energy Thermal Processes. John Wiley, New York (1974). 5. SHRAE 93-77 standard, Methods of testing for rating solar collectors based on thermal performance.
0741-983X/89 $3.00+ .00 Pergamon Press pie
THE WATER-TRICKLE CERAMIC SOLAR COLLECTOR ALl A. BADRAN Mechanical Engineering Department, Faculty of Engineering and Islamic Architecture, Umm A1 Qura University, Makkah, Saudi Arabia
(Received 29 September 1988 ; accepted 16 November 1988) Abstract--A new application for ceramic is investigated. The idea is to use the ceramic as an absorbing surface for fiat plate collectors. In this type of collector, water flows atop the absorbing surface and is exposed to sunrays. The water film is thin (1-2 ram) and it does not touch the glazing. Water is trickled from a header at the top of the collector down the ceramic surface. Solar radiation penetrates the water layer, gets absorbed by the ceramic and the energy is given back to water. Ceramic is selected as a material for the absorbing surface because it is mainly cheap, locally available in developing countries, and easy to manufacture in such countries. Besides, the ceramic surface presents a good solution for several problems practised with solar collectors : (i) It has a surface of granulated nature, which results in good spreading of the water film atop the absorber. (ii) It is superior in resisting the corrosion caused by the moisture inside the collector. (iii) It provides an ample space for the flow to proceed even if some precipitation of minerals carried by the water occurred on the absorbing surface. The collector is tested against a conventional flat-plate-type collector (with tubes atop a steel plate) and the performance of the two is compared. Experimental findings agree with the theoretical analysis of the idea. The instantaneous efficiency of the ceramic collector is about 5--17% higher than that of a certain conventional collector of the fine-tube-type.
1. INTRODUCTION One of the major problems facing flat-plate collectors is the high cost that results from using the fin-tube configuration for the absorber plate. Another problem which is inherent in such a configuration is the loss in the collector efficiency. Many design ideas are used to minimize the loss in fin efficiency. The reduction of spacing between tubes, increasing the tube diameter, improving bond conductance, and using highly conductive absorber plates do increase the fin efficiency but also increase the cost. One of the methods used to reduce the cost is the so-called "water trickle method" [1]. In this method water flows atop the absorber plate and is exposed to the glazing. The plate is corrugated such that the water is trickled down the grooves and the part of the plate that is between the grooves remains dry. The cost is reduced by this method because no tubes are used, but, unfortunately, the efficiency is not improved because :
face in a dropwise form. Dropwise condensation reduces the solar radiation reaching the absorber plate because the drops form a barrier of"lenses" attached to the glazing. The sun rays suffer multiple reflections through these liquid lenses in the off-noon hours. An improvement in the design [2] circumvented the first problem by eliminating the grooves completely. This resulted in a simple design, where the liquid flows atop a flat plate. To keep the uniformity of the flow, a wire mesh is attached to the plate, thereby utilizing surface tension force to spread the flow. The material used is galvanized steel for both the absorber plate and the wire mesh. The new improvement in this research is mainly in the material. The absorber plate is made of ceramic and the wire mesh is cancelled. Ceramic is selected for this rather unusual service because it has many useful characteristics : (a) Water flows uniformly on ceramic surface without the use of a wire mesh to spread it. (b) Ceramic is superior in resisting corrosion and that makes it a superb material for solar energy applications.
(i) the fin-tube configuration is essentially kept in the design, with the only difference being that the liquid is exposed directly to solar radiation and (ii) the liquid vapor is condensed on the glazing sur517
518
ALIA. BADRAN
Aluminium frame /Glazing /Ceramictiles Rubberseal / / /Mortar ,.I.~haa,h,r / / / /cov.,plato. ~ heaoer
Concretebase' / / Insulation// Envelope/ Fig. 1. Longitudinal cross-section in ceramic collector. (c) In areas of hard-water conventional solar collectors of the fin-tube type suffer from the precipitation of salts inside the tubes. (For instance, this has been practised in Irbid, Jordan where this research is carried out.) This problem occurs in conventional collectors mainly because they are subjected to a higher surface temperature. When precipitation occurs, it occurs inside the tubes, which are of limited area. But in water-trickle collectors, the problem of precipitation is not critical because water has enough space to flow through. In fact, with the use of ceramic, the problem is eliminated simply because the surface temperature of ceramic is less than that of steel. (d) Ceramic is a locally available material in almost every country. This feature is important for developing countries because it means less dependence on imported steel. Furthermore, it can be manufactured by non-technical communities from material already used for pottery. (e) Ceramic is an example of a building material that may be used for the construction of roofs. The ceramic collector can be considered, in fact, as a module which is part of a large scale system. A large part of the roof can be covered with glazing to form a large collector. This concept helps in utilizing already built roofs that have slopes facing south. All that is needed is to install the piping work and cover the necessary area with glazing. The ceramic collector is tested against a conventional flat plate type collector of the same area, insulation, glazing and envelope. Results show that this collector is favorably compared with conventional ones.
2. APPARATUS The collector consists basically of a concrete platform upon which ceramic tiles are fitted using cement
mortar to form the absorber part (Fig. 1). The absorber is placed inside a galvanized steel case with a 5 cm thick layer of glass wool insulation in between. A single glass cover, 4 mm thick, fitted to an aluminium frame is placed on the top. Inlet and outlet headers are placed at the top and the bottom made of 2.5 cm dia. galvanized steel pipe but the inlet has 20 holes, each of 2.5 mm dia. and the outlet is slotted. Figure 2 shows a photograph of the collector. It should be noted here that, in the beginning of this work, a great effort was spent in trying to make the absorber completely of ceramic. But large cracks, as wide as 2 cm developed in the clay after it was dried. Attempts were made to fill those cracks with clay but that was not successful either. The problem was finally solved by replacing part of the clay with a reinforced concrete casting and covering it with several pieces of ceramic (32 x 21 × 1 cm) that are made locally. In order to compare the performance of the collector with that of conventional ones, a fin-tube type collector is installed beside it. The conventional collector is chosen arbitrarily from those manufactured locally (Rum Co.). It is made according to the specifications of the Royal Scientific Society, Jordan. Therefore, this collector will be termed the RSS collector. The details of the specifications may be found in Ref. [3]. The test facility, shown in Fig. 3 follows in general what is recommended by A S H R A E 93-77 with minor modifications. Mainly, the direction of flow is reversed to suit the ceramic collector where the flow is from the top to the bottom. 3. PERFORMANCE ANALYSIS
The thermal performance of the solar collector is determined by obtaining: (i) the instantaneous efficiency under steady state conditions and (ii) the time response characteristics under transient conditions. The instantaneous efficiency of flat plate collectors is given [4] by the well-known equation q = Fr(t~)e - F r U u ( t r , - ta)/l. (1) This equation is valid for both collectors under consideration--the conventional, termed the RSS collector and the ceramic one. Since similar glazing, absorbing paint and insulation are used in both, the major difference between the two is in the expression of FR. Another important factor in the equation is Ut.. It is expected, however, that this factor is much less for the case of ceramic collector due to its low mean temperature.
Water-trickle ceramic solar collector
519
Fig. 2. A photograph of the collector.
Pyr'snomGRer \ a.
FR = [GCp/UL][1-exp(-FUL/GCp)]
(2)
and, for the ceramic collector, it can be shown [2] that
Therm~ Hot water I
tank I
I I
FR = [GCp/UL][1 --exp (-- UL/GCp).
By-pau
Flow mater
exchangor
Fig. 3. A sketch of the collector's testing arrangement.
(3)
It is obvious that the difference between the two expressions is that the efficiency factor F ' for the ceramic collector equals 1. Since F ' is always less than or equal to 1, then keeping it at the maximum is an improvement in the collector's efficiency. In the conventional RSS design, the tube spacing is about 10 cm and the diameter is 12 ram. F o r a steel plate of 1 mm thickness, the typical value of F ' ranges from 0.9 to 0.98, depending on the overall heat transfer coefficient. Therefore, it is expected that the ceramic collector shall have up to 10% increase in efficiency, provided that other factors remain the same.
520
ALl A. BADRAN
The time response analysis of the ceramic collector follows exactly those of the conventional ones. The ASHRAE 93-77 standard [5] gives the governing equation as
(CA/Ac) dtf/dT = FRI('c~)e-- FRUL(tfi -- ta)
be greater than 790 W/m2.) Inlet, outlet, and ambient temperatures are recorded, then the incident solar energy is abruptly reduced to zero by shielding the collector from the sun. The inlet and outlet temperatures are plotted as a function of time until the value of the expression
- (rhCp/Ac)(tfo- tr,) (4)
rhCp
FRUL(tf, i--ta) + --Ac (/r,e,T--/f,i) which results in the following expression for the exit temperature of the transfer fluid as a function of time :
FRI('~oOeFR UL(tfi --/a) - - ?~lfP/Ac)(tf,e,T -- tfi) FRI(z~)~ -- FR UL(tn -- ta) -- rhCp/Ac) (tf, e,i - tn) =
exp-
. . . . . . . . . . . . . . . .
FRUL(tr'i--ta) H'- Ac (tfx'initial--lf'i) where
FRI(r~)~ = zero during the test
rmc l (5)
where K = [rnCp/FRULAcl[(F/FR)-- 1].
(7)
~< 0.3
rhCp
then eq. (7) is plotted against time and the time constant is calculated. This is the time required for the above equation to equal l/e = 0,368.
(6)
The quantity KCA/rhC P is known as the time constant and is the time required for the quantity e -~cP/KcA to change from 1.0 to (I/e) = 0.368. It is expected that the time constant of the ceramic collector is one order of magnitude larger than that of the conventional one. The reason is that the heat capacity CA of the collector is much larger than that of the RSS collector. 4. TESTING PROCEDURE AND CALCULATIONS (a) Collector thermal efficiency According to the ASHRAE standard, the testing shall be conducted under steady state conditions. The two collectors are tilted at 47 '~ then the system is primed by the working fluid and then the inlet fluid temperature (t~) is adjusted and held constant. The system is operated with a flow rate of 0.02 kg/m 2 s at the working inlet temperature for 1.5-2 h before taking any data in order to reach a steady state. This period is necessary for the ceramic collector due to its large heat capacity. Then t~, tf,~, t,, and I are recorded each 15 min increment. Data are taken half an hour before and after noon and the average values of tf¢, ta and I are calculated for the same day. This procedure is repeated for different values of the inlet fluid temperature. Then the thermal efficiency and ( t n - ta)/I are calculated every day and plotted. A straight line is obtained and the equation of this line is found by the least squares method.
5. RESULTS The performance tests of the ceramic collector are performed first before the ceramic tiles are painted black. At the same time the RSS collector is tested and the results of both tests are plotted on the same chart (Fig. 4). After that the ceramic collector is painted black and test results are shown on the same chart (Fig. 4). In all cases, a straight line results and the equation of this line is found by the least squares method. It is found that, for the ceramic collector : q = 0.513-5.20 (tf, i-t~)/L before painting the absorber black r/= 0.592-6.22 ( t f j - t , ) / L after painting, and for the RSS collector : q = 0.439- 8.54 (t~,~- t~)/l.
~SO- 80 - Ceramic collector, painted black _ / Ceramiccollector, without painting 70 L I ~ / Conventional collector (RSS) 80
~ 2o
~ 10--~
(b) Collector time constant The test is done according to ASHRAE standard. The system is primed to reach steady state conditions at some inlet fluid temperature. (Solar insolation must
I
1
[
2
I
3
L
4
I
5
I
6
I
7
I
8
I
9
I
10
I
11
(tf. i-ta)/I X 10 - 3 , ~C mzNV
Fig. 4. Instantaneous efficiencycurves for ceramic collector vs conventional collector.
Water-trickle ceramic solar collector
contamination seems likely to take place in the collector. The poor performance observed for the RSS collector may be related to manufacturing defects. The published literature [3] about the original design of this collector reports a much better performance. This performance is reported in the form of the efficiency equation :
°L__ 1.0
0.6
.~ Io
O
•
0.4
0.2 o.o
"e~.Conventional
I 2o
I 4o
collector
I
eo
521
I
so
r/= 0.657-5.79 (Tf,i - Ta)/L I
loo
I
12e
1~
T, min
The selection of the RSS collector for comparison was merely for availability reasons. Most of the collectors in the area follow the same design.
Fig. 5. Time constant of ceramic collector vs conventional collector. 7. CONCLUSION The time constant experimental results are shown in Fig. 5 for both collectors. From the figure, it is found that the time constant is 73 min for the ceramic collector and 13.8 min for the RSS collector.
6. DISCUSSION As expected, the efficiency of the ceramic collector is higher than that of the conventional one, even before the ceramic surface is painted black. Naturally the efficiency is improved when the ceramic surface is painted black such that it became about 17% higher than that of the conventional design. The efficiency could not reach higher values due to the condensation of water droplets on the inner glass surface. This decreases the transmittance of the glass in the off-noon hours and increases the heat losses to the outside. The water flow was uniform on the ceramic surface and the problem of salt precipitation was solved in this type of collector by eliminating the tubes. Even after possible precipitation is assumed, the efficiency is still higher by about 5%. This figure is obtained by assuming that the absorptance of the surface when fully covered by salts is equal to that of the original ceramic surface. The time constant of the ceramic collector is high due to the large heat capacity of ceramic and concrete, which means low response. This is an additional feature for the collector, where the ceramic is utilized for energy storage. During the period of testing, which lasted for about 3 months, no algae growth was observed. It could be suggested that this period was not enough for the anticipated growth, but it is doubtful that the high temperature, which might reach up to 60°C inside the collector will permit such a growth. Therefore, no
The idea of using ceramic as an absorber surface for solar energy is found to be successful. The performance of the collector is superior to conventional ones such that a 17% increase in efficiency is obtained. Practically, ceramic did withstand the tough environment of moisture, heat and salts inside the collector. Economically, the fact that only one model is built makes it hard to conclude that it is really less in cost. There are, however, some problems faced with the ceramic collector. Mainly, the heavy weight (about 250 kg) of the collector made it difficult to handle. This weight, however, mainly belongs to the concrete base. The base does not have to be concrete; wood, sheet metal or any other light weight roofing material could be used. Another problem is that the ceramic surface, when painted, loses a part of its ability to spread the flow. A solution to this could be the use of a dye with the clay itself to make the ceramic black without the need for painting. One more important restriction on this type of collector is that it cannot be thermosyphonic. Therefore, forced circulation is necessary for such a system.
8. RECOMMENDATIONS Other material may be tried using the concept of water trickle collectors. Cement, as well as any other roofing material could be used as an absorber, as long as the surface properties enhance uniform spreading of the flow. A module of glazing, headers, proper frame and sealing medium is recommended to be built. This module could be installed on any sloping type roof to test its potential for solar energy collection. In future testing, it is recommended that the collectors' daily performance characteristics and long term performance characteristics under varying ambient conditions are obtained.
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ALl A. BADRAN
Acknowledgements---The work of Khalaf A1-Sirhan, who built the collector and Tayseer Qunnies, who tested it is acknowledged. Last but not least, the support of Yarmouk University, Irbid, Jordan which made this research possible is deeply appreciated.
NOMENCLATURE Ac CA Cp F' FR G I k rh m ta /re tfi /f.e.i
collector area, m 2 collector heat capacity, J/°C specific heat, J/°C kg collector efficiency factor collector heat removal factor mass flow rate per unit area, kg/m 2 s solar insolation, W/m 2 defined by eq. (6) mass flow rate, kg/s mass of collector, kg ambient temperature, °C outlet fluid temperature, °C inlet fluid temperature, °C initial outlet fluid temperature, °C
lf.e,T
outlet fluid temperature at time T, °C UL overall heat transfer loss factor (zct)~ effective transmittance-absorptance product r/ solar collector thermal efficiency
REFERENCES
1. J. T. Beard, Engineering analysis and testing of watertrickle solar collector. Report No. 2, ERDA ORO/492776, June 1976. 2. A. A. Badran, Utilization of solar energy by using black liquid collectors. S.M. Thesis, Massachusetts Institute of Technology, January 1979. 3. H. E1-Mulki, A. Jaradat, M. Qashou and R. Ta'ani, Solar and wind energy activities in Jordan. Proc. First Arab Int. Solar Energy Conf. Kuwait, 2-8 December 1983 (Edited by H. Alawi et al.), pp. 427-433. 4. J. A. Duffle and W. A. Beckman, Solar Energy Thermal Processes. John Wiley, New York (1974). 5. SHRAE 93-77 standard, Methods of testing for rating solar collectors based on thermal performance.