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Experimental study of a roof solar collector towards the natural ventilation of new houses
ELSEVIER
Energy and Buildings 26 ( 1997) 159-164
Experimental study of a roof solar collector towards the natural ventilation of new houses Joseph Khedari, Jongiit Hirunlabh *, Tika Bunnag Energy
Technology
Division,
School of Energy
and Materials, King Mongkut’s Institute Ban,pkok 10140, Thailand
of Technology
Thonburi,
Bangmod,
Rasburana,
Received 10 November 1995; revised 23 September 1996 -
Abstract The paper discusses the possibility of offering thermal comfort in new housing built in European style and situated in a hot and humid climate, without inducing mechanical energy cost, by means of a constructive element: the Roof Solar Collector (RSC). With this RSC it is possible, on the one hand, to minimize the fraction of the solar flux absorbed by the dwelling (insulation) and, on the other hand, to induce a natural ventilation which improves its thermal comfort. The influence of length and tilt angle of the RSC and local constructing materials used on the performance of the RSC is studied experimentally. The results of the study were that the appropriate materials of the roof solar collector, with regard to the improved natural ventilation, should be CPAC Monier concrete tiles on the outer side and gypsum board on the inner one. The optimum length of roof solar collector must be shorter on the order of 100 cm and tilted at 30”. The induced natural ventilation rate was about 0.08-0.15 m3 s-’ m-‘. 0 1997 Elsevier Science S.A. Keywords:
Roof solar collector;
Natural
ventilation;
Thermal
comfort -
1. Introduction The traditional Thai housewith a high gableroof, extended long eaves and harmonious integration with environment, was well designedto promote natural ventilation which provided a natural feeling of comfort [ I]. However, as it uses wood and special handicrafts, the present day cost of construction is very expensive. Thus, it hashad to be abandoned and hascreateda trend towardsthe useof bricks andconcrete. At present, various models of houses all over the country have been widely adaptedfrom Europeanstyles considering only the beauty of the outside and neglecting the thermal comfort of the residents;cooling is to be accomplishedby the addition of mechanicalair conditioning. Among the problemsof modernhousessituatedin tropical countries, the most important concernsthe heataccumulation under the roof structure making the upstairsroomshot in the afternoon [ 21. In Thailand, the roofs are constructed by using iron structure and CPAC Monier concrete tiles (the most widespread made by the Siam Cement Company LTD.), dark colored, red, blue, and black which can absorbheat well. The advantageousfeaturesof light colors, when used,are considerably * Corresponding
author.
037%7788/97/$17.00 PUSO378-7788(96)01030-4
0 1997 Elsevier
Science S.A. All rights reserved
Fig. 1. Schematic
representation
of a new house.
reducedbecauseof dust and dirt in the air. The lower beam usesconcrete with an iron foundation supportingthe wooden framing of the ceiling covered with microfibre, Fig. 1. Due to the solarradiation, the temperatureof air within the closed enclosureformed by the roof structure will increaseover the day andthe storedheat is transmittedinto the interiorthrough the adjoining parts of the microfibre of the ceiling. This leads
160
J. Khedari
et al. /Energy
and Buildings
to an uncomfortable feeling of warmth of the residents due to the high temperature of the upper space in room. At present, indigenous reflector aluminum foil has been used under the roofing tiles to protect the rooms from the heat stored within the roof enclosure, but the weather of Thailand is hot and humid, and water vapor, dust and dirt in the air make the aluminum foil deteriorate quickly and the heat is then transmitted to the inside of house. Virtually, houses can be designed and constructed to remain cool during hot weather. Depending on tolerance to high temperatures, it is possible to eliminate completely the need for mechanical cooling methods by using passive systems [ 3-51, although this sometimes requires extraordinary measures. By far the most effective means of solar cooling is to keep the sun’s energy out of the building. This is most effectively accomplished, while tropical landscale is away, by using movable devices shading the dwelling’s windows, walls, and roof but most designs deteriorate rapidly in many hot and humid climates. The other passive techniques such as thermal mass in buildings, evaporative cooling, and radiative cooling cannot be advantageous in tropical countries because the mean daily temperature is relatively high. At temperatures below 34°C which is the average summer temperature in many hot and humid conditions, air movement might be one of the most useful and least expensive methods to provide a comfortable indoor climate. The movement of air across human skin creates a cooling sensation caused by heat leaving the skin through convection and by the operation of perspiration. Air movement at a speed of up to 0.25 m/s [6,7] goes unnoticed. The most common way to create air movement without using mechanical power is to open a window and allow breezes to blow into a building. However, the problem with this simple concept is that an open window can admit dust, pollen, and dirt. Many variations of solar chimney have been used widely in the past, and many are being developed again today [ 81. In some active air solar systems, the collectors are vented to the outside during hot summer weather, pulling building air through them to induce ventilation. Using the roof to act as a solar collector is also well known [9-121 but to make air change in housing without mechanical energy cost is a new undertaking. In this study we try to analyse the behaviour of such a construction element, ‘the Roof Solar Collector,’ towards the natural ventilation of housing.
Table 2 Characteristics
of materials
26 (1997)
159-164
Air In (from Ambient: Experiment) (from House: System) Fig. 2. Schematic Table 1 Configurations
representation
of the Roof Solar Collector
(RSC).
studied -
Configuration
Upper material
RSC-M
CPAC Monier tiles CPAC Monier tiles CPAC Monier tiles
1
RSC-M2 RSCM3
Low’er material concrete concrete concrete
plywood lining aluminum foil gypsum board
with
gypsum board lining aluminum foil -
with
2. Roof solar collector, RX The RSC is comprisedmainly of two ‘parts,as shown in Fig. 2. Three configurations were madeby using local materials as indicated in Table 1. The surface area of the RSC was set equal to 2 rn* (2 X 1 m) and the gap between roof and insulator was fixed at 14 cm. The lateral right and left sidesof RX are madeby using wood (200X7 X2 cm) and iron (200X7.62 X0 1 cm). They are covered with aluminum foil and painted dark red color. The ground structure madeof iron allows to adjust the RSC’s tilt. The characteristicsand thermal conductivity of materialsusedare given in Table 2.
3. Experimental methods A hot wire anemometer(range: O-50 m/s) was used to measurethe collector air velocity. The velocity and temper-
used
Material
Dimensions (cm)
( L X W XT)
CPAC Monier tile (dark red) Plywood with aluminum sheet Gypsum board Gypsum board with aluminum sheet
33X42X 1.5 100x200x 1.1 100~200~0.9 100x200x 1.0
Weight (kg/piece)
Measured thermal (W/m K)
4.4 2.5 2.2 2.2
0.1463 0.1542 0.0873 0.1258
conductivity
J. Khedari
et al. /Energy
and Buildings
26 (1997)
159-164
161
60 cm
30 cm
-
L
+
Temperature
A
Air Velocity
200 cm
1,
10
variations
40
of the thermocouples
and anemometers
setting in the RSC.
results
10
13
study: closed roof solar collector
To demonstrate the feasibility of natural ventilation by using a roof, a preliminary experiment was made. The configuration retained was RSC-Ml and tilted at 30”. Both sections of inlet and outlet air had been closed using a wood plate. As indicated in Fig. 4, the temperature of air within the RSC’s enclosure was high enough to establish a density difference with the ambient air and to induce a natural air circulation. Also the stored heat in this enclosure, (the enclosed space in Fig. 1, would be transmitted to the interior of house through the adjoining part of the ceiling. 4.2. EfSect of insulating material Figs. 5 and 6 show that, when the intensity of radiation was relatively similar for the 3 days of experiments, the air
of air temperature
--
14
15
16
of the closed RSC-M
; 1919194 ; 8/l l/94 ; 519194
70
---------
Length
1
5 !! B e $J
1 ( LY= 30”;
at 10a.m. at 1 p.m. at 4 p.m.
130
190
(cm)
Fig. 5. Average temperature of air inside the configurations M2, and RSC-MS vs. length at different times ( LY= 30”). 40
Testing the performance of the RSC had been undertaken on different days corresponding to different ambient conditions. It was not easy to make a comparison between the different results because of the various climatic conditions. However, general and subjective conclusions were formulated. 4.1. Preliminary
12
0 RCS-M2 0 RCS-M3 A RCS-Ml
- 1
ature of ambient air were measured by using a propeller anemometer. Thermocouples of type K (range: O-l 250 “C) were used to measure the temperature of the air, the inner sides of the CPAC Monier tiles and the insulating materials at different points of the RSC as shown in Fig. 3. The RSC was placed facing the south on the top of the building of the school (on the 12th floor) where no shadow shielded the surface area of the collector. Experiments were performed from 9 a.m. up 4 p.m. [ 131.
4. Experimental
I 11
Time (hr)
[w
Fig. 3. Position
I
9
Fig. 4. Hourly 17/08/94).
100 cm l4cm
25 !
0 RCS-M2 0 RCS-M3
; 1919194 ; 8/l l/94
A RCS-Ml
; 519194
----
RSC-M
1, RSC-
13Ocm 190 cm
35
30
E3 25:-9
10
11
12
13
14
15
16
Time (hr) Fig. 6. Hourly variations of the average air temperature inside the configurations RSC-Ml, RSC-M2, and RSC-M3 at two positions ( IY = 30”).
temperature inside the configuration RSC-Ml was lower than that of the other two configurations. This is due to plywood which increases heat losses from the back side. After 1 p.m. (Fig. 6), the temperature decreased rapidly due to the end wind effect. Thus, it was not recommended for constructing the RSC. It can also be seen that the performances of configurations RSC-M2 and RSC-M3 were similar (Fig. 6) up to noon (except 11 a.m.) (the intensity of radiation was relatively similar). Consequently, aluminum sheet was not chosen
162
J. Khedari
et al. /Energy
and Buildings
because it did not improve the performance of the RSC and has a relatively high cost. Figs. 7 and 8 illustrate the longitudinal and hourly variations of the natural air velocity of the roof solar collector for the different configurations studied, respectively. It can be seen that the velocity of air is a close function of the ambient conditions. In fact, the induced ventilation rate is proportional to the difference in temperature at the inlet and the outlet of the RSC and the height of the ventilation path, i.e., vertical distance between the openings. Thus, the hotter the day, the greater the difference between the temperature of outgoing air from the RSC and the ambient air, and the faster the air velocity. From these results, it was seemed that the configuration RSC-M2 was more appropriate for new houses from both the thermal and the economical points of view. More attention was made to analyse its performance. 4.3. Effect of the tilt angle of the roof solar collector With the radiation reaching the surface area of roof held constant, increasing the tilt angle of the RSC tended increase the natural air flow rate. In our study, the heat absorbed by CPAC Monier tiles depends on solar incidence angle, season and tilt angle of the RSC, (Y.The higher the angle (Y,the lower 1.5 1
0 RCS-M2 0 RCS-M3 A RCS-Ml
; 1919194 ; 8/11/94 ; 519194
--------
26 (1997)
159-164
is the energy absorbed. This could explain the variation of air velocity presented in Fig. 9 for different tilt angles. For cx= 15”, the energy absorbed by tiles is maximized (for Bangkok, the annual optimum tilt angle for solar collector is equal to 15”). However, increasing the tilt angle increased the induced air flow rate. For tr=45”, the energy absorbed by tiles decreased considerably, which also reduced the amount of the natural air flow rate. Therefore (Y= 30”, seemed to be a good compromise and led to an optimum natural air flow rate. 4.4. Effect of the length of the roof solar coi’lector From Figs. 5, 6 and 10 which gives the hourly variations of the average temperature of CPAC Monier tiles, air, and gypsum board with collector length, it was found that at lengths greater than 100 cm the air temperature was nearly constant. Thus, the temperature difference between the air inside the RSC and the ambient air was almost constant. Therefore, between 100 to 200 cm, the induced air flow rate increased mainly due to the increasing of the ventilation path, i.e., vertical height. This explained the variations of air veloc-
1.5
atlOa.m. at 1 p.m.
OI-100
10
Length (cm) Fig. 9. Variations of air velocity of the configuration RSC-M2 vs. length for different tilt angles of the roof solar collector at different times.
100 Length Fig. 7. Air velocity of the configurations vs. length at different times ((Y = 30”). 2
1
0 RCS-M2 0 RCS-M3 A RCS-MI
(cm) RX-Ml,
; 19/9/94 ; 8/l l/94 ; 5t9i94
RSC-M2,
and RSC-M3
I
10
11
12
--__ -at
cl Monia 0 Air
at 130cm 190cm
- - - - at 100 cm -at 190cm
0-l
9
190
13
14
Time (hr) Fig. 8. Hourly variations of air velocity of the configurations M2, and RSC-MS at two positions ((Y = 30”).
1.5
RSC-M
16
1, RSC-
9
10
11
12 Time
13
I
Ambient
14
15
Tiles
16
(hr)
Fig. 10. Hourly variations of the average temperatures of CPAC Monier tiles, Air and Gypsum board for the configuration RSC-M2 at two positions (cr=30”; 19/9/94).
J. Khedari
et al. /Energy
and Buildings
26 (1997)
159-164
163
0.18 , 0.16 -
0 ! 9
Fig. 11. Hourly
u 2
350
2 ‘FI SW ij 4
250 200
it! E
150
I
12 13 14 Tie (hr) of the natural air flow rate per unit area of the roof solar collector
variations
lo
11
15
16
at two positions
( cx = 30”; 19/9/94),
air temperature exceeds the human body core temperature ( 37°C)) this ventilation would reduce considerably the rate of the heat transferredto ceiling and might be the mosteffective measurefor saving cooling energy and improving cooling efficiency.
300
W
D 5 d
100
5. Conclusions
50 0 9
10
11
12
13
Time (hr) Fig. 12. Hourly variations of the energy evacuated at two positions ((Y = 30”; 19/9/94)
14
15
16
by the roof solar collector
ity shown in Fig. 9 and those of the natural air flow rate plotted per unit area of RSC at two positions in Fig. 11. By knowing the flow rate of natural air circulation (Fig. 11) and the temperature difference between the outgoing air collector and the ambient air (Fig. lo), the energy evacuated per by the RSC could be estimated, Fig. 12. To optimize natural ventilation, the length of the roof solar collector should be shorter on the order of 100 cm. 4.5. Design and pe$ormance system
of the roof solar collector
By knowing the required rate of ventilation demand,the number of RSC units could be estimated. The RSC units of this systemshouldbe installedin parallel. The adjoining parts of CPAC Monier concrete tiles and gypsum board shouldbe well sealedto protect the air leakage and to avoid damage from rain. One advantageof this systemis its ability to self balance; the hotter the day, the hotter the air of the RSC and the faster the air movement. This outlined strategy would completely be effective in avoiding the needfor air conditioning during winter (November-January) and partially during the monsoon season (June-November) _During Thailand’s summer(FebruaryMay, known as the hot-hot season),although the induced ventilation ceasesto provide effective cooling asthe ambient
The use of the roof to act as a solar collector to induce natural ventilation of housing has been studied experimentally. The effect of insulating material, the tilt and the length of the Roof Solar Collector (RSC) have been investigated. The configuration using CPAC Monier concrete tiles on the outer side and gypsum board on the inner one, the socalled RSC-M2 system, seemedto be the most appropriate for suchapplication. Its optimum dimensionsare the following: short length about 100cm; tilt at 30”; and the spaceplates equalto 14cm. The ratesof natural air ventilation andenergy evacuated by the RSC were about 0.08Xj.15 m3 SK’ m-*, and 150-350 W m-*, respectively. The proposedsystem is also economical becausethere is potentially little extra charge sinceall its elementsarealready usedin construction of the new house. However, how effective sucha device would be in aiding ventilation and reducing interior air temperaturesin a real building will have to await full-scale testing of sucha building using an array of collectors.
References [ 11 T. Buranasomphob, Energy conservation in building design: a case study of a traditional style house, ASEAN-EC Energy Conservation Seminar, Bangkok, 14-18 Dec., 1987, pp. 177-181,. [ 21 D.S. Parker, Measured air-conditioning and thermal performance of a Thai residential building, Energy, 20 ( 1995) 907. [3] A.A.M. Sayigh, Solor Energy Application in Buildings, Academic Press, New York, 1979, pp. 147-166. [4] B. Givoni, Options and applications of passive cooling. Energy Build., 7 ( 1984) 297. [ 51 V.R. Nostron, Passive Solar Design Handbook, Wiley, New York, 1984, pp. 68-75,
164
J. Khednri
et al. /Energy
[6] F.C. Mcquiston and J.D. Parker, Heating Ventilating and Air Conditioning Analysis and Design, Wiley, New York, 1977, pp. 5381. [7] ASHRAE, ASHRAE Standard 62-73: Standards for Natural and Mechanical Ventilation, American Society of Heating, Refrigerating and Air-conditioning Engineers, New York, 1973, p, 10. [ 81 F. Al-Hamadani, L. Caignault and A. Zoulalian, Etude d’un bardage thermique en vue de la climatisation par ventilation dune habitation situee dans une region chaude et humide, Enrropoie, N.139, 1988. pp. 24-34. [9] F.C. O’Brien-Bemini and J.G. McGowan, Performance modeling of non-metallic flat plate solar collectors, Solar Energy, 33 ( 1984) 305.
and Buildings
26 (1997)
159-164
[ 101 C. Choudhury, S.L. Andersen and J. Rekstad, A solar air heater for low temperature applications, Solar Energy, 40 ( 1988) 335. [ 111 D.L. Loveday, Thermal performance of air-heating solar collectors with thick, poorlyconductingabsorber plates,Solnr Energy, 41 ( 1988) 593. [ 121 H. Benali, Y. Sfaxi and M. Grignon, Modelisation thermique d’une toiture solaire-Application a la recuperation de l’tnergie solaire dans I’habitat, Journee Intemationale de Thermique, Alexandtie, Egypt, 1922 April 1993. [ 131 T. Bunnag, A study of a roof solar collector towards the natural ventilation of new habitations, Masters Them, Degree, Ring Mongkut’s Institute of Technology, Thonburi, Thailand, 1995.
Energy and Buildings 26 ( 1997) 159-164
Experimental study of a roof solar collector towards the natural ventilation of new houses Joseph Khedari, Jongiit Hirunlabh *, Tika Bunnag Energy
Technology
Division,
School of Energy
and Materials, King Mongkut’s Institute Ban,pkok 10140, Thailand
of Technology
Thonburi,
Bangmod,
Rasburana,
Received 10 November 1995; revised 23 September 1996 -
Abstract The paper discusses the possibility of offering thermal comfort in new housing built in European style and situated in a hot and humid climate, without inducing mechanical energy cost, by means of a constructive element: the Roof Solar Collector (RSC). With this RSC it is possible, on the one hand, to minimize the fraction of the solar flux absorbed by the dwelling (insulation) and, on the other hand, to induce a natural ventilation which improves its thermal comfort. The influence of length and tilt angle of the RSC and local constructing materials used on the performance of the RSC is studied experimentally. The results of the study were that the appropriate materials of the roof solar collector, with regard to the improved natural ventilation, should be CPAC Monier concrete tiles on the outer side and gypsum board on the inner one. The optimum length of roof solar collector must be shorter on the order of 100 cm and tilted at 30”. The induced natural ventilation rate was about 0.08-0.15 m3 s-’ m-‘. 0 1997 Elsevier Science S.A. Keywords:
Roof solar collector;
Natural
ventilation;
Thermal
comfort -
1. Introduction The traditional Thai housewith a high gableroof, extended long eaves and harmonious integration with environment, was well designedto promote natural ventilation which provided a natural feeling of comfort [ I]. However, as it uses wood and special handicrafts, the present day cost of construction is very expensive. Thus, it hashad to be abandoned and hascreateda trend towardsthe useof bricks andconcrete. At present, various models of houses all over the country have been widely adaptedfrom Europeanstyles considering only the beauty of the outside and neglecting the thermal comfort of the residents;cooling is to be accomplishedby the addition of mechanicalair conditioning. Among the problemsof modernhousessituatedin tropical countries, the most important concernsthe heataccumulation under the roof structure making the upstairsroomshot in the afternoon [ 21. In Thailand, the roofs are constructed by using iron structure and CPAC Monier concrete tiles (the most widespread made by the Siam Cement Company LTD.), dark colored, red, blue, and black which can absorbheat well. The advantageousfeaturesof light colors, when used,are considerably * Corresponding
author.
037%7788/97/$17.00 PUSO378-7788(96)01030-4
0 1997 Elsevier
Science S.A. All rights reserved
Fig. 1. Schematic
representation
of a new house.
reducedbecauseof dust and dirt in the air. The lower beam usesconcrete with an iron foundation supportingthe wooden framing of the ceiling covered with microfibre, Fig. 1. Due to the solarradiation, the temperatureof air within the closed enclosureformed by the roof structure will increaseover the day andthe storedheat is transmittedinto the interiorthrough the adjoining parts of the microfibre of the ceiling. This leads
160
J. Khedari
et al. /Energy
and Buildings
to an uncomfortable feeling of warmth of the residents due to the high temperature of the upper space in room. At present, indigenous reflector aluminum foil has been used under the roofing tiles to protect the rooms from the heat stored within the roof enclosure, but the weather of Thailand is hot and humid, and water vapor, dust and dirt in the air make the aluminum foil deteriorate quickly and the heat is then transmitted to the inside of house. Virtually, houses can be designed and constructed to remain cool during hot weather. Depending on tolerance to high temperatures, it is possible to eliminate completely the need for mechanical cooling methods by using passive systems [ 3-51, although this sometimes requires extraordinary measures. By far the most effective means of solar cooling is to keep the sun’s energy out of the building. This is most effectively accomplished, while tropical landscale is away, by using movable devices shading the dwelling’s windows, walls, and roof but most designs deteriorate rapidly in many hot and humid climates. The other passive techniques such as thermal mass in buildings, evaporative cooling, and radiative cooling cannot be advantageous in tropical countries because the mean daily temperature is relatively high. At temperatures below 34°C which is the average summer temperature in many hot and humid conditions, air movement might be one of the most useful and least expensive methods to provide a comfortable indoor climate. The movement of air across human skin creates a cooling sensation caused by heat leaving the skin through convection and by the operation of perspiration. Air movement at a speed of up to 0.25 m/s [6,7] goes unnoticed. The most common way to create air movement without using mechanical power is to open a window and allow breezes to blow into a building. However, the problem with this simple concept is that an open window can admit dust, pollen, and dirt. Many variations of solar chimney have been used widely in the past, and many are being developed again today [ 81. In some active air solar systems, the collectors are vented to the outside during hot summer weather, pulling building air through them to induce ventilation. Using the roof to act as a solar collector is also well known [9-121 but to make air change in housing without mechanical energy cost is a new undertaking. In this study we try to analyse the behaviour of such a construction element, ‘the Roof Solar Collector,’ towards the natural ventilation of housing.
Table 2 Characteristics
of materials
26 (1997)
159-164
Air In (from Ambient: Experiment) (from House: System) Fig. 2. Schematic Table 1 Configurations
representation
of the Roof Solar Collector
(RSC).
studied -
Configuration
Upper material
RSC-M
CPAC Monier tiles CPAC Monier tiles CPAC Monier tiles
1
RSC-M2 RSCM3
Low’er material concrete concrete concrete
plywood lining aluminum foil gypsum board
with
gypsum board lining aluminum foil -
with
2. Roof solar collector, RX The RSC is comprisedmainly of two ‘parts,as shown in Fig. 2. Three configurations were madeby using local materials as indicated in Table 1. The surface area of the RSC was set equal to 2 rn* (2 X 1 m) and the gap between roof and insulator was fixed at 14 cm. The lateral right and left sidesof RX are madeby using wood (200X7 X2 cm) and iron (200X7.62 X0 1 cm). They are covered with aluminum foil and painted dark red color. The ground structure madeof iron allows to adjust the RSC’s tilt. The characteristicsand thermal conductivity of materialsusedare given in Table 2.
3. Experimental methods A hot wire anemometer(range: O-50 m/s) was used to measurethe collector air velocity. The velocity and temper-
used
Material
Dimensions (cm)
( L X W XT)
CPAC Monier tile (dark red) Plywood with aluminum sheet Gypsum board Gypsum board with aluminum sheet
33X42X 1.5 100x200x 1.1 100~200~0.9 100x200x 1.0
Weight (kg/piece)
Measured thermal (W/m K)
4.4 2.5 2.2 2.2
0.1463 0.1542 0.0873 0.1258
conductivity
J. Khedari
et al. /Energy
and Buildings
26 (1997)
159-164
161
60 cm
30 cm
-
L
+
Temperature
A
Air Velocity
200 cm
1,
10
variations
40
of the thermocouples
and anemometers
setting in the RSC.
results
10
13
study: closed roof solar collector
To demonstrate the feasibility of natural ventilation by using a roof, a preliminary experiment was made. The configuration retained was RSC-Ml and tilted at 30”. Both sections of inlet and outlet air had been closed using a wood plate. As indicated in Fig. 4, the temperature of air within the RSC’s enclosure was high enough to establish a density difference with the ambient air and to induce a natural air circulation. Also the stored heat in this enclosure, (the enclosed space in Fig. 1, would be transmitted to the interior of house through the adjoining part of the ceiling. 4.2. EfSect of insulating material Figs. 5 and 6 show that, when the intensity of radiation was relatively similar for the 3 days of experiments, the air
of air temperature
--
14
15
16
of the closed RSC-M
; 1919194 ; 8/l l/94 ; 519194
70
---------
Length
1
5 !! B e $J
1 ( LY= 30”;
at 10a.m. at 1 p.m. at 4 p.m.
130
190
(cm)
Fig. 5. Average temperature of air inside the configurations M2, and RSC-MS vs. length at different times ( LY= 30”). 40
Testing the performance of the RSC had been undertaken on different days corresponding to different ambient conditions. It was not easy to make a comparison between the different results because of the various climatic conditions. However, general and subjective conclusions were formulated. 4.1. Preliminary
12
0 RCS-M2 0 RCS-M3 A RCS-Ml
- 1
ature of ambient air were measured by using a propeller anemometer. Thermocouples of type K (range: O-l 250 “C) were used to measure the temperature of the air, the inner sides of the CPAC Monier tiles and the insulating materials at different points of the RSC as shown in Fig. 3. The RSC was placed facing the south on the top of the building of the school (on the 12th floor) where no shadow shielded the surface area of the collector. Experiments were performed from 9 a.m. up 4 p.m. [ 131.
4. Experimental
I 11
Time (hr)
[w
Fig. 3. Position
I
9
Fig. 4. Hourly 17/08/94).
100 cm l4cm
25 !
0 RCS-M2 0 RCS-M3
; 1919194 ; 8/l l/94
A RCS-Ml
; 519194
----
RSC-M
1, RSC-
13Ocm 190 cm
35
30
E3 25:-9
10
11
12
13
14
15
16
Time (hr) Fig. 6. Hourly variations of the average air temperature inside the configurations RSC-Ml, RSC-M2, and RSC-M3 at two positions ( IY = 30”).
temperature inside the configuration RSC-Ml was lower than that of the other two configurations. This is due to plywood which increases heat losses from the back side. After 1 p.m. (Fig. 6), the temperature decreased rapidly due to the end wind effect. Thus, it was not recommended for constructing the RSC. It can also be seen that the performances of configurations RSC-M2 and RSC-M3 were similar (Fig. 6) up to noon (except 11 a.m.) (the intensity of radiation was relatively similar). Consequently, aluminum sheet was not chosen
162
J. Khedari
et al. /Energy
and Buildings
because it did not improve the performance of the RSC and has a relatively high cost. Figs. 7 and 8 illustrate the longitudinal and hourly variations of the natural air velocity of the roof solar collector for the different configurations studied, respectively. It can be seen that the velocity of air is a close function of the ambient conditions. In fact, the induced ventilation rate is proportional to the difference in temperature at the inlet and the outlet of the RSC and the height of the ventilation path, i.e., vertical distance between the openings. Thus, the hotter the day, the greater the difference between the temperature of outgoing air from the RSC and the ambient air, and the faster the air velocity. From these results, it was seemed that the configuration RSC-M2 was more appropriate for new houses from both the thermal and the economical points of view. More attention was made to analyse its performance. 4.3. Effect of the tilt angle of the roof solar collector With the radiation reaching the surface area of roof held constant, increasing the tilt angle of the RSC tended increase the natural air flow rate. In our study, the heat absorbed by CPAC Monier tiles depends on solar incidence angle, season and tilt angle of the RSC, (Y.The higher the angle (Y,the lower 1.5 1
0 RCS-M2 0 RCS-M3 A RCS-Ml
; 1919194 ; 8/11/94 ; 519194
--------
26 (1997)
159-164
is the energy absorbed. This could explain the variation of air velocity presented in Fig. 9 for different tilt angles. For cx= 15”, the energy absorbed by tiles is maximized (for Bangkok, the annual optimum tilt angle for solar collector is equal to 15”). However, increasing the tilt angle increased the induced air flow rate. For tr=45”, the energy absorbed by tiles decreased considerably, which also reduced the amount of the natural air flow rate. Therefore (Y= 30”, seemed to be a good compromise and led to an optimum natural air flow rate. 4.4. Effect of the length of the roof solar coi’lector From Figs. 5, 6 and 10 which gives the hourly variations of the average temperature of CPAC Monier tiles, air, and gypsum board with collector length, it was found that at lengths greater than 100 cm the air temperature was nearly constant. Thus, the temperature difference between the air inside the RSC and the ambient air was almost constant. Therefore, between 100 to 200 cm, the induced air flow rate increased mainly due to the increasing of the ventilation path, i.e., vertical height. This explained the variations of air veloc-
1.5
atlOa.m. at 1 p.m.
OI-100
10
Length (cm) Fig. 9. Variations of air velocity of the configuration RSC-M2 vs. length for different tilt angles of the roof solar collector at different times.
100 Length Fig. 7. Air velocity of the configurations vs. length at different times ((Y = 30”). 2
1
0 RCS-M2 0 RCS-M3 A RCS-MI
(cm) RX-Ml,
; 19/9/94 ; 8/l l/94 ; 5t9i94
RSC-M2,
and RSC-M3
I
10
11
12
--__ -at
cl Monia 0 Air
at 130cm 190cm
- - - - at 100 cm -at 190cm
0-l
9
190
13
14
Time (hr) Fig. 8. Hourly variations of air velocity of the configurations M2, and RSC-MS at two positions ((Y = 30”).
1.5
RSC-M
16
1, RSC-
9
10
11
12 Time
13
I
Ambient
14
15
Tiles
16
(hr)
Fig. 10. Hourly variations of the average temperatures of CPAC Monier tiles, Air and Gypsum board for the configuration RSC-M2 at two positions (cr=30”; 19/9/94).
J. Khedari
et al. /Energy
and Buildings
26 (1997)
159-164
163
0.18 , 0.16 -
0 ! 9
Fig. 11. Hourly
u 2
350
2 ‘FI SW ij 4
250 200
it! E
150
I
12 13 14 Tie (hr) of the natural air flow rate per unit area of the roof solar collector
variations
lo
11
15
16
at two positions
( cx = 30”; 19/9/94),
air temperature exceeds the human body core temperature ( 37°C)) this ventilation would reduce considerably the rate of the heat transferredto ceiling and might be the mosteffective measurefor saving cooling energy and improving cooling efficiency.
300
W
D 5 d
100
5. Conclusions
50 0 9
10
11
12
13
Time (hr) Fig. 12. Hourly variations of the energy evacuated at two positions ((Y = 30”; 19/9/94)
14
15
16
by the roof solar collector
ity shown in Fig. 9 and those of the natural air flow rate plotted per unit area of RSC at two positions in Fig. 11. By knowing the flow rate of natural air circulation (Fig. 11) and the temperature difference between the outgoing air collector and the ambient air (Fig. lo), the energy evacuated per by the RSC could be estimated, Fig. 12. To optimize natural ventilation, the length of the roof solar collector should be shorter on the order of 100 cm. 4.5. Design and pe$ormance system
of the roof solar collector
By knowing the required rate of ventilation demand,the number of RSC units could be estimated. The RSC units of this systemshouldbe installedin parallel. The adjoining parts of CPAC Monier concrete tiles and gypsum board shouldbe well sealedto protect the air leakage and to avoid damage from rain. One advantageof this systemis its ability to self balance; the hotter the day, the hotter the air of the RSC and the faster the air movement. This outlined strategy would completely be effective in avoiding the needfor air conditioning during winter (November-January) and partially during the monsoon season (June-November) _During Thailand’s summer(FebruaryMay, known as the hot-hot season),although the induced ventilation ceasesto provide effective cooling asthe ambient
The use of the roof to act as a solar collector to induce natural ventilation of housing has been studied experimentally. The effect of insulating material, the tilt and the length of the Roof Solar Collector (RSC) have been investigated. The configuration using CPAC Monier concrete tiles on the outer side and gypsum board on the inner one, the socalled RSC-M2 system, seemedto be the most appropriate for suchapplication. Its optimum dimensionsare the following: short length about 100cm; tilt at 30”; and the spaceplates equalto 14cm. The ratesof natural air ventilation andenergy evacuated by the RSC were about 0.08Xj.15 m3 SK’ m-*, and 150-350 W m-*, respectively. The proposedsystem is also economical becausethere is potentially little extra charge sinceall its elementsarealready usedin construction of the new house. However, how effective sucha device would be in aiding ventilation and reducing interior air temperaturesin a real building will have to await full-scale testing of sucha building using an array of collectors.
References [ 11 T. Buranasomphob, Energy conservation in building design: a case study of a traditional style house, ASEAN-EC Energy Conservation Seminar, Bangkok, 14-18 Dec., 1987, pp. 177-181,. [ 21 D.S. Parker, Measured air-conditioning and thermal performance of a Thai residential building, Energy, 20 ( 1995) 907. [3] A.A.M. Sayigh, Solor Energy Application in Buildings, Academic Press, New York, 1979, pp. 147-166. [4] B. Givoni, Options and applications of passive cooling. Energy Build., 7 ( 1984) 297. [ 51 V.R. Nostron, Passive Solar Design Handbook, Wiley, New York, 1984, pp. 68-75,
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et al. /Energy
[6] F.C. Mcquiston and J.D. Parker, Heating Ventilating and Air Conditioning Analysis and Design, Wiley, New York, 1977, pp. 5381. [7] ASHRAE, ASHRAE Standard 62-73: Standards for Natural and Mechanical Ventilation, American Society of Heating, Refrigerating and Air-conditioning Engineers, New York, 1973, p, 10. [ 81 F. Al-Hamadani, L. Caignault and A. Zoulalian, Etude d’un bardage thermique en vue de la climatisation par ventilation dune habitation situee dans une region chaude et humide, Enrropoie, N.139, 1988. pp. 24-34. [9] F.C. O’Brien-Bemini and J.G. McGowan, Performance modeling of non-metallic flat plate solar collectors, Solar Energy, 33 ( 1984) 305.
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26 (1997)
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[ 101 C. Choudhury, S.L. Andersen and J. Rekstad, A solar air heater for low temperature applications, Solar Energy, 40 ( 1988) 335. [ 111 D.L. Loveday, Thermal performance of air-heating solar collectors with thick, poorlyconductingabsorber plates,Solnr Energy, 41 ( 1988) 593. [ 121 H. Benali, Y. Sfaxi and M. Grignon, Modelisation thermique d’une toiture solaire-Application a la recuperation de l’tnergie solaire dans I’habitat, Journee Intemationale de Thermique, Alexandtie, Egypt, 1922 April 1993. [ 131 T. Bunnag, A study of a roof solar collector towards the natural ventilation of new habitations, Masters Them, Degree, Ring Mongkut’s Institute of Technology, Thonburi, Thailand, 1995.