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Diurnal pattern of global solar radiation in the tropics: a case study in Malaysia
Renewabh, Energy Vol. 3, No. 6/7, pp. 741-745, 1993 Printed in Great Britain.
0960-1481/93 $6.00+.00 (~; 1993 Pergamon Press Ltd
DATA BANK Diurnal pattern of global solar radiation in the Tropics: a case study in Malaysia MOHD. YUSOF HJ. OTHMAN, KAMARUZZAMAN SOPIAN, BAHARUDIN YATIM a n d MOHD. NOH DALIMIN Solar Energy Research Group, University Kebangsaan Malaysia, 43600 Bangi, Malaysia
(Received 28 October 1992; accepted 25 November 1992) Abstract--Global solar radiation patterns in Bangi (2°56'7.3"N, I05°47'0.2"E), Malaysia are discussed. The frequency of occurrence of the various solar radiation patterns observed over a 5-year period is derived. These observations will provide useful information for the design of solar energy systems and equipment for installation in tropical countries having a similar global solar radiation pattern.
1. INTRODUCTION It is widely known that the information on diurnal variation of global solar radiation at a location is vital before any solar energy equipment can be installed [l]. The information on sunshine hour, solar radiation intensity and diurnal variation of global solar radiation will help the designer to design a suitable solar system for a particular application, such as photovoltaic water pumping system, highway signal, remote communication facilities and many other applications including solar thermal and drying systems. The diurnal variation of global solar radiation received at a location is seldom represented by an idealized sine curve. In countries experiencing significant cloud cover, this idealized curve can hardly be observed. The ideal curve can only be seen if there is no interference from clouds and other objects which obstruct the incoming sunlight to the ground. Therefore, the diurnal variation of sunlight differs from one place to another. In tropical countries like Malaysia, the cloud pattern can be highly variable due to the high humidity and unpredictable weather, especially during the monsoon seasons. In order to understand the climatic conditions and solar radiation pattern, a geographical background of the landmass is briefly described [2]. Mainland Malaysia is a peninsula lying between latitudes 1.30 and 6.60°N, and longitudes 99.50 and 103.30°E. It is divided into the east and west coastal plains by the Main Range running northwest to southeast. Bangi is located in the west coastal plain at latitude 2°56'7.3"N, 105°47'0.2"E. The climate in Bangi, as in many places of the region, is mainly influenced by the system of the Asian monsoons. The Southwest monsoon brings rain and cloud to the northwest coast from June to early October, while the northeast monsoon brings rain and cloud to the east coast from November to March. Therefore the west coast is dry from November to March while the east coast is dry from June to September. During the dry season, the climate is hot and sunny, with intermittent breaks of cloud formation and hence rainfall in the late afternoon due to convection currents. The cold season is absent and seasonality is marked by rainfall rather than temperature. 741
The equatorial climate is characterized by heavy rainfall, constantly high temperature and relative humidity. There are two exceptionally wet belts in the peninsula with a total rainfall of more than 275 cm/year. The first belt extends the entire length of the eastern part of the rainfall in November and December. The other is confined to the northern half of the western side of the peninsula and receives heavy rainfall twice a year, in April-May and Octobe~November. Much of the precipitation occurs as thunderstorms and the normal pattern is one of heavy falls within short periods. Generally, the chances of rain falling in the afternoon or early evening are high compared with that in the morning. Much of Peninsular Malaysia has more than 170 rainy days ; however, an area may have a great number of rainy days and yet receive a lesser amount of rain in a year than another area with a smaller number of rainy days but receiving its rain in heavy spells. Temperature remains uniformly high over the peninsula throughout the year. Average temperatures are between 25.6 and 27.8°C. Most locations have a relative humidity of 8 ~ 88%, rising to nearly 90% in the highland areas, and never falling below 60%. Early studies on aspects of sunshine hours in Malaysia were reported by Dale [3]. The report was based on the record of observations on the duration of bright sunshine using a Campbell-Stokes sunshine recorder maintained by the Malaysian Meteorological Services Department at several stations (mostly near airports) throughout the Malaysian peninsula for several years. The data are presented in terms of total or effective sunshine and relative sunshine only. We present our observations on diurnal variation of global solar radiation in Bangi. The observations are unique because of the geographical background on the land-mass mentioned earlier. The results presented here can also be used to predict the pattern of solar radiation in other tropical countries with a similar weather pattern. It was observed that the instantaneous solar radiation intensity sometimes rises higher than the solar constant, even reaching 1.4 kW/m ~" which is the saturation point of the recording system. Even though such occurrences are rare, this unpredictably high radiation intensity (brought about
742
Data Bank
Dote:
Feb. 0 9 ,
1986.
1.2
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Fig. 1. Global solar radiation for a clear day.
Dote: NOV.
19,
1988.
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1500
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1200
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Fig. 2. Global solar radiation for a fully cloudy day.
0900
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0700
Data Bank
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743
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Fig. 3. G l o b a l solar r a d i a t i o n for a p a r t l y c l o u d y day.
Dote:
Jan.
06,
1986. .°.
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Fig. 4. G l o b a l s o l a r r a d i a t i o n with intensity higher t h a n solar constant.
).8-1
0800
,.61
,.41
0700
744
Data Bank Sept.
Date:
04,
1987.
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Fig. 5. Global solar radiation with rain in the afternoon•
by the random distribution of cloud) can damage a solar photovoltaic system especially the DC/AC inverter and power controller• 2. THE FACILITY Solar flux is monitored using a precision pyranometer model MS-801 EKO with a sensitivity up to 7 mV/kW . m z.
This pyranometer together with the photovoltaic panels for a 1.2 kW peak PV pump testing station is installed on a mounting frame inclined 10° facing south• A detailed description of the data aquisition system is explained elsewhere by Othman et al. [4]. The data were recorded every 10 s on a dot recorder model EH800-01D CHINO• The system operates from 0600 to 1900 h daily. Data collection and records were kept since May 1985. 3. RESULTS Records from 1334 days of data which represent 73.1% of the 5-year period were analysed. The missing days were due to electricity black-out and regular servicing of the system. From the distribution of the global solar radiation, the diurnal variation of the global solar radiation were classified into five patterns as follows.
W e a t h e r p a t t e r n in Bangi (%) Cloudy 51
Rain
Clear 16
14 Sr>Sa
3
Aft, Rain
17
Total day: 1334 (73.1%) Location: Bangi2 56°N, 101 47°E Fig. 6. Percentage of global solar radiation pattern in Bangi, Malaysia•
3.1. Global solar radiation pattern for a clear day Figure 1 shows the global solar radiation pattern on a clear day. The occurrence of this type of radiation pattern is very rare in Malaysia• The global solar radiation pattern shown in Fig. 1 was recorded on 9 February 1986. From this figure, it can be seen that for such a clear day, the maximum instantaneous solar intensity was 971 W/m 2 and the amount of solar energy received during the whole day was 6.957 kWhr/m 2. This value of 971 W/m 2 represents 71% of the solar constant quoted by Iqbal [5]. It is also lower than the value quoted by Meinel and Meinel for the desert area which is 77% [1]. This lower value can be explained because the atmosphere in Malaysia is denser than that of the desert due to the higher humidity. 3.2. Global solar radiation pattern for a fully cloudy day This radiation pattern was defined for the occurrence of cloud cover during the entire day, from early morning until
Data Bank late afternoon, sometimes with occasional rain. The occurrence of this type of solar radiation is also very rare in Malaysia. Figure 2 shows an example of this global radiation pattern, which occurred on 19 November 1988. The maximum solar intensity received on that day was 121.4 W/m 2 at 1330 h and the intensity was <70 W/m 2 for more than 9 h. Total global solar radiation on this day was 0.50 kWhr/m 2. This value is only 7.18% as compared with that of a clear day. 3.3. Global solar radiation pattern for a partly cloudy day This is the familiar type of solar radiation pattern at this location. Figure 3 shows the pattern recorded on 9 March 1988. This pattern of solar radiation is variable and changes according to the degree of cloudiness. However, on this particular day, the maximum solar intensity was 1142,9 W/m 2 and total solar radiation was 4.43 kWhr/m 2. The maximum intensity is much higher than the instantaneous intensity on a clear day as recorded in Fig. I. The reasons for this pattern of solar radiation are discussed in detail by lqbal [5]. The increase in the solar radiation intensity is due to reflection and refraction of solar radiation by water molecules, dust and aerosol in cloud and also diffusion of anisotropic radiation at the horizon. 3.4. Global solar radiation pattern with instantaneous intensity higher than solar constant Figure 4 shows the global solar radiation pattern recorded on 6 January 1986. It can be seen in this diagram that at about 12:15 h the solar radiation intensity may be above 1400 W/m 2 (the maximum intensity the chart could record). The occurrence of this type of solar radiation is very rare. The total amount of solar radiation recorded on that day was 4.16 kWhr/m 2. As explained in Section 3.3 the very high instantaneous intensity is also due to refraction and reflection of sunlight by the cloud and particles in the air [6]. 3.5. Global solar radiation pattern with afternoon rain As a tropical country, Malaysia receives a lot of rainfall. Experience shows that afternoon rain affects performance of the photovoltaic system. Figure 5 shows a typical example of the global solar flux on 4 September 1987. In this diagram, the ground received plenty of sunshine until 13:30 h. Then suddenly, at about 1400 h, rain started to fall until late in the afternoon. The maximum instantaneous solar intensity
745
was 957 W/m 2 and total solar radiation received for the day was 3.00 kWhr/m 2. 4. DISCUSSION In the topical region the exact pattern of global solar radiation on any day cannot be predicted. However, our analysis based on data recorded for 1334 days in an observation period of 5 years enables us to classify the global solar radiation in Malaysia into the five patterns described previously. Figure 6 shows the frequency of occurrence of the various patterns: 51.0% cloudy, 16.5% afternoon rain, 15.7% clear, 13.7% rainy and 2.8% cloudy with occassional solar intensity above the solar constant. Knowledge of the frequency of these patterns is very important in designing a solar system, especially a solar photovoltaic system. The information will help manufacturers and organizations involved in solar energy system design to determine the specifications for the solar system and its components for installation in Malaysia and similar locations. Due consideration must be given to the possibility of the occurrence of solar radiation exceeding the solar constant. The high frequency of occurrence of afternoon rain suggests that the orientation of solar panels should also be considered in order to optimize the utilization of available solar energy in the premeridian. Acknowledgement--The authors wish to thank the Solar Energy Research Group, University Kebangsaan Malaysia, for making their facilities available for the project. REFERENCES
1. A. B. Meinal and M. P. Meinal, Appl. Solar Energy, Addison-Wesley, London (1977). 2. S. Niewolt, The climate of continental South East Asia. In : Climate of Southern and Western Asia. World Survey Climatology (Edited by Takahashi and Arakawa), Vol. 9, pp. 1-38. Elsevier, Amsterdam (1981). 3. W. L. Dale, J. Tropical Geography 19, 2(~26 (1964). 4. M. Y. H. Othman, M. N. Dalimin and B. Yatim, Proc. of the First Regional Seminar on Process Control. UNESCO and UKM, pp. 79-88 (1988). 5. Muhammad Iqbal, An Introduction to Solar Radiation. Academic Press, London (1983). 6. A. A. M. Sayigh, Personal discussion during his visit to University Kebangsaan Malaysia, Bangi, Malaysia, 9 16 February (1990).
0960-1481/93 $6.00+.00 (~; 1993 Pergamon Press Ltd
DATA BANK Diurnal pattern of global solar radiation in the Tropics: a case study in Malaysia MOHD. YUSOF HJ. OTHMAN, KAMARUZZAMAN SOPIAN, BAHARUDIN YATIM a n d MOHD. NOH DALIMIN Solar Energy Research Group, University Kebangsaan Malaysia, 43600 Bangi, Malaysia
(Received 28 October 1992; accepted 25 November 1992) Abstract--Global solar radiation patterns in Bangi (2°56'7.3"N, I05°47'0.2"E), Malaysia are discussed. The frequency of occurrence of the various solar radiation patterns observed over a 5-year period is derived. These observations will provide useful information for the design of solar energy systems and equipment for installation in tropical countries having a similar global solar radiation pattern.
1. INTRODUCTION It is widely known that the information on diurnal variation of global solar radiation at a location is vital before any solar energy equipment can be installed [l]. The information on sunshine hour, solar radiation intensity and diurnal variation of global solar radiation will help the designer to design a suitable solar system for a particular application, such as photovoltaic water pumping system, highway signal, remote communication facilities and many other applications including solar thermal and drying systems. The diurnal variation of global solar radiation received at a location is seldom represented by an idealized sine curve. In countries experiencing significant cloud cover, this idealized curve can hardly be observed. The ideal curve can only be seen if there is no interference from clouds and other objects which obstruct the incoming sunlight to the ground. Therefore, the diurnal variation of sunlight differs from one place to another. In tropical countries like Malaysia, the cloud pattern can be highly variable due to the high humidity and unpredictable weather, especially during the monsoon seasons. In order to understand the climatic conditions and solar radiation pattern, a geographical background of the landmass is briefly described [2]. Mainland Malaysia is a peninsula lying between latitudes 1.30 and 6.60°N, and longitudes 99.50 and 103.30°E. It is divided into the east and west coastal plains by the Main Range running northwest to southeast. Bangi is located in the west coastal plain at latitude 2°56'7.3"N, 105°47'0.2"E. The climate in Bangi, as in many places of the region, is mainly influenced by the system of the Asian monsoons. The Southwest monsoon brings rain and cloud to the northwest coast from June to early October, while the northeast monsoon brings rain and cloud to the east coast from November to March. Therefore the west coast is dry from November to March while the east coast is dry from June to September. During the dry season, the climate is hot and sunny, with intermittent breaks of cloud formation and hence rainfall in the late afternoon due to convection currents. The cold season is absent and seasonality is marked by rainfall rather than temperature. 741
The equatorial climate is characterized by heavy rainfall, constantly high temperature and relative humidity. There are two exceptionally wet belts in the peninsula with a total rainfall of more than 275 cm/year. The first belt extends the entire length of the eastern part of the rainfall in November and December. The other is confined to the northern half of the western side of the peninsula and receives heavy rainfall twice a year, in April-May and Octobe~November. Much of the precipitation occurs as thunderstorms and the normal pattern is one of heavy falls within short periods. Generally, the chances of rain falling in the afternoon or early evening are high compared with that in the morning. Much of Peninsular Malaysia has more than 170 rainy days ; however, an area may have a great number of rainy days and yet receive a lesser amount of rain in a year than another area with a smaller number of rainy days but receiving its rain in heavy spells. Temperature remains uniformly high over the peninsula throughout the year. Average temperatures are between 25.6 and 27.8°C. Most locations have a relative humidity of 8 ~ 88%, rising to nearly 90% in the highland areas, and never falling below 60%. Early studies on aspects of sunshine hours in Malaysia were reported by Dale [3]. The report was based on the record of observations on the duration of bright sunshine using a Campbell-Stokes sunshine recorder maintained by the Malaysian Meteorological Services Department at several stations (mostly near airports) throughout the Malaysian peninsula for several years. The data are presented in terms of total or effective sunshine and relative sunshine only. We present our observations on diurnal variation of global solar radiation in Bangi. The observations are unique because of the geographical background on the land-mass mentioned earlier. The results presented here can also be used to predict the pattern of solar radiation in other tropical countries with a similar weather pattern. It was observed that the instantaneous solar radiation intensity sometimes rises higher than the solar constant, even reaching 1.4 kW/m ~" which is the saturation point of the recording system. Even though such occurrences are rare, this unpredictably high radiation intensity (brought about
742
Data Bank
Dote:
Feb. 0 9 ,
1986.
1.2
1.2-
hO-
I,O-
_o o.8-
o.8-
c
...,.~-~-.,~,~. .7"~
/
\
"
~
..
/
~,
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\
o
%
c
', \
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0.4.
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' .
1800
.
\
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-
,//'~"x
""~ t
~
'~ ////
g
i
J
E
x
1700
1500
1600
'~
1400
1300 Time
\
1200
.
I100
I000
0900
0.2
\
0800
0700
(hour)
Fig. 1. Global solar radiation for a clear day.
Dote: NOV.
19,
1988.
~.2.
1.2-t
i.o
I.o-I
o.s
o.8.1
0.6"
o.6-1
o 0.4o
0.4-1
E
o= o c
g
0.2 J
P
1800
1700
1600
1500
1400
1300
1200
1100
1000
Time (hour)
Fig. 2. Global solar radiation for a fully cloudy day.
0900
0800
0700
Data Bank
Dote:
Mor.
743
1988.
09,
.t •
•. '.
."
• .3[ • ,. "
.\-
__...............~;----....,, ,,..,. /800
1700
; :'~
1500
- -
o.64
e=
.
:: .:.
\:
~:'.
,,
~
;
1400
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Time
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~ 0.8-
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,'..
~
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0900
0800
0700
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1,2t
I.o
1.0-1
{hour)
Fig. 3. G l o b a l solar r a d i a t i o n for a p a r t l y c l o u d y day.
Dote:
Jan.
06,
1986. .°.
".) •..
•
.
d"
E
)
• ,.
•
:
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.
.... ..- ..
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....
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1600
1700
1600
1500
1400
1300 Time
1200
,,,.. \
I I00
I000
-~ o.8_~ _
~ o.6.
.. :...o.,,-
0900
{hour)
Fig. 4. G l o b a l s o l a r r a d i a t i o n with intensity higher t h a n solar constant.
).8-1
0800
,.61
,.41
0700
744
Data Bank Sept.
Date:
04,
1987.
1.2-
I .2'
1.0-
1.0
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. 1800
.
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. 15OO
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\
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,\ ~5
Time
12OO
k,:i .'-.--.
0.2
,\ : IIOO
'~ IOOO
0900
0800
0700
{ hour)
Fig. 5. Global solar radiation with rain in the afternoon•
by the random distribution of cloud) can damage a solar photovoltaic system especially the DC/AC inverter and power controller• 2. THE FACILITY Solar flux is monitored using a precision pyranometer model MS-801 EKO with a sensitivity up to 7 mV/kW . m z.
This pyranometer together with the photovoltaic panels for a 1.2 kW peak PV pump testing station is installed on a mounting frame inclined 10° facing south• A detailed description of the data aquisition system is explained elsewhere by Othman et al. [4]. The data were recorded every 10 s on a dot recorder model EH800-01D CHINO• The system operates from 0600 to 1900 h daily. Data collection and records were kept since May 1985. 3. RESULTS Records from 1334 days of data which represent 73.1% of the 5-year period were analysed. The missing days were due to electricity black-out and regular servicing of the system. From the distribution of the global solar radiation, the diurnal variation of the global solar radiation were classified into five patterns as follows.
W e a t h e r p a t t e r n in Bangi (%) Cloudy 51
Rain
Clear 16
14 Sr>Sa
3
Aft, Rain
17
Total day: 1334 (73.1%) Location: Bangi2 56°N, 101 47°E Fig. 6. Percentage of global solar radiation pattern in Bangi, Malaysia•
3.1. Global solar radiation pattern for a clear day Figure 1 shows the global solar radiation pattern on a clear day. The occurrence of this type of radiation pattern is very rare in Malaysia• The global solar radiation pattern shown in Fig. 1 was recorded on 9 February 1986. From this figure, it can be seen that for such a clear day, the maximum instantaneous solar intensity was 971 W/m 2 and the amount of solar energy received during the whole day was 6.957 kWhr/m 2. This value of 971 W/m 2 represents 71% of the solar constant quoted by Iqbal [5]. It is also lower than the value quoted by Meinel and Meinel for the desert area which is 77% [1]. This lower value can be explained because the atmosphere in Malaysia is denser than that of the desert due to the higher humidity. 3.2. Global solar radiation pattern for a fully cloudy day This radiation pattern was defined for the occurrence of cloud cover during the entire day, from early morning until
Data Bank late afternoon, sometimes with occasional rain. The occurrence of this type of solar radiation is also very rare in Malaysia. Figure 2 shows an example of this global radiation pattern, which occurred on 19 November 1988. The maximum solar intensity received on that day was 121.4 W/m 2 at 1330 h and the intensity was <70 W/m 2 for more than 9 h. Total global solar radiation on this day was 0.50 kWhr/m 2. This value is only 7.18% as compared with that of a clear day. 3.3. Global solar radiation pattern for a partly cloudy day This is the familiar type of solar radiation pattern at this location. Figure 3 shows the pattern recorded on 9 March 1988. This pattern of solar radiation is variable and changes according to the degree of cloudiness. However, on this particular day, the maximum solar intensity was 1142,9 W/m 2 and total solar radiation was 4.43 kWhr/m 2. The maximum intensity is much higher than the instantaneous intensity on a clear day as recorded in Fig. I. The reasons for this pattern of solar radiation are discussed in detail by lqbal [5]. The increase in the solar radiation intensity is due to reflection and refraction of solar radiation by water molecules, dust and aerosol in cloud and also diffusion of anisotropic radiation at the horizon. 3.4. Global solar radiation pattern with instantaneous intensity higher than solar constant Figure 4 shows the global solar radiation pattern recorded on 6 January 1986. It can be seen in this diagram that at about 12:15 h the solar radiation intensity may be above 1400 W/m 2 (the maximum intensity the chart could record). The occurrence of this type of solar radiation is very rare. The total amount of solar radiation recorded on that day was 4.16 kWhr/m 2. As explained in Section 3.3 the very high instantaneous intensity is also due to refraction and reflection of sunlight by the cloud and particles in the air [6]. 3.5. Global solar radiation pattern with afternoon rain As a tropical country, Malaysia receives a lot of rainfall. Experience shows that afternoon rain affects performance of the photovoltaic system. Figure 5 shows a typical example of the global solar flux on 4 September 1987. In this diagram, the ground received plenty of sunshine until 13:30 h. Then suddenly, at about 1400 h, rain started to fall until late in the afternoon. The maximum instantaneous solar intensity
745
was 957 W/m 2 and total solar radiation received for the day was 3.00 kWhr/m 2. 4. DISCUSSION In the topical region the exact pattern of global solar radiation on any day cannot be predicted. However, our analysis based on data recorded for 1334 days in an observation period of 5 years enables us to classify the global solar radiation in Malaysia into the five patterns described previously. Figure 6 shows the frequency of occurrence of the various patterns: 51.0% cloudy, 16.5% afternoon rain, 15.7% clear, 13.7% rainy and 2.8% cloudy with occassional solar intensity above the solar constant. Knowledge of the frequency of these patterns is very important in designing a solar system, especially a solar photovoltaic system. The information will help manufacturers and organizations involved in solar energy system design to determine the specifications for the solar system and its components for installation in Malaysia and similar locations. Due consideration must be given to the possibility of the occurrence of solar radiation exceeding the solar constant. The high frequency of occurrence of afternoon rain suggests that the orientation of solar panels should also be considered in order to optimize the utilization of available solar energy in the premeridian. Acknowledgement--The authors wish to thank the Solar Energy Research Group, University Kebangsaan Malaysia, for making their facilities available for the project. REFERENCES
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