some aspects of solar radiation climatology of iraq

7
Renewable Energy Vol.2. No. 2, pp. 167-173. 1992 0960 1481/92 $5.00+.00 PrintediBGreal Britain. PergamonPressLtd DATA BANK Some aspects of solar radiation climatology of Iraq M. AL-RIAHI, N. AL-HAMDANI and H. AL-SAFFAR Solar Energy Research Center, Jadiriyah P.O. Box 13026, Baghdad, Iraq (Received 7 October 1991 ; accepted 8 November 1991) Abstract--Study of the climatology of global solar radiation is considered very useful for assessing the potential efficiency of systems designed for solar energy utilization. This paper explores some aspects of solar radiation climatology in Iraq. Analysis of the monthly averages global solar radiation and the general atmospheric transparency for theperiod 1971 1985 for three differem climatological zones (Mosul, Baghdad, Nasiriyah) are discussed. The frequency distribution of daily clearness index for each station is determined using histograms of frequencies. The percentage number of days with solar radiation and sunshine duration values below a certain value is analyzed and discussed. The period of successive days having radiation less than 5 MJ/m 2 -day- t and 10 MJ/m:'day ~is examined and presented graphically. INTRODUCTION A full understanding of the radiation climatology Is necess- ary before an efficient and economic apparatus can be designed for the utilization of solar energy. For example, in the construction of a solar water heater, details of the radi- ation regime must be known before the capacity of the auxili- ary water heater and the size of the storage tank can be determined [1]. Analysis of solar radiation data in a way which make them useful in the design of solar house heating systems, in the applications of solar energy to house cooling, and in other applications where energy storage for several days is needed, were discussed by several authors [2 5]. This paper examines in detail some aspects of radiation climatology for three climatologically different regions in Iraq. The results allow one to easily assess the available solar energy for a particular season of solar energy utilization when solar radiation may exceed a particular energy efficiency threshold. The results also provide an estimate of the amount of time and the number of consecutive days when solar energy falls below the efficiency threshold of the solar utilization systems under consideration. OBSERVATIONS Measurements of the daily total global solar radiation and duration of bright sunshine have been carried out in Iraq by the Iraqi Meteorological Organization for many years. Daily observations of duration of sunshine are recorded by Campbell-Stokes type heliograph, and values of global solar radiation are obtained from the recordings of a bimetallic Robitsizch Actinograph. The present work is based on con- tinuous sets of data collected between 1971 and 1985 in Mosul, Baghdad, Nasiriyah and Basrah. From the climato- logical and topographical points of view, the actinometric measurements at these stations are considered to represent the Northern, Central and Southern zones of Iraq. Table 1 gives the geographical locations of these stations. DATA ANALYSIS Monthly average of daily global radiation The amount of solar radiation received by the Earth's surface varies from one place to another owing to the atten- uating properties of the atmosphere and the diverse geo- graphical characteristics of the Earth's surface. Hence, detailed studies of solar radiation under local climatic con- ditions have been carried out for various places [6-11]. Monthly average values of daily total global solar radi- ation on a horizontal surface for Mosul, Baghdad and Nasi- riyah are shown in the upper part of Fig. 1, in which the monthly average values of maximum global radiation (assuming the atmosphere is perfectly clear), and extra- terrestrial radiation values are also shown. Notable points of this figure are : 1. The curves for the three stations are nearly similar in pattern. 2. The average yearly cumulative radiation for Mosul is 177.862 MJ/m z per year. In an average year, the summer period (June-August) contributes about 36.78% of the annual total. The worst month (December) contributes least, being responsible for only about 3.80%. The monthly aver- age of daily incoming total global solar radiation has its maximum value in June (22.896 M j/m2), while the minimum values occurs during December with about 6.757 MJ/m 2. 3. The total global solar radiation received at Baghdad on a horizontal surface is about 214.02 MJ/m z per year, of which 36.12% occurs during the summer months. December is responsible for 4.71%. The highest and the lowest mean monthly total occur in June (26.759 MJ/m z) and December (10.084 MJ/mZ). 4. The mean annual total of global radiation for Nasiriyah is about 201.948 MJ/m 2 per year. The summer period con- tributes about 31.76 MJ/m z. December contributes least, being responsible for only about 4.71%. Over the year, the highest monthly mean total occurs in July (21.708 MJ/m 2) and the lowest occurs in December (9.914 MJ/m~). 167

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Renewable Energy Vol. 2. No. 2, pp. 167-173. 1992 0960 1481/92 $5.00+.00 Printed iB Greal Britain. Pergamon Press Ltd

DATA BANK

Some aspects of solar radiation climatology of Iraq

M. AL-RIAHI, N. AL-HAMDANI and H. AL-SAFFAR Solar Energy Research Center, Jadiriyah P.O. Box 13026, Baghdad, Iraq

(Received 7 October 1991 ; accepted 8 November 1991)

Abstract--Study of the climatology of global solar radiation is considered very useful for assessing the potential efficiency of systems designed for solar energy utilization. This paper explores some aspects of solar radiation climatology in Iraq. Analysis of the monthly averages global solar radiation and the general atmospheric transparency for theperiod 1971 1985 for three differem climatological zones (Mosul, Baghdad, Nasiriyah) are discussed. The frequency distribution of daily clearness index for each station is determined using histograms of frequencies. The percentage number of days with solar radiation and sunshine duration values below a certain value is analyzed and discussed. The period of successive days having radiation less than 5 MJ/m 2 -day- t and 10 M J / m : ' d a y ~ is examined and presented graphically.

INTRODUCTION

A full understanding of the radiation climatology Is necess- ary before an efficient and economic apparatus can be designed for the utilization of solar energy. For example, in the construction of a solar water heater, details of the radi- ation regime must be known before the capacity of the auxili- ary water heater and the size of the storage tank can be determined [1]. Analysis of solar radiation data in a way which make them useful in the design of solar house heating systems, in the applications of solar energy to house cooling, and in other applications where energy storage for several days is needed, were discussed by several authors [2 5].

This paper examines in detail some aspects of radiation climatology for three climatologically different regions in Iraq. The results allow one to easily assess the available solar energy for a particular season of solar energy utilization when solar radiation may exceed a particular energy efficiency threshold. The results also provide an estimate of the amount of time and the number of consecutive days when solar energy falls below the efficiency threshold of the solar utilization systems under consideration.

OBSERVATIONS

Measurements of the daily total global solar radiation and duration of bright sunshine have been carried out in Iraq by the Iraqi Meteorological Organization for many years. Daily observations of duration of sunshine are recorded by Campbell-Stokes type heliograph, and values of global solar radiation are obtained from the recordings of a bimetallic Robitsizch Actinograph. The present work is based on con- tinuous sets o f data collected between 1971 and 1985 in Mosul, Baghdad, Nasiriyah and Basrah. From the climato- logical and topographical points of view, the actinometric measurements at these stations are considered to represent the Northern, Central and Southern zones of Iraq. Table 1 gives the geographical locations of these stations.

DATA ANALYSIS

Monthly average of daily global radiation The amount of solar radiation received by the Earth's

surface varies from one place to another owing to the atten- uating properties of the atmosphere and the diverse geo- graphical characteristics of the Earth's surface. Hence, detailed studies of solar radiation under local climatic con- ditions have been carried out for various places [6-11].

Monthly average values of daily total global solar radi- ation on a horizontal surface for Mosul, Baghdad and Nasi- riyah are shown in the upper part of Fig. 1, in which the monthly average values of maximum global radiation (assuming the atmosphere is perfectly clear), and extra- terrestrial radiation values are also shown. Notable points of this figure are :

1. The curves for the three stations are nearly similar in pattern.

2. The average yearly cumulative radiation for Mosul is 177.862 MJ/m z per year. In an average year, the summer period (June-August) contributes about 36.78% of the annual total. The worst month (December) contributes least, being responsible for only about 3.80%. The monthly aver- age of daily incoming total global solar radiation has its maximum value in June (22.896 M j/m2), while the minimum values occurs during December with about 6.757 MJ/m 2.

3. The total global solar radiation received at Baghdad on a horizontal surface is about 214.02 MJ/m z per year, of which 36.12% occurs during the summer months. December is responsible for 4.71%. The highest and the lowest mean monthly total occur in June (26.759 MJ/m z) and December (10.084 MJ/mZ).

4. The mean annual total of global radiation for Nasiriyah is about 201.948 MJ/m 2 per year. The summer period con- tributes about 31.76 MJ/m z. December contributes least, being responsible for only about 4.71%. Over the year, the highest monthly mean total occurs in July (21.708 MJ/m 2) and the lowest occurs in December (9.914 MJ/m~).

167

168 Data Bank

Table 1. Geographical location of recording stations and durations of records

Latitude Longitude Elevation Records Town N E m used

Mosul 3614 ' 43"09' 223 1971 1985 Baghdad 33~'02 ' 40'~14 ' 34 1971 1985 Nasiriyah 31'05' 46'14" 3 1971 1985 Basrah 3034" 47'47' 2 1971 1980

5. Comparing the various percentages of the average daily of incoming global radiation during the summer period obtained at the three stations, there is a marked decrease in the percentage of incoming global radiation at Nasiriyah. This could be attributed to duststorms invading the south of Iraq during that period of the year.

Some consideration over the attenuation o f global radiation To give an opinion about the transmission phenomena at

the three stations, we calculated the average daily values

of the extraterrestrial radiation H0 for each month using equations given by Iqbal (1983) [12], and the potential radi- ation as that which corresponds to completely cloudless days H c. With these values and those of the monthly average of daily global radiation on a horizontal surface H, we cal- culated the attenuation for an atmosphere of normal com- position, K v = H / H o , and for a clear atmosphere, KTC = Hc/Ho. These ratios could be considered as a good estimate of the irradiance absorbed by the aerosols, clouds etc., plus the irradiance scattered upward by clouds and

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Fig. 1. Monthly average of solar radiation for Mosul, Baghdad and Nasiriyah (upper part), ,~ - ~- = extraterrestrial solar radiation ; . . = clear sky global radiation ; O - - - O = mean daily global radiation for 1971-1985, lower part indicate the clearness index under normal and clear

atmosphere (KT, Klc).

Data Bank 169

gaseous and solid aerosols. The lower part of Fig. 1 shows the mean monthly values of global radiation attenuation for Mosul, Baghdad and Nasiriyah. It is clear that the KTc and KT curves have a similar trend. The smaller differences between Kxc and Kx values occur during summer and never exceed 4% for Mosul and Baghdad, and 8% for Nasiriyah. Values of Kxc are relatively high in winter, due to the atmo- sphere being more clear in winter (in the absence of clouds) than in summer, The quantity of aerosols increases ila the summer, being greater in Nasiriyah. The mean monthly values of KT present a maximum of 55.2% and 57.2% in September for Mosul and Nasiriyah respectively, and a maximum of 65.4% in August for Baghdad. These curves show a minimum of 41.6%, 55.6% and 51.0% in December for Mosul, Baghdad and Nasiriyah, respectively. The Kvc curves show a maximum of 61.8% and 72.0% in January for Mosul and Baghdad, and 68.7% in February for Nasiriyah. The minimum values of 54.2%, 67.7% and 61.2% were observed in October, July and August for the three stations, respectively.

The study of the frequencies distribution of daily clearness

index for each station were determined by using histograms of frequencies. The results are shown in Fig. 2. It can be seen that for all stations, the daily clearness index distribution show a similar behaviour and assimilable by normal dis- tribution with almost negative asymmetry. The interval of frequencies is relatively high in winter and spring. In summer, the negative asymmetry and the absolute interval decrease and the distributions skewe towards high values of daily clearness index. It can also be seen that the peaks of the histograms vary with season and from one station to another. The displacement is a function of the augment of irradiation from winter to summer and the variability is a consequence of the cloudiness.

Number of days with solar radiation below specific values There have been a great many measurements of solar

radiation taken in various ways all over the world. Processing of these data in special ways for practical applications of solar energy, can make the data more useful. For example, to design the heating system properly, it is necessary to know

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Fig. 2. Percentage frequency distribution of the daily clearness index for winter, spring, summer and autumn (1971 1985) for Mosul, Baghdad, and Nasiriyah.

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Data Bank

Solar Utilization Seaaon

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Fig. 3. Solar utilization season lengths above various selected energy levels during the yearly cycle for Mosul, Baghdad and Nasiriyah. Dots indicate beginning and ending dates of solar threshold values for

individual years of study (1971 1985).

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Fig. 4. Accumulative number of days with radiation below indicated values for Mosul, Baghdad and Nasiriyah. winter (Dec., Jan., Feb.) ; . . . . . spring (Mar., Apr., May) ; . . . . . . . summer (June,

July, Aug.); autumn (Sept., Oct., Nov.).

Data Bank 171

the consecutive number of days for which one is likely to need storage or auxiliary heating. Thus, an assessment of the solar radiation variability for specific solar energy thresholds becomes important. An example is presented in Fig. 3. The solar utilization season for a particular radiation threshold is defined as that period between the average first and last date exceeding the threshold value.

Since storage of solar radiant energy is difficult because of the extreme solar radiation variability which may occur dur- ing a given solar utilization season. Therefore, one con- templating the design of a solar utilization system must be concerned with the number of consecutive days when solar radiation values falls below the system's efficiency threshold during a solar utilization season. Figure 4 indicates the num-

ber of days, consecutive or not, on which the radiation is below a certain value for the three sites (Mosul, Baghdad and Nasiriyah). Four curves are shown, one for each season of the year. To illustrate the usefulness of these curves, con- sider for example, a system in which 10 MJ/m 2- day - ~ are needed and assume that a chemical storage battery is used in the system. For such system, Fig. 4 indicates that there would be, in an average winter season, 28 days at both Baghdad and Nasiriyah on which some storage would be required, since the number of days with radiation below 10 MJ /m2-day - ~ is 28. Alternatively, there would be 72 days during winter season on which the radiation would be above 10 MJ /m 2. day ~, so that some storage would be possible on those days.

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Fig. 5. Frequency patterns of successive day periods receiving radiation less than 5 MJ/m 2" day ~ and 10 M J / m2 .day - ~ for Mosul, Baghdad and Nasiriyah.

0 2 i 6 10 0 2 ( 10 Successive days Nssiriyah S u ¢ c ~ s i v e days

172 Data Bank

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Fig. 6. Accumulative number of days with sunshine duration below indicated values at Mosul, Baghdad and Basrah. winter; . . . . spring; • • - summer; autumn.

There are days with low values of global radiation and this may be due to cloud cover or seasonal low altitude of the sun. On these days limited energy is received by the solar appliances and it is worthwhile to look at their occurrence [13]. Two arbitrary levels are considered in this work; firstly days with global radiation below 5 MJ/m2"day ~, these will be referred to as "low level radiation days", and secondly those with global radiation below 10 MJ/m2.day t, which are termed "moderate level radiation days". The number of low and moderate level radiation days were examined, a frequency distribution was established showing a picture of how many times these days come single, in pairs or three or more days in a row. Figure 5 shows the frequency distribution for Mosul, Baghdad and Nasiriyah for low and moderate level radiation days of 15 years of data. In Mosul for example, 25 low level radiation days per annum can be expected on which the value of daily global radiation remains below 5 MJ/m 2. day-L Of these days, 12 are single, three in 2 successive days, and once in three and four successive days might occur. There are 48 moderate level radiation days on which daily global radiation remains below I0 MJ/m 2" day and they are distributed as follows: single days (16 occasions), 2 days (five times), 3 days (three times), 4 days (twice) and 5 days (once). These are average values and from year to year there is a considerable variation to this pattern. It should be pointed out that these histograms are valuable for the utilization of solar energy. For example, in deter- mining the amount of auxiliary heating, necessary allowance should be made for some heating in months where the mean daily solar output exceeds the required amount, since it is usually undesirable to install sufficient storage to cope with more than two successive days of low radiation.

Cumulative JhequemT distribution of sunshine duration The availability of bright sunshine without interruption

for long periods and the frequency distribution of days with various sunshine duration can be used to determine, to a very great extent, the possibility of the utilization of solar energy for practical purposes [14]. In Fig. 6 cumulative fre-

quency distribution curves of days with various sunshine duration have been drawn. At the three stations under con- sideration (Mosul, Baghdad, Basrah). It should be noted that during the 10-year period examined in this figure (1971 1980), June, July, August and September have never had sunless days (i.e. duration = 0) at Mosul and Basrah, in addition to the above months October have never had sunless days at Baghdad. The highest percentage of sunless days occurred during winter months, being 15.5%, 5.9% and 6.8% for Mosul, Baghdad and Basrah, respectively. Beside the sunless days, if we also take into account the days with sunshine duration >~ 8.0 hours, which may be considered as a good day from the view point of sunshine duration, we can draw the following conclusions ; days with sunshine duration /> 8.0 hours recorded in every month of the year for the three stations. The percentage of days with sunshine duration ~>8.0 hours are 67.02%, 72.26% and 72.92% for Mosul, Baghdad and Basrah, respectively.

REFERENCES

I. J. N. Black, New Sources of Ener#y 4, 311. U.N. Pub- lication Sales No. 63.I.38 (1964).

2. H. Tabor, Transaction of the Conference on the Use of Solar Energy: The Scientific Basis 3, Part [, Section A, 24 (1955).

3. B. Y. H. Liu and R. C. Jordan, Solar Energy 7, 53 (1963). 4. R. L. Lougeay and A. J. Brazel, Solar Energy 33, 409

(1984). 5. I. Bennett, Solar Energy 9, 145 (1965). 6. M. A. Abbas and M. K. Elnesr, Pageoph 112, 753 (1974). 7. P. J. Robinson and W. E. Easterling, J. Appl. Met. 21,

1730 (1982). 8. J. D. Kalma, Aust. Meteorol. Mac/. 18, 134 (1970). 9. R. H. B. Exell, Solar Energy 18, 349 (1976).

10. A. Mani and O. Chacko, Solar Energy 14, 155 (1976). 11. A . A . M . Sayigh, Proc. Summer Workshop on the Physics

Data Bank 173

International Solar Ener#y Society, Hamburg, F.R.G., 3887 (1987).

14. A. Akrawi, M. AI-Riahi and Hikmat, Proc. of the 2nd Arab International Solar Energy Conference, Bahrain, 433 (1986).

~[ Non-Conventional Energy Sources, Trieste, Italy, 257 (1983).

12. M. lqbal, An Introduction to Solar Radiation. Academic press, Toronto, Canada (1983).

13. J. Andringa, Proc. of" the Biennial Congress of the