On the measurement of clear day solar irradiance components at Islamabad

Renewable Energy Vol. 2, No, 2. pp. 175 179, 1992

09N~1481/92 $5.00+,00 Pergamon Press Ltd

Printed in Great Britain.

DATA

BANK

On the measurement of clear day solar irradiance components at Islamabad P. AKHTER, A. BAIG and A. MUFTI National Institute of Silicon Technology, No. 25, H-9, Islamabad, Pakistan

(Received 7 May 1990 ; accepted 21 June 1990)

Abstract--An experiment has been designed where only one instrument (pyranometer) has been used to record direct, total and global irradiances on an half hourly basis on every 15th or nearest clear day of each m o n t h and on four special days when the sun was passing through an equinox or solstice. The results have been exploited to calculate the diffuse components of total, diffuse and direct components of global radiation, their percentages and sun hour durations, etc. The correlations between the length of the day, sun hours and noontime intensities of different components have been discussed. Also a simple approach has been developed to observe the changes in the clear day atmospheric turbidity over the day and also over the year.

I. I N T R O D U C T I O N

II. E X P E R I M E N T A L S E T - U P

Reliable insolation data and detailed knowledge of variations in the solar radiation and its precise measurement at different sites of the world is extremely important because of its utilization in m a n y disciplines such as climatology, meteorology, biology, agriculture, architecture, environmental engineering, etc. Above all it is important for solar energy applications, such as photovoltaics which converts the sun energy--the most a b u n d a n t in the w o r l d ~ i r e c t l y into electricity which is the most versatile form of energy. The effective design and utilization of solar energy systems depend largely on the adequate knowledge of the insolation characteristics such as sunshine duration (sun hours), total, global, and diffuse components and their variations on daily and hourly basis. Most of the data available in the world is of global radiation, i.e. the total radiation being received on a horizontal surface. Only some of the stations record diffuse component of the global radiation, i.e. the radiation coming only from the sky and not direct from the sun on to the horizontal surface [1]. Few other stations are equipped with pyrheliometers to measure the direct radiation which is the radiation coming directly from the sun and falling to the plane perpendicular to the sun rays. Such data is then used to calculate the other components such as diffuse or direct radiation falling on horizontal and/or inclined plane. Even at places such as the U.S. network where facilities of measuring both global and diffuse or direct component of radiation are available, these quantities are always measured using two independent instruments which could vary quite widely in their characteristics [2, 3] and hence will introduce large errors while correlating their results. To avoid such errors, we have set up a system which uses only one instrument (pyranometer) to record global, direct and total radiations and the results are then exploited to compute their diffuse components and sun hour durations on clear days.

Epply's precision spectral pyranometer (model PSP), has been used for all radiation measurements. The experimental set up is shown in Fig. l where the pyranometer is fixed on a board which can be set horizontal with the help of levelling screws. The plane of the pyranometer can be tilted at any angle from zero degree (horizontal) to 90'. A metallic needle of length / = 4.5 cm is fixed vertically to the board and its shadow 's', when the pyranometer board is set horizontal, is used to measure the position of the sun, i.e. to measure local zenith angle at the time of observation. Also for the measurement of radiation falling on a plane perpendicular to the sun rays, the plane of pyranometer is tilted so that the needle casts a shadow of zero length. At this position the pyranometer is directly facing the sun and is receiving what is called total radiation. This is composed of two parts. The first part is called direct radiation which is the radiation coming directly from the sun and falling on the inclined plane facing the sun. The second is the diffuse component which is the radiation received by the sun facing the inclined plane and is coming from the sky only after being scattered by the atmosphere. In order to measure the direct normal radiation a collimator has been designed. It is made of 5 cm inner diameter black PVC pipe and makes an angle of 8 ~ and satisfies the equations : tan 0z = (R--r)/L

(1)

R/L = 1/20

(2)

and

as per instrumental standards [4, 5]. Here R and L are the radius of the objective and length of the collimator respectively ; r is the radius of the detector ; 00 and 0s are the angles defined in Fig. l(b). 175

176

Data Bank

Fig. 1. (a) Experimental set up. A, place of observation ; B, pyranometer ; C, collimator ; D, standard solar cell ; E, metallic needle ; F, horizontal plane ; G, levelling screws. (b) Collimator design.

IlL R E S U L T S AND DISCUSSION Using the set-up described in section ll, the global radiation (ig) on the horizontal surface, total radiation (IT) on the inclined surface, direct normal radiation (ID) and local zenith angle (Oz) were routinely recorded on the roof top of the NIST building at Islamabad, Pakistan. Other components such as diffuse radiation (ld) on the inclined surface, direct (iD) on the horizontal surface and diffuse (ij) on the horizontal surface were then calculated using the relations 0z = arctan (s/I)

(3)

Ia = lv--I D

(4)

iD = ID cos 0z

(5)

id = ig -- iD

(6)

w h e r e ' s ' and '/' are defined in section II and are shown in Fig. I ; 0z is the zenith angle. Equations (3)-(6) are evolved from the definitions of different components of the radiation as defined earlier in sections I and It. For further explanation any standard book on solar radiation [6] can be consulted. All the different components were recorded on half hourly basis on the 15th or nearest clear day of every m o n t h and serve as representative (clear) day of that month. Observations were also recorded on the four special days of 21 March and 22 September when the sun is at the equinox and 22 June and 22 December solstices [7, 9]. The results of only four selective m o n t h s of the year 19861987 are shown in Fig. 2 for comparison. All different components of irradiance shown in Fig. 2 show, on average, a symmetry at noon. However some variations due to change of atmospheric turbidity are also observable. The results show that 'direct normal' and ktotal' (on inclined surface) radiations give a broad peak distribution. The peak is much broader in summer than in winter. The global radiation and direct horizontal component give comparatively sharp peak distribution. The diffused components of total (on inclined surface) and global are not shown in these plots. These values can be read from the difference of the total and direct component plots at a given time, i.e. using eqs (4) and (6). The behaviour of the plots clearly show that the values of the diffuse components on both inclined and horizontal surfaces remain practically constant during the major part of the day and then increase

quite rapidly at both ends of the day. The percentage of these diffuse components are also plotted on the same graphs. Their m i n i m u m (noon) values change quite widely with the season. During winter it is as high as 30% and becomes less than 5% during summer except in extreme cases of humidity when it may increase to 40%. The variation in the m a x i m u m (noon) intensities of total, direct normal, global and direct horizontal components, over the year, are plotted in Fig. 3 and show rather interesting results. The values at noon of direct normal radiation shows a m i n i m u m in late September and gives a m a x i m u m during early April. Likewise the total radiation at noon also shows a minima during late September but its value approaches its m a x i m u m during early January and then it stays constant until June. This means that the intensities of total radiation at noon are maxima and stay constant for the first six months of the year. However the global (horizontal) radiation and its components show a different behaviour. The values at noon of both global and direct horizontal component show minima during late December, i.e. end of the year. However the values at noon of the direct horizontal component show a sharp maxima in early April where the global (horizontal) radiation (values at noon) shows a broad m a x i m u m extending from early April to the end of June. Sun hours are usually measured using sunshine recorders which focuses the direct radiation onto a scaled paper and gives a burning mark. The total length of this burn is then taken as the sun hours. The threshold of burning for a clear dry day is about 70 W / m 2. However in opposite extreme conditions, i.e. with high humidity, this value could go as high as 280 W / m 2, i.e. four times the previous value. An average value of threshold is usually taken equal to 210 W / m 2 as standard [5]. This average value is also shown in Fig. 2. The two vertical solid lines cut the direct radiation curve at this average value of 210 W / m 2, so that the time span between these two lines is considered as the m a x i m u m sun hours on the clear day. Figure 2 also shows two other vertical lines (dashed) which are drawn at sunset and sunrise time on that particular day. The difference of these two timings is the (total) length of the day. Both these measured values of "length of the day" and sun hours (maximum) are plotted against the m o n t h s (days) in Fig. 4. Both show maxima in the m o n t h of June and (averaged) run nearly parallel. In order to find some relationship these two quan-

177

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Fig. 2. Solar radiation on 15th or nearest clear day of four selective months. × , total ; O, diffused % of total ; + , global horizontal ; O, diffused % of global ; O , direct normal ; O , direct horizontal. 15

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Fig. 4. Day length and sun hours (maximum) during the solar year.

2

4

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O. , , 10 12 (MONTHS}

Fig. 3. Intensities o f t o t a l direct n o r m a l , g l o b a l a n d direct h o r i z o n t a l r a d i a t i o n at n o o n on clear days.

titles are plotted against each other in Fig. 5 where triangles are the recorded data points and circles the calculated values which will be discussed later in this section. The three solid lines are the plot of the equation : day l e n g t h - m a x i m u m sun hours = n

(7)

for n - 1,2 and 3 respectively. Almost all points lie between the lines for n = l and n = 3. This shows that on any day of the year, m a x i m u m sun hours are always between 1 to 3 hr shorter than the length of day except in very extreme cases

of dryness or high humidity like the ofle marked ' S ' in Fig. 5. Most likely this difference is an average of 2 hr as is clear from the line for n = 2. This means that the intensity of direct radiation approaches the value of 210 W / m 2 about 1 hr after sunrise and remains higher than this value until 1 hr before sunset. However, a true relationship can only be drawn when more such data of m a n y years is available. As discussed earlier and is clear from Figs 3 and 4, in spite of the fact that m a x i m u m sun hours and day length change over the year quite substantially, the total radiation values stay constant for a good part of the year. So it is difficult to show any relationship between these parameters. However a relationship could be developed between day lengths, sun hours (maximum) and noon direct normal.

[78

Data Bank 15 n=l

n=Z

n=3

14 ui 13 12 :Iz 7

~

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9 7 7

11

9

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DAY

15

LENGTH

17

19

(Hrs.)

Fig. 5. Day length and sun hours (maximum) of a clear day.

global, direct horizontal radiations. The plots of their values are shown in Fig. 6. They all show more or less linear behavtour and solid lines are the least square fit to the data. The figures show that the data for global (horizontal) radiation is comparatively scattered primarily because of variation in diffuse horizontal component as discussed earlier in this section. The direct horizontal component data is less scattered and both obey the following empirical relation H = miN+h

(8)

where iN is the intensity of the direct horizontal component at noon in kW/m2; H (hr) is day length or m a x i m u m sun hours, m is the slope of the line and h (hr) is the value of H for which is becomes zero and is given by the y-intercept of the line. The two lines in Fig. 6 (for direct normal component) give the same slope values for both m a x i m u m sun hours [Fig. 6(b)] and length of days [Fig. 6(a)] and is equal to 8_+ 1 hr/kW/m:. However the y-intercepts give h = 7 + 1 hr and 5_+ 1 hr for length of the day and m a x i m u m sun hours respectively. This difference of about 2 hr and same slope values basically tell the same story as shown in Fig. 5 and explained earlier using eq. (7). This means that if the sun trajectory would have been further in the south so that length of the day is equal to 7 hr, then the intensities at noon of direct normal radiation would have been greater than 210 W / m : giving rise to the m a x i m u m sun hours equal to 5 hr, but the zenith angle now will become so large (0z ~ 9 0 ) that projection of intensities at noon of direct normal radiation onto the horizontal surface (i.e. the direct horizontal) component is nearly zero. This seems to be quite reasonable

because for Islamabad, the value at noon of 0z changes on average by 1F' for each I hr change in day length. This can easily be verified by simple calculation using standard equations for 0z [10]. The m i n i m u m day length and corresponding 0z for our location are about l0 hr and 57' respectively. If the day length happened to be 7 hr instead of 10 hr, i.e. 3 hr shorter still, then the corresponding 0z would have been equal to 90 ~ (57~+3 x I I'). Equation (8) can equally and similarly be applied for global (horizontal) radiation curves in Fig. 6 but because of more scattered data, the uncertainty is comparatively larger. Intensities at noon of direct normal radiation also show the similar linear behaviour with m a x i m u m sun hours and day length. But because of large scattering in the data, the results are not shown here. Such correlation could be made with more confidence, when we will have more data for many years. It is also worth remembering that such correlations could be a function of location. So care must be taken not to generalize the results; they should be applied only to a location on the same latitude and more or less similar geometry of the surroundings. We have also compared our recorded data of direct normal radiation with that of extraterrestrial normal radiation (IE) to have an idea about the changes in the clear day atmospheric turbidity over a day and also throughout the year. The values of 1E vary due to the earth's orbital motion and is given by [10] IE = [l + 0.033 cos (360n/365.25)]1o

lo

15 L4

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INTENSITY

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(12)

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NOON

(ll)

where km~ is the m i n i m u m (i.e. along the zenith) attenuation

123

0.1

(10)

where 0z is again the zenith angle which can be calculated theoretically [I0]. Now eq. (10) gives

.-I- GLOBAL

0 M DIR C TNTAL OE RZ [O

12-

H=miN*h

IE exp (-- km).

Here lo is the direct normal radiation ; k is the broad band attenuation co-efficient; m is the optical path length which is further related to m i n i m u m optical length mt (i.e. optical path length along the zenith) by :

(a) 16

(9)

where 1o = 1373 W / m 2 is the solar constant and n is the day of the year, i.e. n = 1 for 1 January and so on. When IE passes through the earth's atmosphere, it is attenuated by a factor exp ( - k i n ) so that its value at the Earth is given by Beer's law [6] as :

( k W l m 2)

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h

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3~1

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0

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;'I=miN.h0 0 ~ ' ~ "

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0:2 0:3 0:4 0:5 0:6 0:7 0:8 0:9 1:0 NOON TIME iNTENSITY (kW/m z)

Fig. 6. (a) Day length and (b) sun hours (maximum) as a function of intensities of global and direct horizontal component at noon.

1.1

179

Data Bank , I I

0-, I ~2 04

0CT.14,1987 ........

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Fig. 7. Changes in minimum (along the zenith) attenuation optical path length on clear days for lslamabad.

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optical thickness. This quantity is independent of inclination of the sun and is a function of atmospheric turbidity. This can be calculated by using the measured values of Io (Fig. 2) and calculated values of IE [eq. (9)] in eq. (12). The results of four selected days (same as in Fig. 2) are shown in Fig. 7 where the lines are the average values. The results show that for clear days the minimum attenuation optical thickness which as a first approximation is the measure of atmospheric turbidity, stays more or less constant (within 10-15%) for a major part of the day during which the direct normal radiation remains higher than the average value of the threshold of burning (as explained earlier in this section and are shown as vertical solid lines in Figs 2 and 7). Near this time of day, the minimum attenuation optical thickness decreases rapidly towards the end of the day. The maximum deviation from the mean value has been observed equal to 20% on clear days. The values of minimum attenuation thickness at noon have also been calculated and plotted in Fig. 8 where the variations in the values of IE over the year [eq. (9)] are also shown. This figure shows the overall changing behaviour of the clear day atmospheric turbidity over the year at Islamabad. The dotted lines show the upper and lower cases whereas

the solid curve shows the average value. The results show that clear day atmospheric turbidity is the most dense in summer with comparatively large variations and is least dense in winter with less spreading of data points. Similar observations have also been reported earlier [11] for locations in U.S.A. Our data [Fig. 8(b)] shows that on average the maximum turbidity in July is about three times more dense than the minimum values in the months of December and January. Knowing the value of km~ (minimum attenuation optical thickness) for a clear day and considering that this value stays practically constant for that day, we have exploited eqs (10) and (11) to find out km (the optical thickness) and then the value 0z at which IE would attenuate to the value of ID = 0.21 W/m z which is the threshold value and is explained earlier. Then using standard equations [10], time of the day at which this could happen can be calculated. The time lapse between these two values is again the maximum sun hours on that day. Such calculated values of maximum sun hours are also plotted against day length in Fig. 5. These values show same pattern as experimentally observed values and satisfy eq. (7). However in most cases the calculated value is lower than that practically measured. This is primarily because the value of km] decreases near the end of the day (Fig. 7) and hence practically longer sunshine hours are expected. Such simple calculations give us more conlidence on the validity of eq. (7). However the true range of values of "n" in eq. (7) will be given only when we have measured data of maximum sun hours for many years.

REFERENCES

1. M. Frere, J. O. Rijiks and Y. J. Rea, Organization Meteorological Mundial. Technical note No. 161, OMN No. 506 (1978). 2. P. M. Nast, Measurements on the accuracy of pyranometers. Solar Energy 31,279 (1983). 3. A. J. Mohr, D. A. Dahlberg and I. Dirmhirn, Experiences with test and calibrations of pyranometers for mesoscale solar irradiance network. Solar Energy 22, 197 (1979). 4. K. Krebs, Energy Standard Procedures for Terrestrial Photovoltaics Performance Measurement. Commission of European Communities (1981). 5. K. L. Coulson, Solar and Terrestrial Radiation. Academic Press, New York (1975). 6. M. Iqbal, An Introduction to Solar Radiation. Academic Press, New York (1983). 7. G. T. Trewartha and L. H. Horn (eds), An Introduction to Climate (5th edn), Chaps I-3. McGraw-Hill, New York (1980). 8. F . K . Lutgens and E. J. Tarbuck, The Atmosphere, Chap. 2. Prentice-Hall, New Jersey (1979). 9. F. L. Whipple (ed.), Earth, Moon and Planets (3rd edn), Chap. 5. Harvard University Press, Massachusetts (1970). 10. A. Rabl, Active Solar Collectors and Their Applications, Chap. 2. Oxford University Press, Oxford (1985). 11. SOLMET Vol. 2, Hourly Solar Radiation--Surface Meteorological Observations. Final Report TD-9724. National Climatic Centre, Ashesrille, North Carolina (1979).