spectral solar irradiance in the range 300–1100 nm measured at valència, spain

Pergamon 0960-1481 (95) 0 0 0 9 5 - X

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Renewable Energy, Vol. 6, No. 8, pp. 997-1003, 1995 Copyright (c) 1995 Elsevier Science Ltd

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Spectral solar irradiance in the range 300-1100 nm measured at Valencia, Spain

J. A . M A R T I N E Z - L O Z A N O , M . P. U T R I L L A S a n d F . T E N A

Depar tament de Termodinfimica, Universitat de Valencia, 46100 Burjassot, Val6ncia, Spain

(Received 16 Apri l 1995; accepted 5 June 1995)

Abstract--A programme of measurements aimed at originating a database of spectral solar irradiance which is representative of a wide range of optical masses and atmospheric conditions is described. A preliminary analysis of the measurements for clear-sky conditions is also presented.

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

Nowadays, an extensive worldwide database containing experimental data for integrated global solar irradiance on the horizontal plane is available. Many stations record direct and diffuse integrated solar irradiance incident at ground level, and this has enabled the development over the last few years of a series of sufficiently contrasted models for the evaluation of the three components of solar radiation on planes of any orientation and inclination. However, the spectral distribution of solar irradiance and its dependence on geographical, astronomical and atmospheric factors has not been sufficiently established. This knowledge is o f fundamental importance not only for properly validating trans- mission models and determining the optical characteristics of the atmosphere, but also for designing and evaluating the efficiency of many systems of energy use of solar radiation, such as, for example, all those based on photovoltaic conver- sion. Furthermore, the effect of ultraviolet radiation on bio- logical systems strongly depends on the wavelength, so that the biologically active exposure must be determined by con- sidering the corresponding function of spectral action and integrating over the suitable region of the solar radiation spectrum [1].

In this work, we present a programme of measurements aimed at providing a database of spectral solar irradiance which can be representative of a wide range of optical masses and atmospheric conditions. This programme is based on a similar one undertaken by Riordan et al. [2] at the N R E L (National Research Energy Laboratory, formerly SERI) in the U.S.A. The use of different values of the optical mass is of fundamental importance because the scattering and absorption processes depend to a great extent on the path taken by the radiation through the atmosphere, However, the dependence of these processes on the optical mass becomes, in turn, a dependence on the location where measurements are taken, the day of the year and the time of day. Furthermore, it is well known that cloudiness, turbidity, precipitable water vapour, ozone and atmospheric pressure are mainly responsible for the fraction of extraterrestrial irradiance which reaches ground level. In Spain, besides the studies carried out by Cachorro et al. [3, 4] in the 1980s, only recently have systematic measurements of spectral solar

irradiance by Fabero and Chenlo [5, 6] and Lorente et al. [7] been presented.

I N S T R U M E N T A T I O N AND M E T H O D O L O G Y

Spectral solar irradiance was measured in the range 300- 1100 nm with an LI-COR 1800 portable spectroradiometer with a 6 nm bandwidth, and controlled by a portable PC and the manufacturer ' s software. The optical receptor of the LI- 1800 is a PTFE-dome cosine receptor with a 2Jz steradian field of view. The monochromator is a holographic grating, motor-driven scanning type which disperses the radiation into its spectral components . At the entrance to the monochromator is a filter wheel with seven filters and an opaque target. The detector, located at the exit slit of the monochromator, is a silicon photodiode operating in the photovoltaic mode. The step size was 2 nm and the scan time for the whole range of wavelengths was 27 s. Two scans were made for each measurement and the average value was saved into computer memory. This process required less than 90 s in total. The measurements were taken on the terrace of the Depar tment building in Burjassot at a height of 40 m above sea level; the spectroradiometer was calibrated every 6 months with a reference lamp (LI-COR optical radiation calibrator). The calibration results show a deviation lower than a 10% for wavelengths higher than 400 rim. For shorter wavelengths, this deviation increases gradually reaching values near to the 30% in the u.v.b. Results concerning to the cosine response of the Teflon dome have been published by Nann [8]. Between 400 and 1000 nm the error was less than 8% up to incident angles of 60 °.

In our work, we have considered as a min imum limit the errors obtained by Riordan et al. [2] that estimate average errors for several LI-1800s at NREL, for all the types of measurements in all orientations. The corresponding values are : 5 15% for the 300-350 nm band ; 5% for 350-450 nm ; 2% for 4650-950 nm and 4% for 950-1100 nm.

For a given optical air mass, the following spectral measurements were carried out : (i) global and diffuse solar irradiance on a horizontal plane ; and (ii) global, direct and diffuse solar irradiance on a plane normal to direct solar radiation. Figure 1 shows the spectral curves at 13:30 h on

997

998 Data Bank

14 July 1993 with an optical mass of 1.1. For the sake of comparison with the s tandard spectra of the ASTM [9, 10], diffuse and global spectral solar irradiance values for an optical mass of 1.5 were also measured occasionally on a 37 ° tilted plane.

For the direct component , a radiance limiting tube with a field of view of 5 ° was coupled to the Teflon diffuser of the spectroradiometer receptor. For the diffuse component , a shade disk was used. The designs of these devices were kindly provided by Fabero and Chenlo [11] and are based on previous work by Cannon [12]. However, the measurements taken with these additional devices (radiance limiter and shade disk) need to be corrected, as shown in Fig. 2. The correction factors depend on the incidence angle and the optical mass, but they do not depend on wavelength. They may also be affected by the reflected irradiance, a l though this influence has not yet been evaluated. However, no satis- factory method for determining their value is yet available.

The spectroradiometer was oriented manual ly on a tripod with the help of a three-axis joint and a system of alignment for the measurements in the direction normal to solar rays. This measurement procedure takes ca 15 min and therefore stable clear sky conditions are necessary.

Global and ultraviolet integrated irradiance, on a horizontal plane were continually recorded in the same location by means of a Kipp-Zonen CM-6 pyranometer and Eppley TUVR, respectively. An Eppley NIP pyrheliometer measured direct irradiance at normal incidence. Measured values were averaged and recorded every 10 min on an LI-1000 data acquisition system and transferred weekly to a mainframe computer. The instrumentat ion used constitutes part of a solar radiation measuring station described in a previous paper by Utrillas et al. [13]. The measurements of integrated irradiance serve two different functions : (i) they confirm the persistence (if any) of atmospheric conditions during the time of measurement and (ii) they allow for the characterization of atmospheric conditions through the indexes k t (ratio between the extraterrestrial global irradiation and the global irradiation at ground level on a horizontal surface), kn (the same as k, but referring to direct irradiation at normal incidence) and ktuv (same as k t but referring only to the 300-400 nm range). In order to estimate the content of atmospheric aerosols, the direct integrated irradiance was filtered by means of Schott OGI , RG2 and RG8 filters on the pyrheliometer, and compared with the direct spectral irradiance which was measured simultaneously. The water vapour content was estimated from relative humidity measurements , as no direct measurements were available. Direct measurements of ozone content were also unavailable.

The spectra were stored in a data base identifying solar time, solar azimuth, optical mass, measurement plane (horizontal, normal, 3 7 ) and type ofmeasurement (global, direct, diffuse). Furthermore, the time-averaged values of kt, k, and ktu,. were determined for every spectrum, as well as the instan- taneous values of k,, for the measurements made with the OGI , RG2 and RG8 filters.

RESULTS AND DISCUSSION

One thousand spectra have been measured and stored since the beginning of 1993. They have all been taken under clear sky conditions with optical masses ranging from 1.05 to 4.50. Half of them correspond to measurements on a normal plane (200 global, 150 direct, and 150 diffuse irradiance measurements) and the other half to measure-

ments on a horizontal plane (400 global and 100 diffuse measurements) .

Figures 3 (morning) and 4 (afternoon) show the dependence of the global spectral irradiance on the optical mass. They correspond to horizontal orientation of the receiver on a clear winter's day 28 January 1994. It is interesting to observe that, even though their shape is similar, the spectral curves for the same optical masses cannot be superimposed. This is clearly seen in Fig. 5, where the spectral curves for the optical masses of 2.04 and 2.17 have been superimposed ; they correspond to the morning and afternoon of the same day. Instead, for a given value of the optical mass, the afternoon irradiance values are significantly higher than the morning ones. This fact is observed for most of the days. The observed asymmetry with respect to solar noon has been previously reported by Cachorro et al. [3] and may be due to very different causes. In our case, the measuring station was located at a distance of 5 km Northwest of the downtown area of Valrncia, and this suggests a greater influence of urban aerosols on the morning measurements. This influence would be more important during winter, when thermal inver- sion does not break until well into the morning. During summert ime, there are morning mists (due to the influence of the sea) which disappear in the afternoon, and this would be reflected as a greater contribution of maritime-type aerosols. This influence of the aerosols is corroborated by the broadband measurements with the Eppley pyrheliometer and filters.

The influence of the optical mass on the spectral distribution is shown in an even more relevant way in the evolution of the direct and diffuse components of solar irradiance. Figure 6 shows the direct irradiance in normal incidence for different optical masses on the same day 28 January 1994. Figure 7 presents the diffuse irradiance on a horizontal plane corresponding to the same set of measurements. It is observed that large wavelengths have a greater relative contribution to direct irradiance than shorter wavelengths as the optical mass increases. This is due to an increase in the Rayleigh scattering in the shorter wavelengths (u.v. and blue) which causes the effect of reddening the solar disk in the hours close to sunrise and sunset.

The programme presented here enables the separate study of the direct and diffuse components of solar radiation. Direct spectral irradiance in normal incidence (see, e.g. Fig. 6), can be later used to evaluate, with the help of models available in the literature, the total transmittance of the atmosphere, as well as the transmissivity factors of the different atmospheric components. Utrillas et al. [14] report the first evaluation of the transmissivity of aerosols in the region of Valrncia.

Clear days are known to be characterized by a very low contribution of the diffuse component to global radiation. However, if only the u.v. spectral region is considered, it can be observed that the contribution of the diffuse component to global radiation is greater than the contribution of the direct component , even under very clear sky conditions (see Fig. 8). This is explained by the fact that, without water vapour, the scattering of radiation follows Rayleigh's law, and it thus become more important at shorter wavelengths. Furthermore, in the u.v.b, region at higher zenith angles, scattering should not only be due to Rayleigh scattering, but it is also caused by atmospheric ozone, which absorbs direct radiation much more than diffuse radiation coming from directions nearer to the zenith. This fact should be taken into account when designing defensive barriers against the effect of not only direct, but also diffuse u.v. solar radiation.

Data Bank 999

2 . 0

~ 1 . 5

1.0

0.5

--a--DIFFUSE (N)

-- GLOBAL (H)

+GLOBAL (N)

DIRECT (N)

---o--DIFFUSE (H)

0°0 r %,,,,..,,,.,-

300 400 500 600 700 800 900 I000 1 I00

~. (rim)

Fig. 1. Spectral distribution of direct, diffuse and global irradiance on the normal and horizontal planes on 13 July 1993, optical air mass 1.2.

. ' - ' 2 . 0 --r

1.5

1.0

0.5

0.0

:. GLOBAL

+DIREC~

A DIFFUSE

: DIRECT+DIFFUSE

.... (DIR+DIF)/GLOBAL

300 400 500 600 700 800 900 I000 1 I00

~. (nm)

Fig. 2. Comparison of the experimental values of global spectral irradiance with the sum of the experimental values of diffuse and direct components on the normal plane on 13 July 1993, optical air mass of 1.2.

1000

1.0

Data Bank

t 0 . 8

0.6

0.4

0.2

0.0

. , ; , , , , . , -

r,. . . . . #

t • m

J

• 08:28 (M=3.70) -- ,L-- 09:58 (M=2.17) ----a-- 10:17 (M=2.04) .- B- 10:59 (M=1.87)

11:46 (M=1.78) - - ~ - 12:31 (M=l.80)

~,'* °

300 400 500 600 700 800 900 1000 1100

7~ (rim)

Fig. 3. Spectral distribution of global irradiance on a horizontal plane for different values of the optical mass on the morning of 28 January 1994.

1.0

0.8

0.6

0.4

0.2

O I

° , ° , ,

-"/"4' ~ ' ° ' ' ° " " . ' o . • •

. .~ t t

e

• • • • .

:. 12:31 (M=l.80) ...A-. 13:20 (M=1•93)

- 13:42 (M=2.04) • . B . 14:00 (M=2.16)

14:32 (M=2.46) -- o . 14:50 (M=2.71)

15:26 (M=3.51) -- , - - 15:48 (M=4.38)

" ~ l ~ . - "" •- - ~ . . . . * L/ , ~

- , ' , . I [ 'F~. I T . . . .

0.0

3 ~ 400 500 600 7 ~ 800 900 1000 1100

7~(nm)

Fig. 4. Spectral distribution of global irradiance on a horizontal plane for different values of the optical mass on the afternoon of 28 January 1994.

Data Bank 1001

. - - , 0 . 8

_ 0.7

0.6

0.5

0.4

0.3

0.2

0.1

O . O

300

p , s l t l " ¢ l I ~ l ¢ / t,e tt/4,. "11 t t'a ~,

~,~ ~ ~*~ 1 I

l~#tl

400 500 600 700

¢ 09:58 (M=2.17)

- 10:.17 (M=2.04)

- - l-- 13:42 (M=2.04)

- - ,-- 14:00 (M=2.16)

800 90O 1000 1 ! 00

~. (rim)

Fig. 5. Compar ison of spectral curves of global irradiance on a horizontal plane corresponding to the same optical mass and day 28 January 1994.

.--. 1.5 ~ • M=l.8

~, • M=2.5

1.2 • .= M=2,8

0.9 ~ u t " ~ ~ ¢ M=3.5

0.6 -~

300 400 500 600 700 800 900 1000 1 lO0

(nm)

Fig. 6. Evolution of spectral distribution of direct irradiance on a normal plane for different optical masses on 28 January 1994.

1002

0.20

Data Bank

0.16

0.12

0.08

0.04

"- M=l.8

- ; M=2.5

--*--M=2.8

c M--3.5

0.00 4 300 400 500 600 700 800 900 1000 1100

(nm)

Fig. 7. Evolution of spectral distribution of diffuse irradiance on a horizontal plane for different optical masses on 28 January 1994.

~--, 1.0 -1

0.8

0.6

0.4

0.2

0.0 - ~

3OO

, DIFFUSE (H)

~ G L O B A L (H)

• DIRECT (H)

320 340 360 380 400

7~ (nm)

Fig. 8. Spectral distributions of global, direct and diffuse u.v. irradiance incident on a horizontal plane on 24 June 1993, optical air mass 1.08.

Data Bank 1003

---, 0.5

"~~ 0.4 1

0.3 i

0.2

0.0 , 300

A 08:06 (M=l.60) - 09:00 (M=1.32)

10:47 0VI= 1.08)

14:00 (M= 1.52) , 16:00 (M=1.64) j,.,. ]

' ' I ' ' ' I , w I ' ' I '

320 340 360 380 400 x (nm)

Fig. 9. Spectral distribution of diffuse u.v. irradiance incident on a horizontal plane for different values of optical mass on 24 June 1993.

Finally, Fig. 9 represents the spectral distribution of the diffuse ultraviolet irradiance on a horizontal plane for different optical masses of a summer day. The curves show clearly different behaviours in the u.v.a, and u.v.b, regions. Diffuse radiation is practically independent of wavelength in the u.v.a, zone, while it grows very steeply in the u.v.b, zone.

R E F E R E N C E S

1. B. L. Diffey, Stratospheric Ozone Reduction, Solar Ultra- violet Radiation and Plant Life. Springer-Verlag, Berlin (1986).

2. C. Riordan, D. Myers, M. Rymes, M. Hulstrom, W. Marion, C. Jennings and C. Whitaker, Spectral solar radiation data base at SER1. Solar Energy 4 2 , 67-79 (1989).

3. V. E. Cachorro, A. M. Frutos and J. L. Casanova, Developing and checking of a spectral solar irradiance measurements system. J. Rech. Atmos. 19, 15-24 (1985).

4. V. E. Cachorro, A. M. Frutos and J. L. Casanova, Medida de la irradiancia solar espectral en el rango 40(~1000 nm: su evoluci6n con diversos parametros atmosf~ricos. Opticapuray Aplicada 18, 135-147 (1985) (In Spanish).

5. F. Fabero and C. Chenlo, Anfilisis de la distribuci6n espectral de la irradiancia solar en Madrid. V. Congreso Ibkrico de Energia Solar (1990) (In Spanish).

6. F. Fabero and F. Chenlo. Analysis of the variation in

the spectral distribution of solar irradiance. Proc. lOth E.C. Photovoltaic S.E. Conf. (1992).

7. J. Lorente, A. Redafio and X. De Cabo, Influence of urban aerosol on spectral solar irradiance. J. Appl. Meteor. 33, 406~413 (1994).

8. S. Nann, Uncertainties in determination of short-circuit current from measures and modeled spectral solar irradiance. Proc. 9th European Photovoltaie S.E. Conf. (1989).

9. ASTM, American Society for Testing and Materials, Standard terrestrial direct normal solar spectral irradiance tables for air mass, E892-82 (1982).

10. ASTM, American Society for Testing and Materials, Standard tables for terrestrial solar spectral irradiance at air mass 1.5 for 37 '~ tilted surfaces, E892-87 (1987).

11. F. Fabero and F. Chenlo, Instituto de Energias Renovables (IER), CIEMAT. Personal communication (1993).

12. T .W. Cannon, Spectral solar irradiance instrumentat ion and measurements techniques. Solar Cells 18, 233-241 (1986).

13. M. P. Utrillas, J. A. Mart inez-Lozano and A. J. Casano- vas, Evaluation of models for estimating solar irradiation on vertical surfaces at Valdncia, Spain. Solar Energy 47, 223-229 (1991).

14. M. P. Utrillas, J. A. Mart inez-Lozano and V. Cachorro, Estudio de la turbiedad atmosf6rica a partir de medidas espectrales en la banda 400 nmq570 nm. VII Congreso Ibkrico de Energia Solar (1994) (In Spanish).

spectral solar irradiance in the range 300–1100 nm measured at valència, spain

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