Global irradiation

Quaschning, V. and Ortmanns, W. (2003). Specific cost development of photovoltaic and concentrated solar thermal systems depending on the global irradiation -a study performed with the simulation environment GREENIUS. The ISES Solar World Congress 2003,14-19 June 2003 Gothenburg, Sweden. Almeria, Spain DLR e.V. 

CIE Publication No. 85 [2] provides data on solar spectral irradiance for typical atmospheric conditions. A condensed version of a table for maximum global irradiance at the equator is given in ISO 4892-1 [3]. Reference solar spectral irradiance can be found in ISO 9845-1 [4] and analytical expressions for daily solar profiles are given in IEC 61725 [5], but this sort of data cannot generally be used to provide simplistic average acceleration factors. 

Irradiation Measurements. An Epply photometer with sensors for total global irradiance, photosynthetic active radiation (PAR), and ultraviolet (UV) radiation (295-385 nm) was used to measure irradiance continuously and averaged a measurement every 0.5 h. 

Figure 3. Average distribution of spectral global irradiance observed in Barcelona during 1989-92 for cloudless conditions and for the four turbidity classes considered (from figure 1 of Lorente at al. 1993).

UV-B spectral global irradiance and total ozone direct sun measurements carried out at Lisbon during 1990 with a MARK 11 Brewer ozone spectrophotometer and with clear-sky conditions were used for comparison at several zenith angles, total ozone and aerosol conditions. However, because aerosol optical depth measurements were not available at this time it was decided to use T5oo=0.0 for the model input. The results of the Model/Brewer ratios for global spectral irradiances are shown in Figure 5. The results are similar and consistent with the earlier LOWTRAN 7 (6) comparison, where the ratio approaches to unity as the wavelength increases,... 

Figure 6. Measured (Brewer) and computed (original and corrected) global irradiance spectra for Lisbon on l July of 1990 (Z=48.3°, 03=312 Dobson units and Tm> =0.3.

Figure 7. Direct, diffuse and global irradiance UV spectra confuted for Lisbon...

Figure 8. CIE erithema action spectrum and effective irradiance computed for global irradiance in figure 4.

Figure 4. Composites of the last 4 QBO cycles (1990-1999) for (a) 30 hPa zonal wind at Singapore, (b) total ozone at Thessaloniki, Greece and (c) global irradiance at 305 nm (solid line) and erythemal irradiance (dashed line). Total ozone and irradiances measurements correspond to clear sky conditions and solar zenith angle of 63° (updated from Zerefos et al., 1998).

In (3) ftg is the direct to global irradiance ratio, fd is the diffuse cosine correction factor and fb the direct cosihe correction factor, which is determined in the laboratory. A full description of the cosine correction procedure along with the uncertainties involved are described in detail in Bais et al., (1998). 

AM (0) sunlight The solar irradiance in space just above the earth atmosphere (air mass, AM, zero). Also called extraterrestrial global irradiance. 

AM (1) sunlight The solar irradiance traversing the atmosphere when the sun is in a position perpendicular to the earth surface. Also called terrestrial global irradiance. 

A second characteristic of UV solar radiation is fhat, even for very clear afmospheres, if is composed in similar amounfs of both beam and diffuse radiation (Hulstrom et al., 1985). The first is defined as the radiation arriving directly from the sun, while the second is the solar radiation that has been scattered by gases and aerosols after entering the earth s atmosphere. This second t)/pe of radiafion reaches fhe ground in a more or less diffuse manner that is, with similar intensity from all directions in the sky. In this respect, the situation encountered in solar photocatalytic reactors is quite different from fhe one encounfered in solar fhermal collectors. The latter are able to use the whole solar spectrum, and in that case diffuse radiation accounts for a much smaller fraction of fhe global irradiance. 

The value of the global irradiation (Eq. 7.3) varies widely with the time of day, the time of year, the latitude, the altitude, and atmospheric conditions. As indicated previously, the maximum solar irradiation incident on the earth s atmosphere averages 1366 W m-2. Because of scattering and absorption of solar irradiation by atmospheric gases (see Fig. 4-5), S on a cloudless day with the sun directly overhead in a dust-free sky is about 1000 W m-2 at sea level. 

We will consider a horizontal leaf exposed to full sunlight (Fig. 7-1) at sea level where the global irradiation S is 840 W m-2. We will assume that the absorptance of the leaf for global irradiation a is 0.60 and that the reflectance of the surroundings r is 0.20. By Equation 7.5, the direct plus the reflected sunlight, cloudlight, and skylight absorbed by the leaf then is... 

The direct solar radiation and the diffuse sky-radiation are combined under the term global radiation. The global irradiance EG of a horizontal area on the ground is made up of the following parts ... 

Figure 9. Increase of spectral global irradiance as a consequence of an increase of albedo by 0.1 for three different levels of albedo.

So far the effect of changing albedo was discussed only on global irradiance, that means on horizontal detector surfaces. If the surface is tilted or if actinic detectors (2n or 4k) are used, then significantly higher effects due to high albedo have to be expected. 

Figure 10. Increase of spectral global irradiance for an increase in altitude by 1000 m, based on measurements of three spectroradiometers (ATI, DEZ, FRG) at different altitudes (1200 m, 1750 m, 2964 m) relative to an instrument at a valley station (Garmisch-Partenkirchen, Germany, 730 m). The solid line is the average of the individual measurements.

Special instrumentation has been provided for the measurement of flie meteorological parameters of CDER site, like horizontal global irradiance-humidity and maximal-minimal ambient temperatures as illustrated in Figs. 3 and 4 respectively. The ambient temperature and relative humidity of the exposure location site is in the range of 18 0 °C and 20-95 % respectively. 

ASTM global irradiance standard solar spectrum (air mass (AM) 1.5) up to wavelength of 1000 nm, incident on a plane tilted 38° facing south, normalized to 1000 W m" for the whole spectrum. 

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