Measurement of Spectral-Spatial Distribution of ...

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May 18, 2011 - Mathias Leers1,4, Alexander Preiss1,2, Nicoletta Ferretti1 and Stefan Krauter1,2,3 ... program to follow every possible ray from the sky sphere.

26th European Photovoltaic Solar Energy Conference and Exhibition






Mathias Leers , Alexander Preiss , Nicoletta Ferretti and Stefan Krauter Photovoltaik Institut Berlin AG, Wrangelstr. 100, D-10977 Berlin, Germany, Phone: +49 (30)814 52 64-0, Fax:+49 (30) 814 52 64-101, e-mail:[email protected] 2 Technical University Berlin, Sec. EM 4, Einsteinufer 11, D-10587 Berlin, Germany 3 University of Paderborn, Institute for Electrical Engineering - Sustainable Energy Concepts, Pohlweg 55, D-33098 Paderborn, Germany, e-mail:[email protected] 4 HTW-University of Applied Science, Wilhelminenhofstraße 76/77, 12359 Berlin, Germany, e-mail:[email protected]


Index Terms – spectral effect, angle of incidence, yield, weak light performance, tilt angle ABSTRACT Accurate determination of power output and energy yield is crucial for profitable operation of PV power plants. Usually the level of global irradiance is used to determine resp. actual power output. However, this method does consider neither incidence angle nor spectral effects in detail. Usually, a rather vague "performance ratio" is used instead. In order to reduce that uncertainty, a simulation program to follow every possible ray from the sky sphere through the module encapsulation into the cell has been set-up, considering reflection at every interface, transitivity, dispersion and angular (resp. spatial) effects. However, this program requires as input the spatialspectral distribution of irradiance to carry out accurate simulations. To provide that data, the spatial distribution of the spectra and luminescence has been measured in an outdoor laboratory via a sky-scanner and a spectrometer. The measured irradiance distribution over the sky-sphere has been compared to standard models ([3], [4]) including spectral distribution; moreover the measured data has been used to calculate the effective irradiance received from different modules during a period of one year. The calculations of the resulting , power outputs, and energy yield shall be compared to actual measurements. The results of this work will become even more relevant while emerging thin film technologies such as a-Si, CdTe and CIGS have rather different spectral responses which cannot be analyzed and compared using the conventional global irradiance approach.

Figure 1: spectral measurement, exposed to ; th , Berlin, 18 may, 2011 between 10:00 AM and 6:00 PM. The global irradiance in measured via crystalline silicon sensors is suitable for yield prediction of crystalline devices. The electrical energy generated from the installed crystalline modules confirmed the predictions over the last decades. The irradiance measured by crystalline silicon cells is not useful for energy yield prediction of thin-film devices. The different spectral responses provide yield variations depending on the used technology. Sometimes even the same cell technologies are showing different performances [1]. There does not exist any general rule to use the irradiation values for thin-film prediction. The reasons for this are basically: spectral effects, annealing effects, degradation of the modules.

INTRODUCTION The prediction of the estimated energy yield, generated by photovoltaic modules is based on the measurement of environmental conditions. The most important of these input values is the incident irradiation measured in the horizontal plane. Applied models of conversion like e.g. Perez [3] or Klucher [4] are also considering module orientations different from horizontal.

Fig.1 shows the measured spectra during one day at the exemplary position (from south) and (out of the horizontal). The measurements have been taken between 10:00 AM and 6:00 PM. All curves in the figure are normalized to the same irradiance, so it is possible to compare the spectral relation. Depending on the spectral response there are different possible resulting yield.


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Figure 3: clear sky conditions, Berlin, May 18 , at 10:05 AM. (a) picture of the hemisphere during measurement, (b)distribution of the wavelength (energy weighted average). Figure 2: light reflection and transmission of multilayer structure [2].

In fig.3, 4 and 5 pictures of the sky conditions and the calculated average wavelength are shown for each condition. A position on figure 3b equates the same position on figure 3a.

MODELING To study the behavior of spectral effects more in detail, a "sky-scanner" has been installed at the Photovoltaik Institut Berlin AG. Single measurement points are taken across the hemisphere with a grating spectrometer. Thereby both the spectra and the irradiance are determined. The for any module type and orientation is calculated considering the spatial acquired spectral irradiance. For this calculation the refraction laws are applied on every incoming ray. (1) (2)



Additionally from the refracted sun rays (formula (8) and (9)), internal reflections will be considered. The calculations for two layers (glass and EVA) includes the internal secondary reflection in the first and the second layer , fig.2 (equations(10) to (13)).

(3) (4)


(5) (6)


(7) (12) reflectance parallel, vertical pol. transmission parallel, vertical pol. resulting reflectance for unpolarized light transmitted irradiation reflected irradiation


SETUP AND IMPLEMENTATION In the first instance the polarization of the light is not considered. The reflection and transmission of the light through the module encapsulation can be determined by the formula (8) to (13). Considering the spectral response ( ) of the cell material, the theoretical short-circuit current ( ) can be calculated.

Initially the weather where graded into three typical conditions. These, three different situations are evaluated for spectral distribution, incident irradiation and resulting short-circuit current for each orientation. For the calculations only the diffuse radiation is considered. In this way it is possible to separate the results between clear and overcast sky conditions.


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Figure 4 partially overcast sky conditions, Berlin, may th 19 , at 11:14 AM. (a) picture of the hemisphere during measurement, (b) distribution of the wavelength (energy weighted average).



Figure 7 incidence of irradiation and ISC visualized for overcast sky. (a) spatial distributed irradiation, (b) calculated for amorphous silicon (a-si)


(b) th

Figure 5: overcast sky conditions, Berlin, May 19 , at 01:15 PM.(a) distribution of the wavelength (energy weighted average), (b) picture of the hemisphere. EXAMPLARY TEST SERIES

The measurements are visualized for the following sky conditions: clear sky (fig. 3), partially overcast sky (fig. 4) and overcast sky (fig. 5).


Figure 8: theoretical for every module orientation: (a) amorphous silicon (a-si), (b) multicrystalline silicon (multi c-Si). possible to normalize the sum of the measured irradiance at each point. The aperture angle for each point is . The sum of the integral of all measured and corrected spectra, results into the spatial distributed irradiation over the hemisphere and should be equivalent to the global irradiance measured via a pyranometer.


The measurements have been taken on three days close to each other. Therefore, spectral effects due to seasonal changes do not occur. The effects shown are based exclusively on the cloud situation and its effect on the diffuse light portion. The hemisphere has been divided into 61 measuring points. By the determination of the horizontal diffuse irradiance with a pyranometer it is

To classify the sky conditions in terms of spectral distribution the concept of “Average Photon Energy (APE)” has been used (14). With this term a red or a blue shift of the measured spectral irradiance in comparison to the AM 1,5 spectrum can be directly observed. (14) Spectral irradiance Electronic charge Photon flux density For the AM 1,5 spectrum the APE results to be (for an integration in the range between 350 and 1050 nm) 1,88 eV. In figures 9 to 11 the APE value calculated for the measured spectra is plotted over the hemisphere.



Figure 6: spatial distributed irradiation, (a) clear sky,

(b) overcast sky.


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Figure 9: clear sky conditions, Berlin, August 18 , at 09:01 AM. (a) picture of the hemisphere during measurement, (b) distribution of the Average Photon Energy (APE).

Figure 10 dark clouded sky conditions, Berlin, August th 29 , at 01:00 PM. A picture of the hemisphere during measurement (left), distribution of the Average Photon Energy (APE) (right).

Figure 12: theoretical spectra for different surface directions of the module.

For the calculation of the short-circuit current the spectral response and the total irradiation incidencing on the cell have been considered. In fig.8a and 8b it is shown how the different module orientations effect short-circuit currents. As only diffuse light is considered, by increasing the module angle the short-circuit current decreases. This effect is due to the smaller portion of sky which irradiates on the tilted module.


Figure 11: light clouded sky conditions, Berlin, August th 29 , at 13:30 PM. picture of the hemisphere during measurement (left), distribution of the Average Photon Energy (APE) (right). For a better evaluation of the spectral effects on the module, each measured spectrum is projected on the module level (equations 1 to 13). For each direction is then possible to calculate the spectrum seen by the module. In Fig. 12 are shown calculated spectra for some exemplary directions and sky conditions. To this spectra the direct spectrum which, can vary depending on the sky conditions, should be also added.

The use of spatial spectral measurement data for energy yield predictions is strongly bounded to a place. The spectral distribution does not only depend on daytime or season, but also on the geographic location. Here the importance of spectral distribution for the determination of energy yield is shown. Therefore, to improve existing models, different locations and different weather conditions should be also considered. Furthermore, the behavior of thin-film technologies gives evidence of the spectral radiance influence. Information about the local spectral distribution could be useful for the production of modules. Especially in the case of multi-junction modules the spectral response could be optimized in order to obtain higher yields. Different spectral conditions may be compensated by varying the cell parameters. The facing direction of the module is also an important issue. For this reason in order to get more direct light the modules are usually directed to the south. Anyway, in the case there is no freedom of choice for the module direction a module with an optimal spectral response should be chosen.


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REFERENCES [1] A.Hügli, T.Hälker, S.Ransome, J.Sutterlüti, I.Sinicco. Outdoor characterization and modeling of thin-film modules and technology benchmarking. Hamburg, 2009. th 24 European PVSEC. [2] Prof. Dr. Stefan C.W. Krauter. Solar Electric Power Generation - Photovoltaic Energy Systems, Springer Press, 2006. [3] R.Seals J.Michalsky R.Stewart R.Perez, P.Ineichen. modeling daylight availability and irradiance components from direct and global irradiance, Solar Energy, 44, 1993. [4] T.Klucher. evaluation of models to predict insolation on tilted surfaces,1978.


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