Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

3. Modelling and analysis (SP4)
3.1 ISAAC SP4 ACTIVITIES (2008)
3.1.1 Validation of the Energy Rating Standard - IEC61853 Draft version (WP4.3)
Currently photovoltaic (PV) modules are compared and characterised under Standard Test Conditions
(STC). This is not sufficient to explain differences in energy production between modules under real
operating conditions. The proposed IEC 61853 standard describes the energy yield with regard to
irradiance, spectral distribution of the light, angle of incidence effects and module operation temperature.
The current draft of this standard consists of four parts. Part 1 describes the test methods to map
module performance over a wide range of temperature and irradiance conditions. Part 2 focuses on
measurements describing spectral and angle of incidence effects as well as a procedure to determine
the module operating temperature as function of irradiance, wind speed, ambient temperature and
mounting structure. The methodology of the energy rating procedure is described in part 3, whereas
part 4 contains the standardized weather conditions for which the energy rating has to be specified. A
short summary of the latest version is given in [2].
The present study investigated the proposed IEC 61853 energy rating standard by using real monitoring
data acquired over a full year and in 1 minute intervals. A full description of the outdoor tests is
given in [REF5]. The aim of the study here was to prove the capability of the standard to confirm differences
in-between modules performing differently under real operating conditions. The energy outputs
for three very different crystalline silicon modules selected out of 3 performance classes (M1: best
c-Si modules; M2: modules with 3-6% lower energy output compared to the best ones, M3: modules
with 6-8% lower energy output compared to the best ones) and a CdTe module (M4), were therefore
calculated and compared to its real outputs. To be able to calculate the energy output the modules
had to be first characterised according to the IEC61853 standard for: (1) irradiance and temperature
dependencies, (2) spectral response, (3) thermal coefficients for different wind-classes and (4) angle
of incidence effects. The measurements were partially executed at SUPSI-ISAAC (1 and 3), JRC-ESTI
(2) and Arsenal Research (4).
To evaluate the weight of part 2 of the standard on the final energy prediction of different modules,
some of the steps of part 2 have been evaluated separately. Following abbreviations are used here to
identify the single steps: Gi (measured in-plane irradiance), DNI (irradiance modelled with the Klucher
model [REF6]), LT (reflection correction), SP (spectral correction), Tm (measured module temperature)
and Ta (calculated module temperature). The numbers obtained in this way are compared to the
results obtained by totally neglecting part 2 (part 1 only - DNI_Tm) or by using directly in-plane irradiance
and module temperature as input (best case - Gi_Tm).
� � �
Gi_Tm�
MBE� RMSE
DNI_LT�
DNI_Tm�
SP_Tm
MBE� RMSE MBE RMSE
DNI_LT�
SP_Ta�
MBE� RMSE�
M1� Ecal�Emes� [%]� �1.6� 3.9� �0.7� 4.8� �0.1� 6.1� 1.4� 6.6�
M2� Ecal�Emes� [%]� �1.0� 3.8� �0.2� 5.0� �0.7� 6.1� 0.0� 6.0�
M3� Ecal�Emes� [%]� 2.0� 4.1� 2.6� 5.2� 2.2� 6.5� 3.4� 7.1�
M4� Ecal�Emes� [%]� 1.5� 4.6� 2� 11.2� 8.4� 24.4� 8.8� 24.6�
M4*� Ecal�Emes� [%]� /� /� /� /� 0.2� 6.9� 0.6� 6.8�
Table 2: Energy rating error (MBE and RMSE) between calculated and measured energy output for a
set of long term data, representing a full year at Lugano site and for 4 different modules (M1-
M3: c-Si, M4: CdTe). For M4* the errors are recalculated with a narrower spectral band for
CdTe of 300-900nm. All other values are calculated for 300-1200nm.
Table 2 gives a summary of the results. It shows that the proposed energy rating standard led to annual
energy prediction accuracy, here described by the MBE value (mean bias error), in the range of
-1.6% to +3.4% and a RMSE value (root mean square error) of 3.8-6.9% when a optimal spectral
range is used for the spectral loss calculations. The implementation of part 2 leads to no major improvements
compared to part 1, but to a slight increase of the RMSE. Improvements could be only
observed for single clear sky days but by leading at the same time to a decline for most other days
and consequentially to an overall drop in accuracy. This is mainly due to the higher uncertainty of the
additional characterisation methods of part 2, also due the fact that they are not regularly used in all
laboratories today and that they are not validated as extensively as STC power measurements. The
PERFORMANCE, G. Friesen, SUPSI, ISAAC-TISO
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