Uncertainty in Photovoltaic performance parameters – dependence on location and material

When considering the system yield, one needs to know the uncertainty in key parameters for the annual yield in order to determine the confidence limit. This requires a consideration not only of the instrumentation but also of the operating environment. The importance of this is demonstrated by carrying out an uncertainty analysis for different locations, technologies and instrumentations. The accuracy of the key parameters is determined with regards to whether the uncertainty margins allow meeting contractual obligations for guarantees of results. It is shown that different operating environments have different boundaries. The main uncertainty is in the irradiance which ranges from 0.6-1.5% and filters into the PR with up to 6% for northern Europe (Site 1)

Accuracy of energy prediction methodologies

In the current market, the specific annual energy yield (kWh/kWp) of a PV system is gaining in importance due to its direct link to the financial returns for possible investors who typically demand an accuracy of 5% in this prediction. This paper focuses on the energy prediction of photovoltaic modules themselves, as there have been significant advances achieved with module technologies which affect the device physics in a way that might force the revisiting of device modelling. The paper reports the results of a round robin based evaluation of European modelling methodologies. The results indicate that the error in predicting energy yield for the same module at different locations was within 5% for most of the methodologies. However, this error increased significantly if the nominal nameplate rating is used in the characterization stage. For similar modules at the same location the uncertainties were much larger due to module-module variations

Photovoltaic performance measurements in Europe: PV-catapult round robin tests

Two sets of modules have been sent around to different testing installations across Europe, one set to laboratories performing indoor calibrations and one set to laboratories performing outdoor power and energy ratings. The results show that for crystalline and polycrystalline devices, a very good agreement between laboratories has been achieved. A lower agreement between laboratories has been achieved for thin film devices and further need for research is identified

Accuracy of Energy Prediction Methodologies

In the current market, the specific annual energy yield (kWh/kWp) of a PV system is gaining in importance due to its direct link to the financial returns for possible investors who typically demand an accuracy of 5% in this prediction. This paper focuses on the energy prediction of photovoltaic modules themselves, as there have been significant advances achieved with module technologies which affect the device physics in a way that might force the revisiting of device modelling. The paper reports the results of a round robin based evaluation of European modelling methodologies. The results indicate that the error in predicting energy yield for the same module at different locations was within 5% for most of the methodologies. However, this error increased significantly if the nominal nameplate rating is used in the characterization stage. For similar modules at the same location the uncertainties were much larger due to module-module variations

Results of the Sophia module intercomparison part-1: stc, low irradiance conditions and temperature coefficients measurements of C-Si technologies

The results of a measurement intercomparison between eleven European laboratories measuring PV energy relevant parameters are reported. The purpose of the round-robin was to assess the uncertainty analyses of the participating laboratories on c-Si modules and to establish a baseline for the following thin-film round-robin. Alongside the STC measurements, low irradiance conditions (200W/m2) and temperature coefficients measurements were performed. The largest measurement deviation from the median at STC was for HIT modules from -3.6% to +2.7% in PMAX, but in agreement with the stated uncertainties of the participants. This was not the case for low irradiance conditions and temperature coefficients measurements with some partners underestimating their uncertainties. Larger deviations from the median from -5% to +3% in PMAX at low irradiance conditions and -6.6% to +18.3% for the PMAX temperature coefficient were observed. The main sources of uncertainties contributing to the spread in measurements were the RC calibration, mismatch factor and capacitive effects at STC and low irradiance conditions as well as the additional light inhomogeneity for the latter. The uncertainty in the junction temperature and the temperature deviation across the module were the major contributors for temperature coefficients measurements

Uncertainty in energy yield estimation based on C-Si module roundrobin results.

Results of the European FP7 Sophia project roundrobin of c-Si module power measurements at STC and low irradiance and temperature coefficients were used to calculate annual energy yield at four sites. The deviation in the estimates solely due to the different measurement results is reported, neglecting the uncertainty in the meteorological data and losses unrelated to the performed measurements. While minimising the deviation in Pmax measurements remains the key challenge, the low irradiance and temperature coefficient contributions are shown to be significant. Propagating the measurement deviation in c-Si module measurements would suggest that expanded uncertainty in energy yield due to module characterization alone can be as high as ±3-4%

Validation of proposed photovoltaic energy rating standard and sensitivity

PV devices are currently compared on the basis of the power measurements, which might not be as meaningful as a comparator as the energy yield. The energy rating standard proposed by the IEC promises to overcome this shortcoming. It has been implemented in three institutes and the issues with the current drafts are evaluated. The data required as the input for the energy rating is normally not available and synthetic datasets will have to be used or parts need to be estimated for validation studies. The validation against outdoor data shows that the uncertainty of the input data, specifically the angular distribution of the diffuse irradiance, makes the energy prediction part virtually not applicable for energy yield calculations. The validation effort shows a reduction of the standard deviation in the measurements, indicating that all environmental effects are considered. The evaluation of the originally proposed standard days shows that there is little information to be gained in their application as they are not representative of realistic conditions; they are enveloping the possible environments. This will in some cases over-emphasise the importance of certain effects, as their contribution to the overall energy yield might be negligible. Overall, the proposed standard represents an important advance on power rating. The standard is able to identify differences in device technologies. Further work might be required to make the output more relevant to a wider variety of users, though

Photovoltaic Performance Measurements in Europe: PV-Catapult Round Robin Tests

Two sets of modules have been sent around to different testing installations across Europe, one set to laboratories performing indoor calibrations and one set to laboratories performing outdoor power and energy ratings. The results show that for crystalline and polycrystalline devices, a very good agreement between laboratories has been achieved. A lower agreement between laboratories has been achieved for thin film devices and further need for research is identified