Diego Pavanello

DETERMINATION OF INTERNAL SERIES RESISTANCE OF PV DEVICES: REPEATABILITY AND UNCERTAINTY

The calibration of photovoltaic devices requires the measurement of their current–voltage characteristics at standard test conditions (STC). As the latter can only be reached approximately, a curve translation is necessary, requiring among others the internal series resistance of the photovoltaic device as an input parameter. Therefore accurate and reliable determination of the series resistance is important in measurement and test laboratories. This work follows standard IEC 60891 ed 2 (2009) for the determination of the internal series resistance and investigates repeatability and uncertainty of the result in three aspects for a number of typical photovoltaic technologies. Firstly the effect of varying device temperature on the determined series resistance is determined experimentally and compared to a theoretical derivation showing agreement. It is found that the series resistance can be determined with an uncertainty of better than 5% if the device temperature is stable within ±0.1 °C, whereas the temperature range of ±2 °C allowed by the standard leads to much larger variations. Secondly the repeatability of the series resistance determination with respect to noise in current–voltage measurement is examined yielding typical values of ±5%. Thirdly the determination of the series resistance using three different experimental set-ups (solar simulators) shows agreement on the level of ±5% for crystalline Silicon photovoltaic devices and deviations up to 15% for thin-film devices. It is concluded that the internal series resistance of photovoltaic devices could be determined with an uncertainty of better than 10%. The influence of this uncertainty in series resistance on the electrical performance parameters of photovoltaic devices was estimated and showed a contribution of 0.05% for open-circuit voltage and 0.1% for maximum power. Furthermore it is concluded that the range of device temperatures allowed during determination of series resistance in IEC 60891 should be further restricted.

Characterization of CPV cells on a high intensity solar simulator: A detailed uncertainty analysis

The authors modified a large area pulsed solar simulator to a high intensity pulsed solar simulator, moving the test measurement plane to various distances close to the xenon lamp. Measurement results reported in this work have confirmed the r−2 dependence of the total irradiance on the cell-to-lamp distance, up to the maximum measured intensity at 3000X. Classification of the solar simulator based on the international standard IEC 60904–9 is reported. Though the standard procedure for current-voltage measurements of non-concentrating cells prescribes a calibrated reference cell to detect the total irradiance independently, a “self-reference” method is usually preferred at high intensities by the CPV community. In this work the authors apply a detailed uncertainty analysis to both the procedures. The result highlights the pros and contra of the two methods, giving a useful tool in the pre-normative work for the preparation of norms and standard procedures for terrestrial CPV cells characterization.

Statistical analysis of weather conditions based on the Clearness Index and correlation with meteorological variables

The Clearness Index and the ratio between diffuse and global irradiance for different time scales are used to define a taxonomy for sky conditions and to study patterns to provide information for production-oriented probabilistic risk analysis studies for utility-level PV arrays. The study is focused on the definition of probabilities and distribution functions for the defined taxonomy of sky conditions. Correlations of the Clearness Index are investigated with different meteorological variables, to finally highlight that the only important correlation besides the irradiance is given by the relative humidity, though with a negative pattern. In conclusion, the analysis has shown that the three constraints of the standard test conditions (1000 W/m2, 25 °C, air mass of 1.5 global) normally applied for laboratory characterisation of PV modules are actually never simultaneously met outdoor.

Clearness-based sky taxonomy for one year irradiance data collected at BNL

The paper presents the analysis of one year irradiance data collected at Brookhaven National Laboratory during 2012, with the purpose to define a sky taxonomy mainly based on the calculation of the sky clearness index Kt. The evaluation is supported by additional meteorological variables like ambient temperature, wind, RH and atmospheric pressure. The analysis will additionally include the ratio of the diffuse-to-total irradiance for good weather conditions. Four sky categories are defined: cloudy, partly cloudy, sunny, and clear. The Kt parameter allows describing the solar climate of a particular location, providing also the basis for the estimation of solar radiation on horizontal as well as inclined surfaces. The statistical analysis defines the clearness index on the daily and yearly basis, but it also looks into hourly values to detect correlations between morning and afternoon conditions. The analysis herein developed is in the context of a statistical evaluation of the cloud coverage probability for application in a probabilistic production-oriented risk assessment analysis for photovoltaic systems.

kWh/Wp measurements & predictions of 13 different PV modules

At the begin of 2009 the Swiss PV module Test Centre at SUPSI-ISAAC started a new measurement campaign investigating thirteen different modules commercially available on the market. Two modules of each type have been exposed outdoors for energy yield monitoring and a third module, stabilised in advance, has been stored indoors as a reference. The modules covered a large range of different technologies ranging from multi-crystalline silicon (mc-Si) of which two with back-contact cells, 3 single-crystalline silicon (sc-Si), 1 hybrid mono-crystalline technology with amorphous silicon layer (HIT), 1 double junction amorphous silicon (a-Si/a-Si), 1 micromorph (a-Si/μc-Si), 1 Cupper-Indium-Sulfide (CIS) and 1 Cupper-Indium-Gallium-Diselenide (CIGS). The aim of the measurement campaign was to assess the quality of current technologies and the understanding of observed differences between technologies. Outdoor and indoor performance of the modules were analyzed over 15 months performing measurements under real operating conditions. The modules were therefore installed on a ventilated rack where each single module was connected to a maximum power point tracker delivering Im, Vm values in minutes intervals. The indoor measurements consisted in regular measurements under standard test conditions (STC) and 200W/m2, to determine the stability of the devices over time, and some initial temperature coefficient measurements and measurements at different irradiance levels. The annual energy output in kWh/Wp was calculated and simulations were performed based on the indoor measurements. The scope of the simulations was to explain the differences in energy output, by quantifying the losses generated by the two primary mechanisms: the temperature effect given by the temperature coefficient and the efficiency loss at low irradiances. Requirements for future energy rating of PV modules are given together with a discussion about the involved measurement uncertainties.