Characterising PV Modules Under Outdoor Conditions: What's Most Important for Energy Yield
Standard parameters in Photovoltaic (PV) such as Final Energy Yield (YF) or Performance Ratio (PR) (IEC 61724) are simply calculated as sums (YF) or averages (PR) of the PV device performance over the measurement period and therefore do not allow for correlations with parameters such as irradiance, temperature or downtime. For a better characterisation and prediction of PV module performance under outdoor conditions detailed studies on the level of I-V parameters would be desirable. However,
... orithms as used in commercial simulation programs were found to be of limited use . Therefore a new "Loss factors model" (LFM) has been developed which determines a module's performance from its I-V curve simply as the product of five physically significant and independent normalized "loss factors" as well as spectral and temperature corrections. The benefits of such a Loss Factors Model will be shown in this paper. It allows validation of technology improvements, also performance distributions in mass production can be checked by quantifying initial losses and separating them from any long term changes. It also benchmarks different modules, production series or technologies at sites with different climates and can be used for Energy yield and Performance predictions. Differences at low light behaviour and temperature coefficients can be checked and validated with the LFM and seasonal changes can be distinguished from module degradation. Previously, the effect of energy yield losses due to RSC and ROC were hard to quantify, now they can be determined easily and the value of improvements can be estimated. In the present paper the LFM has been verified for different PV module technologies (c-Si, HIT, a-Si, CIGS, CdTe and a-Si/uc-Si) in two different climatic conditions (Switzerland and Arizona). Good fits to module performance (ISC, RSC, FF, ROC and VOC and hence PMAX or efficiency) were obtained under a wide variety of weather conditions.