Joint Research Activity 2 : Greater accuracy of PV modules rated power and energy output prediction of PV modules and systems

The power rating of photovoltaic modules is the key for energy and lifetime evaluations. For years, the conventional single crystal technology had a rather well-known behaviour from the peak power viewpoint. With the diversification of the PV products, which means new materials and technologies used, several important facts are observed :  
• The rated peak power of a PV module or system is no longer an absolute reference: the first hours, days or weeks of illumination lead to either a light-induced degradation for some technologies or to an improving light-soaking effect for some others. Therefore the power rating of some technologies is time-, history- and temperature-dependant.
• Two technologies with the same rated power will not provide the same energy at the end of the day or of the year. The difference in some cases can be as high as 20%.
• The installation of PV modules become more and more “building integrated”, which results in various operating conditions

All these phenomena have to be investigated mainly through Round Robin tests, to end up with harmonised criteria, preconditioning and measurement procedures, in order to provide a better accuracy of the rated power and the energy output prediction, as quickly as possible.

This activity is divided in six subtasks :
JRA 2.1 : Measurement technology and Quality management
The various test methods will be compared, in order to obtain guidelines or requirements :
- on data acquisition (time resolution, sampling frequency, averaging methods, etc.) for metrological and electrical data, technical requirements on measuring equipment, .
-  on irradiance and irradiation measurement as reference for the energy yield
- Regarding BIPV additional measurement of non-electrical data e.g. temperatures or air flow rate of backside ventilation becomes necessary. Appropriate requirements will be defined. Results will be used for modelling of PV modules in JRA 4.
The objective is to obtain a systematic evaluation and minimisation of measurement uncertainty, and especially regarding the low irradiance behaviour and the temperature coefficient
In the context of life-time energy and degradation studies the uncertainty must indeed be better than the expected degradation rates, which are in the range of 0-3% per year.

JRA 2.2 : Collection and management of measurement data
- standardized database for the storage of irradiation data and module or system performance data.
- remote access to selected data by internet based communication
- elaboration of guidelines for performance evaluation of PV modules
- software tools for generic data analysis

JRA 2.3: Power rating of innovative technologies : measurement and preconditioning requirements
New technologies such as back-contact cells, thin-films and multi-junctions cells remain very challenging for power measurement. The light-induced degradation phenomenon is also problematic.
The best way to evaluate and improve the measurement accuracy is through exchange of samples between the laboratory and compare the measurement procedures. Two formal Round Robin tests, with both indoor flash-test and outdoor measurements are anticipated during the four year period.
• Indoor test will include I-V light-dark, Qyield, spectral ellipsometry of the solar cells ARC. The IV-measurement will be performed with non-steady state devices using flasher lamps and steady-state devices
• Outdoor operation performed for comparative purpose will be periodically controlled in indoor conditions in order to analyze changes in performance.
New procedures to assess power rating (depending on several preconditioning processes), low irradiance behaviour, and temperature coefficient will be selected in order to minimise the uncertainty. The major goal is to provide access to reliable and comparable IV-characterisation within various research infrastructures.

JRA 2.4: Module Energy output : measurement and prediction
Energy measurement will also be performed during the Round Robin tests. These results will be used to :
1. Recommend the best practices to use
2. Validate energy output prediction models. The goal of this task is mainly to bring energy yield prediction to acceptable uncertainties, whether it is based on indoor measurements or quick outdoor characterisation.

JRA 2.5: Specific requirements for CPV systems
Formation of a spectral network based on component cells
Processing and evaluation of data collected in NA 5: Rating of several CPV modules at different sites to assess discrepancies and uncertainties
Evaluation of the impact of mechanical and tracking errors on the energy performance
Evaluation of the impact of pointing (tracking) / structural looseness errors in the energy performance of CPV systems, independently of the cell technology and CPV system, and development of software tools for its calculation.

JRA 2.6: Specific requirement for BIPV systems
When applied as building integrated product, the energy output of PV modules is affected by the installation method in the building : vertically, horizontally, tilted, with or without air flow and the back side. Especially the thermal behaviour of the module is of major importance. When used in the building envelope, PV modules must be regarded as structural elements providing several functions like weather protection and thermal insulation, which also influences the electrical energy output. Furthermore, the influence of the building integration on the electrical performance depends also on the used PV technology.
Actions to be made :
- Comparative of different BIPV systems in different countries/climates : The additional parameters required and their relevant test procedures will be selected, in order to obtain an harmonized characterisation
- Specific accuracy and uncertainty of measurement procedures and performance predictions of BIPV systems
- Elaboration of models mathematical procedures including thermal behaviour of these systems with the building for error analysis
Results will be used in JRA 4 for the development and verification of models