UL Standard for Safety Photovoltaic System Performance – Part 3: Energy Evaluation Method
|Publication Date:||29 January 2019|
This part of IEC 61724, which is a Technical Specification, defines a procedure for measuring and analyzing the energy production of a specific photovoltaic system relative to expected electrical energy production for the same system from actual weather conditions as defined by the stakeholders of the test. The method for predicting the electrical energy production is outside of the scope of this technical specification. The energy production is characterized specifically for times when the system is operating (available); times when the system is not operating (unavailable) are quantified as part of an availability metric.
For best results, this procedure should be used for long-term performance (electrical energy production) testing of photovoltaic systems to evaluate sustained performance of the system over the entire range of operating conditions encountered through the duration of the test (preferably one year). Such an evaluation provides evidence that long-term expectations of system energy production are accurate and covers all environmental effects at the site. In addition, for the year, unavailability of the system (because of either internal or external causes) is quantified, enabling a full assessment of electricity production.
In this procedure, inverter operation and other status indicators of the system are first analyzed to find out whether the system is operating. Times when inverters (or other components) are not operating are characterized as times of unavailability and the associated energy loss is quantified according to the expected energy production during those times. For times when the system is operating, actual photovoltaic system energy produced is measured and compared to the expected energy production for the observed environmental conditions, quantifying the energy performance index, as defined in IEC 61724-1. As a basis for this evaluation, expectations of energy production are developed using a model of the PV system under test that will serve as the guarantee or basis for the evaluation and is agreed upon by all stakeholders of the project. Typically, the model is complex and includes effects of shading and variable efficiency of the array, but the model can also be as simple as a performance ratio, which may be more commonly used for small systems, such as residential systems.
The procedure evaluates the quality of the PV system performance, reflecting both the quality of the initial installation and the quality of the ongoing maintenance and operation of the plant, with the assumption and expectation that the model used to predict performance accurately describes the system performance. If the initial model is found to be inaccurate, the design of the system is changed, or it is desired to test the accuracy of an unknown model, the model may be revised relative to one that was applied earlier, but the model should be fixed throughout the completion of this procedure.
The aim of this technical specification is to define a procedure for comparing the measured electrical energy with the expected electrical energy of the PV system. The framework procedure focuses on items such as test duration, data filtering methods, data acquisition, and sensor choice. To reiterate, the procedure does not proscribe a method for generating predictions of expected electrical energy. The prediction method and assumptions used are left to the user of the test. The end result is documentation of how the PV system performed relative to the energy performance predicted by the chosen model for the measured weather; this ratio is defined as the performance index in IEC 61724-1.
This test procedure is intended for application to grid-connected photovoltaic systems that include at least one inverter and the associated hardware.
This procedure is not specifically written for application to concentrator (> 3X) photovoltaic (CPV) systems, but may be applied to CPV systems by using direct-normal irradiance instead of global irradiance.
This test procedure was created with a primary goal of facilitating the documentation of a performance guarantee, but may also be used to verify accuracy of a model, track performance (e.g., degradation) of a system over the course of multiple years, or to document system quality for any other purpose. The terminology has not been generalized to apply to all of these situations, but the user is encouraged to apply this methodology whenever the goal is to verify system performance relative to modeled performance. Specific guidance is given for providing the metrics requested for the IECRE certification process, providing a consistent way for system performance to be documented.