There are various inspection methods that can be done to a solar asset. Depending on the dimension and the purpose, you can choose the type of inspection that suits you the most.
Indeed, it is necessary and important to know the solar photovoltaic system condition, both during the construction as the O&M phase.
So, we’ll talk about 3 different inspection methods for solar parks, their benefits, and limitations, namely: I-V Curves, Thermography and Electroluminescence.
In this blogpost we will talk about the I-V Curves.
Inspection Method: What is the I-V curve and how it works?
The I-V Curve is the curve that represents the power as a function of current (I) and voltage (V), produced by the photovoltaic module or string (depending on what you are testing – from now on we will always refer as string). The measurement equipment records the relation between the current and the voltage, varying the load, since the open circuit voltage until the short-circuit current. The graphic below shows the typical I-V curve and the respective power curve.
In the field, it is necessary to record the irradiance level, ensuring that the irradiance is measured at the same inclination and orientation than the string to be tested, and the cell temperature. It is also needed to define the string (number and type of solar panels) and the length and gauge of wire between the PV modules and the PV analyzer. Depending on the test equipment, this information could be made before or after the measurements. I’ll drop a list of some manufactures of I-V curve equipment:
What about the results of this inspection method?
When the I-V curve have a deviation from the standard curve, it means one of two things: or the string have a problem, or the test went bad because of misconnections, wrong equipment configuration or the low irradiance levels. Beside the fact that you must connect the equipment accordingly (may vary from test equipment), the standard IEC 61829:2015 Photovoltaic (PV) array —On-site measurement of current-voltage characteristics says that the minimum level of irradiance should be, at least, 700 W/m2.
Certain problems, such as microcracks, hot spots, manufacturing defects, or PID, can affect individual modules within a string differently. By conducting module-level IV-curve measurements, you can detect these module-specific issues that may not be apparent when assessing the entire string as a whole. On the other hand, conducting inspections and IV-curve measurements at the module level can be more time-consuming, especially in large-scale solar installations. Therefore, a thoughtful and strategic approach to sampling is crucial to balance the need for accuracy with practicality.
Each measurement gives you information about the PR and the FF, where:
PR – Performance Factor – is the most important information that the equipment could give you about the string performance. The values vary between 0% and 100% and a good and wealthy string have PR values above 90%.
Performance Factor (PF, %) = 100 * (Measured Pmax/Predicted Pmax)
FF – Fill Factor – It is the square defined by the quotient of the three important points of the I-V curve. The Isc, the Voc and the Pmáx (Imp,Vmp).
Basically, is the way to, accordingly to this inspection method, classify the efficiency of each module and PV technology. The range of FF is from 0 to 1 and it is dimensionless.
It’s important to note that the actual fill factor can vary depending on factors such as the type of solar technology (e.g., monocrystalline, polycrystalline, thin-film), manufacturing quality, operating conditions (temperature, sunlight intensity), and age of the solar cell or module. While the ideal fill factor varies by technology and application, it’s generally desirable to maximize FF to achieve the highest energy conversion efficiency possible.
A fill factor of 0.5 is considered the lower limit of acceptability. This means that only half of the available energy from sunlight is effectively converted into electricity, indicating significant energy losses within the device. Solar cells or modules with a fill factor below this threshold are typically considered inefficient or may have performance issues. A fill factor of 0.85 represents high efficiency. In such cases, 85% of the available solar energy is efficiently converted into electrical power. Solar cells or modules with a fill factor in this range are considered highly efficient and are desirable for optimal energy production.
How to “read” the curve?
The curve is influenced by 3 essential factors: solar module temperature, irradiance, and conservation state.
When you have higher values of temperature, the values of the YY axis (current) are slightly higher. On the other hand, with the increase of temperature, the voltage output reduces significatively so does the power output.
In its turn, the increase of irradiance levels increases the current output significatively and does not have an impacting effect on the voltage. Consequently, the power output increase with the rise of irradiance levels.
These 2 first factors only change the position of the curve in the axes, with the variations of Isc and Voc. However, the curve could have different shapes, and that is an indicator of the existence of defects.
There are several defects that can be identified by I-V curve analysis, such as module Isc mismatch, tapered shade or dirt dams, shunt paths exist in PV cells, effect of shading or damage cell, effect of series resistance, effect of module degradation or soiling, etc.
The graphic below shows 3 examples of possible defects:
It is a good inspection method to know if the performance of the string or module is in accordance with the expected. It could be quickly analyzed by the PF (should be between 90% and 100%).
However, it isn’t an expeditious method, and it could take some time to evaluate all photovoltaic system strings. Additionally, with the increasing use of string inverters that are able to provide the strings output in real-time through an APP, this inspection method could be used only to discover specific modules of a string previously identified by the inverter’s software string analyses.
All the best,