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101 renewable - Tracking Solar Panel I-V Curve

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Last Updated
12th of October, 2018

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Traking the I-V curve of an existing PV-Panel in a solar installation is paramount in obtaining an accurate diagnostic of what may impede the optimal output and the deviance of the plant's efficiency from the expected values. The focus is on the PV Fill Factor, the I-V Curve dependency of temperature and irradiance, the I-V Curve real profile against the expected values at standard conditions. We are talking about distinct situations where the real I-V curve profile potentially may give up the causes affecting solar panels faulty behavior: wrong type of silicon used, uncompensated temperatures and irradiance values, shading, degradation, soiling, defected cells, series resistance, and wrong bypass diode.

I-V Fill Factor:-

The Fill Factor (FF) is the measure of efficiency deviation from the maximum expected value. The Fill Factor (FF) depends of the type of silicon used in (module) cells, and most of the time is an indicator of defects in solar panel material. The Fill Factor (FF) provides an indication that changes have occurred during the panel time in service and those changes are reflecting silicon material stress and cell aging.

Solar PV Fill Factor diagram

To determine FF value we use following equation:

FF = (Impp * Vmpp) / (Isc * Voc) and it corresponds on the graph to (Area A) / (Area B)

>_Impp: Maximum Power Point Current or the amperage at maximum power point (Mpp) in standard conditions,
>_Vmpp: Maximum Power Point Voltage or the voltage at maximum power point (Mpp) in standard conditions,
>_Isc: Short Circuit Current,
>_Voc: Open Circuit Voltage

Comparing the FF value at different time intervals, it will give us an indicator of material depreciation, respective of defects occurring at cell level.

Temperature and Irradiance:-

Temperature and Irradiance are external factors which are directly influencing the panel efficiency, and the power output delivered to the system.The Voc, open circuit voltage of a PV module varies with cell temperature. As the temperature increases, due to environmental changes or heat generated by internal power dissipation, the open circuit voltage (Voc) decreases. This, in turn, reduces the power output and the current delivered to the system becomes significantly higher. In order to mitigate the effect of the temperature, during the dimension phase of the project, we use the PV module temperature coefficient.

Solar PV Fill Factor diagram

Irradiance is the second external factor which directly affects module performance. A dim light, a reduction of sunlight, will result primarily in a reduction of current pumped outside the solar panel and it will reduce the power output. Voc, the panel output voltage, changes very little with increase in irradiance incident on the panel.

Solar PV Fill Factor diagram

I-V Curve Tracking:-

An I-V curve tracking against the standard conditions allows the commissioners to determine if a curve aberration exists and based on the curve profile to interpret if we have either quality, power performance or installation problems of the solar PV system.

The effect on the curve can spell the cause of the deviation from the expected curve profile, and that can be either due to soiling, shading of the modules, high resistance wiring or connection problems between modules, a mismatch of modules caused by manufacturing or specification differences.

Here are some curve profile and the underline effects that cause the deviation to the expected curve profile in standard conditions:

Solar PV Fill Factor diagram

For the case modules are experiencing shading or having cells experiencing partial malfunctions, the curve's MPP is at the bottom of a pit. Replacement or relocation of the panel may be the appropriate solution.

Solar PV Fill Factor diagram

For the case modules are soiled or time degradation starts to bite, the MPP values are below the expected values, shifting the module performance downward . Appropriate measures are, panel professional cleaning, system repowering using panel power optimizers.

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