By Will White, Fluke Solar Application Specialist
Optimizing the efficiency of bifacial solar modules through precise I-V curve measurement is key to maximizing energy yield and return on investment (ROI) for solar professionals. Compared to traditional monofacial modules, bifacial solar modules have gained popularity in the solar industry due to their ability to capture sunlight from both sides, potentially increasing energy yield.
Despite these advancements, the capacity rating under Standard Test Conditions (STC) remains based only on light from the module's front side. Not including backside irradiance means the additional energy production from the rear side is not fully captured in the standard module power rating.
In this article, we will discuss what to look for when conducting I-V curve tests on bifacial solar modules.
Standard Test Conditions and Bifacial Gain
Standard Test Conditions (STC) for solar modules include an irradiance of 1000 W/m², a cell temperature of 25°C, and an air mass of 1.5 atmospheres. However, these conditions do not account for the bifacial gain, which is the additional power generated by the rear side of bifacial modules.
This gain depends on factors such as ground reflectance and installation height, which are not standardized in STC measurements. Therefore, comparing the measured results to the manufacturer's rating under STC while testing bifacial modules is challenging.
Testing Bifacial Modules with Fluke I-V Curve Tracers
The Fluke SMFT-1000 Multifunction PV Tester/Performance Analyzer and the PVA-1500 PV Analyzer/I-V Curve Tracer capture tilt angle, module temperature, and frontside irradiance during an I-V curve test. Capturing these data points allows the software to translate the measured I-V curve to what the curve would have been under standard test conditions of 1000 W/m2 and 25°C cell temperature.
This information helps compare the actual performance of a module or string of modules to manufacturer recommendations or other test results.
Since bifacial modules gain performance from backside irradiance, which is not included when rated under standard test conditions, their field performance will exceed their rating at STC. This difference can make it challenging to compare the measured results to the manufacturer's rating since the actual performance of the module will often exceed what's expected.
Bifacial modules are usually compared between measurements instead of comparing the measured results translated to STC to the manufacturer's rating. When comparing test results of different modules or strings of modules, it's critical to ensure that they are the same module manufacturer and model, have the same number of modules in series, and are mounted at the same tilt angle and orientation.
Comparing I-V Curve Test Results
To identify outliers and ensure consistent performance across different strings of modules, compare each string's fill factor (SMFT-1000 and PVA-1500), performance factor (PVA-1500 only), and I-V curve. The fill factor (FF) is a metric used to identify curves that are operating less efficiently due to deviations from normal I-V curve shape.
The fill factor is calculated as the ratio of the maximum power (the power at the maximum power point, MPP) to the product of open-circuit voltage (Voc) and short-circuit current (Isc):
(Imp x Vmp) ÷ (Isc x Voc) = Fill Factor
A higher fill factor indicates higher efficiency. A low fill factor may indicate issues with the string of modules, which would require further troubleshooting.
The performance factor (PF) compares the actual power output to the expected power output under actual conditions. This metric is provided in the PVA-1500 PV analyzer and data analysis tool (DAT) software.
Since the performance factor is a ratio of measured vs. expected power based on frontside STC specifications, the performance factor with bifacial modules will typically exceed 100%. This result is not unusual and is to be expected. Performance factor can still be used to identify outliers, modules, or strings with a result lower than those of comparable modules or strings under comparable conditions.
The I-V curve of different tests can be overlaid in the SMFT-1000 and PVA-1500 software to identify outliers quickly. The measured I-V curve translated to STC will typically outperform the manufacturer's expected I-V curve at STC because of the backside gain. Still, they should be similar to other comparable modules or strings.
You can identify any underperforming modules or strings by comparing the fill factor, performance factor, and the I-V curve of different strings. This comparison helps pinpoint issues such as shading, soiling, or module degradation, allowing for targeted maintenance and optimization.
Using the Fluke SMFT-1000 and PVA-1500 for I-V curve tracing of bifacial modules provides comprehensive insights into their performance. Accurate testing and detailed analysis help optimize the installation and maximize the energy yield of bifacial solar modules.
Key Takeaways for Effective Bifacial Module Testing
Testing the performance of bifacial modules provides valuable insights into their effectiveness. While the bifacial factor is not included in modules' STC specs, using frontside irradiance measured with Fluke SMFT-1000 and PVA-1500 allows for a consistent and comparable analysis of bifacial module performance.
By standardizing the approach to testing and comparison, solar professionals can reliably assess the efficiency and quality of bifacial modules, even without accounting for backside irradiance. This method identifies underperforming modules and informs decisions for troubleshooting and maintaining solar arrays.
