Temperature variance over the day, and/or seasons has a significant influence on the production capabilities of the PV-Modules in your array. This article focuses on this effect and how to estimate discrepancies in design, so you can determine if a PV module is compatible with Tigo's TS4 MLPE products.
Note. Only the temperature effect on PV modules has been considered in this article. A truly great solar design will incorporate a more extensive investigation - including topics like: altitude, orientation and the albedo (light reflection) effect.
- Temperature Correction
- Temperature Coefficient
- Comparing Data Sheets
- Case Examples
- Quick Temperature Correction Calculator
The reviewing of the data sheets for both the PV-Modules and Tigo MLPE's is important because the maximum voltage output of a PV-Module may change in a specific location, or under certain conditions. This review process is sometimes referred to as "Temperature Correction."
Historic temperature data (maximum recorded highs and lows) for the system's location, can be used to add precision to your calculations. However, please keep in mind that the required information may not always be available for all locations.
In physics, a coefficient is a multiplier or factor that measures some property. When designing a system, it is important to use the Temperature Coefficient of the PV-Module to calculate the gains (or losses) in voltage due to local ambient temperature changes. This will ensure that the PV module is compatible with the system's voltage specs.
The common practice is to compare the PV-Module's Temperature Coefficient against the lowest recorded temperature for the area. However, solar designers have realized that using a 100-year record-low temp resulted in overly conservative designs. The 2017 NEC 690 added a section that authorized the use of ASHRAE Temperature Data for these calculations since they can be more reflective of average conditions.
Comparing Data Sheets
The Temperature Characteristics section of a PV-Module's data sheet lists the temperature coefficients for power, open-circuit voltage (Voc), and short circuit current (Isc). These values indicate how strongly each component depends on the cell temperature (surface temperature of the PV module).
The power (Pmax) and voltage (Voc) coefficients are negative because they will decrease with increasing temperature. The coefficient of Isc is a positive number since the current slightly increases with a rise in temperature.
Module data sheets usually graph the effects of temperature on these three electrical characteristics. This graph clearly shows these relationships:
Max Power and Voc decrease, while Isc increases (as the cell temperature increases). Since the temperature has such an adverse effect on voltage, we must use the coefficient values to find the worst-case scenario (with respect to Voc), at the array's location.
Using the available data sheets, along with the temperature resources (below), we can confirm whether the PV-Module is compatible with the TS4-A-O:
From these data sheets, you will need to gather four pieces of information:
- Voc (from PV-Module data sheet)
- Temperature Coefficient of Voc (%) (Temperature Characteristics of PV-Module data sheet)
- Max Voc (from TS4 product data sheet)
- ASHRAE temperature for the area (links provided below)
Then, start plugging this information into a resource to calculate the effects on the PV-Module's output.
Step 1: Go to: Maximum Solar Voc and Pmax Online Calculator
Step 2: Fill in the calculator data using the PV-Module data sheet information and the additional temperature information found in the links below.
In this exercise, we used the ASHRAE data provided for Sacramento, California:
Below is what the Maximum Solar Voc and Pmax Online Calculator entries look like for this area:
Note: The Extreme Minimum (-3 ºC) was used for the Worst Case Temperature Value. For string sizing applications, you would also include a 2% average temperature value (as per the NEC 690).
Step 3: Click submit and review results
The two most important results are:
- Max Voltage: 52.94 (under the 80V maximum on the TS4)
- Pmax: 570.96 (under the 700W maximum on the TS4)
When comparing these numbers, we see that the PV-Module easily fits within the limits of the TS4 (even though this PV-Module could make up to 571W in the right conditions!).
Although there is no calculation required for Isc compatibility, we must still verify that the PV-Module's value is below the 15A limit for the TS4-A-O. At 13.57A, this PV-Module is well within limits.
Conclusion: Even in the worst-case scenario of temperature for this area, this module output will not exceed the TS4 tolerances.
Q: What happens if I am using a module that has corrected temperature values for voltage and power that are close to, or exceed the TS4 limits?
The most common scenario where this could happen is when using a Tigo TS4 product that accepts inputs for 2 PV-Modules (like with the TS4-A-2F).
For example: Let's take a quick look at a design using the TS4-A-2F. The data sheet specs are almost identical to the TS4-A-O used in the previous example:
This product has a maximum power value of 1000W total, but since the 2F accepts 2 module inputs, each input is limited to 500W. Let's compare our worksheet results with the TS4-A-2F datasheet:
The maximum Voc is 52.9V (which is still comfortably below the 80V max input voltage), and the max current of the module is also within specs. However, the module is rated for 520W, and the 20 watt overage of the PV-Module could exceed the power rating.
Generally speaking, this module could still be compatible with the TS4-A-2F, depending on the design and geographic location of the system.
Quick Temperature Correction Calculator
At the bottom of this article, Tigo has provided a simple spreadsheet that can help you perform a quick temperature correction. Just fill in the orange cells, and the sheet will automatically show the results.
If you have questions or require design assistance, please contact the Tigo Sales Engineering team, at: firstname.lastname@example.org