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String Sizing Tool - Where do the numbers come from?

First of all, we load the values of each solar panel from the spec sheet from the panel manufacturer. That is where these numbers come from:


 



Module


Activasun SRS170




Max Power Voltage - Vmp


36.1


Vdc


Open Circuit Voltage - Voc


43.3


Vdc


Voltage Temp Coeff - Vtoc


-0.1645


V/°C


STC Rating - Pmp


170


Wstcdc


Max Power Current - Imp


4.71


Adc


PTC Rating


154


Wptc



Next, we load all the values for each inverter. That is where these numbers come from:




Inverter


Xantrex GT3.3N (240)





Pnom


3300


Wac


Idc max


17.5


Adc


Vmptmax


550


Vdc


Voc


600


Vdc


Vmptmin


200


Vdc


Efficiency


95.5


% AvgEfficiency


We look to find how many panels are required to get the Vmp into the range of the MPPT tracking window of the inverter. Based on this alone, the panel and inverter combination above would be between 6 and 13 panels in series.


We then have to look at temperature. We ask the customer for his record low and average high temps.



Temperature Specifications
Celsius or Fahrenheit Celsius
Record low ambient temperature (°C) 14
Average high ambient temperature(°C) 25

When it comes to the temperature, the low side is critical. If we exceed the VOC of the inverter, the inverter will be damaged. Since the VOC goes up when temperature goes down, we calculate as follows on the low side: VOC from the panel table + voltage coefficient, times the number of degrees C below 25. As an example, if the customer puts in that his record low is 14ºC, we would take the difference of 25-14 (11) times the coefficient -0.1645 = 1.8095V. Add this to the VOC of the panel. In this case, the VOC at the low temp becomes 45.1095VDC. Multiply this by the number of panels determined above which gets a range of 270.657 and 586.4235. If after VOC max calculation based on record low temperature exceeds VOC of the inverter, that combination is removed. In this case, the upper number (13) is still displayed, but shows as nearing the 600VDC max. Edge of cloud effect could potentially push it over the 600VDC limit.



Next, we look at average high temps. These also use the coefficient, but this is not a critical value, so it does not put hard stops. We also take into account that the cell temperature is on average 35ºC higher than ambient. To determine the Vmp at max temperature, we take average high plus 35º minus 25º. As an example, with average high being at 25º, that would be 25+35-25=35 times the coefficient of -0.1645 or 5.7575VDC per panel below VMP times the number of panels in series. Vmp being 36.1 ? 5.7575 = 30.3425 times the 6 panels = 182.055 and times 13 panels is 394.45. If any of these numbers are outside of the MPPT window of the inverter, they are also discounted. Notice that 6 panels is below 200VDC, so it is not displayed. The tool only shows 7-13 panels as an acceptable string size with the temperatures of 14-25C.



Finally we display the calculated values for STC, PTC and CEC based on the number of panels in series. These constants are stored in the database per panel, though CEC ratings are not displayed in the information about the panel in the upper left corner. We also calculate the number of parallel strings using Imp to make determinations of how many parallel strings are possible. Notes are added for any configuration that puts the power above the conversion capability of the inverter selected and the cells are Green if good, Yellow if Vmp goes below the MPPT window of the inverter and Red if it is nearing the Voc limits.

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