System Design Criteria / Degradation Explained



Eco Depot USA is committed to providing systems with exceptional performance over the entire life of the system. This page describes the assumptions we use for our Photovoltaic Systems.



Background:

Known factors effect actual array performance and system design. The inverter manufacturers' string sizing programs do NOT include these factors and they request that you accept responsibility (liability) for system design when using their tools.

The result of most of these ignored factors is that the array voltage is sized too low for the inverter to perform well over the life of the array especially on the warmest days of the year as the array ages.


Factors Explained:

Degradation

The biggest of these factors is array degradation over time. We are designing systems to last 20 plus years, so we must look at anticipated voltage degradation when specifying an array design. Data on array degradation is hard to find. By comparing long term testing reports, module warranties, and discussion with manufacturers we have come to the conclusion that it is prudent to expect 0.25% per year loss in voltage and are allowing for 25 years worth of loss in our system designs.

Tolerance

The tolerance could also be called the minimum guaranteed wattage. A module with a -9% tolerance could be rated 9% less than the spec sheet says it is. We assume 1/2 of the power tolerance is due to lower voltage. If you assume the rating of the average module will be 1/2 of the allowed tolerance and that 1/2 of this will be represented in voltage, we come to the decision to take 1/2 of the tolerance off of the design voltage for this factor.

Voltage Drop

Array wiring will always have some voltage drop. We assume a voltage drop of 1% on the array side.

 High Grid Voltage

The grid voltage will be higher than 240 in some locations (close to a transformer). The allowed grid voltage for inverter operation is up to 264V (10% high). When the inverter is pushing power to the grid, any voltage "drop" in the AC wiring of the inverter is seen at the inverter terminals as a voltage rise. We assume the grid voltage will be 2% higher than 240. This is applied as a 2% lower voltage of the array.

Maximum / Minimum Temperatures at Site

The temperature at the site will affect array voltages (variable by site). We assume the temperature minimum will be -13 Deg F and the maximum will be 113 Deg F.


Partial Shade Performance

The average modules we sell have 2 or 3 bypass diodes. When a full cell or more is shaded, the voltage from that bypass diode area is lost while the current is allowed to bypass the shaded area. (ALL of the voltage from every bypass area with a cell shaded is lost in partial shade conditions). We assume no shading, but if you have partial shading at your array site you should select relatively higher voltage array designs.

In Conclusion

By incorporating these known factors into array design Eco Depot USA has more restrictive criteria and a few less designs that we find acceptable when compared to inverter manufacturers’ string sizing tools.

This inconvenience is unfortunate but it means that the designs we approve will perform better over the life of the system.

If you have shading, high grid voltage, greater than 1% voltage drop in the array wiring or more extreme temperatures than -13 to 113 F, let us know and we will review the best design for your particular application.

 

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