Question on Adding Panels to String

hdude

I am having my new system installed this week in S.E. Arizona. It will be 3 strings of 14 - 250w CentroSolar panels combined to use a SMR 11400 TL. Each String will be established on 3 rows of fix tilt racking - 2 sets of 5 panels and 1 set of 4 panels - with one hole open per string.

My question concerns filling that hole.
1.) The SMA calculator shows a max of 14 panels per string are acceptable (590vac max for panels vs 600v limit for converter. A 15th panel exceeds the max. My belief in designing the system was to fill the home either somewhere 10+ years out with another compatible panel - when the system voltage should degrade enough that that panel would size less than 600 volts. Based on what I have learned (or assumed), solar panel will derate about 1/2% per year - 50% on volts and 50% on power. @ 10 years @5% works out to about 20vdc for the string, the additional panel (27vdc) should be within limits . Is this assumption correct?
2.) During the hot summer I understand that the temperature coefficient should drop the available voltages as much as 20%. Would it be feasible (and safe) to annually rewire each string to add that 15th panel during summer and disconnect it back to 14 during the winter?

Near Future expansion will be one more string of 14. Should I order 14 or 18 panels?

inetdog

Re: Question on Adding Panels to String

You have a fundamental misconception about panel behavior in low light and what happens when the panel degrades.
The current at maximum power, Imp, gets reduced with age and with low temperature and with low light. The voltage does not change much except to fall with increasing panel temperature.
You will never be able to fill you holes.

hdude

Re: Question on Adding Panels to String

You will never be able to fill you holes. Too bad for my hopes.

I just found this on the Internet . http://www.homepower.com/articles/solar-electricity/design-installation/ask-experts-voltage-drop-calculations

Every string inverter has a minimum and maximum input voltage range. The maximum may be hit on a record-low-temperature, sunny winter morning when the sun first strikes the array. During this short period at dawn, the current is very low. As soon as the sun’s intensity and array current increase, the modules warm up just enough to drop the voltage below that maximum. Calculating maximum low-temperature VOC is necessary to size the equipment to handle that maximum voltage. The array’s DC voltage will gradually decrease over time due to normal PV module degradation, so this calculation covers the best-case situation when the PV modules are new.
Calculating minimum DC voltage is of greater concern, as all inverters have a minimum acceptable voltage for establishing full maximum power-point tracking (MPPT). Good design practice is to maximize the number of modules in each series string to reduce the potential for the inverter’s DC input voltage to ever drop below the minimum necessary for efficient operation.

What I haven't found is how much VOC can be expected to drop - over what time. I probably will need to test the string voltages over the years to come to my own conclusions.

inetdog

Re: Question on Adding Panels to String

The only reason that semiconductor physics gives us for crystaline, as opposed to thin film amorphous, cells to decrease in open circuit voltage would be an increase in the internal leakage resistance that moves them toward the higher current end of the IV curve without any external load applied. The slope of the IV curve near zero current is very small, at least in high incident light conditions. So I will take that part of the homepower article with a grain of salt.

inetdog

Re: Question on Adding Panels to String

You may find this article on failure mechanisms interesting.
The section on degradation over time lists three likely mechanisms, each of which will primarily affect the current at maximum power but not the open circuit voltage.
A few mechanisms, like cell shorting, will have the effect of lowering the Voc of the panels in question, but are properly addressed by replacing the failed panels instead of just adding an additional panel to the string. And the loss in Voc would be on the order of .5 volts per shorted cell, not enough to allow for an entire additional panel in each string.