This brings up some interesting questions I had yesterday on this subject.
1. If the percentage change for Vmp is greater than Voc but the manufacturer only lists the temperature coefficient for Voc, what would be considered the best practice to calculate the Vmp loss? Use the temperature coefficient for power (typically ~-0.5%/C for crystalline)? Evergreen is the only manufacturer I have found that lists all these coefficients, and as much as I would like it to, one data point doesn't constitute a trend. 2. If I have a module with power tolerance of +-3%, and I presume the power will be at or near the -3%, should I apply -1.5% to the voltage and -1.5% to the current out of the box? Or will one be weighted more than the other? To me it makes sense to do what have heard Bill Brooks advocate for, determine the minimum string length and then add one module. That would take the issue of dropping out of the inverter's window out of the equation. But, as Jeff has brought up, we don't always have the liberty to do that. Ryan From: [email protected] [mailto:[email protected]] On Behalf Of Kent Osterberg Sent: Monday, December 07, 2009 10:25 PM To: [email protected]; RE-wrenches Subject: Re: [RE-wrenches] large array string sizing Jeff, The maximum voltage of the PV modules will probably be a little less than the spec'd value and it will decrease slightly as the modules age. But going over the inverter's voltage rating can (probably will) be catastrophic. It looks like your leaning towards avoiding that possibility by using 12 modules and I think that is the right approach. While inverters do have a minimum operating voltage, the more likely circumstance is that the array voltage drops outside the MPPT window. The inverter will still operate and the output power will drop off but by less than the percentage by which the maximum array voltage drops outside the MPPT window. So if you are loosing 5% of your potential output for 60 hours a year, that's not a serious problem. But you still need to make this decision carefully. In 10 to 20 years the system voltage will drop 5 or maybe 10%. How far are you going to be outside the MPPT window then? Is the inverter still going to operate? Another potential gotcha working against you is that some inverter manufacturers string calculators assume that the peak power voltage coefficient is essentially the same as the open circuit voltage coefficient; that is to say the percentage change applies to both numbers. It doesn't! In fact the peak power voltage changes almost as many volts as the open circuit voltage. Take a look at the voltage/temperature coefficients for the Evergreen modules; Evergreen publishes both coefficients. While most manufacturers only publish one value, the numbers are similar for all poly or mono crystalline cells. Sounds like this module/inverter combination may not be a good match. Kent Osterberg Blue Mountain Solar, Inc.
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