Is there a book, text or FAQ on curve tracing PV Modules?
So far I have read about 1/3 of the messages on the RE-Wrenches list from january 04.
I can theoretically understand Vmpp and Impp and Isc and Voc somewhat but somehing I can read a few times helps.
Do I just put a DVM to measure Voc for example.
I can understand why you need matched panels in a string.

IE lowest amp is a choke point.
different panels with different amperages in series are limited to slitely above the lowest amperage panels.
(4amp12v + 2.5 amp12v would probably make a 2.55 amp 24v at the sum of volatges example??) ie 61 watts not 78 watts???)

It looks slitely better for parallel ones, amps adding and volts averaging but reading material, tests etc

Any directions are appreciated.
Robert

The Curve tracer I used was a 20K piece of test equipment that Xantrex owned.

The Curve tracer sweeps the array from VOC (voltage open circuit) to zero volts and measures the current during the sweep. The sweep takes only a few seconds and when connected to a PC, the data can be stored and plotted

For a home-owner, this is total over kill

All you have to do for a system is remember that you need to match current (series connections) and voltage (paralleling connections).

Using name plate values is generally fine as manufactures are with -3%/+5% range. If one is to attempt to match panels to get even tighter tolerance for maximum performance for an array I would suggest using VOC from a DVM as an indicator of the panel’s potential.

In my most recent array, the photowatt’s came with the flash tester data and I used that to match my panels. Looking at the indivual data sheets, it appears the highest VOC panels also where the strongest in current.

Without tester data, I fear it is a difficult task at best to measure the individual panels. As the panels heat up the voltage drops, if the panels are exactly in the same position to the sun, the voltage and current will be different. Keep this in mind if you do try to test you panels, and I would suggest you measure at least 2-3 times the group, to average out the differences

Interconnecting:

Since most panels are Si (silicon), the voltage parameters are usually the same "12 volt panelsâ€￾ (typically 21 volts VOC, 17 volts mpp), double this for "24 volt" panels. It doesn't make any difference what manufactures makes the panels, Silicon is the same!

The only thing to be careful about is Asi (Unisolar) is a different technology and has different voltage and amperages points. As a general rule, on shouldn't mix strings of Si and Asi ... nothing bad will happen, but the array will no perform to its rated power

So 2 or 4 panels are in series for a nominal 48 volt system, those 2 or 4 panels should be the same current rating. The current WILL be limited by the weakest cell, so DON'T put a 2 amp panel in series with a 4 amp panel, you will only get 2 amps out! (Think water pipe for analogy here)

In DC systems, power is amps X volts for a string, and n1 + n2 ...for the parallel strings for total DC power

For anyone new to solar panels, take a deep breath and if reading the above paragraphs makes sense, you 95% the way to understanding what make a PV system.

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Using name plate values is generally fine as manufactures are with -3%/+5% range. If one is to attempt to match panels to get even tighter tolerance for maximum performance for an array I would suggest using VOC from a DVM as an indicator of the panel’s potential.
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so if i lay the panels in the backyard in full sun at a normal angle, same spot, say propped on a table , and do a Voc immediately this will give me cold Voc (if they are not in sun but equilibrated to outside shade temp, say garage or cold basement
then leave in full sun for 1 hr to equilibrate on a nice hot day more or less normal and again do a Voc w DVM
then again in another hour or 2, same angles natch-u-rally
(which i think is just measuring the volts)
then let them bake for 2 weeks in full sun and do again?
I noticed my ASi's array which was rated at 1376watts (81v per panel in strings of 4, 48"x26") peaked at about 1,550w the first few days, settled down lower and is now down to 1,054 (22+%loss over 6 yrs)
Do single crystal Si have that initial spike of watts followed by a setltling?
Is all this a chase of ghosts and fractions of %

Kinda ... the panels will heat up VERY fast in full sun ... just keep that in mind as your testing different panels



Yup , Asi out of the shoot runs 5-10% high , then degrads at a faster rate than Si. Asi has'nt been around long enough to nkow what happens in 20-25 years ... that's why only Unisolar is left , no oe buys Asi unless thy need it to take bullet holes and still run



NO and from my direct experiance , I haven't seen ANY decline in power output form my 4 Si array or those of others I nkow that data log their systems ... panels getting dirty (like bird poop) is the only thing I've sen case a drop and cleaning the panel takes care of that.

Eveything I have read and from my personal experiance would lead me to beleive there is NO break-down or decline in Si as long as the panels are minimally maintaned ... clean the panels , check the interconnecting screws once a year. The ONLY failures I have seen or been reported on in Si , is ByPass diodes shorting out , never seen a panel just quit !



In your case , I would say yes


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Actually, some of the Si modules have been tracked over many years and it's been found that most of them degrade about 2-4% in the first year and then very little after that. But they do degrade slowly over the years. The ASE modules are one exception to this with very little degradation, I think due to different metallization, solder and encapsulant.

I would think that one would want to use short circuit current to match modules for a series string. I have seen many modules where the higher open circuit voltage does not track the short circuit current very well. And current is the value you're trying to match. Also current is less effected by temperature so may be easier to measure anyway.

Brad

Yes and no ... Current is less effected by temperature is true , BUT , since the testing is done using the sun and current is DIRECTLY proportional to the sunlight , it makes for almost impossible comparisons from panel to panel unless great care can be made to have the same angle to get the same irradiance for each panel tested.

My VOC suggestion was based on the SAME PANEL MODEL, not different manufactures or wattages! The VOC data is from my 36 test data sheets which 100% seem to correspond to higher VOC = higher amps and power for the same panel type

I have also used the short circuit test , a 20 dollar digital dvm from Home depot with a 10 amp scale has worked well for me.

I wanted to be clear and say I agree that using ISC is the best way for match current but needs the panels are already mounted in the same plane and can be tested with seconds from panel to panel.

Since it is almost impossible to get repeatable values but free hand testing of the panels in th sun , and the values recorded likely will be greater variation than the panels -3%/+5%.

This is why I suggested doing the tests a few time to average or use VOC to indicate the stronger panels based on the test data I have seen first hand

I have a few old DVM's
I just got some Sharp 185's from Sun Electronics
They have MC connectors.
If I want to measure Voc and Isc do I just put the DVM leads in the connectors?
I remember smoking an old multimeter when i tried to measure current incorrectly awhile back.

The Home Depot DVM has a 10 amp scale , so yes with this DVM , just put across the panel leads and you will get the ISC value for the panel , at that moment for how ever you have it pointing towards the sun

how far are you from North Orlando
are you ever up for visitors to take a look at your system?