Question about string sizing

vidhan

My buddy and I will be installing his off-grid system. He plans to use two outback VFX3648 inverters and two Outback FM-60 Charge Controllers. We intended to use 30 Evergreen ES195 blemished panels, but we had trouble getting these and ended up with 28 Evergreen ES-A-205-FA2 panels. I am wondering how to size the strings.

With the 30 panels we were going to use 3 strings of 5 panels per charge controller. Can we use two strings of 5 and one string of 4 per charge controller/inverter? If so, what sort of inefficiency will that introduce?

We're new at this, as you can probably tell by my question.

Thanks for your input.

BB.

Re: Question about string sizing

Yes you can connect strings of differing Vmp voltages--however, if the voltage are far apart (as in this case, 1 panel difference)--The end results of your 2x5+1x4 will probably have the same output as either 2x5+0 or 3x4 panels.

The Vmp will try to find the Maximum Power Point (Tracking or MPPT). And you have two peaks... One at 3x4, and another at 2x5--which one will the controller choose? Don't know--but in either case you are "missing" the use of either 4 or 2 panels worth of power. You are probably better off buying the last two panels or figuring an different configuration:

Something like:

Controller A; 3sx5 panels
Controller B; 3sx4 panels

One panel left over (use as spare or small 1 panel backup, or for another cabin, or try and sell, or buy 2-3 panels more).

I am not checking the "details" of your configuration (max Wattage, Vmp hot, Voc cold)--but it looks doable. Need to know the battery bank voltage for further design checking (by somebody who knows more than I about the Outback product line).

-Bill

PS: Sorry, I missed the 48 volt inverters...

crewzer

Re: Question about string sizing

Vidhan,

My analyses indicate that series strings of five ES-A-205 modules are a virtually perfect configuration for PV energy systems using OutBack controllers and 48 V battery banks. The Voc is within spec in the winter (137.71 V at -40 C per 2008 NEC 690.7), and the operational Vmp is high enough in the summer time to reach absorption and EQ target voltages for even flooded-cell batteries.

Considering the voltage drops in the wiring and in the DC-DC buck converter inside the controller, the operational Vmp of just four modules in series will almost certainly be too low in the summer, unless you live in an area that’s fairly cool year ‘round. Accordingly, arrays involving strings of four modules (i.e., 4 x 4 plus 4 x 3, or 5 x 2 + 4 x 1 plus 5 x 2 + 4 x 1) probably will not work satisfactorily.

Your best options may be:

1) Sell or “reassign” three modules, and build two arrays, one 5 (in series) x 3 (series strings in parallel) and the other 5 x 2
2) Buy two more modules and build two arrays, each 5 x 3.

HTH,
Jim / crewzer

vidhan

Re: Question about string sizing

Thanks, guys. We are in Hawaii. The temperature will never dip below, say 60 degrees in the sunlight, and will usually be mid-eighties, ambient temperature. The panels will be flush mounted over a metal roof. Do you think the voltages will drop too low under those conditions for strings of 4 panels?

Aloha,
Vidhan

BB.

Re: Question about string sizing

Sorry, I missed the 48 volt inverter before...

For 4 panels in series, the low temperature is not the problem (Voc cold is the maximum voltage). Vmp (hot) is the one where you may have issues.

Assuming 90F, the minimum Vmp (hot) would be around 15.3 volts (using the Xantrex String Sizing Tool to calculate Voc cold and Vmp hot because the data and tool is handy)...

4x15.3 volts = 61.2 volts

You would want to charge your battery around 58 volts minimum (equalize flooded cell batteries around 60 volts minimum--all needing temperature compensation, i.e., increase in voltage, if the batteries get colder than ~77F--may not be an issue for you).

And add a minimum of ~2 volts for controller drop plus your wiring losses (need copper wire distances and awg to do the math); 61.2V-2Vdrop=59.2V available for battery on a 90F day...

In the end--you are really close on a 90F day. Probably would work pretty well most of the year on 4 panels (Vmp is usually not a sharp peak). 5 panel strings would be better (or ideal as Jim said).

Jim/Crewzer is the expert in the Outback products here (plus a couple others)--so they would be better able to give you the authoritative answer.

-Bill

crewzer

Re: Question about string sizing

The panels will be flush mounted over a metal roof.

The combination of heat reflection from/around the metal roof and little or no cooling air between the array and the roof makes this installation configuration about the worst installation configuration possible. The modules will probably operate at ~35 C above ambient at mid-day. On a 90 F (32 C) day, the modules' cell temperature will reach 65 C or higher -- 40 C above STC spec of 25 C.

At 40 C above STC, figure on a Vmp- and power reductions of ~18% based on Evergreen specs: http://www.evergreensolar.com/upload/011108%20Datasheets%20(SR)/ES-A_200_205_210_US_011108.pdf Then there are previously mentioned voltage drops in the wiring and charge controller.

You might get to 60 V reliably if the array was frame mounted and away from the roof, had very good circulation of cooling air around and behind the modules, and if the modules' cell temp increase was therefore limited to ~25 C above ambient.

I still think five 205 modules in series is the way to go for the 48 V inverter.

What type, brand, and model batteries are you planning on using?

HTH,
Jim / crewzer

vidhan

Re: Question about string sizing

Okay. I think I had better clarify about the mounting. I intend to mount them using Unirac standard rails on L feet attached to SS. hanger bolts, NOT directly on the roof, so they should have good air circulation around them. For the batteries, we were thinking either Trojan L-16HC or Surrette S460 batteries, both of which are available locally at decent prices.

crewzer

Re: Question about string sizing

You may to get even more specific about the "mounting" strategy. In general, an air gap of less than six inches between the array and the roof is treated as no gap at all.

Take a look at this "Mounting Style" discussion from the glossary on SMA's website: http://www.sma-america.com/en_US/no_cache/service/glossary.html?sword_list

HTH,
Jim / crewzer

vidhan

Re: Question about string sizing

I guess I am pretty well sold on the strings of 5. We will just buy another 2 panels and have the array of 30 that we originally intended. As far as the operating temperature of the panels-I think we may try a little experiment to see what it is truly likely to be in windless weather. Usually this site gets those nice cooling trade winds that should be pretty helpful to keep things cool. We can, though, elevate the panels a little more by using longer hanger bolts. Hmm, something else to consider.......... Thanks for your input.

crewzer

Re: Question about string sizing

OK... I realize you're in the Tropics (~20 degrees N lat?), but the real test will come in June when the Sun is directly overhead. I hope you'll post results of your experiments and operations!

BTW, here are links to the manufacturers' charging and maintenance instructions:

Trojan: http://www.trojanbattery.com/Tech-Support/BatteryMaintenance.aspx
Rolls/Surrette: http://www.surrette.com/files/BU-RS-614.pdf

Regards,
Jim / crewzer