Which solar panels should I go for

yrtrnc

Hi

I have a Victron 150/70 mppt charge controller. I have an option of going with two models of solar panels, a 255w and a 280w. I will install them 3 in series 5 in parallel . The 280w panels are a bit more bulky and Im not sure the controller will give me full power. What do you think?

These are the panels
https://www.sunergysolar.com.au/content/pdf/Trina-Honey-Specs.pdf
http://alemasolar.com/files/trina-solar-poly-TSM-PC14_EU.pdf

These are the results for the 280w panel calculations

Max. input voltage				150 V
Max. PV voltage @ min. temperature				144.9 V
Min. input voltage @ MPP				49.0 V
Min. PV voltage @ max. temperature				84.7 V
Max. output current				70 A
Max. current @ MPP min. temp.				70.0 A
				* Power limiting @ low temp.
Max. current @ MPP max. temp.				69.5 A

And these are for the 255w panels

Max. input voltage				150 V
Max. PV voltage @ min. temperature				123.2 V
Min. input voltage @ MPP				49.0 V
Min. PV voltage @ max. temperature				73.6 V
Max. output current				70 A
Max. current @ MPP min. temp.				70.0 A
				* Power limiting @ low temp.
Max. current @ MPP max. temp.				65.7 A

mcgivor

Go the most cost effective route, at the end of the day the difference would be negligible, most off grid users over produce as a safeguard, I'm assuming this is a factor in your calculation. What ambient temperature will the controller itself be subjected to, out of curiosity, running close to, or at the maximum capacity in higher ambient  temperatures could derate the output, something to consider if applicable. 

Estragon

Are the panels the same price?

How much and what size of wire between panel and controller?

The temp adjusted Voc on the 280w strings is getting pretty close to max. A Midnite classic will protect itself (by not charging) from voltage above max to an extent, but assuming the Victron doesn't it might be a bit too close for comfort IMHO.

Estragon

Also, what sort of elevation and climate (eg. coastal, mountains)? Overpanelling likely okay if you probably never see STC output.

Photowhit

Looks like a 48 volt system voltage?

All costs being the same, I'd go for 280 watt panels. You may not utilize all of the available power on perfect days. but it will be rare.

Consider the Normal Operating Cell Temperature value. If you can't find it for the 280 watt panels you can look at the NOCT values for the 255watt panels;


This states that normally you can expect 183 watts from your 250 watt name plate panels. Or about 73%, I like to use 75% as an average.

So you can expect about 280 x .75= 210 watts from your 280 watt panels., since you will have 15 of them, you can expect about (15x210=) 3150 watts from them, so If you brought your system down to about 40% State of Charge and were charging around 48 volts, you would have 3150/48= ~65.5 amps available for recharging, this would be within your chargers capabilities.

The extra potential would be nice on overcast days.

Check to be sure the Victron can be 'over paneled', I'm pretty sure they can. On those odd cold days after a long storm, when the batteries are low, you would have the potential of mid 80's amps available, but the charge controller will likely just limit it to 70 amps.

mike95490

The 255w. 

The 280w is far too high voltage at cold temps to attempt to hang a $500 controller onto.   Voltage drop is a dead end - at cold startup at dawn, there is no current flow, and you just fried the controller. And panels can easily get 20 degrees colder than air - from clear sky radiation, same way ancients made ice   http://blog.stickyrice.net/archives/2008/ice-in-the-desert-a-fridge-without-electricity/

via night time radiation to the sky.

If 200w is going to make or break the system - it's already broken, unless you live in the desert and never have clouds.

Bur more important, you would easily be able to hit 80A on a cool day or intermittent clouds, and not sure what the overload capability is on the 70A controller.

Estragon

Yup, I edited my post to delete the voltage drop for the reason Mike points out. Won't give much room with very little current.

Photowhit

I'll agree with @mike95490 if these values just came from your calculations and not from a tool provided by Victron and perhaps he's right. Not sure where a minimum input of 49 volts came from, I would want a minimum of 30% above the charging voltage, or more like 58 x 1.30= ~75 volts. MPPT type controllers need some head space above the charging voltage to work properly.

As to the math, It's NOT a 200 watt difference, but a 15x (280-255) = 15x25=375 watt difference. Enough to make a difference IMHO. 

yrtrnc

> @Photowhit said:
> I'll agree with @mike95490 if these values just came from your calculations and not from a tool provided by Victron and perhaps he's right. Not sure where a minimum input of 49 volts came from, I would want a minimum of 30% above the charging voltage, or more like 58 x 1.30= ~75 volts. MPPT type controllers need some head space above the charging voltage to work properly.
>
> As to the math, It's NOT a 200 watt difference, but a 15x (280-255) = 15x25=375 watt difference. Enough to make a difference IMHO. 

Thank you for all your input guys.

The calculations are from victrons spreadsheet.

I'm in Cyprus so we have a Mediterranean climate. We hardly ever go below 0 c.

Not sure how ambient temps will be at the moment but the equipment will be in an insulated room below the panels with mechanical ventilation.

Wire is 3 M 35 mm copper wire.

Cost is not an issue as I already own both panels.

I do not think there is any risk on the controller. I'm just trying to figure out if I'll be able to produce that extra 375w with the victron controller and if it will be worth it. The controller has a power limiter at high temps of 70c.

Photowhit

It will more often being extra charging when you are having overcast skies, or if you run a lot of loads during the day.

I have about a 5000 watt array, and mostly never use more than a small portion of it. 

We also don't know your battery bank size. A 70 amp charge coming into a 20KWh (@48 volt 400 amp) flooded lead acid battery bank will rarely if even be fully utilized. The battery bank can only take in about 15% of it's amp hour capacity, maximum. But the ability of your array to produce 7 amps in stead of 6 during cloudy days may be very rewarding. 

yrtrnc

Battery bank is 8 pcs 100A 12.8v lifepo4 batteries wired in 200A 51.2v configuration. Recomend charge voltage is 100A and below. Maximum charge voltage is 400A. So all is good there.

I think you are right, I will probably go for the bigger panels. As I will also be using alot of electricity during the day.

Estragon

Assuming you're in a coastal area, marine haze will likely limit output to some degree. The extra 375w in nameplate capacity would help production on really hazy or light overcast days, and you would likely rarely see the 280w array put out the 70a controller max. In other words, yes, IMHO, you will see a benefit from the extra wattage.

That said, the max Voc is a bit too close for comfort to me.

yrtrnc

> @Estragon said:
> Assuming you're in a coastal area, marine haze will likely limit output to some degree. The extra 375w in nameplate capacity would help production on really hazy or light overcast days, and you would likely rarely see the 280w array put out the 70a controller max. In other words, yes, IMHO, you will see a benefit from the extra wattage.
>
> That said, the max Voc is a bit too close for comfort to me.

@Estragon so cell temperature would have to be about -11c for 150v. Am I right?

Considering the record low temperature was around -2c in my area in 1959. I'd say its a pretty low chance I will see max voc over 150v at - 11c. Thoughts? Also considering the effects of global warming since 1959

Am I missing something? Can cell temperature reach -11c with ambient temps at -2c?

mike95490

yrtrnc said:

Am I missing something? Can cell temperature reach -11c with ambient temps at -2c?

Yep  you didn't read the link .   Passive night time thermal radiation.  Try using a IR heat gun thermometer, and point it at the sky  -400F

Panels can easily get 20 degrees colder than air - from clear sky radiation, same way ancients made ice   http://blog.stickyrice.net/archives/2008/ice-in-the-desert-a-fridge-without-electricity/

via night time radiation to the sky.

Estragon

> @yrtrnc said:
> > @Estragon said:
> > Assuming you're in a coastal area, marine haze will likely limit output to some degree. The extra 375w in nameplate capacity would help production on really hazy or light overcast days, and you would likely rarely see the 280w array put out the 70a controller max. In other words, yes, IMHO, you will see a benefit from the extra wattage.
> >
> > That said, the max Voc is a bit too close for comfort to me.
>
> @Estragon so cell temperature would have to be about -11c for 150v. Am I right?
>

I haven't done the arithmetic, but that sounds about right.


> Considering the record low temperature was around -2c in my area in 1959. I'd say its a pretty low chance I will see max voc over 150v at - 11c. Thoughts? Also considering the effects of global warming since 1959

Global warming is a bit of a misnomer. The term is "climate change", which is not just about warming. For example, in my area (mid-continental Canada), we've had a somewhat cold winter and a very cold spring (~10°C below normal) so far. Possibly climate change related, there has been a tendency for diminished polar winds allowing cold air which is normally confined to polar regions to spill south more regularly and for longer periods. The climate may be warmer on average, but the change may be allowing more extremes more often here. Another example is the weakening of the Atlantic ocean current circulation. If it continues, there's a good chance the climate in Northern Europe could cool considerably.

I was on the Florida panhandle coast in January. They normally have a similar climate to yours, but got an unusual snowstorm and an extended cold period. Stuff happens.
>
> Am I missing something? Can cell temperature reach -11c with ambient temps at -2c?

I've heard the radiation to space thing before. Can't say I've measured the temp of my panels on a clear night though.

In any case, it's a close enough call that I probably wouldn't risk it without something like the Classic "hyperVoc".

Photowhit

Estragon said:

They normally have a similar climate to yours, but got an unusual snowstorm and an extended cold period. Stuff happens.

For the record I was there too, please lets not call that a snow storm... What we got in Tallahassee, is what we would call flurries in Missouri. I've seen better snows when I lived in Florida.

Estragon

True, but it was a novelty for folks on the coast - seeing frozen puddles was apparently pretty unusual there. In Tallahassee, they told me it helps keep the bugs down.

yrtrnc

> @mike95490 said:
> yrtrnc said:
>
>
> Am I missing something? Can cell temperature reach -11c with ambient temps at -2c?
>
>
>
>
>
> Yep  you didn't read the link .   Passive night time thermal radiation.  Try using a IR heat gun thermometer, and point it at the sky  -400F
>
>
>
>
> Panels can easily get 20 degrees colder than air - from clear sky radiation, same way ancients made ice   http://blog.stickyrice.net/archives/2008/ice-in-the-desert-a-fridge-without-electricity/via night time radiation to the sky.

20 degrees centigrade? For real!

Estragon

You'd think a radiant temp gradient that large between panels and air would cause convective currents in the surrounding air, which would mitigate it. I dunno.

I can see evaporative cooling in a dry desert environment making a pretty big gradient though.