Derating 24v Solar Panels?

BFL13 wrote:
You could make a profit on those 140s in today's market I bet.

lol, yeah, I was thinking I could at least break even, especially if tariffs drive up the prices. And as a kit, with the solar30 and 6awg cabling, I should have no problem recovering my costs.

red31 wrote:
• Nominal operating cell temperature (NOCT): 45±2?

looks like that panel gets 25C warmer with 80% sun.

The coefficients are used with temps different than 25C STC

http://pvcdrom.pveducation.org/MODULE/NOCT.htm

Thanks for the link!

Red,

If Tair = 30C, and NOCT = 45C, and S = 80, it gives a result of 55C. Would this be the cell temp for a fairly well ventilated, average panel, at 30C ambient, then?

If so, 55C - 25C = 30C, so the panels I'm considering would lose about 67.5 watts, at 30C ambient? ( 0.45% x 30 = 13.5% x 500w = 67.5w )

(In that case, the peak amps would likely be in the 26a range :))

You could make a profit on those 140s in today's market I bet.

True, I've only done a few charges, but I have a pretty good idea of what the current set-up can produce, having compared pointed to flat several times, under varying sky conditions. I do know 280w/pointed will not suffice, spring and fall, and that 500w/flat will be too much, most of the time in summer. But I need to go big to cover the worse case scenarios.

The 140w/12v panels I bought are no longer available, so adding a third is no longer an option, either.

I'm seriously considering a 24v system, in the future, so the 24v panels will be necessary for that, as well.

I didn't make a log Friday for the A/C test but I did check hourly and flat with room for ventilation the highs I saw was 45a from my three 250w panels in series. I've seen one hit 18.5a tracking but it wasn't 95F like it was Friday. I have seen higher numbers when it was cooler but not drastically higher.

So flat the two 250w panels will push a 30a controller but not as often as you will tracking. Even then not buy much and it is likely designed to clip anyways.

Another point is that unless you pull a big bank down a lot over night, or pull big loads mid day, you will likely be in float by the time you could see peak amps. For us overnight draw mid summer is at it's lowest with the least furnace run and shorter time between nightfall and bedtime when we run the lights and TV/SAT. Both the trailers 95 Ah grp 27 and the inverters 350 Ah 8-D are in float by night fall, the resting voltages the next morning are 12.5+ for the grp 27 and sometimes the healthy 8-D barely burned off it's over voltage. both batteries are in float before 9 am when I split the two 245w's up.

I have only tracked the 250w's a few times and then switched to flat so that I didn't become addicted to the power and believe me, it's addicting. I could point the twin 245's at the spot the sun would come up in and by the time the frost melted off I was getting 23 of the 34a peak they would do when the sun was higher. Even after running the coffee maker the two systems would have the 8-D in float rather quickly and I'd switch one over to the trailers battery for the rest of the day unless we ran the microwave.

Now then, for the rest of the day once the early morning haze burnt off and the sun got a bit higher, that 490w tracking was good for 32-34a until the sun got low again. That was both mid summer and Sept. numbers with average temps in the high 70'sf. As I said the 250w polys hits higher tracking than the 245w mono's.

So you are more likely to max out the controller tracking in cooler temps than flat in the heat.

My Can Solar 60 cell 255w has three diodes. Check the owner's manual for which ones you are looking at. Anyway, you can play with how many cells to cover to get what.

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EDIT Quick Test- 60 cell 255w panel
- all in sun Isc 8.79, three cells one end covered (all on one side for half of 6) Isc still 8.77.

As soon as the fourth along (next to middle of panel) covered Isc dropped to 4.4, then when fifth along (leaving only the corner one open, Isc was 0.2 and with all six of the end covered, still 0.2.

You can get intermediate amounts above 4.4 by covering the first three and then part of the fourth.

I haven't tried other combos, but obviously you can tinker till you get what you are looking for.
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Your requirements are changing and you should have the requirements clear before going to design. :)

You haven't even done part of this summer yet on solar to learn what it can do or not, so maybe you will have more ideas by Fall. Meanwhile do some "masterful inactivity" ? What if you get a job at a shipyard in BC or NS? Anything can happen.

• Nominal operating cell temperature (NOCT): 45±2?

looks like that panel gets 25C warmer with 80% sun.

The coefficients are used with temps different than 25C STC

http://pvcdrom.pveducation.org/MODULE/NOCT.htm

Regarding Heat derating:

››› Electrical Characteristics at STC (Standard Test Conditions: Irradiance 1000w/m², Module Temperature 25°C, Air Mass 1.5):

• Module Type: LDK250W Poly

• Maximum Power at STC(Pmax): 250Wp

• Maximum Power Voltage(Vmp): 30.30V

• Maximum Power Current(Imp): 8.27A

• Open-circuit Voltage (Voc): 37.7V

• Short-circuit Current (Isc): 8.39A

• Power Tolerance (%): 0-3

• Maximum System Voltage: IEC EN:1000V/ UL: 600V

• Module Efficiency (%): 15.50

• Temperature coefficients of Pmax: -0.45%/?

• Temperature coefficients of Voc: -0.32%/?

• Temperature coefficients of Isc: +0.06%/?

• Operating Temperature(?): -40?~+85?

• Nominal operating cell temperature (NOCT): 45±2?


Is it correct to say the above co-efficients are to be used at above 45C panel temp, not 25C? If so, that may not allow for very much heat DE-ration.

And how hot do panels get in the summer, anyway?

BFL,

I slept on it, and while sitting down by the beach this morning, it became obvious I need to keep things as simple as possible... but without eliminating my tilt 'n' twirl plans. So I'm looking to go with the two 24v panels, only (which is what I would have done originally, if planning a rooftop install, from the get go). But I cannot find 24v/200w poly panels to meet the Tracers 390w/12v bank rating.

Yes, I could simply go with a single panel, pointed, at times, but that would reduce peak amps to about 20a... less than what I'm seeing with the two 12v panels in series/mppt/pointed.

Usually I won't need every ounce of energy I can squeeze out of a single 24v/250w panel, in summer, but there may be times when I need to replenish Ah's quickly, or during/after a/c use. At these times I'd like to get the most out of the system, without taxing the controller too hard.

The panels I'm looking at are 60 cell... 6 rows of 10. My understanding is those 6 rows will be separated into 3 double rows of 10 (20 cells/section), by two diodes. So covering one cell would close down 20 cells, or 1/3 of the panel. Two 250w panels would therefore be de-rated to ~ 333w, for about 27a peak; perfect.

What I don't know is how (external) wiring effects things, if at all? Do the diodes operate differently if the panels are wired in parallel vs. series?

(I'll either keep the 12v system as a portable, or sell it.)

The panels should derate themselves in the summer heat. If two 250s is too much at times, switch one of the 250s off?

Do you even need the extra AH in the summer? (not so much furnace and lights on less time with longer days)

Just get one 250 and another 140 12? IMO avoid 100w monos--low amps.

I derated my 130w panel for a time so I could run it temporarily with a 105w limit controller. Just tape over one cell in a corner and it knocks the panel down to about half. That was enough for the job at hand (maintain batts while away from rig for a few days)