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 > Solar Controllers PWM MPPT Temperature Performance

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CA Traveler

The Western States

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Posted: 06/09/20 06:32pm Link  |  Quote  |  Print  |  Notify Moderator

It’s often stated that PWM performs better than MPPT up to 10% at higher temperatures which is a statement that’s largely out of context and basically incorrect. For anything other than for simple solar arrays the performance of the two controllers is the same.

I certainly agree that if you only only consider the panel temperature coefficients panel voltage goes down with increasing temperature and vice versa. The current changes but by by a much smaller percentage. I guess this is the source of the misinformation.

Increasing temperature moves the maximum power point down (less power) and to the left (less voltage) and the IR curve also shifts to the left. This results in decreasing power available for charging and at both lower temperatures and higher temperatures MPPT has the advantage.

IN 2014 when I was gearing up for solar I didn’t fully understand the issue which is explained in the “Victron White Paper Which solar charge controller: PWM or MPPT?”. If interested the document is not long or deeply technical. Enjoy

Click here: https://www.victronenergy.com/blog/2014/........ich-solar-charge-controller-pwm-or-mppt/
Then just under the Victron controller picture click on the link to download the pdf file.


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Lwiddis

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Posted: 06/09/20 06:56pm Link  |  Quote  |  Print  |  Notify Moderator

Interesting article.

“The MPPT controller will harvest more power from the solar array. The performance advantage is substantial (10% to 40%) when the solar cell temperature is low (below 45°C...”

I don't camp much when the temp exceeds 113 degrees F for me or my panels.


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BFL13

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Posted: 06/09/20 07:29pm Link  |  Quote  |  Print  |  Notify Moderator

No idea who often claims that.

It is a fact that panel heating lowers the panel voltage so that typically, you lose 10% of the power because voltage which drops with heat is such a big component of watts compared with amps which go up a bit with heat. So you lose watts for using MPPT which has a buck converter that uses watts in and watts out.

Meanwhile PWM gets the prevailing panel amps to the battery up to and sometimes over the Isc value no matter what the panel heating is.

So for instance with my three 100w panels aimed at high sun each panel rated 6.3 Isc, I got 18.6 amps to the battery with PWM as seen on the Trimetric.

Nostalgic photo--those three are now on top of the Class C in sig.

">][image]

Swapped out the PWM real quick, no change in the sky, for the MPPT controller set to 12-12, no change in wiring and Ta Da! 18.x amps not even 19. So same amps.

I did all kinds of measurements of temperature with my IR gun and tried different things with different panels and measured it all, did all kinds of math with the temp coefficients . I reported a lot of that on here back then. Most of my notes from then are now gone.

Anyway despite those studies that showed (one was Dutch ISTR) that what I was seeing was impossible, I got what I got. Everyone is invited to go out there and do the same thing, swapping controllers, etc, and see what you get.

I have now got a mix of MPPT and PWM controllers in parallel set to the same voltage on my roof arrays and they all behave adding their amps to the right amounts (as expected by me) so I do not have a dog in this race anymore.

I will stick with what I saw with my own eyes for real, never mind what I am supposed to see according to the MPPT enthusiasts, Dutch or not [emoticon] .

Note--I do not have the controller efficiencies for my PWM and MPPTs. the wiring was good for 12v and both controllers were 12-12. Panel temps taken on the back of the tilted panels aimed at the sun showed variations in temp with location on the panel--not the same everywhere, but more to where the sun was and up higher too. So panel temp must be taken as some sort of average and the voltage of each cell is added to get panel voltage remember. Anyway, it was all good fun playing scientist and seeing what was really going on.

* This post was edited 06/09/20 07:36pm by BFL13 *


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red31

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Posted: 06/09/20 09:46pm Link  |  Quote  |  Print  |  Notify Moderator

the full white paper is informative although I believe biased as one can see in the graph fig 15 (chapter 7). The problem is their comparison includes series connected panels for MPPT! Well additional voltage above batt from somewhere!
"Let us now assume that the MPPT controller is connected to a solar array with sufficient cells in series to achieve an MPPT voltage several volts higher than the highest battery voltage."

Full white paper

It is hot here! All I need to do is look at fig 15 and see that PWM is above the 90% line in the temps of concern. The MPPT line should mimic PWM when Vmp is less than batt voltage as seen in chapter 5. Vmp drops with heat!
"Most MPPT controllers cannot transform a lower voltage to a higher voltage, as that’s not what they are made for. If the MPPT voltage Vm becomes lower than Vbat, they will therefore operate like a PWM controller, connecting the panel directly to the battery."


to me the 90% line represents the bucking eff. With regular controller limiting power in abs and float it just don't matter much then you go series (wiring. light and heat) and ya need MPPT.

* This post was edited 06/09/20 09:56pm by red31 *

BFL13

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Posted: 06/09/20 10:14pm Link  |  Quote  |  Print  |  Notify Moderator

Lwiddis wrote:

Interesting article.

“The MPPT controller will harvest more power from the solar array. The performance advantage is substantial (10% to 40%) when the solar cell temperature is low (below 45°C...”

I don't camp much when the temp exceeds 113 degrees F for me or my panels.


Typically panel temp is about 20C above ambient, which I can confirm from my measurements. So when it is nice out at 25C, panel is 55C and down 10% in watts at around 50C

I cannot understand the way they do panel specs at panel 25C which would mean ambient is just above freezing. Sort of weird.

The MPPT guys all talk about power but PWM could care less about power--just the amps. So all that power being wasted by PWM is bogus, since you still get the rated Isc so the power does not matter.

The main thing is that you need the buck converter in the MPPT to do 24-12 with 24v panels, which PWM controllers cannot do. They can do 12-12 or 24-24, while an MPPT can do either 24-12 or 12-12 or 24-24 so you have a choice between series and parallel with 12v panels.

red31

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Posted: 06/10/20 06:44am Link  |  Quote  |  Print  |  Notify Moderator

Many panels have published NOTC.
This test is @ 80% or 800 watts per square meter vs 1000.
20C air blowing on the panel @ 20C.

45C temps in these conditions! with a breeze and space behind the panel for air flow.

"Both conductive and convective heat transfer are significantly affected by the mounting conditions of the PV module. A rear surface which cannot exchange heat with the ambient (i.e., a covered rear surface such as that directly mounted on a roof with no air gap), will effectively have an infinite rear thermal resistance. Similarly, convection in these conditions is limited to the convection from the front of the module. Roof integrated mounting thus causes higher operating temperature, often increasing the temperature of the modules by 10°C."

https://www.pveducation.org/pvcdrom/modu........rrays/nominal-operating-cell-temperature

BFL13

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Posted: 06/10/20 07:40am Link  |  Quote  |  Print  |  Notify Moderator

NOTC does not work for me. I would get full Isc when conditions seemed to be more NOTCish. I assumed when I got full Isc that meant it was like Standard conditions. You don't know how much air pressure is compared with temperature and light. To get all of those variables to line up per NOTC would be difficult. One component will be ahead of the other in real life.

With MPPT, to check it, you first check Isc of the panel, get rated Isc in the prevailing conditions, then connect up and see what amps to the battery are when the battery is low enough to have the MPPT in Bulk. That will be as good as it gets for amps to the battery except for edge of cloud events

My Tracer has a setting for temperature but I just leave it at 25C. Not sure what that is about.

Here is how I had the panel when I was doing the temp meaurements with the IR gun at the back of it. Lots of breezeway. As mentioned, the temp is not the same everywhere so it would be hard to get each cell's temp and do its voltage calculation from the coefficient and add up all those voltages to get your "panel voltage". It sort of works as an average using the middle of the panel. (Not showing are the bungee cords I had all over the back of that to stop it blowing away)

">][image]

My Eco-Worthy has a funny program for seeing how much better it is doing than PWM would be. It is sort of bogus, but it turns out that its PV amps reading is the same as the Isc reading by meter, so you don't have to disconnect the panel to get that figure with the Eco-W.

* This post was edited 06/10/20 08:02am by BFL13 *

red31

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Posted: 06/10/20 09:09am Link  |  Quote  |  Print  |  Notify Moderator

It is not whether or not NOTC works for you, it is that NOTC info shows that panel get hot as you know, they get hot even with 80% 'sun'.

The biased white paper shows when PWM=MPPT, this is a large range, bigger than the blue shaded area, I'll suggest 40-100C

BFL13

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Posted: 06/10/20 10:04am Link  |  Quote  |  Print  |  Notify Moderator

I think what I don't like about NOTC specs is the suggestion that getting 80% is normal, when I would get full Isc aiming at a high sun as being normal. I got the idea they listed the NOTC numbers as some sort of salesman excuse thing. I am muddled in my thinking about all that. "Nominal" does not equal "Normal" I guess.

These guys use "Normal" for NOTC. Only they call it NMOT. they use "nominal" in the warranty statement. They give a graph for low light compared with STC performance

http://www.wegosolar.com/products.php?pr........-310-Watt-60Cell-Solar-Panel-Black-Frame

EDIT--I see my Canadian Solar brand panel specs use "Nominal" for NOTC. It probably makes sense in the original Swahili ?

To me it is so easy. Whatever the conditions or time of day, aim the panel and see what Isc is. Connect up and see that is the same as the amps to the battery with PWM and your equipment is working right.

I found that Isc per panel wattage is about the same so I can estimate what a 255 watt panel ought to do if it were PWM, then see what it really gets. I use a 130w panel with 8.2 Isc as baseline.

255/130 x 8.2 = 16 amps So I thought I would see more than 16 with my MPPT. Not really. So after much poking around it turned out to be panel heating reducing input watts so output watts was lower and divide that by battery voltage and there you are. It did come out a little ahead in the morning with lower Isc at lower sun, but mid-day about the same.

Salvo posted his results back when showing his MPPT did much better than PWM/direct early in the day, then the results got close by high noon.

I had the thought the 20 amp MPPT controller would clip the amps so it would be stuck at 20, but it was doing 18.x with MPPT compared with 18.6 with PWM in that test with three 100s.

Heat coefficients for 12v panels are not the same as with 24v panels so that might throw things off a bit, but not enough to matter.

* This post was last edited 06/10/20 10:55am by BFL13 *   View edit history

ktmrfs

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Posted: 06/10/20 12:42pm Link  |  Quote  |  Print  |  Notify Moderator

now that panel prices have come down, and efficiency up so panels are less expensive and smaller for many cases PWM makes sense given the price.

There is at least one case I often encounter where MPPT and series connected panels pays off vs. PWM or parallel MPPT

I have 3 175ish W portable panels. In a few cases when we camp the only location that gives reasonable sunshine is located 100-150 ft from the trailer. For cabling I made up a bunch of cables using 30A RV cables with anderson connectors and paralling the ground with on of the other two wires. So basically I have a cable with one wire #8 and one wire #10. Now stuff 30ish+ amps down that cable with 3 panels in parallel vs 10A in series with an MPPT controller at the batteries. MPPT wins in the series with about a 10% current gain vs parallel.

As far as my experience with MPPT vs PWM, the experiments I did for comparison did show an advantage to MPPT even with short cables, but not as high as one might think. Many MPPT controllers are most efficient with series connected panels with voltage near the max rated for the controller and least efficient with 12V parallel connected panels. That was born out in my experiments with the 12V parallel connected panels have more output than my PWM controller but on the order of 5%. while the series connected setup with 3 series 12V panels had better performance. I have the data archived somewhere.

In any event, IMHO with todays prices if you have room spending the money on and extra panel and PWM controller will get you more AMPS than spending the money on a MPPT controller.


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