solar system catastrophe and rebuild

[Jayson]

I'm happy that the panels all tested the same and seem to be in functioning order, even after campsite resoldering the busted panel junction boxes.

Getting 5 amps from each panel at 2pm was great. The part I am missing is why aren't the amps doubling to around 10 with the 2 series groups connected in parallel?

I have 640 watts in panels, so I expect to get 450 to 500 watts peak...with this situation I will max out at ~200 with abit of loss like my 180 maximum today.

Is my math incorrect here? With all cylinders kicking I should be getting 41v x 10± amps from the array. But only getting half that in current.

[Bala]

At 2pm you had 5A from one series pair and 4.75A from the other series pair. So at 2pm you could have seen 9.75A total.

At 3pm due to sun angle you had 3.8A from one series pair and 3.4A from the other series pair so 7.2A possible and you said at 3pm you were getting 6 to 7A.

[Jayson]

Gotcha. I'll keep taking tests, at least of the main incoming current.

At the end of the day, I just want to be able to replenish the energy I plan to use. If I could get around 400 watts incoming I think I could achieve that. If my 640 watts of panels is going to max out at 180, I will struggle.

So now that we know the panels are ”good”, why am I only receiving 30ish% of the power?

[Ampster]

.........
So now that we know the panels are ”good”, why am I only receiving 30ish% of the power?

One hypothesis could be temperature and/or orientation. Do you have a way to test that?

[Bala]

As Ampster said, if you want more production you need better sun angle and cooler panels.

Mobile panels so you can chase the sun.

If your panels are mounted direct to the roof with no air gap then they will not cool as good as if they were raised.

[Jayson]

Thanks for the input. I didn't realize that those factors could have that much of an effect.

I did do a fair amount of research on the flexible panels/mounting and opted to mount them on 1/4 inch corrugated plastic sheeting.

Provide some airflow and keeps em off the hot roof. Still, my 2 pm test they were pretty hot to the touch.

I can't think of anything super accurate with a probe or beam that I have here to measure temp. I have a pretty accurate indoor thermometer that I might get a ballpark reading from.

I'll do some reading on...current loss due to panel heating?

Guess I'll start there. Thanks again.

[J.P.M.]

So now that we know the panels are ”good”, why am I only receiving 30ish% of the power?

Two big reasons as Ampster pointed out: Because the solar input is probably only about 30% or less of the STC test input, and your panels are running hot because of less than optimal air circulation around them which lowers their efficiency. The first you can't do much about. The second you can.

[J.P.M.]

Thanks for the input. I didn't realize that those factors could have that much of an effect.

I did do a fair amount of research on the flexible panels/mounting and opted to mount them on 1/4 inch corrugated plastic sheeting.

Provide some airflow and keeps em off the hot roof. Still, my 2 pm test they were pretty hot to the touch.

I can't think of anything super accurate with a probe or beam that I have here to measure temp. I have a pretty accurate indoor thermometer that I might get a ballpark reading from.

I'll do some reading on...current loss due to panel heating?

Guess I'll start there. Thanks again.

On operating cell temps. in general: A reasonably well designed system's cell temp. under high sun and a fairly perpendicular angle between the sun and the plane of the array should run something like 30 - 32 or so C. above the ambient air temp. immediately around the array, with probably a reduction in that temp. diff. of maybe 1 - 2 C. or so for every m/sec. of wind speed up to ~ 10m/sec. wind velocity.. So, with bright sun and no/light wind and a close to normal incidence angle of sunlight, 30 C air temp. and a 30 C delta T will get you a 60 C. cell temp.

Flush mounting a stationary array will decrease circulatory cooling potential and up the cell to panel temp. diff. and so the cell temp. by maybe 8-12 C. and in so doing decrease cell eff. by maybe 4-5 % or so besides probably decreasing cell life a bit.

I use an IR thermometer that I calibrated to measure front and back of panel temps. Get one that has readout that's adjustable for the surface emissivity of what you're measuring. I add ((POA irradiance /1000)*3 ) to my back of panel readings for an est. of cell temp. I believe I get data that makes sense and jibes w/other measurements of electrical parameters.

If you're interested in some semi technical stuff see PVEDUCATION.org or, even though it's a solar thermal text book, chap. 23 of "Solar Engineering of Thermal Processes" by Duffie & Beckman. It's a pretty complete but still concise treatment of solar protovoltaic design with a pretty decent treatment of cell temps.

On current loss due to panel heating: one place to look: current loss/deg. C (or K) is usually/often listed on a panel's data sheet.

[Jayson]

Back in service area and just wanted to pop on and say thanks for all of the time and input from everyone here. After reading a bunch on temp coefficients and voltage losses due to heat, I think you guys were on the money with the diagnosis of power loss due to hot panels.

I've been able to track and test over the last couple days and the variance is quite massive. I think in my case the cheap panels are VERY sensitive to heat. It explains why my best numbers came earlier in the morning with good sun and not too hot yet. Unfortunately I have no safe, reliable way to raise the flexible panels. This is my maiden voyage and largely a test run for everything, so I will look at a rigid panel system for the future.

All that being said, I've been able to run all of my systems without fail and recharge a decent amount each day. As I dial in my last components and run full loads each day, I will probably need some some help better understanding the inverter function, battery bank, and voltage levels (drops) I am experiencing to make sure it's all "normal" activity.

Thanks again, Happy Easter!

[J.P.M.]

Back in service area and just wanted to pop on and say thanks for all of the time and input from everyone here. After reading a bunch on temp coefficients and voltage losses due to heat, I think you guys were on the money with the diagnosis of power loss due to hot panels.

I've been able to track and test over the last couple days and the variance is quite massive. I think in my case the cheap panels are VERY sensitive to heat. It explains why my best numbers came earlier in the morning with good sun and not too hot yet. Unfortunately I have no safe, reliable way to raise the flexible panels. This is my maiden voyage and largely a test run for everything, so I will look at a rigid panel system for the future.

All that being said, I've been able to run all of my systems without fail and recharge a decent amount each day. As I dial in my last components and run full loads each day, I will probably need some some help better understanding the inverter function, battery bank, and voltage levels (drops) I am experiencing to make sure it's all "normal" activity.

Thanks again, Happy Easter!

As a final comment, under sunny skies, unless your panels have an absolutely terrible temp. coeff. of voltage, a poor angle of incidence (and the lower P.O.A. irradiance that comes with it) between the sun and the panels will most likely hurt instantaneous power and daily energy output quite a bit more than high temps. or high (negative) temp. coefficients.

Rough SWAG, the reduction in output due to a lower P.O.A as a result of the lower angle of incidence at 10A.M. vs. noon will be responsible for most, if not all of the decrease in output at that time vs. noon output.

Somewhat ironically perhaps, the panel efficiency (as defined by solar input to the array/array electrical output) at those times, depending on the amb. temp. difference between 10 A.M. and noon and the diff. in P.O.A. irradiance at those two hours may result in panel temps. at those times that are close to one another. All other factors being equal, less P.O.A. irradiance will mean a lower panel temp. and it's also likely that the amb. air temp. will be less at 10 A.M. but if the lower P.O.A might lower output by 15-20%, the lower panel temps might increase efficiency by about 5% so.

Array power levels are more sensitive to and dependent upon P.O.A. irradiance levels and so angles of solar incidence on the array than any other input.

Take what you want of the above. Scrap the rest.

[Jayson]

In a series circuit, opening the circuit in one location, stops current flow. Then it's safe to de-mate and mate the connectors. They are still LIVE with power, but with no flow, there is no destructive arc.

Another simpler measurement, is, while the system is running, measure the PV voltage at the controller input, which should be close to the Vmp of your panels in series, if so, that's a quick check that the MPPT is at least keeping the panels at their Vmp. If you read a voltage close to battery voltage (27v) on the PV side of the controller, you have a problem

Was just ready to post, trying to get Mike's attention, and another lost entry, full of detail.

Quick version, ugh:

Still getting 41 from the panels. When mppt starts working, the voltage fluctuates all day from 29 to 41 volts and won't charge up to float voltage (27.2) where it easily did before.

What problem does this indicate?

It's quite a bit hotter now, other than that I haven't changed much and it's been working great.

[Mike90250]

Still getting 41 from the panels. When mppt starts working, the voltage fluctuates all day from 29 to 41 volts and won't charge up to float voltage (27.2) where it easily did before.

What problem does this indicate?

It's quite a bit hotter now, other than that I haven't changed much and it's been working great.

It indicates the MPPT is not working. If it was working properly, the panels will never drop below their MPPT voltage until the battery is nearly full.

[Jayson]

Okay, thanks Mike. Looking into this a bit more online as there are many users with identical circumstances.

With the DC breaker off (pv to mppt line), I get 41 volts on the pv side. When on, it starts to fluctuate and the voltage reads the same across the entire line.

This info makes it more confusing because this is precisely what is happening:

"A MPPT solar charge controller is the charge controller embedded with MPPT algorithm to maximize the amount of current going into the battery from PV module.

MPPT is DC to DC converter which operates by taking DC input from PV module, changing it to AC and converting it back to a different DC voltage and current to exactly match the PV module to the battery."

Anyways, it seems to charge normally up to about 26.5 then kind of sputters out, not charging much more. The voltage never gets within 2 volts of the current battery voltage, but close...

I'll check my program once more and try to "reset" everything and check all connections tomorrow.

[Mike90250]

> With the DC breaker off (pv to mppt line), I get 41 volts on the pv side. When on, it starts to fluctuate and the voltage reads the same across the entire line.

That sounds to me like PWM. MPPT should only lower the Voc of the panels by a couple % When it starts the sweep, you would see a quick dip in voltage, but the panel voltage should quickly return to something very close to Vmp.

PWM would quickly connect the Panels to the battery, and the panel voltage will drop to the battery voltage, and stay there until the battery is charged, and then the PWM will start modulating the pulse width to not overcharge the battery.