Strange performance of solar blanket

Brodie Allen · Guru

Strange performance of solar blanket

Jaahn · Guru

Hi you have just discovered a well know fact ! Any small shadow on a solar panel will reduce the output by a large amount and it can reduce output to almost zero. That is why some better panels have some bypass diodes built in to the panel connection system to help reduce the downgraded performance somewhat.

That shadow problem is also important if running panels in series into a MPPT reg. It might be worth looking at bypass diodes to help.

Here is an interesting video demonstration of the shade effects. https://www.solarpoweraustralia.com.au/shade-impact

More information on bypass diodes. https://www.pveducation.org/pvcdrom/modules-and-arrays/bypass-diodes

Jaahn

-- Edited by Jaahn on Thursday 27th of July 2023 12:25:05 PM

Are We Lost · Guru

Thanks Jaahn, that video demonstration was useful. It highlights why I have always recommended running panels in parallel. In series with an MPPT regulator will give better results in ideal conditions, but so often conditions are not ideal and when a tree partly shades one of the panels, parallel with PWM does better.

Whenarewethere · Guru

As said, a small shadow will bring a solar panel to its knees.

Some tests with my set-up, panels about 12° off square to the sun.

No cells covered: 127 watts. (After test 3° 129 watts)

No cells covered, full power of 120 watt set-up.

1 cell covered on 1 of the series sets: 85 watts.

1 cell covered on 2 of the series sets: 46 watts.

7473904733253722996 (1).jpg

1 cell covered on 3 of the series sets: 53 watts.

I tried covering 9 cells on 1 panel & it dropped to the same as if only 1 cell was covered on 1 panel.

Let's say you have a very large 300 watt solar panel. It will look like one panel of 144 cells, but the individual solar cells will be grouped into 4 sections of 36 each.

So if a shadow falls on a tiny wee corner, only a 1/4 of the panel stops working. The manufacturer is NOT doing this for you. It is simply using diodes to save the panel from dying due to effectively a short circuit.

This is basically what is happening with my 6 x 20 watt set-up, but because it's only 120 watts & this only 3.8 amps in series ((20 watts 36 cells each x 2 in series 72 cells) x 3 sets 216 cells) I have a fairly low risk of a catastrophic failure, but I am still careful as I don't have diodes.

So basically I have 3 sections & if a tiny shadow falls across a wee bit of one section, I only lose a third of my power.

A message from a bird is enough to ruin your day!

So if you are desperate for every last watt make sure there is simply no shadow no matter how small or you will have warm beer!

Brodie Allen · Guru

just looking at whenarewethere's post -

Currently in good sunlight - meter says 10 amps @ 20 volts.

Is this 200 watts?

From my folding array.

B

Jaahn · Guru

deleated

-- Edited by Jaahn on Saturday 29th of July 2023 07:09:06 PM

Jaahn · Guru

yes 10amps at 20volts is 200watts.

Is the controller a MPPT type ? Otherwise the readings might need to be qualified. The battery will not be accepting 20 V directly from a PWM controller, but the panel voltage may rise that high during the controller switched off time. So the two reading are not at the same moment.

jaahn

Brodie Allen · Guru

Jaahn wrote:
yes 10amps at 20volts is 200watts.
Is the controller a MPPT type ? Otherwise the readings might need to be qualified. The battery will not be accepting 20 V directly from a PWM controller, but the panel voltage may rise that high during the controller switched off time. So the two reading are not at the same moment.
jaahn

Thanks Jaahn.

the readings are straight from the blanket - no controller.

Via an amp meter.

Jaahn · Guru

Brodie Allen wrote:
Jaahn wrote:
yes 10amps at 20volts is 200watts.
Is the controller a MPPT type ? Otherwise the readings might need to be qualified. The battery will not be accepting 20 V directly from a PWM controller, but the panel voltage may rise that high during the controller switched off time. So the two reading are not at the same moment.
jaahn
Thanks Jaahn.
the readings are straight from the blanket - no controller. Via an amp meter.

Hmm to test a panel for power you do need a load for the power to go into ! Then you can measure both the V and A at the same time while it is working. Here are some ideas to test a panel if you wish to play ; https://footprinthero.com/how-to-test-solar-panels

However an easy way is to connect the panel direct to the battery for a short while and do the readings while connected. The V will be the battery voltage, and also some voltage loss in the wires to and from the panel. The ammeter will read the A ok. This will be OK for a short time until the battery voltage gets too high, say 14.5 max then switch off like the regulator does.

So you can see that at battery voltage of 14.5V and say 9 A the power is ~130 Watts . The A will be bit lower as it is not at the optimum point.

A MPPT regulator works by putting a load on the panel to get the Max Power from it and then converts that power to suitable lower V and higher A for the batteries needs at that time.

Cheers Jaahn

Whenarewethere · Guru

That's why I bought a 20 amp Victron MPPT. You can shove in a lot of watts, especially in series. But the output has a fixed maximum amount to the batteries.

I was suspecting that my 120 watts of solar was a bit over the 10 amp output of the controller.

So bought a new 20 amp controller. Up to 11.3 amps output. Not often & also had to be perfect conditions, but nevertheless enough over the 10 amp limit of the previous controller. Why throw any solar resources.

I was a bit surprised that 120 watts of solar was well within the recommendation for a 10 amp controller.

One spends all the money & endless time setting up a system. Then puts in an underperforming controller. A complete waste of the entire system. Might as well tighten up a cable tie around the fishing tackle!

One thing is measuring input from the panels. It may look good. But you need to measure the amps going into the batteries & there could be a good percentage of amps going into heat.

Seems silly having a guillotine between solar & batteries!

Dick0 · Guru

When it comes to shadows on solar panels, it seems to affect serial arrays more than parallel arrays.

Why?

Jaahn · Guru

Dick0 wrote:
When it comes to shadows on solar panels, it seems to affect serial arrays more than parallel arrays. Why?

Hi I might think the two videos I referenced above might give a good explanation ;

Here is an interesting video demonstration of the shade effects. https://www.solarpoweraustralia.com.au/shade-impact

More information on bypass diodes. https://www.pveducation.org/pvcdrom/modules-and-arrays/bypass-diodes

But my explanation might be this.

In parallel each panel can output power independently and each current just adds to the total. If one or more is reduced by shadow, that only affects the individual panels but the rest still output OK. Even a reduced output from a shaded panel may still add some current.

In series the current has to go through all the panels one by one to provide power. If one panel or more, has power reduced by shadow that will reduce the current that can flow through that panel and actually restrict the current flowing if it drops more. So you can see that it is a double whammy for the current flow.

In both cases correct bypass diodes help with shadowing but it is vital for series connection. In big powerful arrays on houses, it also prevents panels being damaged by reverse flow current when shaded which can burnout cells and possibly even start fires up there in extremes !!

Jaahn

Brodie Allen · Guru

Thats easy to understand. Excellent.

Thanks Jaahn.

B

StormCamper · Member

Dick0 wrote:
When it comes to shadows on solar panels, it seems to affect serial arrays more than parallel arrays.
Why?

The most important thing to understand about solar is a; the panels are designed to be installed in a shadefree environment, and if you have shading that's your fault, get out the chainsaw...

Panels are designed for a certain amount of heat cycling, so more shade exposure is a problem, you have to imagine say a branch and a pole acting every day for 20 plus years. Panels can take some hot spots but too much and they can fail and undermine the rest of the system.

b; solar setups are actually suppose to have inspections every so often for safety. There are some dozen ways for hot spots to form from quality issues and degradation of the panels. The main driving reason why rooftop setups use high voltage is simply because of the absurd cost of thick cables. For all practical purposes high voltage adds no benefit over lower voltage, to determine what topology is best is too difficult with many factors like shading, soiling etc. In mobile setups there is no clear consensus.

Some will tell you high voltage starts earlier and finished later giving an overall more energy harvest through the day. The reality is this is a dubious claim usually from salesman. The OCV of a cell collapses with the smallest load applied because the OCV is mearly the product of the small build up of electrons and holes at each end of the pn junction. When you short a cell this build up is completely gone. In other words it only takes a small amount of light intensity to produce a voltage, in other words they are a current source, in other words high voltage has no practical advantage. Low voltage panels work down to about 85degrees from the zenith.... I have never been in a place where the sun is that low.

So for series topology to work well you need at least 4 internal bypass diodes, this of course assumes each panel has its own bypass diodes (more things to go wrong), by the time you get to ~8 series becomes a viable approach. The problem is if a shaded panel gets bypassed too easily the whole panel is lost hence it's not clear as to when series topology is a good idea

Also under proper working conds you don't need blocking diodes for parallel panels. Unless one is so severely shaded, the diffuse light is usually enough to generate an ocv.