Dramatic collection reduction

[SolarSteve]

Hi all,

The snow around here has piled up enough that a strong blowing from the south drifted snow against the bottom 20% of my array. This happens maybe once or twice a year and I trudge out into the field and move the snow. I didn't have the time this week to deal with it and because it was super sunny I thought there was no rush. Then last night I found my bank SOC was sitting at 57%. Obviously not happy about that!

I checked my Outback controller and it said that for last week I only got under 20AH. After clearing the snow away the next morning I got 334AH, which has got to be a record for me as I've never let the banks get drained down so low

I don't understand the physics of why ~20% reduction in capacity resulted in 10 fold decrease in collection. Can someone help me understand what happened? Obviously I'm not going to blow off cleaning the panels the next time this happens.

Thanks,

Steve

[Mike90250]

snow covering a small portion of the panels, can effectively shut down the string, if the tolerances are close. There are bypass diodes often embedded in the panels, that are supposed to limit the reduction to 1/3 of the panel.

At least you caught the problem before your batteries were toast

[peakbagger]

Home Power years ago had an article on how little panel shading could impact a panel. If I remember correctly a horizontal strip across the panel made a lot more impact than a vertical one.

I have to run my snowblower in front of my pole mount or the snow that slides off after a storm can cover the bottom edge. I used to drive by big solar farms in mass that didnt have a lot of clearance between the ground and the bottom edge of a panel, after a big snow storms I could see acres of panel with the bottom edge covered with snow. It was not practical to clean between the rows (no where to put the snow) so they just let them sit there and hope for warm weather.

[SolarSteve]

Thanks very much for the answers! I suspected as much, but it's great to have something to back up a hunch. I'll have to be less laid back about snow cover in the future.

Compounding the problem was my inverter's default 44v low battery cutoff setting. I found a good thread here where people with the same inverter and 48v system discussed the dangers of such a low default. I understand the rationale for it, but it nearly landed me on the wrong side of the 50% capacity threshold. I've bumped my setting to 47v and will see if I get unexpected cutoffs when sudden loads hit the system.

Steve

[Mike90250]

Large inverters have low LVD to allow for some voltage sag in the batteries under some brief heavy load. 46V is more common, than 44v

[SolarSteve]

That's what I learned in the other thread. It makes sense to me, but as you point out that 44v is rather low for the default. The trade off between saving an occasional cutout and potentially ruining thousands of Dollars worth of batteries seems out of whack to me. It seemed that way to the other people with the 44v default

Steve

[inetdog]

That's what I learned in the other thread. It makes sense to me, but as you point out that 44v is rather low for the default. The trade off between saving an occasional cutout and potentially ruining thousands of Dollars worth of batteries seems out of whack to me. It seemed that way to the other people with the 44v default

Steve

One way you can deal with this is to use a separate LVCO circuit and design it to have a time characteristic, i.e. 44V is OK for up to say 1 minute for motor starting, but if sustained for over a minute the threshold is 46V or even higher. I do not know of any commercial product with this type of time characteristic, but by combing an op amp with a voltage reference you could construct one fairly easily. Or use a very small programmable logic controller.
The kicker is that you either have to have an enable/disable input available for the inverter or else have a contactor capable of carrying and interrupting some pretty high current.

[bcroe]

Perhaps just having a single LVCO with a minute or so delay would cover things. Otherwise
several with different delays could be used. The usual delay method here is a 4541 chip (can
be set up for miliseconds to days) directly driving the gate of a big MOSFET, then a relay if
needed. Microwatt operating power and uncritical power.

I would like to get the rest of my panels 40 inches above the ground, to avoid the snow
pile up problem. It does not take lots of snow to cut off a group of panel cells, causing
them to be bypassed and lowering panel voltage by a third. Depending on your charging
arrangement, that could cost you 1/3 energy collection, or nearly all. Bruce Roe