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diy solar

Snow on my panels is killing me

At present tilt, not much tendency to slide.
At 40 degree tilt, if bottom edge on roof rather than being ground-mounted well above snow, will slide until it piles up, won't clear panel.
You are right. The OP has a stepped roof, so it would have been advisable to mount the panel at the edge of the stepped roof or even better, at the edge of the roof anyway so the snow avalanche provoked by warming the panels can have room to pile up.
Of course if you have snow growing until the roof, nothing will help.
 
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A PV panel sized to charge battery won't get backfed except at a very low current. That would accelerate draining battery.
If you connect a nominally 12V (approximately 20 Voc) panel to a 12V battery, when sitting at 13.6V or whatever fully charged battery shows, that is way down from Voc.
Backfeed would be (at most) the difference between Isc of panel, and current at 13.6V on the IV curve.
That is why I suggested an adjustable booster buck converter.
You must increase the battery voltage until you get slightly above Voc at 80% of the nominal panel current.
 
Thanks but again, I wasn't talking about tiddlywink 12 V panels. I was talking about real panels that are used on household and commercial applications. Everything I have seen from about 190W up is 30V Vmp and 37 Voc. The 330s and 400s I have are 45 -57V.
Pretty sure the real panels now can't be backfed...
The proposed 1200W 20A Step up converter features:
  • Input voltage: 8-60V
  • Input Current: 20A
  • Output voltage: 12-80V continuously adjustable
  • Output Current: 20A MAX Over 15A
So where is the problem?

I know a guy that used to complain:
You are not trying to be helpful here, you are just trying to push a position of superiority in how you think things should (not) be done...
Do you need a mirror?
:rolleyes:
 
Just a thought. How about installing some huge windows and mounting the panels inside. You can then adjust the angles at will as needed in a dry warmer environment. Solar heat to boot.

PM
PDX
USA
 
Just a thought. How about installing some huge windows and mounting the panels inside. You can then adjust the angles at will as needed in a dry warmer environment. Solar heat to boot.

PM
PDX
USA
That works for the winter-tilt South facing panels (less extra light loss. Double-pane insulated windows? Even more loss. Also, you may want to keep your batteries warm in winter, but not your panels.
If panel not plastered up against window, window has to be much larger to allow angles of sun.
Summer sun, "window" would be a skylight. Same problem.

I have a "solar road" and a "traffic-powered wind turbine" to sell you!
 
Thanks but again, I wasn't talking about tiddlywink 12 V panels. I was talking about real panels that are used on household and commercial applications. Everything I have seen from about 190W up is 30V Vmp and 37 Voc. The 330s and 400s I have are 45 -57V.
Pretty sure the real panels now can't be backfed even if they could 10 years ago or the little toy ones can.
Op appears to have real, commercial quality panels on his shed of the type I am talking about.

Asking on other forums, couple have people have got back to me and said they have batteries direct connected and not had any problems with discharging.
Still like to see some real world evidence of them being back fed. Must be vids of it out there somewhere and melting snow. I can't find any though.
15 to 20 years ago, 12V and 24V nominal panels were the ones used for household and commercial applications. Some number of PV cells in series and a few bypass diodes (in most cases)

I have some SunPower 36V nominal 327W panels I wanted to measure


... but I'm out of MC4 pigtails.

So I measured a Sharp 24V nominal 165W panel instead.


43.1 Voc, 5.46A Isc, 34.6 Vmp

I have an HP 6627A supply, 50V 0.8A
I set current limit for 0.5A and stepped up 1V at a time.
Result was an exponential increase (or slightly higher order?), hit 0.5A at 39V, so I was delivering 20W.
From data sheet curves, I would have expected 0.5A at 33V, 2A at 39V. That's if there was no IR drop, all diode drop.
There is certainly some resistance in silicon bulk, so voltage out drops off more rapidly as you draw current in operation.
Current delivered in this "electric defroster" test doesn't go up as fast with applied voltage, also due to resistance.
At 28V (about a fully charged 24V battery), leakage current was 24 mA. So that's how much backfeed you would get leaving a "24V" panel attached to a 24V battery without light. About 0.6 Ah per day.

Sharp dark current.jpg
 
Never seen anything that old. Might be far more practical and economical in the long run if people using them got with the times a bit and upgraded. Not like used panels are hard to find or expensive now and they would still be a lot newer than 15 years.
Exactly what I'm doing. My older 12V and 24V panels are smaller, and 12% to 14% efficient. My new SunPower panels are larger and 20% efficient, so I'm in the process of substituting the SunPower to get 50% more power out of the same mounts. Primarily because PG&E rejiggered time of use rates in their favor.

Other than that, no reason at all to "get with the times" or "upgrade". Still making about the same power as day 1.
Better thing to do might be get with the times and upgrade appliances. Higher efficiency air conditioner and heat pump maybe? But why replace perfectly good PV panels and inverters. (I'm only replacing inverters for compatibility with my new battery-backup system)

Let me guess - you buy a new car (or bike?) every few years?
My bike may well be older than you.
I know some of my cars are.


Not going to defrost much snow at that rate.
Like none at all.
Would seem to me the forward current Drop in a diode as has been suggested to stop backfeeding would loose more power while Charging through the course of the day than the panel would take backfeeding at night.

Never said 0.6 Ah per day would defrost. But it would drain your battery during an Alaskan winter's "night"

I said at 39V I delivered 20W to the panel. About the limit of the supply I had for testing.
Projecting the curve I got up to 44V, would deliver about 100W to the panel. Maybe that would do the trick.
As I posted earlier, with multiple parallel PV strings, when voltage goes to Voc because SCC isn't drawing current, a string not covered with snow would deliver heating to a covered one.
 
At 28V (about a fully charged 24V battery), leakage current was 24 mA. So that's how much backfeed you would get leaving a "24V" panel attached to a 24V battery without light. About 0.6 Ah per day.
No need to say, nor to test a lot.
Backfeeding to defrost needs ALWAYS more voltage, than that of the battery.
Somewhat a bit higher than Voc of the panels to reach 0.8 * the nominal current.
That's the very reason for the boost buck converter i have proposed. Or using a separate power supply.
 
I said at 39V I delivered 20W to the panel. About the limit of the supply I had for testing.
You can use your battery to provide the first 24V, with your adjustable power supply in series, you can test up to feeding 0.8 * the nominal current.
The panel should get warm very fast and the voltage reduce noticeably: you imperatively need a CC power supply to keep the current constant!
Until the lower layer of snow melts and you get a snow slide should not take very long.
 
"my knowledge and experience with thermo Dynamics tells me that 100W over a 1.6 Sq Meter area in sub zero conditions is going to be about as effective as urinating on a bush fire."

Eskimos insulate with snow. Bigger heat loss may be underside of panel that doesn't have any, as you say. At least if there's any breeze. It is plastic, though. You might be right that wattage similar to STC of panel won't make it shed snow, but then again it might.

"one of those 16 Hour nights or whatever it is in those horrible frozen places"

No, I meant those 28 day nights. https://en.wikipedia.org/wiki/Polar_night

Maybe ground water source heat pump would help you. Cost of a well, though.
For AC, making ice at night (as you mention). (Either cooler air to dump to, or lower electric rates.) Or that ground water source again.
Those tubes running through ice/water ought to chill their contents to zero C. I would expect the ice to melt as fast as you heat the water above zero.

My panels from 15 ... 17 years ago cost me $5, then $4 per watt. A few years later picked up some more Sharp, used, for $2/watt.
Recently was looking for more to add. After realizing the SunPower put out 50% more per unit area I bought enough to replace 100% of area and paid $0.35/watt. I've seen panels from $0.15 to $0.50 and sometimes more, but selected these as a good price for premium quality. 1/15 the the price I originally paid! And I picked up grid tie inverters for $0.08 to $0.12

My bike is '64 (Super Hawk). Our daily cars/truck are all 2000 and older. And then I have some from the 50's and before. I've pulled and/or overhauled a few engines and transmissions. Maybe that's why I saved enough to be able to go overboard on PV.

My recent appliance acquisitions have been other people's hand-me-downs. A Kitchen Aid fridge for $50, mostly because I was scared by the bad reviews for all new models. A mid-range gas stove to replace the cheapie. My sister had bought a stove with electronic ignition during Enron scam, then had the $500 ignition unit fail twice. I'd rather just have a pilot light, but my "new" stove like the fridge had already proved their reliability with the previous owner. I've bought higher end appliances too, but that was for a rental unit.

As for your air conditioning needs, probably sun load on the house is a lot of it. There was a push for reflective shingles for a while. You said your South roof is clear of panels for cosmetic reasons, which means it gets full sun?
My exposed beam ceiling lets in a lot of heat. Hope to put insulation, maybe reflective layer under the shingles when I re-roof.
 
"Also, melting ice is hard because it's almost self insulating to a point. I was playing with a small setup I was looking at to freeze water at night and run Pipes though it to heat exchange for cooling through the day. You don't need much of a water layer before the water warms up and the ice will stay frozen instead of Phase changing."

It is the other direction where I would expect difficulty.
You don't need much of an ice layer to insulate a pipe so it no longer transfers heat away from liquid at a distance from the pipe. Interferes with building ice.
But melting ice (extracting cold from a block of ice), your pipe melts the ice adjacent to it, then convection transfers heat through the newly melted water, delivering it to the ice (and cooling the pipe.)
 
When you are not there, do the batteries get warm enough in the winter to charge? (over 32 degrees F), I understand it can be really cold in the winter high's & lows in the single digit & below or weeks on end. After a while, does you building get too cold for the batteries to charge?
 
@george65 it sounds like you have a monster PV panel setup.

At what voltage do you pull all that power to the SCC and what SCC’s do you use?

Just curious.
 
I will check out Santan Solar. Thanks!

Here are my CC and panels specs:

CC:

Maximum PV Array Voltage (Operating) 140 VDC
Maximum PV Array Open Circuit Voltage 150 VDC
Array Short Circuit Current 60 ADC maximum
Nominal Battery Voltage 12, 24, 36, 48, 60 VDC
Battery Voltage Range (Operating) 10 VDC to 80 VDC
Maximum Output Current 60 A (for all battery voltages except 60 V)
Maximum Output Power 3500 W
Auxiliary Output 5–13 V, up to 200 mA Tare Loss/Night-time Power Consumption 2.5 W Charger Regulation Method Three-stage (bulk, absorption, float) Two-stage (bulk, absorption)

Panels:

Solarworld SW285 x6
  • STC Power Rating 285W
  • PTC Power Rating 259.1W 1
  • STC Power per unit of area 15.8W/ft2 (170.0W/m2)
  • Peak Efficiency 17%
  • Power Tolerances 0%/+2%
  • Number of Cells 60
  • Nominal Voltage not applicable
  • Imp 9.2A
  • Vmp 31.3V
  • Isc 9.84A
  • Voc 39.7V
  • NOCT 46°C
  • Temp. Coefficient of Isc 0.04%/K
  • Temp. Coefficient of Power -0.41%/K
  • Temp. Coefficient of Voltage -0.119V/K
  • Series Fuse Rating 25A
  • Maximum System Voltage 1000V
Hanwha Q-cell x3

Model NumberQ.PEAK G4.1 300 BFR
STC Rating300.0 Watts
PTC Rating274.5
Open Circuit Voltage (Voc)39.76 Volts
Short Circuit Current (Isc)9.77 Amps
Frame ColorBlack
OriginKorea
Power Tolerance-3 / +3%
Module Efficiency18.0%
Area17.67 ft²
Weight41.45 lbs.
Length64.57 in.
Width39.40 in.
Height1.26 in.

"Six more panels for a total of 15 would be right at my MPPT CC max."

You can generally get away with making more watts available, because CC should limit its output current to what is wants. It would be possible for an MPPT CC to not protect itself, so you could check with manufacturer. PWM CC can't protect themselves that way.

The most important limit is Voc, and it looks like you have plenty of margin for cold weather.
ISC is 60A, so 6 strings all oriented the same would be below that. (different angles would reduce actual current produced.)

So you have Solarworld 3s2p + Hanwa 3s1p,
119 Voc at ambient (plenty of room for cold weather without exceeding 150 Voc. One panel -0.119V/K, other not listed)
29.5A Isc (limit is 60A)
2378W PTC
49.5A into 48V (SCC should self-limit to 60A)
39.6A into 60V

Looks to me like you could over-panel to double the panels, all oriented the same, and be just below 60A Isc
Of could power would be limited part of the day by 60A max to battery.

Putting 9 similar size panels on the wall, angle is different so area facing sun and peak wattage available is reduced, but charge controller will still clip to 60A some seasons. Winter with snow on the roof, the wall mount panels will make a big difference.
 
The roof is too slick to get up there and even if I had a roof rake I think it's too crunchy to remove.
By the way, rake-removing the white loose part of the snow, just before a predicted sunny day, is always effective!
Without it, the sun can reach the dark panel through the transparent ice crust and frequently warm it enough to provoke a slide.
 
"Six more panels for a total of 15 would be right at my MPPT CC max."

You can generally get away with making more watts available, because CC should limit its output current to what is wants. It would be possible for an MPPT CC to not protect itself, so you could check with manufacturer. PWM CC can't protect themselves that way.

The most important limit is Voc, and it looks like you have plenty of margin for cold weather.
ISC is 60A, so 6 strings all oriented the same would be below that. (different angles would reduce actual current produced.)

So you have Solarworld 3s2p + Hanwa 3s1p,
119 Voc at ambient (plenty of room for cold weather without exceeding 150 Voc. One panel -0.119V/K, other not listed)
29.5A Isc (limit is 60A)
2378W PTC
49.5A into 48V (SCC should self-limit to 60A)
39.6A into 60V

Looks to me like you could over-panel to double the panels, all oriented the same, and be just below 60A Isc
Of could power would be limited part of the day by 60A max to battery.

Putting 9 similar size panels on the wall, angle is different so area facing sun and peak wattage available is reduced, but charge controller will still clip to 60A some seasons. Winter with snow on the roof, the wall mount panels will make a big difference.
Thanks Hedges. Since the used panels are so cheap I'm inclined to buy another CC to take full advantage of the additional power. I can get another one for under $500 shipped. It's worth it to me to get another Conext CC because I can remote configure/monitor all my Conext equipment. And the Orian BMS I will be using with my new LifePO4 bank has the ability to shut down down and turn back on the Conext CC and Inverter (so I'm told).
 
I know the struggle, same issue with my shed panels. I gave up on trying to keep the snow off but would sure be nice to get free power year round. We get too much of the crappy stuff now days. The wet/icy snow, instead of just the normal snow that is easy to brush off. I even see stop signs caked with snow every time it snows now, so I feel even vertical panels would have trouble. But I think vertical ground mount might be the best bet still, at least they are easier to reach with a scraper.
 
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