Melting snow off PV..

I have never had to worry about my panels getting snow covered as either I am there to shovel them of, or I install panels flat on the wall for winter exposure so that they don’t get covered.
Here is my dilemma...I have been commissioned to do a system design for a grid tied, remote island location, in Northern MN. As I said, they have the luxury of having grid power even though they are a mile or so off shore. The design criterion is to off set summer seasonal use with winter production. For 8-9 months there will be exactly ZERO loads on the system, summer time the loads are pretty substantial. I will do the calcs to see how big a system would be needed to roughly offset the power needs.
My question however, is that this system is going to be unattended and likely inaccessible in all off the winter season so that no one will be able to routinely shovel or sweep the PV. Given the design criterion, winter harvest is critical for the economic performance of the system. So the question is how best to attack the problem? My first inclination would be to simply put roof rated dielectric heat tape in some pattern around the PV. I know from my own experience, even on a very cold (-30F) day, a bit of black panel exposure, and the panel pretty quickly clears itself.
A number of problems come to mind with this strategy. The first is making sure that sliding snow does not dislodge the heat line. The second is having the system shut off when it is not needed. In a normal heat tape situation, a simple T-stat suffices, but in this case, once the panels are clear, even it if it really cold, the heat line doesn’t need to be on. It would seem to me that a link to the inverter such that based on time of day, if the inverter “should” be producing but isn’t, it could trigger the tape to come on, once the inverter produces X watts (by design) then it would shut off until the next cycle, reducing the parasitic energy cost to production.
A couple of things to note, there is 3 phase power to the site, and I’m guessing at this point we might have 2-3 10KW arrays.
So any ideas both on the hard ware as well as the software would be appreciated.
Tony
Here is my dilemma...I have been commissioned to do a system design for a grid tied, remote island location, in Northern MN. As I said, they have the luxury of having grid power even though they are a mile or so off shore. The design criterion is to off set summer seasonal use with winter production. For 8-9 months there will be exactly ZERO loads on the system, summer time the loads are pretty substantial. I will do the calcs to see how big a system would be needed to roughly offset the power needs.
My question however, is that this system is going to be unattended and likely inaccessible in all off the winter season so that no one will be able to routinely shovel or sweep the PV. Given the design criterion, winter harvest is critical for the economic performance of the system. So the question is how best to attack the problem? My first inclination would be to simply put roof rated dielectric heat tape in some pattern around the PV. I know from my own experience, even on a very cold (-30F) day, a bit of black panel exposure, and the panel pretty quickly clears itself.
A number of problems come to mind with this strategy. The first is making sure that sliding snow does not dislodge the heat line. The second is having the system shut off when it is not needed. In a normal heat tape situation, a simple T-stat suffices, but in this case, once the panels are clear, even it if it really cold, the heat line doesn’t need to be on. It would seem to me that a link to the inverter such that based on time of day, if the inverter “should” be producing but isn’t, it could trigger the tape to come on, once the inverter produces X watts (by design) then it would shut off until the next cycle, reducing the parasitic energy cost to production.
A couple of things to note, there is 3 phase power to the site, and I’m guessing at this point we might have 2-3 10KW arrays.
So any ideas both on the hard ware as well as the software would be appreciated.
Tony
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This is for an environmental non profit, and much of the value of a PV installation (to them) is the PR value, so off site doesn’t not suit thier purpose. Pole mount is an interesting idea, and perhaps I will explore the option. That said, there is no one available through out the winter to sweep them. I would do my “normal” wall mounts on hinges, but the buildings all have trees or over hanging porches.
keep the ideas coming folks,
Tony
https://en.m.wikipedia.org/wiki/Rube_Goldberg_machine
Aim a Raspberry Pi zero with a camera module and sleepy-pi board at the array. IIRC, the sleepy-pi will take ~6-30vdc to power the 5v Zero&camera. Could be a little vertically mounted 12v panel & small powersport battery.
Have the sleepy-pi wake up the Zero at preset time(s) daily, take a pic of the array, and calculate the whiteness of the image. If it's too white, trigger a relay to power the heat tape (and optional paint shaker).
Or you could use racking that can be set vertical for winter.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
https://www.weather.gov/dlh/duluthwinterclimate
And use PVWatts to figure out sun (I just pulled "Fox, Mn" out of a hat), fixed array, 50 degrees tilt from horizontal, 77% system efficiency (Grid Tied), numbers below based on 1 kWatt array (dollar value below based on $0.113 per kWH):
https://pvwatts.nrel.gov/pvwatts.php
And figure out the lost harvest assuming array covered by snow for November through March (may be a worst case/false assumption):
- 1,181 kWH total - (68+56+70+84+116 snow months) = 786 kWH non-snow harvest
- 786/1,181 = ~67% harvest
- 1/0.67 = 1.49x larger array assuming "snow cover months" (probably "overkill" for snow on panel guess?)
PVWatts also includes a spreadsheet download option... If the utility was both seasonal and time of use rate plan... You could download the hourly report and get a better handle on solar $$$ harvest on the hourly basis.And there is looking at the Utility Rate Plan... Looks like there are something like 124 municipal electric companies in Minnesota... So I cannot even begin to guess there. Many utilities offer Summer vs Winter rate plans. In California, summer rates are significantly higher (irrigation, A/C)--Don't know about Minnesota--But it may be interesting dollar wise, to see if winter pricing will depreciate winter production more.
Tony, you have much more snow on panel experience than I (me, none). Any idea how long an array remains snow covered in the far north?
-Bill
- 1/0.75 vertical array derate = 1.33x larger array assuming vertical all year round
Interesting... A vertical mounted array need a 1.33x larger system (fixed mount) vs a 1.49x larger array at 50 degree (optimum year round) mount. While I was going to write off the vertical mount as killing harvest and "not worth" the winter harvest--It turns out that having a year round snow free array vertical mounted is better.I would guess that a mechanically configured 20-30 kWatt array moved 2x per year is not a huge interest to them. Could always look into some sort of 1 axis tilt motor setup (cost/maintenance wise).
And check the electric rates... If it is still ~$0.113 per kWH, then a 30 kWatt array in "optimum" conditions (no snow on panels) is something like (30 kWH array * $133 per kWH = ) $3,990 per year utility offset. Economically, I would guess that the installation and maintenance costs have to be kept on the low side.
-Bill
My panels (at ~65°) seem to get covered in late Nov most years, and would stay that way until spring if not shovelled. Snow on the ground can persist well into April/May, but I think the panels would clear in March.
Besides snow, the other problem is the low sun angle. Trees, hills, etc to the south cast really long shadows around Xmas.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Motors for tracking need to be attended. They will fail if left to their own. The concept of tracking the snow off will eventually fail if the snow turns to ice. I have seen a 100 degree elevation tracker freeze the snow and kill the motor. Heaters seem to be a PITA if the grid is there.
Good Luck Tony.
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I assume they have/will check with the utility on feed-in terms & pricing on that size installation.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
First, the whole point of this exercise is to offset major summer usage with September-May harvest. The PV watts numbers are interesting (thanks Bill) especially the wall angle mount. If it were me, that is exactly what I would do, but I don’t get (much) of a vote. The problem on the site is that there are few south facing walls that do not have covered porches. They also are significantly shaded by trees that owners are loath to lose, roofs are better. On all the off grid installations I have done or designed that are unattended in the winter have at the very least a mix of wall mount and roof mounts. Some have hinged wall mounts which is a great solution...but not here.
Dave, your suggestion is good, except that convincing the owners to spring for that much PV is going to be well neigh impossible.
I still think that heating the array ought not be such a problem, but the mere fact that no one has developed the technology in a consumer version tells me that it is more complicated. There is a company known as “Blizzard solar” that has a technology apparently, but no product! Someone once suggested that introducing reverse current into the PV to induce some heat, simple enough, except for optimizers and blocking diodes. I am would be willing to try it on a beta panel, but not on someone else’s large array.
Once again, thanks for the input so far, and keep the ideas coming forward.
Tony
PS, we see our first snow in mid September, first accumulating snow by early October. By mid October (most years) any roof mount PV would be covered, and would remain so, possibly until mid April, and even into May.
http://www.keisolar.com/automatic-solar-panel-snow-removal/
https://tech.nikkeibp.co.jp/dm/atclen/news_en/15mk/031601972/?ST=msbe
http://www.scirustechnologies.com/product.html
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
But I never heard anything about it again (possibly planned for the Midnte Classic MPPT charge controller family????).
http://midnitesolar.com/
Sometimes boB drops by here and could comment (if he sees this thread).
-Bill
Mcgivor, thanks for the links. I have written to two of them, the Nikkibp is a pv maker which would be a last resort. I’ll report back with any information. Please keep the suggestions coming, as we are getting somewhere.
Tony
It could work if the number of panels to be heated was a smallish proportion of capacity, and ambient not too cold. Off-grid, there would be the rather serious risk of depleting batteries while not successfully clearing to the extent needed to charge.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
http://members.sti.net/offgridsolar/
E-mail [email protected]
That said, I do like the challenge of working in cold climates for remote sites, along with the requisite challenges. These folks are not really dear to my heart, but for a variety of mother reasons I am their go to guy on a number of technical matters.
Nano...this is a grid tied system. If you could provide any simple sketch drawing and or any more detail about how to energize a panel in reverse that would be helpful. Secondly, how would reversing the current affect either optimizers or RSS devices required by 2017 code?
Once again, great ideas folks, keep them coming...
Tony
I did a calculation and if the system was just a non-snow season harvest, the system would have to be 1.49x larger. If it could harvest 12 months of the year (set panels to vertical to shed snow)--Then the system only needs to be 1.33x larger (panels set to optimum year round harvest, no snow, sizing of system = 1.0x).
If the system was sized for 3 month harvest, it would even need to be larger yet... And more costly.
Using the concept of reversing the voltage to the solar panels, then optimizers would be certainly confused, if not damaged (are optimizers one of the new ways to shutdown a solar array--Rapid Shutdown NEC requirement?).
I would guess that the voltage needed to reverse current flow th self heat the panels--Roughly 0.5 to 1.0 volt per "diode drop" (a solar cell simply being a forward biased diode when "heating"). That would mean a 60 cell panel (Vmp~30 volts) would need 30-60 volts to get substantial current flow.
And since diodes are notoriously for "poor" self current regulation (very small voltage between near zero current flow and damaging current flow, also temperature dependent), the "voltage source" would also have current regulation (something like 30-50+ volts output and 1 amp of current (30-60 volts @ 1 amp per parallel string) would be ~30-60 Watts of "heating (P=V*I) per panel. Put 10 panels in series (Vmp-array ~ 300 Volts and 1 amp current for heating) would be 300-600 Watts of self heating per ~2,000 to 3,000 Watts of array--Assuming panels are ~200-300 Watts each).
-Bill
My latest thought for Tony is to have them invest in an energy company in the southwest that uses solar. At least if they are green they could hold their head up and say we are doing this for the planet.
Bill the unlimited energy storage concept of the grid is not working south of here today. I am off after I post to see why the batteries are dead and the grid has been down for 3 days.....
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Avg Snow fall ...
Oct = 1"
Nov = 9" <<< Avg Daily High is 40 degrees = lots of melting
Dec = 12"
Jan = 12"
Feb = 8"
Mar = 10" <<< Avg Daily High is 40 degrees = lots of melting
==============
Total = 52"
9" of snow in Nov, is not very much snow.
10" of snow in Mar, is not very much snow.
And in both months, avg high is well above freezing = melting
If you go vertical, then you have wind load and snow drifts.
I must use the snow blower to clear a path in front of my array - the snow will pile up!
At 50 degrees tilt up from horizontal, the snow will slide off, when warm enough.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
I live in an area that gets 50" of snow avg / year - that is not a lot of snow.
Minneapolis, Min ...
November has avg Highs approx 40F = melting
March has avg Highs approx 40F = melting
Even in Dec, Jan & Feb when the High Temp stays below freezing, I get melting and my panels are clear.
Pretending that we get 0 Watts of production from Nov 1st until March 31st ( as shown in message # 7 ) is not the truth, and helps nobody.
And different people have different experiences... Your panels clear. Another poster (a bit farther north) has experienced late November through March array snow cover...
I was not presenting a "truth"... Just data, an analytical process, and even more questions.
-Bill
Solid ice for days until a human showed up. This was up the hill from me at latitude 37.
Keep the data coming. That investment in a south west power plant is how I would go.
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E-mail [email protected]
They are making a comeback in commercial and at least 4 manufactures that I know of.
Pricey and a history of delam issues but definitely power on the backside. Wind on an island is an enemy of any tall vertical array unattended. Did something like this in Panama and the old guy who bought the system was informed that it would not last more than 10 years if he took care of it. He said, "Dave I only need 5 years"
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Tony
I wonder about heat tape (plus some insulation to 'force heat' toward panel front) sort of ends up with some conflicting heat flow requirements (Want to heat panel in winter, but cool in summer/under direct sun).
As long as you are experimenting, how about a ~20-40%+ methanol+water mix and sprayer bar at top of panel ("real" spray rig would need to be UV resistant, possibly compressed air purge, insect resistant--do bugs like methanol?--Methanol is illegal in California except for winter/mountain snow areas, I think):
https://novosolution.ca/images/Freezing-Points-Methanol.pdf
Maybe look into Aircraft de-ice type 1 (actual de-ice) or Aircraft de-ice type 4 (green slime to let snow/ice slip off as aircraft?
-Bill
PS: https://en.wikipedia.org/wiki/De-icing