Two solar questions: wind breaks and parallel/series

[TheArgobus]

My setup:

- 24V 3.6kWh lifepo4 bank
- 60 amp MPPT AIMS Power charge controller
- 3 x 300W 24V Renogy Solar panels mounted sideways on the roof, such that the long edge is facing the bus front and back beginning right adove the windshield, with the following specs:
- optimum operating voltage: 32.2 V
- optimum operating current: 9.32 A
- open circuit voltage: 38.8

1. Are wind breaks/guards really necessary? It seems to me that the weakest point would be the Silicon solar panel Section itself, which should be able to withstand residential rooftop Storm wind. If so, wouldn’t just a solid Piece of sheet metal or thin ply be sufficient along only the leading edge of the forward panel? Or would the sides need some as well? Has anyone ever heard or seen panels fly off while driving?

2. Since both the panels and battery bank are 24V, would it be more efficient to wire all 3 in parallel to keep their volts as close as possible, or to wire the 3 in series to give the charge controller maximum voltage? I’ve read that mppts are super efficient at converting higher voltage to lower bolts with “current boost” but my non professional intuition says to keep them the same volts. The distance between everything is less than 10 feet and we’ll primarily be camping in the forest for up to a week at a time so shade is an issue

[ermracing]

We have 3 panels mounted sideways, but in the middle of the roof, bewteen the hatches. We've hit 70mph and never had a problem.

[kazetsukai]

Originally Posted by TheArgobus

Has anyone ever heard or seen panels fly off while driving?

I think a few people have seen their panels crack when not driving. These folks AFAIK were boondocking in very wide, open areas. Forest may break your wind for you.

The YouTube channel "Life is a Joy" has me on the side of adding wind breaks to mine:

https://www.youtube.com/watch?v=F7nSHJPfJY8

They had a sideways wind crack a panel... Mine are mounted quite high up compared to most builds and are very large panels. Size and mounting solution are factors for me, they probably will be for you. I have a full roof and there's likely a sail effect going on up there at speed, rather just have the peace of mind.

Originally Posted by TheArgobus

we’ll primarily be camping in the forest for up to a week at a time so shade is an issue

Think you answered your own question. Shade will hurt in series. I did 2S5P for this reason- with a 48V bank and my panels, 2S is the minimum series configuration.

[Iceni John]

1. If shade is a potential issue, wire your panels in parallel. That way if one is shaded (or covered in leaves or bird poop) it won't drag down all the panels' power as much as if they were in series. Besides, the only real reason to have panels in series is when they are a large distance from their charge controller(s), such as a remote array a few hundred feet from a house; a bus simply doesn't run into that issue.

2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio will result in more heat generation, more cooling needs, and a potentially shorter life (heat kills electronics).

3. Why do you think you need a wind break or fairing? I have eight grid-tie panels on my bus, arranged so that the air can freely flow under them at highway speeds when they're stowed down against the roof, and I don't hear any noise at all from them when driving that tells me the airflow below and above them is fairly smooth. As has been discussed many times here, securing the panels well to the roof is of paramount importance - driving at 65 MPH is almost the same as a Cat.1 hurricane's wind force. A few flimsy little aluminum brackets and some inadequate screws (or worse) won't result in a long and happy life for you, the panels, or anyone else driving close behind you. I have a total of seventy six stainless 3/8" and 5/16" bolts through the nineteen roof ribs, all with stainless NyLoc nuts - heck, I could almost lift my bus off the ground by its panel mounts! And when my panels are fully raised, even 60 MPH gusts from any direction won't faze them at all.

John

[FrugalFannie]

Originally Posted by Iceni John

1. If shade is a potential issue, wire your panels in parallel. That way if one is shaded (or covered in leaves or bird poop) it won't drag down all the panels' power as much as if they were in series. Besides, the only real reason to have panels in series is when they are a large distance from their charge controller(s), such as a remote array a few hundred feet from a house; a bus simply doesn't run into that issue.

2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio will result in more heat generation, more cooling needs, and a potentially shorter life (heat kills electronics).

3. Why do you think you need a wind break or fairing? I have eight grid-tie panels on my bus, arranged so that the air can freely flow under them at highway speeds when they're stowed down against the roof, and I don't hear any noise at all from them when driving that tells me the airflow below and above them is fairly smooth. As has been discussed many times here, securing the panels well to the roof is of paramount importance - driving at 65 MPH is almost the same as a Cat.1 hurricane's wind force. A few flimsy little aluminum brackets and some inadequate screws (or worse) won't result in a long and happy life for you, the panels, or anyone else driving close behind you. I have a total of seventy six stainless 3/8" and 5/16" bolts through the nineteen roof ribs, all with stainless NyLoc nuts - heck, I could almost lift my bus off the ground by its panel mounts! And when my panels are fully raised, even 60 MPH gusts from any direction won't faze them at all.

John

what do you have for raising your panels?

[TheArgobus]

Thanks for the replies. I’ll just go with putting them in parallel. Only reason I considered putting a wind block / guard in was I saw it mentioned elsewhere on this forum. But I haven’t really seen any pictures with it.

[Iceni John]

Originally Posted by FrugalFannie

what do you have for raising your panels?

Only the very latest high-tech panel-raising equipment for me - a step ladder, muscles, and an aluminum pole with bolts through it at the correct locations so I can temporarily prop the panel up at each of its three angles while I get the panel's support struts extended (the pole's bolts hook onto the bus's rain gutter above the windows). I toyed with the idea of using electric linear actuators, but that would have been too bourgeois and decadent even for me.

John

[dzl_]

Originally Posted by TheArgobus

My setup:]
2. Since both the panels and battery bank are 24V, would it be more efficient to wire all 3 in parallel to keep their volts as close as possible, or to wire the 3 in series to give the charge controller maximum voltage? I’ve read that mppts are super efficient at converting higher voltage to lower bolts with “current boost” but my non professional intuition says to keep them the same volts. The distance between everything is less than 10 feet and we’ll primarily be camping in the forest for up to a week at a time so shade is an issue

Based on my very limited understanding there are three primary reasons that higher voltage is more efficient.

First, Higher voltage = lower amperage for a given power output meaning your wiring will be substantially more efficient for a given wire gauge and length. In your situation this probably won't be a huge factor.

Second, because your PV array needs to be at a higher voltage than your batteries (in the case of Victron, the PV voltage must be battery voltage + 5V to turn on, and +1V to stay on), so a higher voltage means your controller can start producing earlier in the morning and continue later into the afternoon/evening.

Three, I admittedly have a very shaky understanding on this point and might be misinterpreting something, but I believe the MPPT algorithm is more efficient at higher voltages and lower temperatures.

BUT

The tradeoff with wiring in series is that all panels will be bottlenecked by the lowest producing panel. So a little shade on a single panel can substantially reduce the output of the whole array. In a stationary context this isn't that much of a concern so the benefits of series wiring mentioned above are more attractive. But for a vehicle partial and unpredictable shading is a reality, so parallel is an attractive option as well. Pros and cons to both approaches.

[dzl_]

Originally Posted by Iceni John

2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio will result in more heat generation, more cooling needs, and a potentially shorter life (heat kills electronics).

John I think you are correct regarding the stepdown efficiency, but I also think you are only looking at half the equation here (the DC-DC converter side of the controller).

But on the MPPT side of things higher voltage is more efficient in a number of ways (increasing power output in certain conditions, and extending the productive hours somewhat). See my comment above for a slightly better explanation, or see this Victron Whitepaper for a lot of great but technical info on charge controllers. This excerpt is relevant:

Originally Posted by Victron

Conclusion:
When using an MPPT charge controller there are two compelling reasons to increase the PV voltage (by increasing the number of cells in series):
a) Harvest as much power as possible from the solar array, even at high cell
temperature.
b) Decrease cable cross sectional area and therefore decrease cost.

And

Originally Posted by Victron

If connected to a PV array with a substantially higher nominal voltage than the battery voltage, an MPPT controller will therefore provide charge current even at very high cell temperatures or in low irradiance conditions when a PWM controller would not help much.

[dzl_]

Here is a graph that show's what Iceni John mentioned about efficiency being greatest when the difference between PV voltage and Battery voltage is smallest:



Its hard to read, but the basic takeaway here is that the DC-DC converter (which is a part of your MPPT charge controller) is more efficient when voltages are close together. With a 12.8V battery, and a PV array voltage of 17V the converter has a maximum efficiency of about 96%, whereas at 100V the maximum efficiency is about 92% for this particular controller.


But I don't believe this chart considers factors outside of simple conversion efficiency, runtime, wire loss, low irradiance performance, and maybe MPPT performance (not sure on this last point).

[TheArgobus]

Well now this has me wondering: the bus will largely be stationary under the trees, meaning that most of not all the panels will be in the shade. But driving to and from the location will be in the unrelenting California sunshine. Maybe I’d be better off going with a higher voltage since if one is shaded they’ll all be shaded? And start charging earlier in the morning?

[TheArgobus]

So basically 4% more efficient in that specific case. Well now I think I’m going to lean more toward going in series then, it seems that the series benefits outweigh a 4% ish boost.

Except that means my 60 amp mppt is ridiculously oversized.

[TheArgobus]

Also, it appears the Renogy panels I have have built in diodes to resist the voltage loss from a single panel's shading. Series it is! Anyone want a 60 amp MPPT?

[dzl_]

Originally Posted by TheArgobus

Well now this has me wondering: the bus will largely be stationary under the trees, meaning that most of not all the panels will be in the shade. But driving to and from the location will be in the unrelenting California sunshine. Maybe I’d be better off going with a higher voltage since if one is shaded they’ll all be shaded? And start charging earlier in the morning?

Originally Posted by TheArgobus

So basically 4% more efficient in that specific case. Well now I think I’m going to lean more toward going in series then, it seems that the series benefits outweigh a 4% ish boost.

Except that means my 60 amp mppt is ridiculously oversized.

I follow your logic here, I think there probably isn't one 'right way' to go about it, and that there will be tradeoffs either way, and the specifics of your situation would likely determine what configuration is marginally better. But I see the logic of your thinking here.


I wish I had some actual numbers for the flip side of the coin (advantages of higher voltage), but I don't, basically just passing along what I've read elsewhere. I'm sure like with most things solar, the answer is "it depends.."


If you want to learn more about best practices check out this thread, specifically this comment. I attempted to condense all the info I have come across in my research into a set of semi-simple bite-sized best practices for partial shade.

[TheArgobus]

Those are some great posts, I have been searching that forum and it's been extremely beneficial. It seems that, like you say, there's no right way, just different ways for different people and applications. Sometimes I can't help but think, "I can't wait for my next skoolie, which will be full size and I'll really be able to optimize [solar/water/batteries/hvac/kitchen/insert everything else]!"

[FatBoySTL]

ArgoBus: I'm starting to build out my house electric. What kind of MPPT controller is it, how much do you want for it, and what zip code would you be shipping from?

Thanks,
FB

[TheArgobus]

Unfortunately I ran the numbers, and I think I'm just going to keep this unit. All the other units, both AIMS and Renogy, are limited to 100V DC input from the solar panels. Since I have 3, and each one's open circuit voltage is 38.8, the potential input voltage exceeds 100V... AIMS has a 40 amp one that will accept up to 150V DC input, but it's 2/3 the price I paid for the 60 amp unit, so it's not really worth it to sell this one at a discount to then spend that money on a similar unit. Not to mention they're sold out at the moment. Don't do what I did and wait to buy your components until you know what you'll be doing! This is not the first time I spent more money than I should've, definitely won't be the last.

[kidharris]

Originally Posted by Iceni John

1. If shade is a potential issue, wire your panels in parallel. That way if one is shaded (or covered in leaves or bird poop) it won't drag down all the panels' power as much as if they were in series. Besides, the only real reason to have panels in series is when they are a large distance from their charge controller(s), such as a remote array a few hundred feet from a house; a bus simply doesn't run into that issue.

2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio will result in more heat generation, more cooling needs, and a potentially shorter life (heat kills electronics).

3. Why do you think you need a wind break or fairing? I have eight grid-tie panels on my bus, arranged so that the air can freely flow under them at highway speeds when they're stowed down against the roof, and I don't hear any noise at all from them when driving that tells me the airflow below and above them is fairly smooth. As has been discussed many times here, securing the panels well to the roof is of paramount importance - driving at 65 MPH is almost the same as a Cat.1 hurricane's wind force. A few flimsy little aluminum brackets and some inadequate screws (or worse) won't result in a long and happy life for you, the panels, or anyone else driving close behind you. I have a total of seventy six stainless 3/8" and 5/16" bolts through the nineteen roof ribs, all with stainless NyLoc nuts - heck, I could almost lift my bus off the ground by its panel mounts! And when my panels are fully raised, even 60 MPH gusts from any direction won't faze them at all.

John

Just to clarify, to ease my ignorance,

"2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio...." is referring to the voltage step down occurring in the MPPT from the solar panels to the battery bank. Correct?

[dzl_]

Originally Posted by kidharris

Just to clarify, to ease my ignorance,

"2. MPPT charge controllers' efficiency is greatest when their voltage stepdown is as small as possible. Asking them to step down more than about a 2:1 ratio...." is referring to the voltage step down occurring in the MPPT from the solar panels to the battery bank. Correct?

That is what he is referring to yes.


But I would contend that that is partial and misleading advice.

An MPPT controller is two things (well probably more but for the purpose of this point, two things):

(1) A microcontroller/algorithm, that determines the 'maximum power point' to extract as much power as possible from the PV array.
(2) A DC-DC converter that takes the higher voltage input and outputs at power at your battery bank voltage

I believe John is referring to point 2 when he talks minimizing stepdown voltage. The conversion will be more efficient, but (based on my limited understanding) the overall efficiency of the charge controller will likely be less efficient in non-ideal conditions.

But the MPPT algorithm has more room to work its magic when there is a higher differential between battery voltage and pv array voltage. And the charge controller (in the case of Victron) won't even turn on, until PV array voltage = battery voltage +5V.

As far as I've seen, its a standard practice to maximize array voltage in all situations where parallel wiring in not required for other reasons. Now one major reason for that, is wiring cost/losses. This is much more applicable to large stationary arrays than our little systems so the cost/benefit is a little different, but I still think that the conventional wisdom is to maximize voltage.

I don't want to come off as knowing more than I do, what I'm sharing here is what I've learned so far, and my grasp of the technical details is at best limited. John, if you think I'm misinterpreting something, I'd like to hear it, you are a knowledgeable guy, and have a lot of good insight.

[Iceni John]

Originally Posted by dzl_

That is what he is referring to yes.


But I would contend that that is partial and misleading advice.

An MPPT controller is two things (well probably more but for the purpose of this point, two things):

(1) A microcontroller/algorithm, that determines the 'maximum power point' to extract as much power as possible from the PV array.
(2) A DC-DC converter that takes the higher voltage input and outputs at power at your battery bank voltage

I believe John is referring to point 2 when he talks minimizing stepdown voltage. The conversion will be more efficient, but (based on my limited understanding) the overall efficiency of the charge controller will likely be less efficient in non-ideal conditions.

But the MPPT algorithm has more room to work its magic when there is a higher differential between battery voltage and pv array voltage. And the charge controller (in the case of Victron) won't even turn on, until PV array voltage = battery voltage +5V.

As far as I've seen, its a standard practice to maximize array voltage in all situations where parallel wiring in not required for other reasons. Now one major reason for that, is wiring cost/losses. This is much more applicable to large stationary arrays than our little systems so the cost/benefit is a little different, but I still think that the conventional wisdom is to maximize voltage.

I don't want to come off as knowing more than I do, what I'm sharing here is what I've learned so far, and my grasp of the technical details is at best limited. John, if you think I'm misinterpreting something, I'd like to hear it, you are a knowledgeable guy, and have a lot of good insight.

I was just reiterating Morningstar's info for my two TS-MPPT-60 charge controllers! Yes, obviously the panels' voltage needs to be comfortably greater than the batteries' to allow the MPPT magic to happen, but not by a huge amount. Several folk on the Northern Arizona Wind and Sun forum, whose advice I greatly respect and trust, suggest a 2:1 ratio between input and output voltages as a good rule of thumb for efficient and effective MPPT charge controller function. If they support that notion, then it's good enough for me!

Too many folk get hung up with the perceived cost of wiring their bus PV systems, especially if using a 12V house system. Compared to a remote array for a house, even the largest bus array will need only a small fraction of its wiring, so in the big scheme of things who cares if one spends a few more dollars for heavier cables? One will be (or should be) using honking-great 4/0 cables for the starter motor and the start batteries anyway, so another few more feet of 4/0 for the house system is no big deal. And compared to the overall cost of a typical bus conversion, it's just a drop in the ocean, hardly even quantifiable. Just spend what it takes to do it right!

John