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Old 08-09-2018, 12:50 PM   #1
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Designing a solar system

No, I am not God and I won't be making a sun and planets...just designing a small solar system to power an old Airstream.
I'm a new owner of an old Airstream...a 1965 26' Overlander.
It was gutted by the previous owner so I have a blank slate to start from scratch. No issues with pre-wiring because there is none.
In anticipation of this project I have four purchased four "peel and stick" flexible solar panels that are rated at 135Watts@ 24V output. I made that choice because it was the maximum number of panels that would fit on the roof of my trailer.
I am also intending to use LiFePO4 batteries in spite of their initial high cost because they seem to be very efficient at recharging and deep discharge, and with their high cycle capacity should last for many years.
So my question how to appropriately size the components of the system?
I may be hooking up to shore power on occasion, and I do have a 3200W generator that I would be carrying just in case I needed it.
I will be powering 12V DC equipment and 110V AC appliances.
I am thinking of setting my 4 panels up in a Series/Parallel to send 48VDC to the solar controller (to ease the stress on the inverter and reduce the wire gauge necessary for a safe installation) and set up 4 12V batteries in series to provide 48v to the 110/AC inverter but I also need to have 12v DC available for lights and brakes as well as equipment designed for 12v on a trailer, for example pumps, fans and air conditioner
Any thoughts?
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Old 08-10-2018, 11:46 AM   #2
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Let's first discuss your first decision. Why did you purchase flexible panels and who's the manufacturer? Reports that I've read show that the reliability of flexible panels is terrible - making replacement of the panels a difficult and messy undertaking. One reason I've heard for their early demise is that the outer skin of the airstream doesn't conduct the heat away from the solar panels on hot days and we know heat shortens the longevity of solar cells.

I'm a big fan of reusing the existing 10 AWG wiring but in your case you have a clean slate, so I would run heavier gauge wire and plan on wiring all panels in parallel.

In a new installation like yours, I would probably go with all Victron components, including the LiFePO4 cells and take a systems approach. If all the components are from the same manufacturer then you can be assured there will be fewer to no incompatibilities.
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Old 08-10-2018, 12:01 PM   #3
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I'm not a big fan of flexible panels, but let's assume you go ahead with them.

Why do you want to use 48 volts DC? I don't see any advantage, and it would add unnecessary complexity to the system. You aren't going to have any super long wiring runs that would make fat DC solar wiring impractical from your array to the controller. Why not just wire your panels in parallel and use an MPPT controller capable of serving a 12V battery bank with a 24 volt array? Then you don't have to convert again to 12V for your DC electrical system and you can use standard RV components.
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Old 08-10-2018, 02:06 PM   #4
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Hi

Ok, I agree that flexible panels would not have been my choice, you have them so let's move on ....

135W x 4 = 540W. A 24V panel nominally runs around 32 to 36V. You will have roughly 17A out of them if run in parallel. If they are all in series, you will get a bit over 4A at almost 150V. A Victron MPPT 150/60 solar charge converter would match that up fairly well except for the full series combo. I would not risk the potential for over voltage.

If you go for parallel, number 10 wire will do the trick with very minimal losses. You could get away with number 12 and still not be doing anything crazy. I don't see a big need to put them in series (or even series / parallel).

Battery voltage wise, you *could* go with a 48V system. That's going to be a pain if you need 12V for various loads. The normal approach is to stick with a single voltage for everything. Wire is cheap and reliable. DC/DC converters waste energy and break from time to time. ( We go nuts about a 1.2% drop in a wire, but ignore a 10% loss in a DC/DC .... why? .... no idea ...).

The big question is: how much DC load will there be? If you plan on getting 3KW out of an inverter then there will be a pretty substantial DC load. That means (even with lithium) a lot of battery capacity. Simple answer to the wire issue (in that case) is to keep the inverter right by the batteries.

3KW of 120v run from 12V at 90% efficiency is up around 280A. I'd suggest that around 600AH of lithium battery is about as small as you should go for that sort of load. It's also not a bad match for about 600W of panels.

Inverter wise, the Victron MultiPlus 3000 would be about right.

All this is making about a thousand wild guesses. There's a lot more to designing a system than randomly guessing. So:

1) How much inverter (3KW above....) ?

2) How much battery ( 600 AH above...) ?

3) Any other loads ?

One biggie under "other loads" would be the trailer break away power. That needs 12V and running it on a DC /DC ... maybe not so much ....

Bob
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Old 08-10-2018, 08:20 PM   #5
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Sounds like complete overkill to me. Seems like you’re going to be spending a ton of money on a huge solar power system with lithium batteries and an inverter. Do you really need that large of a system. We boondock and do fine with 2 golf cart batteries, 200 watts of solar and a 1,000 watt inverter.

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Old 08-10-2018, 10:35 PM   #6
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Hi

Ok, I agree that flexible panels would not have been my choice, you have them so let's move on ....
Huh... he can't return them?
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Old 08-11-2018, 09:03 AM   #7
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Originally Posted by TouringDan View Post
Sounds like complete overkill to me. Seems like you’re going to be spending a ton of money on a huge solar power system with lithium batteries and an inverter. Do you really need that large of a system. We boondock and do fine with 2 golf cart batteries, 200 watts of solar and a 1,000 watt inverter.

Dan
Hi

That's the basic gotcha about guessing at everything in the "requirements" side of the design. Maybe there's a need for 3KW of AC for 12 hours a day. Maybe the biggest thing that ever runs on AC is a television at < 100W for < 1 hour a day. Maybe the solar panels can be returned, maybe they can't.

A better way to do a design:

1) This is what I need to do

2) This is what I would *like* to do if I can afford it

3) I can stretch the budget this far ... maybe further ....

4) This is how much space I'm willing to give up for this project

5) I'm willing (and able) to do some / all / none all of the work myself.

6) It absolutely positively needs to be done by this date ... errr ... maybe longer ..

7) I want the latest and greatest / old but well proven / simple and close to stock stuff in the design.

Yes, this requires input from the OP. Not just a little input, but quite a bit of input. One would *assume* that much of the above was considered before the solar panels were purchased. I would freely admit that when I did this the scope changed a bit as the project moved on .....

Bob
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Old 08-11-2018, 10:09 AM   #8
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Not sure why you would ever wire solar panels parallel these days? Use an MPPT controller and run the supply to the controller as high as you can. You can then run smaller wires if needed and experience less voltage drop.

I have a similar system to yours. With an electric fridge its about the minimum size you'd ever want. I thought about running a 48v battery bank. But for the minimal gains, a 12v bank made more sence.

The great joy of lithium is the charging rate. I can pump 60 Amps into them with my generator in 1 hour. That's enought power to last me 24hrs, even if it's cloudy all day

It is also very common for people to go crazy oveboard with wire sizes. Use a voltage drop calculator and use the size that is required. Running huge wires through your roof leads to a higher risk of chafing and when you get to the end of your build. You might find that the space in your roof is fast running out of room for all those new wires you're running to everyrhing
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Old 08-11-2018, 07:44 PM   #9
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staring from scratch

unfortunately, I think I will be staring from scratch because the flexible panels I bought are too long for the tubular length of my trailer. I don't want to even try to wrap them around the end caps... it wouldn't be pretty.
I am bummed because I was hoping to go with panels that would be low profile and applied directly to the skin.
I am measuring 16 feet between end caps. One 3' section and one 6' section in between the two fantastic fans and Dometic AC unit are clear.
I'm shopping for new panels.
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Old 08-11-2018, 10:23 PM   #10
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Flex panels are a drag and sorry you wasted your and others time. Zamp slim lines work well.
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Old 08-11-2018, 11:02 PM   #11
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Quote:
Originally Posted by Shermy1987 View Post
Not sure why you would ever wire solar panels parallel these days? Use an MPPT controller and run the supply to the controller as high as you can. You can then run smaller wires if needed and experience less voltage drop.
A parallel array tends to perform far better than a series array when one or more cells are shaded, which is a very common occurrence on an RV roof.

In my view, there's very little benefit to being able to use small wires for such a short distance unless you're stuck with a factory solar prewire.

Quote:
Originally Posted by Shermy1987 View Post
Running huge wires through your roof leads to a higher risk of chafing and when you get to the end of your build. You might find that the space in your roof is fast running out of room for all those new wires you're running to everyrhing
There's an inch and a half of space between the skins. That's plenty of room to run 6awg or 4awg wires from the roof along with all your other wiring, with plenty of room left for insulation.
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Old 08-12-2018, 08:33 AM   #12
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Thanks TheGretleys, This never crossed my mind.

It will be interesting for me to cut my array in two, and run 24 volts from each side and monitor my results.
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Old 08-13-2018, 06:58 AM   #13
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Originally Posted by Shermy1987 View Post
Thanks TheGretleys, This never crossed my mind.

It will be interesting for me to cut my array in two, and run 24 volts from each side and monitor my results.
Hi

Running the panels independently (so you can monitor each of them) gets you into two solar charger converters. That's not a good way to go.

With only two 24V panels, just wire them in seres into any of the Victron converters and move on. Anything from number 12 wire up to 1.5" cross section bus bar will work for the wiring. Anything you pick will be "within measurement error" in terms of losses.

If losses really are a big deal, test your panels and pick the two best ones. Panels vary by more than the loss numbers we seem to get crazy about.

Bob
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Old 08-13-2018, 08:17 AM   #14
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Running the panels independently (so you can monitor each of them) gets you into two solar charger converters. That's not a good way to go.

Not suggesting it makes a whole lot of sense in this situation, but if one wanted to split the four panels into two separable arrays you can wire them through a switch such as this. Then select the arrays individually or combined.



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Old 08-13-2018, 10:37 AM   #15
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got a great suggestion by PM... suitcase panels!
My purpose in choosing flexible panels in the first place was to be as unobtrusive as possible. Suitcase or free standing panels would be an obvious choice... I wish I thought of it myself.
No worries about having the 'wrong' panels. I can sell or repurpose them.
Upside is that it's a clean, scalable solution... easy enough and relatively inexpensive to add solar capacity.
and as the PM observed- you don't have to move your Airstream to get oriented towards the sun
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Old 08-14-2018, 08:09 AM   #16
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got a great suggestion by PM... suitcase panels!
My purpose in choosing flexible panels in the first place was to be as unobtrusive as possible. Suitcase or free standing panels would be an obvious choice... I wish I thought of it myself.
No worries about having the 'wrong' panels. I can sell or repurpose them.
Upside is that it's a clean, scalable solution... easy enough and relatively inexpensive to add solar capacity.
and as the PM observed- you don't have to move your Airstream to get oriented towards the sun
Hi

Panels that are bolted to the trailer don't go wandering when you leave them out in a campsite. Panels on the roof charge the system while you are on the road (in daylight) or sitting at a Walmart shopping. They also don't take up space inside the trailer (when in transit) and all over the campground (when set up).

Do you have dogs? Is one of them a male? .... how about the neighbor's dogs ...

Indeed portable panels can be pointed at the sun. The sun moves so this can be a full time job. You also can park the trailer in the shade and put them out in the sunny part of the site. That assumes a fairly specific layout to the site. It also assumes they are "out of the way" enough to not be a trip (or drive over) hazard when put "over there".

To me, setup and takedown are my two least favorite parts of doing all this. If I can get rid of one more thing to fiddle with on arrival / at departure ... I'm going for it.

Solar is *slow* compared to a generator. It works great because you have it on all the time. If it takes 10 or 12 hours to get things done ... that's fine. It does it for days and days without any cost. To be useful as a power source, you will be "tending" portables quite a bit.

Lots of things to think about.

Bob
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Old 08-14-2018, 09:29 AM   #17
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Hi



Panels that are bolted to the trailer don't go wandering when you leave them out in a campsite. Panels on the roof charge the system while you are on the road (in daylight) or sitting at a Walmart shopping. They also don't take up space inside the trailer (when in transit) and all over the campground (when set up).



Do you have dogs? Is one of them a male? .... how about the neighbor's dogs ...



Indeed portable panels can be pointed at the sun. The sun moves so this can be a full time job. You also can park the trailer in the shade and put them out in the sunny part of the site. That assumes a fairly specific layout to the site. It also assumes they are "out of the way" enough to not be a trip (or drive over) hazard when put "over there".



To me, setup and takedown are my two least favorite parts of doing all this. If I can get rid of one more thing to fiddle with on arrival / at departure ... I'm going for it.



Solar is *slow* compared to a generator. It works great because you have it on all the time. If it takes 10 or 12 hours to get things done ... that's fine. It does it for days and days without any cost. To be useful as a power source, you will be "tending" portables quite a bit.



Lots of things to think about.



Bob


You have to decide what is best for you- portable or install on the trailer. The cheapest and easiest way to go solar is to go the portable route. I don’t agree with Bob’s statement that for portable solar to be useful as a power source, you will be “tending” them quite a bit. You won’t be tending them any more than if they were on the trailer. It is just since they are portable you can move them or change the angle if you want to improve the performance. If you don’t like the portable setup you can always install them permanently on your Airstream.

Dan
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Old 08-14-2018, 01:27 PM   #18
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Bob's portable panel con summary:

1. Can be stolen,
2. They don't charge while traveling,
3. Dogs can pee on them,
4. Aiming at the sun to achieve better performance takes work,
5. Setup and takedown takes work,
6. They occupy valuable space inside and out.

All good points!
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Old 08-14-2018, 03:19 PM   #19
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Designing a solar system

I opted to go for the portable route for my first dive into solar. Using two Renogy suitcase panels wired in parallel which gives 200 watts total, and can arrange them on my truck bed cover with cable locks for security.

No doubt I’ll end up with a more robust setup in the future but this lets me experiment. For about half the cost of a Honda 2k generator I can see if it serves up the juice.

Main power needs are lights, fan, fridge (when running propane), water heater, and maybe radio or TV (just a movie).

Living in the PNW it doesn’t usually get hot except for a month or two, and thus lets us try out some local boondocking sites.
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Old 08-15-2018, 07:14 AM   #20
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Hi

There is a lot of divergence in opinion when the "how big do you need" part of solar comes up. There are those who are pretty happy with a 80W panel to charge batteries when in storage. There are others who seem to feel that anything under 600W is not worth the effort.

Except for price, the one thing that gets agreement is that "more is always better". Since there is no running cost, there really is no downside to solar. At least with a roof mount, the space taken up is unused otherwise. It really comes down to the good old "how much is in your wallet?" decision. I doubt any of us would turn down a giant solar system that somebody else paid for

Usage wise, the math is pretty simple. A 5X larger system will charge things 5X better than the smaller system. Once you *have* the bigger charge capability, your power management approach changes. Run the inverter all the time? ... sure no problem. Keep the TV on for the dogs to watch? .... why not. How much this helps in your case , who knows. There's no doubt that it does change things.

If you camp in the shade a lot, generators are a better bet. If you camp in the part of the world where there are few trees, it's a great thing. I doubt anybody always is in any one situation.

Starting with portable is cool. Expanding a portable system gets problematic. The total cost of "getting to the destination" is probably highest if you go through the portable route and wind up with rooftop.

Bob
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