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Efficiency of flat fixed solar vs adjustable portable?

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#1 Boonie

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Posted 19 January 2017 - 04:42 AM

OK, I know that a solar panel inclined to a determinable angle is more efficient than a panel laying flat.

I know that a panel that tracks the path of the sun during the day is more efficient than a panel laying flat, therefore, a panel that is inclined and tracks the sun is probably the most efficient that that panel can get and certainly more efficient than a panel laying flat. I also know that in the middle of summer parking your camper in the wide open sun so your solar panels can recharge also results in a very hot camper interior, therefore, being able to place solar panels in the sun while the camper is in the shade is certainly more comfortable. If this sounds like I am convinced that I want portable/suitcase solar panels for my rig, rather than flat panels on the roof, you are correct. I have seen many posts that prove I am not the only one who grasps this concept. I also grasp the concept that is easier to have and you are able to have larger systems fixed to the roof, but my needs are limited.

 

So far the endorsements for portable/suitcase systems have been "it works for me" or "it meets my needs". I cannot find nor am I equipped to figure out how many amp/hrs (if that is the right term) you can expect from a portable 100w panel that is inclined and tracked vs a flat 100w panel. Or in other words a 100w portable/suitcase used as described can produce the same as a (fill in the blank) fixed panel.

 

I hope DrJ, Vic, or others who are educated in solar calculations and efficiency can shed light on this. (pun intended)


Edited by Boonie, 19 January 2017 - 04:43 AM.

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#2 MarkBC

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Posted 19 January 2017 - 05:43 AM

It's not possible to give a single answer to the difference between a perfectly-sun-tracking panel and one that's horizontal because it depends on the latitude and the season.  For example, if you're camping in northern Canada in winter it's going to make a huge difference but if you're in Ecuador at the equinox it won't make nearly as much difference.

 

Did you really plan to move the portable panel all during the day (both altitude and azimuth) -- e.g., have it pointed east, standing vertical, at sunrise, at a lower angle, pointed south at noon, and pointed west, standing vertical, at sunset?  Or did you just plan to have the panel positioned by the standard recommendation for residential installations:  pointed south at an angle with the ground equal to the latitude?  (that is, if you were in Los Angles at latitude 34°N, the panel would be elevated 34° off the ground, pointing south)  In winter you'd tilt it up at a higher angle than latitude, in summer a lower angle than latitude.

 

If you want to calculate the difference in power output -- at any position of the sun -- between a horizontal panel and a panel that's pointed directly at the sun, take the sine of the angle that the sunlight makes with the panel, and that number is more-or-less the % of rated power.  Like, if the sun is pointing directly perpendicular at the panel, at 90°, the sine of 90° is 1 (one), so it puts out 100% of rated panel power (disregarding temperature effects...).  If the sun is striking the panel at the relatively low angle of 30°, the sine of 30° is 0.5, so 50% of rated panel power.  

How do you get the "sine"?  Well, you can use a calculator/app but I just say to my smartphone, for example: "OK, Google, what is the sine of 40°?"  And Ms. Google says, "approximately 0.643".

 

I know I haven't given a great satisfying answer -- I'm sure there are websites that give an average "improvement" over horizontal as a function of latitude...but this was just the knowledge I have in my head without doing a web search. ;)

I have two big panels flat on the roof of my camper.  I actually made their mounts such that I can tilt them, if I wanted to...but I've never actually tilted them (because it's inconvenient). Solar panels are cheap enough that I decided to just get plenty of panel and use the non-optimum horizontal orientation. 

 

But I agree that having movable panels would be handy if I camped in a forest, so I can make sure the panels are in the sun even if the camper isn't.


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#3 PaulT

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Posted 19 January 2017 - 06:18 AM

On our trip to DVNP in Feb/Mar 2015, I had the factory 100 watt flat panel and would get less than 1 amp until late morning. Output would rise slowly. I also had a portable Renogy 100 watt flex panel with me. Starting as soon as direct sun was on the camp site, I set up the portable panel pointed directly at the sun & inclined to get the sun full on the panel. I got 4 + amps additional current from the portable as long as I kept the panel perpendicular to the sun's rays. After the sun was high, I could put the portable flat on the roof and continue to charge as we went hiking.

The disadvantage is that it takes hanging around the camper to keep the panel pointed at the sun to maximize charging. A lot depends on your activities. Another possibility is to mount your panels on tilt mounts and orient the camper such that the panels are tilted towards the south by an angle equal (more or less) to your latitude. In other words, match what a home solar roof array would be like at your campsite. This is a compromise that takes less fiddling around.

As long as you are getting enough solar charging to keep you with safe food and cold beer using solar, truck alternator and shore power for the duration of the trip, you're good. If we get low battery charge with inclement weather, we find a campground for a night or so with electrical hookups & fully charge the batteries.

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#4 Boonie

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Posted 19 January 2017 - 08:28 AM

Ah, variables I did not consider. Well, let's see, since this is Wander the West, half way between Mexico and Canada, then half way between the Mississippi river and the Pacific.... Denver, Colorado. And a date....4th of July. And will I constantly change the angle.... no, just every hour. (I hope that makes it easier to read a sun angle table)


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#5 Advmoto18

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Posted 19 January 2017 - 02:20 PM

YMMV because there are so many variables involved.  Suffice it to say, you will never see the panel's advertised claim.  Your charging will always be degraded to some degree and that is simply part of solar technology as it exists today.

 

But, as you mentioned, a panel that can be oriented to the sun's axis, will always provide a better charge rate then the same panel that is fix mounted to the roof of your camper, or wherever.

 

Check out this article.


Edited by Advmoto18, 19 January 2017 - 02:20 PM.

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#6 ntsqd

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Posted 19 January 2017 - 02:31 PM

My approach, if we needed more solar than we have for the deserts, would be to completely cover the roof in rigid panels with an air gap to the camper roof. This would do two things, it would obviously increase our max watts from solar, but it would also result in a "Safari Roof" to help keep the camper cool. There are a lot of things I'd rather be doing than fussing with portable solar panels. Others certainly feel differently.


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#7 JaSAn

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Posted 19 January 2017 - 03:18 PM

Arizona State University has all the information you need to calculate maximum output for solar panels for any latitude in North America for any time of year.  There are too many variables for a generalized answer of any value.

 

I am away from my camper for large portions of the day and would not feel comfortable leaving panels out.

 

jim


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#8 takesiteasy

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Posted 19 January 2017 - 03:49 PM

...

I am away from my camper for large portions of the day and would not feel comfortable leaving panels out.

 

jim

 

This is my feeling also. I thought about portable panel idea for a long time but eventually I went with an oversized panel on the roof as a compromise solution.


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#9 Stalking Light

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Posted 19 January 2017 - 03:53 PM

I'm on the move a lot during the day so I went with 3 100w panels on the roof. I do have a 60w foldable that I put out sometimes when I need more solar and am in one place for a while, but portables would be too much trouble for me. Plus, I don't have to worry about where to store the roof panels.  ;)


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#10 adamxcl

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Posted 19 January 2017 - 05:42 PM

Portables are nice for certain kind of trips but I like having roof panels first and foremost. They are always charging something whether driving, getting fuel or parked at a store. The portables will be more work setting out, moving around, storing them, leaving them out at a risk as others said.

 

If you really want to stay in one place for longer times or want to climb often, you can also install flat panels on the roof but mount them on a tilt bar option. I installed my own set up. I can climb up the side without getting on top, unscrew a knob and then raise and tilt them however I want. But I rarely have ever done that as I have plenty of power here in the southwest. I have about 350w on the roof and am usually at full power again by 9:30 am. Too much power most of the time but then I needed it a couple times in a rainy Oregon forest when I wasn't getting as much sun.


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