Efficiency of flat fixed solar vs adjustable portable?

[Boonie]

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)

 

[MarkBC]

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.

 

[PaulT]

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.

Paul

 

[Boonie]

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)

 

[Advmoto18]

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.

 

[ntsqd]

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.

 

[JaSAn]

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

 

[takesiteasy]

...
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.

 

[Stalking Light]

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.

 

[adamxcl]

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.

 

[Boonie]

Hummm, I seem to be having trouble getting the question across, so let me fill in a few other details. I am in the process of building my rig. (see my build thread "'81 Grandby on a '91 Jeep YJ-New build") At the start of that thread I stated that all campers are a compromise. One of my compromises is weight. When my camper target weight is 650 pounds adding a second solar panel, 25 pounds, and a second battery, 55 pounds or 80 pounds is huge! So can I make what I have more efficient? How much more efficient can I make it? Based on that information, what is the maximum amperage load that I can safely install in my rig?

What I am trying to do is reverse engineering.

 

[Stalking Light]

My GoPower 100w flex panels weigh under 3# each if weight is an issue, fwiw.

 

[Boonie]

Thanks, Charlie. (That's plan B, but don't tell anyone. I'd like to stick to the question.)

 

[MarkBC]

Because I'm interested in the answer (even though there still isn't a simple answer), I did a search and found the website where you can find something like an answer:

U.S. Solar Radiation Resource Maps: Atlas of the Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors

Using that site, I went ahead and generated some comparison maps of "insolation" -- solar power-per-area.

These two maps compare the average power received in March for a horizontal surface and a full-tracking surface:

As you can see from the maps, these are real averages, not just theoretical calculations based on latitude. They include the effect of days when there's cloud/rain-cover -- which is why the usually-sunny Southwest has the most insolation.

So, for example, if you were camped in Wyoming in March a horizontal panel would receive 4 - 5 kWh/m2/day while a full-tracking panel would receive 7 - 8 kWh/m2/day. Moving your panel dawn to dusk to track the sun would give you, in that "March in Wyoming" scenario, about 67% more energy than a horizontal panel.

More maps, with comparisons for December and June:


In December in Wyoming you'd get (on average) 120% more energy by tracking the sun compared to horizontal, while in June you'd get about 40% more energy .

These maps show why it's kinda hard to answer: "Can I get by with my current system if I move the panel to track the sun all day?" It depends a lot on what season you're camping and where you're camping. A systems that's adequate -- with full tracking -- in June probably would fall short in December, and one that's adequate in Arizona probably wouldn't be adequate in Wisconsin. And if you're not going to add more battery storage then you won't have more back-up for overnight or heavy clouds.

That said, I'm glad I found this insolation resource for my own use if nothing else.

 

[cwdtmmrs]

If you use your 3 way fridge, install led lights and watch your usage of a forced air furnace (or go with a Wave heater) I am not sure you need solar at all unless are staying in one spot for a long time. A 100ah battery with 100w panel should be more than enough even at 50%. At 3# per panel, not much weight added.

cwd

 

[Boonie]

MarkBC,

I think you hit the nail on the head. What you have verified is consistent with the article by Charles R Landua that Advmoto18 linked and a solar calculator that I found. Yes, there are seasonal differences especially in June, but overall a 100w portable/suitcase is able to recharge batteries equally as well as a 160w flat fixed panel.

CWD

I can tell you have been following my other threads. I believe your rig and your thinking are similar to mine, so you have taught me to keep track of who says what. As a local radio host says "You need to know where I sit before you know where I stand"

 

[MarkBC]

...but overall a 100w portable/suitcase is able to recharge batteries equally as well as a 160w flat fixed panel....

I think that's right but I'd add: "...if you're moving the portable to track the sun all day".
(I know that's what you meant, but just to avoid any misunderstanding by someone who reads that one bold line without reading your intention earlier in the thread.)

The biggest difference between horizontal and tracking is in December, when the sun is low in the sky so a low angle to a horizontal panel.

 

[cwdtmmrs]

MarkBC, I just don't think Boonie's per day amp usage is going to going to enough to worry about moving a portable unit more than once a day if at all. If he is in camp, no problem. If he is out of camp for a day or so, nothing will be running anyway.

Boonie, maybe use one of the charts on the forum to figure your total per day amp requirements and work from there. The good thing about the portable solar is that if your trip requires driving every day, you can leave it at home.

cwd

 

[MarkBC]

MarkBC, I just don't think Boonie's per day amp usage is going to going to enough to worry about moving a portable unit more than once a day if at all. If he is in camp, no problem. If he is out of camp for a day or so, nothing will be running anyway.
...

That's probably right, but I wanted to answer his original question anyway for others who may wonder.
And I was glad to find that useful site about insolation, for my own future use.

 

[craig333]

Besides having enough solar even when the conditions aren't optimal, I rarely use my portable anymore after finding it always in the shade. Seemed every time I'd wander by I'd have to move it back into the sun. That got old quick especially if you aren't around camp much.

If he's running a compressor fridge then it is using power even when he's not around.