Solar on a small TT

[eflyersteve]

I'm looking at a small travel trailer (starcraft satellite 17rb) which is only about 20ft long. Roof is composite one piece without much of a flat area. One thing that I absolutely want on my next RV is solar. I would prefer this be roof mounted if possible with enough battery to dry camp and run lights, water pump, TV, stereo, charge phones and laptop and occasionally run the microwave. The small size of the trailer would seem to bring some challenges because there isn't much area for gathering solar. This may require me to use some carry-out portable panels that I feel are less than ideal (theft mainly).

So my question - is there an ideal minimum roof area on an RV to provide enough room for the required solar panels to accomplish what I want? I'm not restricted to such a small travel trailer if I need to go larger to get enough solar I will, but was curious if someone had already considered this and what conclusions were drawn?

Thanks!

[CA_Traveler]

Almot wrote:

Amps of the string of 3... Don't change? I remember discussing this with the maker of my MPPT controller - back in those ancient times 4 years ago when the US-made electronics still existed. His point was that it depends A LOT what part of the panel is shaded. If this is the end of the panel, i.e. all 3 sub-strings are partially shaded, this creates a bottleneck so bad that MPPT has little to grab on. For the entire panel, and thus for the entire string. Output drops to almost nothing.

When the entire panel is shaded all 3 bypass diodes in that panel conduct and continue to pass the amps of the other 2 panels. So the output is 2/3. ie MPPT receives all of the amps and 2/3 of the voltage.

[Almot]

Well, 50% drop of the panel with 3 diodes is hardly possible. It's either 33.3, or 66.6 or 99.9% drop. But let's not be nitpicking.

Now, if 33.3% of one panel is shaded... Voltage of this panel drops by 33%, voltage of series 3 panels drops by 11.1%. Amps of the shaded panel don't change, yes.

Amps of the string of 3... Don't change? I remember discussing this with the maker of my MPPT controller - back in those ancient times 4 years ago when the US-made electronics still existed. His point was that it depends A LOT what part of the panel is shaded. If this is the end of the panel, i.e. all 3 sub-strings are partially shaded, this creates a bottleneck so bad that MPPT has little to grab on. For the entire panel, and thus for the entire string. Output drops to almost nothing.

Back to the perfect scenario with exactly one sub-string shaded, i.e. 33.3% V drop on this panel, 11.1% V drop of 3 panels. Is there such thing as 33.3% R increase of one shaded panel in the string of 3?

PS: Upon running some numbers - 2*24 panels with 500W total could still be wired in parallel using off the shelf MC4 extension cable #8 (yes, there is MC4 #8). On a small trailer/camper the V drop will be under 1.5% on 20ft run. No junction box.

[CA_Traveler]

Almot,

Good link. In the section on Bypass Diodes and figure 5 it's clear that the bird dropping would bypass 20 of the 60 cells. This leaves the full amps at 2/3 of the voltage for that one panel. Or full amps at 8/9 of the voltage for the 3 panels.

[CA_Traveler]

pianotuna wrote:
Hi,

3 panels in series. A bird dumps on one panel and output drops by 50%. Not just for the one panel, but for all three.

Best to find panels that are high voltage and wire in parallel.

Incorrect. The bird dropping activates the bypass diode in one panel section ie you lose 1/3 of the power from that single panel. Or you lose 1/9 of the power from all 3 panels with 3 bypass diodes in each panel. To be clear the amps don't change and the voltage is now 8/9 or 89% of the total.

For parallel that panels voltage drops and it no longer contributes so you are at 67% of the power.

[pianotuna]

Hi,

3 panels in series. A bird dumps on one panel and output drops by 50%. Not just for the one panel, but for all three.

Best to find panels that are high voltage and wire in parallel.

[Almot]

Partial shade is not something that I have to deal with, I'm mostly in the open. Still, here is how I see it:

Bypass diodes help each panel to work - it still produces when partially shaded, but the output drops. When output drops on one panel, it drops to the same level on all 3 panels in series string. So, shaded 30% of one panel results in 30% output drop of the entire array, despite that the shade is only 10% of the total array area.

This is (one of) drawbacks of series wiring of 24V panels. I still prefer 24V in series, eliminates the need to install beefy cables and roof junction box. Again, - this works for me, at sea level and in the open.

Here is series/parallel/bypass etc explanation for dummies, with pictures: Link

(Seems like Don Piano was typing at the same time with me, arriving to the same conclusions)

[CA_Traveler]

Click here to see how bypass diodes work.

[Almot]

Yes, A/C is a bad one, 12"-14" tall usually. Move the panel 10"-12" away if you can. Or, indeed, raise it by 4-5". Or - both. At Lat 35 of Oklahoma city shouldn't be too bad. Leafy trees will harm it more. Heavy rain all day, this is the killer, need more battery capacity for those occasions (and less MW).

[CA_Traveler]

I made a reasonable scaled drawing of my roof including elevation of any object like an A/C etc that could cast a shadow onto a panel. I also made scaled drawings of the various sized (physical size) panels and looked at how they would fit on the roof.

I wanted 500W+ and in the end went with 3x 250W raised panels which avoided rig shadows. This setup with a MPPT controller and 24V panels was only $100 more than a similar wattage setup with 12V panels and PWM controller. 24V panels are less $/watt and more W/sq ft and virtually paid for the higher and better MPPT controller.

In addition my panels have 3 bypass diode each. Wired in series I also have better shade output than parallel panels for external shadows like leafy trees. I get charging when there would be none with the same shade on parallel panels.

[Almot]

eflyersteve wrote:
I'll give this {raised panel} some consideration if I can't find enough shade free area. Since we are still in the decision making process on the TT, I am somewhat intrigued by the models with the wall mounted air option. Sure it takes up otherwise needed cabinet space, but it also reduces the roof clutter. But I hate to purchase a trailer based in that trait alone.

I think CA raised his panel not due to some "shading" (be it shading of one panel by another, or shading by trees etc). He did it due to lack of usable roof space. You might have to do it too.

I had to do the same, only over the plumbing vent, not that big hatch on the photo. Raised by ~4" to clear the vent.

Raised - not necessarily "tilted". Raised panel still remains flat (slightly sloped to the side, to shed the water). You may consider tilt, but this won't be fun to climb up and down and flip it up and down unless you stay put in one place for at least 4-5 fays. 20-30% tilt (you won't need more in summer) will increase your output very little in summer (I already hear BFL objecting to the general term of "summer"), - IMO not worth it at your latitude. There are few tilted designs by members, if you still want to, Westend's design is elegant.

MW makes it more difficult. You might consider other ways to reheat the leftovers, this will make the system simpler and cheaper. 600-700W solar would be the minimum to feel confident about MW without having to carry a generator, in your area. By confident I mean "extending the time between generator/charger sessions" 😉 to, well... up to when you have to go home. Unless it's not the land of liquid sunshine North of latitude 49.

[Tom_M1]

The major drawback of mounting panels on the roof means that you must park in the sun. Not very pleasant on a hot day. Therefore I went the semi-portable route. I have three panels, one on the roof and two mounted on the side of my coach. If I'm parked with the driver side in the sun, the side mounted panels can be left in place and tilted up. Otherwise the panels can be removed and set in the sun. I have two 50 ft. extension cables so that I can set the panels up to 100 ft. away.

[CA_Traveler]

For portable panels consider how and where they will be stored. If they are going to be used along with roof top panels on a single controller then they need to be compatible ie same voltage for parallel.

[eflyersteve]

westend wrote:
I have a 235W panel on a 22' trailer. There is room for a second panel because I don't have roof-top AC. The raised option obviates a lot of concerns about shading.
Yes, microwave usage means more of everything in the general sense but it can be done without too much pain. Right now, I choose to use other options besides microwave or high-draw coffee pot. That may change as my system evolves.

Thanks for the info. Having enough room for 400+ watts on the roof (plus another 200 carry-out if needed) is something I have set as a goal. Much appreciate the real-world experience!

[eflyersteve]

CA Traveler wrote:
Besides panel mounting close to the roof there are raised panel mounting options which can solve your problem. I talked to every owner with raised panels that I could find. Here's one of my panels.

A great suggestion and much appreciated. I'll give this some consideration if I can't find enough shade free area. Since we are still in the decision making process on the TT, I am somewhat intrigued by the models with the wall mounted air option. Sure it takes up otherwise needed cabinet space, but it also reduces the roof clutter. But I hate to purchase a trailer based in that trait alone.