Mounting Solar Panels on RV with Curved Roof

[Generis]

It's had lots of trips so far, and zero bad happened. In fact, nothing at all happened. It's so very stiff and solid this way. I'll be doing this again on the next curved roof.

 

[azozrider]

When they are secured well, its not a concern. Mine are elevated around a foot, allowing a lot of air beneath them.

 

[RoadTurtle]

Jelly!!! I'm a whopping 1" short of legal height, and stuck

 

[K8MEJ]

Low-profile aluminum unistrut, aluminum tilt brackets, and Z-brackets are one tried-and-true way to do it. But more and more we've been installing Merlin Solar panels directly the roof. They don't suffer the hot spot and burning problems that most other (non-CIGS) flexible panels suffer from. They aren't cheap, but if you factor in the much less installation time (labor cost) it works out in the end, AND you don't need to worry about a glass panel coming loose and flying into the windshield of the car behind you. I've never seen that happen if properly installed, but still.....

 

[K8MEJ]

Here is a picture of what they look like installed. I'm still working to convince this customer to eliminate all roof-top vents and A/C so we can fit more solar on the roof. This is one of our commercial customers using 40' long Freightliner chassis. Hopefully the next build they will let me fit 3,000 watts of Merlin panels on the roof

 

[azozrider]

The flexible panels though, especially in a power per square inch factoring (critical on rv roof), are radically inferior. A 72x40 SunPower Maxeon 5 is 425w. And at an efficiency of 22.8%. Takes a lot of flexible panels, eating a lot of real estate, to do that. That roof in pic...10 Maxeon panels would give the client over 4kw of power and he doesn't need to lose A/C. And if you have all 40' of roof on those 2 sides, you could a install .dd 2 more panels, giving them 5.1kw... and retaining A/C... all with the unistrut mounting. Pretty easy install. The Maxeon's aren't cheap, but damn nice panels. Shit would last a LONG time, and give them whopping power. I'm aiming for 8.2kw on my 44' 5th wheel by stacking with slides. I have lots of obstacles in addition to my 3 A/C's.

 

[K8MEJ]

You've made incorrect assumptions, particularly the size of the roof. The entire vehicle is 40' long, not the roof. And these are medical clinics. They don't want them to look like they have an erector set on the roof. There is a huge difference in what corporate customers accept vs recreational customers in that regard. We have DC split A/C units going into unit #1 and three-headed LG 240v split units in the following units. We'll get everything off the roof to get to 3kW. There's no way you'd get 4kW of anything on this roof without it looking like Optimus Prime. We have a 12kW generator on-board and a 300A alternator so solar is an off-set to their use, not total solar & battery at this stage, unfortunately. And these units operate in mostly rural and out-of-the-way places so ready access to shore power is very rare.

Maxeon and SunPower make good panels, but they suffer from critical hot spots in certain scenarios and I can't have them melting or catching fire. I can't openly share that data so you'll either have to trust me or not. Using the Sunpower panels work well with proper airflow, and I have used them to make solar awnings over windows for RVs in the past and I'd use them again. I could use them on my customers' roofs if I had some insulated airflow between them and the roof but then that eliminates the benefits of using the flex panels from a labor standpoint.

 

[azozrider]

With that kind of specialised application, it makes sense. What about glass panels mounted to roof and a short flashing down sides for aesthetics? Flexible panels are just so non energy dense.

I get the looks thing… my trailer stands out with its giant superstructure on the roof, but I dont have to care. Amazing how wrapped in “image” society can be. Guess thats why I prefer developing world and keep time in USA to a minimum. Haha

 

[SeanC]

I'm curious if folks with elevated panels on brackets have noticed a decline in fuel economy while driving?

In my previous setup, the solar panels were installed on a roof rack on my pickup truck (tow vehicle), so the panels were directly overhead. The wind resistance was quite loud and noticeable, especially over 45MPH. I never did a fair before-and-after comparison, but if I had to estimate, my highway MPG dropped by 5-10% once the solar panel wind sail was added up top.

Over the lifetime of that setup, I'm pretty sure the energy wasted in extra fuel consumption was much greater than the solar energy harvested from the panels themselves. Just something to consider when choosing to elevate your panels off the roof. My new rig uses CIGS panels (because I didn't want additional height), but it also has the benefit of less wind resistance. They may be expensive, but so is fuel. Not sure what the break-even point is, but if you spend a lot of time up at highway speeds, wind resistance becomes a huge factor in MPG.

 

[RoadTurtle]

I'm curious if folks with elevated panels on brackets have noticed a decline in fuel economy while driving?

In my previous setup, the solar panels were installed on a roof rack on my pickup truck (tow vehicle), so the panels were directly overhead. The wind resistance was quite loud and noticeable, especially over 45MPH. I never did a fair before-and-after comparison, but if I had to estimate, my highway MPG dropped by 5-10% once the solar panel wind sail was added up top.

Over the lifetime of that setup, I'm pretty sure the energy wasted in extra fuel consumption was much greater than the solar energy harvested from the panels themselves. Just something to consider when choosing to elevate your panels off the roof. My new rig uses CIGS panels (because I didn't want additional height), but it also has the benefit of less wind resistance. They may be expensive, but so is fuel. Not sure what the break-even point is, but if you spend a lot of time up at highway speeds, wind resistance becomes a huge factor in MPG.

I wouldn't think it would be measurable. We're towing heavy and large bricks with very little consideration for aerodynamics. It would be fun to see one in wind chamber, but I'd bet airflow has the air stream going above the panels, even if they are lifted.

 

[Jennifer]

Whoa... thats huge. Thats what the girls say....

…. when looking at someone else apparently (oh snap).

Those with, don’t need to brag, those without, compensate with tacky low intelligence, pre-teen schoolyard caliber inappropriate humor. ?‍

 

[azozrider]

I dont spend as much time on highway lately, but when I did, towing with a 6.7 powerstroke, I saw no real measurable difference in mpg. Towing at 85mph across Dakotas with wind would have me slow 5-10mph once I had the panels elevated.

For me, its not totally about $ saved but also independence and ability to park/camp anywhere I like and have full power. I have land in MT where I sometimes hang out, and my solar setup cost what a grid tie would have. And no monthly power bill… anywhere, even in RV parks. So for me I was money ahead the first month I was operational. And when shit hits the fan, I’m not dependent on the grid or a location.

 

[SeanC]

I wouldn't think it would be measurable. We're towing heavy and large bricks with very little consideration for aerodynamics. It would be fun to see one in wind chamber, but I'd bet airflow has the air stream going above the panels, even if they are lifted.

Air resistance is quadratic while rolling friction (from mass/gravity) is linear. In other words, the faster you go, the more air resistance becomes a factor. It's certainly measurable, otherwise they wouldn't be moving to digital mirrors on vehicles. Semi tractors often have wind deflectors on top; they increase fuel economy by upwards of 10% when towing one of those large bricks. But to your point, it may also depend on how high the panels are elevated. In my case, there was a ~5" gap which is quite significant.

I dont spend as much time on highway lately, but when I did, towing with a 6.7 powerstroke, I saw no real measurable difference in mpg. Towing at 85mph across Dakotas with wind would have me slow 5-10mph once I had the panels elevated.

For me, its not totally about $ saved but also independence and ability to park/camp anywhere I like and have full power. I have land in MT where I sometimes hang out, and my solar setup cost what a grid tie would have. And no monthly power bill… anywhere, even in RV parks. So for me I was money ahead the first month I was operational. And when shit hits the fan, I’m not dependent on the grid or a location.

Elevated panels changed my driving behavior quite a bit. Any form of rapid acceleration felt like I would rip the panels right off the truck. That feeling of "the panels are going to fly off" translates to an aerodynamic problem. I don't think people with trailers will actively notice it because the panels (thus, wind noise) are so far away; I only noticed it on my rig because the panels were directly overhead while driving.

I'm not saying solar panels aren't worth it for the independence; I'm just saying it's worth considering flush solar panels despite the higher upfront cost. The long-term cost savings may make it worth it if aerodynamics are a consideration. Granted, your 6.7l diesel probably doesn't care what it's towing, but my 6.2l gas engine wasn't as thrilled.

At the end of the day, I think it all comes down to how much you move around. If you're largely stationary, then aerodynamics are of very little concern. But if you're constantly rolling around on the highway, even a 3% difference in fuel economy adds up.

 

[RoadTurtle]

you really hit the nail on the head that most people don't understand, wind resistance is NOT linear to speed. to use your example, even 3% is not measurable in real world driving. Wind, elevation, barametric conditions, speed, and even fuel can disrupt the measurements. I'm a recently a retired Licensed professional engineer that worked for an OEM in powertrain development. I also used to race airplanes, and it has always cracked me up, if we do this improvement, we can save x percentage in fuel. Ok, show me the data on the complete vehicle. Well, we calculated the air resistance improvement on the component. Ok, put it on the vehicle, and run it through the wind tunnel. Most of the time it would come back at a much lower value, sometimes not even worth it. Back to your 3% improvement, taking it at face value, I plan for 10 mpg while towing. I've seen ranges from 6 mpg to 13 mpg. 3% is not going to be measurable. At 10 mpg, 3% is 0.3MPG, given all the other factors that influence mpg, how would I measure it?

 

[SeanC]

you really hit the nail on the head that most people don't understand, wind resistance is NOT linear to speed. to use your example, even 3% is not measurable in real world driving. Wind, elevation, barametric conditions, speed, and even fuel can disrupt the measurements. I'm a recently a retired Licensed professional engineer that worked for an OEM in powertrain development. I also used to race airplanes, and it has always cracked me up, if we do this improvement, we can save x percentage in fuel. Ok, show me the data on the complete vehicle. Well, we calculated the air resistance improvement on the component. Ok, put it on the vehicle, and run it through the wind tunnel. Most of the time it would come back at a much lower value, sometimes not even worth it. Back to your 3% improvement, taking it at face value, I plan for 10 mpg while towing. I've seen ranges from 6 mpg to 13 mpg. 3% is not going to be measurable. At 10 mpg, 3% is 0.3MPG, given all the other factors that influence mpg, how would I measure it?

Well now I really wish I gave it a fair before-and-after comparison on my truck. Before the panels, I would average ~13MPG highway (not towing). After the panels, I average ~11MPG highway (not towing). Hence my estimation of a 5-10% decrease in mileage. I talk about the truck like it's in the past, but I still own it. I still drive it. It drives like a sailboat now. In normal driving, the drag is noticeable. In windy weather, it's very noticeable.

3% is just for mirrors, on a Honda Civic. It's an example of an unexpected detail making a difference in fuel economy. Adding up many small differences can amount to a more significant difference. If it's 3% mirrors, it's likely quite a bit more for large, elevated rectangles.

But hey maybe you're right and I'm wrong, and that's totally fine. It just seems odd to be so dismissive of what I'm saying. I'm just sharing my own personal experience after installing elevated panels. On my 2nd rig, I used flush-mounted CIGS panels, but had I gone with regular framed panels, I'd definitely put in some consideration to the aerodynamics. Add some bull-nosing, for example.

 

[RoadTurtle]

Well now I really wish I gave it a fair before-and-after comparison on my truck. Before the panels, I would average ~13MPG highway (not towing). After the panels, I average ~11MPG highway (not towing). Hence my estimation of a 5-10% decrease in mileage. I talk about the truck like it's in the past, but I still own it. I still drive it. It drives like a sailboat now. In normal driving, the drag is noticeable. In windy weather, it's very noticeable.

3% is just for mirrors, on a Honda Civic. It's an example of an unexpected detail making a difference in fuel economy. Adding up many small differences can amount to a more significant difference. If it's 3% mirrors, it's likely quite a bit more for large, elevated rectangles.

But hey maybe you're right and I'm wrong, and that's totally fine. It just seems odd to be so dismissive of what I'm saying. I'm just sharing my own personal experience after installing elevated panels. On my 2nd rig, I used flush-mounted CIGS panels, but had I gone with regular framed panels, I'd definitely put in some consideration to the aerodynamics. Add some bull-nosing, for example.

I get it and have been around a lot of aero studies. My favorite, chief engineer and I were testing a mercedes, and I'm in the pass seat. I'm 1/2 deaf anyway, and there's a whistling driving me crazy. I finally rolled down the window and put my hand in front of the door mirror and it went away. If you were to theoretically analyze the aero causing the whistle, I can guarantee it would decrease the fuel economy... but in real world not so much. I LOVED when on airplanes, people would come out with these improvements to increase speed, but they were not cummulative. I did a lot of them, saw some speed improvement, but nowhere near what mfg claimed. My biggest improvement came from repainting the airplane. As it had aged, the wing had developed a "rough" surface, and a fresh paint made such a difference, I was literally shocked by the improvement.
.
Getting back to RV's, and I'd love to see a modern 5th wheel in a wind tunnel, I'd bet the air hits the the front and lifts right over the top of it. Your experience is a little different, because the auto makers definitely wind tunnel every single design and look for air flow disrupters. I highly doubt any RV mfg does this. I don't know the bill for the OEMS, but suspect it's millions upon millions of dollars to mpg test the vehicles. One of my cars had a an energy savings device to disable cylinders while in steady state speed. For grins, I had a 90 mile trip to a nearby city. Ran one way with it enabled, ran it the other way disabled. While the starting and ending gas pump was exactly the same, the middle one was not. Bottom line, I could justify the different gas mileage purely based on the prevailing wind. Beyond that, it exceeded my ability to calculate. OEMs will spend a lot of money on gas mileage improvements in the 2nd decimal place, because they have to. Even to the point of creating unique wire harnesses based on content, because even slight weight will impact measured fuel economy.

 

[K8MEJ]

On my 2nd rig, I used flush-mounted CIGS panels, but had I gone with regular framed panels, I'd definitely put in some consideration to the aerodynamics. Add some bull-nosing, for example.

Which CIGS panels did you use and what results do you get relative to STC? In absolute best conditions with panels (glass or Merlin Solar) mounted flat on the roof we get 70% of STC when the sun is directly overhead. On a typical sunny day we get about 50% of STC over the course of the day. I'm an advocate of solar on large vehicles, but it's difficult to generate a lot of power without extreme measures.

On my personal travel trailer, I am going to mount two Merlin Solar TBS180L panels side-by-side on my large slideout. I have 1600W of glass panels on the roof from my original installation a couple of years. This will give me nearly 2kW nominal. Even with an LG mini-split heat pump as my only source of air conditioning, I can't go more than one hot cloudy day without wondering about having enough battery. I have 15kWh of LFP battery storage. That's why we still use Onan gensets on our commercial vehicle builds.

I've kept an eye on CIGS panels but I'm not seeing much hit the market yet and I haven't seen much written with real-world results. Maybe I'm just not looking in the right places. Please share your results.

Thank you

 

[SeanC]

Which CIGS panels did you use and what results do you get relative to STC? In absolute best conditions with panels (glass or Merlin Solar) mounted flat on the roof we get 70% of STC when the sun is directly overhead. On a typical sunny day we get about 50% of STC over the course of the day. I'm an advocate of solar on large vehicles, but it's difficult to generate a lot of power without extreme measures.

On my personal travel trailer, I am going to mount two Merlin Solar TBS180L panels side-by-side on my large slideout. I have 1600W of glass panels on the roof from my original installation a couple of years. This will give me nearly 2kW nominal. Even with an LG mini-split heat pump as my only source of air conditioning, I can't go more than one hot cloudy day without wondering about having enough battery. I have 15kWh of LFP battery storage. That's why we still use Onan gensets on our commercial vehicle builds.

I've kept an eye on CIGS panels but I'm not seeing much hit the market yet and I haven't seen much written with real-world results. Maybe I'm just not looking in the right places. Please share your results.

Thank you

I haven't done a thorough analysis, and I've primarily used them in sub-optimal conditions. All I can provide is the anecdote: they're advertised to be better than monocrystalline panels, but I haven't really observed much difference. They're also advertised to perform better in shaded areas, but mine don't seem to do any better than monocrystalline panels. The only two things I really have to compare are my 960W (6x 160W CIGS), and 580W (2x monocrystalline 290W), so it's not entirely fair. I guess to put it in short: I wouldn't buy CIGS panels for the advertised performance increase. They're significantly more expensive for what seems like little-to-no performance gain. I'd buy them for the aerodynamics and ease of installation on vehicles. In non-moving residential or commercial applications, I don't see much benefit to CIGS. But perhaps in high-wind areas it's worth a consideration.

I hope all of that is just because I haven't given them a fair shake out in the desert to see them under optimal conditions, for example. Maybe my opinions of their performance will go up after that.

As far as "it's difficult to generate a lot of power without extreme measures", I agree 100%. I'm always impressed to hear someone run off of a small 400W solar array, but I suspect they don't need to run their inverters 24/7. For most full-time vehicle users, I just don't personally think solar can be treated as the "primary" source of power. I think it should be treated as a supplement to ease the burden on other systems, like alternators or generators like you mentioned. I still think it's worth covering every practical exterior surface with solar (panels or water heat), if the budget permits. More solar = less fuel.

 

[azozrider]

Yeah, true power independence is not happening in these small arrays. On my first trailer, Solar IS my primary, but must be supplemented with a generator at times to bring charge state up. I have yet to see a system as large as mine, and it still needs help. 3kw of Sun Power 250w panels at 22% efficiency rating. 14.3kw lithium storage 48v. Only one A/C to run and use propane for heat. Still needs to be bigger system. But no more real estate on roof.

 

[K8MEJ]

As far as "it's difficult to generate a lot of power without extreme measures", I agree 100%. I'm always impressed to hear someone run off of a small 400W solar array, but I suspect they don't need to run their inverters 24/7. For most full-time vehicle users, I just don't personally think solar can be treated as the "primary" source of power. I think it should be treated as a supplement to ease the burden on other systems, like alternators or generators like you mentioned. I still think it's worth covering every practical exterior surface with solar (panels or water heat), if the budget permits. More solar = less fuel.

Well, it depends on the size of the vehicle, location and time of travel, and whether or not the owner wants to invest in energy efficient appliances. In a commercial mobile setting it is more challenging. Solar can reduce the size and use of a generator and/or supplement energy created by a primary or secondary alternator when the engine is running. We typically use a combination depending on the energy needs. These large mobile clinics have solar, generator, and alternator power to charge batteries and supply loads when energy demand is high.

In a recreational or full-time living scenario, if the roof surface is large enough, they only travel in moderate climates OR replace their heating system with hydronic and air conditioning with split units using an inverter compressor, then it's very possible to live 100% off solar & battery. I still recommend carrying a small 2-3k generator for extended stays in cloudy locations.