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Discussion Starter · #1 ·
I am putting a couple of solar panels on the roof. As you can see, there is corrugation on the roof, probably for strength (you can stand on the roof).

Specs:

S1) 2 Solar panels each 21" x 37.5". Pretty flat, with a rectangular bump in front (like front of van) for wires to come out.


S2) The van is tilted, so that the back of the van is about 4 inches higher than the front. During the length of the solar panel this is about a 2 inch drop from the back to the front.

S3) As you can see below, the corrugations are about 3 inches wide, and half an inch deep. Looking at the 21" mark on the tape, the panel will barely be on top of a couple of corrugations, and span 4 "valleys."

S4) This is a low roof, short (118" wheelbase) Promaster 1500.

Options I have, with problems for each:

O1) Block all 4 front and back "valleys" (with putty-like butyl tape). I want to have some airflow under the panels. These flexible panels work much worse if they get hot, and can't ventilate. Otherwise, I would block both ends and not have to solve any of the issues below.

O2) Block only the front part of each valley. Leave the back 4 valleys open to the air. The hot air can now escape. This is what I was going to do until I remembered the downward tilt. After the first rain, the entire length (37") of the valleys under the panels will be full of water. This water would start loosening my glues, turn into stagnant water, and standing water might even start rusting the slightest imperfection in the paint.

O3) Only glue/tape on the high ground (leave all valleys open). Water and air can flow freely as I drive, or even parked. I worry that the wind flowing under the panels would create a lift, and detach the panels, which would fly off as I am driving and cause an accident. I am not sure if this effect would be enhanced by the same effect that makes airplanes lift off the ground (I am much less sure about this. There is a tilt, and there is that bump in front of the panels, but it is a rectangle).

O4) Get real panels with real rails that are bolted to the roof. This is a non-starter. The van has to fit in the garage, so any solution that raises the height-profile more than an inch would simply not work. I have to use thin, flexible panels.

O5) Block only the back valleys. Now I don't have a problem with stagnant water, it will dribble out. But have even a worse problem than O4, since the wind will come in the front, and have to exit, other than to lift the whole thing up.


So, can you think of any ideas I missed? Are any of my imagined problems overblown? Which Option (or even a better one) would you go with?
 

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Have you seen this thread about the results of trying to seal flex panels on the roof?

I’m not a fan of flex panels attached directly to the roof at all, but since you have the challenge of trying to fit your van in a garage, I understand you don’t have the option of using regular panels. I would seriously look into enlarging the garage entrance to about 4” taller.

If I had to use them, I wouldn’t even consider sealing the roof channels. If anything, I would try to figure out a way to raise them up 1/2”because every mm would help with cooling.
Having them sit right on the roof turns the van into big heat sink, so unless you can fit enough panels to run AC all day, all that extra heat being transferred directly to the interior of the van will only make the hot months of the year more miserable than they already are.
 

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2014, 138WB, High Roof, Gas, SW MT
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Hi,
I would take a look at the way @Baxsie did his flexible panels...

He has had them in place for several years with no problems.

You might also check if a low mounted rigid panel. My rigid panel is mounted only 1/4 inch above the highest part of the roof. The panel thickness is 1.5 inches, so the total loss of clearance the way I mounted my panel is only 1.75 inches. I did this primarily to reduce aero drag.
Details here...


I measured the temperature of my roof under the panel and away from the panel - details here...

While I'm happy with my low profile rigid panel mount, if I were doing it over again, I'd use the flexible panels.

I would use the method that allows airflow under the panel through the channels.

Gary
 

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Hi,
I would take a look at the way @Baxsie did his flexible panels...

He has had them in place for several years with no problems.

You might also check if a low mounted rigid panel. My rigid panel is mounted only 1/4 inch above the highest part of the roof. The panel thickness is 1.5 inches, so the total loss of clearance the way I mounted my panel is only 1.75 inches. I did this primarily to reduce aero drag.
Details here...


I measured the temperature of my roof under the panel and away from the panel - details here...

While I'm happy with my low profile rigid panel mount, if I were doing it over again, I'd use the flexible panels.

I would use the method that allows airflow under the panel through the channels.

Gary
Thanks for posting your great article about your findings Gary! Do you have any data for 90+ degree days by chance? Would the panels and/or roof temps have a lower margin between them due to less efficiency from the higher ambient temps and increased strength of the suns rays?
 

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Discussion Starter · #6 ·
Thank you both. This was eye opening, particularly the first picture in Flex Solar Panels, how not to install them. I am clearly not going to block any of the valleys now.
As you know Promasters lean downwards, so even if I have 3 inches of clearance in the front for a fan, the back of the van has less than an inch. Redoing my garage is out of my skill-set, and would likely cost over $10K (more than double the van's current modifications).

I am impressed that using Sika 221 can hold that large panel with only 4 small standoffs. This gives me hope. I bought Sika 252 before reading this, so I hope to have the same success. I'll use that and mount my panels directly on the high ridges.
 

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I would love to have solar, but I have 1" of clearance on the garage, and so even the thin panels would have their power attachements ripped off. If there were panels that either had the power come out as a pair of ribbon conductors or out the bottom I could consider it.
But then there is the question of heat gain vs. power gain: I can add ca. 300W of power, but if that gets me ca. 600W of heat gain vs. the white roof, am I all that far ahead?
 

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Discussion Starter · #8 ·
I would love to have solar, but I have 1" of clearance on the garage, and so even the thin panels would have their power attachements ripped off. If there were panels that either had the power come out as a pair of ribbon conductors or out the bottom I could consider it.
But then there is the question of heat gain vs. power gain: I can add ca. 300W of power, but if that gets me ca. 600W of heat gain vs. the white roof, am I all that far ahead?
If I were you, I would check to see if you have 1" clearance everywhere. In my case, I had 1" in the back, but almost 4" in the front. I would have preferred a fan in the back, but that is not where the clearance was. I verified this by cutting a cardboard box and putting it where I wanted to put the fan in the front, and I was able to drive in the garage. Then I tested it again, with the fan, before gluing it in.

You make a good point about the heat generated by the solar panels. We'll see.
 

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I would love to have solar, but I have 1" of clearance on the garage, and so even the thin panels would have their power attachements ripped off. If there were panels that either had the power come out as a pair of ribbon conductors or out the bottom I could consider it.
But then there is the question of heat gain vs. power gain: I can add ca. 300W of power, but if that gets me ca. 600W of heat gain vs. the white roof, am I all that far ahead?
Hi,

Here is an estimate of how much additional heat gain you would get if you added the thin PV panels glued to your white roof.
I did test a while back that measured the roof, PV panel and internal temperatures on my van on the PV panel and on the adjacent roof. Details here...
I used these temperatures and the heat loss equation to estimate heat gain to the van in the PV panel area and on the bare white roof.
This is for my 20 sqft, 315 watt PV panel, and for a 20 sqft patch of white roof.

Heat gain for white roof = (20 sqft)(119F - 78F) / R7 = 117 BTU/hr or 34 watts

Heat gain for PV area = (20 sqft)(157F - 78F) / R8 = 198 BTU/hr or 58 watts

where:
20 sqft is the area of the PV panel
119F is the temperature of the white roof
157F is the temperature of the top of the PV panel
78F is the temperature inside the van

So, this says the heat gain to the van for the PV panel area is only about 24 watts more than the same area of white roof. If you just increased the insulation thickness under the PV panel a little it could be reduced to the same or less than the rest of the roof. Insulation is great stuff :)

If you look at the whole thing from an energy balance point of view...

You have about 1500 watts of solar coming into the 20 sqft PV panel (and to the 20 2qft of white roof).

The white roof reflects about 90% of this, or 1350 watts reflected - white is cool!

The remaining 150 watts heats up the roof

The roof keeps heating until it gets hot enough that it can lose the heat by convection to the air and heat transfer down thorugh the ceiling insulation down into the van (as caculated above).
The part of the 1500 watts incoming that ends up going down through the ceiling in this case is about 34 watts, or about 2% of the 1500 watts incoming.

For the PV panel...
Same 1500 watts coming in.

About 90% or 1350 watts is absorbed by the dark colored panel and the solar cells.

The solar cells convert as much as 20% or 300 watts into electricity - so that is energy that ends up in the battery!

Being as the dark PV panel absorbs so much more heat than the white roof, it has to get to a higher temp to lose the heat by convection to the air and conduction down through the ceiling. In this case it reaches equiliberium at 157F. But still, only 58 watts ends up in the van. Insulation!

Gary
 

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Hi,

Here is an estimate of how much additional heat gain you would get if you added the thin PV panels glued to your white roof.
I did test a while back that measured the roof, PV panel and internal temperatures on my van on the PV panel and on the adjacent roof. Details here...
I used these temperatures and the heat loss equation to estimate heat gain to the van in the PV panel area and on the bare white roof.
This is for my 20 sqft, 315 watt PV panel, and for a 20 sqft patch of white roof.

Heat gain for white roof = (20 sqft)(119F - 78F) / R7 = 117 BTU/hr or 34 watts

Heat gain for PV area = (20 sqft)(157F - 78F) / R8 = 198 BTU/hr or 58 watts

where:
20 sqft is the area of the PV panel
119F is the temperature of the white roof
157F is the temperature of the top of the PV panel
78F is the temperature inside the van

So, this says the heat gain to the van for the PV panel area is only about 24 watts more than the same area of white roof. If you just increased the insulation thickness under the PV panel a little it could be reduced to the same or less than the rest of the roof. Insulation is great stuff :)

If you look at the whole thing from an energy balance point of view...

You have about 1500 watts of solar coming into the 20 sqft PV panel (and to the 20 2qft of white roof).

The white roof reflects about 90% of this, or 1350 watts reflected - white is cool!

The remaining 150 watts heats up the roof

The roof keeps heating until it gets hot enough that it can lose the heat by convection to the air and heat transfer down thorugh the ceiling insulation down into the van (as caculated above).
The part of the 1500 watts incoming that ends up going down through the ceiling in this case is about 34 watts, or about 2% of the 1500 watts incoming.

For the PV panel...
Same 1500 watts coming in.

About 90% or 1350 watts is absorbed by the dark colored panel and the solar cells.

The solar cells convert as much as 20% or 300 watts into electricity - so that is energy that ends up in the battery!

Being as the dark PV panel absorbs so much more heat than the white roof, it has to get to a higher temp to lose the heat by convection to the air and conduction down through the ceiling. In this case it reaches equiliberium at 157F. But still, only 58 watts ends up in the van. Insulation!

Gary
Im not sure I understood all that, but I have a question;

Do your calculations “gained heat” inside the van account for solar energy collected & “piped” into the van as electricity?

Other interior heat gain of this energy transfer. The solar chargers I purchased have some pretty large heat sinks. I assume they are used to waste the electrons if not used to charge the battery (another process that creates heat).

I have pondered this when considering an Isotherm HWT & even adding Solar to my cabin.
 

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Have you considered spacing those flexible panels off of the roof using coroplast? Coroplast is a corrugated plastic. It has a thickness of roughly 3/16-1/8" and offers an airspace/insulating layer between the roof of the van and a 160F solar panel. Coroplast is used for all sorts of things like roadside signs and boxes UPS and the USPS uses. Think of it as a plastic version of corrugated box. Under the flex panel, there's an appropriately sized sheet of coroplast-then the panel is held in place by a UV stable tape.
 

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2014, 138WB, High Roof, Gas, SW MT
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Im not sure I understood all that, but I have a question;

Do your calculations “gained heat” inside the van account for solar energy collected & “piped” into the van as electricity?

Other interior heat gain of this energy transfer. The solar chargers I purchased have some pretty large heat sinks. I assume they are used to waste the electrons if not used to charge the battery (another process that creates heat).

I have pondered this when considering an Isotherm HWT & even adding Solar to my cabin.
Hi RV,
Yes, all that electricity that solar puts into the house battery ends up as heat eventually and pretty much all of it ends up inside the van. Heat always has the last word :)

Gary
 

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Hi RV,
Yes, all that electricity that solar puts into the house battery ends up as heat eventually and pretty much all of it ends up inside the van. Heat always has the last word :)

Gary
Thanks @GaryBIS

My question rephrased; That electricity “energy piped” inside the van intended for the battery which eventually turns into heat is in addition to your previous heat increase calculations ?

I can fathom the understanding that when I charge from my alternator it heats up the house battery in the charge process. I also understand when I switch off that alternator charging system that energy’s (& eventually heat) path is cut off from the outside.

I can also understand when I go to use that house battery stored 12vdc that the use will creat heat inside the van.


Further my other question around the solar panels on the roof in relation to electrical energy that is piped to the interior solar controller with large heatsinks is once they have topped up the house battery how do they dispense of the electrical energy that the solar panels are making? Do they open the solar wire circuit? Or do they send all that un-needed solar panel energy to their big heatsinks ,,, there by heating up the interior of the van? And if that is the case, are those calculations of increased heat energy included in those previous calculations?

My example of the Isotherm HWT; is the engine fluids piping heat into the van (If the system can not be isolated with shutoff vaves). If solar controllers, control by sending solar panel energy to the big heatsinks when the batteries are 100% charged, then if one had a 400W array, would thet mean they have about a 400W electric heater by heatsink?

Maybe @HarryN knows how these controllers utilize their heatsinks in such zero charging demand scenarios.
 

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I am putting a couple of solar panels on the roof. As you can see, there is corrugation on the roof, probably for strength (you can stand on the roof).

Specs:

S1) 2 Solar panels each 21" x 37.5". Pretty flat, with a rectangular bump in front (like front of van) for wires to come out.


S2) The van is tilted, so that the back of the van is about 4 inches higher than the front. During the length of the solar panel this is about a 2 inch drop from the back to the front.

S3) As you can see below, the corrugations are about 3 inches wide, and half an inch deep. Looking at the 21" mark on the tape, the panel will barely be on top of a couple of corrugations, and span 4 "valleys."

S4) This is a low roof, short (118" wheelbase) Promaster 1500.

Options I have, with problems for each:

O1) Block all 4 front and back "valleys" (with putty-like butyl tape). I want to have some airflow under the panels. These flexible panels work much worse if they get hot, and can't ventilate. Otherwise, I would block both ends and not have to solve any of the issues below.

O2) Block only the front part of each valley. Leave the back 4 valleys open to the air. The hot air can now escape. This is what I was going to do until I remembered the downward tilt. After the first rain, the entire length (37") of the valleys under the panels will be full of water. This water would start loosening my glues, turn into stagnant water, and standing water might even start rusting the slightest imperfection in the paint.

O3) Only glue/tape on the high ground (leave all valleys open). Water and air can flow freely as I drive, or even parked. I worry that the wind flowing under the panels would create a lift, and detach the panels, which would fly off as I am driving and cause an accident. I am not sure if this effect would be enhanced by the same effect that makes airplanes lift off the ground (I am much less sure about this. There is a tilt, and there is that bump in front of the panels, but it is a rectangle).

O4) Get real panels with real rails that are bolted to the roof. This is a non-starter. The van has to fit in the garage, so any solution that raises the height-profile more than an inch would simply not work. I have to use thin, flexible panels.

O5) Block only the back valleys. Now I don't have a problem with stagnant water, it will dribble out. But have even a worse problem than O4, since the wind will come in the front, and have to exit, other than to lift the whole thing up.


So, can you think of any ideas I missed? Are any of my imagined problems overblown? Which Option (or even a better one) would you go with?
I installed 3 flex panels over 3 years ago and 15k miles and no issues whatsoever. Periodically I will clean the valleys by running a thin piece of vinyl molding wraped in thin t-shirt material with Turtle wax cleaner applied on the material. I installed the 3 flex solar panels with 3M VHB double side foam tape on the high part of the valley's, and eternal bond tape on the front and back of the high part of the valleys. The very front panel I used eternal bond tape all the way across to keep the wind from causing any lift. Never cleaned any rust from the valley's. Just some lite dirt.
Hope this helps.
 

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I installed 3 flex panels over 3 years ago and 15k miles and no issues whatsoever. Periodically I will clean the valleys by running a thin piece of vinyl molding wraped in thin t-shirt material with Turtle wax cleaner applied on the material. I installed the 3 flex solar panels with 3M VHB double side foam tape on the high part of the valley's, and eternal bond tape on the front and back of the high part of the valleys. The very front panel I used eternal bond tape all the way across to keep the wind from causing any lift. Never cleaned any rust from the valley's. Just some lite dirt.
Hope this helps.
 

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I installed 3 flex panels over 3 years ago and 15k miles and no issues whatsoever. Periodically I will clean the valleys by running a thin piece of vinyl molding wraped in thin t-shirt material with Turtle wax cleaner applied on the material. I installed the 3 flex solar panels with 3M VHB double side foam tape on the high part of the valley's, and eternal bond tape on the front and back of the high part of the valleys. The very front panel I used eternal bond tape all the way across to keep the wind from causing any lift. Never cleaned any rust from the valley's. Just some lite dirt.
Hope this helps.
 

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Thanks @GaryBIS

My question rephrased; That electricity “energy piped” inside the van intended for the battery which eventually turns into heat is in addition to your previous heat increase calculations ?

I can fathom the understanding that when I charge from my alternator it heats up the house battery in the charge process. I also understand when I switch off that alternator charging system that energy’s (& eventually heat) path is cut off from the outside.

I can also understand when I go to use that house battery stored 12vdc that the use will creat heat inside the van.


Further my other question around the solar panels on the roof in relation to electrical energy that is piped to the interior solar controller with large heatsinks is once they have topped up the house battery how do they dispense of the electrical energy that the solar panels are making? Do they open the solar wire circuit? Or do they send all that un-needed solar panel energy to their big heatsinks ,,, there by heating up the interior of the van? And if that is the case, are those calculations of increased heat energy included in those previous calculations?

My example of the Isotherm HWT; is the engine fluids piping heat into the van (If the system can not be isolated with shutoff vaves). If solar controllers, control by sending solar panel energy to the big heatsinks when the batteries are 100% charged, then if one had a 400W array, would thet mean they have about a 400W electric heater by heatsink?

Maybe @HarryN knows how these controllers utilize their heatsinks in such zero charging demand scenarios.
A typical solar charge controller (both pwm and mppt types) control the power passing through them by turning a transistor on / off. Depending on the type, this can be pretty fast ) KHz range or fairly slowly - multiple seconds.

When "fully off", they consume very little power - measurable only with sensitive equipment.
When "fully on", they also consume relatively little power.

In the time period where they are going in between "fully on" and "fully off", they are not very efficient and generate heat.

There is a great deal of effort spent finding and developing transistors that do this transition very fast and very efficiently, because the faster they can switch, the smaller the other components in the circuit can be - and the overall size of the device. This applies to literally everything in our lives, cell phones, laptops, cars, and solar charge controllers.

So the heat that needs to be moved from this transistor / (often a FET) goes from the surface of that transistor's package to the air - via the heat spreader is mostly the switching loss that happens from moving power through it.

When there isn't any solar charging going on, the "typical" solar charge controller (such as your Victron or a Bogart ) goes into a sort of standby mode and the transistors are turned "off". Relatively little power is used - mostly just stuff like the display and wireless functions remain on.

So on the solar panel side, when there isn't any power being drawn from it, it will behave as an open circuit and the voltage will rise to ~ Voc (Voltage open circuit). Current goes to ~ 0 amps.

For some reason, the renogy rover controllers still consume a lot of power in this standby state - not sure why but it's more than you expect.

Most good controllers are very efficient and if you use one that has a higher rated current output than it actually needs the heat spreader hardly gets warm.

____
 

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2014, 138WB, High Roof, Gas, SW MT
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Thanks @GaryBIS

My question rephrased; That electricity “energy piped” inside the van intended for the battery which eventually turns into heat is in addition to your previous heat increase calculations ?

I can fathom the understanding that when I charge from my alternator it heats up the house battery in the charge process. I also understand when I switch off that alternator charging system that energy’s (& eventually heat) path is cut off from the outside.

I can also understand when I go to use that house battery stored 12vdc that the use will creat heat inside the van.


Further my other question around the solar panels on the roof in relation to electrical energy that is piped to the interior solar controller with large heatsinks is once they have topped up the house battery how do they dispense of the electrical energy that the solar panels are making? Do they open the solar wire circuit? Or do they send all that un-needed solar panel energy to their big heatsinks ,,, there by heating up the interior of the van? And if that is the case, are those calculations of increased heat energy included in those previous calculations?

My example of the Isotherm HWT; is the engine fluids piping heat into the van (If the system can not be isolated with shutoff vaves). If solar controllers, control by sending solar panel energy to the big heatsinks when the batteries are 100% charged, then if one had a 400W array, would thet mean they have about a 400W electric heater by heatsink?

Maybe @HarryN knows how these controllers utilize their heatsinks in such zero charging demand scenarios.
Hi RV,

I'm pretty sure the solar charge controllers when they sense the house battery is full adjust the voltage the PV panels see so that the PV panels stop generating current - perhaps up to Voc on the IV plot below?
Or, maybe they just open the circuit to the PV panel? Not sure, but I'm pretty sure they would not bring a lot of energy in and just dissipate it in the heat sinks.

Font Slope Rectangle Parallel Electric blue


Gary

edit: just saw that Harry answered your question and that seems sensible to me.
 
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