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Old 06-13-2018, 09:22 AM   #1
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Question Solar Panel Theoretical Maximum ?

Hi all.

So I was wondering if anyone has some insights as to what the theoretical (but still practical.. i.e. using off the shelf, widely available equipment) maximum amount of solar energy one could every hope to collect by mounting them on a standard size (~40ft?) bus frame.

I thought it might be an interesting thought experiment.. if any of you pros want to weigh in i'd love to hear your thoughts.
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Old 06-13-2018, 09:45 AM   #2
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Solar system design needs a "balanced" approach.

Sizing components to work properly together.

I could fit at least 7200 watts of panels on my roof but the properly sized battery bank to go with it would be completely impractical.

A great resource for solar info: Forums - Solar Panels - Solar Panels Forum
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Old 06-13-2018, 09:48 AM   #3
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Using my SunPower E20 435 watt panels as a starting point (cause I have the documentation at hand for them)... They are 1312 sq. inches and produce 435 watts. That is about 1/3 watt per sq. inch (round numbers). A 40' coach is something like 40' long x 8' wide which is 46,000 sq. inches. That times 3 = 15,000 watts (15kW).

Of course, you can't cover every square inch very easily.... So, a more realistic calculation would be fitting 11 of these 41" wide (81" tall) panels which would equate to 4785 watts. Of course, that isn't very realistic either but this is all theoretical, right?!?!
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Old 06-13-2018, 11:56 AM   #4
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I think that by exploring the upper limits of what solar panels can do we can universally help anyone who wants to build their own system.

I always try to start with the ideal and then walk my way back to what my budget allows. In any event maybe someone will be inspired to try and create whatever ideal system we can think up here
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Old 06-13-2018, 12:16 PM   #5
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As I played with system design for my bus I found the limiting factor for me was the cost and weight of the battery bank.

I had hoped to follow JD's example and not have a generator. Unfortunately the battery requirement exceeds my budget by a significant amount.
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Old 06-13-2018, 03:05 PM   #6
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I have a 40' bus. There is 26' of clear space between the two roof hatches, between which I made a central 12"-wide walkway. On either side of this walkway are hinged four support frames, each about 5'6" long, that hold each of the eight Sharp 255W grid-tie panels, and they extend out to a few inches from the edge of the roof. Between the last panels and the rear hatch I have 4' of space that will be for the two eventual water heating panels there. So, I have 2040W of PV occupying 22' of roof, but with a 12"-wide walkway down the center, or occupying about 154 sq.ft. measured in a horizontal plane.

Could I have fitted more panels? Maybe, but A) I don't need more power (during the summer I'll be able to produce way more power than I need), and B) I wouldn't have the walkway that is important for cleaning the panels safely (I put two quick-connect water outlets on it so I can easily and safely wash the panels from up on the roof), and C) I want space for the two water heating panels that will each be about 20 sq.ft. and hinged just like the PV panels. This amount of PV will charge my eventual 900 aH of golfcart batteries at about a 13% charge rate, the most that FLA batteries will accept without too much water loss, and even in winter I should still be able to get a useful amount of power into them, hopefully even to Float each day unless it's pissing down with rain.

Sure, you can cover the entire roof from front to back with panels, but why? And besides, you'd need those flexible panels to do so, and we all know how long they last . . .

John
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Old 06-13-2018, 03:18 PM   #7
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Quote:
Originally Posted by JDOnTheGo View Post
Using my SunPower E20 435 watt panels as a starting point (cause I have the documentation at hand for them)... They are 1312 sq. inches and produce 435 watts. That is about 1/3 watt per sq. inch (round numbers). A 40' coach is something like 40' long x 8' wide which is 46,000 sq. inches. That times 3 = 15,000 watts (15kW).

Of course, you can't cover every square inch very easily.... So, a more realistic calculation would be fitting 11 of these 41" wide (81" tall) panels which would equate to 4785 watts. Of course, that isn't very realistic either but this is all theoretical, right?!?!


Those numbers look amazing, i checked out the pdf specs here https://us.sunpower.com/sites/sunpow...-datasheet.pdf for the sunpower e20 435 and it is indeed beefy. I only wonder on the price point and availability..

I suppose in the interest of theoretical maximums would there be any possible system that you could fit on your roof to get that extra 30% sun power by tracking the suns zenith across the sky ? Perhaps on a raised roof rack? I'm racking my brain trying to imagine optimizations
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Old 06-15-2018, 11:34 PM   #8
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Originally Posted by saveyour View Post



I suppose in the interest of theoretical maximums would there be any possible system that you could fit on your roof to get that extra 30% sun power by tracking the suns zenith across the sky ? Perhaps on a raised roof rack? I'm racking my brain trying to imagine optimizations
Practically, no, there's no realistic way to have two-axis tracking on a bus or RV roof. However, vehicles can easily do something that houses have difficulty doing - they can orientate themselves to maximize solar harvest by simply parking to face either due east or due west. Then you can do what I've done, and have all the panels hinged so half of them can be raised to face the sun better, and the other half are angled down against the roof (which you can't do anything about). It's still better to do this than to have them all flat on the roof, unless you live on the equator where having them flat is OK. My benefit from having all my panels tiltable will be in the winter when the sun is lower (not that I ever intend to be further north in the winter where the sun is lower still); any gain in solar harvest at that time is well worth it, but during the summer I'll probably have more power than I need.

John
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Old 06-16-2018, 05:53 AM   #9
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Adding to John's post re. tilting panels...

After several years of full-timing, I have found that my flat mounted panels achieve a maximum output of about 85% (of rated output) during the summer months and a maximum of about 60% during the winter months. I suspect tilting the panels would help the winter output a great deal.
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Old 06-18-2018, 05:09 AM   #10
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Originally Posted by Iceni John View Post
Practically, no, there's no realistic way to have two-axis tracking on a bus or RV roof. However, vehicles can easily do something that houses have difficulty doing - they can orientate themselves to maximize solar harvest by simply parking to face either due east or due west. Then you can do what I've done, and have all the panels hinged so half of them can be raised to face the sun better, and the other half are angled down against the roof (which you can't do anything about). It's still better to do this than to have them all flat on the roof, unless you live on the equator where having them flat is OK. My benefit from having all my panels tiltable will be in the winter when the sun is lower (not that I ever intend to be further north in the winter where the sun is lower still); any gain in solar harvest at that time is well worth it, but during the summer I'll probably have more power than I need.

John

I saw someone else's post on their diy roof rack linear actuator solar panel tilting system.

It appears to be only 1 axis but i think with a few more linear actuators and
a photo sensor it could be fully automated and the other axis can be
controlled by how you park the bus.

Check it out..

http://www.skoolie.net/forums/f11/so...tml#post133025
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