Ultralight Solar PV USB charger for backpacking - suggestions to improve design?

[Stormwind]

Hi all,

RyanH has made great progress creating ultralight solar PV USB charging devices for lightweight backpacking, greatly beating the power-to-weight ratio of commercially available systems (like the Bushnell PowerSync SolarWrap Mini using Powerfilm cells).

Anyone have any suggestions on improvements? (Please post here, not at forum links below)

A high level summary of components (if you prefer not to follow the links below):

1. SunPower Maxeon cells due to their light weight and solid copper backing, increasing their strength and decreasing need for additional stronger backing. Cells are half or quarter cut to get to V and A needed for DC-DC converter
2. Depron foam backing due to combination of light weight and stiffness (but may not be durable enough)
3. A specific DC-DC buck down converter due to light weight, efficiency, and Apple charging support
4. A lightweight covering/"encapsulant" (spray-on) to reduce weight vs plastic films or glass

RyanH detailed an earlier device (2.75oz, 21W max theoretical, about 10W-12W in real usage) here: Backpacking Light forum


He detailed a newer device (1.7oz, 9.4W max theoretical, about 6W in real usage) here: Facebook group post


ULSolarPVUSBCharger2.jpg


Some thoughts:

1. DC-DC USB converter: If we could find a converter like the Adafruit VERTER 5V USB Buck-Boost converter that could boost lower voltage and operate in a V and A range produced by the cells, the device might also produce power in lower power conditions. (The design avoids a battery to reduce weight since the devices being charged already have their own batteries.)

2. Series wiring of half and quarter cells: The cells are half or quarter cells wired in series to get the V and A into a range supported by the DC-DC converter. Of course, this means any shade cuts off power production, but the high power-to-weight ratio and strong solid copper backing of the SunPower Maxeon cells makes this trade-off worthwhile currently.

3. Carbon fiber vs Depron foam: RyahH reports the foam might not be durable enough after a long hike with a device on his pack. Carbon fiber panels or a combo of carbon fiber and Depron foam (which is disappearing from market due to manufacturer) might be worth the heavier weight trade-off. (Many of the structural ideas are from the lightweight RC airplane community.)

4. RyanH has tried many different lightweight encapsulants/protective coverings for the panels - Freeze-Tite and Krylon Clear Gloss spray paint for example - but reports some scratching on his panels during his last long hike with the panels hanging off his pack. Someone else reported success with Flex Seal Clear. I don't know the light transmission, haze, or chemical interactivity stats of any of these.

Thanks!
-Jason

P.S. I emailed RyanH a link to this post - he's provided lots of great info to me via email. The photo is also from RyanH.

P.P.S. This is DIY/MYOG (Make Your Own Gear) - it used to have more links to actual and possible components used in making the panel but admins considered them advertisement links and removed them.

[Stormwind]

A related question: does anyone know what backing material and cover material/encapsulant is used on the flexible solar panels (using SunPower Maxeon cells) that are commercially available?

Example - ApolloFLEX Mono-TL 300W Flexible panel (there seem to be at least several brands):

ApolloFlexMono300Panel.jpg

From the limited pics I have found, it appears to be a semi-rigid flexible plastic backing panel with clear plastic over the cells.

I'm hoping that information can help us with ideas for the lightweight backpacking USB charging panel.

I found a description of materials for one manufactuer's flexible panel: they use aluminum backing, EVA, and ETFE film and claim others use PET for backing and film:


LensumFlexiblePanel.jpg

Which leads to the question: for a needed strength/rigidity, would carbon fiber, aluminum, or PET be lightest (and at what thickness would be needed)?

[PNjunction]

P.P.S. This is DIY/MYOG (Make Your Own Gear) - it used to have more links to actual and possible components used in making the panel but admins considered them advertisement links and removed them.

And for good reason. Surely the members at the backpacking forum are aware of phony participation with the sole intent of providing marketing links to attract web-search rating hits, and not to truly participate or promote the forum they are spamming in any intelligent manner.

I also note the backpacking forum doesn't even allow posting unless you subscribe! It is READ-ONLY, until you pay up for a subscription.

Generally, on most forums, one can refer to another forum/blog etc, but it is usually frowned upon so you aren't seen as a promoter from the other site - ESPECIALLY if it involves a subscription or other form of monetary compensation to make a post. In other words, don't do it unless you absolutely have to in order to keep some semblence of professionalism for all involved.

Would RyanH be upset if I paid for a subscription, and posted nonsensical questions about whether square pockets are better than circular pockets on my diy backpacks made from discarded ladies' handbags along with a link to where you can get them?

See what I'm saying? Maybe ask the moderators of that forum how they feel about getting free advertising from a solar site that is not based on a subscription model to even make a post.

I'm not trying to be a self-appointed forum cop here, but this is just basic general forum courtesy that has been in effect for many many years on most forums...

At the very least, if and when you do this, extend the courtesy in the other direction by posting a link directly back to SolarPanelTalk *publicly*, and not to this thread, but to the site in general at least. Some of us may be watching to see if that link is removed.

[Stormwind]

And for good reason. Surely the members at the backpacking forum are aware of phony participation with the sole intent of providing marketing links to attract web-search rating hits, and not to truly participate or promote the forum they are spamming in any intelligent manner.

No one is trying to give anyone free advertising - I posted here because (1) there is much more solar PV knowledge here that could help iterate the design, (2) RyanH isn't even subscribed to the BPL forum anymore so he can't post there but could post here (and the second link was a Facebook post), and (3) I prefer open forums and non-paid ones myself so I was hoping the discussion would occur here instead.

I also did not want to "copy and paste" RyanH's posts (I saw a previous post on this forum where someone was criticized for it), which is why I put in a link to the post rather than copying the contents - as links are meant to be used. And it would be wrong to copy-and-paste rather than link without RyanH's permission.

At the very least, if and when you do this, extend the courtesy in the other direction by posting a link directly back to SolarPanelTalk *publicly*, and not to this thread, but to the site in general at least. Some of us may be watching to see if that link is removed.

There was no intent here other than starting the discussion here with knowledgeable, interested solar PV folks and iterate on the design.

I added a top-level link to SPT on the BPL post (of which I have no affiliation other than a poster), but I'm not going to remove the link to the post and force people to try and find the post themselves (very user unfriendly and discouraging of participation) rather than just click a link to it.

I hope other new users get a warmer welcome on this forum. This was posted merely in the spirit of cooperation and people working together on making things - not competition between forums/web sites.

[Mike90250]

As long at this is not a scheme to drive traffic to another site, I'm ok with it.

Now, having perused the other site and looked at the construction, I'm going to say those panels will have a limited lifetime. None of the techniques will prevent water vapor from getting inside the enclosure and eventually damaging the cells.
The other way the cells are going to fail, is from micro cracking. When wafers are cut, there is damage done to the edge, laser cut, diamond saw, score and snap, all initiate stress cracking along the edge, that propagates across the entire wafer, damaging it. For a DIY experiment, not a problem, but if you are in backcountry, and rely on solar power, you will be disappointed sometime,

[Stormwind]

Now, having perused the other site and looked at the construction, I'm going to say those panels will have a limited lifetime. None of the techniques will prevent water vapor from getting inside the enclosure and eventually damaging the cells.

Any ideas on a lightweight method to prevent water vapor permeation? One crazy idea I saw online - use Flex Seal Clear to encapsulate the whole cell (though I don't know the light transmission and haze %'s or its chemical compatibility with all the other components).

Would keeping the device out of the rain in a water-proof enclosure (only using it when sunny/no rain) prevent water vapor damage, or does humidity by itself cause it?

The other way the cells are going to fail, is from micro cracking. When wafers are cut, there is damage done to the edge, laser cut, diamond saw, score and snap, all initiate stress cracking along the edge, that propagates across the entire wafer, damaging it. For a DIY experiment, not a problem, but if you are in backcountry, and rely on solar power, you will be disappointed sometime,

Six uncut cells in serial would be about 3.5V at 5.9A - so a less efficient DC-DC boost converter would be needed (if a component exists that can boost that to 5V can be found), instead of the buck-down converter, to avoid cutting the cells at that size of a device.

For cut cells, I wonder which method is the least damaging - I'd guess laser cut. I wonder what method all the small flexible panel manufacturers (mostly in China from my searches) use on their SunPower cells when cutting.

Another line of thought - if the device only lasts 5 years that might be okay - by that time I hope solar PV cells would have made efficiency and power-to-weight ratio improvements (maybe OPV cells) to the point we'd want to build a new device anyway.

[Mike90250]

Humidity over just a couple % sends water vapor right though most paint, rubber, plastic, epoxy. It takes a mm or so of tight metal (not porous aluminum or cast iron)
Tedlar is an engineered plastic that is vapor tight. Glass is also. In commercial panels. tedlar and glass make up 99% of the barriers.

[PNjunction]

Stormwind - I meant no offense.

This forum is renown for getting to the point very quickly, mainly due to helping others avoid a major financial setback, or outright physical danger. This can be overlooked in more social-club forums until it is too late 60 messages down the line.

So I'll get to the point quickly - it is just UNCOOL to cherry-pick another forum's threads or maker projects, no matter where you go. That only weakens / disperses the original community who may not want to change forums, and will miss the involvement of their members. It results in a disjointed effort jumping back and forth, and may be especially unsettling for those who paid for posting services.

Do you know how many of Sunking's posts in other forums would be worthy of discussing here? Plenty, but it would be uncool to the other board to cull his ideas or posts over here, splitting the communities of both.

Anyway, good luck with your project. I can't add much more than to say read the DIY Solar Panel sub-forum here for additional clues as to why this type of project is not really practical, but can be more of an educational technical exercise, which is always a good idea.

[Stormwind]

Stormwind - I meant no offense.
So I'll get to the point quickly - it is just UNCOOL to cherry-pick another forum's threads or maker projects, no matter where you go. That only weakens / disperses the original community who may not want to change forums, and will miss the involvement of their members. It results in a disjointed effort jumping back and forth, and may be especially unsettling for those who paid for posting services.

The original posts are on two completely forums by original maker himself. Also, in the open source software/hardware community (and similarly the lightweight backpacking community), new projects re-use libraries/components from so many other projects (by design of the other projects) that cross-posting and linking (to ask questions, get expert input for the components, and share improvements) is the norm. For example, check out projects on GitHub and other open source portals to see how it works. In this case, we need more solar PV expertise so we go where we can find it and learn.

Anyway, good luck with your project. I can't add much more than to say read the DIY Solar Panel sub-forum here for additional clues as to why this type of project is not really practical, but can be more of an educational technical exercise, which is always a good idea.

Thanks, I'll read that forum more too.

As for practicality, there are Chinese manufacturers already selling lightweight flexible panels with USB chargers so we know it is practical: we are just trying to reduce the weight further (since every ounce matters when hiking for a week with everything carried on your back).

Examples of such devices/panels commercially available (found on AliExpress):
1. 13W 5V flexible panel with USB charger
2. 6.5W 5V flexible panel with USB charger
3. Backpack with 10W 5V folding panel with USB charger

From the above examples, it would appear the supporting structure and encapsulant are the main areas of opportunity to decrease weight.

[Stormwind]

Humidity over just a couple % sends water vapor right though most paint, rubber, plastic, epoxy. It takes a mm or so of tight metal (not porous aluminum or cast iron)
Tedlar is an engineered plastic that is vapor tight. Glass is also. In commercial panels. tedlar and glass make up 99% of the barriers.

Thanks @Mike90250!

Researching Tedlar, it appears there is also transparent Tedlar film available (though I haven't yet found light transmission or haze data for it yet).

References:
1. DuPont Tedlar datasheet that describes clear Type 2, 3, 4, and 5 Tedlar forms
2. DuPont Tedlar Adhesive and Lamination Guide

I wonder if a sandwich of clear Tedlar, adhesive, solar cell, adhesive, and standard Tedlar backing film would work, be a proper water vapor moisture barrier (assuming properly sealed), and be lightweight. My current lack of access to a vacuum pump or autoclave may make the prototyping process difficult, though.

Edit: I found some light transmission stats for clear Tedlar forms and they look fine (compared to panel plate glass and the light wavelengths, 400nm-1200nm, used by Sunpower cells):

Sunpower wavelengths:
sunpower-wv.PNG

Tedlar transmission % for wavelengths:
tedlar-wv.PNG

.. and that somehow led me to 3M Ultra Barrier Solar Film which is actually sold as a flexible solar PV glass replacement, especially for flexible panels. Apparently "flexible solar front sheet" is the search term I've been looking for.

[Mike90250]

Another quick thought. SunElec.com is giving away de-installed solar roof tiles, U pay shipping.
Freebies not likely to last long, a couple ought to do for camping, if you hurry.
contact John directly per his site:

[PNjunction]

Also, in the open source software/hardware community (and similarly the lightweight backpacking community), new projects re-use libraries/components from so many other projects (by design of the other projects) that cross-posting and linking (to ask questions, get expert input for the components, and share improvements) is the norm. For example, check out projects on GitHub and other open source portals to see how it works. In this case, we need more solar PV expertise so we go where we can find it and learn.

I'm familiar with the process, trust me having run Unix boxes for decades and compiling the code from various repositories. Long before Linux became a thing.

BUT what we don't do is spray code all over the place, hoping that the various communites (gnu / linux, freebsd, Openbsd, Netbsd and all the rest) magically come together from disparate codebases. Hence your reference to github.

Thing is, Solarpaneltalk is not github, nor should it be for pet projects picked from other communities, dividing their membership. People and software are different things.

What I smell coming is a big agenda.

Let's review:

1) You want an ultralight solar panel to charge your cellphone while trekking.
2) Commercial units, which are already more efficient / longer lasting, already exist as commodity items.
3) "Development" has stopped or stagnated on your pet project from somebody else's work.
4) You want us to do all your engineering for you, having demonstrated nothing of your own in this field, but mere links to the work of others.

The most practical answer to your maker-toy, is to just eat another bite of a Snickers / energy bar, and carry a proven commercial commodity device with you to compensate for those extra ounces.

[Stormwind]

Another quick thought. SunElec.com is giving away de-installed solar roof tiles, U pay shipping.
Freebies not likely to last long, a couple ought to do for camping, if you hurry.
contact John directly per his site:

Thanks @Mike90250!

I found some interesting videos during my research - the most informative one I think was a video from Lensun (a Chinese flexible panel manufacturer) layering (on top of what look like PET backing and EVA film) strings of Sunpower cells, then EVA film, then the ETFE front sheet film. They then rack it, in preparation for heating (to melt the EVA) in a vacuum.

https://www.youtube.com/watch?v=i1eiqMoxqR4

They uploaded some other interesting videos also, including laser cutting of cells, at: https://www.youtube.com/channel/UCcy...Dt0Kl7w/videos

Now I just have to wait for all the materials to arrive for my next attempt (using vacuum cleaner and heat gun or oven). I've also been waiting on some pre-made (but heavier than desired) 5V 6.5W and 13W flexible panels with USB charger to arrive, to compare construction.

My current thoughts:
1. Encapsulant: I wonder if there is an encapsulant lighter than EVA film. (Any suggestions?)
2. Back sheet: Starting with Tedlar to see if it is lightweight yet strong enough without a PET or other backing material. (Any suggestions for anything lighter?)
3. Front sheet: Going to try clear Tedlar, and also try to get some ETFE film, 3M Ultra Barrier Solar Film 510-F, and other flexible front sheet material to compare. Unfortunately weight per square meter is not commonly included on the data sheets - I might have to buy and weight them myself to compare. (Anyone does so already, have suggestions?)

[Mike90250]

1. Encapsulant: I wonder if there is an encapsulant lighter than EVA film. (Any suggestions?)

EVA is pretty magical "glue" It is UV resistant I believe, but not water vapor resistant (that's the job of glass and tedlar)

2. Back sheet: Starting with Tedlar to see if it is lightweight yet strong enough without a PET or other backing material. (Any suggestions for anything lighter?)

EVA film glues tedlar to the back of the cells, and via the gaps in cells, to the front side EVA, when you have a vacuum laminater.

3. Front sheet: Going to try clear Tedlar, and also try to get some ETFE film, 3M Ultra Barrier Solar Film 510-F, and other flexible front sheet material to compare. Unfortunately weight per square meter is not commonly included on the data sheets - I might have to buy and weight them myself to compare. (Anyone does so already, have suggestions?)

"Film" is about as light as you will get, Thicker/heaver = sheet. No idea about actual weight

[PNjunction]

Um, ok I guess we're going in this direction now -

I suppose we need a background on testing the claims of various solar power usb charging panels, battery banks, AND wiring considerations. By far the best to weed out exaggerated claims is by this guy, MJlorton:

STC, PTC and spectra

https://www.youtube.com/watch?v=UBUwcTrJRf0

Part 2:

https://www.youtube.com/watch?v=Fij5Iw6F7SQ

General notes about some commercial solar 5v chargers

https://www.youtube.com/watch?v=MgRloFSeWoE

We'll note that he set up extensive testing rigs to find out if manufacturers are exaggerating from either infrared or ultraviolet sensitivity. Many of these panels do "ok" with visible sunlight, but may have much more sensitivity to utraviolet or infrared spectra. You can follow his testing with multiple series of these videos, and actual meet and greets with the panel manufacturers.

He's about the only guy that gets down to real testing, and not just the usual run of the mill backyard camping marketer type which exist by the hundreds. (yawn.