Experiments in ultralight solar

@Bradley

Thanks for posting that info on the Powerfilm. I've looked at those before but I don't think I saw the RC Aircraft ones. Not having to use a backing is super cool!

Is my math right though? If you consider the weight of a SunPower cell with a backing, isn't the PowerFilm still over twice as heavy per watt?

I think you're right that buck converters stop working once voltage drops to around 5v. I don't have any experience with PowerFilm, but the idea to use a higher 10v cell to guard against voltage drop is totally impractical for regular mono and poly cells. The problem is the current. If the light is bad enough that a cell is only operating at half its potential voltage, there will be practically zero (or zero) current flowing through the cells.

"…the amperage can be increased as desired simply by adding more panels in shady conditions"

That's easy to say off the cuff, but in reality if your current is only flowing at <1%, you'd literally need hundreds (if not thousands) of additional cells to produce any real wattage.

To clarify though, even if you did want to attempt this route, you could do it with the SunPower cells as well. I ran 12 cells together to get me close to 7 volts. You could easily add more cells if you want more voltage. Keep in mind though, the higher the voltage, the more inefficient the buck converter will be when it does happen to be sunny.

If you want to build a really big array to power a laptop, like you were saying, you could wire up 12-14 SunPower cells. You wouldn't even need to cut them. You could build a 42-49 watt panel and it should only weigh 5-6 ounces.

Hello Bradley. Can you share how you output USB from your 10 watt array, and the success you have with different devices? I have experimented with a variety of unregulated wild cells and they sometimes can charge a variety of devices, and sometimes not, depending on the circuitry of the device being charged and the intensity of the sun, with the iPhone being the most picky. Yes, the larger the array, the more chance for enough power. But I have had more luck charging with all the different cells when there is a controller that regulates the voltage to a more limited range.

"The problem is the current. If the light is bad enough that a cell is only operating at half its potential voltage, there will be practically zero (or zero) current flowing through the cells."

I should have been more clear about this, but this is the whole point of why I want to use amorphous cells like the Powerfilm as opposed to mono- or polycrystaline cells.
The current doesn't dip to near-zero in amorphous cells as soon as they get a bit shady or don't have direct sun. They behave a lot more like you would intuitively expect a solar panel to – if there's half the light reaching it, you get about half the overall power out of the cell.

Amorphous cells only reach about 5% efficiency per surface area, as opposed to the 20+% of monocrystaline cells, but they'll stay around 5% efficient regardless of the level of sun exposure.

As for the weight, you're right, they are nearly twice as heavy, as you say, entirely because you need so many to reach good power levels… what I really meant was they're still in a "lightweight" class far away from anything else I've discovered except the cells you're using here, which seem almost miraculously light. It's easy to build something that is 1/4 the weight or less than a comparable commercial product.

Given how cheap your cells are I am definitely going to buy a few and play around with them. I am curious about wiring in series to reach 10-12 volts through that buck converter (aside: have you tested your buck converter's efficiency? What IC is it using? It also looks to be almost miraculously efficient compared to most) then just eating the apparently-small efficiency cost in the converter in exchange for usability in lower light. But, as you say, current becomes a big issue in monocrystaline cells in low light with a logarithmic dip.

Cameron, I'm not completely sure what you're asking, but if you're not aware, you just need to provide a DC-DC converter (like the one OP linked to, or any other of the countless "car adapter" USB chargers), which can be near-weightless. Then feed sufficient power into the converter from your cells. What is sufficient?

– Must have an input voltage within the range that the DC-DC converter expects. Check documentation on each, but usually this is 5.5-18 volts.
– Must provide sufficient current for the task at hand. This is part of what we're discussing. In good sun exposure, the answer is always "add more cells in parallel until happy." In less-ideal sun exposure, it gets complicated.
– Make sure there's a diode somewhere in the flow so that charge doesn't flow backwards into the cells when you hit a shady spot. Most USB chargers have one already in their circuitry.

You do need to hit a certain current level for most USB devices to be happy and start charging. Like you said, iPhones can expect a fair amount. Just increase input current (cell wattage) as necessary. My 10-watt Powerfilm abomination could charge my 5S even in overcast conditions, albeit slowly. Very fast in direct sun.

I also have a 12-watt mono crystalline panel that won't even light the USB charger's LED in overcast conditions.

@Bradley

I'm not sure about the IC of that buck converter, nor have I done any efficiency testing. I'd be curious to know some numbers though if you're able to run any tests at varying voltages.

I just found this thread.

@Ryan – 21W in 2.75oz is impressive!

Have you made any improvements to the design or found even better panels since your last post?

Another idea, though probably impractical for small MYOG quantities: using Parylene or a conformal coating on the solar cells instead of Freeze-Tite to reduce weight.

Some references:

1. https://www.sparkfun.com/news/2213
2. http://news.mit.edu/2016/ultrathin-flexible-solar-cells-0226
3. http://www.jimonlight.com/2008/12/30/parylene-saviour-to-leds/

I’ve posted over at Solar Panel Talk’s forums to see if they have any suggestions:

https://www.solarpaneltalk.com/forum/off-grid-solar/solar-living/334411-ultralight-solar-pv-usb-charger-for-backpacking-suggestions-to-improve-design

Solar Panel Talk is at https://www.solarpaneltalk.com

Man, that guy PNjunction on the other forums is a bit of a dick eh?

Wow no kidding^

Hi – this is my very first post on BPL, so hello!

Regarding apple charge rates – there are 4 connections in USB world, with the middle two pins being data.  Usually charges and devices are agnostic about what these are supposed to be (some people wire the data pins to ground, others to +5v) and most non-apple devices will work out that it’s not plugged into a PC and “suck it and see” what the max current draw is.  This is why your phone, etc, knows if it is plugged into an “AC Charger” or not.

Apple – being Apple – decided there was money to be made in the cable and charger market.  They include resistors inline on the data pins which identify it as being a “genuine” apple charger.

Now, if you want to fake this in your solar panel design all you need to do is to tie them to 5v with a couple of resistors.  Schematic below, hope this helps!

<p style=”text-align: left;”>Hi,</p>
Thanks for the excellent posts on your project Ryan. Can anyone give update on how much you’ve used your original panel and how it’s holding up? Im working a sunpower array based on this project and im trying to figure out how much work to put into encapsulating the cells to keep moisture out.

Looks like encapsulant is available in small quantities on ebay etc but purpose-manufactured front sheet material for flexi panels is hard to find and mostly sold in very large rolls. Just wondering how the freezer wrap covered sunpower cells hold up to use and exposure to the environment.

Thanks!

More details on my planned project:

I’m planning to make a ~40 watt array with 12 sunpower cells and run this through a buck converter to charge cell phone, gopro, and aux battery for extended backpacking and paragliding trips. Planning to use 2mm depron foam to reinforce each cell. Just trying to figure out how much money and effort to put into sealing/encapsulating each cell, freezer wrap vs. something fancier.

I’m hoping to make an inflatable backing (think mini air mattress) for the array to make it easy to wear the on top of my pack/head or attach it to my paragliding harness so I can keep it out in the sun while on the move. Planning to use 70d heat sealed tpu nylon from diypackraft for the inflatable backing construction. In my (dissapointing) experience with goal zero and power film solar chargers its crucial to be able to keep the panel in the sun while on the move since I’m usually not sitting still in the prime mid- day charging hours.

Stiff backing material. By far the stiffest lightest sheet material I can make is carbon fiber(CF) and epoxy. It’s easy to work with and you can make flat sheets just by laying the carbon fiber material on saran wrap over something flat.  (I’ve used 6 ounce because that’s what I bought for my kayak).  Squeegee the excess epoxy off. Because you just need it for stiffness, you can squeegee out most of the epoxy and leave the warp and weave texture of the CF, just make sure no uncoated fabric. I’m pretty sure I could easily make a clipboard that was about 1/16″ thick. Sand the edges.  Also if you put some ridges in your panels you can make them stiff and even lighter. Just lay string or wire underneath the saran wrap and press down on the top with a foam panel.

A lot of sources say you can increase solar panel output by using reflectors. 30% seems a common number. Maybe try some aluminum foil or mylar space blanket material.

I think a great backpacking solar panel is one that you could hang on your pack or self while hiking without it being too cumbersome.