Foldable 50 W ultra-portable solar system for $150

all future updates for this project will probably be posted here: https://hackaday.io/project/5623-ultra-portable-sustainable-electric-generator

I put two blue led voltmeter displays on the voltage converter board and a fan on the input. It seems to run pretty cool (at least without a load). More pics soon.

I finally found a reasonably priced source. Who would have guessed it - McMaster Carr has something at a comparatively reasonable price, for once.

I got a combination buck / boost DC-DC converter for $13 shipped on ebay.

It might need a cooling fan, we'll see. I will use it to convert the 15-20V from the panel to 12V to charge a standard 12V battery, or to charge a 18V or 19V laptop (there will be a way to switch between the two modes and connect the battery or the laptop. If the output is not stable and goes over 18 or 19 V, I will add an 18 - 20 V adjustable regulator circuit to the output to protect the laptop. When the main converter is in 18-19V mode, I will presumably be using it with a laptop, so I can charge USB devices from the powered ports.

When charging a 12V battery, I'll use a $4 12V-to-5V module to charge USB devices from the battery.

I'll also put $2 LED voltmeters on everything.

With $49 spent on the cells, I still need the following to finish the panel:

• about 2.5 m^2 EVA (STR 15420P) encapsulant (ebay)
• about 1.25 m^2 of TPT (Tedlar-polysomething-Tedlar) backing (ebay)
• about 1.25 m^2 of fluoropolymer (or other suitable) front sheet (ETFE, PFA, FEP, etc)

This should also be done for under about $150 total, since otherwise I might as well just buy a UniSolar 68 W panel which uses the same cells and ETFE coating, for under $200. However that panel only rolls up (does not fold) and probably weighs about 10 pounds alone. The purpose of the encapsulant film is to keep moisture out of the cells. The front and back films also help seal oxygen and moisture out.

Research on fluoropolymer front sheets (the most expensive sheet part AFAIK):

• non-cementable FEP is probably the best.(http://www2.dupont.com/Teflon_Industrial/en_US/tech_info/techinfo_compare.html)
• I can get it at http://www.durafilmshop.com/teflon-FEP-film-500A-p/500a.htm for fairly cheap, if I can convince them to waive the $60 spooling fee.
• Tefzel ETFE scraps on ebay are way overpriced

to complete the project, I also need:

• power controller or DC/DC converters, to output 5V 1.0A (USB), 5V 2.0A (USB), 12V, 19 V for charging 18 and 19V laptop
• small battery or supercapacitor to smooth out power output?

I learned that you need the right encapsulating polymers or your solar cells will fail after a year or two or three instead of lasting 20 or 30 or even 50 years. The wrong materials will corrode your cells, introduce water vapor, acetic acid, etc. which will kill your cells. It's also important to seal each panel or module properly to make sure there's no water vapor that gets trapped inside. Also, I decided that it doesn't make sense to use super brittle, non-flexible solar cells in a foldable, portable device for backpacking. That left me with CIGS or amorphous Si cells; CIGS cells were very attractive but the necessary sealing films are very hard to find in low quantity and CIGS cells are very sensitive to moisture. So, I ordered some a-Si cells and some EVA encapsulating film.

semi-flexible thin-film solar cells (CIGS, CdTe, etc):

• - cheap-ish option ($0.65/W at 50 W scale) with no bus wire needed
• - pretty damage resistant (there are videos on YT of CIGS cells taking bullets in every single cell and continuing to function at 90+% power....)
• - SUPER LIGHT WEIGHT (about half a pound for 50 W) - If I get my hands on the right encapsulating materials, I will have to grab some of these in the future.
• - take up a lot of room because they are somewhat inefficient (around 10-11%)
• - example product: 25 pieces NanoSolar "NanoCell" 2.6 W CIGS cells sold on ebay for $43 total shipped for a total of 65 W according to spec (since they "may vary slightly from stated specs" as they are liquidation surplus - and I saw one complaint saying the power was off). It has two contacts on the back, so no bus wire is needed, but the aluminum contacts may be a bit tricky to solder to.)
• - degrades a little bit faster (about 1.0% decrease in power/year)
• - not compatible with EVA sealing film! (need PVB film or ionomer products from Dupont such as PV5412 or maybe even UV-resistant Surlyn - these seem hard to find at low quantity!)
• - very sensitive to the use of the wrong encapsulating material!
• - very sensitive to moisture!

semi-flexible amorphous Si:

• - slightly higher cost?: about $0.8/Watt (at 50 W scale)
• - somewhat damage resistant; flexible
• - somewhat light-weight
• - take up more room because they are pretty inefficient (around 6-7%)
• - example product: 10 pieces Uni-Solar L-Strip 6.2 W - ebay - $50 shipped (I got this)
• - non-standard cell voltage, but I can deal with that no problem
• - degrade a little bit faster (about 0.9% decrease in power/year)
• - you can cut them between the lines, parallel to them, but you'll need to solder new contacts on
• - can encapsulate with well known EVA material which is field-proven to last at least 20-30 years when applied correctly

(poly- or) mono-crystalline Si:

• - low cost: about $0.7/Watt (at 50 W scale)
• - most efficient (17-22%)
• - somewhat light weight?
• - easily damaged/cracked/broken (this is just the wrong application!!)
• - degrade the slowest (mono - about 0.4%/yr, poly - about 0.6%/yr; use mono Si if you are building a residential or commercial solar power system...)
• - uses standard .5 V cell voltage, but this can be a disadvantage if you want to get the 18 to 20 V required to charge most laptops from fewer than 40 or 50 solar cells
• - require soldering bus wire to front & back – requires bus wire, flux pen, solder, & labor!
• - example product: 20 pieces 5"x5" 2.6 W mono solar cells from China for $35 shipped, PLUS a bus wire, flux pen, and solder kit for $10-20
• - can encapsulate with EVA

I decided to go for the flexible amorphous silicon cells, since they will probably last much longer than the CIGS cells, and because a compatible encapsulating material (EVA) is available for them.

The reason I need to encapsulate the cells in EVA is that it is non-corrosive, UV-resistant, flexible, clear, non-scratch, anti-reflective, and thus seals everything from the elements. The basics of the process are that you melt it on while pulling a vacuum on the cells to eliminate any air bubbles, but I will have to learn.

Once I've wired together the cells and encapsulated them, I will sew/glue/melt the excess encapsulating film around the border of each cell into the fabric that I will use for the backing of the device. Then I will fold it all up and stick it in a rigid box and it will basically be finished. Oh and I need a power controller to get 18/19V (laptop), 12V (lots of things run on 12V), and 5V (USB).

If the a-Si cells won't give me 50 W, I'll either get more of the same cells or start over with either CIGS or mono-Si cells.

How to calculate power output:

The maximum (ideal) solar flux at earth's surface is 1050 W / m^2, but there are seasonal effects that decrease this, and it's not ideal when the sun is not at the zenith, and there are effects related to being at about 40ºN where it is always cloudy...

degradation data - http://www.nrel.gov/docs/fy12osti/51664.pdf

https://www1.eere.energy.gov/solar/pdfs/pvmrw2011_32_tf_coyle.pdf

insolation info - http://www.mpoweruk.com/solar_power.htm

research efficiency (not matched by commercial modules) - http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

dupont presentation on encapsulants: http://www2.dupont.com/Photovoltaics/ko_KR/assets/downloads/pdf/presentations/06_DPVS_Latest_developments_thin-film_JIKim.pdf

more nrel on encapsulants: http://www.nrel.gov/docs/fy09osti/44666.pdf

dupont on front sheets: http://www2.dupont.com/Teflon_Industrial/en_US/assets/downloads/k15778.pdf