I’ve always been fascinated by solar. The idea of getting free and unlimited energy from the sun is just plain cool! The problem with current commercially-available solar panels is their weight. They just aren’t light enough to fit into an ultralight backpacker’s pack. To my knowledge, there just aren’t any truly light weight options out there (with any real wattage at least).
I’ve been playing around with MYOG solar panels over the past year and wanted to share some of my findings. I’ve built half a dozen panels out of different materials, using varying techniques and have learned quite a bit. I’ve honestly had mostly failures, but have finally arrived at what I think is a very promising ultralight solution.
Pretty much all solar panels are made up of the same components: solar cells, a stiff backing material, a plastic cover to protect the cells, wire and a usb power module. The heaviest component here is typically the backing material. Regular solar cells are super fragile and will shatter apart when bent/stressed. This is why very stiff (and heavy) materials must be used.
I stumbled across Sunpower solar cells after doing a bit of research last year. Sunpower cells are unique in that they run all of the bussing and power contacts on the back side of the cell. This does a few things. It allows the cell to produce more power at a physically smaller size which is cool, but more importantly, the copper backing prevents the cells from shattering apart. You can bend a Sunpower cell in half and it will still continue to produce power. Watch the first three minutes of this demo video to see what I mean.
Using Sunpower cells is the key to building an ultralight solar panel as we can now use lighter materials for the backing. I’ve tried unsuccessfully to use cardboard, balsa wood and carbon fiber sheets for backing. These materials are either not waterproof, too heavy and/or too expensive.
I’ve settled on using Depron foam which has been working great. Depron is commonly used in the hobby industry to build things such as RC airplanes, etc. It has a very tight cell structure, is waterproof, and is super light weight. It comes in a variety of thicknesses (1mm, 2mm, 3mm, 6mm).
The next heaviest component of a solar panel is typically the plastic covering that sits on top of the cells. This helps keep the cells free of dirt and moisture. Instead of using heavy plastics, glass or EVA film, I’ve settled on using heavy duty freezer wrap (Freeze-Tite). This is similar to plastic wrap but a lot tougher.
A solar panel is typically made up of multiple “modules” or groupings of solar cells. For example, in the commercial panel below, there are four modules.
The graphic below shows the weights of the various parts that make up each of my modules.
Each 5” x 5” module contains a full 3.5 watt Sunpower cell. This cell can be cut into smaller pieces depending on the size and output of the panel we’re making. I don’t want to get too technical on the electrical specs right now, but the goal is to get to around 7 volts with our panel. We then use a step-down module to bring the volts down closer to 5 which falls within the USB specification. Each solar cell is rated at 0.58 volts so if we run 12 of these together in a series, we’ll end up around 7 volts total. Read more about serial and parallel wiring here if you’re not familiar with it.
So with the goal of having our solar cells cut into 12 pieces, we can assemble varying sizes of solar panels such as the configurations below. Each module weighs approximately 0.41 oz, and each solar panel is assumed to have a single USB output. Additional USB outputs can be added at a cost of 0.14 oz each.
An important note on wattage. Solar panels are typically rated by the maximum potential wattage of their cells under ideal sun conditions. In our case, each whole 5” x 5” Sunpower cell should be able to produce around 3.5 watts. Unfortunately, in real word conditions we get much less final power output coming into our USB-powered devices when all is said and done. This is due to a variety of reasons, some of which include:
- Solar cell not facing directly towards the sun. This could be due to poor panel aiming and low sun angle in the sky which is common in shoulder seasons.
- Hazy/cloudy skies will reduce the strength of the available light
- Putting anything over a raw solar cell, even something as thin as Freeze-Tite, will block/reflect part of the light. I estimate a 5% loss when using Freeze-Tite.
- The kerf of the blade when cutting cells removes power-producing material! I estimate 2% loss (per cut) when using a diamond cutting disc.
- Energy loss during the step-up or step-down process in the USB power module. This is the biggest cause of energy loss.
We have rainy skies here in Portland this week so I haven’t had much of a chance to test my latest panel, but in general, you can assume to get around 50-60% of the rated wattage under good sun conditions. So if the panel is rated for 21 watts, assume you’ll actually be getting around 10-12 watts of usable power. Fortunately this is still plenty of power to work with to charge things like mobile phones in the back country. Most wall chargers for mobile phones charge at 5 watts (1 amp).
Finished 21 watt solar panel (2.75 oz)
Two usb power modules illuminate when there’s enough light.
Sunpower solar cell cut in half. The foam is cut a little wider, a touch fatter than 5”.
Each individual module is about 3.5mm thick so all together it’s about 21mm or 13/16” thick with six modules in this solar panel.
Final weight of completed solar panel is about 2.75 oz. (78 grams). The panel weighed 2.6 oz before adding a second usb module.
The USB step-down power module can easily handle 2 amps of output (10 watts) and works with Apple products. Plastic milk carton cap was cut down and used to cover the module.
Photo below shows the depron after being cut. There are six 2mm sheets and six 1mm sheets. Foam for each module averages just under 0.1 oz each.
Radiused-corners were sanded down by hand (mainly for aesthetics). A drywall sanding brush worked really well. I used a tupperware lid as a guide. Depron sands really nicely.
Make-shift dremel table with diamond disc used to cut solar cells and depron.
Solar cells after being cut in half. Six full cells would normally weigh about 1.5oz, but with the blade kerf removing material, we’re a bit lighter now.
Cells with wire soldiered on. I used solid 26awg copper wire.
Freeze-Tite wrap being taped on to protect the front of the solar cells. I had some 3M 9485 double-sided tape laying around from some past cuben projects and it worked great (this is the tape Zpacks sells). My tape was 1/2” wide so I cut it in half for 1/4” strips. The wrapping process is a little tricky. You want to get the wrap tight but not pull too hard as to crack the solar cells. I had a hard time getting the wrap totally taught which gave me a few small wrinkles. This is purely an aesthetics thing though and doesn’t affect operation.
Modules after being wrapped with Freeze-Tite. The wrap got a little poofy around the radiused-corners so I used some cuben tape to pull it back.
A strip of cuben fiber is used as a hinge that holds the two modules together. The yellow wire is super flexible stranded wire that works well for the constant opening and closing of the panel. Regular solid/stranded wire is brittle and will break with only a few folds back and forth. This is 26awg wire that has 64 strands of 44 gauge wire. Running the wire in an “S” configuration helps mitigate the bending force on the wire.
Cutting out holes for the two USB modules.
Anyway, I think that’s about it. I hope this info is useful to someone. If there’s interest, I can put together a materials list. Let me know.
Help needed from the community:
- Ideas for stronger/lighter materials
- More efficient voltage/USB module combinations
- Cheaper alternatives to any materials
- Cleaner ways to cut the cells (laser cut?)
Thanks!
Ryan
–[UPDATE]–
Materials List
Sunpower solar cells:
I’ve purchased most of my cells from TG Solar off of eBay. He ships priority mail out of Ohio in the US which means you can get the product in just a few days. He only sells ten packs though. There are other Sunpower cells for sell on ebay from China or elsewhere that will sell individual cells for a little more per unit. Definitely buy one or two extra though in case you mess one up while cutting.
Cost: $4.50 per cell
Sources:
TG Solar on eBay
Other eBay
Depron foam:
I’ve purchase all of my depron online through rcfoam.com. RC Foam is awesome except for their shipping is crazy expensive. They have to carefully wrap the product and ship it in large boxes so shipping ends up being like $13+. It’s hard to get the depron to your door step for less than $20. I think there are a few suppliers that ship out of the UK that might be cheaper if you live in that part of the world. I have not used them, but eBay also has some pre-cut depron available. This might be a cheaper route to go depending on how much you need. 1mm depron is more expensive than 2mm. If you want to save money and have a stronger panel, you can just buy 2mm depron and use it for both depron layers. I mess up half the time when cutting depron. Buy more than you need.
Cost: $15-25
Sources:
RC Foam
eBay
USB power module:
This is hands-down the best module I’ve found and it happens to be the smallest and lightest too. I’ve only found these shipped from China. Usually takes 7-10 days to reach me in Portland, OR.
Cost: $3.12
Sources:
eBay
Freeze-Tite:
I have not experimented with other heavy duty plastic wraps, but this could easily be substituted if you can find something cheaper.
Cost: $10.74
Sources:
Amazon
26awg solid wire:
I typically use solid copper wire for soldering onto the solar cells. Stranded wire if fine too if you have some laying around. If you’re doing a small panel, you can probably get away with even 28awg.
Cost: $2.73
Sources:
eBay
26awg super flexible stranded wire:
This is the yellow wire I used to connect each module. It’s CRITICAL that you use super flexible wire for this application. Regular wire is not designed to be bent back and forth thousands of times and will eventually break leaving your panel dead in the water. The only source for this wire that I have found is this random dude online. I was a little hesitant, but paypal’d him $9 and he shipped it out next day. $9 is a lot for what it is, but I don’t currently have a cheaper source. Please share if you have a better place to buy wire. Just make sure it’s similar with 64 strands of 44 gauge wire.
Cost: $9
Sources:
Random dude
Double stick tape:
3M 9485 double stick tape. This tape is really expensive, so if you don’t already have some laying around, you’re probably better off finding a cheaper alternative.
Cost: $28.95
Sources:
ZPacks
Cuben fiber:
0.51oz or 0.74oz cuben used for the hinge between each module and also for cuben tape. Again, this is really expensive and some other material could be substituted if you don’t already have some on hand.
Cost: $29.95
Sources:
ZPacks
Foam Tac glue:
I used this glue to attach the solar cells to the depron. It goes on wet and has a minute or two of working time which allows you to slide the cell around and get it centered nicely. I haven’t tried it, but regular silicone from Home Depot would probably work just as well and be much cheaper.
Cost: $3.95-$12.95
Sources:
eBay
RC Foam
Home Depot
Spray adhesive:
I used 3M Super 77 for gluing on the final depron panel. I think the Scotch brand is probably the same thing and is cheaper for a small can or maybe you can find cheaper locally. Other general spray adhesives may work fine too. Do some testing on scrap pieces to make sure the adhesive doesn’t eat the foam. 3M Super 90 will not work!
Cost: $6.88
Sources:
Amazon
Voltage Tester:
To accurately see how much power your panel is producing, you’ll need something to test it with.
Cost: $6.95
Sources:
eBay
Misc. items needed:
Hole punch
Solder/flux
Soldering iron
Disposable rubber gloves
Multi-meter
Milk jug cap or other plastic bottle cap
Exacto/utility knife
Dremel/rotary tool with diamond disc (check Amazon for discs)
Sand paper
