The idea for the Solar Umbrella Cover came to me while I was sitting at Starbucks under one of their bistro umbrellas (checking my email) and I noticed my phone battery was a little low. I though, "Wouldn't it be great if I could top off my phone while I'm sitting here?" And since I've been a solar enthusiast and tinkerer for years, my thoughts went straight to solar.
My idea was to build a "solar cover" for bistro umbrellas. It would consist of an array of round solar cells wired together to provide power for charging mobile devices. The cover could be placed on top of existing umbrellas. A cable (which contains multiple charging ports) could be routed with clips from the edge of the umbrella, up underneath, and then down the shaft where users could plug in their devices.
This Instructable describes how I built a working prototype for this invention.
Step 1: Design
I thought 15W output would be a good target for my first prototype. That should be enough power to charge two phones at the same time. After looking around on-line for round solar cells, I found some 4" round cells rated at 4V x 80mA. If I wired 45 of those cells in parallel, it would give me approximately 14-15W of solar power.
I decided to wire the cells together in a "net" pattern of concentric circles. That seemed to be the best way to ensure it would lay flat on top of an umbrella. Wiring them in parallel should allow them to provide a constant voltage even if some cells are shaded or not in direct sunlight. Of course the current would be reduced, but at least they will continue to charge the devices.
The biggest challenge was to wire the cells together in parallel while maintaining a matrix pattern that would also physically support the array. I tried lots of different strategies and finally settled on the one pictured.
Since my voltage output would only be 4V and USB requires 5V, I needed a booster circuit. I also wanted to include 2 USB ports for charging multiple devices. I searched the web for a suitable circuit board and I settled on a Dual USB Charging Circuit from Brown Dog Gadgets (see my Materials page).
This circuit takes voltages as low as 2.5V and boots it up to 5V. The
circuit can provide a max power of 3A (charged between the USB ports). Works with most USB gadgets including iPhones.
Step 2: Materials
45 - 4" round solar cells rated at 4V x 80mA (higher wattage cells would be even better)
1 - Duel USB Charging Circuit from BrownDogGadgets http://www.browndoggadgets.com/collections/charge-...
1 - Radio Shack Enclosure Box https://www.radioshack.com/products/radioshack-pro...
1 - Roll of 16 Gauge stranded hookup wire
8" - 2-conductor, 16 Gauge speaker wire
2 - cans of spray on rubber coating (Plasti Dip)
Step 3: Assembly
- I laid out the cells in accordance with my design and numbered each cell with a sticky label. That made it easier to follow my wiring diagram.
- I soldered the cells together using the 16 gauge wire. I tried to keep a consistent distance between cells.
- Once the cells were all soldered together, I soldered a length of speaker wire across one of the cells to feed the charging station.
- In an effort to keep the wiring from coming loose and also make the assembly more sturdy, I sprayed the backs of the cells with several coats of Plasti Dip.
- To assemble the charging station, I first cut a notch in the enclosure box. Then, I hot glued the charging circuit board in and soldered connectors to the input terminals. I used a rubber grommet to run the speaker wire into the enclosure box.
Step 4: Testing and Deployment
Next I took the wired matrix of solar cells out to the backyard for testing. It was a perfect sunny day and my results were awesome!
In direct sunlight, I got 4.34V x 3.48A = 15.1W of power!
Now, I was ready to try out the Solar Umbrella Cover in it's intended environment. So, I deployed it on top of a bistro umbrella in my backyard. I ran the speaker wire from the edge of the umbrella, across one of the blades, to the center. I secured the cable using wraparound Velcro strips. Then, I ran the cable down the shaft of the umbrella to about the right height from the ground and secured the cable again with Velcro straps.
I setup a comfortable sitting area underneath the umbrella and connected some devices. And sure enough they started charging! We tried a couple of iPhones as well as Android phones. Everything worked great until I plugged in an iPad. It sucked up so much power that the phones couldn't charge at the same time.
All in all, I consider it a great success!
Step 5: Conclusions and Challenges
The Solar Umbrella Cover was a great success. I took it from an idea while sitting at Starbucks to an actual working device. I enjoyed the process from design to deployment.
But although I'm very happy with the results, there are still several challenges I would like to continue to work on and maybe turn it into an actual product.
1. I was able to charge two phones concurrently, but larger devices like iPads could only charge one at a time. I plan to use higher wattage cells and probably more cells on my next prototype. I'm going to shoot for at least twice the power on my next try.
2. The device works great in direct sunlight, but when some or all of the cells get shaded by a cloud passing over for example, the current drops and some devices stop charging. I plan to add a battery backup circuit to my next prototype to keep things charging during temporary loss of sunlight.
3. The device is really portable. You can basically ball it up and carry it around in a shopping bag, which means it could be used for more than just bistro umbrellas. I'm thinking the beach, camping, or any other outdoor venue. One problem is, once you bunch it up, it tends to get tangled and can be difficult to deploy. I plan to work on the design some more and try to improve that aspect.
4. When the device is deployed on a bistro umbrella, all is fine until you close the umbrella. Some umbrellas are pointy at the top and some (like mine) have a plastic disc. On umbrellas with pointy tops, I need some way to attach the top of the Solar Cover so it doesn't just slide down to the ground. On all umbrellas, I need to figure out some way to keep the device in place so it doesn't get tangled when the umbrella is reopened.
If anyone has ideas or advice for any of these issues, I would greatly appreciate your input.