Wow, the solar super capacitor system works beyond expectations.
Today is my first outdoor test of the system. It is an overcast winter day with an occasional appearance by the sun. You can see there isn't much shadow, indicating the sun is behind clouds.
The capacitors started off at 3405 mV, which is close to the LDO dropout voltage.
I figured I would just drop the Hammond box out in the snow to see what happens.
I was thinking to check in on the capacitor voltage every couple of hours to see if they are actually charging.
I need the capacitors to charge about 350 mV during the day to ensure the device will run all night.
When I checked the voltage after the first 2 hours the capacitors were already fully charged (over 5700 mV) and the capacitor overvoltage protection circuit was limiting further charging.
This proves that the solar cell and super capacitors are easily adequate and in fact overkill for this configuration.
So I brought the box in and altered the firmware to keep Bluetooth on 100% and updating temperature every couple of seconds whenever the system is charging. This was mainly to provide a some activity in the video. Ultimately the "Bluetooth on" duty cycle should be proportional to the current from the solar cell.
The video below was shot after I put the system back out and the temperature was falling to get back to the snow temperature. Once it reaches steady state there isn't much to see.
Near the end you can see the Hammond box in the snow through the window at more or less the same time as the android display which is inside. You may be able to see shadows in the snow which would indicate this was at a time when the sun was out. The snow was so bright, the camera would have trouble showing both the display and the box at exactly the same time. You may also see that some snow has blown onto the clear lid, but this did not have much effect on the charge rate.
I haven't decided where the system will be mounted yet. I could put it on one of the tables on my deck (see below), but as you can see the snow is deeper than my boots, so the firmware needs to be truly finalized before I wade out there.
Conclusions & Discussion
I am really stoked that this project exceeded expectations in being able to run a computer 24/7 without a battery or mains power supply. I am very happy I took the plunge and invested the time, effort and money to build a very slick platform that can not only implement this weather station, but can be used to experiment with other solar applications. It is pretty cool that a low-cost solar cell of this size can generate enough energy to run a microcontroller all day.
I think the solar cell and clear lid have so much excess capacity, they can be mounted vertically or almost vertical, which would prevent precipitation from accumulating on the surface, which translates to far less maintenance (cleaning). Additionally in the winter when the days are shorter and the sun is at a low angle and there is snow on the ground, snow would actually reflect extra sunlight onto a vertical solar cell, increasing its output precisely when it is needed most. It conjures up the possibility of fashioning a parabolic dish out of snow .... but it wouldn't really be needed.
The Hammond case is an ideal enclosure for this project, with its clear lid and weatherproof capability, but it is also instrumental in making the system a general platform for other projects. It does this in 2 ways, one is by having 8 threaded metal inserts that allow the lid and chassis to be repeatably and easily taken apart. The other is the rubber gasket, which means the system avoids using glue or calking to seal out moisture. Taking apart a glued case is difficult, messy, and probably compromises the seal, so the gasket is a tremendous feature. In enclosures that aren't designed to accommodate a gasket, it is usually not feasible to add one later, not to mention how hard it would be to find a gasket of the correct size. Having the gasket in a clear lid meant I did not have to worry about how good my seal was or go through some possibly destructive testing to determine if the seals worked. And even if an immersion test was called for, the case would not need to be disassembled to prove if the seal held.
I hope everyone understands that these Hammond enclosures are as durable as they come, because my objective is not to try to destroy them to prove they are durable. I have used Hammond enclosures for decades in all sorts of torturous environments and never had any issues with durability. In this project I tossed the enclosure into the snow, confident that it would not suffer any damage or experience moisture ingress in this hostile environment, and indeed it is still in pristine condition.
Using this durable good-looking enclosure transformed this project from being a messy problematic struggle of an experiment to being an attractive project with mechanical elegance that cleanly demonstrates the concept being investigated. It is not always obvious how much struggle and effort is saved by having an enclosure that does such a good job of meeting all requirements. I really hope the many benefits of using this Hammond enclosure come across in the blog because those benefits did play an important role, and demonstrating the utility of these enclosures is an important objective of this whole design challenge.
Now that the fundamental battery-less solar power concept has been proven and is understood, it opens up the prospect of future projects. One possibility is to use LoRa to get the data back to an MQTT server, but that will have to wait until I can figure out a way to economically implement a LoRa gateway. Bluetooth low energy is another option, but that will have to wait until I have a BLE tablet.
Although I did not need all the components included in the kit to implement this project, I will definitely find good uses for them in future projects.
I would like to thank Hammond Manufacturing and element 14 for the opportunity to participate in this design challenge. It has provided me with an excuse to try this solar project and the chance to learn what is involved in both solar cell technology and super capacitor technology and to re-acquaint myself with the benefits of Hammond enclosures.
Relevant Links
Just Encase Solar Super Capacitor Experiment - Blog 1
Just Encase Solar Super Capacitor - Project Outline - Blog 2
Just Encase Super Capacitor Solar Weather Station - Design - Blog 3
Just Encase Super Capacitor Solar Weather Station - Assembly - Blog 4
Just Encase Super Capacitor Solar Weather Station - Bluetooth - Blog 5
Just Encase Super Capacitor Solar Weather Station - Indoor Operation - Blog 6
Just Encase Solar Super Capacitor Weather Station - Outdoor Tests - Blog 7
Just Encase Design Challenge
Tube Amp using Hammond transformers
GPS location and distance tracking using LoRa on a MKR WAN 1300
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