Hi everyone,

I'm currently a Master's student in electronics engineering, and I’m doing an internship within an association promoting sustainable mobility. My project is to design a system that generates and converts electricity from a bicycle, for educational and awareness purposes.

I don't have any technical supervision during this internship, so I’d rather ask here, rather than make a poor design decision, especially since I’m working with supercapacitors, which I know can be dangerous.

I am quite concerned about the way I've managed the supercapacitors, and I do not know if the protection I've added are enough or not.

For the energy storage I've planned to use 12 supercapacitors of 2.7 V / 500 F, which I often see in diy projects. I don’t fully understand why theses values are choosen most of the time, so I'm open to suggestions if better values exist for my system.

My idea was to use an esp32 to send data via Bluetooth, and then display them in a mobile app and allow user to control the outputs

Any feedback would be a huge help, so if anything in the schematic seems off or incorrect, please don't hesitate to let me know !

Thanks a lot in advance

 

Firstly, welcome to AAC!
Secondly, this comes under the heading of 'homework help' and the moderators are likely to move it there. As it is 'homework help' we cannot give you the answers, only guidance.

For the record I supervise a number of MSc student projects IRL, so happy to do so here.

In principle what you describe is 'do-able' but the devil, as always, is in the detail. What's missing from your project is what you expect to achieve ie your outcome in technical terms. For example, what power output and for how long?
If I understand your diagram correctly, you plan to take human effort on a static bike and generate 'mains' electricity, with some intermediate storage in super-capacitors. Have you run some numbers on this? If so, how about sharing them? If not, this would be a good first step.

Your ideas around 'protection' need some work, but that's secondary right now.

 

BTW, the 500F @ 2.7v comes from, AFAIK, approx the same storage capacity as a CR2032 coin cell, about 400mWh. Between 2.7 and 1v the capacity is approx 500 * (2.7^2 - 1^2) /2 = 1573J -> 1573/3600 = 0.43Wh

You may find this useful: https://www.we-online.com/catalog/media/o157040v410 AppNote_SN009_EN.pdf

 

E = 1/2 C V^2 1/2 x 500 x 3.7^2 = 3422 J

An AA cell holds about 14000 J.

So 12 of them can store less energy than 3 AA cells. About 11.6 Wh. I calculate 11.4 for your supercaps.

A person on an exercise bike can produce about 100W without too much sweat, so at say, 50% efficiency, they could charge it in about 14 minutes.

With 80% efficiency for the inverter, it could then operate a 9W LED bulb for an hour.

Is that the kind of demo you envision?

 

@BobTPH Max voltage of supercap is 2.7 not 3.7, so 1822J

Re demo, I was sort of hoping the TS would get to that conclusion themselves.... since this is technically 'homework help'

 

Adding to the overall losses, supercapacitors connected in series require a balancing circuit.
Said balancing circuit is not optional, but mandatory.

 

@BobTPH Max voltage of supercap is 2.7 not 3.7, so 1822J

Re demo, I was sort of hoping the TS would get to that conclusion themselves.... since this is technically 'homework help'

Yeah, I see that now. So it is actually only 2.7^2 / 3.7^2 = 53% of what I calculated.

Sorry, I didn’t think the back of envelope calculation was giving too much info, but I see your point. It is the first thing he should have done. I honestly do not have an opinion on whether that is what he expected.

 

Adding to the overall losses, supercapacitors connected in series require a balancing circuit.
Said balancing circuit is not optional, but mandatory.

True, but lets get past a few fundamentals first...