This isn’t free energy.
Harvesting byproducts very slowly can power a microcontroller and sensor wakes from deep sleep just to take a reading and broadcast the results. I’ve looked into it but these are mostly just placeholders for now. If you can charge up a supercapacitor then it can provide power for days, we’d just be filling it up as it was used. These are all big power, small power we are talking about ambient body heat, and the movement of your wrist powering mechanical watches.
I learned using TI, but they didn’t have an easy wireless solution. Their MSP432 has 95uA/MHz and 850nA when in deep sleep forgetting everything. Possibly having one WiFi base with an RF receiver and have sensor nodes using low power RF to talk to it. MPPT maximum point power tracking is a thing to know.
Mechanical: think car or bridge vibrational.
Heat: Doubtful you’re using all 360º of a fire, Peltier cells are the most basic of this. Apply energy and get one side hot and the other cold, apply hot and cold and you get energy.
Light: Ambient solar
Low power modes can be used in everything so it would be good to make it a best practice sooner rather than later.