I really like this design, however I think it has a problem:
According to the datasheet the TPS610986DSET has no feature to disconnect the load at low battery voltages. I think the result will be that e.g. a ESP32 or other power hungry chips will not power up properly. The TPS63031 might therefore be a better fit.
btw.: Do you still sell assembled versions of this design? Would you be willing to create a new version with the TPS63031 and probably also with 10 or 20F capacitors or an board to solder your own capacitors.
@NeverDie Use additional battery is an interesting idea. My only concern is that the harvester IC will drain it if connected to input. On the other hand it looks like the IC will work just fine as long the STORE has some voltage. And the STORE is connected to battery, which is charged by the harvester IC - i.e. the rechargeable battery is exactly the battery used to prevent IC from shutting down. But perhaps I missed something - still have no boards for test.
Regarding solar cells from a calculator - yeah, that was my first intent, but have no solar calculator out there. Taking in account that such a calculator costs three times less than this SolarBIT thing I have no excuse for not to buy one from a local store - going to fix that tomorrow
Started designing the enclosure for 3d printing ... I will use PET-G as it's pretty strong and not very flammable or so it seems after some tests. I don't know about electrical insulation properties yet ...This all very depends on what other compounds are used when it's manufactured.