What Solar panel voltage, Supercapacitor, & Diodes needed for Nano IoT33, ESP32

I was provided this link in another post to Nick Gammon's site for powering a bare bones Atmel 328p via a solar panel and supercapacitor.


I've made a spreadsheet in LibreOffice to help with calculations
if let's say a more new MCU is used that uses 3.3V.

The two I have in mind along with some ball park figures of regular current consumption:
(I found out via ESP32 forum that when using Wifi or ESP_NOW the current actually peaks up to 500 mA during initial packet startup/transmission.)

Arduino Nano 33 IoT ~ 160 mA (WiFi + blinking led + IMU in use)

ESP32-PICO-KIT ~ 240 mA

I'm will try to deep sleep them (will have to practice some more coding) but for now just curious to see what kind of supercapacitors can I use to power them all day and night before the sun rises again.
So trying to avoid the use of batteries.

My Libreoffice spread sheet can't be attached so here is a link to it.
Solar Panel Charging Super Capacitor and Powering MCU Calculations, R1

The screenshot below is a verification of Nick's calculation to show that the sheet is calculating things properly.

So here are a few things I'm wondering about:

Why are there 2 diodes being used?


A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" when a certain set reverse voltage, known as the Zener voltage, is reached

If this is the case why is the Zener diode being used?
I've read that a Zener diode can be used for voltage regulation is this why it's being used?

I noticed both the diodes have a forward voltage drop of (1.1V and 1V) - based on specs. I found online
So shouldn't the final voltage to the capacitor be 4V if it was a fully sunny day and the solar panel is at 6V?

Are my numbers right regarding 220 ohm resistor and current limitation?
6V/220ohm = .0273 A
What determines an appropriate current to be sent to the capacitor?

I'm thinking of using a 2.7V supercapacitor, since a lot of them at this voltage are low cost and come with high Farad capacity values. There are also other low cost ones near 2.5V and 3V.
Digikey has great filters to single them out.
Digikey filter: +50F supercapacitors, ROHS compliant, active parts, starting at ~$2.50, most near 2.5V,2.7V,3V ratings

From here I thought about using this solar panel from Sunpower:
0.6V, 3.6 Watts, 6A, Sunpower 22%+ efficiency
125x125 mm area

I will have to boost voltage 2 times.

  1. From 0.6V or less (depending on sun) to ~2.7V or slightly less (for super capacitor)
  2. Boost 2.7V to 3.3V to run MCU.

There are modules I've found from TI and Analog devices that do this, but they are limited by input voltage and current requirements. Pololu has some voltage boosters too but same limitations.

Is there a simpler circuit I can use on a breadboard?
Will there be significant losses in efficiency?

Are there diodes with lower voltages similar to the circuit above?
How do I determine their values?

How do I determine the low end of the voltage range for the 2.7V supercapacitor for energy calcs?
Do I just use 0 and let it drain?

I'll keep searching Digikey, but I've read too low of a forward voltage can't be a good thing sometimes due to back emf I think???

So with respect to the Sunpower 0.6V solar panel/3.6W (so 6 amps output)
This is the smallest zener diode I could find:

Zener diode, digikey, less <= 1V

Although I don't think it'll be able to handle the power, since they are rated to 500 mW (0.5 W).

I managed to find this:
PS61022 8-A boost converter with 0.5-V ultra-low input voltage

But I wanted to avoid making a circuit and need to figure out how to hook this up.