A capacitor is from the energy harvesting point of view is a “sort of” battery. It can store energy, that is current at a voltage – because energy = current * voltage. A battery can produce energy, the capacitor can only store energy. But a rechargeable battery seems indeed like a capacitor. Nowadays the “normal” battery has far more power than the biggest “normal” capacitors, but supercaps are developed which will bridge the gap.
A capacitor is not letting current through in a static DC case, it does in the AC – alternating current case. But in the limiting case of starting/ending a DC current it will allow “some current” to flow for a short time.
In normal electronics a capacitor has many other functions, like a filter – for example it stands over the plus and the minus (not being a shortcut) and it is like a reservoir of water where water can flow in when there is temporarily a bit too much and flow out if there is a bit too little. Like this it will prevent peaks in voltage. Peaks in voltage are occuring when you turn a switch on. Peaks are not really appreciated by your chips, so a cap can protect against that.
Together with a coil component interesting things can happen: the coil/capacitor act together like a “bouncing spring” for current. This can become a frequency for a transmitter or a boost for Voltage, together with some smart switching. This last one becomes our “boost” circuit, which is used in the solar harvesting chip ltc3105.
A capacitor comes in a wide range of values in pico, micro and milli Fahrad. That means that we only use very small values of what was once “the unit” a Fahrad. Nowadays with the supercap this Fahrad unit is suddenly becoming available and common again.
For the piezo charging you typically charge a smaller capacitor, because the piezo is only producing small energies. If you insert a supercap for storing the charge you won’t see much of an effect of piezo pinching.
Charging the capacitor is one thing, then you have energy. But then how to do something with this energy? How to discharge decently? This can also be done with the LTC3588 chip.
You charge a capacitor at the left side (connected to the Vin) using the piezo pinching. When this is charged you can enable the attiny85 at the right which is in a sleep mode. It can start up and just for example send some information through the RF transceiver before the energy runs out and the charging process starts again.
This is the setup on a breadboard:
After pinching the piezo the voltage goes above 1.7V and the PGOOD pin of the harvesting chip wakes up the attiny85 and the LED blinks, you can see that here:
This is the C code, where the _delay_ms(3); is critical: if you do it longer, the cap is drained too much.