**6. Future Work and Discussion**

The proposed storage unit is suitable for energy-harvesting-based, wireless sensor node (WSN) applications. The unit can autonomously supervise the charging and discharging of two supercapacitors (SCsmall, SCbig) and a backup battery, exploiting the energy provided by a harvester module. It can be combined with any type of energy harvesting module, such as photovoltaic, triboelectric, piezoelectric, or thermoelectric micro/nano-generators. Depending on the used harvester, a step-up/down converter and/or an AC-DC rectifier might be required (e.g., BQ25504 in case of the small photovoltaic panel or thermoelectric harvester). The proposed storage unit provides two separate power outputs. The SCsmall output, which provides a regulated output voltage (50 mV ripple) has the highest charging priority. Thus, it is considered as the main power output of the unit and should be used for the primary system circuits. Additionally, the SCbig output, provides an unregulated output voltage, however within an adjustable voltage window, with a minimum ∆V of 200 mV. This output has low charging priority and should be used for the secondary circuits of the system. This way extended operation is achieved for the primary system circuits, and additionally, the idle consumption of the secondary ones is eliminated. For embedding an RF transceiver module, an MCU, as well as the required sensors, and in case that real-time acquiring of the sensor data is not mandatory, the SCsmall output can be used for the MCU supply and the sensors supply (for local data logging) and the SCbig output can be used for the power demanding RF transceiver module (so the transmission of the data will take place during energy-rich periods of time).

In the case of continuous energy availability (e.g., continuous vibrations), a battery is not required and can be omitted. However, to ensure undisrupted operation a backup non-rechargeable battery can be added. For ultra-low-power WSN systems and sparse energy starvation periods, a multiyear duration is expected from a coin-size battery cell (e.g., cr2032 3V 230 mah). In case of discontinuous energy availability conditions and for a scenario that the undisrupted operation of the supported system is crucial, the use of a battery is mandatory. Depending on the use-case application and the power requirements, the use of a separate external battery charger module should also be considered. Finally, in systems embedding an MCU, the direct power supply of the MCU from the SCsmall is possible. This way the MCU supply has the highest priority and the longest possible power autonomy. A future work WSN approach is shown in Figure 16.

**Figure 16.** Future work WSN topology.

In the WSN shown in Figure 16, an indoor light harvester based on the BQ25504 IC was used. Additionally, an ultra-low-power MCU (e.g., MSP430FR5969 consuming 20 nA in deep sleep mode and 250 nA with calendar and timekeeping ON) is directly supplied by the small supercapacitor. Since continuous energy availability is not expected and an MCU is used, a rechargeable battery and a battery charger are added to the topology. The MCU is measuring the voltage levels of the two supercapacitors and the battery, in order to select a preferable power plan that matches the available energy. For example, only calendar and timekeeping during very-low energy levels, sensor reading during low energy levels, data transmission during high energy levels and battery charging during very-high-energy levels. The MCU is also controlling all the peripheral modules (sensors, RF transceiver and battery charger), as well as the load control signals (en1, en2) of the proposed storage unit.
