*5.2. Energy Harvesting*

The storage unit was also tested during the energy harvesting operation. A BQ25504 energy harvesting IC combined with an 80 mm × 72 mm PV cell were utilized, harnessing energy at office lighting conditions (approximately 600 Lux of light intensity). The measurement setup is depicted in Figure 12. As this is a low power demanding use case scenario, the values of SCsmall and SCbig were set at 1 mF and 4.4 mF (two 2.2 mF capacitors connected in parallel) respectively.

Figure 13 presents the charging phase of the storage unit, with zero initial stored energy in the two supercapacitors. Since the available current is about 100 uA, the charging of the small supercapacitor requires about 30 s.

As stated, the supercapacitor's values should be properly selected, according to the power supply requirements of the target system. For this test case, smaller capacitors can reduce the charging time duration, at the expense of extended energy autonomy or connection of demanding external loads.

**Figure 12.** Experimental setup for testing the storage unit in indoor light energy harvesting conditions.

**Figure 13.** SCsmall (CH1) and SCbig (CH2) charging via BQ25504.

The proposed unit was also tested with a piezoelectric transducer providing power at the input. Figure 14 depicts the measurement setup. A V25W PZT transducer from MIDE is used to convert the vibrations to electrical energy and a vibrating speaker is used to trigger the piezoelectric transducer. A mass of 1.9 g. is fixed on the tip of the transducer, to further increase the power offered to the storage unit. A rectifier comprised of four Schottky diodes is connected between the output of the PZT element and the proposed storage unit. The vibrations frequency is set at 25 Hz and the open circuit peak-to-peak voltage is measured at 20 V. The observed mechanical displacement of the tip of the transducer was observed at 1 cm peak-to-peak. The values of SCsmall and SCbig were set at 1 mF and 4.4 mF respectively, and also, loads of 100 Ω are connected at the supercapacitors outputs. Finally, Figure 15 presents the charging phase of the two supercapacitors.

**Figure 14.** Experimental setup for testing the storage unit with mechanical vibrations harvesting.

**Figure 15.** SCsmall (CH1) and SCbig (CH2) charging phase from the piezoelectric transducer.
