## 3 Experimental Studies and Discussions
### Self-Powered CO\({}_{2}\) Monitoring  
To validate the performance of the proposed PEH for self-powered CO\({}_{2}\) monitoring, the designed wireless sensor node was connected to the output of one of the LTC-3588 power converters. A CO\({}_{2}\) sensor and a Zigbee wireless transmission module were integrated. The data were transmitted wirelessly to a host computer. The host computer was powered on continuously to receive data at a random time point. Due to the limited energy output of the PEH, a control algorithm of the sensor node was designed, as shown in Figure 8.  
The control algorithm of the self-powered wireless sensor node is shown in Figure 8. Note that the PEH provides limited electrical energy, and the STM32 microcontroller will be in stop mode most of the time. The microcontroller will wake up when the LTC-3588 outputs sufficient energy. Here, the wireless sensor node requires a 3.3 V power supply. Additionally, the LTC-3588 will output 3.3 V when the voltage in the energy storage capacitor is around 4.73-5.37 V according to the datasheet. Serial ports of the microcontroller were used to connect the wireless transmission module to the register. When there is sufficient energy in the capacitor, the procedures of CO\({}_{2}\) measurement and wireless data transmission will be triggered. Firstly, the CO\({}_{2}\) sensor will be powered on, and the data will be recorded. Then, the sensor will be shut down for power saving, and the wireless module will be powered on for the wireless data transmission. Note that the sensor and wireless data transmission modules will still consume energy when connecting to power sources, even if they are not in working conditions. Four GPIO ports were connected in parallel to power the sensor and wireless modules. The modulus will be completely shut down when sensing and data transmission tasks are not required. It is worth mentioning that multiple time delay functions were tested and integrated for the initialization and operation of the sensing and wireless transmission modules during the program debugging. The self-powered wireless sensor node was connected to the capacitor of the vibration energy harvesting module under an excitation of 2.98 g. The electrical energy charging in the energy storage capacitor and the energy consumption of the wireless sensor node during discharge are described by measuring the voltage in the capacitor and the time of recharging, as shown in Figure 9. A typical excitation of a transformer substation is sinusoidal due to the constant operational frequency of the AC power line [53]. We can  
Figure 8: Control algorithm of the wireless sensor node. easily simulate the working conditions in the laboratory. Therefore, sinusoidal excitation was provided in this research to validate the proposed system conceptually under 2.98 g excitation. Figure 9 shows the voltage of the capacitor, representing the process of charging, as well as the power consumption of the wireless sensor node. The charging process starts from a state of zero electrical energy storage in the capacitor. It can be deduced that it takes about 1220 s to charge the capacitor for the first data measurement and data transmission. It is worth noting that in a previous study, it took 8000 s to charge the capacitor and power a similar wireless sensor node with a single piezoelectric transducer PEH [40]. In other words, the advantages of multiple piezoelectric cantilevers with enhanced power output capabilities could be confirmed. In addition, a one-time recharging takes about 630 s in the following cycles. The voltage drop from 5.14 V to 3.76 V, which represents the power consumption of the processes of powering up, sensor data reading, and wireless data transmission. In this research, 16000 \(\mu\)F large capacitors were adopted, and thus, the wireless sensor node will consume 0.0983 J per sensing cycle, with a power consumption of 16\(\sim\)18 mW. In addition, choosing a larger capacitor would provide more energy for each cycle of sensing and data transmission.