## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Item** & **Unit** & **Value** \\ \hline Cell & - & Polycrystalline \\ Operating voltage & V & 6 \\ Working current & mA & 183 \\ Maximum output power & W & 1.1 \\ Open circuit voltage & V & 7.3 \\ Short circuit current & mA & 204 \\ Size & mm & \(112\times 84\times 3\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: PV panel specifications. Figure 4: PV panel and weather station. Here, the environmental conditions that prevailed in the experimental field are presented. A knowledge of environmental conditions is necessary for an understanding of the behavior of the solar-based energy harvesting system. Figure 7 shows the solar irradiance acquired by the weather station. The solar radiation reached high values during the days with increased sunshine, while it decreased and reached minimum values on cloudy days. It is obvious that there were several days with limited sunshine and therefore during these days the energy production from the PV panel was also limited. Figure 8 shows the air temperature acquired by the weather station. The air temperatures exceeded 30 \({}^{\circ}\)C in only two cases, so the changes in the air temperature do not seem to affect the energy production. Figure 9 shows a comparison between the power level of the energy storage device of sensor node 1 and solar irradiance on different days. It is obvious that on days with increased sunshine (for example on 1 September 2020 and 4 September 2020) the level of stored energy was close to 95%. On the contrary, on cloudy days (for example on 18 September 2020) the level of stored energy fell below 80%. Figure 6: Remaining power of the energy storage devices of the 4 sensor nodes. Figure 7: Solar irradiance readings during the experiment. In recent years, there has been a great interest in developing wireless sensor networks for agricultural applications such as soil water monitoring. Wireless sensor nodes have become increasingly complicated. Sensor nodes have evolved gradually into small embedded systems. Attributes such as low power consumption module design or the integration of modules in a chip are essential for energy-constrained portable applications. Figure 8: Air temperature readings during the experiment. Figure 9: A comparison between the power level of the energy storage device and solar irradiance. An energy-harvesting system converts usually solar energy into electrical energy for direct use by the sensor node or indirectly by the node energy storage device. The main devices that are used to store harvested energy for powering sensor nodes are rechargeable batteries or capacitors [17]. Sensor nodes need to have long-lifetime energy storage devices due to the inherent difficulty in physical access to replace or recharge them. The experimental results confirmed the suitability of the energy-harvesting system and energy storage device. The small PV panel successfully charged the rechargeable battery of the solar-powered node. The battery voltage increased gradually from the day we installed the sensor nodes and was maintained at a certain level during all days of the experiment. This also satisfied our expectations of charging the rechargeable battery and keeping the sensor node running without problems. Moreover, the system was not affected by the increased wireless communication (24 activations in 24 h) during the last phase of the experimental procedure. The comparison between the solar-powered and battery-powered nodes showed that solar power distinctly outperformed the battery in outdoor applications. Finally, the experimental results showed that two of the nodes powered by rechargeable batteries started gradually discharging. The third node began to discharge sharply after day 73. A better solution for the nodes would be having two separate energy supply units: a capacitor as the main source and a rechargeable battery as the backup source [18]. For sensor nodes operating continuously, only solar energy harvesters can offer a suitable solution.