## 6 Experimental Results
### Case Study: Periodic Surface Flow Monitoring
#### 6.4.2 Analysis of Self-Powering Capability  
Starting from the outcome of the previous section, we analyze now how the most energy-efficient device behaves when self-powered according to the setup outlined in the previous sections. Specifically, we consider the Raspberry Pi 4 in its optimal SIMD and Multicore configuration, processing color and monochrome images at full and half resolution. From Table 1, we can easily infer that our target device has the following average current drained, assuming a constant 5.1 input voltage, in the four following configurations:  
* Raspberry Pi 4, Color images, Full resolution: 133.9 mA/h;
* Raspberry Pi 4, Monochrome images, Full resolution: 95.8 mA/h;
* Raspberry Pi 4, Color images, Half resolution: 101.2 mA/h;  
Figure 11: Power consumption for the first 200 s of a 15-min measurement cycle with a Raspberry Pi 4 at Full and Half resolution with color and monochrome images. The area under each curve represents the energy consumed during each measurement cycle.  
Raspberry Pi 4, Color Monochrome images, Half resolution: 74.7 mA/h.  
To estimate battery life, we consider a subset of options available for the PiJuice Hat device. Specifically, we select batteries with a capacity of 5000 and 12,000 mA/h. By assuming fully charged batteries in ideal conditions, the following table reports the number of measurement cycles allowed.  
For the successive considerations, we consider as a reference the city of Bologna in Italy. Its most extended daylight occurs on 21 June, and its length is 15 h, 33 min, and 1 s. Consequently, in such a scenario, it would be necessary to perform 63 measurement cycles per day. Looking at Table 2, assuming fully charged batteries, we can notice that such target is always fulfilled with the 12,000 mA/h battery, in any configuration. In contrast, with the smallest 5000 mA/h battery, the target number of cycles would be obtained only when processing monochrome images at half resolution. Interestingly, a fully charged 12,000 mA/h battery would allow for about two days of measurements. Nonetheless, even in this circumstance, battery charging is crucial, and we assume to carry it out with a PiJuice Solar panel, available in the 6, 12, 22, and 40 Watt options. According to the specifications available, the former can nominally provide up to 1000 mA/h when hit by appropriate sunlight, while the latter doubles this value. Therefore, the 6 Watt solar panel would allow for recharging the 5000 mA/h battery in 5 h and the 12,000 mA/h in 12 h. On the other hand, using a 12-Watt solar panel would allow for halving both recharging times. Although these are rough estimations, a setup made of a 12,000 mA/h battery coupled with a 12-Watt solar panel would probably be enough to handle even the worst-case scenario. For the city of Bologna, it could occur around 21 December, during the shortest daylight. In these circumstances, with a daylight length of 8 h 49 min e 43 s, a large battery and solar panel could suffice to enable the 36 measurements per day required even against extended sunlight shortages. Additionally, an oversized power supply system would also account for unavoidable inefficiency concerning the energy harvesting process that we have not considered in our estimation discussed so far, and in degradation of the battery and the solar panel.  
Finally, it is worth highlighting that, although our analysis focused on solar energy at hand and widely deployed when facing stream monitoring, other self-powering strategies could be seamlessly deployed or integrated with it.