## 6 Behavior on a Real Volcano
Since the measurement of the different variables is performed every four minutes, the consumption profile is shown as peaky rather than pulsed, as can be observed in green in Figure 11b. The measurement of this consumption is being performed at the output of the battery, or in other words, at the input of the PRT boost converter that adapts the battery voltage to \(5\,\mathrm{V}\), so that the consumption of all the charges is considered. Thus, the voltage of the consumption coincides with the battery one in Figure 11c. In order to satisfy the previous demand, four thermoelectric modules were installed. Nevertheless, the generation supplied by only one module is enough for this purpose. As can be observed in Figure 11b, the generation of the four thermoelectric modules, in red, is slightly lower than the generation of M2, in dark blue, while the generation of M1, in light blue, is null. This means that out of the four thermoelectric modules installed, only M2 is in operation. The generation of the individual modules is measured before the MPPT. Therefore, the difference between the total generation and the individual generation of M2 is due to the conversion efficiency of the MPPT, which can be rated as \(85.7\%\) in this case. Figure 10: Location of the prototype node and the gateway, and their associated elevation profile. Images taken from Google Earth ©. The total generation is around 0.47 W, higher than the consumption, which presents an average value of 0.37 W. Hence, the battery is charged with the excess of energy produced, until it reaches 4 V (Figure 11c). At this moment, the MPPT opens the circuit of M2 for security reasons, leading to a null intensity and increasing the voltage of M2 from 2.18 to 2.82 V approximately. This open-circuit value is very similar to the one displayed by M1. When all the modules are in open-circuit, the battery is progressively discharged, reducing its voltage. When this voltage reaches 3.74 V, M2 activates again, starting a new cycle. The threshold at which the thermoelectric modules activate is determined by the MPPT. Nonetheless, there exist slight differences in the used boards. M2 has the highest threshold and therefore, it is the first one that starts its operation. It would be necessary to have a battery voltage lower than 3.74 V for the activation of other thermoelectric modules. If the ambient temperature increases, the available temperature difference decreases, as so does the gradient between the sides of the thermoelectric modules, and therefore, the generation. Figure 12 depicts the variables measured between 19 and 23 May 2020, when temperatures close to the typical maximums at Teide volcano were recorded [67]. In this period, the ambient temperature presents values of around 8 \({}^{\circ}\)C during the night and up to 22 \({}^{\circ}\)C during the day. Consequently, the temperature difference between the sides of the modules has decreased from the previous case, with an average of 49 \({}^{\circ}\)C.  
As shown in Figure 12, the total generation obtained with the former gradient is of just 0.35 W, lower than the average consumption. Therefore, the battery is gradually discharged although one thermoelectric module is in operation. When the voltage of the battery is lower than 3.73 V, another thermoelectric module starts its operation. The individual generation of this module is not being monitored, but it can be seen that the total generation, in red, exceeds the individual generation of M2, while M1 remains in open-circuit. When the battery voltage reaches 3.99 V, this second thermoelectric module switches to open-circuit, and again M2 is the only module in operation. On 20 May, the total generation with only M2 was higher than the consumption, completely charging the battery a couple  
Figure 11: **(a)** Temperature of the ground \(T_{ground}\), the hot and the cold side of two thermoelectric modules M1 and M2 (\(T_{H}\) and \(T_{C}\)) and the ambient \(T_{amb}\), **(b)** power, **(c)** voltage and **(d)** intensity measurements during typical operation.