## 6 Behavior on a Real Volcano
of hours after the disconnection of the second module. Nevertheless, on the 21, when the second module switched to open-circuit, total generation with M2 was similar to the consumption. Hence, an equilibrium between generation and consumption can be observed, discharging the battery during the day and charging it during the night, but without reaching the upper nor the lower thresholds to switch to open-circuit or force the operation of a new module respectively. During this equilibrium period, the influence of ambient temperature in generation can be perfectly seen. This influence is more detailed in Figure 13, which shows the ambient temperature \(T_{amb}\) (left axis), the total power generation and the power generated by M2 (right axis) between 21 and 23 May 2020. Thus, during the day temperature increases, diminishing the available temperature difference and therefore reducing generation. During the night, this temperature decreases and generation increases. Figure 12: (**a**) Temperature, (**b**) power, (**c**) voltage and (**d**) intensity measurements during several hot days. The ambient temperature during the days depicted in Figure 12 is very similar to the one used in the laboratory characterizations. Hence, assuming that the wind conditions at Teide are similar to the laboratory ones (in forced convection), an estimation of the heat flux, the thermal resistance of the heat exchangers, and the efficiency can be performed. Table 3 summarizes the average results of M2 on 19 May between 11:00 and 20:00 h. As can be observed, at Teide, ground temperature is almost 10 \({}^{\circ}\)C higher than the laboratory's heat source's one. Consequently, the heat flux increases, reaching 40 W in this case and raising the hot side temperature to 72.5 \({}^{\circ}\)C. Since the gradient between the sides of the module is higher, so is generation, with an average value of 0.41 W, which entails an efficiency of 1.02%. Based on the previous data, it is also possible to calculate the thermal resistance of the hot side heat exchanger. Hence, with Equation (4) a value of 0.27 K/W is obtained, 23 % lower than at the laboratory, where natural convection with water was the heat transfer mechanism. Heat transfer between the ground and the hot side heat exchanger was one of the greatest unknowns, complicating its simulation at the laboratory. Nonetheless, it has been experimentally demonstrated that this heat transfer is really acceptable, with a thermal resistance quite similar to the cold side one. The fumaroles heat up and moisten the ground in their ascent to the surface. Thus, heat transfer is not produced by convection with the gaseous fumaroles but by conduction with the hot and wet ground, which permits obtaining better heat transfer coefficients. Since its installation in December 2019, the prototype has undergone multiple kinds of meteorological conditions, from severe snowfalls and frosts to days with the typical Saharan air layer. On the one hand, Figure 14 shows its operation between 21 and 24 January 2020, when ambient temperature reached values down to \(-10\) \({}^{\circ}\)C. As can be seen, due to the greater difference between ground and ambient temperature, the gradient between the sides of the thermoelectric modules  
Figure 13: Ambient temperature \(T_{amb}\) (left axis), power generated by M2 and total generation (right axis) between 21 and 23 May 2020. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{2}{c}{**Temeratures**} & \multicolumn{2}{c}{**Generation**} & \multicolumn{2}{c}{**Thermal Resistances**} \\ \hline \(T_{ground}\) & 83.44 \({}^{\circ}\)C & \(P\) & 0.41 W & \(R_{HHE}\) & 0.27 K/W \\ \(T_{H}\) & 72.53 \({}^{\circ}\)C & \(Q\) & 40.00 W & \(R_{CHE}\) & 0.24 K/W \\ \(T_{\text{C}}\) & 24.77 \({}^{\circ}\)C & \(\eta\) & 1.02\% & & \\ \(T_{amb}\) & 15.12 \({}^{\circ}\)C & & & \\ \hline \hline \end{tabular}
\end{table}