## 4 Laboratory Characterization  
This section details the characterization of the previous prototype at the laboratory. First, it describes the characterization of the cold side heat exchanger. Afterward, the complete characterization of the whole thermoelectric generator is developed. The characterization of the cold side heat exchanger (CHE) refers to the determination of its thermal resistance for different heat fluxes. For this purpose, a 40 \(\times\) 40 mm\({}^{2}\) heating plate has been used to provide the desired heat flux \(\dot{Q}_{CHE}\) in each experiment, and the temperature at the base of the heat exchanger \(T_{b}\) and in the climatic chamber \(T_{amb}\) have been measured. Thanks to these measurements, the thermal resistance has been calculated according to Equation (1). Each experiment has been repeated three times and the uncertainties have been obtained with [50], considering thermocouples with precision \(\pm\)1 \({}^{\circ}\)C. Insulation was also added to ensure that all the heat flux went through the heat exchanger. \[R_{CHE}=\frac{T_{b}-T_{amb}}{\dot{Q}_{CHE}}=\frac{T_{b}-T_{amb}}{V\cdot I} \tag{1}\]  
Figure 3 shows the thermal resistance of the heat exchanger with respect to the heat flux for the two conditions studied: natural and forced convection, the latter with a wind velocity of 4 m/s. As can be observed, in both cases the thermal resistance decreases as the heat flux increases. This occurs because the properties of the internal working fluid, which is cyclically experimenting phase change, improve with temperature, thus decreasing the thermal resistance for higher heat fluxes. The former decrease is more noticeable for natural convection, where the thermal resistance diminishes from 0.68 K/W with 20 W, to 0.49 K/W with 100 W. In contrast, for forced convection the thermal resistance barely decreases 0.01 K/W from 20 to 120 W, presenting an average value of 0.24 K/W. The heat transfer coefficient in natural convection strongly depends on the temperature difference between the surface of the heat exchanger and the environment, improving with higher gradients. Nevertheless, in forced convection, this effect blurs in favor of the influence of wind velocity. Considering that this paper considers an outdoor application, the values of thermal resistance with forced convection are more representative. Apart from the characterization of the cold side heat exchanger, it is important to determine how much power can be generated with the designed prototype. At the laboratory, the experiments have been performed using the thermal bath depicted in Figure 4 as the heat source, while its analysis under real conditions is detailed in Section 6. This thermal bath is composed of a 10 L water heater, an insulated 30 L container, and a pump for the recirculation of the water used as the heat carrier. The designed prototype has been introduced in the insulated container, in direct contact with the water, which has been maintained at a practically constant temperature of 75 \({}^{\circ}\)C.  
Figure 4: Thermal bath used as heat source in the laboratory experiments to determine the power generated by the designed prototype. Figure 3: Thermal resistance of the cold side heat exchanger for different heat fluxes, considering natural and forced convection. Utilizing the previous temperature constant heat source and with an ambient temperature of 16.5 \({}^{\circ}\)C, each thermoelectric module has been individually connected to an electrical load resistance and its voltage and intensity have been measured for the calculation of the power generated. More specifically, short-circuit, 1 \(\Omega\), 2.2 \(\Omega\), 3.2 \(\Omega\), 4.7 \(\Omega\) and open-circuit conditions have been experimented and the uncertainties calculated again with [50]. In the experiments, the temperatures of the heat source \(T_{source}\), the hot and the cold side of the thermoelectric modules \(T_{H}\) and \(T_{C}\), and the ambient temperature \(T_{amb}\) have also been measured.