## 5 Energy Harvesting Evaluation
### Peltier Cell  
The main objective of using Peltier cells is to check whether or not they are capable of generating energy from temperature differences produced in the tunnels, as in these environments, solar panels cannot be used due to the total absence of light. The technical characteristics of the cell used in our tests are shown in Table 10. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Test** & **Box** & **Piezoelectric** & **EHE004 Configuration** & **Voltage** & **Coulombs (C)** & **mAh** \\ \hline
1 & Metal & V20W & PN & 2.7 V & 0.00054 & 0.00015 \\
2 & Metal & V20W & PN & 5 V & 0.001 & 0.00027 \\
3 & Metal & V20W & SS & 7.7 V & 0.00152 & 0.00042 \\
4 & Metal & V21B & SS & 5.4 V & 0.00108 & 0.00030 \\
5 & Plastic & V20W & SS & 4.4 V & 0.00088 & 0.00024 \\ \hline \end{tabular}
\end{table} 
Table 9: Piezoelectric tests. The test goal is to check whether the temperature difference established between the exterior of the platform casing in contact with the concrete of the slab track and the inside of the casing will be used by the Peltier cell to generate energy. The bottom of the casing was modified to embed the Peltier cell and the edges where it fits were sealed with silicone (Figure 9). This way we achieve stability in the thermal difference and a firm clamping of the cell. The environment of the slab track has been simulated using a box filled with sand on which the platform casing has been placed (Figure 10) so that the face of the Peltier cell was in contact with the sand. \begin{table}
\begin{tabular}{c c} \hline \hline  & **PELTIER CELL TEC1-12710 HTS 100 W 40 \(\times\)40 mm** \\ \hline
**Power** & 100 W \\
**Maximum Voltage** & 15.4 V \\
**Connectors** & No (conventional positive \\  & and negative cables) \\
**Current** & 10.5A \\
**Size** & 40 \(\times\)40 \(\times\)3.3 mm \\
**Temperature range** & 66 degrees Celsius \\
**Price** & 22.74 \(\copyright\) \\
**Image** & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Technical characteristics of the Peltier cell used. Figure 9: Peltier cell installation. Two experiments were defined:  
_a) To place the platform indoor, in a similar environment to railway tunnels._  
To simulate the environment of a railway tunnel, the sand box was placed in a basement where the temperature is low and stable, which is a serious drawback for the performance of this device. Figure 11 shows the voltage generated by the Peltier cell over a period of 20 h. After installation it was observed how a small voltage was generated probably due to the initial temperature of the sand, which was below room temperature. As time passes the temperature difference decreased until it was practically 0, hence the voltage generated was also 0. _b) To place the platform outside, to appreciate the possible obtained differences_  
This test consisted of placing the sand box in a window of the room where it went through periods of direct light, shadow and absence of light. Each of these three phases is reflected in Figure 12. From 10:00 h to 12:08 h in the morning the box was in shade and therefore the temperature difference was 0 \(\;\mathrm{C}\), hence the voltage generated was also 0. From 12:08 h to 14:00 h the sun's rays begin to warm up the box and as the sand temperature was kept fresh a temperature gradient was produced and thus  
Figure 11: Energy generated by the Peltier cell inside a “tunnel”. Figure 10: Peltier cell installed inside a plastic case and buried in the sand. voltage was generated. After this period of time, the sun started to move away (from 14:00 to 17:15 h), and this caused the temperature of the interior of the platform case began to fall until it reached the same temperature as the sand. Finally there was another period (17:04 to 18:32) in which the Peltier cell began to generate power again. This is probably because the sand retained the temperature during the period of time it was exposed to the sun while the interior of the platform casing began to cool.