## 5 Energy Harvesting Evaluation
### Solar Panel  
Solar panels are highly suitable for outdoor systems. However, slab tracks can be installed in indoor environments, as for example, tunnels. Both situations were studied to see whether the panels can be installed in the whole track or whether it was necessary to integrate different energy harvesting systems. Tables 6 and 7 show the technical characteristics of the solar panel and the battery used to perform our tests, respectively.  
\begin{table}
\begin{tabular}{c c} \hline  & **1.5 W Solar Panel 81 \(\times\)137** \\ \hline
**Supplier** & Cooking hacks \\
**Type** & Cristal \\
**Power** & 1.5 W \\
**Voltage** & 5 V (peaks up to 10 V) \\
**Efficiency** & 17\% \\
**Price** & 6.65 € \\
**Size** & 81 \(\times\)137 \(\times\)2 mm \\
**Connector** & JST PH-2 \\
**Image** & \\ \hline \end{tabular}
\end{table}
Table 6: Technical characteristics of the solar panel used.  
The whole system can be seen in Figure 6. A shield Xbee Arduino containing Xbee PRO 868 and an Arduino Mega 2560 board module has been used. The Xbee module is used to send the results of voltage from the board to a PC where the data is collected and stored. Moreover, a special board responsible for efficiently transferring energy captured by the solar panel to the battery is also needed.  
To study the impact of solar panels on the energy consumption issue, three tests were performed:  
* Test 1. It does not use solar panels at all, in order to estimate the duration of the battery without using energy harvesting mechanisms. In this test, the device sends 100 byte packets for 10 s. After that, the node changes to the reception mode for 60 s and it returns again to the transmission mode and so on to drain the battery. This test simulates the situation where the device detects the train and begins transmitting data for 10 s and switches to the reception mode to collect data captured by the neighbor nodes.
* Test 2. Like the previous situation but connecting the solar panel.
* Test 3. It also uses the solar panel, but in this case it simulates the real behavior of the monitoring node (scheduled to the train timetable). This node wakes up once every hour, reads  
\begin{table}
\begin{tabular}{c c} \hline  & **063450** \\ \hline
**Manufacturer** & Unionfortune Electronic \\
**Voltage** & 3.7 V \\
**Charge** & 1000 mAh \\
**Price** & 9.1 \(\upepsilon\) \\
**Size** & 53 \(\times\)33 \(\times\)5.7 mm \\
**Connector** & JST PH-2 \\
**Image** & \\ \hline \end{tabular}
\end{table}
Table 7: Technical characteristics of the battery used.  
Figure 6: The whole system used to carry out solar panel tests.  
the accelerometer data and sends packets for 10 s. Then, the node changes to sleep mode for one hour, and so on.  
Figure 7 shows the results of tests 1 and 2. When the solar panel is used, the battery lasts 164 min more and is able to transmit 10.952 additional packets.  
Figure 8 shows how the solar panel is able to generate enough power to keep the node indefinitely on. Throughout the day the battery enters a discharging (9/7/14 6:00 p.m.) or charging (10/7/14 8:24 a.m.) phase depending on the way the sun's rays affect the solar panel. In both phases different stepped zones can be appreciated. These correspond to small voltage drops which occur every hour during transmission mode.