## 4 Experimental
### Simulation Experiments and Results Using the Energy-Harvesting Module  
The energy consumption of one node is studied by varying different parameters and their contribution to system behavior. To perform this evaluation, we have set a basic scenario:  
1. Network topology is set to a star. Sensor nodes communicate to one single node that acts as the network coordinator and packet sink. 2. The network coordinator power-supply is uninterrupted. The rest of the sensor nodes are powered using an energy harvesting system. 3. The MAC protocol selected is ALOHA. Although throughput can be severely restricted, channel access time is deterministic. No acknowledgement (ACK)messages are sent in order to minimize network traffic. 4. Sensor measurement frequency is configurable in each node. 5. Nodes can transmit and receive information from the coordinator. For that purpose, sensor nodes will listen to the channel and wait for incoming packets. Figure 6: Sensor node simplified state diagram. Remaining time in determined state values are taken from Table 1 and S is the node sleep time (60 s by default). Basic operation of the sensor node under these circumstances is specified in Figure 6, where n is the number of TX + RX cycles that are performed by the node before going to SLEEP mode again and m is the number of consecutive times the radio remains in receiving state. As part of the experiments, the energy stored in just one node is analyzed, since the rest present the same behavior. Node current consumption of the various states (Figure 6) is shown in Tables 2 and 3. The duration of each state is given in Table 1.  
Capacitor capacitance was set to 50 farads, starting voltage to 0 V and maximum voltage to 2.3 V. Solar radiation data corresponds to the radiation measured by AEMet at Valencia (Spain) on 3 April 2011. In all scenarios, the simulation stop time was set to 48 hours, and the sensor is only enabled during the sensor-enabled state. During this state, the radio remains in sleep mode. Figure 7 depicts super-capacitor charge and discharge curves during 48 hours. In the node behavior state diagram in Figure 6, the \(n\) parameter is reconfigured to vary the number of TX + RX loops per period. At sunrise, charge curves match similarly in three cases (n = 10, 15 and 20 loops). The \begin{table}
\begin{tabular}{l l l} \hline
**Task** & **Symbol** & **Time** \\ \hline Sensor setup & \(sens_{setup}\) & 33 ms \\ Sensor measure & \(sens_{meas}\) & 485 \(\mu\)s \\ MCU data processing & \(MCU_{proc}\) & 10.5 ms \\ Node transmission mode & \(t_{TX}\) & 570 \(\mu\)s \\ Node receive mode & \(t_{RX}\) & 102.5 ms \\ \hline \end{tabular}
\end{table} 
Table 1: Sensor states duration time. MCU, microcontroller. \begin{table}
\begin{tabular}{l l} \hline
**MODE** & **I** \\ \hline Sensor enabled & 6 mA \\ Sensor disabled & 0 mA \\ \hline \end{tabular}
\end{table} 
Table 3: Sensor current consumption. zoom during daylight corresponds to the 10 loop simulation and shows that discharge during daylight is negligible. Discharge curves start at sunset and differ in each studied case. With 10 and 15 TX/RX loops, the node never stops its operation, since capacitor voltage never falls below 1 V. The energy consumed at night at 20 loops is more than the super-capacitor can provide, and the node stops operation before sunrise. At sunrise on the second day, the super-capacitor starts receiving energy from the solar panel. During super-capacitor recharge, the node is still transmitting and receiving. This activity can be observed in the figure. However, the node consumes very little energy, and the capacitor can be recharged. Despite increasing the number of loops, experiments show that nodes receive a quick recharge. Figure 8 shows other experiments where the energy demands at sunrise are greater than the energy provided by the panel, and consequently, the node is unable to recover. In this experiment, we define the duration of the RX state as \(m\times 102.5\)\(ms\), where \(m\) is a configurable variable for each experiment set as \(m=4\) and \(m=5\).