# Autonomous Active Tag Using Energy Harvesting Strategies
## 4 Discussion  
As shown in the results obtained in a real scenario, solar energy is dominant over mechanical, since it obtains more energy in an environment that is always changing and is unpredictable. This technique is the bottleneck of our device because the weather cannot be estimated accurately, so the amount of energy obtained cannot be predicted, it is, that is the point with the most uncertainty. In the shady zones, the device might not work continuously due to all the hours of sunlight the device is in the shade. Consequently, for the final device, it is important to minimize or eliminate the hours in which the device is in a shady zone. However, the limiting case of solar energy comes from the voltage drop during the night, as can be checked in Figure 4. This means that if we want the device to stay on continuously, it will be important to characterize those conditions in future implementations because this application is related to an enclosed framework, where the sun is needed for a positive energy balance. If the weather of the place where the tag is implemented is usually cloudy, or the days are short during a great part of the year, this solution will not be optimal, although a previous study must be done to corroborate this. These conditions can be considered as the constraints of the work since the conditions limit the energy obtained to work. Focusing on the board, there is no concrete limit for consumption, energy or size, the work pursuit the lowest power consumption, the most energy obtained, and the smallest size (as a portability and environmental matter). Nevertheless, for mechanical energy, it can be seen that the system is not capable of working in those environmental conditions. The maximum values of collected mechanical energy (in SYS), which are probably due to an increase in road traffic, are not enough to turn on the system if this technique were the only one working in the device. When a vehicle passes near the tag, the voltage in the supercapacitor increases and then drops quickly. The average collected energy is not enough to charge the supercapacitor. The same applies to the BAT output, although the voltage is maintained longer for BAT, because this output is connected to the supercapacitor. So, the contribution of this  
Figure 11: Power consumption of the tag after receiving a “write” command at 4.2 V in the supercapacitor. As in other images, the numbers represent which element is the one that has the largest contribution to the consumption of the tag in that interval in each case: 1 is for the transceiver, 2 is for the EEPROM, and 3 is for the microcontroller. The EEPROM is used twice, once to find where to save the datum and a second time to save it. After that, the processing associated with the command ends. The microcontroller remains active while the checks are being made and the data is saved. technique to the charge and running of the system is very low. A higher inflow of cars and with higher speeds is necessary to allow the system to use only this technique as the main technology and to consider an option to include it in future versions. In a final device, for this application, mechanical energy will not be included. However, for other applications, it can be very interesting to consider this energy. Consequently, whether to include it or not should be valued depending on the scenario. As for the processing module, it is shown that the consumption is adequate for applications such as the one at hand. The energy consumption peaks correspond to transmissions and receptions, which can be sustained by the energy collected in the supercapacitor, as the system is still working after these said peaks. Consequently, the device will remain on over time for whatever scenario. Staying in one scenario or another is variable and depends on the application. Such new applications may require a different supercapacitor, or only one of the two available energy harvesting modules, etc.