# Autonomous Active Tag Using Energy Harvesting Strategies
## 4 Discussion
Including a battery can cause a malfunction due to the fact that it is not capable of handling transmission and reception peaks as well as the supercapacitor. Furthermore, it requires a long time to charge and decreases the lifetime of the system because it has fewer cycles. Therefore, a battery should not be included in the next designs. For this first prototype, in the low power consumption scenario of this application explained above, with the supercapacitor fully charged and no external source of power, the tag is able to function autonomously for 7 min, as can be checked in Figure 7a. When the tag is placed on the traffic sign, with the energy harvesting modules connected. An autonomy of 10 h is achieved, and this is a parameter needed to characterize this device and really prove that it is autonomous. This is due mainly to solar energy, which can recharge the supercapacitor after the peaks of reception that are acceptable to the tag. However, it has to be considered that it depends on the environment and the application. The device, for this application, will work during daylight hours where the dominant mode is the low power consumption scenario with a duty cycling of 50%. However, the duty cycling can be reduced to 0.2%, which means that the consumption is 25 times less and thus increases the lifetime. This value for duty cycling is considered the optimal value for the device because it is the lowest value for which the transceiver is able to work correctly. Therefore, in the final application, this will be the value for the reception mode. This tag prototype turns off during the night, but the previous change in reception values and the option to replace the supercapacitor with a larger one will give the tag full autonomy for the whole day, although a study should be done. If there is enough energy, the scenarios work regardless of the time of day. The usability of this tag as a self-powered device can be guaranteed thanks to Figure 5, where the device supports every peak of the transceiver and is able to continue working. Assuming the change in the duty cycling and the solar source power, the autonomy and viability of using this device as the final self-powered device will have to be checked again. The abovementioned points highlight the need for a specific solar panel, piezoelectric, and supercapacitor, as well as the configuration of the built-in low power consumption modes according to the end application for an optimal optimization of the device. This also requires a prior study of the conditions of the set-up location, under what conditions the application will be used and which requirements the tag will need. This prior work will allow the maximum exploitation of the energy harvesting submodules and the regulation and storage submodule to obtain the best performance regarding energy and lifetime. Although no comparison has been done to check the optimization of the processing module, we have made an effort to find the best configuration of the different components in order to achieve the minimum consumption according to the manufacturer's information. This study is performed to cover the autonomous active tags applications, which have a mode called low power consumption, where the consumption is as low as 31.85 \(\mu\)A (66.89 \(\mu\)J), with peaks of 7 milliamperes for reception (14.7 m)), which is a good low consumption. Taking into account the energy that the supercapacitor is available to store when is full charged and looking at the energy requirements of the device in every scenario, it is clear that the energy balance is positive. The command that needs more energy is \"all data\" in reception scenario due mainly to the energy wasted between transactions, something that has to be checked for the final application. Moreover, the reception peaks have a high energy consumption, this point supports the previous idea of reducing the duty cycle. The tag could be used in almost every environment initially considered, which turns this device into a useful system.