Design and Implementation of a Multi-Source Energy Harvesting System for Smart Agriculture Applications

Wireless sensor networks (WSNs) are of great interest in agriculture in analyzing and monitoring the effects of soil quality, irrigation, fertilization, and environmental conditions on plant growth and health. Currently, WSNs are battery-powered. The use of batteries leads to potential problems, given their limited lifespan and the need for replacement. Supercapacitors coupled with energy harvesting solutions are a possible substitute for batteries in WSNs. Therefore, this thesis aims to discover the energy harvesting capabilities of plant microbial fuel cells (PMFC) for long-term usage in WSNs. The PMFC is a technology that produces bioelectricity by utilizing plants and bacteria. Plants in the PMFC produce organic matter, while bacteria are responsible for oxidizing this organic matter to release electrons, protons, and carbon dioxide. These electrons are transferred to an anode electrode and coupled by an external load to a cathode electrode. Due to the potential difference between the electrodes, these electrons flow from the anode to the cathode through the external load and produce electricity. In PMFC, selecting electrode material with high electrical conductivity is crucial for maximizing performance. For this reason, metal is a proper choice; however, it comes with a corrosion challenge, which decreases the lifespan of PMFC. This thesis presents electrode material selection and determination of a load value that provides stable output power and increases the lifetime of PMFC. Electrodes material is selected according to its high immune level to corrosion and electron attraction capability. In this thesis, the output current is limited to protect the PMFC from corrosion to increase its lifetime. Moreover, a novel three-electrode PMFC is presented, which can provide three different output voltage levels according to the selected electrode material. By alternating the working electrodes, it is possible to recover their corrosion and extend the PMFC's lifetime. The PMFC provides a stable output voltage; however, its power capabilities are limited. Therefore, the PMFC is a good choice for multi-source energy harvesting architecture. Moreover, PMFCs are well suited in combination with solar cells since their performances do not depend on weather or time of the day and perform better than a solar cell at night. For that reason, a solar cell is included as a second source of the system. The two energy sources are combined to maximize the harvested energy. However, only one source must be the active one. A circuit is designed to make this selection autonomously and provide continuous energy harvesting. The harvesting sources are connected to a power management system (PMS) to charge a supercapacitor. The selected PMS employs a maximum power point tracking (MPPT) circuitry to maximize power extracted from the energy harvester source. An additional circuit is introduced to set the correct working point of the MPPT: the circuit selects the best value for the active energy source autonomously. The complete system, which includes energy harvesting sources, designed circuitries, and PMS, was tested with an end-node device to transmit data using the LoRa protocol, showing the possibility of being adopted instead of batteries.