IoT-ready precision agriculture powered by microbial fuel-cells

Monitoring the level of phreatic aquifers is very important to protect and to preserve underground freshwater, which plays a key role in many human activities, particularly in agriculture. Aquifers have a process of natural recharge supplied by meteoric water that penetrates the ground and accumulates, but if withdrawals are greater than the refilling process, they shrink and may generates several issues. Measurement stations are off-the-shelf available and parameters are usually assessed with periodic and man-made measurements, which makes the whole process expensive and uncomfortable. To overcome these limits, a complete IoT system for continuously and remotely monitoring of phreatic aquifers has been developed (Figure 1). It consists of ultra-low power hardware that exploits smart strategies to store and save energy. We used the MSP430FR5739 ultra-low power microcontroller which embeds a non-volatile Ferroelectric-RAM (FRAM) memory, a step-up DC/DC converter to raise the input voltage up to a suitable level (from ≈130 mV to more than 4.5 V), a storage capacitor, a smart phreatimeter and the Semtech SX1276 radio chip equipped with a LoRATM modem. This radio interface allows to transmit acquired data kilometers away also in a noisy environment—conversely to what happens for example in a ZigBee or other WSN scenario—exploiting low complexity network topology and a simple network protocol. The smart phreatimeter exploits the change in capacitance of a cable made of two parallel conductors when it is in contact with water that has a relative permittivity (εr) higher than air (81 and 1 respectively). We measured the capacitance using the measurement process summarized in Figure 2 and, for example, we were able to appreciate variations from 174.54 pF for 80 cm of immersion to 307.36 pF in case of 160 cm (Figure 3 presents the extracted characteristic). The power supply is generated in an eco-friendly and zero emission manner by means of a terrestrial Microbial Fuel Cell (MFC) that ensures a very long energy autonomy. An MFC is an electrochemical bioreactor (cfr. Figure 5) which generates electrical energy exploiting the energy contained in chemical bounds of organic nutrients by means of electrogenic bacteria in an anaerobic environment. To cope with transient and intermittent power supply, the FRAM and “Hibernus” library have been used to be compliant to the recent Transient Computing paradigm (Figure 4). Finally, by carefully characterizing the MFC’s average production and the power consumption of system single components’ (Figure 6), we implemented a simple simulator to evaluate the most suitable storage size for each application (Figure 7) and the tradeoff among speed of charge (few mF) and a longer IoT operations (1F); depending also on the LoRATM modem setup (bitrate, BW, etc.).