Soil-Monitoring Sensor Powered by Temperature Difference between Air and Shallow Underground Soil

Energy harvesting (EH) technologies are useful for the semi-permanent operation of wireless sensor networks, especially, for agricultural monitoring as the networks need to be installed in large areas where power supply is unavailable. In this paper, we propose a battery-free soil-monitoring sensor for agriculture, which leverages the temperature difference between near-surface air and shallow underground soil using a thermoelectric generator (TEG). The performance of systems driven by the TEG mainly depends on the average temperature between the hot and cold sides of the TEG (T) and the temperature difference across the TEG (ΔT). If T is low and ΔT is small, it is challenging to earn enough power to drive wireless microcontroller unit; however, with our dedicated electric circuit, and thermal designs including impedance matching of thermal circuit and suppression of heat loss, the sensor can harvest more than a hundred microwatt on average from the temperature difference between the air and underground soil at a depth of 30 cm. The performance of the energy harvester is evaluated both by numerical analysis using temperature data collected from various farm fields and by a prototype implementation. Moreover, the prototype was deployed to farm fields in Japan and India. Our field experiment results revealed that the prototype could harvest 100 μW-370 μW on average, and drive a wireless microcontroller unit to perform soil monitoring.