Soil-monitoring sensor powered by temperature difference between air and shallow underground soil

Abstract

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 (T). If T is low and T 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 μ W-370 μ W on average, and drive a wireless microcontroller unit to perform soil monitoring.