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 µW-370 µW on average, and drive a wireless microcontroller unit to perform soil monitoring. CCS Concepts: • Hardware → Renewable energy; Sensor applications and deployments; • Human-centered computing → Ubiquitous and mobile computing systems and tools.

In this paper, we present a novel approach to realizing a battery-free soil pro le probe that uses the temperature difference between the near-surface air and the underground soil as its power source. Temperature changes in the underground soil are slower than that in the near-surface air, and thus a large temperature difference exists between the near-surface air and the underground soil for most of the day. We develop a sensor prototype driven by a thermoelectric generator (TEG) that directly converts this temperature difference into electricity. Simulations are performed using real eld data, and results show that our prototype can harvest an average of several tens to several hundreds of microwatts. Because the typical sensing interval of a soil pro le probe is 1 h, the average power consumption (e.g., for a Texas Instruments CC2650) is about 5 µW, which is much lower than the expected amount of harvested energy. Furthermore, the results of an experimental implementation of the prototype proved that when the temperature difference between the near-surface air and the underground soil is only 3 K, which is much lower than the average temperature difference in an actual eld, the measured output power exceeds 80 µW.

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In this paper, we present a novel approach to the realization of a battery-free soil profile probe that uses the temperature difference between the near-surface air and the underground soil as a power source. The temperature change in the underground soil is slower than that in the near-surface air, and thus a large temperature difference occurs between the near-surface air and the underground soil for most of the day. Hence, we developed a sensor prototype driven by a thermoelectric generator (TEG) that directly converts this temperature difference into electricity. The results of an experimental implementation of the prototype proved that when the difference in temperature between the nearsurface air and the underground soil is only 3 • C, which is much lower than the average temperature difference in an actual field, the measured output power is about 80 µW. Because the typical sensing interval of a soil profile probe is 1 h, the average power consumption (e.g., for a Texas Instruments CC2650) is about 5 µW, which is much lower than the expected amount of harvested energy.