We present here the design and fabrication of a self-powered and autonomous fringing field capacitive sensor to measure soil water content. The sensor is manufactured using a conventional printed circuit board and includes a porous ceramic. To read the sensor, we use a circuit that includes a 10 kHz triangle wave generator, an AC amplifier, a precision rectifier and a microcontroller. In terms of performance, the sensor’s capacitance (measured in a laboratory prototype) increases up to 5% when the volumetric water content of the porous ceramic changed from 3% to 36%, resulting in a sensitivity of S=15.5 pF per unity change. Repeatability tests for capacitance measurement showed that the θv sensor’s root mean square error is 0.13%. The average current consumption of the system (sensor and signal conditioning circuit) is less than 1.5 μA, which demonstrates its suitability for being powered by energy harvesting systems. We developed a complete irrigation control system that integrates the sensor, an energy harvesting module composed of a microgenerator installed on the top of a micro sprinkler spinner, and a DC/DC converter circuit that charges a 1 F supercapacitor. The energy harvesting module operates only when the micro sprinkler spinner is irrigating the soil, and the supercapacitor is fully charged to 5 V in about 3 h during the first irrigation. After the first irrigation, with the supercap fully charged, the system can operate powered only by the supercapacitor for approximately 23 days, without any energy being harvested.
Thermoelectric generators (TEG) can harvest solar energy during the day using solar flat panels. They can also benefit from the use of a material that stores solar energy to generate additional power at night, when the panel cools down and the energy stored in this material travels back, through the TEG. The soil can be used as the material that stores solar energy, but the performance of such systems, with the heat sink buried in the soil, depends on the ambient and the soil temperature, parameters which can change drastically with the latitude of the location where the TEG is installed. We present an experimental study with the comparison of the potential energy that can be collected from a TEG system with heat sink buried at different depths and at different latitudes: Campinas, Brazil − 22 ∘ 54 ′ 20 ′ ′ S; and Mataró, Catalonia, Spain − 41 ∘ 32 ′ 17 ′ ′ N. The potential of energy harvesting calculated during 32 winter days in Campinas is 72% of the total calculated during 205 days in Mataró. Experimental results obtained from a complete TEG system showed that in Campinas, during one day, it was possible to store 34.11 J of electrical energy in a supercapacitor. Notably, we demonstrate that the energy generated during the night by the heat stored into the soil can be as high as the energy generated during the day.
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