Delivering power through the soil for IoT sensors
Researchers in the US have developed a technique to send energy through the soil to power wireless sensors in the Internet of Things
The wireless power transfer developed at the Centre for Energy Systems Research, Tennessee Technological University uses conduction currents “through the soil” (TTS) to transfer power to surrounding devices.
The system was simulated and a prototype system is not line of sight dependent and is robust. A major benefit is that each sensor module does not need an individual battery. Instead, batteries can be placed at a central location and the energy distributed radially around that location without wires.
The geometry of the TTS system is similar to a water well, offering a possible way to integrate this WPT technique into existing farming infrastructure at very little cost to the user. The proof of concept uses a horizontal receiver geometry with four IoT devices without batteries across a 0.8 hectare field.
The TTS transmitter (Tx) uses a minimum of two electrodes in direct contact with the soil, where one electrode resided at the soil surface and the other resided at a vertical distance below the surface, defined as a vertical geometry. The top electrode was formed from a well casing that was installed around the bore-hole of the Tx.
The well casing was made of low carbon steel that was approximately 15m long while the bottom electrode was a 15m section of brass tubing 50 mm in diameter, located 75m below the surface.
The brass tubing was connected to a high density poly ethylene (HDPE) tubing of the same diameter that ran from the brass to the surface. A 12-gauge insulated wire was fastened to the inside of the brass tubing at the bottom and ran to the surface on the inside of the HDPE.
The receiver (Rx) uses a horizontal geometry, where both 70cm long electrodes sit at the surface for the ease of deployment and measurement. Future investigations will explore vertical topologies for the Rx, whose placement will become more permanent.
Due to soil strata layers being mostly horizontal, currently injected into the ground will follow strata layers of lower resistivity and not spread (or fringe) as far within the surrounding soil medium. This makes horizontal Tx geometries more prone to changes in conductivity due to weather and less effective at transmitting energy over the surrounding area.
A vertical Tx structure exhibits greater current fringing as moving charge must move across all the strata layers, making the vertical geometry more suitable. For the Rx, the only geometric constraint is that its electrodes must sit at different equipotential lines created by the Tx in order for a voltage to be received.
The design was simulated on the Ansys Maxwell tool and future work will use this model for investigating better electrode designs to determine transmitter drive magnitudes and the maximum distance sensors can be placed based on the Rx electrode spacing.
A small IoT network of commercial agricultural sensors were used to demonstrate the ability to wirelessly transfer power without batteries or large electric storage elements.
The sensor modules include an integrated moisture and temperature sensor with a ATMega32u4 with a LoRa communications IC and a 1mF capacitor for dc filtering and to provide energy for the 200mW bursts the sensor required when transmitting data. The sensors were all powered simultaneously within a radius tested between 10 to 20 m around the Tx.
The power required to operate four sensor modules was 800mW when transmitting LoRa data on 900 MHz.
Prior to installing the ground rods, the system required a peak input power of 500 W at 60 Hz to operate the sensor network at a distance of 10 m from the Tx. After adding the grounding rods, the same power delivery of 0.8 W was achieved using a 250W peak, a two-fold improvement in efficiency with an addition of only six extra low-cost grounding rods.
Experiments are underway to determine how low the Tx impedance can be made before the bottom electrode would need modification. In the experiment, the field was energized for 1 min every hour and shutdown for the remaining 59 min, leading to an average power of only one sixtieth of the peak power.
The paper is at ieeexplore.ieee.org/document/10061422
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