R&D teams are showing interest in underground sensors, accelerating the commercial development potential for WUSNs.
Wireless devices have become ubiquitous, with new applications continually emerging to address the needs of a technology-driven world. Wireless devices can now be found virtually everywhere; from underwater sensor networks that warn of seismic activities that could lead to tsunamis, to ground sensors that measure changes in the permanent ice fields of Antarctica; to airborne sensors that monitor high altitude atmospheric conditions. The last domain for wireless sensors is the earth’s subsurface.
Underground sensors are attracting significant interest from academic, government, and corporate research and development teams, which, in turn, is accelerating the commercial development potential for Wireless Underground Sensor Networks (WUSNs). The list of potential applications for WUSNs has grown to include security (intruder detection), environmental monitoring, precision agriculture, sports field maintenance, earthquake and landslide monitoring, as well as infrastructure monitoring.
Understanding the Technical Challenges
For WUSNs to proliferate, several technical challenges need to be overcome. First, efficient and reliable low power communications protocols must be established that allow buried underground sensor nodes to communicate with one another and to interface with above ground communications networks. Specifically, many of these WUSNs will be designed as redundant and self-healing wireless mesh networks (WMNs) that can properly manage the propagation of electromagnetic (EM) waves within the subsoil. The transmission of EM waves can be complicated by environmental factors such as burial depth, temperature, soil composition, and soil moisture. These environmental factors can affect the strength and reliability of EM waves, including issues related to reflection, refraction, and multi-path fading effects. Wireless device manufacturers are currently developing solutions to minimize EM wave channel losses due to environmental conditions.
The second technical hurdle is to ensure reliable two-way communications between the underground sensor nodes and above ground data interfacing devices. Ensuring a strong two-way communications platform requires the use of specialized antennas that can transmit data through the subsoil, or exit points that are hard wired to antennas located above ground.
The third main technical challenge, which is the main focus of this article, is to develop reliable power supplies that permit long-term, maintenance-free operation of WUSNs.
Learning from the Experiences of AMR/AMI Devices
The power requirements of WUSNs should closely parallel meter transmitter units (MTUs) currently utilized by the water utility industry. Virtually all leading brands of water meter reading MTUs are powered by lithium thionyl chloride (LiSOCL2) batteries, including devices that were still operable on their original batteries after 28 years in the field. LiSOCL2 chemistry is ideal for underground sensors because it offers the highest capacity and highest energy density of any lithium chemistry, along with an extremely low annual self-discharge rate (less than 1% per year), the widest possible operating temperature range, and a glass-to-metal hermetic seal.
Extremely low annual self-discharge permits LiSOCL2 batteries to operate for up to 40 years. However, design engineers need to be aware that not all bobbin-type LiSOCL2 batteries are created equal. For example, an inferior quality LiSOCL2 battery may deliver 10-year operating life with an annual self-discharge rate of 2-3% per year, while a superior grade LiSOCL2 battery can feature a much lower annual self-discharge rate of 0.7% per year, thus permitting up to four decades of maintenance-free operation.
Bobbin-type LiSOCL2 batteries can be modified for extreme underground environments, including placement within arctic permafrost, as these batteries can withstand prolonged exposure to -80°C temperatures, accomplished in part because bobbin-type LiSOCL2 cells are non-aqueous. LiSOCL2 batteries can also be located in down hole oil wells using a version of this chemistry that has been modified to withstand 150°C temperatures.
In addition, bobbin-type LiSOCL2 batteries can be modified using patented hybrid layer capacitors (HLCs) or by other means to deliver the high pulses required to power advanced two-way communications. To maximize battery life, low power communications protocols will need to be utilized. In addition, the devices will need to operate mainly in a ‘standby’ mode that requires little or no power, periodically switching to an ‘active’ mode for data acquisition and transmission.
Thermal Energy Generators Permit Rechargeable Battery Solutions
Until recently, underground environments were ill suited for energy harvesting. However, thermal energy generators now allow energy to be captured from the daily warming and cooling of the upper soil subsurface, thus enabling the use of rechargeable lithium batteries to store the harvested energy.
Consumer grade lithium ion (Li-ion) batteries could offer an inexpensive solution for WUSN devices that are not intended to operate for more than 5 years and 500 full recharge cycles. Consumer grade Li-ion cells are limited by a gradual degrading of the cathode, which causes these batteries to become less receptive to successive recharging, and thus further reduces battery life. Li-ion batteries are also made with crimped seals, which are prone to leakage and corrosion.
If the WUSN needs to operate for decades, then an ideal solution may be to power the device with a TLI Series industrial grade Li-ion battery. These ruggedly constructed cells are capable of delivering up to 20 years of service life and 5,000 full recharge cycles. TLI Series batteries also feature a very low annual self-discharge rate, the ability to be recharged in extreme temperatures (-40°C to 85°C), are capable of delivering up to 15A pulses from an AA-sized cell, and have a glass-to-metal hermetic seal to better withstand harsh environments.
WUSNs face several technical challenges, the least of which is finding a power supply that is well adapted to underground environments.
Filed Under: Aerospace + defense, Capacitors, Energy management + harvesting