Recent passage of the $325 billion Transportation Bill paves the way for the revitalization of major portions of America’s critical bridge, tunnel, and rail infrastructure, with advanced remote wireless sensors being utilized to identify potential deficiencies and monitor ongoing safety conditions.
To address America’s deteriorating infrastructure, the US Congress recently passed a $325 billion Highway and Transportation Funding Act that paves the way for construction projects encompassing all 50 states.
This work is long overdue, as the American Society of Civil Engineers (ASCE) found that nearly nine in ten of our nation’s bridges are structurally deficient, requiring hundreds of billions of dollars in repair and maintenance. Meanwhile, the national electrical grid faces an equally daunting challenge to upgrade a largely antiquated system that could become a growing threat to national economic security.
While the price tag for nationwide infrastructure repairs and improvements continues to rise, remote wireless technology offers a relatively inexpensive solution for monitoring the revitalization effort by offering ‘truly wireless’ solutions that no longer need to be hard wired into the power grid, which can get very pricey, in some cases exceeding $100 per liner foot. Relatively low cost wireless technology effectively empowers federal, state and municipal transportation agencies to gain unparalleled insight into the real-time status of vital infrastructure.
The vast majority these remote wireless devices will be powered by bobbin-type lithium thionyl chloride (LiSOCl2) batteries, with certain applications suitable for energy harvesting in conjunction with rechargeable lithium-ion (Li-ion) batteries.
Remote Sites Demand More Robust Power Supplies
The tragic collapse of I-35W bridge in Minneapolis, MN in 2007, which killed 13 people, injured 145 more, and disrrupted travel throughout the region, demonstrates the clear need for proactive monitoring to look out for structural decay caused by temperature, humidity, strain, pressure deformation, or other causes.
Wireless monitoring systems permit long-term, maintenance-free operation, as battery replacement can require the use of special harnesses and scaffolding to protect worker safety, thus causing labor expenses to far exceed the cost of the battery itself.
If the application draws low rate current for extended periods, then the most commonly preferred choice is a bobbin-type lithium thionyl chloride (LiSOCL2) battery, which delivers high capacity, high energy density, along with a very low annual self-discharge rate. Bobbin-type LiSOCL2 batteries are also non-aqueous, thus permitting an extended temperature range of -55°C to +85°C.
The operating life of a LiSOCl2 battery can vary based on the quality of raw materials and the manufacturing processes used to construct the cell. For example, a lower grade LiSOCL2 battery may have an annual self-discharge rate of 2-3 percent per year, thus limiting operating life to 10 years. By contrast, a superior grade LiSOCL2 battery can feature an annual self-discharge rate of just 0.7 percent per year, thus permitting maintenance-free operation for up to 40 years: significantly reducing the total cost of ownership.
If the device is designed for wireless communications and/or remote shut-off capabilities, then the bobbin-type LiSOCl2 battery may need to be modified by adding a hybrid layer capacitor (HLC). The standard LiSOCl2 battery works to deliver the low continuous current needed when the device is in ‘sleep’ mode, and the HLC delivers the high pulses needed to initiate data interrogation and transmission.
Supercapacitors can also be used to deliver the high pulses. However, supercapacitors are mainly restricted to consumer grade applications due to inherent limitations, such as a high self-discharge rate (up to 60 percent per year), and a limited temperature range that prohibits their use in extreme environments. A supercapacitor made up of two 2.5 V capacitors also requires balancing circuits, which adds self-discharge and expense.
Here are several real-life examples involving the use of bobbin-type LiSOCl2 cells to power structural sensors:
Monitoring the Structural Integrity of a Tunnel
Multiple-Input, Tiny, Enhanced Wireless Instrumentation Systems (MITE-WIS) devices are utilized to monitor repaired concrete sections within a tunnel. A MITE-WIS unit is embedded into the concrete repair patch each time a concrete repair is made. Fireproofing maintains its integrity, and there is no need for large cables to pass through the concrete, which would otherwise create weak points for heat to reach the primary tunnel wall.
MITE-WIS units identify potential structural failures by sensing strain variations across the boundaries of concrete patches. Lithium batteries provide up to two years of service life for a sampling rate of three separate channels per minute. Each unit can store up to three months of data, which downloads during monthly maintenance procedures.
Monitoring Bridges’ Safety
SenSpot wireless sensors are utilized to collect and transmit vital data that gauges stress, tilt, and other conditions that can damage bridge infrastructure. SenSpot sensors require a robust power management solution that can support wireless communications, offering outstanding flexibility (as no two bridges are the same) and long-term reliability in extreme environmental conditions (since the major structural components of a bridge are often highly inaccessible).
Use of a LiSOCL2 battery permits SenSpot devices to operate for decades while continuously monitoring the status of critical bridge infrastructure.
Energy Harvesting Plays A Role
The underside of a bridge is continually shady, and thus not suited for photovoltaic (PV) energy harvesting. However, the future is bright for energy harvesting, especially with the emergence of the Industrial Internet of Things (IIoT) and the self-driving vehicle.trucure ications rather that indstial grade devices
Energy harvesting devices use rechargeable lithium ion batteries to store the harvested energy. However, standard commercial rechargeable lithium ion batteries have a relatively short life expectancy of approximately 5 years, with a limited temperature range, making them ill-suited for long-term deployment. Fortunately, an industrial grade rechargeable lithium ion battery has been developed that operates for up to 20 years and 5,000 recharge cycles, and offers a wider temperature range than standard rechargeable batteries.
These industrial grade Li-ion batteries were recently deployed by Southwire Company, LLC for use in a wireless line/connector sensor that mounts directly on a bare overhead transmission conductor to measure conductor temperature, sag on the transmission line, and electrical current on the line.
Energy harvested from the magnetic field of the electrical line permits readings to be transmitted, typically every 30 seconds, to a base station using 2.4 GHz RF communication. The line/connector sensor requires sufficient line current to fully recharge, so the energy harvesting device needs to be supported by a AA-size industrial grade rechargeable Lithium-ion (Li-ion) battery to permit roughly 45 days of continuous sensor operation, even where there is no line current or when the strength of the magnetic field drops below the threshold required for energy harvesting. The use of a 20-year industrial grade Li-ion battery results in a highly economical long-term solution.
Conclusion
Application-specific requirements dictate will dictate the ideal choice of power supply. Long-life bobbin type LiSOCl2 batteries and industrial grade Li-ion batteries providing design engineers with highly robust power supply options that are inexpensive to install and maintain, providing decades of maintenance-free operation to reduce the total cost of ownership.
