Energy Harvesting – Powerful Options for Remote Wireless Devices
Lithium thionyl chloride batteries, or energy harvesting devices coupled with rechargeable lithium-ion batteries, deliver reliable, long-term power for remote wireless devices.
Wireless sensors are becoming increasingly essential to everyday life, with new applications emerging that offer enhanced system functionality, such as periodic two-way communications and remote shut-off capabilities. Satisfying the power-hungry demands of these devices requires the careful choice of a power management system – a decision that takes on added importance if the device is intended for long-term use in an extreme environment and/or a hard-to-access location.
Two common options include a primary lithium thionyl chloride (LiSOCL2) battery, or an energy harvesting device combined with a rechargeable lithium-ion battery for energy storage.
Among the different battery chemistries available for extended use in remote locations, bobbin-type LiSOCL2 chemistry is preferred because of its high-energy density, wide temperature range, and low annual self-discharge rate.
Bobbin-type LiSOCL2 batteries have a proven track record of success in powering remote wireless sensors, including automatic meter reading devices used by the utility industry that were installed in the mid-1980’s, but were still operational after 28+ years on their original batteries. Achieving such incredibly long life demands an extremely low annual self-discharge rate, as, in many instances, the total lifetime self-discharge rate of the battery can be greater than the total amount of energy consumed by the device itself.
However, not all battery brands are alike in terms of annual self-discharge. Tadiran’s  (New Hyde Park, NY) XOL series of bobbin-type LiSOCL2 batteries can deliver an annual self-discharge rate of approximately 0.7 percent per year, retaining about 70 percent of its initial capacity after 40 years of self-discharge, while other brands of LiSOCL2 batteries could have a self-discharge rate of 2.5 to 3 percent per year, able to retain 70 percent of their initial capacity after 10 years of self-discharge.
In order to deliver enhanced functionality without sacrificing operating life, engineers design the wireless device to operate mainly in a “sleep” mode, during which time daily power consumption ranges from nil to a few microamps, followed by brief periods where the device is in an “active” mode that requires high-current pulses of up to several amps to energize the device for data sampling and communications.
Wireless sensors that are used in low temperatures, which periodically require high-current pulses, may experience lower transient voltage readings during the initial phases of battery discharge. This phenomenon, known as transient minimum voltage (TMV), is correlated to the chemical make-up of the electrolyte and/or the design of the cathode. Tadiran has developed two innovative solutions to combat TMV: PulsesPlus batteries for high-current pulse applications, and TRR Series batteries for moderate current pulse applications.
PulsesPlus lLiSOCL2 batteries are suited for high-current pulse applications, combining a long-life bobbin-type LiSOCL2 cell with a patented hybrid layer capacitor (HLC), which stores and generates high-current pulses.
For applications requiring moderate current pulses, Tadiran Rapid Response TRR Series batteries offer a cost-effective solution, because they do not require the use of an HLC, but still deliver high capacity and high-energy density without experiencing voltage drop or power delay. When a standard LiSOCl2 battery is first subjected to load, voltage can drop temporarily before it returns to its nominal value. TRR Series batteries eliminate this voltage drop under pulse (or transient minimum voltage level), resulting in zero delay during the voltage response. The batteries also use available capacity more efficiently, and can potentially extend the operating life of the battery by up to 15 percent under certain conditions, especially in extreme temperatures.
The emergence of energy harvesting technology is linked to the development of low power communications protocols such as ZigBee, Green Power, Bluetooth LE, and 6LowPan. These low power protocols enable certain wireless sensor networks to operate within a peak power range of 10 µW to 100 mW, which is the sweet spot for energy harvesting devices that draw energy from a variety of sources, including light, heat, RF/EM, motion, or vibration.
Energy harvesting technology is rapidly evolving and each type presents unique performance advantages and disadvantages, so design engineers need to perform careful due diligence to specify the correct power management solution based on application-specific requirements, especially when high reliability is a concern.
Energy harvesting devices need to be coupled with rechargeable lithium batteries that store the harvested energy. While consumer-grade rechargeable lithium-ion batteries have improved dramatically in recent years, these standard cells still have inherent drawbacks for remote wireless applications, including short operating life (max 5 years), low cycle life (max 1,000 cycles), high annual self-discharge (up to 60 percent per year), and limited temperature range (0° to 60°C) with no possibility of charging at low and high temperatures.
To address the limitations of consumer-grade rechargeable lithium batteries, Tadiran recently introduced TLI Series batteries that use the same technology found in their patented hybrid layer capacitor (HLC), which stores the high-current pulses required for wireless communications, and has been field-proven in millions of cells. The batteries modify this technology to deliver reliable, long-term performance under extreme environmental conditions, and offer unique performance features, including:
- The ability to deliver high-current pulses of up to 15 A for AA cell.
- A low annual self-discharge rate of < 5%).
- Up to 5 times more life cycles (5,000 full cycles).
- A longer operating life of 20 years.
- A wider operating temperature range from -40° to 85°C, with storage up to 90°C.
- Charging possible at extreme temperatures.
- A glass-to-metal seal (others use crimped seals that may leak).
Available in AA and AAA diameters and custom battery packs, the batteries can be recharged using DC power or by energy harvesting devices.
The following examples illustrate typical situations where remote wireless devices can be powered by energy harvesting devices supported by lithium-ion rechargeable batteries. Also highlighted is a wireless sensor application that is better suited to 40-year LiSOCL2 batteries for long-term power.
Logimesh (Fort Collins, CO) manufactures wireless sensors that monitor the engines used to drive natural gas production compressors, using energy harvested from the vibration generated by these compressors to offer a self-contained power management system. The sensors detect real-time vibration and exhaust temperatures, monitor the operation of individual cylinder valves, and produce valuable data that helps ensure safe operation and generates predictive models for long-term maintenance. Since natural gas compressors are often located in pipeline found in deserts and other inhospitable environments, a more ruggedized rechargeable lithium-ion battery was required instead of a standard consumer-grade battery.
The IPS Group (San Diego, CA) manufactures solar-powered, wirelessly networked parking meters that use TLI Series rechargeable lithium-ion batteries for energy storage and emergency back-up power requirements. The parking meters incorporate multiple payment system options, access to real-time data, integration to vehicle detection sensors, and user guidance and enforcement modules, all linked to a comprehensive web-based management system. The batteries are ideal for this application due to their ability to deliver high pulses and to operate continuously at extreme temperatures of -40° to 85°C, ensuring many years of maintenance-free system reliability.
Powercast (Pittsburgh, PA) designs and develops wireless sensor networks and devices for low-power applications employing both energy harvesting and LiSOCL2 battery technologies.
In one instance, Powercast developed a sensor that runs on RF energy harvested from broadcast radio or television signals, and/or RF transmitters located within 45 feet of the wireless sensor, a solution that was viable because the particular sensor required micro amps of power to operate, well suited to the RF energy sources that were nearby.
However, for a different application, the WSN-1101 wall-mounted sensor, Powercast chose to power the device with a hybrid LiSOCL2 battery instead of using energy harvesting, which was not applicable.
The WSN-1101 wall-mounted sensor measures a variety of parameters, including indoor temperature, humidity, and other variables in HVAC, lighting control, energy management, industrial monitoring, and medical applications. Designed for indoor use in temperatures ranging from -20° to +50°C, the WSN-1101 can transmit data once per minute for more than 25 years to the Powercast WSG-101 wireless gateway, which interfaces with wired Building Automation Systems (BAS) networks via industry-standard protocols.
Use of a long-life LiSOCL2 battery enables Powercast to offer a highly cost-effective and reliable 25-year solution that instantly converts any building into a smart building, offering an ideal upgrade for older structures with concrete or cinder block walls that cannot be easily retrofitted for hard-wired solutions.
These examples demonstrate the exciting possibilities presented by energy harvesting devices supported by lithium-ion rechargeable batteries, as well as the continued use of bobbin-type LiSOCL2 batteries, providing a dynamic set of options for powering remote wireless devices.
For more information visit www.tadiranbat.com .
This article originally appeared in the January/February print issue. Click here to read the full issue .