Low Voltage Energy Harvesting and Storage for Wireless Condition Monitoring
Introduction
There is extensive literature regarding the use of high capacity rechargeable batteries as energy storage for EV’s and backup power applications, and for 12V rechargeable batteries for use with Solar cells. Less is written about application of rechargeable energy storage technology for wireless sensors (IoT). Applications more commonly discussed might be smart agriculture or smart city applications transmitting relatively low data content such as environmental conditions, remote control signals, UI data from PC peripherals and pricing data from electronic shelf labels. But what about more demanding industrial IIoT applications such as wireless Condition Monitoring?
As deployment of IIoT systems such as asset health monitoring accelerates, battery replacement as a cost of ownership is under increasing scrutiny. Despite this interest, there is still relatively little useful discussion of the operational life of the replaceable primary cell. While there is an expectation of continued improvement in primary cell battery life, the question is often asked – why don’t we see more energy harvesting (EH) technology use to reduce the cost of ownership for industrial IoT?
IIOT Cost of Ownership
In the drive toward the efficiencies of the digital factory, industry has increased the deployment of wireless Condition Monitoring to monitor the health of their assets, with increased visibility to the hidden cost of replacing batteries. Every consumer is familiar with the inconvenience of replacing the smoke alarm battery in the middle of the night, but what about the maintenance manager who has to keep 100’s of wireless sensors up and running in an industrial facility, many deployed in hazardous environments or locations that are difficult to reach?
“Our experience with short battery life has been costly.”, says Tim Weilbaker, Vice President & General Manager of Allied Reliability’s Reliability Solutions group, a leading Predictive Maintenance services company. “With more than 20,000 wireless sensors of various brands deployed over the past 5 years, our data indicate that wireless sensor consumers should budget 8-10% of the initial acquisition cost as an ongoing operating expense for the maintenance and upkeep of the wireless devices. About 90% of the ongoing maintenance costs are attributable to battery life not achieving expected longevity.”
IOT Operation and Energy Consumption
A wireless sensor will have several operational modes managed by the radio SOC (microcontroller), generally guided by the radio and protocol specific firmware. When not measuring, connecting or transmitting the device microcontroller will instruct all functions to enter a ‘sleep’ state.
IoT operation impacts the selection and effectiveness of energy storage in more than one way. For example, frequent relatively high current pulses associated with device power on or radio transmission may degrade some Li-Ion battery technology capacity over time.
For this reason when the load consists of a periodic high current period (measurement and transmission), designers often add an EDLC supercapacitor in parallel to the primary battery to deliver the power needed during transmission, which is then recharged by the primary battery cell during Sleep mode. The capacitor acts as a filter for the current surge, minimizing the sudden load on …
