Sizing Flexible Supercapacitors for IoT Applications

Energy harvesting, sensing, and wireless communication technologies have been enabling the rapidly growing Internet of Things (IoT) market. IoT devices are often found in applications such as wearable devices, meter readers, warehouse lighting systems, etc. Devices that are in hard to reach places (i.e out in the field, in the corner of a building, on top of a stop light, etc…) are often battery powered, which can be a nightmare to maintain. The financial burden can add up quickly as batteries powering the IoT device are replaced overtime.

This burden stems from the IoT device’s power source, the battery. Batteries have short operating lives and are not ideal candidates to provide the bursts of power needed to make a wireless transmission, which are critically important in IoT devices. One solution is to pair, or even replace, these batteries with a reliable and low maintenance option, such as an energy harvesting device (like a solar cell) and Capacitech Energy’s Cable-based Capacitor (CBC).

There are important design elements to consider. For example, the proper number of CBCs to be used and in what arrangement will rely on certain factors such as what the solar panel is capable of harvesting and the minimum power consumption of the IoT device being used. Not enough CBCs and the IoT device may not be powered, too many CBC’s or improper format and the energy harvesting device will not adequately charge the CBCs in a timely manner.

Characteristics to Consider in IoT Energy Harvesting

First consider the minimum power requirements and typical power profile of the device. The minimum power requirements are likely to be what the device will draw when measuring data or processing data. The maximum power requirement will be seen when that data is being sent via Bluetooth, Wi-Fi, or Cellular networks. Now ask yourself how about the timing of these events. How frequently do we need to collect data? How often does data need to be sent somewhere off the board? The answer paints a picture of your power profile. To complete the initial exercise, consider where the device will be physically located, and which sources of energy can be harvested. If the application is indoors, an indoor solar cell could be a great fit, but that same solar cell in an outdoor application may succumb to its environment. You may also want to consider the application and function. Does this IoT device need to be powered 24/7 or only when a certain requirement is met (factory lights are on, workers present, etc…).

Sizing a Flexible Supercapacitor Module for IoT

The key advantage of flexible supercapacitors like the CBC is that they can be used in really interesting ways and in places supercapacitors never have been used before. Instead of restricting supercapacitors to a printed circuit board, the CBC can be hidden away inside of a wire harness, coiled into a corner of an electronics enclosure, or run alongside the leads/terminals from the energy harvesting device to the circuit.

At this stage in the design, the question is how to determine the correct number of supercapacitor CBCs are needed in the application. We always start with voltage. What is the maximum voltage the CBCs will see in your design? This could be directly from the energy harvester (such as a solar cell’s open circuit voltage) or what a power management circuit is designed to charge the energy storage component to.  

An individual cell of the CBC can be charged to a maximum of 1.6V. For applications that would subject the CBC to higher voltages, cells should be connected in series. This is because the maximum voltage that can be applied to a module of supercapacitor CBCs increases with series connections. For example, two CBC cells in series can be charged to a maximum of 3.2V and three cells can be charged to 4.8V (each cell added in series increases the maximum voltage by 1.6V). It does not hurt to overestimate what the max voltage applied to the supercapacitor module might be. Doing so will allow for a comfortable margin and will reduce the voltage applied to each cell.

After identifying how many solar cells are required in series, the question becomes how much power is needed from the supercapacitor and for how long? What is the peak power requirement? This will show you how much current needs to be drawn from the CBCs. Each series strand of the CBC can deliver up to 1.25A. Two series strands in parallel can deliver 2.5A. Three strands, 3.75A and so on. Also, how intermittent is the energy harvesting device? This is to ask how long the energy storage bank might need to power the IoT device on its own. We cover calculating the energy stored in a supercapacitor module in Designing (flexible) Supercapacitor Modules with the CBC.

Sizing Energy Harvesting Modules – Solar Cell Edition

Additionally, the proper size (and even quantity) of the solar panel(s) needs to be determined. Most IoT devices do not need a large panel, but instead the right panel for the purpose of the IoT device. Location of the IoT device(s) needs to be understood, as different lighting scenarios may breed various requirements. The indoor lighting of a warehouse is not the same as an office space directly next to a large window, and neither are comparative to the direct sunlight in an open field. Using an outdoor solar cell inside will leave performance on the table and using an indoor solar cell outdoors may permanently damage the solar cell. Not only does the environment need consideration, but also form factor. Do the solar cells need to be flexible, or can they be rigid? If the plan is to install the solar cell on the top of an electronics enclosure, then a rigid solar cell is perfectly suitable. If the plan is to wrap the entire product around the handle of a bike, something more flexible is required.

Solar cells can also be connected in series to increase their voltage output and in parallel to increase their current input. These characteristics may be critically important for the proper operation of a power management device or a maximum power tracking device.

Conclusion

Each design has its unique challenges. For some applications, it may make sense to use additional energy harvesting devices and fewer energy storage devices. For others, it may be the opposite. Additional information can be found in a previous blog post describing how flexible supercapacitors impact energy harvesting technologies in IoT to include various technologies, power constraints, and intermittency of power. 

 This article is only an introduction. There are many components of an energy harvesting and IoT design that should be considered. CBCs can be arranged and modified for a litany of applications. Please reach out with any questions.