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Energy Storage – Batteries or Capacitors

Energy supply is a central concern in the design of IoT solutions. Most IoT devices run on batteries, and if the battery cells are unable to be recharged then a limit is in some cases set on the operational lifetime of the device itself. Energy supply also influences performance, limiting the number of measurements that can be taken, for example, and thus reducing the meaningful insight that can be derived from data analysis.

It is possible to harvest energy and mitigate both these problems. Energy can be retrieved through photo-voltaic harvesters, for example, and recent developments in the use of gallium arsenide to replace traditional photovoltaic cells makes solar harvesting more efficient than ever.[1] A recent paper that forecasts future developments in IoT energy storage describes the balance that must be achieved here. Enough energy has to be kept in reserve to power the device for longer, and enough energy has to be used to provide valuable insight:

[1] Shaikh and Zeadally (2016) outline other nascent technologies that could harvest energy from vibration and heat.

The design challenge turns into balancing energy harvesting and storage against the energy usage schedule. The energy cost for unit operations such as data sampling, computation and communication and the harvesting and storage capacity of different technologies then become important (Haggstrom and Delsing, 2018: 43).

As the authors suggest, other factors will limit battery life, including temperature. Despite this, batteries remain the commonest means of powering IoT devices because they allow for easy installation and mean that the device can be placed nearly anywhere. Most common among these are primary lithium ion batteries, since they are reliable, and have a far higher energy output per volume than their zinc oxide equivalents.

Ceramic capacitors are often seen as an alternative for long-life IoT devices, largely because they have increased in capacity and energy density in recent years. New electrode materials mean that supercapacitors are also able to hold more charge, and while they tend not to have the same energy density as lithium ion batteries (at best about 130Wh/kg compared to about 200Wh/kg for batteries), they are able to deliver charge more quickly, and tend to wear down less quickly.[1]

In the interests of minimising the number of on-site maintenance visits, to reduce emissions and reduce disruption to occupants, it will be important that IoT devices themselves become increasingly efficient – sending and receiving data over wireless networks using as little energy as possible. But advances in battery and capacitor technology are already allowing for technology solutions that are more ergonomic than ever before. Supplementing batteries with photovoltaic or piezoelectric or thermoelectric harvesting to recapture small amounts of energy from the surrounding environment would certainly help to lengthen the operational life of devices and ensure, in remote locations, that the risk of running power-dependent assets is reduced.

[1] https://spectrum.ieee.org/nanoclast/semiconductors/materials/supercapacitors-take-huge-leap-in-performance

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