Energy harvesting, also known as power scavenging, is the term used to describe methods for powering IoT devices from its local environment, rather than by mains power or primary batteries. The main sources of environmental power are photovoltaic, thermoelectric, kinetic, and radio frequency. These are complement by energy harvesting and power storage systems.
A key misconception is that people equate power scavenging with perpetual life, that device will run forever. However, all systems have limitations. For example, a rechargeable cell powered by a solar panel will die after a period of time or a set number of cycles. So the intelligent design of energy harvesting systems is important, and this may or may not include a battery.
Kinetic energy harvesting systems are powered by physical motion. Available wherever thing are moving. Examples range from sources of micro-power, such as switches/buttons and watches/wearables through to larger sources such as wind and water.
The micro-sources produce a small spike of energy that is just enough to send a small piece of information. The larger sources do not have to be traditional wind power or hydroelectric systems. From an IOT perspective, it is possible to create miniature devices that fit inside pipes to power a single device.
It is possible to fit energy harvesting devices inside pipes with moving water to power an IOT device measuring the flow in remote locations.
Thermoelectric energy harvesting systems are powered by differences in temperature, usually between a source at a higher or lower temperature and the ambient environment. Thermoelectric sources are often available in industrial settings which often have, for example, cold or hot pipes. There are even products that can generate power from the difference between skin temperature and the surrounding air, to power a wearable device.
Solar, also known as optical energy, has been used for a long time has been used in many different applications because the power density that can be generated from a solar cell is reasonable significant for its size. The main challenge with optical energy is to model how big a solar panel, and associated power storage system, needs to be to make sure that an IoT system will function through natural variations in light levels and in the worst case scenario.
RF energy harvesting system, and the closely related induction charging, can extract energy from radio waves, in the same way that old crystal set radios extracted enough energy from AM broadcasts to listen to them without a batter. However, this approach has the lowest efficiency of all the harvesting techniques because the amount of power that must be broadcast in order to get a tiny little bit of power exchange over even a small distance is huge.
The most useful example of this technique is the use of passive RFID tags, which normally consist of a tiny chip and very thin antenna. As the RFID tag passes through a gate or scanner, there is a wireless power exchange that's very short range. The main reason RFID tags can be manufactured for few cents and last such a long time is because have no battery.
The main engineering challenge is knowing when it is appropriate to use energy harvesting. There are a small number of applications where energy harvesting just makes sense, such as switches and some solar cells on devices that are visited regularly. However, many people fall into the trap of including energy harvesting in their IoT design because they can, when it fact it might not make sense to use it. For example, a kinetically charged dog tracking collar is possible but a battery may much more cost effective.
- Possible applications where energy harvesting does make sense are:
- Unusual form factors –e,g, where you've got to get something really thin, woven into clothing etc.
- Massive deployment applications – e.g. where it's not commercially feasible to replace or recharge batteries.
- Inconvenient locations – e.g. places that are really difficult to get to.
Power storage option range from batteries through super-capacitors to solid-state options. The main factors to consider are cycle life, before the component needs to be replaced, the rate at which it goes flat, the overall storage capacity and the length of time the charge is available to execute the IoT device’s function.
A comparison of common power storage options. Diagram curtesy of Simon Blyth, LX Group.
High density rechargeable battery technologies generally have a self-discharge problem and can be hard to charge up using the small sources of power available via some sources of energy harvesting. Super capacities obviously only hold their charge for a very short time but provide an alternative in the right contexts, particularly where the device is being charged/discharged frequently. Examples may be on rotating equipment etc.
Energy harvesting chips
Many manufacturers are now making chip-based solutions that make it easier to design an energy harvesting system into an IoT device.
Comparison of a range of chip-based energy harvesting systems. Diagram curtesy of Simon Blyth, LX Group.
Selection of the right energy harvesting chip would relate to the overall architecture and design of the IoT device.
Key suppliers of energy harvesting technologies include:
- Infinite Power Solutions
Edited by Tim Kannegieter