The development of a sensor-based economy is contingent on our ability to install sensors and actuators in a great variety of contexts. These contexts may vary from our own bodies to concrete structures or remote industrial sites. These devices need to be powered in order to fulfill their function and mains electricity is not always available, nor is it always feasible to replace batteries. Because of this, researchers and businesses across the globe are looking into ways for these devices to harvest sufficient energy from their environment.
- Many IoT devices may have to be installed in places where it is too expensive, undesirable or downright impossible to attach a power cable or replace batteries. Such use cases include medical devices (i.e. so-called insideables), the monitoring of remote infrastructures and sensors buried in roads or bridges.
- Energy harvesting can rely on light by means of solar cells that operate even at low light conditions (200 lux, as in a typical living room). Self-powered cameras can combine image sensors with solar cells to harvest energy from the light they capture through their lens. These no longer have to rely on a separate solar panel (even to send footage to a base station) and this reduces their overall weight and size.
- Piezo-electric (for acceleration or oscillating movements) and tribo-electric (for continuous movement) power generators can harvest electricity from kinetic energy. Applications in development include smart tires that can tell what kind of surface they are on (e.g. icy conditions) or sensors that monitor a patient’s recovery after a joint surgery, which harvest energy from the movement of that same joint. Similar technology is used by parking space sensors that detect passing cars and harvest energy from the cars driving over them.
- Thermoelectric devices (making use of the Seebeck effect) can produce a current based off a temperature difference. These can thus draw energy from engines, bodies or other heat-producing things.
- The market for this kind of energy harvesting technology is estimated to grow to $662m by 2023 at a CAGR of 10%.
- In cases where sensors are only needed for a limited period (e.g. hardening concrete or tracking wildlife), small non-rechargeable batteries will suffice and these may even become fully biologically degradable to take away concerns over the environmental impact of such disposable sensors.
Connecting the dots
From relatively simple sensors to more complex (smart) devices such as microcontrollers, micro-electro-mechanical systems (MEMS) and larger actuators, they all need electric energy to perform their tasks. Some can be connected to the electric grid or make use of batteries, but in many instances, this is too expensive and unpractical. This includes applications in remote areas or places that are difficult to reach (e.g. inside our bodies) or even impossible to get to once they are in place (e.g. buried in the concrete structure of a bridge). For these devices to operate over longer periods, recent technological developments could enable them to extract sufficient energy from their direct environment.
This so-called energy harvesting can make use of different forms of energy, including (sun-)light, temperature differences, movement (e.g. vibration), pressure or radio signals. The amount of energy that can be harvested varies in this order as well; solar energy is the most powerful and radio signals are the weakest. Also, none of these methods are entirely new (e.g. PV solar panels have been around for more than half a century), but improved efficiencies and miniaturization will expand the range of applications and make it possible to power a host of tiny smart devices.
There are roughly two approaches to harvesting energy for sensors. In the first, energy is harvested by a dedicated power generator and used by a sensor and, typically, a data transmission unit. In the second, and most elegant and robust solution, a sensor generates power on its own by “simply” harvesting what it measures.
To illustrate, a piezo-electric sensor can measure acceleration or vibration only because it converts mechanical energy into electric energy and this energy may be used to process the output and/or send it to some kind of base station. This integrated solution is preferable as it reduces costs, is more reliable (i.e. it has fewer components) and takes up less space and weight. However, the energy that can be harvested from measurements alone is not always sufficient or, in some cases, much more harvestable energy may be available in other forms. To illustrate, a tire pressure sensor could possibly harvest some energy from minute tire pressure changes, but much more energy can be harvested from tires rotating or accelerating.
While some applications are already commercially available (e.g. sensors for smart home or office systems), others are still in the laboratory or pilot phase. In those cases, higher energy efficiencies are necessary to harvest sufficient power or reduce the size of the harvesting device. In other words, mass commercialization of these kinds of self-sustaining sensors and other IoT devices is still some years away. Until that point, the rollout of the IoT is somewhat hampered by practical and financial concerns over power supply by traditional means. More interestingly, once the size and cost of these self-powered devices is lowered sufficiently, one can easily imagine how consumers, businesses and governments will start measuring, recording and controlling more than they ever thought possible or desirable, let alone necessary.
- Energy harvesting will go hand in hand with energy savings on the application end. This includes modes of low-power transmission of data over 5G (under so-called MMTC specifications – mass machine-type communication – or LoRa networks) or Bluetooth low energy.
- Even in contexts where energy is readily available (e.g. a building or vehicle), energy harvesting may be cost-effective as it reduces maintenance (e.g. replacing batteries) or saves the cost and trouble of wiring (e.g. a car contains kilometers of heavy and expensive wires) and self-powered sensors would also enable a more modular approach in designing and assembling.
- Self-powered sensors will provide a boost to surveillance initiatives of all kinds. One potentially problematic feature will be that these cannot be shut down altogether by a user (or anyone else). To illustrate, St. Louis University plans on placing Amazon Echo dots in all of its students’ homes, but students are free to use them or to shut them down. In a world full of self-powered devices, the ultimate means of shutting down a machine, unplugging it, will no longer be available.