Written by Asian Scientist Magazine, a content partner of 5G Talk Asia.
From connected home hubs and smart thermostats to the plethora of industrial and agricultural devices, the internet of things (IoT) has infiltrated almost every aspect of our lives. And resistance is futile—this increase in connectivity is expected to continue its rise with worldwide usage of IoT devices forecasted to triple and reach more than 25 billion devices before 2030.
Essential to enabling such IoT devices is the internet and electrical power. Currently, many devices draw energy from single-use, non-rechargeable batteries for cost and convenience. However, with the ever-increasing speed of adoption of IoT devices outpacing efforts to extend battery life, it is clear that single-use batteries which have to be replaced are incompatibile with a sustainable IoT ecosystem.
To tackle the challenges posed by the impending avalanche of IoT devices, innovators have turned to energy harvesting. Here, we explore the challenges and opportunities of energy-harvesting methods that have the potential to drive a sustainable IoT-powered future.
Collecting and converting energy
The working principle of an energy harvester is straightforward. It transforms the energy available in its immediate surroundings into electricity, which feeds the IoT device and powers it—eliminating the need for battery replacement. With the advancement of materials and technologies, we can expect several energy sources, namely mechanical motion, electromagnetic radiation and thermal gradients, to be harnessed to power IoT devices.
Among the available methods, perhaps the most straightforward is kinetic energy harvesting—converting mechanical energy of motion or vibration into electrical energy. This method is particularly applicable for IoT appliances that are used in motion or attached to vibrating objects. For example, a triboelectric smart mat enhances users’ exercise experience and keeps them safe, while serving as a power source for other IoT devices in the vicinity.
Similarly, the invisible radio frequency (RF) radiation that underpins wireless text-messaging and movie streaming services may also prove to be a viable power source. Dubbed RF-energy harvesting, this method converts the energy received from incident RF radiation to fuel devices.
RF energy can be harnessed from ambient RF signals used in wireless data transfer, like Wi-Fi signals, or from dedicated RF radiation generated for wireless charging, like those from a dedicated wireless power transfer network. Various innovative technologies, such as RF energy harvesters can be designed to obtain the energy from a central hub and convert it to direct current. It can be applied for long-range wireless power charging to extend a sensor’s operating lifetime.
Alternatively, heat flux can prove to be successful too. Thermoelectric generators (TEGs) are designed to produce electrical output through the Seebeck effect, where power is generated through the temperature gradients between two surfaces. For example, the MATRIX Prometheus TEG can produce electrical output from small temperature gradients, making it ideal for wearables, industrial process monitoring and waste heat harvesting.
Striving for sufficient energy streams
Despite the potential for these energy harvesting techniques to transform the IoT ecosystem, such methods come with challenges that prevent them from replacing the need for batteries.
One of the hurdles is the varying and often low strength of ambient power sources, which can lead to uncertainty in the power output from an energy harvester. Thus, a power conditioning circuit or boost regulator that accepts an ultra-low input voltage is required before the appropriate voltage can be generated and supplied to an IoT device, or transferred to an energy storage element.
Another significant barrier to overcome is the possibility that an IoT device operation would be interrupted when the fluctuating ambient energy becomes insufficient. To address this issue, an energy storage element, such as a rechargeable battery or a supercapacitor, is needed to continue powering these devices.
For example, a solid-state ultrathin rechargeable battery which has high energy density may be used. The flexibility and bendability of this battery also allows it to be easily integrated into small IoT devices.
The energy harvester can be used to charge the rechargeable batteries, so that there is no need for an external charger or frequent change of batteries. There are integrated circuits that provide an interface to the harvested energy sources, rechargeable batteries and IoT device, which can form a complete battery charging and power management system.
Ultimately, to overcome the inherent instability of the harvested ambient power source, a hybrid solution combining energy harvesting technology and energy storage elements is required.
Energy harvesting has the potential to power long-lasting IoT ecosystems that can operate unattended for extended periods of time. Though several challenges limit deployment of more energy harvesters today, innovative solutions can lead the way towards maintenance-free, perpetually powered IoT devices.