Energy harvesting shows the way to a sustainable future
Batteries are essential for billions of IoT products worldwide, and demand looks set to grow. The global Li-ion battery market was estimated to be worth over $40 billion in 2020 and is expected to increase to more than ten times this amount by 2030 according to analyst Statista.
Batteries are especially important to IoT devices used in remote monitoring solutions. Many of these products are designed with power consumption in mind and can last for months or years between battery replacements. However, when multiplied by millions, even the occasional need to replace a device’s batteries is time-consuming and expensive. And the disposal of millions of batteries generates a significant environmental impact.
Moreover, because many batteries contain lithium - typically sourced from mining or extraction from salt deposits—production can also have a large environmental impact. According to an article in Wired magazine, mining lithium on the salt flats of South America requires 2.27 million liters of water per tonne of lithium.
Limiting environmental impact
Extending battery life (or even eliminating cells) is becoming more practical due to advances in battery technology and the efficiency of IoT chips such as Nordic’s SoCs, SiPs, and companion ICs. Such advances make it feasible to charge batteries by harvesting energy from ambient sources (such as solar energy, vibrations, thermal energy, and wind energy). Harvesting energy mitigates some of the environmental impacts of batteries and introduces a more sustainable option.
A recent technical report (Energy Budgeting for Embedded IoT Solutions Relying on Energy Harvesting) from the KTH Royal Institute of Technology, Sweden’s largest technical university, detailed the viability of harvesting energy for a heart rate sensor. This medical device was designed to detect panic attacks—recognized through an abnormal surge in the user’s heart rate—before transmitting an alert and the patient's identity to healthcare workers.
The report found that the heart rate monitor—which uses Nordic's ultra-low power nRF52840 SoC for computation and wireless connectivity—was able to collect and transmit the data using "energy partially collected from a vibrational energy harvester and stored in a supercapacitor ."The harvester works by gathering vibrational energy generated by the wearer’s heartbeat.
An alternative to batteries
Effective energy harvesting is vital to such applications. Still, how this energy is stored can also significantly impact the end product's overall viability and eventual capability. For instance, the energy-storage device must retain some power to ensure functionality when there’s little or nothing to harvest from the environment. Consider a solar-powered asset tracking device, for example. There might be times when it needs to transmit data at night or on a cloudy day.
Batteries are a good option for energy storage but present a key engineering challenge; rechargeable devices such as Li-ion cells degrade slightly each time they’re recharged. This means the batteries will still need to be replaced eventually - with the associated inconveniences of maintenance and disposal.
Supercapacitors promise a solution. The component is essentially a high-capacity capacitor capable of storing energy for long periods. It is capable of fast charging and discharging to provide power to the end product when harvested energy is unavailable. And supercapacitors can be recharged many more times than Li-ion batteries. The heart rate monitor described in the KTH Royal Institute of Technology report employed a lithium-hybrid supercapacitor, which provided a rated lifetime of five times that of a Li-ion cell at 50,000 cycles.
However, IoT medical devices like this heart rate monitor tend to require power peaks due to complex processing requirements and/or frequent transmission. The KTH Royal Institute of Technology report concluded that although the harvester contributed positively to the power of the device, it could not produce enough energy to charge the supercapacitor fully and would likely need the resources of a backup battery.
Energy harvesting operation
While the heart rate monitor’s vibrational energy harvester fell short of eliminating batteries, today's low-power wireless devices can run from some harvested energy sources alone. One example is SODAQ’s solar-powered asset tracker. Using Nordic’s nRF9160 SiP—which can enter ultra-low power sleep modes between radio transmissions—the SODAQ TRACK SOLAR device employs a half-watt solar panel. It features an accelerometer, light sensor, and temperature sensor. It can send location data to the Cloud using GNSS, Wi-Fi, or cellular IoT.
Similarly, AquaSensing’s Leak Sensor 1.0 can function entirely using energy collected from the environment. It works by harvesting energy from any fluid ingress to power the ultra-low power nRF52832 SoC. Using the SoC’s Bluetooth connectivity, the device can send an alert to the user’s smartphone (via the accompanying app) that a leak has been detected.
Battery-free devices point to the future
Energy harvesting from the environment can be beneficial in many ways. For example, it uses sustainable energy sources and reduces the manufacturing and disposal challenges generated by batteries. While batteries will remain important for some time, today’s energy harvesting solutions can significantly extend their lives.
As detailed in the KTH Royal Institute of Technology paper demonstrated, there are still some limitations for certain forms of energy harvesting, such as vibration. But as wireless chips and power management become more efficient, energy harvesting will likely fulfill its promise and become part of a brighter future.