Nature is exceptionally efficient. It maximizes available and additional resources by using as much of it as possible. Humans are now beginning to follow in nature’s footsteps. Doing this allows us to improve performance, thereby reducing waste and minimizing cost. One of the methods in use today is energy harvesting. We can power electrical devices using ambient energy. For devices operating on batteries, it is possible to use energy harvesting for extending the useful life of the battery, or even replace the energy contribution of the battery entirely.
We have ultra-low-power microcontroller units or ULP MCUs as the logical choice for demonstrating energy harvesting. Many devices like wireless sensors, wearable technology, and edge applications use ULP MCUs because it is essential for these devices to extend their battery lives. Reviewing the working practice of energy harvesting is important to understand its value to ULP MCUs.
The principles of energy harvesting are simple. It must overcome the finite nature of the primary source of energy, here, the battery. However, as no process can be one hundred percent efficient, there will be losses when converting the source power to usable energy, even when there is boundless ambient energy available for capture. This is evident in wind turbines, a renewable large-scale energy source. The wind provides the turbines with potential energy, making the blades rotate. This movement turns a generator, producing electrical power. Other similar large-scale ambient energy sources also exist—geothermal heat, oceanic waves, and solar.
Wearables and other similar small-scale devices harvest thermal, kinetic, or environmental electromagnetic radiation energy. However, each of these makes use of different mechanisms for converting the source power to useful usable energy. It is necessary to consider the utility and practicality of each conversion mechanism, as the application defines the size and mass of the energy conversion technology.
For instance, making use of thermal radiation is more suitable for wireless sensor applications, as the sensor placement and design can take advantage of both forms of energy. Likewise, vehicles can use sensors that make use of radiant heat emanating from the road surface. As engine components like wheels are high-vibration locations, it is possible to harvest motion energy from near them. For wearables using ULP MCUs, harvesting the kinetic energy from the human user’s motion provides the most practical means of conversion to usable energy.
In wearable technology, the primary application of the ULP MCU is to process the edge data gathered by the sensor. And, it is critical to process this data with the minimum power consumption. Energy harvesting supplements the power from the battery, which has a finite amount of energy, and requires periodic replenishment in the form of recharging or replacement as its power depletes. There are three ways of capturing energy for ULP MCUs—using piezoelectric, electromagnetic, or triboelectric generators.
Kinetic forces compressing piezoelectric materials can make it generate an electric field, which can add as much as 10 mW to the battery. Harvesting energy from electromagnetic radiation like infrared, radio, UV, and microwaves can contribute about 0.3 mW of harvested power. Triboelectric generators use friction on dissimilar material surfaces rubbing together from mechanical movements like oscillation, vibration, and rotary motions to generate 1-1.5 mW of electricity.
