26 Jul Future Directions in High-Power Wearable Biomechanical Energy Harvesting Technologies
Energy harvesting has been attracting attention as a technology that is capable of replacing or supplementing a battery with the development of various mobile electronics. In environments where stable electrical supply is not possible, energy harvesting technology can guarantee an increased leisure and safety for human beings.
Wearable devices that can be worn by humans in daily life or in special environments have been highlighted with the development of portable electronic devices and IoT (Internet of Things) technologies. These devices are of various types and are available in forms such as smart glasses, smart clothes, biometric sensors, artificial joints, laptops, and mobile phones. They require power ranging from several mill-watts to several tens of watts for operation. The use of wearable devices consuming high amounts of power increases the weight of batteries that are carried together and requires periodic charging. For example, in modern warfare, soldiers carry a 7 kg-battery for 72 h operation of GPS devices, telecommunication equipment, and other equipment. Heated clothing is used to warm the body in outdoor activities at cold temperatures, which requires a power of ~10 W and about 1 kg of Lithium-ion battery for a 10 h-usage. Therefore, energy harvesting devices have been studied as an assistant energy source for batteries or independent energy sources for the permanent use of wearable devices without restrictions associated with power consumption. Human energy can originate from a chemical or a physical energy source. Typical sources of physical energy include the thermal and kinetic energy of the human body. Wearable thermoelectric devices convert heat from the human body into electricity of several μW continuously without affecting the human body. The human body generates kinetic energy in various forms by using muscles, such as foot strike; motions of joints such as ankle, knee, hip, arm, and elbow; and centre-of-gravity (COG) motion of the upper body. Among the human body motions, lower limb motions, such as ankle, knee, and hip motions induce high biomechanical energy because these joints generate a larger torque than other human joints. In particular, during walking or gait motion, the abovementioned motions are periodically repeated, indicating that energy can be harvested continuously. The frequency of gait motion is normally about 0.5–3 Hz which corresponds to a walking speed of 1.3–7.8 km/h. Therefore, many studies have focused on harvesting biomechanical energy generated during gait motion.
Energy harvesting principles for mechanical kinetic energy include piezoelectric, triboelectric, and electromagnetic energy harvesting. Electromagnetic generators can be categorized into two types, namely inertial induction, and gear-and-generator. Piezoelectric energy harvesting is based on the piezoelectric effect, in which an electric charge accumulates in certain materials in response to applied mechanical stress. Piezoelectric materials include crystals, ceramics, polymers, and proteins. Triboelectric energy harvesting is based on the triboelectric effect, in which a material become electrically charged when it comes into frictional contact with another material. Once the two materials are charged, the displacement between two can generate an electric current through an electrode connecting them. The farther the two materials are in the triboelectric series, the more charge can be obtained. Electromagnetic energy harvesting generates electricity based on Faraday’s law. In inertial-induction-type energy harvesters, a permanent magnet is made to vibrate or oscillate relative to a coil using human motion. In gear-and-generator-type energy harvesters, the human motion is amplified by a gear train and the amplified motion is used to operate a rotary generator. Among these principles, inertial induction, piezoelectric, and triboelectric generators harvest relatively low amounts of power, of the order of several mW or lower, albeit with high power per unit volume or unit mass. Meanwhile, gear-and-generator-type energy harvesters are bulky and lead to high consumption of metabolic energy, but they can generate the highest power, up to 10 W.
To directly drive or to assist a mobile electronic device of watt-level power consumption, an energy-harvesting device must generate watt-level electric power. Considering only generated power, the gear-and-generator-type electromagnetic energy harvester is the best candidate, and other types of energy harvesters cannot be used individually to generate high power. However, considering growing power requirements and the applications of various power ranges, combining energy harvesters based on two or more different principles is more effective than using a gear-and-generator type alone.
Electromagnetic Energy Harvester: Gear-and-Generator Type
The method of operating a rotary electric generator is a classical power generation technique. Devices based on this technique generate significantly higher power than those based on other techniques. With such devices, it is possible to harvest energy by using the centre-of-gravity movement of the upper body, the foot-strike, lower limb movement, and knee joint motion is possible, and watt-level power can be generated at normal walking speed. To develop an appropriate voltage of approximately 5–24 V from a portable rotary generator, it is necessary to achieve a rotation speed of 3000–6000 rpm by amplifying the speed of the human body motion using the gear train. Linear motion of inertial mass, such as foot-strike and COG movement, is converted into a rotational motion of the generator by a rack gear. Rotational motion such as knee joint motion, is amplified using a multi-stage gear train to match the rated speed of the generator, thus optimizing power conversion efficiency. In the case of gear and rotary generators, reaction torque developed at the time of power generation is transmitted to the human body, leading to consumption of a corresponding amount of metabolic energy.
Despite their large reaction torque and bulky structure, gear-and-generator-type energy harvesters have become meaningful in biomechanical energy harvesting owing to the emergence of research on using negative work in human gait motion. Negative work refers to metabolic energy consumed by joints during a decelerating motion. The mechanical energy conversion efficiency of metabolic energy during gait motion exceeds that of all other body motions because negative work is stored in and reused by the muscles. If a generator that absorbs negative work is placed on the knee, it is possible to harvest electrical energy and simultaneously reduce the burden on the muscles.
In the future, WBEHs will be developed along two major directions. The first is low-power and low-burden energy harvesters to supply power to wearable devices such as implantable power sources in humans/animals. Such devices should be light weight, and their consumption of metabolic energy should be negligible to ensure the wearer does not feel discomfort. Flexible and stretchable devices would be suitable for these applications. Thermoelectric, triboelectric, piezoelectric, and inertial induction methods will be used, and the outputs will range from several μW to several mW. Miniaturization of power conversion circuits and battery systems could be of interest. The second direction is high-power WBEHs for charging mobile electronic devices such as laptops, cell phones, and radios. The gear-and-generator method is a promising solution to generate more than 10 W.
The resulting high-power WBEHs would be essential for military purposes from the viewpoint of enhancing safety, efficiency, and survival. In addition to high power, it is essential to ensure low metabolic energy consumption (COH, TCOH). To improve power output and efficiency of WBEHs, their mechanical design should be optimized, for example, by developing gear trains with small form factors and low friction, and though precise alignment of shafts and bearings. Such WBEHs must be sufficiently light weight to reduce metabolic energy consumed in carrying them. To this end, the weight of bearings, gears, and frames, which are essential mechanical elements of these devices, should be reduced. New materials such carbon fibres, ceramics, and composite polymers could be considered as materials for chassis, gears, and bearings. Generative braking or the use of negative work can be expanded to other human motions such as those of the ankle or hip. Finally, combinations of different types of WBEHs can be used viably for stable power generation under various circumstances.
This is an excerpt of the journal article: Wearable Biomechanical Energy Harvesting Technologies, by Choi, Young-Man; Lee, Moon; Jeon, Yongho. Published: September 27. 2017 in Energies 2017, 10(10), 1483; DOI: 10.3390/en10101483 under a Creative Commons Attribution License (CC BY 4.0).