Frictional and electromagnetic energy harvesters have wide applications in harvesting vibrational mechanical energy due to their respective material advantages and output characteristics. However, vibrations in daily life are often random, disordered, and directional. Harvesting mechanical energy in a specific direction can significantly reduce the efficiency of the device when it vibrates in other directions. Therefore, developing an energy harvester that can effectively harvest mechanical energy from vibrations in multiple directions is of great significance in establishing a self-powered system.
Recently, Professor Zhang Haixia's research group at the Institute of Microelectronics, School of Information Science and Technology, Peking University, proposed an electromagnetic triboelectric hybrid energy harvester capable of harvesting mechanical energy from vibrations in any direction within a plane. The related research results, titled "Hybridgenerator based on freestanding magnet as all-direction in-plane energy harvester and vibration sensor," were published in NanoEnergy, a leading journal in nanoscience and technology. Doctoral student Chen Xuexian is the first author of the paper. This hybrid energy harvester uses a spring-fixed suspended magnet as a slider and utilizes FPCB technology to fabricate thin-film triboelectric generator electrodes and electromagnetic coils with an eight-lobed structure. Any two opposing electrodes form a freestanding triboelectric generator, allowing the device to simultaneously generate electromagnetic and triboelectric signals when sliding in any direction, thus improving the efficiency of mechanical energy utilization. Furthermore, the spring and magnet form an in-plane resonant system, enabling the device to effectively generate electrical signals within a low-frequency vibration range of 10Hz. Benefiting from the large output current of the electromagnetic generator and the high output voltage of the triboelectric generator, under combined charging conditions, the device can rapidly charge a 20μF capacitor to 7V within 200s. By attaching the device to a person's lower leg or a bicycle wheel, this composite energy harvester can effectively harvest the mechanical energy generated during running or bicycle braking and illuminate 40 LEDs, thus serving as an energy harvesting device for self-powered running lights or bicycle brake lights. Furthermore, through further analysis of the output signals from the four freestanding triboelectric generators, the device can also function as an active direction sensor, identifying the direction of sinusoidal or pulsed vibrations, showing broad application prospects in environmental monitoring, self-driven sensing systems, and human-computer interaction.
Figure 1: Overview of the structure of the electromagnetic friction hybrid generator
(a) Schematic diagram of the device structure;
(b) Schematic diagram of the internal structure of the slider;
(c) Eight-petal electrode structure of a triboelectric generator;
(d) Photographs of the device structure;
(e) Photograph of the internal structure of the device.
Figure 2: Working principle and simulation analysis of the composite generator
(a) Working principle of the device;
(b, c) Simulated potential distribution on the friction electrode when the device slides along the 11' direction and the direction between 11' and 44';
(d, e) Output of the four freestanding triboelectric generators when the device slides along the 11' direction and the direction between 11' and 44';
(f) Simulated magnetic field strength distribution of the electromagnetic generator;
(g) The change in magnetic flux in the electromagnetic coil when the magnet slides from one end of the device to the other.
Figure 3: Output performance of the composite generator under sinusoidal signal excitation
(a) Schematic diagram of slider motion under sinusoidal signal excitation;
(b, c) Voltage and current output of E11' triboelectric generator under sinusoidal signal excitation;
(dg) Comparison of the output of four triboelectric generators when the device vibrates in four directions;
(h, i) Voltage and current output of the electromagnetic generator under sinusoidal signal excitation;
(j) Comparison of the output of electromagnetic generators when the device vibrates in four directions.
Figure 4: Output power and charging capacity of the hybrid generator
(ab) Output power of the triboelectric power generation section and the electromagnetic power generation section under different loads;
(cd) Circuit diagram and charging curve of the composite generator charging the capacitor.
Figure 5: Direction recognition of pulse vibration signals by a composite generator
(a) Schematic diagram of the motion of the slider under pulse vibration;
(b) Output signal of electrode E11' under pulse vibration in the 11' direction;
(cf) Output signals of the four triboelectric generators when the device vibrates in eight directions;
The (g, h) difference method is used to determine the direction of vibration.
Figure 6: Application demonstration of the composite generator as a self-driven vibration direction sensor and energy harvester
(ab) Using the device as a vibration direction sensor to realize the whack-a-mole game;
(cf) Voltage output signal of the friction part of the device when the foot moves in 8 directions;
(h, i) Fix the device to the human lower leg to collect running mechanical energy;
(j) Fixing the device to a bicycle wheel to achieve a self-driving brake light.
Addressing the random, disordered, and directional nature of mechanical energy in daily life, an electromagnetic-triboelectric composite energy harvester has been developed, utilizing a resonant system composed of springs and magnets to collect mechanical energy from vibrations in any direction within a plane. Through a rational structural design, the resonant frequency of the device is lowered, enabling efficient energy conversion at lower vibration frequencies. Combined with the high output current of the electromagnetic generator and the high output voltage of the triboelectric generator, the device's charging capacity is significantly enhanced. Furthermore, thanks to the unique electrode structure design of the triboelectric generator section, the device can identify pulse vibrations in eight directions, thus serving as a self-driven vibration direction sensor and demonstrating significant application potential in environmental monitoring, self-driven sensing systems, and human-computer interaction.
