Compact, robust, and regulated-output hybrid generators for magnetic energy harvesting and self-powered sensing applications in power transmission lines

Abstract

Establishing maintenance-free wireless sensor networks for online monitoring of power transmission lines is crucial for realizing smart grids, exhibiting necessitating energy harvesters with compact volume, excellent robustness, and efficient, well-regulated output. This study introduces a hybrid magnetic energy harvester (HMEH), seamlessly integrating a magneto-mechanical energy conversion module, a non-contact rotational triboelectric nanogenerator (TENG) module, and an electro-magnetic generator (EMG) module for magnetic energy harvesting and self-powered sensing in power transmission lines. The HMEH converts magnetic energy into synchronized mechanical rotation using the magnetic phase difference principle and smoothly transforms it into a regulated electrical energy output. With a compact size of 5 cm × 5 cm × 3 cm and a light weight of 56 g, the HMEH showcases unprecedented volume and weight output power densities of 48.9 μW cm⁻³ and 65.5 μW g⁻¹, respectively, outperforming common current transformer (CT) energy harvesters in power transmission lines while avoiding issues like magnetic core saturation, leading to self-overheating or vulnerability to load breakdown due to current fluctuations. Furthermore, the non-contact rotational design ensures minimal performance degradation in both EMG and TENG modules, even after continuous operation for 1200 000 and 600 000 cycles, respectively. Environmental robustness testing verifies the IPX7-rated waterproof capabilities of the HMEH, along with its resilience against high temperatures, humidity, and vibration. All these demonstrate the exceptional robustness of the HMEH. Finally, a self-powered wireless power transmission line's temperature sensing system is demonstrated as a proof-of-concept. This work offers an effective strategy for self-powered wireless sensor networks within power grid transmission lines.

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Broader context

The development of smart grids, relying on renewable energy like wind and solar power, is crucial in tackling the global energy crisis and climate change. Power transmission lines, as the lifeline of the system, face risks of outages due to challenges brought by renewable energy sources' inherent volatility, intermittency, and wide distribution. Developing massive self-powered wireless sensor networks is vital for real-time monitoring of the status of power transmission lines, which highly requires compact, robust energy harvesters with efficient, well-regulated output. However, there is always a trade-off for self-powered technologies in power transmission lines including solar cells, CT energy harvesters, and ambient mechanical energy harvesters, among small size and weight, safety risks of current fluctuations, efficient and regulated energy output, and overall robustness. In this study, we develop a regulated-output hybrid magnetic energy harvester (HMEH) for fully self-powered sensing of power transmission lines, which showcases unprecedented volume and weight power density outputs of 48.9 μW cm⁻³ and 65.5 μW g⁻¹. Additionally, other significant features including compact size (5 cm × 5 cm × 3 cm), lightweight (56 g), IPX7-rated waterproof, resistance to current fluctuations, and environmental robustness are equally highly attractive. This work offers an effective strategy for self-powered wireless sensor networks within power transmission lines.

Introduction

The vigorous development of smart grids, which rely on renewable energy sources such as wind and solar power, holds the key to addressing the pressing global energy crisis and climate change. Power transmission lines, serving as the lifeline of the power system, carry the ever-present risk of costly power outages due to potential defects or unforeseeable faults. The integration of new energy sources into the grid, with their inherent volatility, intermittency, and wide distribution, further compounds the challenges to maintaining power grid stability. Against this backdrop, the deployment of a massive multi-parameter wireless sensor network,¹ as illustrated in Fig. 1A, emerges as critical to ensuring power safety by continuously monitoring the status of power transmission lines. Yet, the high voltage and vast geographical expanse of these lines pose significant obstacles to the direct power supply of these sensors. Moreover, the limitations in the capacity and lifespan of conventional batteries threaten the long-term operation and effectiveness of these sensors in their vital task.

Fig. 1 Hybrid magnetic energy harvester configuration. (A) Schematic illustration of the HMEH for self-powered wirelessly sensing applications of power transmission lines. (B) Expanded structural diagram of the HMEH, comprising two parts: the stator and rotor, and three core modules: the MMEC module, EMG module, and TENG module. The stator part includes a Si sheet, short circuit ring, Cu-coil, electrode, and fur. The rotor part includes a polytetrafluoroethylene (PTFE) film, polyamide (PA) film, and magnets. (C) Photographs of the stator, rotor and encapsulated HMEH device. (scale bar is 5 mm).

Self-powered technology effectively converts environmental energy into electricity, addressing the persistent power supply challenges faced by grid sensors.² Solar cells^3–5 and CT^6–11 energy harvesters are the go-to technologies for power grids. While solar cells offer impressive output, their size and weight limit their use in high-voltage towers. CT energy harvesters convert magnetic energy in power transmission lines into stable and regulated electricity, making them most commonly used in power transmission lines. However, several challenges, including inadequate output power, device damage resulting from high current magnetic core saturation, current fluctuations that can easily harm load components, and low-current dead zones, hinder the broader adoption of CT energy harvesters. Recently, advanced mechanical energy harvesting technologies like TENGs,^12–21 PENGs,^22–25 and EMGs,^14,15 converting wind and vibration energy around power transmission lines into electrical energy, demonstrate compelling advantages, such as high power density, lightweight, high current impact resistance, and low cost making them highly desirable solutions.

For instance, Hu et al.²⁰ innovated a highly efficient TENG device that harvests vibration energy from power transmission lines, generating 4.31 mW output power at 4 Hz frequency and mitigating wind-induced vibrations effectively. Gao et al.¹⁵ developed a self-powered power transmission line galloping sensor using hybrid TENG and EMG devices, generating 16.7 mW output power at 1.7 Hz galloping frequency, facilitating real-time monitoring of power transmission line galloping. However, mechanical fatigue and random wind/vibration challenge the long-term use of these mechanical energy harvesters due to poor stability, limited lifespan, and difficult-to-utilize disorderly electrical energy output. Thus, compact and lightweight energy harvesters with high environmental robustness, large current resistance, and high and regulated power output are greatly desired to achieve a cost-effective and maintenance-free smart grid.

Herein, we present a hybrid magnetic energy harvester (HMEH) designed for fully self-powered sensing applications in power transmission lines. The HMEH seamlessly integrates a magneto-mechanical energy conversion (MMEC) module, a ternary dielectric triboelectrification based non-contact TENG module, and an EMG module within a compact size of 5 cm × 5 cm × 3 cm and a light weight of 56 g. Leveraging the magnetic phase difference principle, the MMEC module drives a rotor to produce mechanical rotational motion synchronized with the frequency of the power transmission lines' magnetic field. The hybrid TENG and EMG modules then convert this rotation into regulated and continuous electrical output. With an efficient power management circuit, the HMEH outputs unprecedented volume and weight power density values of 48.9 μW cm⁻³ and 65.5 μW g⁻¹ among the reported power transmission lines energy harvesters, along with exhibiting exceptional resistance to high-current impacts. Furthermore, the environmental robustness testing verifies the IPX7-rated waterproof capabilities of the HMEH, along with its resilience against high temperatures, humidity, and vibration. Finally, a fully self-powered wireless power transmission line's temperature sensing system is demonstrated as a proof-of-concept. This work offers a compelling solution for self-powered wireless sensor networks within power grid transmission lines.

Results and discussion

Hybrid magnetic energy harvester configuration

Fig. 1B shows the schematic design of the HMEH, which mainly consists of a stator, a rotor and an encapsulation shell. The manufacturing process of these components is outlined in Fig. S1 (ESI†). For the rotor component, a cylindrical shell with dimensions of Φ1 cm × 2.5 cm is 3D-printed. This shell features six rectangular grooves for magnet placement. Permanent NdFeB magnets are then inserted into these grooves in an N–S alternating order, and polytetrafluoroethylene (PTFE) and polyamide (PA) thin films are alternately attached to the surface of the rotor after high voltage polarization treatment. For the stator component, a stator shell is created through 3D printing technology with dimensions of 5 cm × 5 cm × 3 cm. This shell incorporates internally designed symmetrical grooves, specifically tailored to house a pair of silicon steel sheets with a short-circuit ring, copper coils, and the rotor component. Four copper electrodes, measuring 2 cm × 1.6 cm, are affixed to the inner wall of the cylindrical groove, maintaining a 2 mm gap. Polyester fur is applied to fill the symmetric gaps between the copper electrodes. Finally, the rotor is securely fixed and suspended inside the stator through a rotating shaft. Simultaneously, the device is comprehensively packaged by utilizing a 3D printed shell and PDMS.

Fig. 1C showcases the physical photographs of the rotor component, stator component, and the fully encapsulated HMEH device. This HMEH device features a compact and fully enclosed structure, with a size of 5 cm × 5 cm × 3 cm and a light weight of 56 g (Fig. S2, ESI†), making it convenient to install on transmission cables. Furthermore, the PDMS encapsulation layer will endow the HMEH with excellent environmental robustness, waterproofing, and operational stability, as demonstrated in Video S1 (ESI†). These significant characteristics are especially attractive for distributed energy harvesters.

The components of the HMEH can be categorized into three modules based on their different functions: the magnetic-mechanical energy conversion (MMEC) module, the electromagnetic generator (EMG) module, and the triboelectric nanogenerator (TENG) module. The process of converting magnetic energy from the power transmission lines into electrical energy comprises two essential steps: firstly, the MMEC module converts the magnetic energy to synchronized mechanical rotational motion with the frequency of the magnetic field of power transmission lines; subsequently, the hybrid TENG and EMG modules generate regulated electrical output via this mechanical rotation. The subsequent sections will delve into the detailed structure and working mechanism of each module of the HMEH.

Operating mechanism and simulation analysis of the MMEC module

Fig. 2A depicts the simplified schematic diagram of the MMEC module in the HMEH, comprising a symmetrical pair of silicon steel sheets with a short-circuit ring and a rotor with magnets. Generally, most countries around the world use a power transmission frequency of 50 Hz for their grid transmission lines, except for the United States and some Asian countries with a frequency of 60 Hz. According to Faraday's law of electromagnetic induction, the power frequency current will generate a 50 Hz alternating magnetic field. Fig. 2B displays the simulated magnetic field distribution surrounding the power transmission lines. The strength of the magnetic field varies sinusoidally with time and is directly related to the current strength in the power transmission lines (Fig. S3, ESI†). The silicon steel sheet can concentrate the magnetic field surrounding the power transmission lines, enhancing the magnetic field strength around the rotor, as depicted in Fig. 2C. A short-circuit ring, mounted at one end of the silicon steel sheet, generates a magnetic phase difference at both ends of the silicon steel sheet via the law of electromagnetic induction (Fig. 2D). By mounting two sets of symmetrical silicon steel sheets with short-circuit ring, a bias magnetic field is produced around the rotor.

Fig. 2 Working principle and simulation analysis of the MMEC module. (A) Simplified structural components of the MMEC module. (B) Simulation of the magnetic field distribution in the cable (cross-sectional view) using COMSOL. (C) Simulation of the magnetic field distribution in the MMEC module with and without short-circuited rings (front views). (D) Comparison of the magnetic flux density at both ends of silicon steel sheets with and without short-circuit rings during a full current cycle. (E) Magnetic field distribution between the silicon steel sheets of the MMEC module at four cable current phases stages (90°, 180°, 270°, 360°). Simulation of torque values applied to the rotor under a full cable current cycle at (F) different current intensities, (G) different magnetic flux densities of magnets, and (H) different numbers of magnets.

Fig. 2E illustrates the simulated magnetic field distribution around the rotor at different phases (90°, 180°, 270°, and 360°) of the power transmission lines. It reveals the generation of a vortex-like magnetic field within an AC current cycle, which interacts with the magnets and facilitates the rotor's continuous rotation. Fig. S4 (ESI†) demonstrates the impact of silicon steel sheets on rotor torque in the MMEC module. Without the short-circuit ring, the magnetic field between the silicon steel sheets generates a sinusoidal torque on the rotor within an AC current cycle, but the total torque is zero, preventing rotor rotation. However, adding the short-circuit ring biases the magnetic field, generating a unidirectional torque on the rotor that continuously drives its rotation. Video S2 (ESI†) showcases the process of transitioning the rotor from a stationary state to rotation, including the stages of asynchronous acceleration and synchronous stable rotation. Fig. S5 (ESI†) exhibits the simulated torque variation with rotor speed, depicting an initial increase, subsequent decrease, and eventual zero torque when the rotor reaches a maximum rotational speed.

To enhance the driving capability of the MMEC module, we conduct a comprehensive study of the effects of power transmission lines, magnets, and short-circuit rings on rotor torque through simulation. Initially, as illustrated in Fig. 2F, an increase in the current within the power transmission lines—from 200 A to 1000 A—leads to a proportional rise in rotor torque, elevating from 1.1 N m to 1.28 N m. This is attributed to the heightened magnetic field strength within the rotor region due to increased power transmission line current. Secondly, demonstrated in Fig. 2G and H, augmenting the strength and number of magnets in the rotor significantly improves rotor torque. However, increasing the number of magnets inversely correlates with the maximum speed of the rotor (Fig. S5, ESI†). The relationship between the maximum speed and the number of magnets can be calculated as n = 60 × f/p, where n is the maximum speed of the rotor, f is the current frequency of the power transmission lines, and p is the number of magnet pairs. For example, under a 50 Hz AC transmission line, the maximum speed of a 4-magnet rotor is 1500 r min⁻¹. The detailed deduction process is presented in Note S1 (ESI†). Finally, Fig. S6 (ESI†) illustrates the rotor torque outcomes across various short-circuit ring diameters, areas, and turns. The diameter and area of the short-circuit ring have a minimal influence on rotor torque. Conversely, an increase in the number of turns in the short-circuit ring tends to result in a reduction of rotor torque. This effect arises from an excess of turns diminishing the magnetic induction intensity at the end of the short-circuit ring, thereby expanding the torque dead zone.

Integrating the MMEC module with mechanical energy harvesters like EMG and TENG enables the conversion of magnetic energy from the power transmission lines into regulated, stable, high-power electrical energy.

Optimization and characterization of the EMG module

Fig. 3A displays the simplified structure diagram of the EMG module, consisting of a rotor with magnets aligned in N–S alternating order and copper coils surrounding the rotor. Fig. 3B illustrates the working principle of the EMG module for converting mechanical energy into stable electrical energy. The distribution of the magnetic field is computed using COMSOL. Initially, a magnet aligns with the coil, maximizing the magnetic flux without current flow. As mentioned above, the MMEC module converts the magnetic energy in the power transmission lines and drives the rotor to rotate. As the rotor rotates from 0° to 30°, the magnetic flux decreases, inducing a forward current. Continuing from 30° to 60°, the magnetic flux increases and reaches its peak upon alignment. During the rotation from 60° to 90°, the declining magnetic flux induces a reverse current in the coil. Finally, a 30° rotation returns the system to a similar state, completing the power generation cycle and producing an AC electrical output.

Fig. 3 Optimization and characteristics of the EMG module. (A) Simplified structural components of the EMG module. (B) Schematic diagram illustrating the working principle of the EMG module under the relative rotation between the rotator and the copper coils in an angle degree of 30, respectively. (C) and (D) Short-circuit current and open-circuit voltage of the EMG module with different numbers of magnets. (E) Amplitudes of the short-circuit current and open-circuit voltage of the EMG module with 6 rotor magnets at various cable current intensities. (F) Dependence of the output current and power output of the EMG module on various external loading resistances ranging from 50 Ω to 1 GΩ. (G) Cycling characteristics of the EMG module for over 1200 000 working cycles and continuous operation for 24 hours.

For the EMG module, according to Faraday's law, the open-circuit voltage (V_oc) and short-circuit current (I_sc) can be expressed as follows:

where B is the magnetic field strength, Φ_B is the total magnetic flux in each coil, N is the number of turns in the coil, and R is the internal resistance of the coil.

The output performance of the EMG module is obviously influenced by magnetic flux and induction coils. Firstly, we examine the impact of the number of magnets in the rotor on the output performance of the EMG module. With 4, 6, and 8 magnets, we observe corresponding V_oc values of 1.29 V, 0.82 V, and 0.59 V, and I_sc values of 7 mA, 4.4 mA, and 2.6 mA, respectively (Fig. 3C and D). The frequency of these output signals is consistently 50 Hz (Fig. S7, ESI†), in line with our previous theoretical prediction (Fig. S5, ESI†). Increasing the magnet number in the rotor enhances driving force but reduces the rotational speed during synchronous rotation, ultimately leading to a lower electrical output. Subsequently, we investigate the effect of induction coils using Fig. S8 and S9 (ESI†). Decreasing the wire diameter reduces the coil weight but also diminishes electrical output performance. On the other hand, a higher number of turns in the induction coil results in a higher induction voltage. Considering the weight and output performance of the EMG module, we select a coil with 2000 turns and a wire diameter of 0.1 mm, paired with a rotor consisting of 6 magnets, as the optimal configuration.

Fig. 3E and Fig. S10 and S11 (ESI†) present the output performance of the EMG at different current intensities (100 A to 800 A) of the power transmission lines. The results indicate an increase in V_oc from 1.15 V to 1.33 V and I_sc from 4.93 mA to 5.59 mA. This is due to the coupled influence of the magnetic field of both the rotor magnets and the power transmission line current on the magnetic flux variation of the induction coil. The surrounding magnetic field intensity increases along with the power transmission line's current intensity. Fig. 3F and Fig. S12 (ESI†) display the voltage, current, and power outputs of the EMG module at various load resistances (50 Ω to 1 TΩ). Increasing the load resistance results in a gradual rise in voltage and a decline in the current of the EMG module. The maximum output power reaches 1.4 mW with a matching impedance of 100 Ω, calculated using the equation P = I²R, where R represents the load resistance, and I represents the instantaneous current. Furthermore, the EMG module exhibits exceptional operational and long-term stability, maintaining a constant electrical output even after 1.2 million duty cycles (over 24 hours) and a long operating period of over 42 days, as shown in Fig. 3G and Fig. S13 (ESI†).

Optimization and characterization of the TENG module

For the TENG module,^26–29 to achieve high output and ultra-stability, we implement a previously reported ternary dielectric triboelectrification based and rotational non-contact mode structure design³⁰ (Fig. 4A). The rotor surface features alternating high voltage polarized PTFE and nylon films as dielectric layers. The stator includes four copper electrodes as triboelectric induction electrodes, with polyester fur applied to the copper gaps for soft contact with the dielectric layer. The softness, large surface area, and mechanical stability of polyester fur function as a long-term charge emitter for dielectric PTFE and PA layers, facilitating greater charge transfer and assisting in replenishing dissipated charges over time. As a result, this design effectively resolves the issues of low electrical output and performance decay over time often experienced in non-contact TENGs.^31,32 Moreover, the rotor and stator parts remain non-contact, which will ensure an ultra-long service life for the TENG module.

Fig. 4 Optimization and characteristics of the TENG module. (A) Simplified structural components of the TENG module. (B) Schematic diagram illustrating the working principle of the TENG module under the relative rotation between the rotator and the copper coils in an angle degree of 45, respectively. (C)–(E) Short-circuit current, open-circuit voltage, and short-circuit charge of the TENG module with 6 rotor magnets at various cable current intensities. (F) Dependence of the output current and power output of the TENG module on various external loading resistances ranging from 50 Ω to 1 GΩ. (G) Cycling characteristics of the TENG module during over 600 000 working cycles and continuous operation for 12 hours.

The working principle of the TENG module involves two stages: the initial state and the stable state (Fig. 4B). In the initial state, the contact between the polyester fur and the nylon film creates the separation of charges due to the difference in triboelectric series. The polyester fur becomes negatively charged, while the nylon film becomes positively charged (stage i). As the polyester fur slides onto the more electronegative PTFE film (stage ii), electrons transfer from the fur to the PTFE, leading to a positive charge on the fur. This process continues until the subsequent nylon film contacts the fur. This helps the PTFE film to obtain more electrons and is the key to achieving high output for the TENG module (stage iii). As the sliding continues, the next nylon film also acquires a positive charge through charge transfer (stage iv). By employing electrostatic induction, the charge redistributes through an external load to generate continuous AC output,^33–35 ultimately entering a stable state after a few cycles of saturation. The working principle of the stable state is demonstrated in Fig. S14 (ESI†).

Fig. 4C–E illustrate the electrical performance of the TENG module featuring a 6-magnet rotor, showcasing consistent output with I_sc measuring at 3.6 μA, V_oc at 110 V, and Q_sc at 44 nC, across a range of cable currents from 100 A to 800 A. Notably, unlike the EMG module, the output of the TENG module remains unaffected by magnetic field variations, emphasizing its dependence on the rotor speed for optimal functionality. The consistent output frequency validates the constant rotational speed of the TENG module during synchronous rotation under diverse cable current levels (Fig. S15, ESI†), thus resulting in nearly identical electrical performance. Fig. 4F and Fig. S16 (ESI†) further depict the electrical output of the TENG module across a spectrum of resistance values (50 Ω–1 TΩ). The voltage and current output exhibit a parallel trend with the EMG module, increasing with load resistance. At a load resistance of 300 MΩ, the TENG module achieves a maximum power output of 1.2 mW. It is noteworthy that the matching resistance of the TENG module surpasses that of the EMG module significantly. The TENG module also demonstrates exceptional and long-term stability, maintaining a constant electrical output even after undergoing 0.6 million working cycles (12-hour duration) and a long operating period of over 42 days, as shown in Fig. 4G and Fig. S17 (ESI†).

Both the EMG and TENG modules showcase outstanding electrical output performance, presenting highly regulated electrical signals. However, achieving simultaneous maximization of the power output for the HMEH device in practical applications is challenging due to their impedance mismatch.

Characterization and robustness testing of HMEH

To maximize the power output of the HMEH device, independent power management for the EMG module and TENG module is essential. As illustrated in Fig. 5A, the output of the EMG module is processed via a transformer and rectifier bridge, while the TENG module output is processed through a rectifier bridge and a previously reported fractal design-based switched-capacitor-converter (FSCC).³⁶ Both module outputs are then connected to the energy storage module, such as the capacitor and Li-ion battery.

Fig. 5 Working principle and simulation analysis of the MMEC module. (A) Schematic of the power management circuit of the HMEH. (B) Charging curves of a 470 μF loading capacitor using the EMG module with various transformers. (C) Charging curves of a 470 μF loading capacitor using the TENG module with various FSCCs. (D) Charging curves of a 470 μF loading capacitor by TENG, EMG, TENG with a 2^4 FSCC module, an EMG with 36 : 2 step-up transformer module, and an HMEH with the complete power management circuit module. (E) Demonstration of powering a commercial calculator using an HMEH and its charging voltage curve as a function of the charging time. (F) Demonstration of powering a 500 lm LED bulb using a HMEH. The maximum output energy of an HMEH per cycle under various working environments, namely (G) different working temperatures from 30 °C to 90 °C, (H) different humidity levels from 30–90 Rh%, (I) different vibration amplitudes from 1 mm to 10 mm at a vibration frequency of 10 Hz.

Fig. 5B depicts the charging curves of the EMG module using various transformers (from 36 : 1 to 36 : 4) for a 470 μF capacitor. Elevating the voltage to an optimal level, the transformer not only alleviates energy losses from the low voltage in the EMG module but also markedly enhances the charging efficiency. The FSCC power management circuit, known for high conversion efficiency, minimal output impedance, and adaptability, efficiently mitigates the low power density and overall energy transfer inefficiencies caused by the high impedance of the TENG module. Fig. 5C depicts the charging curves of the TENG module for a 470 μF capacitor via various FSCCs. The TENG module attains a charging voltage of 0.2 V within 60 seconds without the FSCC circuit, but with the inclusion of a 2^4 order FSCC circuit, it achieves a remarkable 10-fold enhancement in charging efficiency, reaching the same voltage level. Ultimately, after weighing factors like charging rate, voltage stability, and energy loss, we select a 36 : 2 transformer and a 2^4 order FSCC for the whole HMEH power management circuit.

The V–Q curve is typically employed to assess the overall energy transfer performance of energy harvesters. Fig. S18 (ESI†) illustrates that, after undergoing the above power management circuit, the power outputs of the EMG module, TENG module, and HMEH are approximately 1.44 mW, 2.28 mW, and 3.67 mW, corresponding to volume and weight power density values of 19.2 μW cm⁻³ and 25.7 μW cm⁻³, 30.36 μW cm⁻³ and 40.7 μW g⁻¹, and 48.94 μW g⁻¹ and 65.5 μW g⁻¹, respectively. Fig. 5D demonstrates that HMEH has the highest efficiency for charging a 470 μF capacitor. Moreover, as depicted in Fig. 5E and F, HMEH not only offers a stable power supply for portable electronics (Video S3, ESI†) but also drives a 500-lumen LED bulb (Video S4, ESI†), providing ample illumination for reading text in complete darkness.

Ensuring the environmental robustness of distributed energy harvesters, encompassing resistance to factors such as temperature, humidity, vibration, and waterproofing, is equally crucial. Previous studies reveal that the typical operating conditions of power transmission lines are primarily influenced by climate, wind-induced vibrations, and Joule heating within the transmission line itself. Specifically, these parameters encompass a temperature range of 0–60 °C, a humidity range of 30–80 RH%, and vibrations with amplitudes ranging from 1 to 10 mm and frequencies in the range of 10 Hz–10 kHz¹. Notably, variations in environmental conditions, including elevated temperatures, humidity levels, and shock loads, exert a direct influence on the performance and lifespan of energy harvesters in power transmission lines. As depicted in Fig. 5G–I and Fig. S19 (ESI†), the output of the HMEH device exhibits minimal change under simulated operating conditions, including temperatures ranging from 30 °C to 90 °C, humidity levels between 30% and 90%, vibration amplitudes from 1 to 10 mm, and vibration frequencies from 10 Hz to 10 kHz. This simulation was conducted using a heating plate, humidifier, and vibration table to replicate the working environment of the HMEH. Furthermore, the HMEH showcases its IPX7 waterproofing capability, as demonstrated in Video S1 (ESI†). After being submerged in water at a depth exceeding 1 meter for more than 30 minutes, HMEH can still illuminate a 500-lumen bulb.

Table 1 highlights the superior performance of an HMEH compared to traditional CT energy harvesters, with the highest power density of energy harvesters for power transmission lines. Moreover, it effectively overcomes limitations faced by mechanical energy harvesters like TENGs and PENGs, such as ease of fatigue damage and inefficient energy utilization due to unregulated output. The HMEH offers regulated output, compact size, light weight, scalability, easy installation, environmental robustness, and a long lifespan. Seamlessly integrating with wireless sensors, the HMEH can emerge as a robust and maintenance-free solution for comprehensive monitoring of power transmission lines.

Table 1 Comparison of the reported energy harvesters for power transmission lines of power grids

Ref.	Energy type	Energy harvesting technology	Power	Weight	Power density in weight (μW g^−1)	Volume (cm^3)	Power density in volume (μW cm^−3)	Output power
Our work	Magneto-mechanical energy	TENG & EMG	3.67 mW	∼56 g	65.5	75	48.9	Regulated
6	Magnetic field of the transmission line	Current transformer	612 μW	∼2 kg	0.31	292	2.1	Regulated
9		Current transformer	720 μW	∼50 g	14.4	20	36
7		Current transformer	4.5 mW	∼2 kg	2.25	620	7.28
8		Current transformer	5.57 mW	∼2 kg	2.78	2704	2.06
37	Electric field of the transmission line	Electric field energy	16 mW	∼2 kg	8	1570	10.2	Regulated
22	Magneto-mechanical energy	PENG	90 μW	∼10 g	9	3.63	24.8	Unregulated
12		TENG	0.78 mW	∼100 g	7.8	134	5.8
13	Wind energy around the transmission line	TENG	1.3 mW	∼200 g	6.5	230	5.6	Unregulated
14		TENG & EMG	16.7 mW	∼500 g	33.4	400	41
23	Vibration energy around the transmission line	PENG	0.09 μW	∼1 g	0.09	2	0.045	Unregulated
24		PENG	10.4 μW	∼10 g	1.04	1.63	6.38
15		TENG & EMG	16.4 mW	∼500 g	32.8	1140	14.4
16		TENG	6.67 mW	130 g	51.3	205	32.5
17		TENG	460 μW	∼10 g	46	2646	0.17
19		TENG	2.5 mW	∼100 g	25	169.6	14.74
20		TENG	4.31 mW	∼100 g	43.1	100	43.2

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Application in a self-powered wireless temperature sensing system of power transmission lines

Elevated temperatures in power transmission lines can hasten cable insulation aging, induce thermal deformation, create internal voids, and ultimately result in cable damage. As demonstrated in Fig. 6A, we integrate the HMEH with a commercial wireless temperature sensor, establishing a self-powered wireless power transmission line's temperature sensing system. The system comprises a self-powered wireless temperature sensing module on the power transmission lines, a wireless receiving module, and terminal-installed online temperature monitoring software.

Fig. 6 Demonstration of a self-powered wireless temperature sensing system for power transmission lines. (A) Schematic diagram of the fully self-powered wireless online-monitoring system based on HMEH devices for comprehensive perception of electricity and environmental information of power transmission lines. (B) The workflow diagram of the self-powered wireless temperature sensing system of power transmission lines. (C) Photograph of the self-powered wireless temperature sensing system of power transmission lines. (D) Schematic of the self-powered, wireless temperature sensing unit. (E) Photograph of the temperature online-monitoring software interface of power transmission lines. (F) Temperature sensing curve for 48 consecutive hours. (G) and (H) Demonstration of a high temperature warning for power transmission lines.

Fig. 6B and C illustrate the working logic and complete physical setup of the system. The HMEH converts the magnetic energy of the power transmission lines into electrical energy, ensuring a consistent power supply to the wireless temperature sensor through a power management circuit. The wireless temperature sensor collects temperature data of the power transmission lines in real time, transmitting them wirelessly to the computer-connected wireless receiving module. Finally, the online monitoring software on the terminal displays real-time temperature data and enables online fault monitoring.

Fig. 6D illustrates the schematic diagram of the self-powered wireless temperature sensing module (SWTSM). The SWTSM incorporates a snap-fit design for convenient installation on the power transmission lines, comprising upper and lower compartments. The upper compartment houses the HMEH, while the lower compartment integrates the power management module and the wireless temperature sensor module. The wireless temperature module is strategically placed in proximity to the power transmission lines for real-time monitoring of its surface temperature. With dimensions of 5 cm × 8 cm × 3 cm and sealed for protection, the SWTSM showcases advantages such as compact size, lightweight, easy installation, and outstanding environmental robustness.

LabVIEW-based online monitoring software is developed, featuring three main functional areas outlined in Fig. 6E: (1) a real-time temperature display area that presents the ongoing temperature of the power transmission lines; (2) a statistical temperature data area displaying historical average and maximum temperatures, as well as average and maximum temperatures in the last 5 minutes; and (3) a fault alarm area for high-temperature warnings. Fig. 6F illustrates the continuous 48-hour temperature curve obtained from the SWTSM, confirming the ability of the HMEH to consistently provide stable power to wireless temperature sensors. The simulation of power transmission lines by heating using a heating plate is shown in Fig. 6G and H and Video S5 (ESI†). Once the temperature exceeds the alarm threshold (60 °C), the online monitoring software interface promptly issues a temperature fault alarm, enabling timely actions to mitigate potential risks.

Our HMEH prototype, still in its early stage, presents a promising solution for the self-powering technology for grid sensors with notable advantages in output performance and environmental robustness yet still has many aspects that can be improved on. Firstly, enhance the output power density by optimizing the coupling magnetic field of silicon steel sheets and adjusting the shape and placement of magnets and coils in the current HMEH device, significantly boosting its power output while reducing overall volume and weight. Secondly, high-voltage direct current power transmission lines provide significant benefits for large-capacity and long-distance power transmission. Coupled with the magneto-mechanical energy conversion mechanism based on the magnetic phase difference of the HMEH, the potential for direct energy harvesting from these power transmission lines is expected, a capability beyond the reach of traditional CT energy harvesters.

Conclusions

In conclusion, we developed a compact, lightweight, effective hybrid magnetic energy harvester for self-powered sensing applications in power transmission lines. Integrating MMEC, non-contact TENG, and EMG modules, the HMEH measures just 5 cm × 5 cm × 3 cm and weighs 56 g. Utilizing the magnetic phase difference principle, the MMEC module can produce stable, synchronized rotational motion with the magnetic field frequency of power transmission lines. Integrated with the MMEC module, both the EMG module and TENG module generate consistent electrical signals, demonstrating excellent stability. With an efficient power management circuit, the HMEH delivers unprecedented volume and weight power density values of 48.9 μW cm⁻³ and 65.5 μW g⁻¹, respectively, resistant to high-current impacts. Furthermore, with IPX7-rated waterproofing ability and being resilient to temperature, humidity, and vibration, the HMEH passes rigorous environmental robustness testing. Finally, integrating the HMEH with a commercial wireless temperature sensor creates a self-powered wireless temperature sensing system, demonstrating its capability for continuous power supply to wireless grid sensors. This innovative HMEH showcases exceptional performance, stability, and resilience for diverse applications.

Experimental

Modification of PTFE and PA Films

(1) The PTFE film (100 μm thickness) and PA film (50 μm thickness) were ultrasonically cleaned with ethanol and deionized water for 20 min, respectively. (2) The PTFE film and PA film were dried in an oven for subsequent use. (3) The samples were placed in high-voltage polarization corona apparatus (Electro-Technic, BD-20AV) and subjected to polarization treatment for 15 minutes with a discharge voltage of 40 kV.

Fabrication of the hybrid magnetic energy harvester

As depicted in Fig. 1B, the HMEH device consists of a rotor and a stator. The stator is made up of a 3D printed housing, a silicon steel sheet, a short-circuit ring, copper coil, a copper electrode, and polyester fur. The rotor consists of a 3D printed housing, a PTFE film, a PA film, and NdFeB magnets. The fully assembled HMEH device is then waterproofed, dustproofed, and made more resilient by encapsulating and sleeving it using PDMS. A detailed step-by-step guide to the preparation process of the HMEH is given as follows:

Preparation of the stator part. (1) A housing with dimensions of 40 mm × 35 mm × 30 mm was 3D printed, ensuring that several grooves are preserved for silicon steel sheet, copper coils, and the rotor. (2) Two pieces of the silicon steel sheet were cut measuring 80 mm × 30 mm × 0.5 mm. The short-circuit ring was installed at one end and then the silicon steel sheets were placed symmetrically into the 3D printed stator housing. (3) 2000-turn copper coil was inserted into the 3D stator housing. (4) Four pieces of copper foil were laser-cut and affixed to the inner wall of the stator housing with a 2 mm gap. (5) The polyester fur was cut and glued to the gap between the copper electrodes.

Preparation of the rotor part. (1) 3D printing was used to create a cylindrical rotor housing with dimensions of Φ10 mm × 25 mm, preserving six grooves for the NdFeB magnets. (2) Six NdFeB magnets were placed measuring 4 mm × 3 mm × 25 mm into the rotor housing, ensuring alternating positive and negative poles. (3) A total of four alternating pieces of the polarized PTFE film and PA film measuring 16 mm × 25 mm was affixed onto the outer wall of the rotor housing.

Encapsulation of the HMEH device. (1) The rotor was manually assembled into the stator. (2) A shell with a snap-fit structure was 3D printed to securely hold the stator and rotor. (3) A PDMS elastomer was coated onto the surface of the HMEH device for waterproofing and dustproofing encapsulation. (4) The snap-fit structure of the encapsulation shell enables easy installation of the HMEH device on power transmission lines, allowing for subsequent electrical characterization and magnetic energy harvesting.

Simulation analysis of the magnetic-mechanical energy conversion process

Simulations were conducted using COMSOL Multiphysics software, version 6.1. A transient research model was developed to simulate the conversion of magnetic to mechanical energy, utilizing the AC/DC module from COMSOL. The mechanical structure of the device was initially created in Solidworks software and then imported into COMSOL. The relative permeability constant of silicon steel was 2000, and the short-circuit rings were modeled as perfect conductors with a conductivity of 5.81 × 107 S m⁻¹. Additionally, the current in the power transmission lines was set to an AC current with a frequency of 50 Hz and a range of 0–1000 A based on the magnetic-mechanical energy conversion model. Finally, the simulation included the magnetic field distribution between the cable and the magnets, as well as the torque exerted on the rotor.

Power management of HMEH

The EMG unit first went through a transformer (220 V:12 V) for initial boosting, and then rectification through a rectifier to minimize energy loss. On the other hand, the TENG unit was rectified with a rectifier and passed through the FSCC power management circuit as in a previous report³⁶ to enhance the output efficiency. Finally, both units were connected in parallel with the energy storage system to expedite charging speed and stabilize the output voltage. The diode model used in the FSCC is 1N4007.

Electrical measurements

Characterization of the energy harvesting performance. (1) An AC power source (Anmtake, ATC70005) and a cable with a cross-sectional area of 240 mm² were used to simulate the current and magnetic field conditions of the power transmission lines. (2) A programmable electrometer (Keithley 6514) and a high sensitivity acquisition card (National Instruments BNC2110) were used to measure the typical electrical signals of the TENG, EMG, and HMEH. (3) Q–V curves were measured by two electrometers (Keithley 6514) simultaneously through a dual-channel test system, which measured Q and V, respectively. The grounding terminals of the measuring line of the two testing devices were connected together to gain accurate data. (4) All the output signals and curves were recorded using a self-programmed software written in LabVIEW.

Environmental robustness testing of the HMEH. (1) For temperature measurement, the HMEH was placed on a heating plate (IKA, C-MAG HS 7) and the working temperature was controlled using a knob and displayed in real time. (2) For humidity measurement, the HMEH, a humidifier, and a humidity sensor were placed in a confined space. The humidifier and the humidity sensor jointly control the applied humidity of the confined space. (3) For vibration measurement, the HMEH was placed on a shaker (YMC VT-500), which was amplified using a signal generator (Tektronix MDO34) and a linear power amplifier (YMC LA-800). (4) For the IPX7 waterproof test, the HMEH was immersed in 1-meter deep water for 30 minutes and then removed and wiped dry for electrical measurements.

Demonstration of a wireless self-powered temperature sensing system for power transmission lines

The wireless self-powered temperature sensing system comprises a 3D printed hollow enclosure with a snap-on design that houses an HMEH device for power provision and a wireless temperature sensor (Acrel, ATE200) that detects and transmits cable temperature. The second component is a repeater (Acrel, ATC600) that picks up the temperature data and sends it to a PC for real-time temperature display and high-temperature alarm warnings through a software terminal written in LabVIEW.

Author contributions

Q. L.: conceptualization, investigation, data curation, methodology visualization, software development, and writing – original draft. L. Z.: supervision, conceptualization, methodology, formal analysis, and writing – review & editing. C. Z., Y. T. and Y. F.: analysis and data curation. B. L. and Z. A.: formal analysis. D. L.: supervision. Z. L. W.: writing – review & editing and supervision.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 62105238).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ee04563c

‡ Q. Li and L. Zhang contributed equally to this work.

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