Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Charges induced in triboelectric process are usually referred as a negative effect either in scientific research or technological applications, and they are wasted energy in many cases. Here, we demonstrate a simple, low cost and effective approach of using the charging process in friction to convert mechanical energy into electric power for driving small electronics. The triboelectric generator (TEG) is fabricated by stacking two polymer sheets made of materials having distinctly different triboelectric characteristics, with metal films deposited on the top and bottom of the assembled structure. Once subjected to mechanical deformation, a friction between the two films, owing to the nano-scale surface roughness, generates equal amount but opposite signs of charges at two sides. Thus, a triboelectric potential layer is formed at the interface region, which serves as a charge “pump” for driving the flow of electrons in the external load if there is a variation in the capacitance of the system. Such a flexible polymer TEG gives an output voltage of up to 3.3 V at a power density of ∼10.4 mW/cm 3 . TEGs have the potential of harvesting energy from human activities, rotating tires, ocean waves, mechanical vibration and more, with great applications in self-powered systems for personal electronics, environmental monitoring, medical science and even large-scale power. <s> BIB001 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Harvesting energy from our living environment is an effective approach for sustainable, maintenance-free, and green power source for wireless, portable, or implanted electronics. Mechanical energy scavenging based on triboelectric effect has been proven to be simple, cost-effective, and robust. However, its output is still insufficient for sustainably driving electronic devices/systems. Here, we demonstrated a rationally designed arch-shaped triboelectric nanogenerator (TENG) by utilizing the contact electrification between a polymer thin film and a metal thin foil. The working mechanism of the TENG was studied by finite element simulation. The output voltage, current density, and energy volume density reached 230 V, 15.5 μA/cm 2 , and 128 mW/cm 3 , respectively, and an energy conversion efficiency as high as 10−39% has been demonstrated. The TENG was systematically studied and demonstrated as a sustainable power source that can not only drive instantaneous operation of light-emitting diodes (LEDs) but also charge a lithium ion battery as a regulated power module for powering a wireless sensor system and a commercial cell phone, which is the first demonstration of the nanogenerator for driving personal mobile electronics, opening the chapter of impacting general people's life by nanogenerators. <s> BIB002 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Triboelectrification is an effect that is known to each and every one probably since ancient Greek time, but it is usually taken as a negative effect and is avoided in many technologies. We have recently invented a triboelectric nanogenerator (TENG) that is used to convert mechanical energy into electricity by a conjunction of triboelectrification and electrostatic induction. As for this power generation unit, in the inner circuit, a potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons are driven to flow between two electrodes attached on the back sides of the films in order to balance the potential. Since the most useful materials for TENG are organic, it is also named organic nanogenerator, which is the first using organic materials for harvesting mechanical energy. In this paper, we review the fundamentals of the TENG in the three basic operation modes: vertical contact-... <s> BIB003 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> We report a triboelectric nanogenerator (TENG) that plays dual roles as a sustainable power source by harvesting wind energy and as a self-powered wind vector sensor system for wind speed and direction detection. By utilizing the wind-induced resonance vibration of a fluorinated ethylene–propylene film between two aluminum foils, the integrated TENGs with dimensions of 2.5 cm × 2.5 cm × 22 cm deliver an output voltage up to 100 V, an output current of 1.6 μA, and a corresponding output power of 0.16 mW under an external load of 100 MΩ, which can be used to directly light up tens of commercial light-emitting diodes. Furthermore, a self-powered wind vector sensor system has been developed based on the rationally designed TENGs, which is capable of detecting the wind direction and speed with a sensitivity of 0.09 μA/(m/s). This work greatly expands the applicability of TENGs as power sources for self-sustained electronics and also self-powered sensor systems for ambient wind detection. <s> BIB004 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract We present a self-powered flexible printed circuit (FPC) board with an integrated zigzag-shaped triboelectric generator (TEG). This is the first time the fabrication of a TEG has been adapted for industrial production, making the fabrication procedure efficient and low-cost. Multiple friction pairs were integrated in a single TEG cell. The TEG with 10 integrated friction pairs has achieved a voltage of about 620 V with a current density of 45.1 μA/cm 2 . Continuous periodic output was generated under periodic impacts under frequencies ranging from 2 Hz to 15 Hz. Most importantly, the FPC technology makes it simple to integrate electronic components with the TEG to form a self-powered system. As an example, 100 LEDs mounted on the FPC board can be directly illuminated by finger pressing. <s> BIB005 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Harvesting mechanical energy is becoming increasingly important for its availability and abundance in our living environment. Triboelectric nanogenerator (TENG) is a simple, cost-effective, and highly efficient approach for generating electricity from mechanical energies in a wide range of forms. Here, we developed a TENG designed for harvesting tiny-scale wind energy available in our normal living environment using conventional materials. The energy harvester is based on a rotary driven mechanical deformation of multiple plate-based TENGs. The operation mechanism is a hybridization of the contact-sliding-separation-contact processes by using the triboelectrification and electrostatic induction effects. With the introduction of polymer nanowires on surfaces, the rotary TENG delivers an open-circuit voltage of 250 V and a short-circuit current of 0.25 mA, corresponding to a maximum power density of ∼39 W/m2 at a wind speed of ∼15 m/s, which is capable of directly driving hundreds of electronic devices such... <s> BIB006 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> The triboelectric effect is known for many centuries and it is the cause of many charging phenomena. However, it has not been utilized for energy harvesting until very recently.(1-5) Here we developed a new principle of triboelectric generator (TEG) based on a fully contacted, sliding electrification process, which lays a fundamentally new mechanism for designing universal, high-performance TEGs to harvest diverse forms of mechanical energy in our daily life. Relative displacement between two sliding surfaces of opposite triboelectric polarities generates uncompensated surface triboelectric charges; the corresponding polarization created a voltage drop that results in a flow of induced electrons between electrodes. Grating of linear rows on the sliding surfaces enables substantial enhancements of total charges, output current, and current frequency. The TEG was demonstrated to be an efficient power source for simultaneously driving a number of small electronics. The principle established in this work can ... <s> BIB007 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Harvesting mechanical energy is an important route in obtaining cost-effective, clean and sustainable electric energy. Here we report a two-dimensional planar-structured triboelectric generator on the basis of contact electrification. The radial arrays of micro-sized sectors on the contact surfaces enable a high output power of 1.5 W (area power density of 19 mW cm(-2)) at an efficiency of 24%. The triboelectric generator can effectively harness various ambient motions, including light wind, tap water flow and normal body movement. Through a power management circuit, a triboelectric-generator-based power-supplying system can provide a constant direct-current source for sustainably driving and charging commercial electronics, immediately demonstrating the feasibility of the triboelectric generator as a practical power source. Given exceptional power density, extremely low cost and unique applicability resulting from distinctive mechanism and structure, the triboelectric generator can be applied not only to self-powered electronics but also possibly to power generation at a large scale. <s> BIB008 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> A triboelectric generator (TEG) for scavenging flow-driven mechanical -energy to directly power a wireless sensor node is demonstrated for the first time. The output performances of TEGs with different dimensions are systematically investigated, indicating that a largest output power of about 3.7 mW for one TEG can be achieved under an external load of 3 MΩ. <s> BIB009 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract In this article, we present an electrode-free triboelectric generator for harvesting biomechanical energy from human motions through triboelectrification between shoe sole and ground. A thin film of porous ethylene-vinyl acetate copolymer (EVA) is designed and pasted on shoe sole to serve as the friction surface. Electricity is generated by the contacting friction of shoe sole with ground surface during walking, which makes no initial vertical gap needed in shoe sole. At the same time, human body is employed as the natural electrode instead of fabricated electrodes, and the electricity can be obtained from any part of human body. In addition, output performances are characterized by using three different conductors as reference electrode. The output voltage of 810 V is achieved and charge of over 550 nC is transferred in one step of an adult׳s walking. Meanwhile, 76 blue LEDs are lighted through a watch as output node. Owing to the advantage of body electrode, it has potential applications in wearable electronics and implantable devices. <s> BIB010 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract We demonstrated a sort of self-powered vibration sensor based on a triboelectric generator (TEG) using triboelectrification and electrostatic induction effects. The as-prepared TEG delivered an output voltage of 34.06 V and short-circuit current density of 22.5 mA/m 2 corresponding to a maximum power density of 0.77 W/m 2 , which was capable of driving five LEDs directly and continuously with working frequency of 50 Hz without any rectification circuit or energy storage unit. The vibration sensor has a detection range of 0–500 Hz, and high accuracy. Besides, it could realize detection under non-contact mode, which would protect the device and the detected object in operation. Moreover, the stability and repeatability could also be well retained. The TEG based vibration sensor possessed great potential in machines operation monitoring, process control, and security applications in unreachable and access-denied extreme environments. <s> BIB011 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Triboelectric nanogenerator collects kinetic energy from human activities and endows sustainable operation of wearable electronics. This work reports a very natural design of triboelectric nanogenerator for harvesting low-frequency walking energy. In the protocol, the human body and ground serve as electrodes of the triboelectric system, which helps the rubber soles to collect mechanical energy during walking motion of humans and produce pulsed alternative currents. The average open-circuit voltage and short-circuit current produced by the system at low frequency are about 400 V and 12 μA, respectively. Considering the very simple structure, this natural triboelectric nanogenerator can be easily integrated into wearable electronics and take full advantage of human motions as power source and as self-powered systems. <s> BIB012 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Wind energy plays an important role in new-energy market, and it has the potential to be one of the promising green electric energy sources. Thus, it is very important for scientists and researchers to develop alternative method for harvesting wind energy. Here, we introduce a farm structure consists of triboelectric nanogenerators (TENGs) for large-scale energy harvesting from natural wind. Depending on contact electrification based freestanding mode between two disks with patterns of micro-sized circular sectors, the TENG farm that works with the wind flow converts the low and high wind energy into electricity. Finally, the performance of the TENG and Darrieus turbines is compared both experimentally and theoretically, predicting the unique advantage offered by TENG at low rotation speed. Given the convincing characteristics, such as having little weight, low cost, environmentally friendly, and effortlessly connected, the TENG farm can be an effective way towards large-scale green energy harvesting from the wind. <s> BIB013 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Energy from wind flow is very common in ambient environment which can be harvested by triboelectric generator effectively. Herein, electret based triboelectric generator (E-TriG) with both electrostatic and triboelectric effects are investigated with enhanced performance comparing with the traditional triboelectric generator based on only contact electrification. Electret materials like PTFE, CYTOP, TOPAS, and COC are prepared with different methods and charged under positive or negative conditions to optimize the material property. It is proved that the performance of the triboelectric generator can be improved by negatively charged electrets, while with positively charged electrets, the power output is weakened. As a demonstration, the E-TriG has been successfully applied for wireless temperature sensing. At a wind flow rate of 18 m/s, a storage capacitor can be fully charged by three E-TriGs devices within 15 s, and afterwards, wireless temperature signal could be read and delivered to the internet every 5 s. An average power of 400 μW is therefore harvested. With enhanced performance from the corona charged electret, the triboelectric generator shows promising application for the future wireless sensor networks. <s> BIB014 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Paper-based triboelectric nanogenerators (TENGs) represent an affordable, green, and eco-friendly energy harvesting methodology. In this work, we propose a novel X-shaped paper TENG (XP-TENG) formed from a ballpoint ink layer coated by painting with a commercial brush pen. The paper and Teflon tape serve as the triboelectric pairs, with the paper serving as the supporting component. Moreover, the XP-TENG is capable of providing two working patterns that can yield an expanded scope of practical applications. A stacked XP-TENG was proposed for increasing the output performance and harvesting the mechanical energy generated by motion of the human body. This staked XP-TENG can directly light up 101 blue high-power LEDs with working voltage of 3.4 V. The fabricated XP-TENG can be used for harvesting mechanical energy from human walking. <s> BIB015 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract When portability, low cost, and green energy requirements are considered, the search for an electricity generator harvesting environmental energy based on available cheap commercial materials and simple fabrication techniques becomes significantly important. In this study, the capability of ultrasoft and cuttable paper-based triboelectric nanogenerator (P-TENG) to harvest mechanical energy is demonstrated. The P-TENG maintains the excellent softness of tissue paper and has the characteristics of light weight (~ 87 g/m 2 ), high electric conductivity (6 Ω/square), and low cost (~ $3.00/m 2 ). More importantly, the P-TENG can be cut by the end-user to modify its size and shape and still function properly. The mechanical energies available during cleaning processes, the energy associated with the body's motion, sound energy, and wind energy can be directly harvested by using the P-TENG. The high portability of the P-TENG, the simple and scalable fabrication processes, low cost, and its ability to harvest mechanical energy make the P-TENG important for the development of green, portable, energy-harvesting devices. <s> BIB016 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Mechanical energy in a rolling tire is strong and stable, but it is always wasted and there is no effective means for harvesting. Herein, we demonstrate an array of compressible hexagonal-structured triboelectric nanogenerators (CH-TENGs) for harvesting the mechanical energy from a rolling tire. The CH-TENG units can be stacked in parallel connections and fixed in a rubber tire. The energy harvesting performances are systematically investigated and the maximum instantaneous power is 1.9 mW with the CH-TENG in 8 units, a weight load of 10 N and a speed of 2.51 m/s. The CH-TENGs have stable and durable characteristics for a 30-day continuous work and can drive a wireless tire pressure sensor once every 14 min when the moving speed is 2.51 m/s and weight load is 10 N. On the basis of the measured output power of the CH-TENG, it is expected to produce at least 1.2 W for a standard tire with the CH-TENG in 500 units and the running speed of 100 km/h. This work not only provides a promising solution to harvest the wasted mechanical energy from rolling tires, but also demonstrates the potential applications of the CH-TENG in sustainable automotive electronics. <s> BIB017 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Triboelectric Energy Harvesting <s> Abstract Collecting mechanical energy in the surrounding environment to power small electronic devices is an ideal method as a green and clean power source. Based on the working mechanism and the advantages of triboelectric nanogenerator (TENG), a robust rolling friction contact-separation mode TENG (CS-TENG) has been fabricated for harvesting vertical rotation energy by utilizing the integrated cylindrical surface with the conjunction of rolling contact electrification and electrostatic induction. The output current of the CS-TENG can maintain 96% (origin: 1.72 μA, final: 1.66 μA) after two days continuous work a rotation speed of 300 r/min. Furthermore, an electromagnetic and triboelectric hybrid contact-separation mode nanogenerator (HCS-TENG) has been further designed by coupling magnets in the rolling cylinders with copper coils as an electromagnetic generator (EMG) in the acrylic cylinder to implement the multi-functional properties. The gear transmission structure makes the device more facile to be installed on the rotation objects. The output performances of CS-TENG and EMG under various rotation speeds were systematically studied. It shows that the CS-TENG with six units can deliver an output power of 0.15 mW/cm 2 and the 100 μF capacitor can be charged to 6 V in less than 1 min by the hybridized nanogenerator at a rotating rate of 700 r/min. In addition, utilizing the output signal's intrinsic characteristics of CS-TENG, a self-powered rotation speed and displacement sensor using the dual cylinder structure TENG has been achieved for these rotating mobile devices that are inconvenient to get an additional power supply in the grimmest circumstances such as moon car or other celestial and tough environment detection equipment. This multifunctional TENG device reveals a significantly potential application in the grimmest circumstances as power source, self-powered electronics and sensor systems. <s> BIB018
Two objects manufactured with different materials can generate electron flow in a specific direction when they periodically contact or separate from each other by external forces. This electricity production process is called the triboelectric effect which is illustrated in Figure 13  . External mechanical energy can be converted into electricity based on the triboelectric effect. Wang, in his reviewer paper BIB003 , gave a particular table of triboelectric series which shows the order of various materials arranged according to their ability to donate or accept electrons in his review. Two objects manufactured with different materials can generate electron flow in a specific direction when they periodically contact or separate from each other by external forces. This electricity production process is called the triboelectric effect which is illustrated in Figure 13  . External mechanical energy can be converted into electricity based on the triboelectric effect. Wang, in his reviewer paper BIB003 , gave a particular table of triboelectric series which shows the order of various materials arranged according to their ability to donate or accept electrons in his review. The triboelectric EH technique was developed later than other technologies. Many engineers have investigated triboelectric generators because of the ubiquitous ambient motions (such as wind or water flow BIB009 BIB004 BIB014 and human motion BIB010 BIB012 ) and generators' characteristics of scalability, reliability, high efficiency and low cost BIB001 BIB002 . BIB005 and BIB015 describe two triboelectric harvesters designed and fabricated with similar structures, an integrated Zigzag shape and a stacked X-shape,. The power density can reach up to 27.96 mW/cm 2 and 542.22 μW/cm 2 , respectively. Additionally, an ultra-soft and cuttable paper-based harvester is made with cheap commercial materials to scavenge the energy associated with the body's motion, sound energy and wind energy in BIB016 . Another powerful triboelectric generator with the compressible hexagonal structure is designed and installed inside the car tire to power a wireless tire pressure sensor by Guo et al. BIB017 . Moreover, some researchers have found success in triboelectric harvesters using rotational energy because of less abrasion of materials. For example, Xie et al. BIB006 , Zhu et al. BIB008 and Ahmed et al. BIB013 designed three different rotating triboelectric generators respectively and all three prototypes show high performance in harvesting wind energy. A novel linear-grating triboelectric generator is proposed by Zhu et al. BIB007 , which can be manufactured as three different configurations to harvest energy from rotation, rolling and linear piston motion. Yang et al. BIB018 describes a model designed for rotating machinery. Figure 14 shows its architecture consisting of six contact-separation mode triboelectric units shown in Figure 14a , and three electromagnetic and triboelectric hybrid units shown in Figure 14b . The fabricated prototype works with the gear transmission structure to implement EH on the rotating machines. Generally, triboelectric energy harvesting is combined with other technologies to capture enough energy to support the actions of wireless monitoring systems, such as BIB011 . The triboelectric EH technique was developed later than other technologies. Many engineers have investigated triboelectric generators because of the ubiquitous ambient motions (such as wind or water flow BIB009 BIB004 BIB014 and human motion BIB010 BIB012 ) and generators' characteristics of scalability, reliability, high efficiency and low cost BIB001 BIB002 . Ref BIB005 BIB015 describe two triboelectric harvesters designed and fabricated with similar structures, an integrated Zigzag shape and a stacked X-shape,. The power density can reach up to 27.96 mW/cm 2 and 542.22 µW/cm 2 , respectively. Additionally, an ultra-soft and cuttable paper-based harvester is made with cheap commercial materials to scavenge the energy associated with the body's motion, sound energy and wind energy in BIB016 . Another powerful triboelectric generator with the compressible hexagonal structure is designed and installed inside the car tire to power a wireless tire pressure sensor by Guo et al. BIB017 . Moreover, some researchers have found success in triboelectric harvesters using rotational energy because of less abrasion of materials. For example, Xie et al. BIB006 , Zhu et al. BIB008 and Ahmed et al. BIB013 designed three different rotating triboelectric generators respectively and all three prototypes show high performance in harvesting wind energy. A novel linear-grating triboelectric generator is proposed by Zhu et al. BIB007 , which can be manufactured as three different configurations to harvest energy from rotation, rolling and linear piston motion. Yang et al. BIB018 describes a model designed for rotating machinery. Figure 14 shows its architecture consisting of six contact-separation mode triboelectric units shown in Figure 14a , and three electromagnetic and triboelectric hybrid units shown in Figure 14b . The fabricated prototype works with the gear transmission structure to implement EH on the rotating machines. Generally, triboelectric energy harvesting is combined with other technologies to capture enough energy to support the actions of wireless monitoring systems, such as BIB011 . From the references, it is apparent that triboelectric harvesters are normally designed as nanogenerators with advantages of robustness, easy integration, high conversion efficiency and low cost. However, the friction and the inevitable wear of the two triboelectric layers results in a limited lifetime, also rotating contact can reduce the material losses to some degree. The wear of materials From the references, it is apparent that triboelectric harvesters are normally designed as nanogenerators with advantages of robustness, easy integration, high conversion efficiency and low cost. However, the friction and the inevitable wear of the two triboelectric layers results in a limited lifetime, also rotating contact can reduce the material losses to some degree. The wear of materials leads to lower conversion efficiency and gradually cannot meet the energy requirement. Furthermore, heat generated by the wear may cause catastrophic accidents in some environments.