Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Temperature gradients and heat flow are omnipresent in natural and human-made settings and offer the opportunity to harvest energy from the environment. Thermoelectric energy harvesting (or energy scavenging) may one day eliminate the need for replacing batteries in applications such as remote sensor networks or mobile devices. Particularly, attractive is the ability to generate electricity from body heat that could power medical devices or implants, personal wireless networks or other consumer devices. This chapter focuses on the design principles for thermoelectric generators in energy harvesting applications, and the various thermoelectric generators available or in development. Such design principles provide good estimates of the power that could be produced and the size and complexity of the thermoelectric generator that would be required. <s> BIB001 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> The study of thermoelectric energy harvesting on people presented in this paper shows that although power generation is affected by many factors such as ambient temperature, wind speed, clothing thermal insulation, and a person's activity, it does not directly depend on metabolic rate as shown in the experiment. The relevant thermal properties of humans measured at different ambient conditions are reported. Several thermopiles are either attached with a strap directly to the skin or integrated into garments in different locations on human body, and power generation is extensively studied at different ambient conditions. Textile covering thermopiles is found not to essentially decrease power generation. Therefore, a hidden energy harvester is integrated into an office-style shirt and tested on people in real life. It generated power in 5–0.5 mW range at ambient temperatures of 15 $^{\circ}{\rm C}$ –27 $^{\circ}{\rm C}$ , respectively. The thermoelectric shirt with such an energy harvester produces more energy during nine months of use (if worn 10 h/day) than the energy stored in alkaline batteries of the same thickness and weight. <s> BIB002 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> In recent years, wireless sensor networks (WSN) have been considered for various aeronautical applications to perform sensing, data processing and wireless transmission of information, without the need to add extra wiring. However, each node of these networks needs to be self-powered. Considering the critical drawbacks associated with the use of electrochemical energy sources such as narrow operating temperature range and limited lifetime, environmental energy capture allows an alternative solution for long-term, deploy and forget, WSN. In this context, thermoelectricity is a method of choice considering the implementation context. In this paper, we present hands-on experience related to on-going implementations of thermoelectric generators (TEG) in airliners. In a first part, we will explain the reasons justifying the choice of ambient energy capture to power WSN in an aircraft. Then, we will derive the general requirements applying to the functional use of TEG. Finally, in the last section, we will illustrate the above issues through practical implementations. <s> BIB003 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract Presence of the heat gradients onboard of an aircraft luckily provides an opportunity for powering the autonomous sensors. Moreover, the opportunity for an implementation of thermoelectric generators can be observed in an electric power backup of various low-power systems. The backup might be only used in the failure state of onboard power line of an aircraft. Thus a supplied system is not autonomous in the true sense. The presented work deals with the development of a backup electric power source based on the MEMS thermoelectric module. System-level point of view on this energy harvester includes the MEMS thermoelectric module itself, power management, power conditioning and energy storage element. Concept of the backup thermoelectric generator for aerospace applications (BTEG) was tested on the proof-of-concept technology demonstrator. <s> BIB004 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract This paper reviews recent developments in energy harvesting technologies for structural health monitoring (SHM) in aeronautical applications. Aeronautical industries show a great deal of interest in obtaining technologies that can be used to monitor the health of machinery and structures. In particular, the need for self-sufficient monitoring of structures has been ever-increasing in recent years. Autonomous SHM systems typically include embedded sensors, and elements for data acquisition, wireless communication, and energy harvesting. Among all of these components, this paper focuses on energy harvesting technologies. Actually, low-power sensors and wireless communication components are used in newer SHM systems, and a number of researchers have recently investigated such techniques to extract energy from the local environment to power these stand-alone systems. The first part of the paper is dedicated to the different energy sources available in aeronautical applications, i.e., for airplanes and helicopters. The second part gives a presentation of the various devices developed for converting ambient energy into electric power. The last part is dedicated to a comparison of the different technologies and the future development of energy harvesting for aeronautical applications. <s> BIB005 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract Thermoelectric materials have drawn vast attentions for centuries, because thermoelectric effects enable direct conversion between thermal and electrical energy, thus providing an alternative for power generation and refrigeration. This review summaries the thermoelectric phenomena, applications and parameter relationships. The approaches used for thermoelectric performance enhancement are outlined, including: modifications of electronic band structures and band convergence to enhance Seebeck coefficients; nanostructuring and all-scale hierarchical architecturing to reduce the lattice thermal conductivity. Several promising thermoelectric materials with intrinsically low thermal conductivities are introduced. The low thermal conductivities may arise from large molecular weights, complex crystal structures, liquid like transports or high anharmonicity of chemical bonds. At the end, a discussion of future possible strategies is proposed, aiming at further thermoelectric performance enhancements. <s> BIB006 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> A thermoelectric generator (TEG) can be used to harvest electrical energy from human body heat for the purpose of powering wearable electronics. At the NSF Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST), TEGs are one of the enabling technologies being explored to advance the center’s mission of creating wearable, self-powered, health and environmental monitoring systems. As part of this effort, an exploration of the relevant parameters for maximizing the wearable TEG power output from the body heat and maintaining the body comfort is particularly important. For this purpose, the heat from the body must be directed into TEG with minimal loss, the generator must be designed for maintaining a high temperature differential across the thermoelectric material, and the generator must have a small form factor to maintain the body comfort. In order to address these requirements, an optimum TEG design was developed and experiments were conducted both on a temperature-controlled hot plate and on different body locations including the wrist, upper arm, and chest. The TEG was further fabricated into a T-shirt and the power was recorded for different human activities. Comparison of the experiments on various body locations and on the T-shirt yielded the highest to lowest power generated on the upper arm, wrist, chest and T-shirt, respectively. The prospect of powering a wearable electrocardiogram sensor by a TEG on the upper arm is discussed. <s> BIB007 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> In this paper, a manual dispenser printing-based fabrication technique has been developed to synthesize a flexible thermoelectric generator (FTEG). Fabricated FTEGs, printed on polyester fiber clothe, convert the thermal energy from the human body into electrical energy using the Seebeck effect. Two flexible prototypes (prototype A and prototype B) were fabricated using a manual dispenser printing technique with n-type (0.98Bi,0.02Sb)2(0.9Te,0.1Se)3 and p-type (0.25Bi,0.75Sb)2(0.95Te,0.05Se)3 thermoelectric (TE) materials. The fabricated prototypes consisted of 12 pairs of n-type and p-type legs connected by silver conductive threads. The experimental investigations were conducted to determine the characteristics and the electrical outputs of the fabricated prototypes. The open circuit voltage and power output of prototype A and prototype B were 22.1 mV and 2.21 nW, and 23.9 mV and 3.107 nW, respectively, at 22.5 °C temperature difference. The fabricated prototypes were also tested on the human body at different body conditions and were found to be very flexible, twistable, and durable with the substrate as well as conforming well to the human body. <s> BIB008 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Ocean thermal energy is formed by temperature difference between sea surface and deep sea. Underwater glider is a new kind of autonomous underwater vehicle. It ascends and descends between the sea surface and deep sea periodically by adjusting its net buoyancy. Thermal underwater glider can harvest ocean thermal energy through the utilization of phase change material (PCM) and convert it into mechanical energy for buoyancy-driven. In this paper, the fundamental principle of this thermal engine is unveiled. A nonlinear model for relationship between system pressure and phase change rate is established. Based on the established nonlinear model, influencing factors for system pressure and stored energy are analyzed comprehensively. Selected PCM is Hexadecane. Value range for air solubility in liquidus PCM is 0.06–0.1, and for residual air in the system is −0.02 to 0.05. The influence of these two factors on relative energy storage is similar. For typical values of air solubility 0.08 and residual air 0.005, utilization ratio of the thermal engine is less than 50%. Through experiment in the lake, the nonlinear model was verified, maximum system pressure was 12.5MPa, and average stored energy of each cycle was 2.48kJ. A prototype of thermal underwater glider has also been tested in the South China Sea. It had worked continuously for 29days without any failure. Total number of working profile was 121 and total cruising range was 677km, total stored energy was 300kJ. High reliability and performance was validated by the sea trial. <s> BIB009 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract A thermoelectric energy harvesting system designed to harvest tens of microwatts to several milliwatts from low-voltage thermoelectric generators is presented in this paper. The proposed system is based-on a two-stage boost scheme with self-startup ability. A maximum power point tracking technique based on the open-circuit voltage is adopted in the boost converter for high efficiency. Experimental results indicate that the proposed system can harvest thermoelectric energy and run a microcontroller unit and a wireless sensor node under low input voltage and power with high efficiency. The harvest system and wireless sensor node can be self-powered with minimum thermoelectric open-circuit voltage as 62 mV and input power of 84 μW. With a self-startup scheme, the proposed system can self-start with a 20 mV input voltage. Low power designs are applied in the system to reduce the quiescent dissipation power. It results in better performance considering the conversion efficiency and self-startup ability compared to commercial boost systems used for thermal energy harvesting. <s> BIB010 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Humans generate remarkable quantities of energy while performing daily activities, but this energy usually dissipates into the environment. Here, we address recent progress in the development of nanogenerators (NGs): devices that are able to harvest such body-produced biomechanical and thermal energies by exploiting piezoelectric, triboelectric, and thermoelectric physical effects. In designing NGs, the end-user's comfort is a primary concern. Therefore, we focus on recently developed materials giving flexibility and stretchability to NGs. In addition, we summarize common fabrics for NG design. Finally, the mid-2020s market forecasts for these promising technologies highlight the potential for the commercialization of NGs because they may help contribute to the route of innovation for developing self-powered systems. <s> BIB011 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> This work analyzes the results of measurements on thermal energy harvesting through a wearable Thermo-electric Generator (TEG) placed on the arms and legs. Four large skin areas were chosen as locations for the placement of the TEGs. In order to place the generator on the body, a special manufactured band guaranteed the proper contact between the skin and TEG. Preliminary measurements were performed to find out the value of the resistor load which maximizes the power output. Then, an experimental investigation was conducted for the measurement of harvested energy while users were performing daily activities, such as sitting, walking, jogging, and riding a bike. The generated power values were in the range from 5 to 50 μW. Moreover, a preliminary hypothesis based on the obtained results indicates the possibility to use TEGs on leg for the recognition of locomotion activities. It is due to the rather high and different biomechanical work, produced by the gastrocnemius muscle, while the user is walking rather than jogging or riding a bike. This result reflects a difference between temperatures associated with the performance of different activities. <s> BIB012 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract Studies concerning the use of thermoelectric effect for the conversion of thermal energy into electricity have become increasingly numerous since applications of the thermoelectric phenomenon can be limitless. Thermoelectric Generators (TEG) have shown promising results in combustion vehicles, with an overall efficiency increment from 33% to 57%, in addition to an increase of 6% in the available power in an automobile. In the aerospace industry, thermoelectric devices have been employed due to the need of making the most of the available energy. Thermoelectric modules are also used in the harvest of waste heat in wood stoves, furnace walls, and industrial chimneys. In Bioengineering, there are applications for small-scale generation including the use of heat from the human body to feed microelectronic devices and autonomous sensors. Therefore, the article shows a review of the use of thermoelectricity for the energy harvesting, beginning with the main concepts and definitions on the subject, followed by the state of the art containing the main methods and advances in science and technology for the development and application of solid-state micro-generators to capture residual energies. <s> BIB013 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract The figure of merit of thermoelectric materials is temperature dependent, and thus the local compatibility factor changes significantly along the thermo-element length. A local optimization method to maximize the efficiency of a function graded thermoelectric generator was proposed and discussed in this paper. By adjusting the cross-sectional area and segment's thickness, the reduced current equaled the compatibility factor of the material at every local thermo-element layer. This method can use the full potential of existing materials by maximizing the efficiency at every local thermo-element segment. For such a TEG working in a temperature range of 300–1100 K, the efficiencies of P-type segmented Bi0.5Sb1.5Te3/BiSbTe/-PbTe/FeNbSb thermo-element and a N-type segmented Bi2Te2.79Se0.21/Bi2Te2.9Se1.1/SnSe/SiGe thermo-element were 25.70% and 21.73%, respectively, much higher than the conventional segmented thermo-elements. The overall efficiency of the device was more than 23.72%, making it a promising technology to harvest energy from medium and high-temperature industrial components. The optimized TEG can be fabricated by SLS/SLM technology. <s> BIB014 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Abstract A TEG module with 199 thermoelectric (TE) couples was simulated with constant and variable property parameters respectively. It is indicated that the simulative results with variable property parameters are closer to the experimental results. Meanwhile, it is found that the open circuit voltage and output power are proportional to the number of TE couples. The parameterized rectangle-TEG(R-TEG) and circle-TEG(C-TEG) modules with 2 TE couples were established respectively; it was found that the performances of TEG have not relevant to the shape of TE couple. When the hot side temperature is constant, the internal resistance, the open circuit voltage and the efficiency increased but the output power decreased with the decrease of cross sectional area; when the cross sectional area is constant, the open circuit voltage, output power and efficiency increased with the increase of the hot side temperature. The intersection of the respective output power and efficiency of R-TEG and C-TEG are regarded as an optimized value. The performances of the optimized R-TEG and the original TEG are compared, it can be seen that their output power and efficiency increase with the increase of hot side temperature. The maximum output power and efficiency of the optimized R-TEG can reach to 91.88 W and 4.5% respectively. <s> BIB015 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermoelectric Energy Harvesting <s> Conventional internal combustion engines exhibit efficiency in the range of 30%-40%. A significant portion of the remaining energy is dissipated as exhaust heat, some of which can be recovered using thermoelectric generators to reduce fuel consumption and CO2 emissions. The current study evaluates the benefits of using thermoelectric electric generators in hybrid electric vehicles over a prescribed drive-cycle. Specifically, a hybrid electric bus was first modeled over a realistic urban drive-cycle. Engine operating parameters such as torque and speed were extracted and provided as inputs to an engine simulation model. A virtual thermoelectric generator was then modeled using the Matlab/Simulink architecture to evaluate the quantity of energy that can be recovered based on inputs from the engine simulation model. Simulations were carried out to accurately assess the amount of fuel savings achieved due to the use of the thermoelectric generators system. From the analysis, it was observed that a fuel saving of 7.2% and 6.5% can be achieved with the use of Skutterudite and Silicon Germanium-based thermoelectric generator systems, respectively. It was also observed that the additional weight of the thermoelectric generator system had a negligible effect on the fuel consumption. (C) 2018 Elsevier Ltd. All rights reserved. <s> BIB016
Thermoelectric energy harvesting BIB001 is a conventional technique for converting wasted thermal energy into the electrical power by means of thermoelectric generators exploiting the Seebeck effect. A thermoelectric generator consists of a series of PN junctions connected end to end as shown in Figure 9a . Its aim is to increase the action area to increase the amount of the electricity harvested. Figure 9b gives the working principle of each PN junction. When a temperature gradient is established between the hot and cold ends of the ceramic plates, free electrons and holes will move towards their specific directions and generate a potential difference in proportion to the temperature difference between the N-type and P-type semiconductors. This phenomenon is known as the Seebeck effect. Figure 9a . Its aim is to increase the action area to increase the amount of the electricity harvested. Figure 9b gives the working principle of each PN junction. When a temperature gradient is established between the hot and cold ends of the ceramic plates, free electrons and holes will move towards their specific directions and generate a potential difference in proportion to the temperature difference between the N-type and P-type semiconductors. This phenomenon is known as the Seebeck effect. Thermoelectric generators are widely utilized in wearable devices because the human body is a constant heat source with a temperature difference with the ambient temperature most of the time BIB012 . Wearable thermoelectric generators also contribute to clinical medicine through harvesting energy from the human body to support wearable electronics for health condition monitoring to avoid frequent battery replacement BIB002 BIB007  . Since thermoelectric generator is a heat engine, its conversion efficiency is limited by the Carnot cycle efficiency, which is one minus the ratio of the cold and hot temperatures. The generated power is usually insufficient for the requirements of devices. To increase the power output, new thermoelectric generators modules are designed with compact structures BIB013 BIB014 . Another approach is exploring new high-performance materials. For instance, some designers BIB008 have fabricated a flexible thermoelectric generator with special N-type and P-type thermoelectric materials relying on nanotechnology and obtained about 23.9 mW power at a 22.5 ℃ temperature difference. Because of its flexibility, this thermoelectric generator attaches to the heat source surface with an increase in the output power. This shows that the nanostructures of thermoelectric converters not only reduce volume and save materials but also effectively improve the performance of traditional thermoelectric generators. According to the advancement of the nanotechnology and fabrication techniques, numerous new promising nanomaterials and the Thermoelectric generators are widely utilized in wearable devices because the human body is a constant heat source with a temperature difference with the ambient temperature most of the time BIB012 . Wearable thermoelectric generators also contribute to clinical medicine through harvesting energy from the human body to support wearable electronics for health condition monitoring to avoid frequent battery replacement BIB002 BIB007  . Since thermoelectric generator is a heat engine, its conversion efficiency is limited by the Carnot cycle efficiency, which is one minus the ratio of the cold and hot temperatures. The generated power is usually insufficient for the requirements of devices. To increase the power output, new thermoelectric generators modules are designed with compact structures BIB013 BIB014 . Another approach is exploring new high-performance materials. For instance, some designers BIB008 have fabricated a flexible thermoelectric generator with special N-type and P-type thermoelectric materials relying on nanotechnology and obtained about 23.9 mW power at a 22.5°C temperature difference. Because of its flexibility, this thermoelectric generator attaches to the heat source surface with an increase in the output power. This shows that the nanostructures of thermoelectric converters not only reduce volume and save materials but also effectively improve the performance of traditional thermoelectric generators. According to the advancement of the nanotechnology and fabrication techniques, numerous new promising nanomaterials and the optimisation of structures and circuits  BIB015 BIB010  have been investigated to improve the thermoelectric harvesters. Proto et al. BIB011 have provided a comprehensive review on nanogenerators for human body energy harvesting. The machine in service is another heat source with temperature differences from the surroundings. Because of the features of safety, reliability and durability, as well as the extensive existence of temperature difference, thermoelectric generators have been widely applied in vehicles, aerospace, ships and other industrial operations BIB016 BIB004 BIB009 . Some of the generators aim to reduce fuel consumption and CO 2 emissions by converting exhaust heat into an alternative energy as an electric power backup. Others are designed to provide electricity for WSNs using for structural health monitoring (SHM) or system condition monitoring of machines BIB005 BIB003 . It is confirmed that spatial temperature gradients are available, continuous and stable in mechanical systems. More importantly, thermoelectric harvesters have characteristics of high stability, durability and safety. However, relatively low conversion is one of the disadvantages of thermoelectric energy harvesting. It is determined by temperatures of the cold and hot ends, as well as the figure of merit (ZT, properties of thermoelectric material). The maximum of ZT is about 2.6 leading to the maximum thermoelectric conversion rate of about 20% presently BIB006 . But the development of innovative structures, promising nanotechnology and emerging nanomaterials can promote the conversion rate to achieve high efficiency, flexibility and conformability in the field of machine condition monitoring. Another disadvantage in applying thermoelectric generators is that the temperature difference found in many machine systems is small, leading to small power gains. Even the usage of the heat sink can increase the temperature fluctuation, but the harvesting device may be too bulky and too difficult to install, which is a challenge in designing the structure of thermoelectric generators used for maintenance-free machine condition monitoring in the future.