Environmental pollution, global warming and increasing energy demands are urgent challenges facing society. Governments all over the world have set a national policy target for the transition to a zero or low carbon dioxide economy. As a result, scientists and engineers in industry and academia are working to develop cleaner, alternative and sustainable energy production technologies. One technology that has potential in this green technology transition is thermoelectric generators (TEGs), traditionally used off-grid and isolated from things such as stand-alone solar–thermal cells for military and aerospace applications such as missile-testing systems and space telescope cameras. However, future applications based on home entertainment, security systems and smart metering applications are imminent. Key limitations to this are low efficiency, high costs and self-heating with low thermal conductivity. Hence, this study aims to examine the current state of the art of TEGs and identify future research directions to achieve support for the green technology transition. The key findings of this study show that present successes will fulfill the future advancement of thermoelectric technology by supporting a low carbon dioxide economy.
Notation
- Qc
outgoing heat flow at the cold side of the TEG
- Qh
incoming heat flow at the hot side of the TEG
- Rin
internal resistance
- RL
load resistance
- T
temperature (K)
- V
voltage source
- VTEG
voltage at the TEG
- ZT
figure of merit
- α
Seebeck coefficient (V/K)
- ΔTTE
temperature difference between the thermocouples
- ΔTTEG
temperature difference between the two substrates of TEG
- κ
thermal conductivity (W/(m K))
- σ
electrical conductivity (Ω m)
1 Introduction
One of the reasons for global warming today is the waste heat emitted by machines that are used in industrialization. While these machines are used in product development and production, they also emit waste heat into the environment.1 Besides, the excessive waste heat of these machines causes a decrease in energy efficiency. Thermoelectric generators (TEGs) bring hope of a more sustainable environment and development of highly energy-efficient machines. With the development of technology and nanostructured materials, the efficiency and usage temperatures of TEGs have started to increase. Although the efficiency of commercially used TEGs is low, it is seen from studies that the efficiency of TEGs that are being developed in laboratory environments is quite high. TEGs could be used to achieve a low carbon dioxide (CO2) (‘low-carbon’) economic society.2 A thermal difference is utilized to generate electrical energy in TEGs.3 Initially, Thomas Seebeck used the presence of thermal differences between two dissimilar materials to develop an electrical potential.4 This phenomenon is known as the Seebeck effect. Later, dissimilar materials were joined at their ends to prepare a thermocouple for manufacturing a TEG.5 A TEG can also be used in cooling and heating using the effect known as the Peltier effect. The Seebeck effect can be used in electric power plants for power generation by using waste heat recovery management.6 On the other hand, the Peltier effect is the presence of cooling and heating when a voltage is applied across joined conductors. These two effects, one for power generation and the other for refrigeration, are utilized in thermoelectric module (TEM) technologies.7Figure 1 shows the general structure and equivalent circuit of a TEG.
TEGs can be one of the promising solutions for a low-carbon economy. Moreover, TEGs are active devices made with dissimilar materials based on the Seebeck effect. TEGs can directly convert thermal energy to electrical energy. Developing TEG devices with superior performance are interrelated with the Seebeck and Peltier effects. Table 1 shows the currently developed TEG devices for manufacturing.
TEG devices currently developed for manufacturing by researchers
| Inventors | Materials | Power |
|---|---|---|
| Yang et al.8 | BixSb2−xTe3 | 259.3 mW |
| Haidar et al.9 | Bismuth telluride–Sb2Te3 | 1.3 mW |
| Shen et al.10 | Bismuth–tellurium | 15.24 μW |
| Hwang and Jang11 | CNTs–CNPs | 4.8 μW |
| Jung et al.12 | EOG–BTS | 1.65 μW |
CNTs–CNPs, carbon nanotubes–carbon nanoparticles; EOG–BTS, edge-oxidized graphene–Bi2.0Te2.7Se0.3
To date, TEGs have been broadly utilized in various sectors due to their energy efficiency, low-cost maintenance, scalability and long lifetime.13 TEGs have also attracted more interest regarding energy harvesting for different purposes depending on dimensions, power and materials, such as biomass cookstoves,14 concentrating solar generators,15 microprocessors16 and three-dimensional (3D) packaging environments.17 The voltage between the TEG ends depends on the temperature difference between the surfaces. As the temperature difference increases, the voltage rises in direct proportion. p- and n-type junctions of these materials can be used to create any solid-state heat engine. Until now, only a very small number of materials have been discovered as thermoelectric (TE) elements.18 The most usable TE materials are bismuth telluride (Bi2Te3)-based materials. These are suitable for the manufacturing of TEG devices under low-temperature conditions. The TE performances of these materials have a low figure of merit (ZT).19 Materials scientists and engineers have recently concentrated their efforts on high-temperature and high-TE performance conditions. Waste heat sources can be divided into three categories – namely, low- (∼250°C), medium- (250–650°C) and high-temperature-condition (above 650°C) sources.20 Commercialized TEGs composed of bismuth telluride alloys based on bismuth (Bi) in combination with antimony (Sb), tellurium (Te), selenium (Se), idaite (Cu3FeS4) and argyrodite-type (Cu7PSe6) materials is suitable for low-temperature conditions. Other bimetallic and ternary materials such as zinc antimonide (Zn3Sb2), magnesium silicide (Mg2Si), lead telluride (PbTe), lead selenide (PbSe), copper (I) telluride (Cu2Te), copper (I) selenide (Cu2Se), SiGe, tin (II) selenide (SnSe) and doped tin (II) selenide (SnSe) are suitable for medium and high-temperature conditions.21 Advanced TE materials are being developed in order to support a low-carbon economy.22 Some semiconductor compounds, such as zinc antimonide (Zn4Sb3), germanium (II) telluride (GeTe) and tin (II) selenide-based materials (germanium (II) sulfide (GeS), germanium selenide (GeSe), tin (II) sulfide (SnS), tin telluride (SnTe)), possess an exceptionally low thermal conductivity, are relatively inexpensive and exhibit a high ZT value within a different temperature gradient. Other new materials of interest include skutterudites, tetrahedrites and rattling ion crystals.23 TE properties have been improved by using semiconductor compound elements.24,25 Modern TEG manufacturers can use these semiconductor elements. TEGs find wide usage areas in waste heat recovery, contributing to energy efficiency since TEGs are scalable semiconductor devices. Vehicle exhaust systems,26,27 steel processing,28 airborne engines29 and photovoltaic (PV) hybrid systems30 are examples of these areas. The research can be applied to modern aviation and helicopters to increase their efficiency. Other applications include producing enough power in various locations with temperature gradients and developing TE hybrid power sources to reduce carbon dioxide emissions (‘carbon emissions’).30 The conversion of thermal energy into electrical energy can be carried out by using TEGs. Their advantages in the recovery of waste heat are quite numerous, such as requiring minimum maintenance, high reliability and long lifetimes. TEGs are very suitable for use in areas that are hard to reach and where there is no electricity network. Examples of applications for supplying the energy of a system with TEGs are radio communication, telemetry and wireless sensor networks. TEGs are the most robust power-generation sources due to their advantages, such as not having moving parts, not being affected by climatic conditions, being able to perform continuously and not needing batteries. Solar PV panels can be installed in remote areas as an energy resource. They may not be suitable for places in which solar radiation levels are low, areas with lots of cloud or a large tree canopy cover, dusty deserts and forests. In these circumstances, TEGs can be a more suitable solution for power supply. Recently, researchers have considered whether the waste heat energy of microprocessors can be recycled. Many space probes produce electricity by using radioisotope TEGs, whose heat source is a radioactive element. The recent achievements will lead to an exciting next generation using nanostructured TEGs. Worldwide, the next generation’s energy demand may be fulfilled by using these devices.
TEGs are more favorable to researchers because they can be used for waste heat recovery, electrical energy generation in remote areas and powering microsensors. In their earlier reports,31–35 the authors suggested that bismuth telluride and bismuth telluride-based nanostructure materials are suitable for TEG device manufacturing for TE applications. Furthermore, the authors demonstrated that nanostructured materials are more suitable for device manufacturing compared with thin films, according to their previous reports.36–39 As long as the need for energy is not over, generating and using this energy in the most efficient way will continue to be a hot research topic for both researchers and engineers. TEGs, which are used in waste heat recovery and contribute to energy efficiency, can are preferred for meeting small energy needs in areas far from the electricity grid. A review of research showing that TEGs can be used in electrical energy generation was published in 2003 by Riffat and Ma.40 After a certain period of time, another study showing other TEG applications was conducted by Ahiska and Mamur in 2014.41 In addition to these, a study including new applications was conducted by Champier in 2017.42 In relation to this, a review study that explains systems designed with TEGs to generate electricity from waste heat from biomass stoves was carried out.43 Moreover, Siddik et al.44 showed that the TEG studies used for the recovery of waste heat from humans in 2017 as wearable TEGs. Considering the time that has passed since these studies, some advances in technology and nanostructured materials45 have gained a significant momentum. This study emphasizes the developments in nanostructured TEGs in recent years that can be used in the future.
This review paper discusses the extensive state of the art of TEGs and also investigates current TE materials and keys to achieving high efficiency and power factors with various TE material preparations. Furthermore, the authors present the utilization of TEGs in low- and high-power applications. Moreover, the basic technologies of TEGs are given in this review paper. Existing and future technologies are also presented. In addition, this paper highlights which technology would be satisfactory for enhancing the TE performance of a TEG device. Ultimately, it also introduces a new idea for developing the technology of TEGs that will have more impact on the advancement of TEGs utilized for energy harvesting. The paper may provide a way to explore high-performance TEGs that could easily be used for energy generation and conversion to produce thermoelectricity. It is regarded as one of the most encouraging waste heat recovery elements. An effective substantial process for the better utilization of TEGs can maximize the temperature flow across TE specimens. Currently, thermoelectricity is a very advantageous sustainable power resource; it is not only in a development phase but, soon, it may be discovered as a peerless sustainable resource.46 As a result, the goal of this study is to examine the current state of the art of TEGs and identify future research directions to aid in the green technology transition. The study was undertaken as a desktop review of the published literature in different journal articles and discusses the present research and industrial approaches for future TEG technologies. Finally, the authors propose a new opportunity to advance TEG technology for minimization of carbon emissions. The paper has four sections. Section 1 introduces TEG development and summarizes the current start of the art of TEGs; Section 2 reviews the different technologies, such as design and fabrication criteria of TEGs; Section 3 discusses the limitations and challenges of TEGs; and Section 4 provides the conclusion and future research direction.
2 Technologies of TEGs
Waste-heat-management systems are one of the world’s most underutilized energy resources.47 Until now, most TE materials could not be operated at high temperatures to recycle waste heat into electrical energy.48–50 Recently, researchers have found a solution in terms of some economical, efficient and cleaner TE materials for the conversion process under high-temperature conditions.51 This reveals that challenging goals can be achieved, such as better fuel economy of vehicles, lower carbon emissions in the world and lower manufacturing costs of TEGs.52 The automobile industry benefits greatly from high-temperature vehicle technology.53 This technology has the potential to have multiple impacts and improve fuel economy. TEGs can be developed according to several technologies, such as design approaches and fabrication approaches.54,55
2.1 Design approaches
Various design approaches for TEGs can be employed due to the different shapes of the heat-recovery-management surface. TE and power-generation properties are enormously influenced by the design of TEGs. The primary objectives are to minimize heat loss and contact thermal resistance with consideration of any TEG design. Based on their design, TEGs are classified as planar and multilayer thin or thick film, metallic, portable, flexible and so on.
2.1.1 Planar and multilayer thin or thick films
In performing the TEG operation based on the Seebeck effect, when the terminals of two dissimilar material junctions are joined and replaced at different heat sources, an electromotive force (EMF) takes place.56 When numerous TE elements are connected in series (thermopiles), they can produce a significant EMF. Researchers have presented planar thermopiles for the fabrication of TEGs. Yuan et al.57 presented the modeling, design, fabrication and characterization of a planar micro-TEG. Using the model TEG, they could convert waste heat into direct-current electrical energy. The model TEG was designed and fabricated into a planar form. The TEG had a large thermal resistance. With a high thermal resistance value, the TEG can provide better application under high-heat sources. Also, the TEG was made of two periodically etched silicon (Si) substrates that were respectively utilized as temperature concentrators and evacuators. The embedding of this TEG in a multilayer membrane of the poly-silicon-based thermopiles consisted of large lengths of thermoelement legs. Thus, it prevented direct heat losses from a concentrator to an evacuator by using thick air cavities etched into the effective substrates. Additionally, the performance of the TEG was improved by employing 3D simulations. This kind of TEG involves thermal input power instead of a temperature difference across the chip, which is known as the efficiency factor. It has a more effective advantage due to the thermal resistance of the TEG. The obtained application data indicate that the TEG can be used at a high-temperature difference of ∼267 K with a thermal resistance of 78 K/W. Finally, the authors estimated a maximum output power of 138 μW/cm2, whereas the input power was 4 W/cm2 with an efficiency factor of 865 μm2/W. The utilization of conventional TEGs is restricted owing to their high cost and complex manufacturing. Fan et al.58 demonstrated a thin-film TEG on zinc (Zn)-based TE materials. Their TEG had a low cost. They used a flexible substrate and cost-effective zinc-based TE materials for developing the TEG. They obtained a maximum output power of 246.3 μW. Their TEG consisted of ten thermoelements that operated at a temperature difference of ∼180 K. Finally, the authors found that the various shapes of cost-effective thin-film TEGs on flexible substrates show excellent output power for micro- and nanoenergy utilization. Cao et al.59 presented a flexible screen-printed bismuth telluride-based thick-film TEG with reduced material resistivity by using cold isostatic pressing. They fabricated a prototype TEG on a flexible polyimide substrate with eight thermoelements with dimensions of 20 mm × 2 mm × 78.4 μm and measured the resistivity and Seebeck coefficient of the printed materials with the applied pressure. They estimated the prototype TEG voltage to be 36.4 mV with a maximum power of 40.3 nW at a temperature gradient of 20°C. Figure 2 shows schematic diagrams of planar, vertical and mixed micro-TEGs.
Schematic diagrams of (a) planar, (b) vertical and (c) mixed micro-TEGs; adapted from Jaziri et al.60
Schematic diagrams of (a) planar, (b) vertical and (c) mixed micro-TEGs; adapted from Jaziri et al.60
Researchers are facing some problems in generating a sufficient EMF because it depends on the Seebeck coefficient and the temperature difference between the hot and cold junctions.61 There are two steps to improving the EMF of TEGs. One is the high performance of TE materials that are used for the manufacturing of thermoelements to increase the Seebeck coefficient, and the other is to increase the number of thermopiles inside the TEG. Thin- or thick-film technologies expand the range of TE materials available.62 It could be different compositions of materials, such as semiconductors, metal oxides and multilayer thermopiles.63 The developed technologies for operating predictable multilayer connections allow construction processes consisting of a few dozen layers. A large TEG fabricated structure contains thousands of miniature thermoelements spread across a multilayer. This process can generate a sufficient EMF to power electronic microcircuits, and it can be categorized as energy harvesting. Figure 3 shows the concept of a multilayer micro-TEG.
2.1.2 Metal-based TEGs
The main problem with the use of a medium-temperature-range TEG is its low performance. This problem is caused by the moderately low thermal stability of TEG interconnects. To solve this problem, researchers can use metal-based TE materials in this technology.65 A metallic TEG is different from a conventional TEG in terms of the high power density of TE devices. It does not use any junction technology in the n- or p-type layer and, thereby, does not manipulate or reduce the thermal conductivity of materials. It is built with a thin film with an array structure on a plane surface. Lastly, it is fabricated by using a simple fabrication process. It is tied up with another layer built on top of its original layer embodiment as a tandem mode. The TEG has built a multi-layered thin-film TE array structure in tandem mode. By means of the formation, it reproduces regenerative cycles to capture and convert more electrical energy for high performance. Metal-based TEGs have advanced technology and are prepared to modify production costs and efficiency. They can be easily integrated with pipe insulation for powering wireless sensors on the internet of things (IoT). Lead telluride-based TEMs were prepared through sputter deposition to form the reaction couple of copper (Cu)/thin-film metallic glass (TFMG)/lead telluride and copper/titanium (Ti)/lead telluride by Yu et al.66 The samples were annealed at 673 K between 8 and 96 h. It was revealed that the prepared samples effectively blocked interdiffusion by not having grain boundaries that allowed this diffusion between the metal electrodes and the TEM substrate. The electrical resistivity remained in the range between 3.3 and 2.5 × 10−9 Ω m2. The authors recommended the use of TFMG as a forceful diffusion barrier layer for these modules. Finally, they concluded that titanium-based TFMG has strong thermal properties and low electrical resistivity that allow bonding with lead telluride. Lead telluride-based TEMs also showed more promise as a replacement diffusion barrier layer for the medium-temperature range. Meanwhile, a high-performance lead telluride-based nanostructure metal-based TEM delivering high conversion efficiency was demonstrated by Hu et al.67 They fabricated metal-based modules such as lead telluride–magnesium telluride (MgTe) (p type)–lead telluride (n type) and bismuth telluride/lead telluride–magnesium telluride (p type)–bismuth telluride/lead telluride (n type). In their process, they investigated temperatures between 570 and 810 K to find the ZT from 1.4 to 1.8. They obtained a maximum conversion efficiency of 11% for the nanostructure metallic module at these temperatures. Finally, they concluded that the maximum conversion efficiency of their TEG could reach around 15% by improving the electrical resistance and thermal contacts of the TEG.
Besides the studies mentioned earlier, a study was conducted by Iezzi et al.68 They manufactured a cost-effective screen-printed metal-based TEG by using silver and nickel inks. With a couple of advanced analyses, the TEM can be used on commercial steam pipe insulation for the IoT. Vibrations and thermal ripples could be measured and monitored by combining the TEG with the IoT. Thus, some measures can be taken in advance to prevent damage to the process. The fabricated TEG consists of 420 TE junctions comprising 12 modules that produce a power of 308 μW at a 127 K temperature difference. This power was enough for wireless communication capabilities and for operation of a temperature-sensing circuit. Moreover, the TEG was implemented on a device. The device had an RFduino microcontroller transferring temperature data every 30 s through Bluetooth to a cell phone. The device was charged within the TEG for 4 h. After being charged, the device sent the data in 10 min. Ultimately, the authors suggested that after being optimized, the TEG could be utilized in industrial wireless sensing networks.
2.1.3 Portable TEGs
Researchers are very interested in portable TEGs for rapid development. Advances in electronic technology and battery-management systems increase this interest even more. A TEG has small packages and generates low electrical energy. These advantages allow TEGs to be easily operated as personal energy backpackers. They generate direct-current electricity. They can give a small power output that can be used by a stove. Their use is very advantageous for areas without a grid for charging of mobile phones, which are indispensable devices. Moreover, they do not require repair or maintenance and have a long lifetime. It features portable technology to carry. With a TEG, a meal can be prepared on a camping night. Most importantly, a TEG can charge universal serial bus (USB)-enabled mobile devices as quickly as a standard outlet does when a flame-resistant USB cable is used with the TEG. Recently, this type of TEG has become commercially available. Figure 4 shows a typical portable TEG with 5 W USB power.
An efficient prototype portable micro-TEG with an integrated microcombustor was developed by Aravind et al.70 A new design is suggested in their report to integrate microcombustor generating power. The device has high temperature uniformity. Three backward parts in a recirculation hole that are laminated in a heating medium of aluminum (Al) material were used in its configuration. The authors achieved a significantly higher power density of 0.12 mW/mm3 of system volume with a maximum conversion efficiency of 4.03%. Consequently, they successfully designed a compact prototype system with a high conversion efficiency, which indicates the possibility of its utilization for portable power applications. Meanwhile, a portable TEG whose input energy was taken from a catalytic combustor was designed and assembled by Fanciulli et al.71 The portable TEG was a safe power application. Hydrocarbon fuel was selected for its input power. A fast-charging system was realized owing to the development of the portable TEG. When its cold-side temperature was below 40°C, the output electrical power was about 1 W. The portable TEG developed was tested at various flow rates. The authors emphasized its efficacy for process optimization and efficiency enchancement in their report. In this work, the TEG was a simple design that used common materials and components.
A 100 W portable TEG for low geothermal temperatures was designed and developed by Ahiska and Mamur.72 The authors examined and monitored the properties of the TEG through a supervisory control and data acquisition system. The system would mark a new stage in the examination of portable geothermal TEGs. The testing system examined geothermal areas. It had a hot and a cold water inlet. They both generated electricity, and the authors measured the efficiency of their portable TEG. At a temperature difference of 67°C, a maximum power of 41.6 W was obtained. The authors also measured the maximum power point (MPP) value. For this reason, they continuously varied the load resistance of the TEG. An MPP was captured when the load resistance of the TEG was about 15 Ω. The experimental results show that the efficiency of the portable TEG was about 3.9%.
2.1.4 Flexible TEGs
In the past few years, most TEG research has significantly improved by focusing on inorganic bulk TE materials and their characteristics. Recently, a flexible TEG was fabricated where the use of bulk material was prevented by its inherent toughness.73 The progress of research on flexible TEGs has included fundamental principles of TEGs, conducting polymer TE materials, nanocomposites consisting of inorganic nanostructures, inkjet-printed materials and fully inorganic flexible TE materials in nanostructured thin films. There is a high demand for flexible TEGs due to the advancement of flexible power electronics and wearable sensors for powering and monitoring. The flexible TEG technology is rapidly expanding, with numerous promising applications for these purposes. Flexible TEG technologies provide this capability without sacrificing performance when compared with rigid TEGs. Polymers and composite materials can be used to develop flexible TEGs. These TEGs are fabricated by using innovative manufacturing processes, such as additive manufacturing, printing and thermal spraying processes. A high-power flexible TEG was manufactured for energy acquisition from both planar and non-planar surfaces by Sugahara et al.74 They prepared the TEG by using organic thin-film materials. The TEG was developed for wireless sensor networks and wearable power applications. The TEG had 250 pairs of thermoelements (n and p types) successfully fabricated on a flexible 50 × 50 mm2 substrate. The authors investigated the output power, mechanical strength and bending characteristics at different bending cycles and heat gradients of the TEG. Their experimental results showed that the maximum output power density was 158 mW/cm2 at a temperature difference of 105 K and that an efficiency of 1.84% was obtained. The flexible TEG was reliable and stable during the mechanical tests. The authors concluded that the experimental results showed the great potential of portable, flexible TEGs. Figure 5 shows the schematic design of the flexible TEG.
Schematic design of a flexible TEG module: (a) full-scale module; (b) cross-sectional images of A-A′; (c) cross-sectional images of B-B′; (d) conventional approaches of thermoelectric module; (e) large-scale flexible thermoelectric module; adapted from Sugahara et al.74
Schematic design of a flexible TEG module: (a) full-scale module; (b) cross-sectional images of A-A′; (c) cross-sectional images of B-B′; (d) conventional approaches of thermoelectric module; (e) large-scale flexible thermoelectric module; adapted from Sugahara et al.74
Other materials with good TE properties are polyaniline composite polymers. These exhibit good TE performance that can be further optimized with conductive additives. Regarding these materials, single-walled carbon nanotube/polyaniline composites were developed by Li et al.75 The authors used ethanol treatment. The fabricated flexible TEG generated an about 2.86 μW power output. Moreover, the TEG had a high power density of 745 μW/cm2 when the temperature difference was 50 K. The authors concluded that the output power and density could be improved by the treatment of composite TE materials. In conclusion, they found that polyaniline composite polymers have facile processing, are cost effective and can be utilized for TE conversion systems. Sargolzaeiaval et al.76 created a high performance flexible TEG using all of the aforementioned factors. The authors used rigid p- and n-type bismuth chalcogenide TE legs. The process operates in series with stretchable liquid metal interconnects, having low resistivity with self-healing. The TEG would be powered by the human body. The thermal energy of the human body would be converted into electrical energy by the flexible TEG. This has placed the TEGs in an important place in wearable electronic device applications. Moreover, the authors highlighted that the thermal conductivity of the TEG could be easily increased by encasing liquid metal lines in a new stretchable silicon elastomer doped with both graphene nanoplatelets and the main prepared sample eutectic gallium indium (EGaIn). Lastly, the authors achieved a device power level of 30 μW/cm2 at an air velocity of 1.2 m/s. To make flexible TEGs,77,78 other researchers developed a simple way to form thin, soft and flexible liquid metal interconnections for human body heat harvesting. The authors found maximum power factors of 180 and 110 μW/(m K2) for bismuth telluride and Bi0.5Sb1.5Te3 nanowires, respectively. They were able to achieve a maximum power of 127 nW despite the 32.5 K temperature difference. Even microforms of TEG studies may be found in applications and production. In this context, Beretta et al.79 reported an experimental set-up for the characterization of flexible micro-TEGs. It can easily measure the conversion efficiency and the power generated by the developed TEG under mechanical stresses and deformation. The set-up was in an atmospheric environment at a temperature interval of 293–423 K under vacuum. The function of mechanical pressure and load resistance exhibits uncertainty at temperature differences of approximately 0.02 K. The set-up was used to examine commercial rigid devices. The power generated and conversion efficiency had repeatability within 5 and 3%, respectively. The authors attribute the accuracy of the measurement to the minimization of all potential sources of heat flux losses.
2.2 Fabrication criteria
Fabrication of TEGs employs a bottom-up process that considers the ZT, the thermoelement arrangement and different temperature conditions. The aim of these fabrication criteria discussed here is to highlight the complexity and reduce production costs.
2.2.1 Figure of merit
The main advantages of TEGs are the possibility of performance without maintenance, high reliability and a long lifetime. The main disadvantage is very low efficiency due to more heat losses. The technological progress in TEGs depends on the efficiency increase. If productivity increase can be achieved, the use of TEGs will increase tremendously. Techniques for efficiency increase are investigated, and some recommendations are suggested for TEG manufacturers. To fulfill this requirement, when creating TEG sectional schemes, every section should be arranged in its operating temperature range. In such a scheme, every section uses its own TEMs, whose construction and materials are optimized for the operating temperature range of each section. The efficiency of TEGs based on sectional design is investigated. Thus, TEG efficiency would be increased for the next step. To do so, one primary requirement is to increase the quality of TE materials, and their ZT is ensured. The performance of a device, or a system, relative to that of its alternatives is characterized by using a quantity known as ZT. It is often defined for particular materials or devices to observe their relative utility for utilization in technology. Furthermore, it is used to determine the efficiency of thermoelements and TEGs. The ZT of any material is expressed as ZT = (α2σ/κ)T, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature in kelvin and κ is the thermal conductivity. The value of the dimensionless parameter ZT gives the TE material efficiency or TE material performance.
The main issues are TEG manufacturing and how to increase the ZT value to achieve higher output power. The optimization of ZT appears to be a very challenging issue80 because three characteristics, the Seebeck coefficient, the electrical conductivity and the thermal conductivity, affect each other a lot. The nanostructured TE materials used in TEG manufacturing have ZT values that lie between 1 and 1.82, whereas some materials have a ZT ≥ 2.81,82 Although several promising TE materials have been discovered in recent years,83 bismuth telluride-based materials are still the most widely used for TE applications.84,85 The main TE applications are industrial waste-heat-management processes, weather stations installed in hills, telephone transmitter stations in remote mountains, border security power stations and energy-generation devices for aviation. Other applications include fuel and natural gas combustion heat, transmission and off-grid control systems. These applications illustrate the TEG technology’s broad research areas, which have had great practical significance in recent years.
The value of the dimensionless parameter ZT, which gives the TE material efficiency or TE material performance, was investigated for TE materials by Wei et al.86 Based on theoretical calculation, the authors found that the ZT value reaches 14 for bismuth telluride nanowires with a diameter of 5 Å, and graphdiyne has a ZT value of 4.8 at 300 K. The dimensionless ZT value of n-type tin (II) selenide reached 2.8, as revealed by experiments. The authors focused on the updated theoretical and experimental achievements of bismuth telluride-based, tin (II) selenide-based, copper (II) selenide (CuSe)-based and germanium (II) telluride/lead telluride-based series TE materials. They also studied multicomponent oxides, half-Heusler alloys and organic–inorganic composite TE materials. Furthermore, they described the preparation process, structural and TE properties and device manufacturing of these materials. They researched the performance of these materials and recommended further research development. The optimization of the ZT is always attracted by the high ZT value of TE materials. The ZT value was significantly improved. Doping and different processing technologies have applied the modulating electrical and thermal conductivities and Seebeck coefficients of some of these materials. Obtaining a high ZT value at room temperature is too hard. However, most high-performance doped materials are expensive and toxic. Moreover, the preparation of TEGs with high conversion efficiencies has also faced some problems. The authors provided a wealth of information and cutting-edge data for researchers interested in the field of TE technology.
Various strategies for improving the value of dimensionless ZT were discussed by Aswal et al.87 Firstly, a high ZT could be achieved by improving the power factor through band structure modification, degeneration of multiple valleys, creation of electronic resonance states, electron energy barrier filtering, highly mismatched isoelectronic doping, modulation doping and depressing of the bipolar effect. Secondly, the authors discussed that a high ZT could be obtained by reducing thermal conductivity to the point where scattering photons within the unit cell point defects, rattling atoms, interstitials, vacancies, matrix–precipitate band alignment and a coherent interface are possible. Thus, a higher ZT value could be achieved either by enhancing the power factor α2σ or by reducing the thermal conductivity κ of TE materials. The fabrication cost of TEGs and their potential applications in different areas were highlighted by the review report. Finally, the authors concluded that TEGs provide clean energy that is noise-free and virtually maintenance-free and continuously produces power for a long time under ambient temperature conditions. They discussed that TEG technologies are suitable for operation under different temperature conditions. Figure 6 shows a summary of the temperature dependence of the values of the dimensionless parameter ZT of p- and n-type TE materials.
Summary of the temperature dependence of the ZT values of (a) p- and (b) n-type TE materials. PEDOT:PSS, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate). Adapted from Rull-Bravo et al.88
Summary of the temperature dependence of the ZT values of (a) p- and (b) n-type TE materials. PEDOT:PSS, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate). Adapted from Rull-Bravo et al.88
The amount of heat in the environment reaches up to about 100°C, and this waste heat is released into the environment. The development of high-efficiency bismuth telluride will provide a significant advantage in recovering this waste heat as electrical energy. However, it is a great challenge that the electrical conductivity of a TE material is very high, while its thermal conductivity is very low. For this reason, it is very difficult to develop the value of the dimensionless parameter ZT for these temperatures.
Bi2−xSbxTe3 TE nanostructure materials with significant ZT values were fabricated by using a hydrothermal synthesis process by Liu et al.89 This was followed by cold-pressing and sintering in an evacuated and encapsulated ampoule. The prepared samples, which were the size of x = 1.55 with nominal composition, exhibited a dimensionless ZT value between 1.75 and 1.65 at 270–290 K. This value was significantly improved by ∼70% at room temperature compared with those of commercial state-of-the-art bismuth telluride materials. The authors suggested that ZT could be raised by reducing the thermal conductivity, which is mainly relevant to the increased phonon scattering in the nanostructure materials. Another researcher stated that variance in ZT values improves module efficiency. Figure 7 shows the variation of TEM efficiency for different ZT values under low-temperature conditions.
Variation of TEM efficiency for different ZT values under low-temperature conditions; adapted from Kumar et al.90
Variation of TEM efficiency for different ZT values under low-temperature conditions; adapted from Kumar et al.90
Significant volumes of the waste heat produced by fossil-fuel-based economy can be developed into usable electricity. However, the widespread use of traditional TE nanoparticles is being hampered by their low efficiency, unavailability, high cost and inadequate production scalability. Silicon is a very effective method for improving the TE performance of readily available and inexpensive materials. However, using these nanostructures in practice remains a significant issue, particularly when it comes to covering the vast distances needed for significant waste heat recovery. A family of nano-enabled materials consisting of scalable and affordable large-area paper-like structures. There has been a five-fold improvement in the TE figure of merit in p-type silicon nanotubes. This opens up a market for waste heat recovery and demonstrates outstanding power densities above 100 W/m2 at 700°C. The adaptability of the method, which allows it to be used with a variety of nanomaterials, and the high tunability of the nanosized features, such as the wall thickness of the nanotubes, which regulates ZT enhancement by significantly reducing thermal conduction, can lead to further developments through the use and optimization of additional materials and nanostructures. There is still an opportunity for development, and the techniques used are both affordable and scalable, which highlights the potential effects of this paradigm on the use of TE nanomaterials in commercial applications in the future.
2.2.2 Thermoelement arrangement
Researchers focus on designing and fabricating thermoelements of a TEG to achieve maximum performance with the highest ZT value of TE materials based on the heat gradient. p- and n-type TE materials can be arranged in different ways, such as ordinary and segmented, to prepare the thermoelement of TEGs.91,92 Researchers have used many conductive and semiconductor TE materials for fabricating thermoelements. TEG structures are formed at the optimal operating temperature for the segment of TE materials. It is essential to select these materials carefully for significant electrical energy performance, such as high output voltages and electric power and low internal resistance. The conversion efficiency is related to the value of the dimensionless parameter ZT. TE materials should have a high Seebeck coefficient and low thermal conductivity for the best performance. Traditionally, metal materials have low ZT values, and semiconductor materials have high ZT values. Commercially, the most commonly used TE materials are bismuth telluride-based materials that provide ZT values around unity under low-temperature conditions. However, the heterogeneity between differently stacked materials can greatly affect the compatibility of segmented TE materials, which affects TE efficiency. Consequently, maximum TE efficiency can be achieved when the current density is near the compatibility factor of segmented specimens. The TEG architectures within the increment for thermoelement length have contrary trends in the maximum output power and conversion efficiency. It was concluded that segmented TEGs are more efficient in recovering waste-heat-management systems than traditional TEGs. The performance of segmented TEGs depends on the ratio of materials, the design of thermoelements and the ZT values of the two materials.
In recent years, the increase in environmental pollution and the emphasis on the global energy crisis occurring in the world have encouraged the use of renewable energy sources significantly. Continuous developments in vehicles operating on fossil fuels cause their gases to spread even more. It is also important to increase their energy efficiency. The temperature of the waste gases emitted by these vehicles is very high. Converting a certain part of this to electrical energy can be done with TEGs. Shu et al.93 optimized the configuration of segmented modules using an exhaust-based 3D numerical model engine for developing a TEG for waste heat recovery management. They considered the thin heat exchanger and moderate inlet dimensions to minimize the background pressure and incremental weight. They also compared the performances of the two modes with different forms and configurations of TEGs. With single-mode TEGs, the output power increased by about 13.4% with a wide temperature gradient in the radial direction. On the other hand, using multi-TE modules, the output power rose by about 30.8% compared with that of the original model. When these values are taken into account, there is great energy recovery. Finally, the authors concluded that the extended fins in the heat exchanger enhance heat transfer and improve temperature uniformity. In this circumstance, they concluded that the multi-TE modules proved to be more effective. Figure 8 shows geometric models of ordinary and segmented TE elements.
Geometric models of (a) ordinary and (b) segmented TE elements; adapted from Shu et al.93
A typical TEM is made of the material bismuth telluride. A TEM has a (56 × 56 × 5) mm size and has 126 electrically linked TE couplings in series. Two p- and n-type semiconductor sectors linked by a conductive substance such as copper constitute a TE pair. As seen in Figure 8, the ceramic has a thermal isolation effect on the top and bottom sides to maintain the temperature difference. It was discovered that temperature-dependent TE material physical properties were more accurate than constant properties. Medium-temperature skutterudite (P1/N1, (Zn0.9975Ge0.0025)Sb3/Ba0.4In0.4Co4Sb12) and low-temperature bismuth telluride (P2/N2) semiconductor materials are arranged in two-layer arrangements in the structure of a segmented TE element, and their lengths are equal. When the temperature is lower than about 480 K, bismuth telluride performs noticeably better than the other material based on the Seebeck coefficients of both materials.
2.2.3 Low-temperature-condition materials
The heat difference between the two dissimilar junctions is proportional to the EMF produced through TEGs. The TEGs are solid-state devices and are made of p- and n-type junction TE materials. Suitable low-temperature-condition TE materials include bismuth telluride, bismuth telluride-based materials, CsBi4Te6, tin (Sn)-doped bismuth antimonide (BiSb), Bi85Sb14Pb3, AgSbTe2, AgSbSe2, copper (I) selenide, Cu2Se1−xIx, Cu3FeS4 and Cu7PSe6. Determining TE material behavior at low temperatures is very important for TEG manufacturing. The use of a shape-memory alloy as an effective mechanism for recovering heat at low-grade temperatures was demonstrated by Kumar et al.94 The researchers who conducted this study used the transformation and superelasticity properties of the TE materials. They built a thermal motor to make the most of waste energy by effectively utilizing the convection, conduction and radiation modes of heat transfer. The authors performed heat-transfer and material investigation to achieve high-frequency, efficient and sustainable operation of their engine. The engine generated 234 kW/m3 of electricity from an active specimen. Finally, the authors concluded that generating power could potentially reduce carbon emissions. In this way, this technology can slightly alleviate concerns about climate change around the world. Table 2 shows a summary of different TE materials under low-temperature conditions. The materials have been used in some applications and research.
Summary of different TE materials under low-temperature conditions
| Materials | Majority carriers | Temperature: K | ZT value | References |
|---|---|---|---|---|
| Bi85Sb14Pb1 | Holes | 190–210 | 0.11–0.12 | Chen et al.95,96 |
| Bi77.1Sb22.9 | Holes | 240 | 0.13 | Jin et al.97 |
| Bismuth telluride | — | 423 | 0.61 | Kim et al.98 |
| AgSbSe0.25Te1.75 | Holes | 520 | 0.65 | Wojciechowski and Schmidt99 |
| Bismuth telluride | Electrons | 300–475 | 0.68–1.16 | Guo et al.,100 Bao et al.,101 Akshay et al.,102,103 Yang et al.,104 Nour et al.,105 Saleemi et al.,106 Wu et al.107 |
| CsBi4Te6 | Holes | 225 | 0.82 | Chung et al.108 |
| (Bi2Te3)0.9(Bi1.9Cu0.1Se3)0.1 | Electrons | 417 | 0.98 | Cui et al.109 |
| Silicon | Holes | 200 | ∼1.00 | Boukai et al.110 |
| Bi2(Te,Se)3 | Electrons | 300 | 1.01 | Tan et al.111 |
| MgAgSb0.99In0.01 | Holes | 525 | ∼1.10 | Ying et al.112 |
| Bi2Te2.7Se0.3 | Electrons | 300–480 | 0.85–1.27 | Yan et al.,113 Hong et al.,114 Tan et al.,115 |
| (Bi2Te3)0.24(Sb2Te3)0.76 | Holes | 350 | 1.14 | Jiang et al.116 |
| Bi2Te2.5Se0.5 | Electrons | 300–463 | 1.18–1.28 | Xu et al.,117 Tan et al.118 |
| Bi2Te2.3Se0.69 | Electrons | 450 | ∼1.20 | Zhai et al.119 |
| Bi2Te2.79Se0.21 | Electrons | 357 | 1.20 | Hu et al.120 |
| Bi2Te2.88Se0.15 | Electrons | 298–423 | 1.18–1.22 | Kim et al.121 |
| Bi0.3Sb1.7Te3 | Holes | 380 | 1.30 | Hu et al.122 |
| (Bi2Te3)0.20(Sb2Te3)0.80 | Holes | 398 | 1.33 | Di et al.123 |
| BiSbTe bulk alloy | Holes | 300–373 | 1.20–1.40 | Poudel et al.124 |
| Bi0.4Sb1.6Te3 | Holes | 300–398 | 1.20–1.41 | Jimenez et al.,125 Chen et al.,126 Yeo and Oh127 |
| (Bi,Sb)2Te3 | Holes | 273 | 1.41 | Fan et al.128 |
| (Bi,Sb)2Te3 bulk composites | Holes | 440 | 1.47 | Cao et al.129 |
| Bi0.48Sb1.52Te3 | Holes | 390 | 1.50 | Xie et al.130 |
| Bi0.52Sb1.48Te3 | Holes | 300 | ∼1.56 | Xie et al.131 |
| Bi0.50Sb1.50Te3 | Holes | 300–323 | 1.56–1.80 | Jiang et al.,132 Lv et al.133 |
| PbSnSeTe | — | 300 | 2.00 | Harman et al.134 |
| Tellurium-embedded bismuth telluride | Electrons | 375 | 2.27 | Choi et al.135 |
| Iodine-doped copper (I) selenide | — | 400 | 2.30 | Liu et al.136 |
| Bismuth telluride/Sb2Te3 superlattice | Holes | 300 | ∼2.40 | Venkatasubramanian et al.137 |
Majority carriers: holes (p type) and electrons (n type)
An energy recovery process was carried out for ultra-low waste heat temperature by Sulaiman et al.138 The authors used a proton-exchange membrane (PEM) fuel cell as an energy source. The PEM system consists of a combined TEG, heat pipe and heat sink system. The authors analyzed various features of the system. They also did a steady-state analysis according to a mini-scenario determined by their system. Their system had a PEM fuel cell with a TEG-generated power of 2 kW. They also developed a thermal resistance network model. In this system, TEG cooling modes and the orientation of the TEG toward the heat flow were used as variables. The authors obtained the highest voltage and output power of 25.7 mV and 218 mW, respectively, at a 37°C waste heat temperature through normal flow orientation and forced convection cooling. The outcomes were extremely satisfactory. If this system is used in groups in a vehicle, there will be a very low level of waste heat output from the PEM fuel cell. This situation is very satisfactory in terms of the environment and energy cycle. In conclusion, the authors successfully validated the proposed model. The low-temperature-condition TE materials were utilized by Liu et al.139 to harvest low-enthalpy thermal work by using the TE effect. A 500 W low-temperature TEG application was carried out. The authors investigated the system properties under different conditions, such as various inlet temperatures and temperature differences between the hot and cold sides. Furthermore, they used different TE materials to manufacture the TEG modules. They designed their systems in such a way that they could obtain 500 W of electrical energy at a temperature difference of about 200°C. The system efficiency reached 4.5%. Finally, the authors argued that their developed system was more cost effective than PV and wind power systems. It is very important to characterize the properties of TEGs at low temperature values, because the efficiency of TEGs is very low. Therefore, knowing these values provides great benefits to users. Karabetoglu et al.140 characterized a bismuth telluride-based TEG at a low-temperature range of 100 to 375 K. They determined the characteristics of the TEG with the experimental set that they developed. Among the properties of TEG were the Seebeck coefficient and electrical conductivity. The authors observed that the critical mean operating temperature for their module was 250 K. Lastly, the authors determined that TEGs made from bismuth telluride-based TE materials performed quite well at low temperatures.
A study on the low-temperature waste-heat-harvesting process was carried out by Hsu et al.141 They constructed a system that converted waste heat released from the exhaust pipe of an automobile into electrical energy. They conducted both simulation and experimental works to observe the operation of TE materials. According to the simulation results, the authors designed a sloping block that could improve the performance of TEG modules. The system was characterized by the temperature difference with the open-circuit voltage and the maximum output power of this TEG.
2.2.4 High-temperature-condition materials
The efficiency and performance of TEGs depend on the TEG module and TE material characteristics. For a TEG manufacturer, TE materials are available, cheap, non-toxic and chemically stable and have sufficient ZT values. Besides, the performance of TE materials used in TEG production depends largely on the properties of these materials, such as the Seebeck coefficient and electrical and thermal conductivity. TEGs have been constructed by using bismuth telluride and bismuth telluride-based materials based on low-temperature TE materials. Moreover, they have been formed from skutterudite materials based on medium-temperature TE materials. The performance of the TE materials from which TEGs are made is low for low or medium temperature values. For this reason, researchers focus on high-temperature regions where higher performance is achieved for TE materials.
For constructing a TEG, magnesium telluride, lead telluride, zinc antimonide, magnesium silicide, SiGe, tin (II) selenide and doped TE materials are suitable for high-temperature conditions. By means of these TE materials, ternary or quaternary forms can be obtained. TEGs have a high temperature gradient, which provides significant opportunities for energy harvesting. Today, TEG studies done at high temperatures are progressing on the laboratory scale to perform passive heat transfer. It is very important to recover heat at high temperatures, because released waste heat produces a lot of greenhouse gases and has a lot of energy. Table 3 shows a summary of different TE materials under high-temperature conditions.
Summary of different TE materials under high-temperature conditions
| Materials | Majority carriers | Temperature: K | ZT value | References |
|---|---|---|---|---|
| Bi2O1.96Te.04Se | Electrons | ∼773 | ∼0.69 | Pan et al.142 |
| In0.10Co4Sb12 | — | 573 | ∼0.79 | Wang et al.143 |
| Lead telluride–silicon | Electrons | 675 | 0.90 | Sootsman et al.144 |
| Si80Ge20 | H or E | 1073 | 0.95–1.10 | Joshi et al.,145 Usenko et al.146 |
| Yb0.19Co4Sb12 | Electrons | 600 | ∼1.00 | Nolas et al.147 |
| BiCuSeO | Holes | 923 | ∼1.10 | Li et al.148 |
| In0.25Co4Sb12 | Electrons | 573 | ∼1.20 | He et al.149 |
| Pb9.6Sb0.2Te3Se7 | Electrons | ∼650 | 1.20 | Poudeu et al.150 |
| Na0.48Co4Sb12 | — | 850 | 1.25 | Pei et al.151 |
| Yb0.20Co4Sb12.3 | Electrons | 800 | 1.26 | Li et al.152 |
| NaPb18BiTe20 | Holes | 670 | ∼1.30 | Guéguen et al.153 |
| SiGe | Electrons | 1173 | 1.30 | Wang et al.154 |
| Lead telluride–lead (Pb)–sulfur (S) | Electrons | 750 | 1.40 | Girard et al.155 |
| Lead telluride–lead–antimony | Electrons | 650–700 | 1.40 | Sootsman et al.156 |
| ZrCoBi0.65Sb0.15Sn0.20 | Holes | 973 | ∼1.42 | Zhu et al.157 |
| In4Se2.78 | Electrons | 705 | 1.48 | Rhyee et al.158 |
| FeNbSb | Holes | 1200 | ∼1.50 | Fu et al.159 |
| Tin telluride | Holes | 873 | 1.50 | Li et al.160 |
| Sb2Te2Se | Electrons | 1000 | 1.50 | Xu et al.161 |
| Ag0.8Pb22.5SbTe20 | Electrons | 700 | 1.50 | Zhou et al.162 |
| In4Se2.67Cl0.03 | Electrons | ∼700 | 1.53 | Rhyee et al.163 |
| Sb2Si2Te6/Si2Te3 | Holes | 823 | 1.08–1.65 | Luo et al.,164 Huang and Zhao165 |
| Lead selenide–tin (II) selenide | — | 873 | ∼1.70 | Tang et al.166 |
| Lead telluride–strontium telluride (SrTe) | Holes | ∼800 | 1.70 | Biswas et al.167 |
| Ba0.08La0.05Yb0.04Co4Sb12 | — | 850 | 1.70 | Shi et al.168 |
| Sn0.978Ag0.007S0.25Se0.75 | Holes | 823 | ∼1.75 | Cai et al.169 |
| Ge0.93Bi0.07Te | Holes | 773 | ∼2.00 | Wu et al.170 |
| Na-doped (PbTe)0.86(PbSe)0.07(PbS)0.07 | Holes | 823 | 2.00 | Korkosz et al.171 |
| Pb0.94Na0.02Sr0.04Te | H or E | 800 | ∼2.10 | Kim et al.172 |
| Sn0.97Ge0.03Se | Holes | 873 | ∼2.10 | Chandra and Biswas173 |
| Ge0.93In0.01Bi0.06Te | Holes | 723 | ∼2.10 | Perumal et al.174 |
| Potassium (K)-doped PbTe0.7S0.3 | Holes | 923 | ∼2.20 | Wu et al.175 |
| Magnesium telluride-doped PbTe0.8Se0.2 | Holes | 820 | ∼2.20 | Fu et al.176 |
| Pb0.98Na0.02Te–SrTe | Holes | 923 | 2.50 | Tan et al.177 |
| Tin (II) selenide | Holes | 850–923 | 1.00–2.60 | Wei et al.,178 Zhao et al.179 |
Majority carriers: holes (p type), electrons (n type) and H or E (p type or n type)
Considering that electrical energy generated from TEGs increases in direct proportion to the temperature difference between the surfaces of TEG, using TEGs at these high temperatures will allow significant energy recovery.
Industrial high-temperature waste heat recovery using a heat pipe TEG was carried out by Wang et al.180 The authors operated a heat pipe and TEG system at high temperatures to meet the electrical energy needs of the devices. The authors made use of a potassium heat pipe in their system. The pipe was a skutterudite TEG. Thermal management achieved this structure of the pipe. Thus, the passive heat transfer of the system with a TEG at high temperatures was easily investigated. About 630°C was achieved in the heat pipe when their system was operated. In this case, the effective thermal conductivity increased to 35 831 W/(m K). As a result, they proposed that the TEG could be used to evaluate the effective thermal conductivity and Seebeck coefficient for system-level performance by simulation. Finally, the authors concluded that effective thermal conductivity rises with increasing operating temperature in the heat pipe.
One of the applications with high temperatures is aircraft propulsion systems. The devices used in these systems are required to be reliable, lightweight and easy to write. Conventional wired sensing processes used in aircraft systems are sometimes not sufficient. For these circumstances, Zaghari et al.181 developed a high-temperature self-powered sensing process for a smart bearing in an aircraft jet engine. The authors created a wireless process that could integrate self-power-bear condition monitoring with a TEG at high temperatures. The obtained results demonstrated that the TEG could ensure adequate power for a wireless sensing process in the engine environment. Finally, the authors concluded that the power output in the jet engine could significantly improve with high-temperature gradients when compared with that of other TEGs within heat sinks. A stable thermal interface material with high-temperature resistance and thermal conductivity was developed as a factor that minimizes the total thermal resistance of the system of the temperature flow path operating under high-temperature situations (∼500°C) of a TEG module by Kim et al.182 The developed system increased the TE energy through a high-temperature heat inlet to strengthen thermal interface conduction. The authors produced a thermal interface material that could perform stable temperature conduction under high-temperature conditions in a combination of multidimensional filler compounds with a polyimide matrix. The developed thermal interface materials were also improved for power generation. After that, the thermal interface materials were compared with cases without a thermal interface material and with conventional graphite foil thermal interface material by the reduction of the interface thermal resistance. Their system confirmed the thermal cyclic testing for resistant and heat-conduction features of the developed thermal interface material. Finally, the authors concluded that the developed thermal interface materials clearly show a potentially significant influence on the performance of TEGs.
The ability of TE materials to convert heat into electricity can increase fuel efficiency and provide a reliable alternative energy source in many applications by collecting wasted heat and, thus, aid in the search for new energy sources. Superior TE materials must be sought through diverse tactics to build high-performance TE devices. High-performance TE device development has the potential to increase interest in TE material research by expanding the market for TE applications. It focuses on the most important novel approaches to producing TE materials with good performance in temperature gradients as well as their applications. While nanostructure engineering and defect engineering can significantly lower thermal conductivity as it approaches the amorphous limit, manipulating the carrier concentration and band structures of materials is beneficial in improving electrical transport characteristics. TE devices are currently used to generate electricity in remote missions, solar–thermal systems, implantable or wearable devices, the automotive sector and many other fields. They are also used as temperature sensors, controllers and even gas sensors. The trend for the future is to optimize and integrate synergistically all the efficient variables to increase TE performance further, making highly efficient TE materials and gadgets more useful in daily life. The development of high-ZT TE materials is still a top focus in the TE area to match theoretical projections and lower manufacturing costs, making TE technology more competitive as a promising alternative energy source with a wide range of real-world applications. Due to the expanding market for alternative energy sources, achieving a higher ZT and using TE materials are mutually inefficient. Bismuth telluride/skutterudite segmented modules have recently attained a high efficiency of about 12%, which is highly encouraging but is still not competitive with those of conventional energy sources. For their practical applications to increase, more focus is needed on creating highly efficient TE materials and rationally designed TE devices. Due to their durability and dependability, radioisotope TEGs will continue to offer significant power support for remote missions but with a higher specific power or a smaller size and a lighter weight. To improve fuel efficiency and lower carbon emissions, TEGs will become more significant in the automotive sector. They will take part in temperature conditioning and exhaust waste heat recovery. Besides, it is urgently needed for low-cost, large-scale TEGs to be used widely in industrial waste heat recovery, solar PV/TEG power plants, geothermal energy generation, green building solar heat generation and air-conditioning or to perform admirably as temperature sensors or controllers in a variety of settings, including aerospace missions, meteorological devices, medical devices, precise instruments and machinery and food or wine storage, such as wine cellars. Researchers should concentrate on TE materials made of low-toxicity, earth-abundant elements to realize the industrial application of TE devices. For instance, because tin telluride and poisonous lead telluride have comparable application temperatures, tin telluride can be thought of as a substitute. When creating high-performance TE materials for industrial applications, the mechanical strength, processability, durability and stability of materials must also be adequately improved.
3 Limitations and challenges
TEGs have less efficient performance compared with other energy conversion technologies. The phenomena mean that a minimum amount of heat is converted to electrical energy for significant thermal energy input to the generator. TEGs are a promising way to generate electrical energy directly by using the waste-heat-recovery-management process. There is a significant amount of heat waste from power cycles that can be developed by using TEGs. The goal of this utilization is to convert this waste heat into high-performance electrical energy while minimizing fuel costs. The efficiencies of TEGs are very limited due to their thermal and electrical behavior being dependent on each other. On the other hand, recent TE nanostructure material technology has made them more efficient in some selective applications. TEGs that are applied to hot exhaust manifolds in automobiles would help reduce fuel consumption. This technology would help support low carbon emissions. When the above are evaluated together, it is clear that TE material efficiencies depend on ZT values and operating temperature conditions. It is one of the biggest challenges that the factors affecting the ZT value are interconnected. These increase the electrical conductivity while decreasing the thermal conductivity. The TEG technology would require a significant ZT value that could be competitive by using a nanostructured single-crystal TE semiconductor material. The entire TE community has placed a lot of emphasis on the quest for a high ZT, in particular, a high peak ZT, using various phonon engineering and electron engineering approaches. However, since it will not produce high efficiency, a high peak ZT within a constrained temperature range is not sufficient for use in real applications. Additionally, a high output power density is equally significant as efficiency when the heat source capacity is enormous, such as solar heat, or the cost of the heat source is not a major consideration, such as waste heat from automobiles and the steel industry. Researchers must either boost the power factor or shorten TEG legs to attain higher power densities for given heat source and temperature boundaries. On the other hand, shortening the legs can have serious negative effects, such as an increase in heat flux that will raise the cost of heat management at the cold end, an increase in parasitic loss that will reduce output power generation and an increase in thermal shear stress that will cause device failure. Therefore, the optimal course of action is to raise the power factor of the TE material before optimizing the related physical design parameters. Researchers can use the power factor as a criterion for looking for novel TE materials with high output power because it is a pure material property.
The mutual dependencies between the electrical and thermal conductivity are accompanied by charge carriers. The thermal and electrical conductivity of solid materials can be detected by both the lattice and electronic contributions. There are numerous other factors to be considered to increase the power efficiency of TEGs as well.
Advanced TEG technology research has developed some TE materials that are suitable for temperature conditions. It is essential that an ideal TE semiconductor material have a wide temperature gradient with significant ZT values. The carrier and mechanical behavior of these materials also must be significant. The development of TEG devices can use doped bimetallic junction TE semiconductor nanostructure materials. The approaches of these materials should fulfill the following criteria: high-quality, cost-effective, scalable and tunable TE characteristics compacted for machining or device-controllable behavior; compacted nanoscale features; enhanced ZT value over those of the bulk materials; and high thermal stability for an extended period. Researchers can advance the search for this type of material for the modern development of TEGs. TEGs are known for the following remarkable requirements: direct energy conversion, low thermal conductivity, high electrical conductivity, no moving parts with high reliability, noiseless operation and free maintenance, power generation in a wide range, compact size with the embedded existing set-up, long life span with no scale effect, no working fluids inside TEG and high TE efficiency. A good contact material for one material may not be the best option for another due to the complexity and diversity of TE material systems. Since each application may have various needs and surroundings, each one needs a unique analysis and design. Bismuth telluride modules are the best example. The operating temperatures for cooling and power-generation applications differ. As a result, contacts need to be prepared differently. Last but not least, evaluating system stability takes a lot of time because it involves repeated temperature cycling and aging experiments.
Continued research in this field to gain a more quantitative understanding is necessary to allow the rational design and preparation of technology.183–185 This research field will optimize nanostructured TE semiconductor materials and accelerate the wide adoption of TE technologies in power-generation and cooling systems. Recently, the use in TEG devices of organic and hybrid (organic/inorganic) TE materials that are compatible with low-cost scalable printing and coating processes has attracted interest. Higher-performance TE materials can be achieved by understanding material characterization and design procedures. Finally, researchers should take into account at least three conditions for TEGs to be a competitive technology for power-generating applications: efficiency, effectiveness and reliability.
4 Conclusion and future research
It is difficult for researchers to manage a ZT value of 3 or above. This needs impressive development in material compositions with temperature conditions. It also depends on thermal and electrical conductivity. Nanotechnology seems to play an important role in increasing the ZT of TE materials. Technologically, the characteristics of the best-performing TE materials depend strongly on the material of nanostructure form and its device construction and growth approach technique. If the materials are manufactured in nanostructure form, the ZT value will increase, and then material efficiency could be improved. Until now, researchers have continued their significant research on TE nanostructure materials with narrow energy gaps, heavy material doping, point defect loading and different temperature gradients. This review paper highlights the present technologies of TEGs in a broad range of end-user sectors to worldwide markets. One of the issues encountered in relation to conversion efficiency is the technology of TEGs. The paper recommends systematic and intensive research work on the development of nanostructure TE materials with the new composition Bi2−xSbxTeySe3−y (0.10 ≤ x ≤ 1.90; 0.10 ≤ y ≤ 2.90) for understanding the material in TE applications. There are now more thermoelements in TEGs as a result of researchers’ diligent efforts to improve conversion efficiency by utilizing new technologies. Researchers will try to design TEMs and thermoelement legs with lower-internal-resistance materials. Finally, it is believed that recent successes will fulfill the future advancement of TEG technology, which could lead to exciting next-generation thermoelectricity demand. Further research is going to be recommended to study other significant technologies, which function at high efficiency with temperature gradients for large-scale production. TE materials are converted into nanostructure single crystal form with a significant temperature gradient to achieve the best manufacturing purposes of TEGs. Providing TEGs with higher electrical energy with the requirement of utilizing new material compositions with high efficiency is necessary for further research into this technology.
Acknowledgement
Author Mohammad Ruhul Amin Bhuiyan is grateful to the Islamic University, Kushtia, Bangladesh, for providing support for this research.
