A comprehensive review of thermoelectric technology: Materials, applications, modelling and performance improvement
Introduction
The drastic changes in climate have driven the need for increasing research into alternative sources of energy. Those rapid changes in climate are mainly attributed to the use of fossil fuels for transport and energy generation. Due to climatic challenges, several countries around the world have pledged to reduce primary energy consumption through an increase of efficiency in production, distribution and end-use, limit carbon dioxide emissions and increase the utilization of renewable energy sources [1]. The rapid development of power electronics technologies has enabled the realization of high energy-efficient systems such as electric vehicles [2]. The U.S. Energy Administration in 2011 estimated that almost two-thirds of total demand for petroleum is from the transportation sector. With an assumption that daily production of petroleum holds steady at 63.5 million barrels, global oil reserves are conventionally predicted to last approximately fifty years [3]. In the French industry, 75% of the final energy is used for thermal purposes such as furnaces, reactors, boilers and dryers. However, around 30% of this heat is assumed to be wasted in form of discharged hot exhaust gas, cooling water and heated product [4]. Therefore, the recovery and utilization of the waste heat is believed to contribute some amount of energy to the energy needs of the society.
Research and development have been promoted on thermoelectric (TE) modules which convert heat energy directly into electrical energy. TE devices are semiconductor devices that have the ability to either generate a voltage when exposed to a temperature gradient, exploiting the Seebeck effect, or produce a temperature gradient when supplied by electricity, exploiting the Peltier effect [5]. A number of currently available and applicable low-grade waste heat recovery methods include plant/district/water heating, direct power generation (TE and piezoelectric), absorption cooling, indirect power generation (steam and organic Rankine cycle), desalination/clean water and biomass co-location [6]. TE technology is seen as one of the most promising direct power generation technique used to recover waste heat energy because of the direct conversion from thermal energy to electrical energy, unlike the organic Rankine cycle, believed to have been discovered more than 150 years ago [7]. Heat energy can be harvested or recovered using two direct electricity generation strategies: thermoelectricity and pyroelectricity. Whereas thermoelectricity is the generation of electricity using thermoelectric harvesting systems which exploit the Seebeck effect for conversion of heat energy i.e., generation of electricity due to difference in temperature of two dissimilar conductors or semiconductors connected together at two junctions, pyroelectricity exploits specific materials whose structures are modified when heat is applied on them and in turn the polarization of the material is changed, thus creating electric potential [8].
TE modules offer low cost electricity, and green energy technology without the use of moving parts or production of environmentally deleterious wastes [9]. However, the optimal performance of TE modules depends on several factors like material selection and operation strategy. A study has used an irreversible model to analyse the performance of a thermoelectric generator (TEG) with external and internal irreversibilities, providing some significant instructions for their optimal operation including the information about optimal device-design parameters, efficiency, maximum power output, etc. [10]. Most of the research by 2005 had concentrated on electronics cooling applications especially on the variation of the geometric features such as shapes, sizes, and orientations to the flow in heat transfer systems but in later years, research on TE technology has gained a lot of momentum [11].
Although energy-related GHG emissions from fossil fuel combustion account for 70% of total emissions come from heat supply and electricity generation, a relatively large portion is generated from the transportation industry [12]. Although electric vehicles (EV) powered by renewable energies are seen as a potential solution to curtail GHG from transport industry [13], TE technology is also continuously introduced in low carbon vehicles to extract waste energy from various points of the vehicle. For a typical gasoline-engine vehicle, about 40% of the fuel energy is discharged from the exhaust pipe; about 30% is lost into the cooling system, clutch, gearbox and differential [14]. Hence, energy-harvesting systems which recover this heat energy to convert it into electricity with the use of TEGs are being studied and introduced [15], [16].
There are challenges surrounding TE technology which call for more research to address them. The assembly of TEG devices is still particularly challenging, and consequently these devices have been applied only in niche applications [17]. With regard to TE materials, there is a problem of wide lattice thermal conductivity whereby nanostructuring has been considered as an effective approach to enhance the TE performance of materials by reducing their lattice thermal conductivity [18]. The introduction of large concentrations of lattice vacancies in silicon nano-films is noted for creating more than a 20-fold reduction in thermal conductivity, while Seebeck coefficient and electrical conductivity are largely maintained [19]. With such challenge in mind, the impact of the substrate on the efficiency of thin film thermoelectric technology is assessed [20]. It is found that as the thickness ratio between substrate and thin film increases, the parameter ZT sharply tends to zero; this effect causes a serious problem to overcome by the thin film TE technology, more especially at nanoscale. Frequency dependency of ZT is another challenge especially in cylindrical thermoelectric nano devices. A study indicates that regarding the relative values of the particles' mean-free path and the characteristic size of the system, the performances of the thermoelectric devices are reduced in those situations [21]. However, it is found that non-local effects may be used as an aiding tool to have less marked reductions in those performances.
With TE Cooling (TEC) devices, cooling hot-spots with high heat flux is becoming one of the most important technical challenges in integrated circuit industry, calling for more aggressive thermal solutions, than those required for uniform heating, to mitigate this problem [22]. One efficient method of hotspot thermal management is to use super lattice TECs which can provide on demand and localized cooling. A detailed 3-D thermal model of a stacked electronic package is developed to investigate the efficacy of TECs in hotspot cooling for 3-D technology where thermal contact resistances between dies inside the TEC module, and between TEC and heat spreader are shown to have a crucial effect on the TEC performance [23]. Regarding TEG devices, product development requires solving coupled challenges in materials development and systems engineering to enable new thermoelectric materials and material performance improvements for development of thermoelectric generators for numerous applications [24].
The objective of this work is to carry out a comprehensive review of thermoelectric technology encompassing the materials, applications, modelling techniques and performance improvement. A brief background of the TEG devices and materials has been covered first followed by the applications of these technologies. The rest of the paper is organised as follow: Section 2 deals with different TE materials while Section 3 describes the application of TE devices. TE modelling techniques are discussed in Section 4 whereas various structures of TEG systems are clarified in Section 5. 6 Methods of efficiency improvement of TEG system, 7 Description of the reviews on thermoelectric materials, parameters and applications elaborate the methods of efficiency improvement in TEG and the description of the reviews on TE technology, respectively. The paper ends with a concluding remark.
TE modules can be used for either cooling or power generation, as shown in Fig. 1. A TEG is a solid-state device that can convert heat directly into electrical energy when a temperature difference is placed across it [25]. The TE module consists of arrays of N and P type semiconductors in which, by applying a heat source on one side and a cooler heat sink to the other side, electric power is produced and vice versa. Electric power can be converted to cooling or heating by reversing the current direction [26].
While choosing TEGs for application in varying conditions, it is necessary to select an appropriate semiconductor with acceptable performance in the temperature range of that condition [27].
The figure of merit (Z) is a parameter generally used to gauge the performance of a TE material:where is the Seebeck coefficient of n-type or p-type material. is the electrical conductivity of the material in p-type or n-type in Siemens per meter whereas is the thermal conductivity [27].
In general, for obtaining maximum efficiency, the important characteristic for thermoelectric material is the dimensionless measurement thermoelectric performance figure of merit ZT [26].where S, σ, T and are the Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity, respectively. In order to get high thermoelectric efficiency, the figure of merit should be large.
In a thermoelectric material there are free electrons or holes which carry both charge and heat. The electric potential (Voltage) produced by a temperature difference is known as the Seebeck effect and the proportionality constant is called the Seebeck coefficient. If the free charges are positive (the material is p-type), positive charge will build up on the cold end which will have a positive potential. Similarly, negative free charges (n-type material) will produce a negative potential at the cold end [28]. Fig. 2 illustrates the movement of electrons and holes in p- and n-type materials respectively due to Seebeck effect.
The features of heat transfer are calculated using the mechanism of the Seebeck effect as shown in Fig. 3. The compressed air heated by a heater with flow rate FH, heat transfer coefficient h1 and at inlet temperature Ti flows through the heat pipe to supply heat to the high/hot temperature side of the TE module. Similarly, the jet air with flow rate FC, heat transfer coefficient h2 and at temperature Tj provides cooling to create the cold side of the module. The waste heat energy is transferred to the heat absorber with temperature Ti,s by heat convection while the internal heat sink absorbs some of the waste heat energy, and the thermal grease with thermal conductivity ktg guides the heat to the hot side of TEG with TTEG,H temperature. The heat collection and heat dissipation distinctly at the hot and cold sides of TEG create the temperature difference between the both sides of the module to generate electricity due to Seebeck effect. Simultaneously, the internal Joule heating happens in the TEG due to the internal electric resistance Rint. More details of the heat transfer mechanism are found in [30].
The derivation of heat conduction equation is based on the internal energy generation of the TE module [30]:where x is any thickness of the TEG material, is heat energy generated per unit volume and KTEG is the thermal conductivity of the TEG.
Assuming the low and high temperature sides are taken to be same, the boundary conditions are expressed as T (0)=Ti, s at x=0 and T (δ)=To, s at x=δ.
The total generated power can be calculated as follows:
Putting the Seebeck effect into consideration, the thermal power QH and QC from the high and low temperature sides can respectively be computed as:where δ is the thickness of the TEG and I is the current. In this case, ATEG has two meanings; first as the area of the heat sink for the high-temperature side, and as the area of the TE module for the low temperature side [30].
The maximum electric power generated from the TEG is expressed as [31]:where λ is the biot number of the TEG, is the heat transfer ratio of the heat absorber to the heat sink and is the Seebeck parameter of the TEG.
In another formwhere Em is the electromotive force (EMF) generated by Seebeck effect. Pmax occurs when the RL equals to TEG module internal resistance Rint.
For the physical properties of TE materials, some previous studies assumed that they are temperature-independent, however, it has been demonstrated experimentally [28] that the physical properties of the TE materials are temperature dependent. This dependence can be very significant and strongly affect the TEG performance [32]. The physical properties of Bi2Te3 are calculated as:
The heat transfer coefficient is also expressed as:where AB is the heat sink base area.
The parameters that affect the performance of TE devices can be physical or nonphysical. The most important optimization parameters for TEG include TE element leg length, leg area ratio between n- and p-type legs, fill fraction, load resistance and module spacing. The effect of various leg geometries on thermo-mechanical and power generation performance of thermoelectric devices are investigated by modelling leg geometries and analysed using finite-element method [33]. Thermal stresses in the legs, power outputs, temperature distributions and conversion efficiencies are evaluated and the results show that significant differences in sizes and distributions of the thermal stresses in the legs occur due to varying leg geometries. The different output parameters for TEG device are identified as power generation, thermal efficiency, current, thermal and electric fields, etc. [34].
It is also noted that the influences of the effective Seebeck coefficient and carrier density variations on the performance of TE system are generally neglected, resulting in an overestimation of the power generator performance under strong-ionization temperature region [35]. Therefore, researchers suggested and proved the use of finite element tool to solve such complexity, whereby the characteristics of TEG and their relationship to its performance can be well analysed. Additionally, the behaviour of power generation from TEG exposed to a transient heat source on the hot side and natural convection on the cold side is studied [36]. It is stressed that modelling TEGs under these conditions is complicated compared to TE coolers because of the non-linearities and the unknown electric currents in a closed-loop circuit. Therefore a transient TE model which comprises Seebeck, Thomson, Peltier, and Joule effects using finite-difference techniques is suggested and the incorporation of Thomson effect plays a significant role in precisely predicting the generated power. Fateh et al. used a finite difference model to investigate the interdependencies among these optimization parameters for TE elements included with an exhaust gas heat exchanger [37]. Different researchers look at the optimum parameters differently depending on the method used in the parameter analysis. The influence of geometric features on TEG device characteristics is done using multi-objective evolutionary algorithms [38]. The parameters assessing geometric features include pin length size and shape factor while operating parameters include external load and temperature ratio parameter. The influences of the thermal conductivity ratio, the external load parameter, the conductance ratio and the figure of Merit on the output power, the efficiency, and the entropy generation rate are predicted for various device parameters [39]. A technical solution for recycling blast furnace slag flashing water heat based on TEG is also presented [40]. The effects of key parameters like the TE element length, slag washing water temperature, the packing factor of the TE module and heat exchanger flow passage length on the performance of the TEG device are examined.
While studying the optimum design of TE devices, dimensionless groups are used to properly define and represent important parameters of the TE device [41]. According to this study, the optimum design comprises the optimum parameters such as efficiency, current, power, geometry or number of thermocouples, and thermal resistances of heat sinks. The geometry effects on thermoelectric properties of silicon nanowires based on electronic band structures are also investigated where it is found that thermoelectric properties significantly depend on nanowire geometry such as different sizes, shapes, and orientations [42]. As nanowire size decreases, TE properties of nanowires can be enhanced and as a result, triangular nanowires with side length of 1 nm have the best results of ZT and it can be improved to 1.5 for n-type and 0.85 for p-type nanowire. The study on TE Properties of Ge Nanowires (Ge NWs) which is based on electronic band structures also reveals that thermoelectric properties vary significantly depending on the band structure of the Ge NWs of different sizes, cross-sectional shapes, and orientations [43]. Moreover, the thermodynamics and thermal stress analysis of TEG is also done to assess the influence of pin geometry on device performance [44]. With finite element method, it is established that thermal efficiency increases for certain geometric configuration of TEG device and the maximum thermal stress in the pin decreases slightly representing improved life expectancy of TEG.
The power density (power generated per unit surface area) of TEG strongly depends on the TEG module spacing. The optimization of the TEG module spacing and its spreader thickness, for TEG modules attached to a rectangular chimney plate for venting hot flue gases is also done [45]. Therefore, finding the optimal module spacing for the TEG system used in waste heat recovery is crucial to ensure optimum power generation from the system. The predicted numerical data obtained using finite difference method and a simplified conjugate-gradient method has demonstrated a good power vs. current curve. It is important to note that heat transfer in the TEG is one of the performance measures that need to be investigated before using TEG. Some methods like the longitudinal vortex generators have been used to improve heat transfer performance as well as power output and thermal conversion efficiency [46].
Section snippets
Different materials used for TEG/TEC
The material used in the construction of a TEG or TEC plays an important role in controlling the performance of these devices. There are many features that describe the performance of these materials to make them suitable for use in TEG device manufacture. In this section, various TEG/TEC materials will be discussed and the selection criteria of these materials for use in thermoelectric power generation as well as for cooling are clarified.
Application of thermoelectric devices
Thermoelectric device applications are concerned with environmentally-friendly refrigeration and power generation in industrial utilities, transportation tools, military devices, space applications and medical services [59]. TEGs have also found application in biomass gasifiers to further improve the recovery of heat [60]. TE devices are either used for cooling purposes or for power generation from either low heat recovery applications or as stand-alone heat to electric energy conversion
Thermoelectric modelling methods
Thermoelectric effect is the direct conversion of temperature difference into electric voltage and vice versa [82]. The term "thermoelectric effect" embraces three separate effects: the Seebeck, Peltier, and Thomson. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not. Joule heating occurs when an electric current flows through a resistance or a conductor and it induces temperature gradient in microfluidic device [83]. If it occurs in graphene
Different structures of TEG unit/module
In this section, we look at the TEG structure itself and how different modifications have been made in order to improve the performance. The modifications are done by altering different TEG components, for example by stacking materials of different properties to form a segmented TEG device. First, the basic structure of TEG is highlighted followed a detailed discussion of the progress on structural improvements.
Methods of efficiency improvement of TEG system
The efficiency of TE device can be improved through various methods including proper matching of internal resistance of the TE device with external load, adjusting the boundary temperature and the number of TEG units, use of dc to dc converters etc. The performance and efficiency of TE conversion system can also be improved by minimizing the heat losses to the ambient medium. A three-dimensional numerical model of TEs with consideration of coupling of temperature field and electric potential
Description of the reviews on thermoelectric materials, parameters and applications
In this section, different reviews of TE technology found in literature are discussed. This is aimed at helping the readers to find the necessary information that could not be found in this piece of work.
Fairbanks discussed the history and progress of thermoelectric applications in vehicles, from the discovery of Seebeck effect by Thomas Johann Seebeck in 1821 to the modern improvement of thermoelectric technology in the 21st century [49]. He described the progress in thermoelectric materials
Critical observations and recommendations for further research on TE technology
In this section some observation are highlighted from the present work with regard to the current status of various aspects of TE technology including improvements and challenges. Consequently, the areas for future research on the subject area are suggested accordingly.
On the TE materials, a tremendous improvement on ZT has been registered in recent years with ZT of more than 7 reported. This is justified by the increased application of TE devices for heating and cooling which requires high ZT.
Conclusion
With near exhaustion of fossil fuel resources coupled with desire to reduce global warming, it has become increasingly necessary to exploit alternative and renewable energy sources to diversify the energy supply. Thermoelectric (TE) devices are regarded as alternative and environmental friendly technology for harvesting and recovering heat which is directly converted into electrical energy. In this work, a comprehensive review of thermoelectric technology encompassing the materials,
Acknowledgement
The authors acknowledge the scholarship from Islamic Development Bank (IDB) and University of Nottingham.
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