High-temperature Thermoelectric Generation Research Articles

Investigation of a novel multigeneration system driven by an ammonia-methane Brayton cycle with partial production and utilization of ammonia, methane, and hydrogen: Energy and carbon footprint assessments

In this paper, to address gaps in the development of low-carbon energy systems, an innovative concept of an ammonia-methane Brayton cycle is proposed. This system is interconnected with various subsystems, including the Kalina cycle, high-temperature steam electrolysis, thermoelectric generators, and ammonia and methane synthesis reactors. The primary aim of this system was to reduce dependence on external resources by establishing local sources for water and fuel, thus reducing the environmental impact and enhancing energy sustainability. A thermodynamic model of an energy system was developed via EES codes which leads to a thoughtful perception into the interactions of the proposed system parameters. Also, comprehensive investigations involved the examination of different energy system scenarios. Modeling outputs of the proposed multigeneration system discovered excellent thermal efficiency and power production, with figures of 56.5 % and 17 MW, respectively, while the Brayton system showed the results of 23.7 % and 3.8 MW. This study also highlighted the environmental benefits of the multigeneration system by demonstrating a carbon dioxide emission of 0.058 kg/kWh, representing its significant positive impact on environmental sustainability. Also, the produced ammonia and methane within the system was 0.37 % and 9 %, of fuel in combustion chamber, respectively. Also, the rest of demand fuel was provided from the external resources.

Numerical Investigation of a Novel Heat Exchanger in a High-Temperature Thermoelectric Generator

A cylindrical thermoelectric power generator for high-temperature flue gas was designed, and a distributor was installed to enhance heat transfer by affecting the jet on the hot side. The influence of the different distributor diameters and jet hole diameters on the temperature distribution of the hot and cold sides of the thermoelectric module was studied. The corresponding temperature field, velocity field, and exhaust pressure drop of the device were also obtained. The results indicated that the temperature difference between the hot and cold ends of the thermoelectric module was increased, and the uniformity of the temperature distribution was improved with the increasing diameter of the distributor and the decreasing diameter of the jet hole. The performance of the thermoelectric power generator was further improved by the jet hole with a gradient diameter. The number and distance between jet holes were sensitive to pressure drop.

Evolution of Thermoelectric Generators: From Application to Hybridization.

Considerable thermal energy is emitted into the environment from human activities and equipment operation in the course of daily production. Accordingly, the use of thermoelectric generators (TEGs) can attract wide interest, and it shows high potential in reducing energy waste and increasing energy recovery rates. Notably, TEGs have aroused rising attention and been significantly boosted over the past few years, as the energy crisis has worsened. The reason for their progress is that thermoelectric generators can be easily attached to the surface of a heat source, converting heat energy directly into electricity in a stable and continuous manner. In this review, applications in wearable devices, and everyday life are reviewed according to the type of structure of TEGs. Meanwhile, the latest progress of TEGs' hybridization with triboelectric nanogenerator (TENG), piezoelectric nanogenerator (PENG), and photovoltaic effect is introduced. Moreover, prospects and suggestions for subsequent research work are proposed. This review suggests that hybridization of energy harvesting, and flexible high-temperature thermoelectric generators are the future trends.

Convection-effect-enhanced micro thermoelectric module directly heated by catalytic combustion

A novel micro thermoelectric power generation module directly heated with localized catalytic combustion has been proposed. Arrays of high-temperature plate-type thermoelectric generator (TEG) elements, of which the hot junction is directly heated by exothermic catalytic oxidation of hydrocarbon fuel, are designed. Heat losses caused by heat conduction through the TEG element are recovered by the convection effect with unburned mixture flows. Catalyst layers were prepared by impregnating Pd or Pt catalyst on porous Al2O3 supports coated by using the suspension plasma spray. A series of one-dimensional numerical analyses considering the catalytic combustion in addition to the conductive and convective heat transfer were made to examine the temperature field of the TEG module and to investigate how various parameters affect the convection effect. A model combustor using quartz plates was developed, which mimics the temperature fields of the SiGe TEG module. It is found that, with the aid of the convection effect, temperature differences over 700 °C between the hot/cold junctions can be achieved, while the unreacted fuel concentration is less than 10%. Finally, 40 mm-long Ni-based alloy TEG elements have been fabricated using laser cutting and spot welding techniques, which ensure high operating temperatures and reliability. In a power generation demonstration using butane as a fuel, an open circuit voltage of 0.64 V has been obtained from a single TEG module with a peak output power and a power generation density as high as 10.1 mW and 10.8 kW/m3, respectively.

Electronic and phonon contributions to the Thermoelectric properties of newly discovered half-Heusler alloys XHfPb (X= Ni, Pd, and Pt)

In this work we calculate the thermoelectric figure of merit of XHfPb (X= Ni, Pd, and Pt) by computing the both the power factor and the lattice thermal conductivity by first principles. We make reasonable approximations: we use the Constant Relaxation Time Approximation (CRTA) to compute the electron transport contribution and the modified Debye–Callaway model to calculate the thermal lattice conductivity. We also report the dielectric properties of these semiconductors and the mode Grüneisen parameters. Not surprisingly we find that the average Grüneisen coefficient correlates with the thermal conductivity. Next, we consider a realistic relaxation time τ and carrier concentration n from experimental data on ZrHfPb and obtain the figure of merit ZT as a function of temperature. Our main finding is that despite the Pt is isoelectronic with Ni and Pd, the ZT of PtHfPb is larger and behaves differently from the other two materials, suggesting that PtHfPb is better suited for high temperature thermoelectric generators.

Potential of Recycled Silicon and Silicon-Based Thermoelectrics for Power Generation

Thermoelectrics can convert waste heat to electricity and vice versa. The energy conversion efficiency depends on materials figure of merit, zT, and Carnot efficiency. Due to the higher Carnot efficiency at a higher temperature gradient, high-temperature thermoelectrics are attractive for waste heat recycling. Among high-temperature thermoelectrics, silicon-based compounds are attractive due to the confluence of light weight, high abundance, and low cost. Adding to their attractiveness is the generally defect-tolerant nature of thermoelectrics. This makes them a suitable target application for recycled silicon waste from electronic (e-waste) and solar cell waste. In this review, we summarize the usage of high-temperature thermoelectric generators (TEGs) in applications such as commercial aviation and space voyages. Special emphasis is placed on silicon-based compounds, which include some recent works on recycled silicon and their thermoelectric properties. Besides materials design, device designing considerations to further maximize the energy conversion efficiencies are also discussed. The insights derived from this review can be used to guide sustainable recycling of e-waste into thermoelectrics for power harvesting.

Conformal High-Power-Density Half-Heusler Thermoelectric Modules: A Pathway toward Practical Power Generators.

Thermoelectric generators (TEGs) exploiting the Seebeck effect provide a promising solution for waste heat recovery. Among the large number of thermoelectric (TE) materials, half-Heusler (hH) alloys are leading candidates for medium- to high-temperature power generation applications. However, the fundamental challenge in this field has been inhomogeneous material properties at large wafer diameters, insufficient power output from the modules, and rigid form factors of TE modules. This has restricted the transition of TEGs in practical applications for over three decades. Here, we successfully demonstrate large diameter wafers with uniform TE properties, high-power conformal hH TE modules for high-temperature application, and their direct integration on flue gas platforms, such as cylindrical tubes, to form large area flexible TEGs. This new conformal architecture design provides a breakthrough toward medium-/high-temperature TEGs over the conventional BiTe- and polymer-based flexible TEG design. A variable fill factor and greater flexibility due to the conformal design result in higher device performance as compared to conventional rigid TEG devices. Modules with 72-couple hH legs exhibit a device high-power-density of 3.13 W cm-2 and a total output power of 56.6 W under a temperature difference of 570 °C. These results provide a promising pathway toward widespread utilization of thermoelectric technology into the waste heat recovery application and will have a significant impact on the development of practical thermal to electrical converters.

Li2CO3 as Protection for a High-Temperature Thermoelectric Generator: Thermal Stability and Corrosion Analysis

In most steelmaking processes, huge amounts of waste heat at high temperature (700–800 °C) are thrown into the environment without any use. An alternative use for this waste heat is electricity generation through thermoelectric generators. However, these high temperatures, as well as their fluctuations over time, affect not only the conversion rate of the thermoelectric generator but also its useful lifetime. The incorporation of a latent thermal energy storage (TES) system could be a solution; nevertheless, the thermal stability and corrosive effect of the (PCM) phase change material are key aspects for the thermal storage system definition, in terms of durability. In this work, developed in the framework of the European project “PowGETEG” (RFSR-CT-2015-00028, funded by the Research Fund for Coal and Steel), a high-temperature analysis (700–800 °C) of the Li2CO3 thermal properties, thermal stability and corrosive effect on the AISI 304 and AISI 310 stainless steels is carried out. The results show that the eutectic salt Li2CO3 exhibits high thermal stability with neither change in its thermal properties nor material degradation. This work shows that lithium carbonate Li2CO3 and AISI 310 make a very good combination for the definition of a thermal storage system able to protect a high-temperature thermoelectric converter from temperature variations, making it more reliable.

U-type unileg thermoelectric module: A novel structure for high-temperature application with long lifespan

Strong thermal stress caused by high temperature and difference of thermal expansion coefficient (CTE) will negatively influence the lifespan of the thermoelectric module. In this work, a new high-temperature CaMnO3-based U-type unileg thermoelectric module, combining a unileg structure with pn-junction, is proposed and investigated. The novel design avoids the device failure due to different CTEs and high temperature gradients. As a result, the maximal thermal stress (σmax,TEM) of 3.31 GPa and fatigue life of 41686 cycles are 46 % and 132 % of those of traditional modules at 6 W and 300 K, respectively. To further relieve stress concentration, the effect of rounded corners (ru, rl), Ag layer thickness (HAg) and length of right legs (LR), have been studied. It has been found that larger ru, and rl are suitable to relieve the local stress concentration, and the lowest σmax,TEM and highest power (Pmax) are achieved at (ru,rl)=(0.1,0) and (0,0.5). Moreover, larger LR and HA are beneficial for mechanical properties by decreasing the peak stress and dispersing the high thermal stress regions, while module performance is improved at lower LR and HAg. Results obtained from this U-type unileg thermoelectric module should influence and guide the design and optimization of high-temperature thermoelectric generators.

Comparative performance and thermoeconomic analyses of high temperature polymer electrolyte membrane based two hybrid systems

The main objective of this study is to compare the two systems in terms of the thermoeconomic and the performance. The first one is called hybrid I and consists of high temperature polymer electrolyte membrane and thermocapacitive cycle. The second one is named hybrid II, which is composed of high temperature polymer electrolyte membrane and thermoelectric generator. Thermocapacitive cycle and thermoelectric generator have various advantages, such as generally lower cost and higher power density. So, they have good potential to utilize waste heat. The performance parameters of the considered hybrid systems include power density, energy efficiency, exergy efficiency and exergy destruction rate. The results have shown that hybrid I is more advantageous than hybrid II. The maximum power density values for hybrid I and hybrid II are obtained to be 2536.91W and 2049.62W while their energy efficiencies are 77.4% and 76.8%, respectively.

Performance enhancement of a natural-gas-fired high-temperature thermoelectric generation system: Design, experiment and modelling optimization

High-temperature thermoelectric generation (TEG) system that driven by combustion is an efficient way for simultaneously providing heat and electric power, but its application prospect is restricted by very low ratio of electric power to heat supply. In this work, an annular high-temperature TEG system is extended by adding medium-temperature TEG modules, and correspondingly a novel two-stage annular multi-hole burner with baffle construction is specially designed to replace original single-stage annular multi-hole burner. The optimal geometric dimensions of the new burner are obtained by conducting a simplified 2D model in ANSYS. Experiment results show that, due to the new burner design, the inner-wall temperature of the high-temperature TEG module is averagely increased by 73 K, bringing an electric power rise of 14.8%; in comparison with the single-stage high-temperature TEG system, the total electric power generation of the high- and medium-temperature coupled TEG system is greatly improved by 73.8%. To further improve the electric performance of the high-temperature TEG module, a well-developed multi-physics field coupled model which containing 2D combustion reactions in the new burner and 1D multi-physical thermoelectric (TE) effects is established to reveal the optimal length and number of TE element, and to assess the optimization of cold-side heat sink.

High performance (ZT>1) n-type oxide thermoelectric composites from earth abundant materials

Oxide-based thermoelectric materials have the advantages of high-temperature stability, low toxicity and low processing cost comparted to other metal-based systems. However, achieving ZT > 1 remain elusive especially for n-type bulk oxide thermoelectrics. Here, we report the experimental demonstration of ZT > 1 in n-type oxides by synthesizing composites of Nb-doped SrTiO3 (STN) with natural graphite. Introduction of conductive graphite inclusions in STN matrix has led to a surge in electrical conductivity due to 21-times increase in weighted mobility (μw) of electrons resulting in high thermoelectric power factor ~5400 µWmK2, which is ~16-times higher than that of pure STN. Furthermore, we could restrain the increase in thermal conductivity by attaining enhanced Umklapp scattering along with phonon-glass-like temperature-independent phonon mean-free-path above Debye temperature. We have achieved the maximum ZT ~1.42 in STN+ 0.5 wt% G composite, which is 15-times enhancement compared to STN. Our proposed way of designing rare-earth-free composites with graphite can potentially open up the possibility of fabricating novel high-temperature thermoelectric generators.

Evaluation of Thermoelectric Properties of Doped β-Iron Disilicide Prepared by the Powder Metallurgy Technique

Thermoelectric generator (TEG) offers a potential application in harvesting waste-heat energy into usable electrical power. β-iron disilicide (β-FeSi2) thermoelectric is applicable in elevated temperature systems such as hydrogen plants, copper reverberatory furnace and copper refining furnace. In this work, high TE performance Mn doped p-type and Co doped n-type thermoelectric (TE) β-iron disilicide compacts were fabricated for TEG using powder metallurgy route. The effects of variation in type and quantity of dopants [p-type dopant: Mn: stoichiometry Fe1−xMnxSi2 (x = 0.04 − 0.12) or n-type dopant: Co: stoichiometry Fe1−yCoySi2 (y = 0.01 − 0.05)] on thermoelectric properties were studied. Significant improvement in ZT was observed compared to earlier research works for this material system. This result is a positive indication for the potential use of β-FeSi2 in the high temperature TEG. In this work, the working efficiency was also evaluated which was not mentioned in the earlier research works.

A Die Attach Design for Thermoelectric Generator Packages for Automotive Applications

The requirement to achieve high reliability for the die attach in thermoelectric generator (TEG) package design is key to their viability and safe operation in automotive applications. In this study, the reliability of die attach material Al 718 (88% Al, 12% Si) during high temperature operating conditions is discussed. A novel approach is outlined to determine the damage parameter for accurate reliability predictions using a scalable test vehicle fabrication, testing, and finite element analysis (FEA) simulations. In addition, a new TEG package design is proposed that can withstand extreme environmental conditions and exhibit enhanced mechanical reliability by minimizing the likelihood that sublimation and oxidation of the semiconductor dies occurs. The proposed design of TEGs is analyzed using finite element simulations. Prototype models were fabricated and tested to validate the computational predictions. Continuum damage mechanics was used to predict the fatigue life of TEG packages using computational modeling of cyclic damage evolution. The results demonstrated that the proposed die attach design can be commercially viable for high-temperature TEG applications.

Fuel-burning thermoelectric generators for the future of electric vehicles

Ragone plot showing power density (kW/kg) and energy density (kWh/kg) is widely used for batteries, fuel cells and other energy conversion technologies. This is particularly useful for transportation and mobile applications where the weight is a key limiting factor. Here, we present for the first time, Ragone plot for scalable fuel-burning thermoelectric power generators. Optimizing thermoelectric leg geometry, the highest power and energy densities nearly scale with the square root of the material figure-of-merit (ZT). Internal heat recovery can improve fuel economy by 350% (reaching an efficiency >20% for a temperature independent ZTavg ~0.5), however, there is a trade-off due to the additional mass of heat exchanger hence the reduced power density. Prospects for electrification of particularly compact and unmanned vehicles are discussed. We foresee the potential of using fuel to drive fully-electric vehicles with high temperature thermoelectric generators. Development of the technologies for high heat transfer, reliable and robust contacts, and high ZT over a wide temperature range presents opportunities in the future energy landscape for mobile applications.

Hybrid carbon thermal interface materials for thermoelectric generator devices

Thermal interface materials (TIMs) are extensively used in electronic devices as efficient heat transfer materials. We fabricated all-carbon TIMs by hybridizing single-wall carbon nanotubes (SWCNTs) with graphite and demonstrated their performance by applying them to a thermoelectric generator (TEG) device. The hybrid carbon TIM exhibited maximum thermal conductivity when the SWCNT content was near 10 wt%. The TIM thermal contact resistance measured by a home-made calorimeter setup was 2.19 × 10−4 m2K/W, which did not vary with temperature but decreased with applied pressure. Post-treatment of the TIM with a silane coupling agent further reduced the TIM thermal contact resistance by 30%. When the TIM was placed between a TEG device and a copper heat reservoir, the TEG output power increased with the temperature difference across the TEG and applied pressure. Moreover, the post-treatment of the TIM enhanced the output power of the TEG device by up to 18.5%. This work provides a simple and effective pathway towards a carbon-based TIM that can be applied to a high temperature TEG.

Energy, exergy, and economic analyses of an innovative energy storage system; liquid air energy storage (LAES) combined with high-temperature thermal energy storage (HTES)

Liquid air energy storage is one of the most recent technologies introduced for grid-scale energy storage. As the title implies, this technology offers energy storage through an air liquefaction process. High energy storage density, no geographical limitation, and applicability for large-scale uses are some of the advantages of this technology. To improve the performance and environmental friendliness of the conventional design of this technology, a novel liquid air energy system combined with high-temperature thermal energy storage, thermoelectric generator, and organic Rankine cycle is proposed in the present article. The thermal energy storage unit removes the need for the conventional combustion chamber and thereby reduces greenhouse gas emission. The organic Rankine cycle and the thermoelectric generator recover the generated heat of the system during the charging process to further improve the efficiency of the system. During peak demand periods, around 9.6 MW power and 2.5 kg/s domestic hot water could be generated at round trip energy and exergy efficiencies of 61.13% and 52.84%, improving by 6.59% and 3.28% in comparison to the stand-alone design of this technology. The results indicate that the reference system operates with an air storage energy density and an occupied space energy density of 839 and 104 MJ/m3. The economic analysis represents a payback period of 3.91 years and a net profit of about 18.6 $M during the useful lifetime of the system.

Engine exhaust manifold with thermoelectric generator unit

Thermal energy recovery systems are subject of intensive research activities in automotive engineering. They are becoming appealing because of the energy savings, low maintenance requirements and compact designs. Another element is the increase in working temperature ranges over the previous maximum of 400 °C. This was limiting the positioning of these devices to mid and rear locations of the exhaust systems. They also contained a specially designed cooling system for the cold side of the thermoelectric generator unit, which effectively preformed the requirements of the cold junction point (Seebeck effect) between the “P” and “N” semiconductors. Currently, thermoelectric generator manufacturers are producing devices that could operate at the temperatures over 850 °C which is opening up different research and application opportunities. This research report shows a continuation of previous research work carried out, to test 850 °C thermoelectric generator units, using the performance specifications and measurements conducted in our labs. We have extrapolated and predicted power output further. In the research, we covered the concept of placing high-temperature thermoelectric generator units directly into the exhaust port of the engine, just outside the cylinder head. Novel development solution is presented, where a new thermoelectric generator unit is designed to be adaptable to the profile of the exhaust port. Computational fluid dynamics analysis was conducted on a real-world engine and concept models, confirming consistent thermal heat transfer from the combustion chamber of the engine into the exhaust manifold thermoelectric generator unit assembly. Cooling of the device is achieved by direct coolant flow from the cylinder head. It could also be realized using an external cooling system.

The use of materials based on Si3N4 in the composition of composites for the design of high-temperature thermoelectric generators

The possibility of reducing the thermal conductivity of silicon nitride as a basis of high-temperature electrical converters was investigated in the thesis. Also, the values of thermoelectric figure of merit and efficiency of thermoelectric current generator for the case of refractory oxygen-free composites were simulated. During the study, the dependence between the m coefficient, which determines the maximum possible efficiency of the thermoelectric generator and the ZT thermoelectric figure of merit, was determined. It was shown that the coefficient of thermal conductivity of the studied materials ranges from 1,2 to 4·106 m2/s and is characterized by a negative temperature coefficient over the entire temperature range. It was found that the thermal conductivity of Si3N4-based materials varies from 2,1 to 5,1 W/(m·K) depending on the type of sintering activator. The use of Al2O3 as an activator makes it possible to obtain a 25% lower thermal conductivity value comparing to materials with the addition of MgO. For the first time, it was proved that currently it is not possible to achieve an efficiency of 0,5hT in Si3N4-based materials used as a composite basis for high-temperature thermoelectric generators development.

Development and multi-objective optimization of a newly proposed industrial heat recovery based cascaded hydrogen and ammonia synthesis system

The industrial flue gas emitted to the atmosphere is considered not only harmful to the environment but also a waste of plentiful resources of thermal energy. The thermal energy extracted from the industrial flue gas can be employed for multiple purposes. This study proposes a new configuration to integrate the thermal management of industrial flue gas for thermochemical copper-chlorine (Cu-Cl) cycle based ammonia synthesis. A reverse osmosis desalination unit is employed to supply the freshwater required by the thermochemical Cu-Cl cycle. To recover the heat from high-temperature oxygen stream, thermoelectric generators (TEGs) and organic Rankine cycle (ORC) are integrated with the proposed configuration to utilize the low-grade waste heat for power production. A portion of produced hydrogen through the thermochemical Cu-Cl cycle is supplied to the cascaded system for ammonia production. A double-stage cascaded ammonia synthesis system is integrated with the proposed configuration to achieve high fractional conversion. A multi-objective optimization using genetic algorithm is implemented to the proposed system using the MATLAB to investigate and determine the best-operating temperatures and pressures for the ammonia synthesis system. The proposed configuration produces 518.4 kmol/day of hydrogen and 226.8 kmol/day of ammonia. The overall exergetic and energetic efficiencies are found to be 28.7% and 40.8%. Moreover, the results obtained from the comprehensive sensitivity analyses are presented and discussed.