Beside Head of LIMM, Pham Kim Ngoc, the investigative team of this project is from the Research Group of Center for INOMAR, VNU-HCM and University of Science, VNU-Hanoi.
1. Introduction to Thermoelectricity
Renewable energy sources and energy efficiency-enhancing technologies are expected to play a crucial role in the future energy revolution. However, several challenges need to be addressed, including achieving reasonable efficiency, ensuring that materials meet sustainability standards such as high abundance, non-toxicity, and long-term stability to align with current environmental criteria. Thermoelectric materials and devices are promising solutions for advancing sustainable energy systems. An estimation diagram of energy generation, consumption, and waste in the USA for the year 2018 can provide valuable insights into the current energy landscape and the potential impact of thermoelectric materials on improving energy efficiency.
An estimation diagram of energy generation, consumption, and waste in the USA for the year 2018 can provide valuable insights into the current energy landscape and the potential impact of thermoelectric materials on improving energy efficiency.
Thermoelectric materials offer a unique capability to convert waste heat into electrical energy, thereby enhancing the efficiency of energy systems and contributing to sustainability. These materials are evaluated based on their dimensionless figure of merit (zT), which determines their efficiency in converting thermal to electrical energy. The ideal thermoelectric material should have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Metal oxides, due to their stability, abundance, and non-toxicity, are among the promising candidates for thermoelectric applications.
Thermoelectricity involves the direct conversion of thermal energy into electrical energy through the thermoelectric effect. A temperature gradient across a solid (except superconducting materials) generates an electrical voltage between the hot and cold ends. This phenomenon, discovered in 1821 and named the Seebeck effect, has extensively been utilized in thermocouples for temperature measurement. While the voltage (thermoelectromotive force,TMF) generated by metals is generally less than 50 μV/K, semiconductors can generate the TMF of several hundreds of μV/K. Since n- and p-type semiconductors generate the TMF of the opposite signs, and thereby doubles the voltage when combined, a semiconductor element (unicouple) as shown in Figure. This process is utilized in thermoelectric modules, where n-type and p-type materials are electrically connected in series and thermally in parallel. These thermoelectric modules are assembled into thermoelectric generators, which have demonstrated high reliability historically. For instance, the thermoelectric generators in the Voyager I and II space missions used radioactive decay as an energy source for over 30 years. This technology for direct conversion of heat to electricity is called thermoelectric conversion or thermoelectric power generation.
2. Thermoelectric Materials
Conventional thermoelectric materials developed until 1990s having been selected by these guiding principles are intermetallic compounds and alloys with covalent bonding characters (for higher mobility), and consist of heavy elements (for lower κph) such as Bi, Te, Pb. Representative materials are Bi2Te3 for room temperature up to 200°C, PbTe for 400600°C, and SiGe alloys for 4001000°C. However, these materials are incapable of wide commercialization because of their shortcomings such as poor durability at high temperature in air, low abundance and high cost of the comprising elements, and high toxicity.
Significant advancements have been made in improving the performance of thermoelectric materials. Techniques such as nanostructuring and doping are used to enhance their properties. Metal oxides, with their inherent advantages, are being extensively researched and have shown promising improvements in their thermoelectric performance.
Metal Oxides
Metal oxides are promising thermoelectric materials due to their high thermal stability and relative robustness. They are non-toxic and stable over a wide temperature range, making them interesting candidates for thermoelectric applications. However, for widespread use, their thermoelectric efficiency needs to be improved.
Notable oxide thermoelectric materials include CaMnO3-based perovskites, Al-doped ZnO, layered cobalt oxides such as NaCo2O4 and Ca3Co4O9, and SrTiO3-related phases. These materials have seen significant advancements in thermoelectric performance over the past two decades.
3. Applications
One of the most important applications of thermoelectric technology is in waste heat recovery. Heat can be supplied by combustion processes, biomass, solar energy, or industrial waste heat, making applications as a component in waste heat recovery systems viable.
Space Applications
As previously mentioned, thermoelectric generators have been used in space missions such as Voyager I and II, where they provide power from radioactive decay for over 30 years. This demonstrates the durability and reliability of thermoelectric technology in harsh applications.
4. Thermoelectric Equations and Calculations
The efficiency and performance of thermoelectric materials and devices can be understood through several key equations and parameters. The primary figure of merit for thermoelectric materials is the dimensionless figure of merit, zT, defined as:
where:
The Seebeck coefficient, SSS, is a measure of the voltage generated per unit temperature difference across the material. It is given by:
Where ΔV is the voltage difference and ΔT is the temperature difference.
This parameter indicates the electrical power output per unit temperature gradient and material volume.
where:
5. Thermal Conductivity and Electrical Conductivity
Thermal conductivity, κ\kappaκ, has contributions from both the lattice (phonons) and the electronic part:
where κlat is the lattice thermal conductivity and κe is the electronic thermal conductivity. The Wiedemann-Franz law relates the electronic thermal conductivity to the electrical conductivity:
where L is the Lorenz number, typically 2.45×10−8 WΩK−2
where:
6. Challenges and Opportunities
Despite their great potential, thermoelectric technologies face several challenges, such as the need for inexpensive, non-toxic materials and cost-effective preparation techniques. Metal oxides, due to their abundance and stability, could be a viable solution to these challenges, especially when fabrication techniques like spark plasma sintering can be applied to prepare large quantities of materials from simple precursors represent a promising field of research and application in converting waste heat into useful electrical energy. This opens new opportunities for sustainable development and energy efficiency improvements.
Journal Articles
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• S is the Seebeck coefficient (V/K),
• σ is the electrical conductivity (S/m),
• κ is the thermal conductivity (W/(m·K)),
• T is the absolute temperature (K).
The power factor, an important parameter for evaluating thermoelectric performance, is defined as:
The efficiency of a thermoelectric generator, η, can be expressed as:
• Th is the hot-side temperature,
• Tc is the cold-side temperature,
• ZTm is the average dimensionless figure of merit of the material over the temperature range.
The electrical conductivity, σ\sigmaσ, can be described by the relation:
• n is the carrier concentration,
• q is the charge of the carriers,
• μ is the mobility of the carriers.
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