🔋Thermoelectric Materials and Devices Unit 13 – Thermoelectric Energy Harvesting Applications

Key Concepts and Principles

• Thermoelectric energy harvesting converts waste heat into usable electrical energy based on the Seebeck effect
• Seebeck effect generates a voltage when a temperature gradient is applied across a thermoelectric material
• Peltier effect is the reverse process where applying a voltage creates a temperature difference used for cooling applications
• Figure of merit ($ZT$) quantifies the efficiency of a thermoelectric material higher $ZT$ values indicate better performance
• Thermoelectric materials require high electrical conductivity, low thermal conductivity, and a large Seebeck coefficient to maximize efficiency
  • Balancing these properties is challenging as they are often interdependent
• Carnot efficiency sets the upper limit for thermoelectric energy conversion determined by the hot and cold side temperatures
• Thermoelectric generators (TEGs) consist of multiple thermoelectric couples connected electrically in series and thermally in parallel

Thermoelectric Materials Overview

• Ideal thermoelectric materials exhibit a combination of high electrical conductivity, low thermal conductivity, and a large Seebeck coefficient
• Common thermoelectric materials include bismuth telluride ($Bi_2Te_3$), lead telluride ($PbTe$), and silicon germanium ($SiGe$) alloys
  • $Bi_2Te_3$ is widely used for near-room-temperature applications (refrigeration, waste heat recovery)
  • $PbTe$ and $SiGe$ are suitable for mid to high-temperature applications (automotive, industrial waste heat recovery)
• Nanostructuring techniques (quantum dots, superlattices) can enhance thermoelectric properties by reducing thermal conductivity through phonon scattering
• Doping with impurities or alloying can optimize carrier concentration and improve electrical conductivity
• Organic and polymer-based thermoelectric materials offer advantages such as flexibility, low cost, and ease of processing
  • Examples include PEDOT:PSS and carbon nanotube composites
• Skutterudites, clathrates, and half-Heusler alloys are promising high-temperature thermoelectric materials with complex crystal structures

Device Structure and Components

• Thermoelectric devices consist of multiple thermoelectric couples connected electrically in series and thermally in parallel
• Each thermoelectric couple comprises a p-type and n-type semiconductor leg connected by metallic interconnects
• P-type legs have positive Seebeck coefficients and conduct holes, while n-type legs have negative Seebeck coefficients and conduct electrons
• Ceramic substrates provide electrical insulation and structural support for the thermoelectric legs
• Thermal interfaces (heat exchangers, heat spreaders) facilitate efficient heat transfer between the thermoelectric device and the heat source/sink
• Electrical contacts and interconnects are made of highly conductive materials (copper, gold) to minimize electrical losses
• Protective coatings and encapsulation ensure device stability and prevent oxidation or degradation of thermoelectric materials

Energy Conversion Mechanisms

• Thermoelectric energy conversion relies on the Seebeck effect, where a temperature gradient induces a voltage across a thermoelectric material
• Charge carriers (electrons in n-type, holes in p-type) diffuse from the hot side to the cold side, creating a net current flow
• The generated voltage is proportional to the temperature difference and the Seebeck coefficient of the material ($V = \alpha \Delta T$)
• In a thermoelectric generator, the temperature gradient drives the current through an external load, producing useful electrical power
• The Peltier effect, the reverse of the Seebeck effect, is used for thermoelectric cooling and temperature control applications
  • Applying a current causes heat absorption at one junction and heat release at the other, creating a temperature difference
• Thomson effect describes the reversible heat absorption or release in a single conductor with a temperature gradient and an electric current
• Joule heating, an irreversible effect, occurs due to electrical resistance and reduces the overall efficiency of thermoelectric devices

Performance Metrics and Efficiency

• Efficiency of a thermoelectric device depends on the material properties, temperature gradient, and device geometry
• Figure of merit ($ZT$) is a dimensionless quantity that characterizes the performance of a thermoelectric material
  • $ZT = (\alpha^2 \sigma / \kappa) T$, where $\alpha$ is the Seebeck coefficient, $\sigma$ is electrical conductivity, $\kappa$ is thermal conductivity, and $T$ is the absolute temperature
• Higher $ZT$ values indicate better thermoelectric performance, with values above 1 considered suitable for practical applications
• Power factor ($\alpha^2 \sigma$) is another metric that focuses on the electrical properties of a thermoelectric material
• Thermoelectric conversion efficiency ($\eta$) is the ratio of the electrical power output to the heat input
  • $\eta = \frac{T_H - T_C}{T_H} \frac{\sqrt{1 + ZT} - 1}{\sqrt{1 + ZT} + \frac{T_C}{T_H}}$, where $T_H$ and $T_C$ are the hot and cold side temperatures, respectively
• Carnot efficiency sets the upper limit for thermoelectric conversion efficiency, which is lower than the ideal Carnot limit due to irreversible losses
• Thermal and electrical contact resistances, as well as parasitic heat losses, can significantly reduce the overall device efficiency

Applications and Use Cases

• Waste heat recovery is a primary application of thermoelectric energy harvesting, converting unused heat into useful electricity
  • Examples include industrial processes, automotive exhaust systems, and power plants
• Space applications utilize radioisotope thermoelectric generators (RTGs) for long-duration power supply in remote locations
  • RTGs convert heat from radioactive decay into electricity and have been used in various space missions (Voyager, Cassini)
• Portable and wearable electronics can benefit from thermoelectric energy harvesting, using body heat to power devices
  • Thermoelectric generators can be integrated into clothing, watches, or other accessories
• Thermoelectric cooling modules (Peltier coolers) are used for temperature control in applications such as electronics cooling, medical devices, and scientific instruments
  • Advantages include compact size, no moving parts, and precise temperature control
• Thermoelectric air conditioning and refrigeration systems offer an alternative to traditional vapor-compression systems, with potential for higher efficiency and reduced environmental impact
• Remote sensing and wireless sensor networks can utilize thermoelectric energy harvesting to power sensors in hard-to-reach or inaccessible locations

Challenges and Limitations

• Low conversion efficiency of thermoelectric materials compared to other energy conversion technologies (solar, mechanical)
  • Current high-performance materials have $ZT$ values around 1-2, limiting practical efficiency to 5-10%
• Scarcity and high cost of some thermoelectric materials (tellurium, germanium) hinder widespread adoption
  • Developing cost-effective and abundant alternative materials is an ongoing challenge
• Thermal management and heat transfer issues can limit the performance of thermoelectric devices
  • Efficient heat exchangers and thermal interfaces are crucial for maintaining the required temperature gradient
• Electrical contact resistance and compatibility between thermoelectric materials and electrodes can cause performance degradation over time
• Mechanical stresses due to thermal expansion mismatch can lead to device failure, especially in high-temperature applications
• Optimizing thermoelectric materials often involves trade-offs between conflicting properties (electrical conductivity, thermal conductivity, Seebeck coefficient)
  • Nanostructuring and complex material design are needed to decouple these properties

Future Trends and Research Directions

• Nanostructured and complex thermoelectric materials with enhanced $ZT$ values through phonon scattering and energy filtering
  • Examples include quantum dots, superlattices, and hierarchical structures
• Exploration of new material systems and compositions, such as organic semiconductors, metal-organic frameworks (MOFs), and hybrid materials
• Development of high-temperature thermoelectric materials for waste heat recovery in industrial processes and power generation
  • Focus on materials with high thermal stability and mechanical robustness
• Flexible and printable thermoelectric devices for wearable and conformable applications
  • Investigating novel fabrication techniques like 3D printing and roll-to-roll processing
• Integration of thermoelectric devices with other energy harvesting technologies (photovoltaics, piezoelectrics) for enhanced performance and functionality
• Computational materials design and machine learning approaches to accelerate the discovery and optimization of new thermoelectric materials
• Addressing scalability, manufacturing, and system-level integration challenges for widespread deployment of thermoelectric energy harvesting technologies