1.4 Basic principles of energy conversion in thermoelectric systems

Thermoelectric devices convert heat to electricity or vice versa. They use temperature differences to generate power or electrical input to create cooling. These devices rely on special semiconductors that move heat and electricity in useful ways.

Energy conversion in thermoelectric systems follows key principles. The Seebeck and Peltier effects drive power generation and cooling. Efficiency depends on material properties and operating conditions, with the ZT figure of merit indicating performance.

Thermoelectric Devices

Fundamental Components and Types

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• Thermoelectric generator (TEG) converts thermal energy directly into electrical energy using temperature differences
• Thermoelectric cooler (TEC) uses electrical energy to create a temperature difference, enabling cooling or heating
• Thermoelectric module consists of multiple thermoelectric couples connected electrically in series and thermally in parallel
• N-type and P-type semiconductors form the basic building blocks of thermoelectric devices
  • N-type semiconductors have excess electrons as charge carriers
  • P-type semiconductors have excess holes as charge carriers
  • Pairing N-type and P-type materials creates a thermoelectric couple

Operational Principles and Applications

• TEGs harness the Seebeck effect to generate electricity from waste heat (industrial processes, automotive exhaust)
• TECs utilize the Peltier effect for precise temperature control (portable coolers, electronics cooling)
• Thermoelectric modules can operate in both power generation and cooling modes depending on the applied voltage or temperature gradient
• Semiconductor doping alters electrical properties
  • Enhances charge carrier concentration
  • Optimizes thermoelectric performance (ZT figure of merit)

Energy Conversion Principles

Thermodynamic Foundations

• Carnot efficiency represents the theoretical maximum efficiency of a heat engine operating between two temperatures
  • Calculated as $η_c = 1 - T_c/T_h$, where $T_c$ is the cold reservoir temperature and $T_h$ is the hot reservoir temperature
  • Sets an upper limit for thermoelectric device efficiency
• Heat flux describes the rate of heat transfer per unit area
  • Governed by Fourier's law: $q = -k∇T$, where $k$ is thermal conductivity and $∇T$ is the temperature gradient
  • Crucial for understanding thermal transport in thermoelectric materials

Electrical and Thermal Interactions

• Electrical current flows through the thermoelectric material when a temperature gradient is applied (Seebeck effect)
  • Current density given by $J = σ(-∇V - S∇T)$, where $σ$ is electrical conductivity, $∇V$ is voltage gradient, and $S$ is Seebeck coefficient
• Energy conversion efficiency quantifies the ratio of useful output energy to input energy
  • For TEGs: $η = \frac{P_{out}}{Q_{in}}$, where $P_{out}$ is electrical power output and $Q_{in}$ is heat input
  • For TECs: Coefficient of Performance (COP) = $\frac{Q_c}{P_{in}}$, where $Q_c$ is heat removed and $P_{in}$ is electrical power input
• Thermoelectric figure of merit ZT determines device performance
  • $ZT = \frac{S^2σT}{k}$, where $S$ is Seebeck coefficient, $σ$ is electrical conductivity, $T$ is absolute temperature, and $k$ is thermal conductivity
  • Higher ZT values indicate better thermoelectric performance (current commercial materials achieve ZT ≈ 1)