Electronics Guide

The Seebeck Effect

The Seebeck effect generates an electromotive force when a temperature gradient exists across a conductor or semiconductor. The voltage produced is proportional to the temperature difference, with the proportionality constant called the Seebeck coefficient, measured in microvolts per kelvin. Different materials have different Seebeck coefficients, which can be positive or negative depending on whether holes or electrons are the majority charge carriers.

Thermoelectric generators use pairs of p-type (positive Seebeck coefficient) and n-type (negative Seebeck coefficient) semiconductor elements connected electrically in series and thermally in parallel. This configuration doubles the effective Seebeck coefficient compared to a single material and allows efficient collection of generated current. Multiple such pairs are typically connected to achieve useful voltage and power levels from modest temperature differences.

Figure of Merit

The efficiency of thermoelectric materials is characterized by the dimensionless figure of merit ZT, which equals the square of the Seebeck coefficient times the absolute temperature, divided by the product of electrical resistivity and thermal conductivity. High ZT requires large Seebeck coefficient, low electrical resistance to minimize Joule heating losses, and low thermal conductivity to maintain temperature gradient across the material.

These requirements conflict because electrical and thermal conductivity typically correlate in materials. The best thermoelectric materials balance these properties, often through nanostructuring that scatters phonons more effectively than electrons, reducing thermal conductivity while preserving electrical conductivity. Commercial thermoelectric materials achieve ZT values around 1, while laboratory materials have reached 2 or higher at specific temperatures.

Carnot Efficiency Limits

Thermoelectric generators operate as heat engines, subject to the Carnot efficiency limit determined by the hot and cold side temperatures. For a temperature difference of 50 K with a cold side at room temperature, the Carnot efficiency is approximately 15%. Actual thermoelectric efficiency is further reduced by material properties captured in the ZT value, with practical efficiencies typically 5-10% of Carnot efficiency for current commercial materials.

While thermoelectric efficiency is lower than many other heat-to-electricity conversion methods, thermoelectric devices offer advantages in simplicity, reliability, and scalability. With no moving parts, they can operate for decades without maintenance. They scale effectively from milliwatts to kilowatts and can be integrated into compact form factors unsuitable for mechanical heat engines. For many low-power harvesting applications, the convenience and reliability outweigh efficiency limitations.

Operating Characteristics

Thermoelectric generators produce DC output with voltage proportional to temperature difference and current limited by internal resistance. Maximum power transfer occurs when load resistance matches internal resistance, at approximately half the open-circuit voltage. The relatively high internal resistance of thermoelectric modules, compared to other power sources, requires careful matching with power conditioning circuits.

Power output depends on the square of the temperature difference for a given module, making even modest increases in gradient highly beneficial. Maintaining the temperature gradient against thermal conduction through the module and achieving good thermal contact at the hot and cold interfaces are critical for performance. Thermal interface materials and heat sink design often limit practical system performance more than the thermoelectric material itself.

Bismuth Telluride Alloys

Bismuth telluride (Bi2Te3) and its alloys with antimony and selenium are the dominant thermoelectric materials for near-room-temperature applications. These materials achieve ZT values approaching 1 at temperatures from 200-400 K, making them well-suited for environmental temperature gradient harvesting. Both n-type and p-type compositions are available, enabling complete thermoelectric couples from this material family.

Commercial thermoelectric modules typically use bismuth telluride alloys due to their performance at ambient temperatures and established manufacturing processes. The materials are relatively expensive due to tellurium scarcity, motivating research into alternative materials. Despite this cost, bismuth telluride remains the standard for low-temperature thermoelectric harvesting applications.

Lead Telluride and Skutterudites

Lead telluride (PbTe) and skutterudite compounds provide good thermoelectric performance at intermediate temperatures of 500-900 K. These materials are used for waste heat recovery from automotive exhaust, industrial processes, and other mid-temperature sources. Skutterudites, with the formula CoSb3 and related compositions, can be filled with rare earth atoms to reduce thermal conductivity while maintaining electrical properties.

The higher operating temperatures of these materials enable higher Carnot efficiency but require thermal management to protect other system components. Applications include automotive waste heat recovery, where exhaust temperatures naturally fall in the optimal range for these materials, and industrial process monitoring in elevated temperature environments.

Silicon-Germanium Alloys

Silicon-germanium (SiGe) alloys provide thermoelectric performance at high temperatures above 900 K. These materials have been used extensively in radioisotope thermoelectric generators (RTGs) that power deep space missions, where their stability at high temperatures and radiation resistance are essential. For terrestrial applications, SiGe finds use in high-temperature industrial waste heat recovery.

The compatibility of silicon-based thermoelectrics with standard semiconductor processing offers potential advantages for integrated harvesting devices. Research on nanostructured silicon thermoelectrics aims to improve performance while leveraging established silicon manufacturing infrastructure. Low-dimensional silicon structures can achieve significantly higher ZT than bulk silicon through phonon scattering at boundaries.

Emerging Materials

Research continues on new thermoelectric materials with improved performance. Half-Heusler compounds offer tunable properties through compositional variation. Organic thermoelectrics based on conducting polymers provide flexibility and potentially low cost, though current performance lags inorganic materials. Hybrid organic-inorganic composites attempt to combine the processability of organics with the performance of inorganic thermoelectrics.

Nanostructuring approaches including superlattices, nanowires, and quantum dots can enhance ZT by reducing thermal conductivity more than electrical conductivity. These structures exploit quantum confinement effects and interface scattering to decouple normally correlated transport properties. While manufacturing challenges remain, nanostructured materials offer paths to ZT values exceeding 2 that would significantly expand thermoelectric harvesting applications.

Heat Source Coupling

Effective heat transfer from the heat source to the thermoelectric module is essential for maintaining temperature gradient and power output. Thermal interface materials fill microscopic gaps between surfaces that would otherwise impede heat flow. Options include thermal greases, phase-change materials, thermal pads, and metallic foils, each with trade-offs in thermal conductivity, ease of application, and long-term stability.

Surface preparation significantly affects interface thermal resistance. Flat, smooth surfaces minimize the gap that interface materials must fill. For irregular surfaces, conformable interface materials or intermediate heat spreaders may be needed. In high-temperature applications, thermal interface stability over time and under thermal cycling must be considered.

Heat Sink Design

The cold side of a thermoelectric generator requires a heat sink to dissipate absorbed heat and maintain temperature difference. Natural convection heat sinks provide passive cooling suitable for low-power applications, while forced convection with fans enables higher power density but requires additional power consumption. Heat pipe and vapor chamber technologies provide efficient heat spreading for uneven heat loads.

Heat sink design involves trade-offs between thermal resistance, size, weight, and cost. Finned heat sinks increase surface area for convective cooling. The optimal fin geometry depends on the cooling medium and whether flow is natural or forced. For applications where the cold side contacts a thermal reservoir such as soil or water, direct contact may provide superior cooling without additional heat sink components.

Thermal Concentration

Low thermal gradients in the environment can be enhanced through thermal concentration using heat-collecting surfaces, thermal diodes, and passive concentration structures. Large-area heat collectors on the hot side increase total heat input for a given temperature difference. Selective surfaces that absorb solar radiation while minimizing thermal emission can create elevated temperatures from sunlight.

Thermal diodes, which conduct heat preferentially in one direction, can prevent back-flow of heat during periods when the gradient reverses. This enables harvesting from fluctuating gradients while protecting stored energy from dissipation. Phase-change thermal storage can also buffer temperature variations, providing more consistent hot-side temperature despite environmental fluctuations.

System-Level Optimization

Maximum power extraction requires matching between thermal and electrical system characteristics. The thermal resistance from source through module to sink must allow adequate heat flow to maintain temperature gradient while supplying energy for conversion. Too little thermal resistance reduces temperature difference; too much limits heat flow and power output. Optimal thermal design depends on source and sink temperatures and the thermoelectric module properties.

Electrical load matching through maximum power point tracking adapts to varying thermal conditions. As temperatures change, both open-circuit voltage and optimal load resistance shift. Active MPPT circuits track these changes, maintaining near-optimal power extraction across varying conditions. For stable thermal environments, fixed-ratio DC-DC converters may suffice at lower cost and complexity.

Building-Environment Gradients

Buildings maintain interior temperatures different from outdoor conditions, creating harvestable gradients at the building envelope. Wall, window, and roof surfaces exhibit temperature differences between inside and outside that vary with weather, season, and HVAC operation. These gradients can power building automation sensors at locations where wiring would be difficult or expensive.

The magnitude of building thermal gradients depends on insulation quality, HVAC setpoints, and outdoor weather. Well-insulated buildings maintain larger interior-exterior differences but have less heat flow available for harvesting. Thermal bridges where heat flow concentrates offer higher power density at the cost of reduced overall building efficiency. System design must balance harvesting potential against building energy performance.

Geothermal Gradients

Shallow geothermal gradients exist between the Earth's surface, which varies with air temperature and solar heating, and the subsurface, which maintains relatively stable temperature year-round. This gradient reverses seasonally in many climates: the surface is warmer than subsurface in summer and cooler in winter. Thermoelectric harvesters can generate power from both polarities with appropriate power conditioning.

Deeper geothermal gradients from the Earth's internal heat provide more stable temperature differences independent of surface conditions. The geothermal gradient averages approximately 25-30 degrees Celsius per kilometer depth in typical continental crust, though it varies significantly with local geology. Deep borehole installations can access larger gradients but require substantial infrastructure investment.

Water Body Gradients

Lakes and oceans exhibit temperature stratification, with surface temperatures varying with weather while deeper water remains stable. This stratification creates gradients that can be harvested by thermoelectric devices positioned across the thermocline. Tropical oceans with stable surface warming and cold deep water offer the largest and most consistent gradients, forming the basis for ocean thermal energy conversion.

Smaller water bodies including ponds, rivers, and even pipes carrying water at different temperatures create local gradients for small-scale harvesting. Water's high heat capacity and convective heat transfer properties enable efficient thermal coupling to thermoelectric devices. Submerged or floating harvesters can access water body gradients for powering aquatic sensors and monitoring systems.

Body Heat Harvesting

The human body maintains core temperature near 37 degrees Celsius, creating a temperature difference with ambient air that can power wearable devices. Body heat harvesters typically use thermoelectric generators positioned against the skin, with heat sinks exposed to air. The available gradient depends on ambient temperature, clothing, and body part, with exposed skin on extremities often providing the largest differences.

Wearable thermoelectric harvesters have demonstrated power levels of tens to hundreds of microwatts per square centimeter of body contact. This power can supplement battery-powered fitness trackers, medical monitors, and smartwatches. The continuous nature of body heat provides power whenever the device is worn, reducing charging frequency and extending operational duration.

Industrial Waste Heat

Industrial processes release enormous quantities of waste heat from furnaces, boilers, and chemical reactions. Much of this heat is at temperatures suitable for thermoelectric harvesting, providing power for process monitoring sensors without requiring additional electrical infrastructure. The harsh industrial environment demands robust thermoelectric systems resistant to temperature cycling, vibration, and contamination.

Hot pipes, ducts, and equipment surfaces represent distributed sources of waste heat throughout industrial facilities. Strap-on thermoelectric generators can be retrofitted to existing equipment, harvesting energy from surfaces that would otherwise simply radiate heat to surroundings. This approach enables incremental deployment without modifying process equipment or interrupting operations.

Automotive Applications

Internal combustion engines convert only about 30% of fuel energy to useful work, with the remainder lost as heat through exhaust and cooling systems. Thermoelectric generators on exhaust systems can recover a portion of this waste heat, improving overall vehicle efficiency. Temperature differences of 200-500 K between exhaust and ambient create significant harvesting potential.

Automotive thermoelectric systems face challenges including wide temperature variation, mechanical vibration, and limited space. The additional weight and cost must be justified by fuel savings over the vehicle lifetime. Development continues on integrated exhaust heat recovery systems that combine thermoelectric generation with improved thermal management for the powertrain and cabin.

Electronics Cooling

Electronic devices generate heat during operation that must be removed to prevent damage. The temperature difference between hot electronic components and cooling systems represents harvestable energy. Thermoelectric harvesters integrated into electronic cooling systems can power sensors, fans, or other auxiliary functions from the waste heat of the electronics themselves.

Data centers present particularly attractive opportunities due to the large amount of heat generated in concentrated areas. Server room cooling moves enormous amounts of thermal energy that could partially be recovered through thermoelectric harvesting. The power recovered could offset some cooling energy costs, improving overall data center efficiency.

Combined Heat and Power

Any combustion or heat-generating process can incorporate thermoelectric harvesting as part of a combined heat and power system. Solid fuel stoves with thermoelectric generators can power fans for improved combustion and heat distribution. Portable combustion heaters with thermoelectric outputs can charge devices in off-grid situations. The same fuel provides both heat for comfort and electricity for electronic needs.

Biomass and waste combustion systems can integrate thermoelectric harvesting to power monitoring and control systems without external power connections. This is particularly valuable for remote installations where grid power is unavailable. The self-powered monitoring enabled by waste heat harvesting improves system safety and performance in off-grid combustion applications.

Wireless Sensor Nodes

Thermoelectric harvesters enable truly autonomous wireless sensor nodes that operate indefinitely without battery replacement. Industrial sensors monitoring temperature, pressure, and vibration on hot equipment can harvest power from the same thermal conditions they monitor. This eliminates maintenance visits to hazardous or hard-to-access locations while providing continuous monitoring coverage.

Building automation sensors powered by envelope thermal gradients enable smart building functionality without wiring to every sensor location. Window and wall temperature sensors, air quality monitors, and occupancy detectors can all be powered by thermoelectric harvesting where sufficient gradients exist. The distributed nature of temperature gradients throughout buildings supports distributed sensor deployment.

Remote and Pipeline Monitoring

Oil and gas pipelines traverse remote areas where grid power and regular maintenance are impractical. Thermoelectric harvesting from the temperature difference between pipeline contents and ambient conditions can power monitoring systems that detect leaks, corrosion, and operational anomalies. The critical nature of pipeline safety justifies investment in reliable, autonomous monitoring.

Remote well sites, pump stations, and process equipment in the petroleum industry similarly benefit from self-powered monitoring. The waste heat from compressors, engines, and processing equipment provides ample thermal energy for harvesting. Wireless communication of monitored data enables remote oversight of distributed infrastructure without on-site visits.

Wearable Electronics

Wearable devices for fitness, health monitoring, and personal communication can supplement battery power with body heat harvesting. Thermoelectric generators integrated into watchbands, wristbands, or clothing patches generate continuous power while worn. This extends operational duration between charges and may eventually enable battery-free wearable operation for low-power devices.

Medical monitoring devices that must operate continuously for extended periods particularly benefit from body heat harvesting. Implanted devices can potentially draw power from body heat, though thermal gradients inside the body are smaller than at the skin surface. External medical monitors worn for days or weeks can reduce or eliminate battery concerns through thermoelectric harvesting.

Space Power Systems

Radioisotope thermoelectric generators have powered spacecraft since the 1960s, using the heat from radioactive decay to generate electricity through thermoelectric conversion. The reliability, long life, and independence from solar illumination make RTGs essential for deep space and planetary surface missions. The Voyager spacecraft, launched in 1977, continue to operate on RTG power after nearly five decades.

Future space missions may incorporate improved thermoelectric materials for higher conversion efficiency. Dynamic conversion systems using Stirling engines compete with thermoelectrics for space power applications, offering higher efficiency at the cost of moving parts. Thermoelectric systems remain attractive for missions prioritizing reliability over efficiency.

Material Performance Improvement

The efficiency of thermoelectric harvesting remains limited by material ZT values. Achieving ZT values of 2 or higher across broad temperature ranges would dramatically expand economically viable applications. Research on nanostructured materials, new compound families, and novel physical effects continues to push performance boundaries. The complex interactions between electrical and thermal transport properties make rational material design challenging.

Cost Reduction

Current thermoelectric materials, particularly tellurium-containing compounds, are expensive due to raw material costs and complex processing. Alternative materials using abundant elements could reduce costs significantly. Improved manufacturing processes including continuous production and reduced waste would also help. As costs decrease, thermoelectric harvesting becomes viable for more applications.

System Integration

Practical thermoelectric harvesting requires optimized integration of thermoelectric modules with thermal interfaces, heat sinks, and power conditioning. Off-the-shelf components often fail to fully exploit available thermal gradients. Application-specific integrated designs that co-optimize thermal and electrical systems can achieve significantly better performance than assemblies of general-purpose components.

Flexible and Printable Devices

Flexible thermoelectric generators that conform to curved surfaces would enable harvesting from a wider range of heat sources. Printable thermoelectric inks and additive manufacturing approaches could reduce fabrication costs while enabling customized geometries. Organic and hybrid thermoelectric materials offer inherent flexibility, though current performance lags rigid inorganic devices.