How Is Heat Transferred To Electrical Energy?

Thermoelectric generators are devices that can convert heat directly into electrical energy via the thermoelectric effect. The thermoelectric effect refers to phenomena where temperature differences create electric voltage and vice versa. Thermoelectric generators allow the capture of waste heat and its conversion into usable electricity. This can improve efficiency and provide power generation solutions for applications where solar, wind, hydroelectric, or other renewable sources are unavailable. Given that vast amounts of excess heat are produced worldwide for which there is no recovery, thermoelectric generators offer an important pathway to improve energy sustainability through waste heat recycling.

This article will provide an in-depth look at how thermoelectric generators work and the science behind them. We’ll explore thermoelectric materials, module construction, efficiency limits, heat source considerations, current applications, challenges, and the latest research in this field. By the end, you’ll have a comprehensive understanding of how heat energy can be harvested for electricity generation using thermoelectric devices.

How Thermoelectric Generators Work

Thermoelectric generators work through the Seebeck effect, which states that when two dissimilar electrical conductors or semiconductors are placed in a temperature gradient, electricity will flow between them. This effect is used to convert heat directly into electrical energy.

Thermoelectric generators consist of p-type and n-type semiconductor materials connected thermally in parallel but electrically in series. The p-type material has an excess of positive charge carriers called “holes”, while the n-type material has an excess of negative charge carriers called “electrons”.

When one end of the joined p-type and n-type materials is heated, the heat flow drives the charge carriers in the semiconductors to diffuse from the hot end towards the cold end. The electrons in the n-type material and the holes in the p-type material move to opposite ends, building up a voltage difference across the module.

The diffusion of charge carriers under a temperature gradient leads to the creation of voltage, enabling the direct conversion of heat into electricity via the Seebeck effect. The voltage can then be used to power electrical devices and equipment.

Thermoelectric Materials

The efficiency of thermoelectric devices depends heavily on the properties of the thermoelectric materials used. Good thermoelectric materials need to have both high electrical conductivity to minimize electrical resistance losses, and low thermal conductivity to maintain a temperature gradient.

Some of the best thermoelectric materials are heavily doped semiconductors such as bismuth telluride (Bi2Te3). Doping improves the electrical conductivity, while alloying and nanostructuring help suppress heat conduction through the lattice vibrations known as phonons.

There is often a tradeoff between electrical and thermal properties that needs to be balanced. For example, measures to increase electrical conductivity like doping often also increase thermal conductivity. Researchers are working to optimize these materials through techniques like bandstructure engineering.

Module Construction

Thermoelectric generators are made up of basic building blocks called thermoelectric modules that can be scaled up as needed for the application. The key components in a thermoelectric module are:

• Thermocouples – These are pairs of n-type and p-type semiconductor materials that are connected electrically in series but thermally in parallel. Having dissimilar materials is what enables the thermoelectric effect of converting heat to electricity.
• Ceramic Plates – These hold the thermocouples in place and provide electrical insulation between the hot and cold sides. The plates need to efficiently transfer heat while preventing electrical shorts.
• Electrical Interconnects – This is the wiring that connects all of the thermocouple junctions in series. The interconnects are attached to the ceramic plates on each side.

To generate more power, many of these modules can be assembled together into larger thermoelectric generator units. The number, size and arrangement of modules can be tailored to match the heat source and required power output.

Efficiency Limits

The efficiency of thermoelectric generators is limited by the thermoelectric figure of merit (ZT) of the materials used. ZT depends on three interrelated material properties: electrical conductivity, thermal conductivity, and the Seebeck coefficient. Achieving high ZT values is challenging because the properties often have opposing effects. For example, increasing electrical conductivity typically increases thermal conductivity, which reduces ZT. The best bulk thermoelectric materials have reached ZT values between 1-2.5. Nanostructuring techniques and new complex crystal structures have enabled some high ZT values above 2, but there are still difficulties scaling up such approaches.

There are two main strategies being researched to improve efficiency by increasing ZT: reducing thermal conductivity by scattering phonons while maintaining good electrical properties, and enhancing the power factor through higher Seebeck coefficient and electrical conductivity values. Advanced thermoelectric materials using these approaches include nanocomposites, superlattices, thin films, quantum dot superlattices, and low-dimensional materials like 2D transition metal dichalcogenides. With continued improvements in ZT through new materials and structures, the efficiency limits of thermoelectric generators can be increased.

Heat Source Considerations

The key requirement for thermoelectric generators is a heat source. Potential heat sources that can be leveraged include:

• Industrial waste heat from factories and power plants
• Vehicle exhaust heat from cars, trucks, aircraft engines
• Solar irradiation and solar thermal collectors
• Geothermal heat sources
• Stoves and cooking appliances
• Human body heat
• Friction surfaces in braking systems
• Decay of radioactive materials

The higher the temperature difference between the hot and cold sides of the thermoelectric generator, the greater the efficiency. Ideal heat sources provide sustained high temperatures over 200°C. However, even low-grade waste heat sources around 50-100°C can be utilized.

It’s important to match the heat source properties to the optimal operating conditions and material limits of the thermoelectric modules. Generally, steadier heat sources enable better performance than fluctuating or intermittent sources.

Applications

Thermoelectric generators have a wide range of current and potential applications in industries like automotive, power generation, and electronics cooling.

In automotive applications, thermoelectric generators are being used to convert waste heat from the exhaust into usable electricity. This can improve overall fuel efficiency by capturing energy that would otherwise be lost. Car manufacturers are investigating how to integrate thermoelectric materials into exhaust systems and engine blocks to generate electricity to power accessories and recharge batteries.

For power generation, thermoelectric materials can convert heat from sources like geothermal vents, industrial processes, and solar radiation into electricity. This allows for waste heat recovery and small-scale power generation in remote locations. Thermoelectric generators are already being used to power sensors and weather monitoring equipment in harsh environments.

Thermoelectric coolers are also widely used in electronics to dissipate heat and maintain stable operating temperatures for components like laser diodes and microprocessors. Thermoelectric materials can pump heat efficiently from hot electronics into heat sinks and provide precision temperature control without moving parts or refrigerants.

Challenges

Thermoelectric generators face some key challenges that limit their widespread adoption and use. Some of the main challenges include:

Issues with stability at high temperatures – Many promising thermoelectric materials are unstable at higher operating temperatures above 500-600°C. This limits the heat sources that can be used and the overall efficiency. New material development is needed.

Manufacturing costs – The materials used in thermoelectric generators, like tellurium and selenium, can be expensive. In addition, fabrication of the modules requires specialized equipment and precision to create a tight thermocouple junction. This makes large-scale manufacturing costly. Improvements in materials and processes are needed to reduce costs.

System integration – To work effectively, thermoelectric generators require integration with heat sources and the electrical system. This balance of system engineering can be challenging, from dealing with high temperatures to impedance matching the output power. More turnkey solutions are needed.

Latest Research

Researchers are actively working to improve the efficiency and viability of thermoelectric generators through exploring new materials and designs. Some key areas of focus include:

New Materials: Scientists are developing new thermoelectric materials that can convert heat to electricity more efficiently. Examples include organic materials like conducting polymers, nanostructured materials, skutterudites, clathrates, and thin film superlattices. These aim to provide higher electrical conductivity and lower thermal conductivity.

Nanostructuring: Structuring thermoelectric materials on the nanoscale can reduce thermal conductivity while maintaining good electrical conductivity. Approaches include incorporating nanoinclusions, nanocomposites, nanowires and quantum dots.

Low-Dimensional Materials: 2D materials like graphene and topological insulators are being studied for their potential to achieve high conversion efficiency. The goal is to exploit quantum effects at reduced dimensions.

Hybrid Modules: New module designs integrate different thermoelectric materials in an optimal configuration to maximize efficiency and power output. Examples include segmented and cascaded designs.

Printable Materials: Printable organic semiconducting inks and thermoelectric composites enable low-cost, large-scale fabrication of modules on flexible substrates. This could expand applications.

Waste Heat Harvesting: Modules are being optimized to effectively capture waste heat from industrial processes, vehicle exhaust, power plants, and more. This provides a way to improve energy efficiency.

Conclusion

The conversion of heat into electricity using thermoelectric generators represents an important renewable energy technology for a more sustainable future. Thermoelectric devices take advantage of the Seebeck effect to generate an electric current from a temperature gradient. Key points summarize that:

Solid-state thermoelectric devices have no moving parts, provide maintenance-free operation, and can be adapted to various heat sources and power needs.

Thermoelectric materials like bismuth telluride alloys provide the best performance in terms of high Seebeck coefficient, low resistivity, and thermal conductivity.

Module construction with n-type and p-type semiconductor legs connected thermally in parallel and electrically in series optimizes the thermoelectric effect.

Improvements in thermoelectric efficiency depend on optimizing the materials’ figure of merit through advances in materials science and nanostructures.

Waste heat recovery presents a major opportunity to improve energy efficiency and sustainability through thermoelectric generation.

Thermoelectrics face challenges in cost and competitiveness with other technologies, but continued research promises enhanced efficiency and viability.

Overall, thermoelectric generation has the potential to play a key role in a clean energy future by converting untapped heat into usable electricity.