Trystan Casey
Thermoelectric devices are used to convert heat energy into electrical energy and vice versa. The first useful thermoelectric device was the thermocouple, invented in 1821 by German physicist Thomas Johann Seebeck. Thermoelectric generators (TEGs) have no moving parts, making them reliable and durable. They can be used in a variety of applications, such as powering spacecraft or cooling blood plasma during storage. TEGs can also be integrated into existing technologies to boost efficiency and reduce environmental impact by producing usable power from waste heat. The design of TEGs can vary depending on their application, and they are made from materials such as bismuth telluride, skutterudites, and nanocomposites.
| Characteristics | Values |
|---|---|
| Name | Thermoelectric generator (TEG) |
| Other Names | Seebeck generator |
| Function | Converts heat to electrical energy |
| Effect | Thermoelectric effect, specifically the Seebeck effect |
| Efficiency | 5-8% |
| Applications | Spacecraft, medical storage, radioisotope pairing, powering remote equipment |
| Materials | Bismuth telluride, skutterudites, nanocomposites, functionally graded materials |
| Design | No moving parts, solid-state |
| Heat Source | Hot exhaust flue, radioisotopes, etc. |
| Heat Mechanism | Heat is absorbed on the "cold-side" and ejected out the "hot-side" |
| Heat Sink | Required for the "hot-side" |
| Limitations | Requires a large temperature gradient, inefficient compared to grid power |
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What You'll Learn
Thermoelectric generators (TEGs)
The thermoelectric effect refers to the direct conversion of temperature differences into electric voltage and vice versa via a thermocouple. Thermocouples, composed of metals like iron, nickel, and copper, are widely used in science and industry due to their accuracy and ability to operate across a broad temperature range. TEGs, as thermoelectric devices, utilise this effect to generate electricity, measure temperature, and modify the temperature of objects.
TEGs require a large temperature gradient to function, which can be challenging to achieve in practical scenarios. They exploit the Peltier effect, where multiple junctions in series experience heat loss or gain, to operate as heat pumps or cooling devices. The efficiency of TEGs typically ranges from 5% to 8%, with newer devices using doped semiconductors like bismuth telluride and lead telluride to achieve higher efficiency.
TEG design considerations include managing thermal losses due to material interfaces and avoiding substantial pressure drops between heating and cooling sources. The compatibility of materials in segmented TEGs is crucial to ensure efficient operation. TEGs can be designed for various applications, including handheld devices that use body heat and flexible wearables made with novel polymers. They are particularly useful for generating power from waste heat, such as in power plants, factories, and automobiles, and can be integrated into existing technologies to improve efficiency and reduce environmental impact.
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Thermoelectric modules
The performance of thermoelectric modules depends on the efficiency of the semiconductor materials used. This efficiency is determined by the material's electrical conductivity, thermal conductivity, and Seebeck coefficient, which change with temperature. Bismuth telluride and its solid solutions are effective thermoelectric materials at room temperature and are commonly used in refrigeration applications. Skutterudites, with their chemical composition of LM4X12, where L is a rare-earth metal, M is a transition metal, and X is a metalloid, have also shown potential for use in multistage thermoelectric devices.
The design of thermoelectric modules can vary depending on the specific application. They can be made compact and durable, making them suitable for powering remote equipment such as Arctic weather stations or navigation buoys. Thermoelectric generators have no moving parts, making them reliable and low-maintenance. Additionally, their durability and environmental stability have made them a preferred choice for NASA's deep space exploration missions.
One common design consideration for thermoelectric modules is the selection of a heat sink. The heat sink is attached to the "hot-side" of the module and plays a crucial role in ejecting excess heat. However, choosing a heat sink based solely on the module's heat pumping capacity is a common mistake. Instead, a balance must be struck between heat ejection and the current rating of the power supply.
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Thermoelectric materials
Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and are often used in refrigeration applications around 300 Kelvin. Skutterudites, with a chemical composition of LM4X12, where L is a rare-earth metal, M is a transition metal, and X is a metalloid, also exhibit suitable thermoelectric properties and can potentially be used in multistage thermoelectric devices.
Superlattices, with film thicknesses ranging from a few micrometers to about 15 micrometers, have also been used to create high-performance microcoolers and other devices. The Bi2Te3/Sb2Te3 superlattice material, in particular, has demonstrated enhanced thermoelectric capabilities. Nanocomposites are another promising class of materials for bulk thermoelectric devices, but challenges remain in making them suitable for practical applications.
Functionally graded materials offer the potential to improve the conversion efficiency of existing thermoelectric devices. These materials have a non-uniform carrier concentration distribution and, in some cases, a solid-solution composition. By utilising these materials, thermoelectric devices can operate across a broader temperature range, improving their overall efficiency.
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Thermoelectric effect
The thermoelectric effect is a phenomenon where temperature differences are directly converted into electric voltage and vice versa via a thermocouple. This effect forms the basis of the functioning of thermoelectric devices, which can be used for power generation, temperature measurement, and temperature control.
The thermoelectric effect is responsible for the direct conversion of temperature differences into electrical energy. This phenomenon occurs in a thermocouple, which consists of two different metals joined together. When there is a temperature difference on each side of the thermocouple, a voltage is created, and this voltage can be used to generate electrical power. Conversely, when a voltage is applied to the thermocouple, it can transfer heat, resulting in heating or cooling effects.
Seebeck Effect
The Seebeck effect, named after German physicist Thomas Johann Seebeck, is a specific form of the thermoelectric effect. In 1821, Seebeck discovered that joining strips of two different conducting materials into a loop could produce this effect. The Seebeck coefficient is one of the key factors influencing the efficiency of thermoelectric materials, along with electrical conductivity and thermal conductivity.
Peltier-Seebeck Effect
The Peltier-Seebeck effect combines the Peltier and Seebeck effects. The Peltier effect describes how some junctions in a series of junctions in a Peltier heat pump lose heat while others gain heat. By combining these two effects, the Peltier-Seebeck effect can drive a heat engine or refrigerator closer to Carnot efficiency, which indicates thermodynamic reversibility.
Applications of Thermoelectric Effect
Thermoelectric devices have a wide range of applications due to their ability to convert heat into electrical energy and vice versa. They are used in power generation, especially in remote locations with moderate power requirements, such as Arctic weather stations or navigation buoys. Thermoelectric devices are also valuable for temperature-sensitive tasks, such as keeping blood plasma cool during storage and powering spacecraft like Cassini on its mission to Saturn. Additionally, they have advantages in durability and environmental stability, making them attractive for NASA's deep space exploration.
Materials for Thermoelectric Devices
The efficiency of thermoelectric devices depends on the materials used. Bismuth telluride and its solid solutions are effective thermoelectric materials at room temperature, making them suitable for refrigeration. Skutterudites, with a chemical composition of LM4X12, where L is a rare-earth metal, M is a transition metal, and X is a metalloid, also show potential for use in multistage thermoelectric devices. Nanocomposites are a promising class of materials for bulk thermoelectric devices, but challenges remain in making them practical.
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Applications of thermoelectric devices
Thermoelectric devices have a wide range of applications across various sectors. Here are some of their uses:
Power Generation and Temperature Control in Extreme Environments
Thermoelectric devices are used in power generation and temperature control in challenging environments, such as deep space exploration. For instance, NASA's Mars rover Curiosity is powered by a Multi-Mission Radioisotope Thermoelectric Generator, which uses plutonium dioxide to produce electricity. Thermoelectric devices are also used in nuclear power plants as a secondary system to recover electricity from waste heat.
Temperature Control in Electronics and Imaging Devices
Thermoelectric devices are employed to manage the temperature of sensitive electronics like computer CPUs and graphics cards, and scanning electron microscopes. They help prevent damage caused by excessive heat and improve the reliability and lifespan of these components.
Cooling and Refrigeration
Thermoelectric cooling is used in various applications, including refrigerators in RVs, submarines, and trucks for pharmaceuticals. These devices are known for their small size, quiet operation, and low maintenance. They are also used in food dispensers in restaurants to keep perishable items cool.
Military and Defence
Thermoelectric technology is used in military clothing to keep soldiers cool. Additionally, thermoelectric devices are crucial in controlling the temperature of sensors and sensing circuits in military equipment, ensuring accurate measurements and performance. They also play a role in temperature control systems for missiles and space vehicles.
Medical and Wearable Devices
The human body can be a sustainable heat source for thermoelectric devices, making them useful for wearable health monitoring systems and implanted medical devices. These devices can draw small amounts of thermal energy from the body without requiring batteries, enhancing their reliability and convenience.
Telecommunications and Sensors
Thermoelectric devices are used in telecommunications equipment, where they help maintain stable temperatures for optimal performance. They are also incorporated into sensors, ensuring accurate temperature measurements and fixed-temperature control for sensitive instruments.
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Frequently asked questions
A thermoelectric device, also known as a thermoelectric generator (TEG) or Seebeck generator, is a solid-state device that converts heat into electrical energy through the thermoelectric effect.
A thermoelectric device creates voltage when there is a temperature difference on each side, converting temperature differences into electric power and vice versa. This effect can be used to generate electricity, measure temperature, or change the temperature of objects.
Thermoelectric devices have a variety of applications, including power generation, temperature control, and cooling. They are used in spacecraft, medical equipment, and laboratory devices for DNA amplification. TEGs can also be integrated into existing technologies to boost efficiency and reduce environmental impact by producing usable power from waste heat.
The usefulness of a material in thermoelectric systems is determined by its electrical conductivity, thermal conductivity, and Seebeck coefficient, which change with temperature. Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature, while nanocomposites and functionally graded materials show promise for improving conversion efficiency.
One important consideration is the need to reconcile the contradiction between high electrical conductivity and low thermal conductivity. Additionally, thermal expansion can introduce stress and cause fractures, so this must be managed. The selection of a heat-sink for the "hot-side" of the device is also critical to its performance.
