A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat flux (temperature differences) directly into electrical energy through a phenomenon called the Seebeck effect (a form of thermoelectric effect). Thermoelectric generators function like heat engines, but are less bulky and have no moving parts. However, TEGs are typically more expensive and less efficient.
Thermoelectric generators could be used in power plants to convert waste heat into additional electrical power and in automobiles as automotive thermoelectric generators (ATGs) to increase fuel efficiency. Radioisotope thermoelectric generators use radioisotopes to generate the required heat difference to power space probes.
In 1821, Thomas Johann Seebeck rediscovered that a thermal gradient formed between two dissimilar conductors can produce electricity. At the heart of the thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference. In 1834, Jean Charles Athanase Peltier discovered the reverse effect, that running an electric current through the junction of two dissimilar conductors could, depending on the direction of the current, cause it to act as a heater or cooler.
Thermoelectric power generators consist of three major components: thermoelectric materials, thermoelectric modules and thermoelectric systems that interface with the heat source.
Thermoelectric materials generate power directly from the heat by converting temperature differences into electric voltage. These materials must have both high electrical conductivity (σ) and low thermal conductivity (κ) to be good thermoelectric materials. Having low thermal conductivity ensures that when one side is made hot, the other side stays cold, which helps to generate a large voltage while in a temperature gradient. The measure of the magnitude of electrons flow in response to a temperature difference across that material is given by the Seebeck coefficient (S). The efficiency of a given material to produce a thermoelectric power is simply estimated by its “figure of merit” zT = S2σT/κ.
For many years, the main three semiconductors known to have both low thermal conductivity and high power factor were bismuth telluride (Bi2Te3), lead telluride (PbTe), and silicon germanium (SiGe). Some of these materials have somewhat rare elements which make them expensive.
Today, the thermal conductivity of semiconductors can be lowered without affecting their high electrical properties using nanotechnology. This can be achieved by creating nanoscale features such as particles, wires or interfaces in bulk semiconductor materials. However, the manufacturing processes of nano-materials are still challenging.
Thermoelectric generators are all-solid-state devices that do not require any fluids for fuel or cooling, making them non-orientation dependent allowing for use in zero-gravity or deep-sea applications. The solid-state design allows for operation in severe environments. Thermoelectric generators have no moving parts which produce a more reliable device that does not require maintenance for long periods. The durability and environmental stability have made thermoelectrics a favorite for NASA's deep space explorers among other applications. One of the key advantages of thermoelectric generators outside of such specialized applications is that they can potentially be integrated into existing technologies to boost efficiency and reduce environmental impact by producing usable power from waste heat.
A thermoelectric module is a circuit containing thermoelectric materials which generate electricity from heat directly. A thermoelectric module consists of two dissimilar thermoelectric materials joined at their ends: an n-type (with negative charge carriers), and a p-type (with positive charge carriers) semiconductor. Direct electric current will flow in the circuit when there is a temperature difference between the ends of the materials. Generally, the current magnitude is directly proportional to the temperature difference:
In application, thermoelectric modules in power generation work in very tough mechanical and thermal conditions. Because they operate in a very high-temperature gradient, the modules are subject to large thermally induced stresses and strains for long periods. They also are subject to mechanical fatigue caused by a large number of thermal cycles.
Thus, the junctions and materials must be selected so that they survive these tough mechanical and thermal conditions. Also, the module must be designed such that the two thermoelectric materials are thermally in parallel, but electrically in series. The efficiency of a thermoelectric module is greatly affected by the geometry of its design.
Thermoelectric generators are made of several thermopiles, each consisting of many thermocouples made of a connected n-type and p-type material. The arrangement of the thermocouples is typically in three main designs: planar, vertical, and mixed. Planar design involves thermocouples put onto a substrate horizontally between the heat source and cool side, resulting in the ability to create longer and thinner thermocouples, thereby increasing the thermal resistance and temperature gradient and eventually increasing voltage output. Vertical design has thermocouples arranged vertically between the hot and cool plates, leading to high integration of thermocouples as well as a high output voltage, making this design the most widely-used design commercially. The mixed design has the thermocouples arranged laterally on the substrate while the heat flow is vertical between plates. Microcavities under the hot contacts of the device allow for a temperature gradient, which allows for the substrate’s thermal conductivity to affect the gradient and efficiency of the device.
For microelectromechanical systems, TEGs can be designed on the scale of handheld devices to use body heat in the form of thin films. Flexible TEGs for wearable electronics are able to be made with novel polymers through additive manufacturing or thermal spraying processes. Cylindrical TEGs for using heat from vehicle exhaust pipes can also be made using circular thermocouples arranged in a cylinder. Many designs for TEGs can be made for the different devices they are applied to.
Using thermoelectric modules, a thermoelectric system generates power by taking in heat from a source such as a hot exhaust flue. To operate, the system needs a large temperature gradient, which is not easy in real-world applications. The cold side must be cooled by air or water. Heat exchangers are used on both sides of the modules to supply this heating and cooling.
There are many challenges in designing a reliable TEG system that operates at high temperatures. Achieving high efficiency in the system requires extensive engineering design to balance between the heat flow through the modules and maximizing the temperature gradient across them. To do this, designing heat exchanger technologies in the system is one of the most important aspects of TEG engineering. In addition, the system requires to minimize the thermal losses due to the interfaces between materials at several places. Another challenging constraint is avoiding large pressure drops between the heating and cooling sources.
If AC power is required (such as for powering equipment designed to run from AC mains power), the DC power from the TE modules must be passed through an inverter, which lowers efficiency and adds to the cost and complexity of the system.
Only a few known materials to date are identified as thermoelectric materials. Most thermoelectric materials today have a zT, the figure of merit, value of around 1, such as in bismuth telluride (Bi2Te3) at room temperature and lead telluride (PbTe) at 500–700 K. However, in order to be competitive with other power generation systems, TEG materials should have a zT of 2–3. Most research in thermoelectric materials has focused on increasing the Seebeck coefficient (S) and reducing the thermal conductivity, especially by manipulating the nanostructure of the thermoelectric materials. Because both the thermal and electrical conductivity correlate with the charge carriers, new means must be introduced in order to conciliate the contradiction between high electrical conductivity and low thermal conductivity, as is needed.
When selecting materials for thermoelectric generation, a number of other factors need to be considered. During operation, ideally, the thermoelectric generator has a large temperature gradient across it. Thermal expansion will then introduce stress in the device which may cause fracture of the thermoelectric legs or separation from the coupling material. The mechanical properties of the materials must be considered and the coefficient of thermal expansion of the n and p-type material must be matched reasonably well. In segmented thermoelectric generators, the material's compatibility must also be considered to avoid incompatibility of relative current, defined as the ratio of electrical current to diffusion heat current, between segment layers.
A material's compatibility factor is defined as
When the compatibility factor from one segment to the next differs by more than a factor of about two, the device will not operate efficiently. The material parameters determining s (as well as zT) are temperature-dependent, so the compatibility factor may change from the hot side to the cold side of the device, even in one segment. This behavior is referred to as self-compatibility and may become important in devices designed for low-temperature operation.
In general, thermoelectric materials can be categorized into conventional and new materials:
Many TEG materials are employed in commercial applications today. These materials can be divided into three groups based on the temperature range of operation:
Although these materials still remain the cornerstone for commercial and practical applications in thermoelectric power generation, significant advances have been made in synthesizing new materials and fabricating material structures with improved thermoelectric performance. Recent research has focused on improving the material’s figure-of-merit (zT), and hence the conversion efficiency, by reducing the lattice thermal conductivity.
Researchers are trying to develop new thermoelectric materials for power generation by improving the figure-of-merit zT. One example of these materials is the semiconductor compound ß-Zn4Sb3, which possesses an exceptionally low thermal conductivity and exhibits a maximum zT of 1.3 at a temperature of 670K. This material is also relatively inexpensive and stable up to this temperature in a vacuum, and can be a good alternative in the temperature range between materials based on Bi2Te3 and PbTe. Among the most exciting developments in thermoelectric materials was the development of single crystal tin selenide which produced a record zT of 2.6 in one direction. Other new materials of interest include Skutterudites, Tetrahedrites, and rattling ions crystals.
Besides improving the figure-of-merit, there is increasing focus to develop new materials by increasing the electrical power output, decreasing cost and developing environmentally friendly materials. For example, when the fuel cost is low or almost free, such as in waste heat recovery, then the cost per watt is only determined by the power per unit area and the operating period. As a result, it has initiated a search for materials with high power output rather than conversion efficiency. For example, the rare earth compounds YbAl3 has a low figure-of-merit, but it has a power output of at least double that of any other material, and can operate over the temperature range of a waste heat source.
To increase the figure of merit (zT), a material’s thermal conductivity should be minimized while its electrical conductivity and Seebeck coefficient is maximized. In most cases, methods to increase or decrease one property result in the same effect on other properties due to their interdependence. A novel processing technique exploits the scattering of different phonon frequencies to selectively reduce lattice thermal conductivity without the typical negative effects on electrical conductivity from the simultaneous increased scattering of electrons. In a bismuth antimony tellurium ternary system, liquid-phase sintering is used to produce low-energy semicoherent grain boundaries, which do not have a significant scattering effect on electrons. The breakthrough is then applying a pressure to the liquid in the sintering process, which creates a transient flow of the Te rich liquid and facilitates the formation of dislocations that greatly reduce the lattice conductivity. The ability to selectively decrease the lattice conductivity results in reported zT value of 1.86, which is a significant improvement over the current commercial thermoelectric generators with zT ~ 0.3–0.6. These improvements highlight the fact that in addition to the development of novel materials for thermoelectric applications, using different processing techniques to design microstructure is a viable and worthwhile effort. In fact, it often makes sense to work to optimize both composition and microstructure.
The typical efficiency of TEGs is around 5–8%. Older devices used bimetallic junctions and were bulky. More recent devices use highly doped semiconductors made from bismuth telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide (Ca2Mn3O8), or combinations thereof, depending on temperature. These are solid-state devices and unlike dynamos have no moving parts, with the occasional exception of a fan or pump.
Thermoelectric generators (TEG) have a variety of applications. Frequently, thermoelectric generators are used for low power remote applications or where bulkier but more efficient heat engines such as Stirling engines would not be possible. Unlike heat engines, the solid state electrical components typically used to perform thermal to electric energy conversion have no moving parts. The thermal to electric energy conversion can be performed using components that require no maintenance, have inherently high reliability, and can be used to construct generators with long service-free lifetimes. This makes thermoelectric generators well suited for equipment with low to modest power needs in remote uninhabited or inaccessible locations such as mountaintops, the vacuum of space, or the deep ocean.
The main uses of thermoelectric generators are:
Besides low efficiency and relatively high cost, practical problems exist in using thermoelectric devices in certain types of applications resulting from a relatively high electrical output resistance, which increases self-heating, and a relatively low thermal conductivity, which makes them unsuitable for applications where heat removal is critical, as with heat removal from an electrical device such as microprocessors.
While TEG technology has been used in military and aerospace applications for decades, new TE materials and systems are being developed to generate power using low or high temperatures waste heat, and that could provide a significant opportunity in the near future. These systems can also be scalable to any size and have lower operation and maintenance cost.
In general, investing in TEG technology is increasing rapidly. The global market for thermoelectric generators is estimated to be US$320 million in 2015. A recent study estimated that TEG is expected to reach $720 million in 2021 with a growth rate of 14.5%. Today, North America captures 66% of the market share and it will continue to be the biggest market in the near future. However, Asia-Pacific and European countries are projected to grow at relatively higher rates. A study found that the Asia-Pacific market would grow at a Compound Annual Growth Rate (CAGR) of 18.3% in the period from 2015 to 2020 due to the high demand of thermoelectric generators by the automotive industries to increase overall fuel efficiency, as well as the growing industrialization in the region.
Small scale thermoelectric generators are also in the early stages of investigation in wearable technologies to reduce or replace charging and boost charge duration. Recent studies focused on the novel development of a flexible inorganic thermoelectric, silver selenide, on a nylon substrate. Thermoelectrics represent particular synergy with wearables by harvesting energy directly from the human body creating a self-powered device. One project used n-type silver selenide on a nylon membrane. Silver selenide is a narrow bandgap semiconductor with high electrical conductivity and low thermal conductivity, making it perfect for thermoelectric applications.
Low power TEG or "sub-watt" (i.e. generating up to 1 Watt peak) market is a growing part of the TEG market, capitalizing on the latest technologies. Main applications are sensors, low power applications and more globally Internet of things applications. A specialized market research company indicated that 100,000 units have been shipped in 2014 and expects 9 million units per year by 2020.