Thermoelectric Generator: How the Seebeck Effect Turns Heat Into Electricity
A thermoelectric generator, or TEG, is one of the few technologies that turns heat straight into electricity with no turbines, no fluids, and no moving parts. Its operating principle, the Seebeck effect, was discovered two centuries ago, yet the same physics now powers spacecraft billions of kilometres from Earth and is being explored to reclaim waste heat from engines and factories. This reference explains how thermoelectric generators work, the materials and the figure of merit that govern them, their honest efficiency limits, and where they genuinely earn their place.
What a thermoelectric generator is
A thermoelectric generator is a solid-state device that produces an electrical voltage whenever one of its sides is hotter than the other. Unlike a conventional power plant, which burns fuel to spin a turbine, a TEG contains no rotating machinery and no working fluid. It is a block of specially engineered semiconductors sandwiched between two ceramic plates: hold one plate against a heat source and the other against something cooler, and current flows.
This directness is the whole appeal. With no moving parts, a TEG can run silently and maintenance-free for decades, tolerate vibration and vacuum, and start delivering power the instant a temperature difference appears. The trade-off is efficiency: TEGs convert only a modest fraction of the heat passing through them into electricity. That balance of extreme reliability against limited efficiency defines where the technology makes sense.
The Seebeck effect: the physics behind the voltage
The heart of every thermoelectric generator is the Seebeck effect, discovered in 1821 by the Baltic-German physicist Thomas Johann Seebeck. He joined two dissimilar conductors, bismuth and copper, into a loop, warmed one junction, and found that a nearby compass needle deflected. Seebeck interpreted it as magnetism, but the real cause was a voltage driven by the temperature difference between the junctions.
The underlying mechanism is straightforward. In a conductor, charge carriers at the hot end have more thermal energy and diffuse toward the cold end, piling up there and building an electric field that opposes further flow. The result is a voltage proportional to the temperature difference. The constant of proportionality is the material's Seebeck coefficient (S), measured in microvolts per kelvin; a larger coefficient means more voltage for the same temperature gap.
The Seebeck effect belongs to a family of thermoelectric phenomena. Its reverse, the Peltier effect (Jean Charles Athanase Peltier, 1834), uses a current to pump heat and is the basis of solid-state coolers. William Thomson (Lord Kelvin) later unified the two with the relations that bear his name, showing they are two faces of the same physics.
How a thermoelectric generator is built
A practical TEG is not a single junction but many wired together. The building block is a thermocouple pair: one pellet of n-type semiconductor (doped to carry negative electrons) and one of p-type (carrying positive holes). The two pellets are joined at the top by a metal strip that sits on the hot side, and connected at the bottom to the external circuit on the cold side.
When heat flows through the pair, electrons drift in the n-type leg and holes drift in the p-type leg, and because the two are wired in series their voltages add rather than cancel. A single couple produces only a fraction of a volt, so a module packs dozens or hundreds of them electrically in series and thermally in parallel, sandwiched between two ceramic plates that conduct heat while insulating the circuit electrically.
The output of a module depends entirely on maintaining the temperature difference. The hot side must stay hot and the cold side must shed heat, usually through a heat sink or fluid loop. If both sides drift to the same temperature, the voltage collapses to zero, no matter how much total heat is present.
Materials and the ZT figure of merit
A good thermoelectric material must do three things at once that normally conflict: conduct electricity well, generate a large Seebeck voltage, and conduct heat poorly so the temperature difference is not short-circuited. Physicists capture this balance in a single dimensionless number, the figure of merit ZT = (S²σ / κ) T, where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the absolute temperature.
The higher the ZT, the closer a device can approach the theoretical Carnot efficiency for its temperature range. For decades the workhorse material has been bismuth telluride (Bi₂Te₃), which reaches a ZT of about 1 near room temperature and dominates commercial modules. For higher temperatures, lead telluride (PbTe) and silicon-germanium (SiGe) alloys take over, the latter surviving the roughly 1,000 °C hot-side temperatures inside spacecraft generators.
Pushing ZT higher is the central research challenge. Nanostructuring, which scatters heat-carrying phonons while letting electrons pass, has produced laboratory reports of ZT above 2, though such peak values are often narrow in temperature and not always independently reproduced. Reaching a broad, reliable ZT of 3, which would lift conversion efficiency toward 30%, remains a long-standing goal rather than an achieved result.
Honest efficiency: how much power you really get
Commercial thermoelectric generators typically convert about 5 to 8% of the heat flowing through them into electricity. Advanced modules and multi-stage designs can reach into the low double digits under large temperature differences, but there is no credible TEG that turns most of its input heat into power. This ceiling is set by material ZT and by the temperature difference available, not by poor engineering.
It is worth stating plainly what a TEG is not. It does not create energy; it taps a heat flow that already exists and converts a fraction of it, obeying the first law of thermodynamics, which holds that energy is conserved and can only be converted, never created. Remove the temperature difference and the output stops. Any description of a thermoelectric device producing power from nothing, or running with unlimited output, contradicts the thermodynamics the technology is built on.
Because of this, TEGs win on grounds other than raw efficiency: reliability, silence, longevity, and the ability to harvest heat that would otherwise be wasted. The right question is rarely how efficient a TEG is in isolation, but whether recovering a modest slice of otherwise-lost heat, with zero maintenance, is worth it for a given application.
Real applications: from deep space to your wrist
The most celebrated use of thermoelectric generators is in radioisotope thermoelectric generators (RTGs), where the heat comes from the natural decay of plutonium-238. NASA's twin Voyager probes, launched in 1977 and still operating in interstellar space, each carry three RTGs that together produced about 470 watts at launch using silicon-germanium couples. The Mars rovers Curiosity and Perseverance each run on a single MMRTG delivering roughly 110 watts of electrical power at the start of their missions, using lead-telluride-based couples. TEGs are chosen here precisely because they have no parts to wear out over decades in a place no engineer can reach.
Closer to home, thermoelectric generators are used for industrial and automotive waste-heat recovery, capturing energy from exhaust pipes, furnace walls, and flue gases that would otherwise vanish. They also power remote sensors, gas-pipeline cathodic protection, and off-grid instruments where a reliable trickle of watts matters more than efficiency. At the smallest scale, body heat can drive wearable and Internet-of-Things devices, replacing batteries for ultra-low-power electronics that need only microwatts to milliwatts.
These honest power scales matter. A TEG is a source of watts and milliwatts, not megawatts. Its value lies in reliability and in reclaiming heat at points where no other converter is practical, a form of energy harvesting rather than bulk generation.
Where thermoelectrics meet neutrinovoltaic research
Thermoelectrics belong to a broader field concerned with converting ambient energy that is normally lost. That field is where the research of the Neutrino Energy Group in Berlin sits. Their neutrinovoltaic concept is distinct from thermoelectrics: rather than harnessing a temperature difference, it investigates whether a patented multilayer of graphene and silicon can extract usable current from multiple ambient sources at once, including thermal and electromagnetic fields and the faint momentum imparted by particles such as neutrinos.
It is important to be precise about status. Neutrinovoltaic is early-stage research in development, not a finished or purchasable product, and it makes no claim to create energy from nothing; like any converter, it would only redirect energy already flowing through its surroundings. It is scientifically anchored in the 2015 Nobel Prize in Physics for the discovery that neutrinos have mass, the 2017 COHERENT experiment that first measured coherent elastic neutrino-nucleus scattering (confirming that neutrinos transfer a tiny but real momentum to matter), and work by Thibado and colleagues in 2020 on charge separation in freestanding graphene. Where TEGs are a mature, well-understood technology, neutrinovoltaic sits at the exploratory frontier of the same ambition: making better use of the energy already flowing through the environment. Both fit within the wider search for renewable energy innovations.
Frequently asked questions
How do thermoelectric generators work?
A thermoelectric generator works through the Seebeck effect. When one side of a junction between two different semiconductors is kept hotter than the other, charge carriers diffuse from the hot side to the cold side, creating a voltage. Wiring many n-type and p-type pellets in series builds this into a usable current, with no moving parts involved.
What is the Seebeck effect?
The Seebeck effect, discovered by Thomas Johann Seebeck in 1821, is the appearance of a voltage across a conductor or semiconductor when its two ends are held at different temperatures. The size of the voltage per degree is the material's Seebeck coefficient. It is the fundamental principle behind every thermoelectric generator.
How efficient is a thermoelectric generator?
Most commercial TEGs convert about 5 to 8% of the heat passing through them into electricity, with advanced designs reaching into the low double digits under large temperature differences. Efficiency is limited by the material's ZT figure of merit and the available temperature difference, so a TEG never turns most of its input heat into power.
What materials are thermoelectric generators made from?
The most common material is bismuth telluride (Bi₂Te₃), which performs best near room temperature with a ZT of about 1. For higher temperatures, lead telluride (PbTe) and silicon-germanium (SiGe) alloys are used; SiGe couples power spacecraft generators that run near 1,000 °C on the hot side.
What is the ZT figure of merit?
ZT is a dimensionless number that ranks how good a thermoelectric material is, defined as ZT = (S²σ/κ)T, combining the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature. A higher ZT means higher possible conversion efficiency. Most practical materials sit near ZT = 1, and reaching a broad ZT of 3 remains a research goal.
What are thermoelectric generators used for?
They power deep-space probes and Mars rovers through radioisotope thermoelectric generators, recover waste heat from engines and industrial exhaust, run remote off-grid sensors, and can drive low-power wearable electronics from body heat. They are chosen for reliability and the ability to harvest otherwise-wasted heat rather than for high efficiency.
