A thermoelectric generator (TEG), also called a Seebeck generator, converts a sustained temperature difference directly into electrical energy. It can recover some electricity from waste heat without moving parts, but it does not generate useful power from temperature alone: heat must keep flowing from a hot side to a cooler side, and the cooler side must be able to reject that heat.
How a thermoelectric generator works
A temperature difference drives charge carriers through thermoelectric materials. This produces a voltage known as the Seebeck voltage. A practical module connects many thermocouples electrically in series, to build voltage, and thermally in parallel, so heat passes through them together.
In simplified form, the open-circuit voltage is proportional to the effective Seebeck coefficient and the temperature difference across the module: Voc ≈ Seff × ΔT. Here, ΔT is the difference between the module’s hot-side and cold-side temperatures—not simply the temperature of the heat source. Once a load is connected, current flows and the voltage at the terminals falls.
The module is only one part of the generator. A working system needs a heat source, good thermal contact to the module, and a cold-side heat sink or other route for heat to leave. Interface resistance, wiring, electrical resistance inside the module, and poor heat rejection all reduce the useful output.
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What temperature difference does a TEG need?
There is no single temperature-difference threshold that applies to every TEG. A small gradient can produce a voltage, but whether it produces enough power for a particular job depends on the module’s materials and geometry, its internal resistance, how much heat is available, and the electrical load. A module’s rated hot-side temperature and maximum temperature difference are separate limits; check both for the specific module.
The gradient must also be sustained under load. If the cold side warms because its heat sink is inadequate, or the hot source cannot supply enough heat, the temperature difference across the module shrinks and its output falls. A hot source by itself is not enough: the system must maintain a cooler side while heat continues to flow through the module.
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How much electricity can a TEG produce?
Output varies widely with the application and test conditions. A 2025 review in Sensors reports that many autonomous-sensor applications it reviewed produce a few milliwatts to tens of milliwatts. He et al.’s 2024 review in Applied Thermal Engineering reports the following literature ranges:
| Application or system measure | Reported figure | How to interpret it |
|---|---|---|
| Wearable TEG power density | Below 100 μW/cm² | A review range for wearable applications; it is not a guaranteed output for a particular module. |
| Industrial TEG power density | 25–300 mW/cm² | A review range across industrial applications and conditions. |
| Geothermal TEG power density | 20–130 mW/cm² | A review range across geothermal applications and conditions. |
| TEG system efficiency | 2.5%–6.5% | A system-level literature range; the boundary and operating conditions differ among studies. |
| Estimated TEG system cost | US$2,000–15,000 per kW | An estimated range reported in the review, not a current quote for a module or installation. |
These figures cover different heat sources, temperature differences, module sizes, and system boundaries. Power density is not the same as total power: the total depends on the active module area and the conditions it can actually maintain. Treat a module’s nominal voltage as only one specification, not a prediction of usable watts.
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How to match a TEG to an electrical load
A TEG can be approximated as a voltage source with an internal resistance. Maximum power transfer occurs when the external load resistance matches that internal resistance. At this maximum-power point, the voltage across the load is approximately half the open-circuit voltage, as described in the 2025 Sensors review. A changing heat source or temperature gradient can move that operating point.
- Find the module’s electrical and thermal specifications. Check its open-circuit voltage, internal resistance, hot-side temperature limit, maximum temperature difference, and the conditions used for any rated output.
- Estimate the operating gradient in the assembled system. Use the expected hot-side and cold-side temperatures at the module interfaces, not just the source temperature. Account for the heat sink and contact interfaces.
- Compare the load with the module’s internal resistance. For a simple resistive load, matching the resistance is the maximum-power condition. If the load voltage differs from what the TEG can provide, a power-management circuit may be needed.
- Choose conversion and storage around the real load. A boost converter, energy storage, and control may be useful when the TEG voltage is low or varies as the gradient changes. Their own operating requirements and losses affect what reaches the device.
- Recheck under the intended heat flow. Confirm that the cold side stays cool enough and the heat source sustains the gradient while the load is connected. Open-circuit voltage alone does not establish loaded power.
In the simplified Thevenin model, if the open-circuit voltage is Voc and internal resistance is Rint, the ideal matched-load maximum is Pmax = Voc2/(4Rint). This is a circuit-model result, not a promise of system output: thermal limits, changing conditions, converter losses, and heat-transfer constraints can lower delivered power.
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Where TEGs are useful—and what constrains them
Reviews discuss TEG use in autonomous sensor nodes, Internet of Things devices and wireless sensor networks, wearable and medical equipment, automotive and industrial waste-heat recovery, aerospace systems, and geothermal applications. The fit depends on whether a site has a durable temperature gradient and whether the recovered electricity justifies the thermal hardware.
- Small sensors: Milliwatt-scale output may suit low-power monitoring or intermittent operation, often with energy storage and power management.
- Wearable devices: Body heat offers a small gradient, so the power density reported for wearable systems is much lower than industrial ranges.
- Industrial or geothermal recovery: These settings can have more substantial heat flows, but they also require suitable interfaces, heat exchangers or sinks, and a system design matched to the source.
TEGs are solid-state and silent, with no moving parts in the module, which can make them attractive where low maintenance matters. The trade-offs are modest conversion efficiency and system cost. Thermal integration is central: contact resistance, heat-sink design, material durability, operating environment, geometry, and maintenance can matter as much as the thermoelectric material itself.
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What to check before choosing a TEG module
- Hot-side temperature limit and maximum temperature difference.
- Expected sustained temperature difference and available heat flow in the installed system.
- Open-circuit voltage, internal resistance, and output conditions.
- Cold-side heat-sink or heat-exchanger requirements.
- Module dimensions and whether rigid or flexible geometry fits the installation.
- Durability, material toxicity, and suitability for the operating environment.
- Module, installation, and maintenance costs, compared with the value of recovered power.
Recent development areas include flexible micro-TEGs, improved thermoelectric figure of merit, segmented or cascaded materials, better contact and interface engineering, geometries suited to curved surfaces, and improved thermal management. The practical challenge remains connecting the module to a real heat source and electrical load without losing too much of the temperature gradient at interfaces, in wiring, or through inadequate heat rejection.
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