Thermoelectric generators

In plain language

A thermoelectric generator turns a temperature difference directly into electricity. When one side of a small device is hotter than the other, charge carriers in the material move from the hot side towards the cold side, and this builds up a voltage. The device has no moving parts, so it can run quietly for a long time, but its output depends on keeping a temperature gap across it.

Two uses are well established or being tested seriously. Radioisotope thermoelectric generators use heat from the natural decay of their fuel to make electricity for spacecraft, and NASA's overview lists New Horizons, the Curiosity and Perseverance Mars rovers, and the Voyager spacecraft among the missions they have powered. Researchers are also building thin, flexible devices that sit on the skin and use body heat to power small sensors. Those laboratory results are small, from about 2 to about 15 microwatts per square centimetre, and they only work where there is a useful temperature gap.

Technical explanation

The performance of a thermoelectric material is usually summarised by the dimensionless figure of merit ZT. Higher ZT means more of the heat flowing through the material is converted into electrical power rather than simply conducted away. Even a good material cannot convert all the heat. The efficiency is a fraction of the Carnot limit, which is set by the temperature difference across the device. A 2024 review in the journal Theory and Building Practice reports, in its abstract, that thermoelectric generator efficiency is limited to half of the Carnot value for ZT values between 1 and 3. Snyder and Toberer's 2008 review in Nature Materials, titled 'Complex thermoelectric materials', is a standard reference on the materials route to raising ZT. A single peak ZT value is not a reliable guide to device efficiency, because material properties change with temperature. A 2018 arXiv preprint by Ryu, Chung and Park argues this and proposes tuning segmented legs to gain efficiency; that work is a preprint and has not been checked against its final published version.

For space power, the NASA overview describes the radioisotope thermoelectric generator as using thermocouples to convert heat from radioactive decay into electricity, with no moving parts. It states that the general purpose heat source module used in these systems is nominally designed to produce 250 watts of thermal power at the beginning of a mission. That is heat, not electricity, and the overview gives no electrical output or efficiency figure, so this draft does not state one.

For wearables, a 2022 study in Advanced Science measured a body-heat-only generator with a temperature difference of 5.5 degrees Celsius and a power density of 2.19 microwatts per square centimetre. The same device reached 15.33 microwatts per square centimetre when it also used light. These tests used an insulating film at a 35 degree surface temperature with a 25 degree ambient, not live skin. Waste-heat recovery from vehicles and industry is the other main route. This draft has not yet verified a peer-reviewed performance figure for it.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Conversion efficiency is low. A 2024 review reports that thermoelectric generator efficiency is limited to half of the Carnot value for ZT values of 1 to 3 (Redko and colleagues, 2024).
• Usable power depends on the temperature difference. In the 2022 wearable study, body-heat-only output was 2.19 microwatts per square centimetre at a 5.5 degree difference and fell almost to nothing when ambient temperature reached 35 degrees (Jeong and colleagues, 2022).
• Wearable power figures are hard to compare. Results depend on skin or surface temperature, airflow, wearing pressure and how power is normalised; the 2022 tests used a film rather than live skin.
• A single ZT value does not accurately represent device efficiency, according to a 2018 preprint (Ryu and colleagues).
• The NASA overview consulted gives thermal power for radioisotope heat sources but no electrical output or efficiency figure, so electrical performance of these systems is not stated in this draft.
• No peer-reviewed waste-heat recovery performance figure has been verified for this profile yet.

Possible future uses

Product concepts: Body-heat wearable charger, Rechargeable drinks bottle that cools itself

Timeline

1. 2008Snyder and Toberer publish the review 'Complex thermoelectric materials' in Nature Materials (volume 7, pages 105 to 114). Sources: [1]
2. 26 Oct 2018Ryu, Chung and Park post a preprint arguing that a single peak ZT does not accurately represent thermoelectric device efficiency. Sources: [2]
3. 24 Feb 2022Advanced Science reports a wearable generator that harvests body heat and light, reaching 15.33 microwatts per square centimetre with both sources and 2.19 microwatts per square centimetre from body heat alone in laboratory tests. Sources: [3]
4. 2024Redko and colleagues review thermoelectric energy converter efficiency and state that efficiency is limited to half of the Carnot value for ZT of 1 to 3. Sources: [4]

Futures this could help bring about

How far away?

Cooling without climate-warming refrigerants

Engineering prototypes · 4 of 7 steps done

How far away?

Electricity from waste heat

Engineering prototypes · 4 of 8 steps done

Glossary

Seebeck effect

The voltage that appears across a material when its two ends are held at different temperatures.

Figure of merit (ZT)

A dimensionless number that combines a material's electrical and thermal properties; higher values indicate better thermoelectric performance.

Carnot efficiency

The maximum efficiency any heat engine can reach when working between two temperatures; it depends only on those temperatures.

Radioisotope thermoelectric generator (RTG)

A power source that uses heat from the natural radioactive decay of a fuel, converted to electricity by thermocouples.

Sources

1. review article
   Complex thermoelectric materials
   G. Jeffrey Snyder, Eric S. Toberer. Nature Materials, Feb 2008. DOI: 10.1038/nmat2090. Accessed 21 Oct 2026.
2. preprint
   Thermoelectric degrees of freedom determining thermoelectric efficiency
   Byungki Ryu, Jaywan Chung, SuDong Park. arXiv, 26 Oct 2018. Accessed 21 Oct 2026.
3. peer reviewed paper
   Operation of Wearable Thermoelectric Generators Using Dual Sources of Heat and Light
   Myeong Hoon Jeong, Kwang-Chon Kim, Jin-Sang Kim et al.. Advanced Science, 24 Feb 2022. DOI: 10.1002/advs.202104915. Accessed 21 Oct 2026.
4. review article
   Improving the energy efficiency of thermoelectric energy converters - review
   Andriy Redko, Oleksandr Redko, Ihor Redko et al.. Theory and Building Practice (Lviv Politechnic Publishing House), 2024. DOI: 10.23939/jtbp2024.02.007. Accessed 21 Oct 2026.
5. government report
   Radioisotope Power Systems: Overview
   NASA Science, 28 Apr 2025. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.