TE Generators

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Peltier elements can also be used to convert heat energy into electrical energy by directing a thermal heat flow through the element. This flow is created by a temperature difference (ΔT) and the natural tendency towards thermal equilibrium within the element. When two bodies at different temperatures are in contact, the warmer surface loses heat while the cooler surface gains heat—until both bodies reach the same temperature.

These devices are referred to as thermoelectric generators or thermoelectric generators (TEGs).

Depending on the material, these generators have an efficiency of around one to five percent. They are particularly useful for generating electricity from waste heat, such as from incinerator exhaust streams (without removing so much heat that the exhaust tower’s draft stops) or from various industrial and IT processes.

Physical Effect and Structure

Thermoelectric generators primarily utilize the Seebeck effect, a thermoelectric effect where a temperature difference between two metals creates a voltage. As a result, even a temperature difference of less than 20K can generate several mW of power. The Seebeck effect can also be used to measure heat flow or temperature differences.

Each Peltier element functions as a thermoelectric generator. When heated, it generates electrical voltage, with heat flowing from the hot to the cold side, driven by the temperature difference. The resulting energy can be tapped directly via the electrical connections. However, heat energy must be continually supplied to maintain the temperature difference.

The heat flow power is calculated as: Q = ΔT / R_th

Q represents the power flowing from the warm side to the colder side. To sustain this flow, the warm side must continuously receive power to prevent a temperature drop, and the cold side must be able to release this energy to avoid heating up. Otherwise, the temperature difference, and thus the heat flow and energy generation, will quickly disappear.

Power Generation and Economic Considerations

The generator requires about 10W/cm² of heat flow. Maximum efficiency is achieved when the load resistance matches the module’s internal resistance. Additionally, generated voltage depends on the temperature and the number of leg pairs. Efficiency can be further increased by using materials with excellent thermoelectric properties.

Examples include bismuth-telluride alloys (up to 300°C) or bismuth-lead alloys (up to 360°C). For higher requirements, a homogeneous lead-telluride blend, stable up to 500°C, is suitable.

This energy generation process is practical only if the element has sufficiently robust cooling. Depending on application and heat flow per TEC, the cooling element might need to be as large as a dictionary if using passive convection. Forced-air cooling could reduce the size but would consume much of the generated energy to power the fan.

TEC use is more practical in micro energy harvesting (MEH), where battery-operated systems can be replaced by independent power supplies. TEGs can generate enough power to support sensor systems—a valuable asset for the Internet of Things (IoT). TEGs are also increasingly used in hard-to-access locations to avoid the need for network connections or frequent battery replacements.

Requirements for using a TEC include:

Two areas with different temperature levels
A continuous source of energy
Consistent heat dissipation

During project planning, it’s essential to assess the following requirements:

What causes the high temperature?
How can heat flow be directed through a Peltier element?
How can the energy be dissipated?
How will the source temperature respond to a continuous heat flow?
How will the sink temperature respond to continuous energy intake?

Only after thorough analysis can it be determined whether to use the heat and how to size a thermoelectric generator setup.

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Rules for Thermoelectric Generators

1. Thermoelectric generators produce direct current with an efficiency of around 5% when there is a temperature difference.

2. Only a fraction of the heat is converted into energy. Efficiency increases with the temperature difference.

3. Power is generated only when a temperature difference exists; no power is produced if both sides are at the same temperature.

4. To maintain generator function, one side must remain heated while the other remains cooled.

5. Any Peltier element can be used as a thermoelectric generator. However, material and build quality are critical to efficiency and lifespan.

6. Seamless integration into the thermal environment optimizes energy output.

7. The open-circuit voltage of a thermal generator is directly proportional to the driving temperature difference.

8. The current with load connected is calculated as open-circuit voltage divided by the sum of internal and load resistance: I_Load = U₀ / (R_i + R_L).

9. Each leg pair of a Quick-Ohm generator or Peltier element generates 0.4 mV of open-circuit voltage per Kelvin temperature difference. For example, if the QC-241-1.0-3.0 Peltier element has temperatures of 20°C and 100°C on either side, an open-circuit voltage of 0.4 mV/K x 241 x 80K = 7.7 V can be obtained at the terminals.

10. The thermal generator acts as a voltage source, so the load resistance should match the internal resistance.

11. Integrating a thermoelectric generator into an existing system affects system conditions, potentially altering temperatures.

12. It should be evaluated whether the secondary generation effect interferes with the primary system effect, resulting in an overall disadvantage.