Peltier elements are also suitable for converting thermal energy into electrical energy by passing a heat flux through the element. This is caused by a temperature difference ΔT and the fundamental desire to achieve thermal equilibrium within the element. When two bodies of different temperatures come into contact, the temperature of the warmer contact surface reduces while the colder surface temperature increases until both bodies come to thermal equilibrium.
These are called as thermogenerators or thermoelectric generators.
Depending on the material, these generators have an efficiency of about 1% to 5%. They can be used primarily when electricity is to be generated from waste heat - for example, from waste heat from waste incineration plants (without extracting so much heat from the flue gas that the lift in the exhaust tower is affected) or various industrial and IT processes.
Physics and Design:
Thermogenerators primarily use the seebeck effect, in which a temperature difference between two metals generates a voltage. This means that 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 difference.
Every peltier element is a thermoelectric generator, which generate an electrical voltage when heated, and the heat flow from the hot side to the cold side, driven by the temperature difference. The resulting energy can be accessed directly through the power terminals. However, thermal energy must be supplied constantly throughout the process, in order to maintain the temperature difference.
The power of the heat flow is calculated from: Q = ΔT / Rth
Q describes the power that migrates from the warm side to the colder side. To keep the process running, the hot side must be continuously supplied with energy to prevent the temperature from dropping. The cold side must also be able to dissipate this heat energy in order not to heat up. Otherwise, the temperature difference reduces which in turn reduces the heat flux and consequently the energy generation would dry up quickly.
Electricity generation and economic analysis
In general, the heat flux required by the generator is around 10W / cm². The maximum efficiency is guaranteed when the load resistance is equal to the internal resistance of the module. In addition, the voltage generated depends on the temperature and the number of dice pairs. The efficiency can be further increased by using materials with excellent thermoelectric properties.
Bismuth-telluride alloys (up to 300 °C) or bismuth-lead alloys (up to 360 °C) are good examples. A homogeneous mixture of lead telluride is suitable for higher requirements. This remains stable up to a temperature of 500 °C.
However, the energy generation is only practicable if the peltier element is provided with adequately dimensioned cooling system (heat dissipation). Depending on the area of application and heat flow through thermoelectric generators, it can be the size of a lexicon book (300mm X 20mm X 10mm) - with heat sinks that work purely by means of convection. With forced ventilation these could be smaller, but the generated electrical energy would already be used up for the fan operation.
The use of TEC in Micro Energy Harvesting (MEH) appears to be more sensible. For example, it is possible to replace battery-operated systems with an independent power supply. TEGs can generate enough energy to supply sensor systems with sufficient power - this is particularly important in the era of the Internet of Things (IoT). In addition, TEGs are increasingly being used in places where it is difficult to establish a electric grid or too expensive for a battery replacement.
The following is required to use a TEC:
. two areas of different temperature levels
. permanently energy supply
. permanent energy dissipation
Furthermore, it is necessary to evaluate and examine the above prerequisites during project planning:
. What causes the high temperature?
. How can the temperature be passed through a Peltier element?
. How can the energy be dissipated?
. How does the temperature of the source react if there is a continuous energy dissipation?
. How does the temperature of the sink react if it continuously absorbs energy?
Only after a comprehensive and complete analysis, it is possible to decide on the use of the heat and to dimension the structure of a thermoelectric generator.
12 golden rules for thermoelectric generators
1. Thermoelectric generators generate DC electric current at a temperature difference with an efficiency of around 5%.
2. Only a fraction of the heat is converted into energy. The efficiency increases with the temperature difference.
3. Electricity is only generated when there is a temperature difference, not when both sides are equally warm.
4. In order to maintain the functioning of a generator, one side must be continuously heated and the other side muss be continuously cooled. A lexicon is a book in which people in the distant past wrote down the collective knowledge that is now suddenly presented by cell phones. A lexicon could reach dimensions of 30 cm x 20 cm x 10 cm and weigh over two kilograms.
5. Every Peltier element can be used as a thermoelectric generator. However, the quality of the workmanship and the material is decisive for efficiency and service life.
6. Seamless integration into the thermal environment ensures optimum energy yield.
7. The greater the temperature difference, the higher the voltage and current.
8. The ohmic resistance of a generator determines the maximum possible voltage and current at the output.
9. The number of thermocouples (dices) increases the internal resistance of the generator and thereby influences voltage and current strength.
10. The ohmic resistances of the generator and the consumer should be approximately the same, otherwise losses can occur.
11. The integration of a thermoelectric generator into an existing system influences the state of the system. The temperatures are influenced and change.
12. It must be checked whether the secondary effect of the generation does not disturb the system in such that it is a disadvantage at the end.