Thermoelectricity involves the conversion of heat to electricity or electricity to heat using the Seebeck effect and the Peltier effect.
When two metals are in electric contact with one another, electrons flow from the metal in which they are loosely bound and into the other metal, where they are tightly bound. The electron binding on the metal is measured by a Fermi level, which represents the energy difference between the occupied energy levels and the unoccupied ones; the higher the level, the less the binding. At the Fermi level, the energy of an electron is –W, relative to a free electron located outside the metal.
The electron flow between the two conductors in contact will continue until the change in electrostatic potential brings the Fermi levels of the two metals to the same value, W1 and W2 respectively. The electrostatic potential (contact potential) is given by the equation;
ⅇΦ12 = W1 –W2 , where ⅇ = 1.6 x 10-19 Coulomb
When a loop is formed from two different metals (a thermocouple), there will be 0 voltage because the two contact potentials oppose one another hence no current will flow. If the temperature in one of the junctions is raised relative to the second, there will be a current and a net electromotive force. To maintain this temperature difference, heat must enter at the hot junction and leave at the cold junction. The choice of a thermocouple is dependent on: the medium chemical properties, the measured temperature and the sensitivity required.
The generation of a thermal electromotive force at a junction is the Seebeck Effect. The electromotive force is approximately linear with the temperature difference at the junction. An application of the Seebeck effect is in measurement of temperature.
The absorption or release of heat at a junction in which an electric current flows is the Peltier Effect. The Seebeck and Peltier effects occur at a junction between semi-conductor and metal or two semi-conductors.
Semi- conductor thermocouples made up of n-type and p-type Bi2Te3 have been used in refrigeration (using the Peltier effect). The thermocouples are connected electrically serial and thermally in parallel. When current flows, a temperature difference develops at the junction. The temperature of the hotter junction is kept low by removal of heat while the second junction remains colder, acting as a refrigerator.
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Peltier refrigerators are utilized in cooling small compact bodies that lack mechanical moving parts and can be regulated to precise, stable temperatures. Some applications of thermoelectric modules include: calorimeters, environmental analyzers, infrared detectors, compact heat exchangers, water and beverage coolers, infrared detectors and laser collimators.
Advantages of thermoelectric systems
- Temperature control- precise temperatures can be set using closed loops in the range ±0.1°C.
- Reliability- due to its construction (in ideal conditions), the life span is >200,000 hours.
- Eco friendly- the systems do not use or emit harmful particles or gases and any wasted heat energy can be recycled.
- Heating and cooling modules present in same system- not required to have two separate modules.
- Compactness- it is lightweight and small, with no moving parts, hence maintenance free.
- Reliable source of energy- only electric current and electromotive force are required.
- High scalability- can be applied in heat sources of any size.
- Cost effective- the production cost is low.
Disadvantages of thermoelectric systems
- The energy conversion efficiency is very low (5-8%).
- The module requires a relatively constant heat source to work effectively.
- The resistance output is high.
The limitations of thermoelectricity include: lack of enough industrial education, slow technological progression and adverse thermal pollution (in the event of occurrence), damaging water ecosystems. The limited primary energy sources has caused environmental concern due to emissions, leading to demand for new technologies to generate electrical power.
Thermoelectric power generators can be used as green technology due to their ability to convert waste heat energy into electrical energy, which can be used to improve the overall module efficiency. Waste heat being discharged from industries can be utilized.
Thermoelectric power generation is in a developmental phase, with low efficiency but outstanding advantages, it could be a feasible energy source in the future.
More research on thermoelectric generators is needed to find suitable thermoelectric materials that can withstand higher temperatures of various heat sources at a reasonable cost and performance.
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Int. Journal of Applied Sciences and Engineering Research (2012) Vol. 1, No. 2
The Editors of Encyclopaedia Britannica, William L. Hosch (Feb 03 2009). Electricity: Thermoelectricity by Johann Wilhelm Ritter. http://www.britannica.com
Walker, Kris. (2019). How Can Thermo Electrical Generators Help the Environment? AZoCleantech. Retrieved November 19, 2020