Here is an essay on ‘Thermoelectric Generator’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Thermoelectric Generator’ especially written for school and college students.
Essay on Thermoelectric Generator
Essay Contents:
- Essay on the Introduction to Thermoelectric Generator
- Essay on the Principle of Operation and Performance Analysis of Thermoelectric Generator
- Essay on the Selection of Materials for Thermoelectric Generator
- Essay on the Cascading Operation of Thermoelectric Generator
- Essay on the Applications of Thermoelectric Generator
- Essay on the Limitations and Effects of Thermoelectric Generator
Essay # 1. Introduction to Thermoelectric Generators:
ADVERTISEMENTS:
A loop of two dissimilar metals develops an e.m.f when the two junctions of the loop are kept at different temperatures. This is called Seebeck effect. This effect is used in a thermocouple to measure temperature.
Thermoelectric generator is a device which directly converts heat energy into electrical energy using the Seebeck thermoelectric effect. The device is very simple but thermal efficiency is very low of the order of 3%. Efficiency of thermoelectric generator depends upon the temperature of hot and cold junctions and the material properties. The semiconductor materials have more favourable properties which can withstand high temperatures and can give reasonable efficiency.
The probability of developing peak load power stations of the order of 100 MW working at 20 percent thermal efficiency is high. Where cheap fuels are available thermoelectric generators can be developed for base load and standby power generation also.
Another important application is the use of radioactive decay heat to generate power in space and other remote locations. The use of solar energy to supply heat for generating electricity can be an attractive application of thermoelectric devices if high efficiency materials can be developed.
ADVERTISEMENTS:
The operation of a thermoelectric generator is shown in fig 14.1.
The net useful power output is given by:
The magnitude of potential difference depends on the pair of conductor materials and on the temperature difference between the junctions.
For a loop made of copper and constant wires, the value of Seebeck coefficient α is 0.04 mV/K. For a temperature difference of 600K between the junctions, a voltage of 24 mV will be developed. In order to achieve higher potential difference many generators have to be connected in parallel. For increasing the useful power output, parallel and series connections are used.
Essay # 2. Principle of Operation and Performance Analysis of Thermoelectric Generator :
ADVERTISEMENTS:
The thermo-elements of a thermoelectric generator are made up of semiconductors p and n type. Heat is supplied to the hot junction and from the cold junction heat is removed. Both the junctions are made of copper, see fig. 14.3.
When a current (7) flows through the junction of two elements, Peltier heat is produced. This is called Peltier effect. The Peltier coefficient.
From Ist Law of Thermodynamics as applied to upper plate (as control volume), the temperature difference (T1 – T0) will generate a Seebeck voltage, αpn(T1 – T0). There will be an electrical current I who will flow through the external load RL.
The heat Q̇1will flow into hot junction and conducted into the two legs, Q̇k.
The Peltier heat Q̇p will be produced at the junction due to current flowing through the circuit.
Joule heat Q̇j/2 will flow into the junction. It is assumed that half Joulean heat appears at each junction.
Example 1:
For a thermoelectric power generator following parameters are given:
Temperature of the hot reservoir = 600K
Temperature of the hot sink = 300K
Figure of merit for the material, z = 2 × 10-3 K-1. Determine the efficiency of the thermoelectric generator. What will be the Carnot efficiency?
Solution:
Essay # 3. Selection of Materials for Thermoelectric Generator:
The efficiency of a thermoelectric generator depends upon suitable properties of the elements. The material should have high value of figure of merit Z and also capable of operation at very high temperature.
i. The thermal conductivity of thermoelectric element should be as low as possible. The value of k can also be reduced by introducing suitable impurities.
ii. The mobility of current carriers (electrons or holes) should be as high as possible or electrical conductivity should be raised by introducing suitable impurities.
iii. One element should be purely p-type and the other n-type. The semiconductor should have low ionization energy and narrow forbidden band.
iv. The thermo-elements should have variable impurity content so that electron concentration should depend upon the operating temperature.
v. The material should be corrosion resistant, should have high strength, elasticity so that it does not crack due to thermal stresses.
vi. The bridge material should have high thermal and electrical conductivity and stable against thermal stresses.
vii. The use of variable properties of thermoelectric elements becomes very important when thermoelectric pile or cascaded operation is required. The working temperatures can be different.
1. Metals:
(i) For most metals, the value of Seeback coefficient is less than 10 × 10-6 V/K.
The ratio of electrical conductivity (1/ρ) to thermal conductivity (k) for all metals can be predicated from quantum mechanics.
(ii) The Figure of merit [Z = α2/ρk] at an average temperature of 500 K works out close 8.2 × 10-6.
(iii) At T1 = 700K and T0 = 300K, efficiency is 8.5 × 10-4 or 0.085 % which very low for practical application.
(iv) For the best pair of metals Bi-Sb,
2. Semi-Conductors:
Semiconductors have higher values of Seebeck coefficient. The figure of merit Z exceeds 1 × 10-3 and efficiency approaches 10%. These have higher melting point, thus permitting operation at higher temperature. The efficiency of 15% or more is attainable. Therefore, semiconductors are more suitable materials for thermoelectric generators than metals.
3. Figure of Merit:
Bismuth telluride, lead telluride, germanium and other semiconductors have properties suitable for thermoelectric generation. Alloying and “doping” make it possible to produce p-type and n-type materials. The figures of merit of some thermoelectric materials are given in Table 14.1.
High Temperature Semiconductors:
The melting point of Bi2 Te3 is 584° C and that of Pb Te is 922° C. Silicates are considered the most promising of all the high temperature materials used for thermo-electrodes. With Mn Si2, with T1 = 1300 K and T0 = 300 K, the thermal efficiency is as high as 9.7 % Boride and carbide base high temperature materials can give an efficiency of 15 to 20%.
Essay # 4. Cascading Operation of Thermoelectric Generator:
M The materials may have best figure of merit at high temperature than at low temperatures. But certain materials have favourable figure of merit at lower temperatures. The problem can be solved by cascading where heat rejected from one part of system becomes the heat input to another part. The size of device decreases from one stage to another as the average temperature is lowered. The heat input to each stage is less than to the stage above.
The overall efficiency of the system is given by:
The greater the number of stages, the greater the efficiency. If each stage of thermoelectric cascade generator is optimized for geometry and resistance ratio, each stage efficiency can be improved.
The output voltage can be increased by putting a number of thermocouple is series (Fig. 14.2). Such an arrangement is called a thermopile.
Essay # 5. Applications of Thermoelectric Generator:
The thermoelectric generators are already recognized as very convenient direct energy conversion systems.
i. These are very simple in construction.
ii. Very compact.
ii. Absence of moving parts.
iv. Suitable for remote and space applications.
v. A promising system for waste heat recovery.
1. Nuclear Reactor with Thermoelectric Fuel Elements:
A thermoelectric generator as incorporated in the fuel elements of a nuclear reactor is shown in fig. 14.4. This will help in obtaining large power outputs.
2. Combined Thermoelectric and Steam Power Plant:
Thermoelectric generator can be used as topping plant to a steam power plant. The overall efficiency of the combined plant will increase due to higher source temperature. The scheme is shown in Fig. 14.5.
3. Thermoelectric Waste Heat Stack:
The waste heat from gas turbines, diesel engines and stack gases can be used to produce electricity by a thermoelectric generator. The metal stack consists of a series of rings of two alternate metals connected at the inner and outer annular edges alternately. These rings are thermally and electrically insulted. A schematic diagram is shown in fig. 14.6.
4. Decay Heat of Radioactive Isotopes:
The decay heat of radioisotopes has been used for the operation of small (0.1 kW) thermoelectric generators. Based on heat of decay of Strontium 85, remote generators for signaling have been used.
5. Solar Energy:
A combination of thermoelectric generator and solar collector can be employed to generate electricity from solar energy.
Essay # 6. Limitations and Effects of Thermoelectric Generator:
Limitations:
The main limitation of thermoelectric generators is low thermal efficiency. Lot of work is needed to develop thermocouple elements which can give reasonable efficiencies.
Thermoelectric Effects:
Thermoelectric effects involve interchange between thermal energy and electrical energy.
There are four thermoelectric effects:
(a) Seebeck effect.
(b) Peltier effect.
(c) Thomson effect.
(d) Joule effect.
These phenomena form the basis for the design and analysis of thermoelectric generators.
(a) Seebeck Effect:
Presence of a temperature gradient in a conductor induces an electric potential gradient even when current flow is zero.
α is called the Seebeck coefficient for a single material. It has the units of volt per degree temperature difference. The value of a depends upon the materials used. The maximum induced potential gradient is called Seebeck e.m.f.
αpn is called the differential Seebeck coefficient or thermoelectric power. The variation of Seebeck coefficient with temperature is shown in Fig. 14.7. The Seebeck coefficient is positive for p type semiconductor and negative for n type semiconductor.
Therefore,
The Seebeck e.m.f will be very high if thermoelectric generator is made from p type and n type semiconductors. The heat loss due to Fourier effect will also be reduced as the thermal conductivity of semiconductor is less than that of metal.
(b) Peltier Effect:
When an electric current (I) flows through the junction of two materials, Peltier heat is produced. This is called Peltier effect. The peltier effect is reversible effect because if heat is produced when current flows in one direction, the same amount of heat is absorbed at the junction if the current flow is reversed.
Peltier heat per unit time:
Qp, n = πp, n. I
where,
πp, n is called Peltier coefficient.
Peltier coefficient is the heat produced at the junction per unit current flow, per unit time.
(c) Thomson Effect:
When an electric current flows through a material having a temperature gradient, there is production (or absorption) of heat. This phenomenon is called Thomson effect. It is also a reversible process because reversing the direction of current flow reverses the direction of heat transfer without change in magnitude.
Thomson coefficient (σ) is defined as the Thomson heat produced per unit time per unit electric current and per unit temperature gradient.
The Seebeck effect, the Peltier effect and Thomson effect are more pronounced in semiconductors as compared to metals.
(d) Joule Effect:
When a current flows through a material with resistance R, an amount of heat is produced per unit time. This is called Joule heat.
Qj = I2R
This is irreversible process.