In this article we will discuss about:-1. Principle of Thermoelectric Power Generation 2. Thermoelectric Materials in Thermoelectric Power Generation 3. Thermoelectric Power Generator 4. Applications.
Principle of Thermoelectric Power Generation:
Thermoelectric power generation process is based on the Seebeck effect which states that loop of dissimilar metals will develop an emf if the two junctions are kept at different temperatures. This principle is already being used in thermocouples for measurement of temperature. However the recent advances in material technology have made possible experimental thermo-electric generators of a few kW size.
The simplest thermo-electric power generator consists of a thermocouple, comprising a P-type and N-type thermo-element connected electrically in series and thermally in parallel. Heat is pumped into one side of the couple and rejected from the opposite side. An electrical current is produced, proportional to the temperature gradient between the hot and cold junctions.
The net useful power output is given as:
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Pout = I2R watts …(7.14)
Where I is the current flowing through the circuit in amperes and R is the external load resistance connected across output terminals of the thermo-electric generator in ohms.
The current in the circuit is given by:
I = αΔT /(Rin + R) amperes… (7.15)
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where α is Seebeck coefficient in V/k, ΔT is the temperature difference between hot and cold junctions in degrees absolute, K and Rin is the internal resistance of thermoelectric generator in ohms.
The magnitude of potential difference depends on the pair of conductor materials and on the temperature difference of the junctions.
For a loop made of copper and constantan wire, the value of Seebeck coefficient is 0.04 mV/k. For a temperature difference of 600 K between the junctions, a voltage of 24 mV will be produced.
Thermoelectric Materials in Thermoelectric Power Generation:
The efficiency of thermoelectric generator depends upon suitable properties of the elements. It was later in 1909 and 1911 that Altenkirch showed that good thermoelectric materials should have large Seebeck coefficients, high electrical conductivity and low thermal conductivity. A high electrical conductivity is necessary to minimise Joule heating, whilst a low thermal conductivity helps to retain heat at junctions and maintain a large temperature gradients.
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These three properties were later embodied in the so-called figure-of-merit, Z. Since Z varies with temperature, a useful dimensionless figure-of-merits can be defined as ZT. The thermal conductivity of thermoelectric materials can be reduced by introducing suitable impurities.
The other requirements of thermoelectric materials are:
1. The mobility of current carriers (electrons or holes) should be as high as possible. The electrical conductivity can be raised by introducing suitable impurities.
2. One element should be purely P-type and the other N- type. The semiconductor material should have low ionization energy and narrow forbidden band.
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3. The thermo elements should have variable impurity content so that electron concentration should depend upon the operating temperature.
4. The thermoelectric material should be corrosion resistant, should have high mechanical strength and elasticity so that it does not crack due to thermal stresses.
5. The bridge material should have high thermal and electrical conductivities and stability against thermal stresses.
6. The use of variable properties of thermoelectric elements becomes very significant when thermoelectric pile or cascaded operation is required. The operating temperature may be different.
Thermoelectric Power Generator:
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Thermo-generators are the devices that convert heat (temperature differences) directly into electrical energy. For the most part, this term is synonymous with “thermoelectric generator” and rarely used. Older Seebeck-based devices used bimetallic junctions and were bulky while more recent devices use semiconductor P-N junctions that can have thicknesses in the millimeter range.
These are called solid state devices and unlike dynamos have no moving parts other than sometimes a fan. Fuel such as natural gas, propane or kerosene can thus be used for generating dc which can be converted into ac by using inverter.
The principle of thermoelectric generation is shown in Fig. 7.7. The electrodes I and II are bridged at hot junction and are connected to the output terminals at the cold junction. The hot junction is kept at high temperature by concentrating sun rays on it. The cold junction is kept cold by water cooling. The electric current is set up due to difference in temperatures of hot and cold junctions and completes its path through the external load circuit.
Though it is based on direct conversion of heat into electric power but its efficiency is quite low (1 to 3%). Like any heat engine its efficiency depends upon the temperatures of the hot and cold junctions. A two stage device with hot and cold junction temperatures of 1,500 K and 300 K has been developed to give an overall efficiency of 13%. A lot of research is being done to find new materials capable of working at high temperatures. Special attention is being paid to semiconductor materials.
The voltage and electrical power output can be increased by increasing the temperature difference between the hot and cold ends. 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 as shown in Fig. 7.8. The direct current generated can be changed into alternating current by using inverter. The alternating voltage is then increased to the desired value with a transformer.
Thermoelectric generators have been built with power outputs ranging from a few watts to kilowatts. An important application is the use of radioactive decay heat to generate power in space and other remote locations.
Semiconductors are good thermal insulators, so these can be used as the basis for thermal converter designs capable of withstanding large temperature gradients and hence generating larger thermo emfs. At present, semi-conductor materials like lead telluride, zinc antimonite, bismuth telluride, germanium silicide and the various selenides produce power with an efficiency of about 10%.
Ternary compounds (such as silver-antimony telluride, lead- tin telluride) and quaternary compounds composed of bismuth, tellurium, selenium and antimony, called as neelium, are promising ones and higher efficiencies are expected to be achieved in the future, Extensive studies are currently being carried out with the aim to develop semiconductors capable of operating at high temperatures. Basically, thermal converters can employ liquid semiconductors. However, the main drawback of using liquid semiconductors is that the rate of heat transfer from the hot to cold junction is increased by convection.
Energy used for heating the hot junction of the thermal converter may be energy content of chemical fuels or heat generated in nuclear reactors besides solar radiant energy.
The main advantages of thermal converters are absence of moving parts, non-involvement of high pressures and use of any heat source. They are readily serviceable and quiet in operation. Thermal converters are preferred over other power sources from the point of view of reliability, compactness, portability and ease of use. These are widely employed as power sources on space crafts, submarines, missiles, beacons, and other equipment of different kinds.
The main drawbacks of thermal converters are high cost, low efficiency and low outputs.
Applications of Thermoelectric Power Generation:
The thermoelectric generators are recognized as very convenient direct energy conversion systems. These are quite suitable for remote and space applications and promising systems for waste heat recovery.
1. Nuclear Reactor with Thermoelectric Fuel Elements:
A thermoelectric generator, as incorporated in the fuel elements of a nuclear reactor. This will help in obtaining large outputs.
2. Combined Thermoelectric and Steam Power Plant:
Thermoelectric generator can be employed 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. 7.10.
3. Thermoelectric Waste Heat Stack:
The waste heat from gas turbines, diesel engines and stack gases can be used to generate 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 insulated. A schematic diagram is shown in Fig. 7.11.
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 signalling have been used.
5. Solar Energy:
A combination of thermoelectric generator and solar collector can be used to generate electrical energy from solar energy.