Thermo Electric conversion system

Thermo Electric conversion system

Thermo electric Conversion System:

The quest for a reliable, silent, energy converted with no moving parts that transforms heat to electrical power has led engineers to reconsider a set of phenomena called the Thermoelectric effects. These effects, known for over a hundred years, have permitted the development of small, self contained electrical power sources.

Seebeck (thermoelectric) effect:

The German Scientist Seebeck (in 1822) discovered that if two dissimilar material are joined to form a loop and the two junctions maintained at different temperatures, and e.m.f will be set up around the loop. The magnitude of e.m.f. will be E = aDT where DT is the temperature difference between the two junctions and ais the Seebeck co-efficient. This effect has long been used in thermocouples to measure temperatures.

Thermoelectric Power Generator:

Figure shows a schematic diagram of a thermoelectric power generator. The thermocouple materials A and B are joined at the hot end, but the other ends are kept cold; an electric voltage or electromotive force is then generated between the cold ends. A.D.C (Direct Current) will flow in a circuit or load connected between these ends. The flow of current will continue as long as the heat

is supplied to the hot junction and removed from the cold ends. For a given thermocouple, the voltage and electric power output are increased by increasing the temperature difference between the hot and cold ends.

In a practical thermoelectric converter, several thermocouples are convected in series to increases both voltage and power as shown in figure. If the output voltage is insufficient to operate a particular device or equipment, it can be increased, with little loss of power, by an inverted transformers combination. The direct current generated by the thermocouples is first changed into alternating current of essentially the same average by means of an inverter. The alternating current and voltage in then increased to the desired value with the help of a transformer. The high voltage alternating current can be reconverted into direct current if required, by the use of a rectifier.

The source of heat for a thermoelectric generator may be small oil or gas burner, a radio- isotope or direct solar radiation.

Figure: Thermoelectric generator.

Figure: Thermocouples in series (to increase voltage).

A typical couple operating with hot and cold junction temperatures of 600°C and 200°C could be designed to give about 0.1 V and 2 A i.e., about 0.5 W, so that a 1 kW device could require about 5000 couples in series.

Taking into account mechanical characteristics, stability under operating conditions and ease of fabrication, Bismuth telluride appears to be most suitable material. It can be alloyed with such materials as Bismuth selenide, Antimony telluride, Lead selenide and Tin telluride to give improved properties.

Research is being carried out on the possibility of using thermoelectric devices within the core of a nuclear reactor. The hot junction would be located on the fuel element and the cold junctions in contact with the coolants.