Thermoelectric & Thermionic conversions Page 124

By Electromagnetic force 전자기력 (Energy) Current 전류 Flow of electrons When electrons flow? Potential difference [VOLTAGE (+ & -)] path is provided  By Electromagnetic force 전자기력 (Energy)

Basics of thermoelectric conversion Korean definition available at Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Korean definition available at http://happy8earth.tistory.com/434

(Thomas Johann Seebeck, 1821) Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Solid device Thermal energy Electrical energy By Seebeck effect (Thomas Johann Seebeck, 1821)

Temperature difference produce voltage Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Circuit made from two dissimilar (different-다른) metals When junctions maintained at different temperature Temperature difference produce voltage How to know the circuit? By deflection (change the position) 처짐 of compass magnet. Magnitude of deflection Proportional (ratio-비율) to temperature different & type of conducting material Not depends on temperature distribution along the conductor

Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Two dissimilar materials joined to form a loop Two junctions maintained at different temperatures EMF set up around the loop. Magnitude of current depends on conduction materials (Metal A and B) What is loop? A shape produced by a curve that bends around and crosses itself. Temperatures measured by thermocouples heat Seebeck effect – The principle of thermocouple

Seebeck effect Thermocouple 열전대 a sensor used to measure temperature Used in thermocouple 열전대 & spacecraft 우주선 Thermocouple Induced emf (electromotive force) is generated together with flow of current Temperature difference of two metals at two ends in closed circuit Continues current generation until temperature difference maintained between two ends High thermal efficiency achieved by semiconductors as they withstand at higher temperature. Any wastage source of heat [steam, gas, chemical fuel, solar energy & heat from nuclear reactor] used for heating

The magnitude of EMF (E-Energy) E = αs ΔT αs = Seebeck coefficient ΔT = Temperature difference between hot & cold junction

Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Reversible effect of Seebeck effect When electric current flows across isothermal (constant temperature) junction of two dissimilar (different) materials, either generation or absorption of heat at the junction. Heat generated by current flows in one direction Heat absorbed by reversed current flow.

Peltier effect (Peltier, 1834) When current passed between two dissimilar metals Reverse 반대말 process of Seebek effect Convert electrical energy into temperature difference of two ends Lenz, 1938 Heat absorption or generation at junctions depends on direction of current flow This effect applied to portable electric food coolers/warmers of small size.

Peltier’s coefficient α1-2 is Heat produced or absorbed at junction/unit current flow/unit time αp1-2 = αp1-αp2 = QP/I QP = αp1-2I I = Peltier heat per unit time

Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Thomson effect (Thomson, 1851) Heating or cooling of homogeneous conductor resulting from the flow of electrical current in the presence of temperature gradient (temperature reduction)

Positive Thomson effect Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect The absorption or evolution of heat energy, if a current is allowed to flow in a conductor having it's different parts at different temperatures is known as Thomson Effect Heated in middle point C Current flow from A to B Copper bar AB When no current flow, point M & N equidistant from C are at the same temperature When current passed A to B Temperature: N > M Similarly, Temperature B > A Sb, Ag, Zn, Cd   current passed from B to A, M will show higher temperature compared to N From A to C heat is absorbed and from C to B heat is evolved Positive Thomson effect

Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Iron bar AB heated in middle point C & current flowing from A to B. When no current is flowing, point M & N equidistant from C are at the same temperature. Similarly, A will show higher temperature as compared to B. When current is passed from A to B, M shows higher temperature compared to N. It means from A to C heat is evolved and from C to B heat is absorbed. This is known as Negative Thomson effect Similar effect is observed in the case of Pt, Bi, Co, Ni, Hg.

Therefore, in the case of lead, Thomson effect is nil. Heat it at the middle point C Current is flowing from A to B or from B to A. Lead (Pb) M & N equidistant from C same temperature Therefore, in the case of lead, Thomson effect is nil.

Thomson coefficient () dQ1/dx = -------------- dT/dx Where, dQ1/dx – Heat interchange/unit time/unit length of conductor dT/dx – Temperature gradient So, Thomson heat per unit time dQ1 dT ----- =  I ----- dx dx

Types of thermoelectric effects Seebeck effect Peltier effect Thomson effect Joule effect Joule Effect Heat energy found when electric current flows through a resistance 저항 전기. In a closed electric circuit if the current flows through a resistor R, the heat generated (Q) by the resistor is equal to I2R Q = I2R I – Peltier heat per unit time Electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. 

Construction of thermoelectric generator By using special semiconductor materials for optimization of seebeck effect. Components Thermocouple p-type & n-type material connected electrically in series & thermally in parallel Burner box Exhaust

Thermionic conversion system Emission of electrons from heated metals & some oxides surfaces is known as thermionic emission Small current production Required high temperature (1000 oC) Page 130

Consists of Metal electrodes – cathode (tungsten) & anode cesium coated tungsten Sealed container – ionized gas or cesium vapour Heat the cathode (1000 oC) emitted electrons move to anode Production of number of electrons depends on temperature Voltage developed between the electrodes Current flow in external circuit Heat converted electrical energy Accumulation of electrons in anode – Space charge Current density J = AT2 e-b/T A/m2 b = e/K Where, T – temperature of the surface K A – constant e – natural log base b – oK constant K - Boltzmann constant