Thermo Electric Power Generation.pdf

By R Naresh MED,GITAM SoT GITAM UNIVERSITY Hyderabad campus

 Thermo

electric power generation  Thermo ionic power generation  Magneto hydro dynamic systems

The pioneer in thermoelectric was a German scientist Thomas Johann Seebeck (1770-1831) Thermoelectricity refers to a class of phenomena in which a temperature difference creates an electric potential or an electric potential creates a temperature difference. Thermoelectric power generator is a device that converts the heat energy into electrical energy based on the principles of Seebeck effect Later, In 1834, French scientist, Peltier and in 1851, Thomson (later Lord Kelvin) described the thermal effects on conductors

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In the purer metallic conductors outer electrons, less connected to others, can move freely around all the material, as if they do not belong to any atom. These electrons transmit energy one to another through temperature variation, and this energy intensity varies depending on the nature of the material. If two distinct materials are placed in contact, free electrons will be transferred from the more “loaded” material to the other, so they equate themselves, such transference creates a potential difference, called contact potential, since the result will be a pole negatively charged by the received electrons and another positively charged by the loss of electrons.

When the junctions of two different metals are maintained at different temperature, the emf is produced in the circuit. This is known as Seebeck effect.

The material A is maintained at T+∆T temperature The material B is maintained at temperature ‘T’. Since the junctions are maintained at different temperature, the emf ‘V’ flows across the circuit.

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The electric potential produced by a temperature difference is known as the Seebeck effect and the proportionality constant is called the Seebeck coefficient.

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If the free charges are positive (the material is p-type), positive charge will build up on the cold which will have a positive potential.

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Similarly, negative free charges (n-type material) will produce a negative potential at the cold end.

Whenever current passes through the circuit of two dissimilar conductors, depending on the current direction, either heat is absorbed or released at the junction of the two conductors. This is known as Peltier effect.

 Thomson  Heat

effect

is absorbed or produced when current flows in material with a certain temperature gradient. The heat is proportional to both the electric current and the temperature gradient. This is known as Thomson effect.

 Irreversible

conversion of electrical energy into heat when a current I flows through a ressistance R.

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Qj=I2R

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Thermoelectric power generation (TEG) devices typically use special semiconductor materials, which are optimized for the Seebeck effect.

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The simplest thermoelectric power generator consists of a thermocouple, comprising a p-type and n-type material connected electrically in series and thermally in parallel.

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Heat is applied into one side of the couple and rejected from the opposite side.

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An electrical current is produced, proportional to the temperature gradient between the hot and cold junctions.

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• • • •

Therefore, for any TEPG, there are four basic component required such as Heat source (fuel) P and N type semiconductor stack (TE module) Heat sink (cold side) Electrical load (output voltage)

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As the heat moves from hot side to cold side, the charge carrier moves in the semiconductor materials and hence the potential deference is created.

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The electrons are the charge carriers in the case of Ntype semiconductor and Hole are in P-type semiconductors.

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In a stack, number of semiconductors is connected.

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A single PN connection can produce a Seebeck voltage of 40 mV.

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The heat source such as natural gas or propane are used for remote power generation

P-type

and

N-type

 Power

P= I2RL

V=IR 2   s12 T  I= V/R = P    RL  R  RL 

  s12 2 T 2    P max = (when R=RL) = P    4R   

Figure of merit

  s12 2  Z=   R   

Efficiency of the generator =

Energy provided to the load Heat energy absorbed at the hot junction

w I 2Rl   qh (1 2 )ITh  KT  0.5I 2R 

Rl m R

Max. Ideal efficiency  Th Tc  1 ZTm 1   max    Th  1 ZTm Tc / Th 

(1   2 ) 2 Z [(k1 /  1 )1/ 2  (k 2 /  2 )1/ 2 ]2

Tm 

(Th  Tc ) 2

(1   2 )T I R  Rl where: w is the power delivered to the external load and qH is the positive heat flow from source to sink

(1   2 ) 2 Z KR

(kA) K l ( l ) R A k KR  k  

 Figure

of merit

 Diagram  The

shows

charge buildup at cold side

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A high electrical conductivity is necessary to minimize Joule heating and low thermal conductivity helps to retain heat at the junctions and maintain a large temperature gradient. A large Seebeck coefficient is advicable.These three properties were later put together and it is called figure-of-merit (Z).

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The good thermoelectric materials should possess 1. Large Seebeck coefficients 2. High electrical conductivity 3. Low thermal conductivity

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The example for thermoelectric materials • • • •

BismuthTelluride (Bi2Te3), Lead Telluride (PbTe), SiliconGermanium (SiGe), Bismuth-Antimony (Bi-Sb)

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ZT is maximized when the product RK is minimized, where R is the total couple resistance and K is the couple thermal conductance. This is accomplished when:

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and the figure of merit for the couple is given by

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The efficiency (n) (power generation mode) of the thermoelectric couple is given by the power input to the load (W) over the net heat flow rate (QH), where QH is positive for heat flow from the source to the sink and is given below:

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Proportional to (1+ZT)1/2

Cold side

N-type

Hot side

Heat flow

V Electron flow

Easy maintenance: They works electrically moving parts so they are virtually maintenance free.

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without

any

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Environment friendly: Thermoelectric generators produce pollution. Therefore they are eco friendly generators.

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Compact and less weight: The overall cooling system is much smaller and lighter than a mechanical system.

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High Reliability:Thermoelectric modules exhibit very high reliability due to their solid-state construction • No noise: They can be used in any orientation and in zero gravity environments. Thus they are popular in many aerospace applications. •

Convenient Power Supply: They operate directly power source.

no

thermoelectric comparable

from

a

DC

Cryogenic IR Night Vision

Water/Beer Cooler Si bench

TE

Electronic Cooling

Laser/OE Cooling

Cooled Car Seat

31

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The standard material we work with is BiTe. The best efficiency that can be achieved with this material is approximately 6%.

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But once the material is constructed into a module, efficiency drops to 3 to 4% because of thermal and electrical impedance. No other semiconductor material can perform as well as BiTe as far as efficiency is concerned. Other material such as PbTe are used but are far less efficient, and must be used at significantly higher temperatures (450°C- 600°C) hot side and are not commercially available!

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Thermoelectric Seebeck effect modules are designed for very high power densities, on the order of 50 times greater than Solar PV!

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Bismuth telluride is the best bulk TE material with ZT=1 Trends in TE devices: • Superlattices and nanowires: Increase in S, reduction in k • Nonequilibrium effects: decoupling of electron and phonon

transport • Bulk nanomaterial synthesis 

Trends in TE systems • Microrefrigeration based on thin film technologies • Automobile refrigeration • TE combined with fluidics for better heat exchangers

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To match a refrigerator, an effective ZT= 4 is needed To efficiently recover waste heat from car, ZT = 2 is needed

 Thermionic

emission is the basis for the working of this system.  In 1873, the Britain professor Frederic Guthrie invented the Thermionic phenomenon.  In 1883, Thomas A. Edison observed that the electrons are emitted from a metal surface when it was heated. This effect is called Edison effect.  Later in 1904, a British physicist John Ambrose Fleming developed two-element vacuum tube known as diode.

 Thermionic

power generator (TPG) is a static device that converts heat energy into electrical energy by boiling electrons from a hot emitter surface (= 1800K) across a small inter electrode gap (< 0.5 mm) to a cooler collector surface (= 1000K)

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The idea of electrons leaving the surface

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A thermionic energy converter (or) thermionic power generator is a device consisting of two electrodes placed near one another in a vacuum.

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One electrode is normally called the cathode, or emitter, and the other is called the anode, or plate.

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Ordinarily, electrons in the cathode are prevented from escaping from the surface by a potential-energy barrier.

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When an electron starts to move away from the surface, it induces a corresponding positive charge in the material, which tends to pull it back into the surface.

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To escape, the electron must somehow acquire enough energy to overcome this energy barrier.

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At ordinary temperatures, almost none of the electrons can acquire enough energy to escape.

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However, when the cathode is very hot, the electron energies are greatly increased by thermal motion.

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At sufficiently high temperatures, number of electrons are able to escape.

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The liberation of electrons from a called thermionic emission

a

considerable

hot surface is

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For the electrons to travel, the unit is at vacuum. This limit the size of the generator.

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Electron emission is inhibited by space charge, small quantity of Cesium metal is introduced into the evacuated vessel

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Molybdenum, tantalum, tungsten impregnated barium oxide.Uranium carbide, zirconium carbide.

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Prototype combustion-heated thermionic systems for domestic heat and electric power cogeneration

 Advantages • •

Higher efficiency and high power density Compact to use

 Disadvantages • • •

There is a possibility of vaporization of emitter surface Thermal breaking is possible during operation The sealing is often gets failure

 An

MHD generator is a device for converting heat energy of a fuel directly into electrical energy without conventional electric generator.

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In advanced countries MHD generators are widely used but in developing countries like INDIA, it is still under construction, this construction work in in progress at TRICHI in TAMIL NADU, under the joint efforts of BARC (Bhabha atomic research center), Associated cement corporation (ACC) and Russian technologists.

Magneto hydrodynamics (MHD) (magneto fluid dynamics or hydro magnetics) is the academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water. The word magneto hydro dynamics (MHD) is derived from magnetomeaning magnetic field, and hydromeaning liquid, and -dynamics meaning movement. The field of MHD was initiated by Hannes Alfvén , for which he received the Nobel Prize in Physics in 1970 Hannes Alfvén

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This effect is a result of FARADAYS LAWS OF ELECTRO MAGNETIC INDUCTION. (i.e. when the conductor moves through a magnetic field, it generates an electric field perpendicular to the magnetic field & direction of conductor). The induced EMF is given by Eind = u x B where u = velocity of the conductor. B = magnetic field intensity. The induced current is given by, Iind = C x Eind where C = electric conductivity The retarding force on the conductor is the Lorentz force given by Find = Iind X B

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The conducting fluid flow is forced between the plates with a kinetic energy and pressure differential sufficient to over come the magnetic induction force Find.

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An ionized gas is employed as the conducting fluid.

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Ionization is produced either by thermal means I.e. by an elevated temperature or by seeding with substance like cesium or potassium vapors which ionizes at relatively low temperatures.

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The atoms of seed element split off electrons. The presence of the negatively charged electrons makes the gas an electrical conductor.

90% conductivity can be achieved with a fairly low degree of ionization of only about 1%.

 Open

cycle MHD

 Closed

cycle MHD Seeded Inert gas system. Liquid metal system  Temperature of CC MHD plants is very less compared to OC MHD plants. It’s about 1400oC.

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The fuel used maybe oil through an oil tank or gasified coal through a coal gasification plant

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The fuel (coal, oil or natural gas) is burnt in the combustor or combustion chamber.

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The hot gases from combustor is then seeded with a small amount of ionized alkali metal (cesium or potassium) to increase the electrical conductivity of the gas.

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The seed material, generally potassium carbonate is injected into the combustion chamber, the potassium is then ionized by the hot combustion gases at temperature of roughly 2300’ c to 2700’c.

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To attain such high temperatures, the compressed air is used to burn the coal in the combustion chamber, must be adequate to at least 11000c.

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A lower preheat temperature would be adequate if the air is enriched in oxygen. An alternative is used to compress oxygen alone for combustion of fuel, little or no preheating is then required. The additional cost of oxygen might be balanced by saving on the preheater.

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The hot pressurized working fluid leaving the combustor flows through a convergent divergent nozzle. In passing through the nozzle, the random motion energy of the molecules in the hot gas is largely converted into directed, mass of energy. Thus , the gas emerges from the nozzle and enters the MHD generator unit at a high velocity.

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In a closed cycle system the carrier gas operates in the form of Brayton cycle. In a closed cycle system the gas is compressed and heat is supplied by the source, at essentially constant pressure, the compressed gas then expands in the MHD generator, and its pressure and temperature fall. After leaving this generator heat is removed from the gas by a cooler, this is the heat rejection stage of the cycle. Finally the gas is recompressed and returned for reheating.

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The complete system has three distinct but interlocking loops. On the left is the external heating loop. Coal is gasified and the gas is burnt in the combustor to provide heat. In the primary heat exchanger, this heat is transferred to a carrier gas argon or helium of the MHD cycle. The combustion products after passing through the air preheater and purifier are discharged to atmosphere.

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Because the combustion system is separate from the working fluid, so also are the ash and flue gases. Hence the problem of extracting the seed material from fly ash does not arise. The flue gases are used to preheat the incoming combustion air and then treated for fly ash and sulfur dioxide removal, if necessary prior to discharge through a stack to the atmosphere.

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The loop in the center is the MHD loop. The hot argon gas is seeded with cesium and resulting working fluid is passed through the MHD generator at high speed. The dc power out of MHD generator is converted in ac by the inverter and is then fed to the grid.

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When a liquid metal provides the electrical conductivity, it is called a liquid metal MHD system.

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An inert gas is a convenient carrier

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The carrier gas is pressurized and heated by passage through a heat exchanger within combustion chamber. The hot gas is then incorporated into the liquid metal usually hot sodium to form the working fluid. The latter then consists of gas bubbles uniformly dispersed in an approximately equal volume of liquid sodium.

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The working fluid is introduced into the MHD generator through a nozzle in the usual ways. The carrier gas then provides the required high direct velocity of the electrical conductor.

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After passage through the generator, the liquid metal is separated from the carrier gas. Part of the heat exchanger to produce steam for operating a turbine generator. Finally the carrier gas is cooled, compressed and returned to the combustion chamber for reheating and mixing with the recovered liquid metal. The working fluid temperature is usually around 800’c as the boiling point of sodium even under moderate pressure is below 900’c.

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At lower operating temp, the other MHD conversion systems may be advantageous from the material standpoint, but the maximum thermal efficiency is lower. A possible compromise might be to use liquid lithium, with a boiling point near 1300’c as the electrical conductor lithium is much more expensive than sodium, but losses in a closed system are less.

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It has no moving parts & the actual conductors are replaced by ionized gas (plasma). The magnets used can be electromagnets or superconducting magnets. The plasma temperature is typically over 2000 °C, the duct containing the plasma must be constructed from nonconducting materials capable of withstanding this high temperature. The electrodes must of course be conducting as well as heat resistant.

Superconducting magnets of 4~6Tesla are used. Here exhaust gases are again recycled & the capacities of these plants are more than 200MW. Non-conducting walls of the channel must be constructed from an exceedingly heat-resistant substance such as yttrium oxide or zirconium dioxide to retard oxidation

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Ionization of GAS:

 Various methods for ionizing the gas are available, all of which depend on imparting sufficient energy to the gas. The ionization can be produced by thermal or nuclear means. Materials such as Potassium carbonate or Cesium are often added in small amounts, typically about 1% of the total mass flow to increase the ionization and improve the conductivity, particularly combustion of gas plasma

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In MHD the thermal pollution of water is eliminated. (Clean Energy System)

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Use of MHD plant operating in conjunction with a gas turbine power plant might not require to reject any heat to cooling water.

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These are less complicated than the conventional generators, having simple technology.

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There are no moving parts in generator which reduces the energy loss.

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These plants have the potential to raise the conversion efficiency up to 55-60%. Since conductivity of plasma is very high (can be treated as infinity).

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It is applicable with all kind of heat source like nuclear, thermal, thermonuclear plants etc. Extensive use of MHD can help in better fuel utilization.

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The construction of superconducting magnets for small MHD plants of more than 1kW electrical capacity is only on the drawing board.

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Difficulties may arise from the exposure of metal surface to the intense heat of the generator and form the corrosion of metals and electrodes.

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Construction of generator is uneconomical due to its high cost.

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Construction of Heat resistant and non conducting ducts of generator & large superconducting magnets is difficult.

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MHD without superconducting magnets is less efficient when compared with combined gas cycle turbine.