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Basic Principle, Selection, Properties and Applications of Thermoelectric Materials Abraham Shibu¹, Ashish Noble Jacob V², Jom Abraham John³, Viswanath Reddy S⁴ ¹ ² ³ &⁴Department of Mechanical Engineering, Karunya University, Coimbatore, India ARTICLE INFO Abstract history: 14 November 2016 Keywords Smart materials, thermoelectric materials, ZT value, seebeck coefficient, thermoelectric generators, thermal insulating box ABSTRACT Smart materials have properties that react to changes in their environment. This means that one of their properties can be changed by an external condition, such as temperature, light, pressure or electricity. This change is reversible and can be repeated many times. Thermoelectric materials are used to build devices that convert temperature differences into electricity and vice versa. The working principle of thermoelectric material is based on three effects, discovered by Seebeck, Peltier and Thomson. The amount of power generated using thermoelectric material depends on a dimensionless figure of merit called ZT, which depends upon seebeck coefficient, thermal conductivity, temperature, electrical conductivity. Here we discuss about SnSe (thermoelectric materials) and its behaviour at various temperatures .This paper will be explaining how and where this material is used in our day to day practices. For the application of the thermoelectric material a hot and cold reservoir is necessary. A set up of consisting of an thermal insulating box in which the thermo electric material is placed in the middle in such a way that there is no heat transfer between the two regions. Thermal conductive metal such as aluminium is used to transfer the heat from the two reservoirs to the thermo electric material. INTRODUCTION Thermoelectric material is the most essential and primary input for almost all activities now a days. One can’t even imagine a day without electricity. As we know, electricity can be generated through mechanical energy, wind energy, nuclear energy, etc. Now a day, energy is produced through non renewable resources like coal, which causes pollution to the environment. There are so many other energy sources which can be converted into electrical energy. Energies are inter-convertible; like mechanical energy can be converted into electrical energy and wind energy can be converted into electrical energy and vice versa. Similarly, we can also convert the energies that are in the form of light, sound, heat, etc into electrical energy. Light energy can be converted into electrical energy by solar panels. Sound energy and heat energy can also be converted into electrical energy by using smart materials. Heat energy is getting wasted in most of conventional power plants, vehicles, home appliances, etc. Here we discuss about the harnessing of waste heat energy by using smart materials. Thermal energy can be directly converted into electrical energy by using a smart material called thermo-electric material. BASIC PRINCIPLE Thermoelectric materials are used to build devices that convert temperature differences into electricity and vice versa. The working principle of thermoelectric material is based on three effects, discovered by Seebeck, Peltier and Thomson. A thermocouple is a device used extensively for measuring temperature. The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows: 1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals. 2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit. 3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit. In most of the cases the emf suggested by the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple. The basic theory behind this Thermo Electric Generator is "seebeck effect". Seebeck effect was discovered by Thomas Seebeck in 1821. When a temperature difference is recognized between the hot and cold junctions of two dissimilar materials (metals or semiconductors) a voltage is generated, this voltage is called Seebeck voltage. Indeed this phenomenon is applied to thermocouples that are extensively used for temperature measurements. When a Thermoelectric material (Thermoelectric Module or Thermocouple) held in-between temperature gradient it generate some voltage . In fact, this phenomenon is applied to thermocouples that are extensively used for temperature measurements. Base on this Seebeck effect, thermoelectric devices can act as electrical power generators. Temperature. Mostly Ceramics material for this which is Al2O3. It also transfer temperature to the modules from hot side. It should be thick. Thermoelectric Power Generator (TEG) is a solid state device which converts Heat Energy into Electrical Energy. All the exciting conventional power generators convert Thermal Energy into Mechanical Energy then to Electrical Energy. So here no mechanical work ( no moving parts). So it produce less noise and no pollution when compare to conventional power generators. TEG is working by Thermo Electric Effect (seebeck) effect. When TEG held between temperature gradients (Hot end, Cold end) it produce some voltage this voltage is called seebeck voltage.TEG has Modules which is semiconductors (p,n). Here electrons acting as a thermoelectric power fluid (working medium). Pair of p-type semiconductor and ntype semiconductor is called as a Module. These semiconductors highly doped by pollutants in order to increase the Electric conductivity.TEG has shield it avoid modules damaging due to high temperature. The efficiency of TEG and voltage generated by TEG is directly proportional to semiconductor material and temperature gradients. So selections of semiconductor based on electric conductivity of the material and try to increase the temperature difference value. This semiconductor is coupled by copper electrode. Increasing no of modules and no of stages and coupling no of TEG increase overall efficiency and voltage output. Exciting efficiency of TEG is 4.2% to 6%. When using stages it increases the efficiency to7%. The thermoelectric property of a thermoelectric material depends on the dimensionless figure of merit called ZT. Thermal Fin It is used here for increase the thermal gradient value. When we increase the Thermal gradient value it increase the seebeck voltage generated by TEG. This FIN also transfers the heat from Thermoelectric Module. It is made by Aluminum metal. When we include Thermal fin it increase the efficiency of the TEG. Important Parameters taken into account are: Seebeck Coefficient is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material. Thermal conductivity (often denoted k, λ, or κ) is the property of a material to conduct heat.Electrical conductivity or specific conductance is the reciprocal of electrical resistivity, and measures a material's ability to conduct an electric current. SEEBECK EFFECT Open circuit voltage, V = T α (Th – Tc) Seebeck coefficient, α = dV /dT units: V/K Seebeck coefficient = 1/q x entropy Q /T MEASURING SEEBECK COEFFICIENT Components of TEG : 1. Thermoelectric Module 2. Thermoelectric shield 3. Thermal Fin 4. Copper electrode Thermoelectric Module It is semiconductor which is highly doped by pollutants to increase the electric conductivity of the semiconductor. Good semiconductor has electric conductivity in between 200µV/K - 300µV/K. When choosing semiconductor it has to withstand that much high operating temperature. Some of the good thermoelectric module semiconductors are Bi2Te3, CaMnO, Ca3Co4O9, Sb2Te3, and PbTe Thermoelectric Shield It is a material which protects the modules damage due to high Physically heat one side of sample, Thermocouples top and bottom to measure ΔT, Cold sink at the other side of sample and 4 terminal electrical measurements DIMENSIONLESS FIGURE OF MERIT The ability of a given material to efficiently produce thermoelectric power is related to its dimensionless figure of merit given by: Z T = (σ S²T)/λ which depends on the Seebeck coefficient S, thermal conductivity λ, electrical conductivity σ, and temperature T. THERMOELECTRIC EFFICIENCY POWER GENERATING The typical efficiency of TEGs is around 5–8%. Older devices used bimetallic junctions and were bulky. More recent devices use highly doped semiconductors made from bismuth telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide (Ca2Mn3O8), or combinations thereof, depending on temperature. These are solid-state devices and unlike dynamos have no moving parts, with the occasional exception of a fan or pump. greater difference in temperature greater is the energy produced, so as this material can not with stand higher temperatures the second was selected which is not upto the mark but it gives a better ZT value compared with the first one. The third material taken for testing was tin selenide which showed a better performance than all the other materials. It gives a ZT value of 2.6 at a temperature of 923K which is a good value compared with the others. Tin selenide, also known as stannous selenide, is an inorganic compound with the formula (SnSe), where Tin has a +2 oxidation state. Tin(II) selenide is a narrow band-gap (IV-VI) semiconductor and has received considerable interest for applications including low-cost photovoltaics and memory-switching devices. Tin(II) selenide is a typical layered metal chalcogenide ; that is, it includes a Group 16 anion (Se2−) and an electropositive element (Sn2+), and it is arranged in a layered structure. Tin(II) selenide exhibits low thermal conductivity as well as reasonable electrical conductivity, creating the possibility of it being used in thermoelectric materials. In 2014, a team at Northwestern University has established the world record performance for thermoelectric material efficiency. Name and identifiers Other names -Tin(II) selenide CASR Number- 1315-06-6 Properties Chemical formula – SnSe Molar mass - 197.67 g/mo Appearance - steel gray odourless powder Density - 6.179 g/cm3 Melting point - 861 °C (1,582 °F; 1,134 K) Where Tc is the sink temperature and Th is the heat source. Thermodynamic efficiency: the competition- ZT of 4 start to become seriously competitive Solubility in water - insoluble Band gap - 0.9 eV (indirect), 1.3 eV (direct) Structure SELECTION OF MATERIALS Crystal The below mentioned are some of the materials selected for testing: Related compounds 1. β-Zn4Sb3 - ( The ZT value is 1.4 at 400°C) 2. Ge0.55Pb0.45Te – ( ZT of 1.55 at 723 K) 3. SnSe – ( ZT of 2.6 ± 0.3 at 923 K) * The first material has a ZT value of 1.4 at 673K, so for the thermodynamic efficiency to be competitive the ZT value must really reach upto 4 as mentioned above, and this ZT value is not given at a higher temperature. Therefore the first material could not give sufficient energy which we required because as there is a structure- Orthorhombic, Dangerous for the environment (N) . Toxic(T), Other anions Tin(II) oxide,Tin(II) sulphide ,Tin telluride Other cations Carbon monoselenide, Silicon monoselenide Germanium selenide,Lead selenide. THERMOELECTRIC SETUP PRACTICAL LIMITATIONS The proposed thermoelectric mainly consists of few components: Besides low efficiency and relatively high cost, practical problems exist in using thermoelectric devices in certain types of applications resulting from a relatively high electrical output resistance, which increases self-heating, and a relatively low thermal conductivity, which makes them unsuitable for applications where heat removal is critical, as with heat removal from an electrical device such as microprocessors. Thermoelectric materialTin(II)selenide, conductor rods (Aluminium or copper rods), fibre glass box. This proposed model has been modelled using pro-e. The two long conductor rods are attached to the two opposite sides of the thermoelectric material. This combined material has been inserted in a cuboid with two holes at the two faces , hole dia little greater than that of the conductor rods. The one end of the rod is connected to the heat producing agent and the other end to the colder side, this temperature difference will make the thermoelectric material produce electricity. Greater the temperature difference greater is the voltage developed. Thermoelectric material and the conductor rods Fibre glass box Assembled view of Product High generator output resistance: In order to get voltage output levels in the range required by digital electrical devices, a common approach is to place many thermoelectric elements in series within a generator module. The element's voltages add, but so do their individual output resistance. The maximum power transfer theorem dictates that maximum power is delivered to a load when the source and load resistances are identically matched. For low impedance loads near zero ohms, as the generator resistance rises the power delivered to the load decreases. To lower the output resistance, some commercial devices place more individual elements in parallel and fewer in series and employ a boost regulator to raise the voltage to the voltage needed by the load. Low thermal conductivity: Because a very low thermal conductivity is required to transport thermal energy away from a heat source such as a digital microprocessor. The relatively high thermal conductivity of a generator module relative to copper and aluminum thermal conductors used in heat sinks means the thermoelectric generator impedes the waste heat removal causing the silicon device temperature to rise significantly. Cold-side heat removal with air: In aircooled thermoelectric applications, such as when harvesting thermal energy from a motor vehicle's crankcase, the large amount of thermal energy that must be dissipated into ambient air presents a significant challenge. As a thermoelectric generator's cool side temperature rises, the device's differential working temperature decreases. Ast the temperature rises, the device's electrical resistance increases causing greater parasitic generator self-heating. In motor vehicle applications a supplementary radiator is sometimes used for improved heat removal, though the use of an electric water pump to circulate a coolant adds an additional parasitic loss to total generator output power.Water cooling the thermoelectric generator's cold side, as when generating thermoelectric power from the hot crank case of an inboard boat motor, would not suffer from this disadvantage. Water is a fare easier coolant to use effectively in contrast to air. APPLICATION For the application of the thermoelectric material a hot and cold reservoir is necessary. A set up of consisting of an thermal insulating box in which the thermo electric material is placed in the middle in such a way that there is no heat transfer between the two regions. Thermal conductive metal such as aluminium is used to transfer the heat from the two reservoirs to the thermo electric material. Water heater of primary energy which is equal to 42.9% of total primary energy consumption. About 70% of India's electricity generation capacity is from fossil fuels. India is largely dependent on fossil fuel imports to meet its energy demands. During the fiscal year 2014-2015, the per capita electricity generation in india was 1,010 kWh. The quantity of waste heat is nearly equal to the electric power generated. Therefore, when thermoelectric power generation is applied to all the cogeneration systems, the energy recovered will be 1,010 kWh x15% = 151kWh. Assuming that thermoelectric power generation is introduced to all the new cogeneration systems in 2018, it will be applied 2,34,000 MW. This corresponds to the saving of approximately 3,41,800kl of crude oil per year and the reduction of approximately 9,20,000 tons of CO2 per year. Saving the government crores in the generation of electricty. CONCLUSION The proposed product is attached to the top of the water heater one end of rod connected to hot and the other to the cold side. Air-Conditioner By harnessing the wasted heat energy and converting it into electricity, we can drastically reduce the cost and increase the power generation.More than 19000 villages in our country doesn’t have access to electricity. By installing this setup we can generate enough electricity for powering almost all the places in our country.` REFERENCES Thermoelectric materials: energy conversion between heat and electricity by Xiao Zhang, LiDong Zhao. Waste heat energy harvesting using thermo electric generator by A Jack delightus peter, Balaji D, D Gowrishankar. One end of the rod in the proposed model will be connected to the evaporator, that is cold side and the other end connected with the condenser which eject heat outside. This temperature difference can produce the electricity. And after getting the required voltage the voltage has to be stepped up using a step up transformer and through this electricity is given to different plug points from where current can be drawn. STATISTICAL ANALYSIS Concerning the energy-saving effect of thermoelectric power generation, we shall study the application of a high-efficiency thermoelectric system to a heat based power plant for cogeneration. We estimate the overall effect in India as follows. India's net imports are nearly 144.3 million tons of crude oil, 16 million tons of LNG and 95 million tons coal totalling to 255.3 million tons Role of thermo electric generator in recovery of waste heat of automobiles by Om Prakash, Mukesh Pandey, Anurag Gour, Savita Vyas.