PRODUCTION OF ELECTRICITY FROM EXHAUST GAS

PRESENTED BY S.BHUVANESHWARI(10EIR021)

T.KEERTHANASHREE(10EIR051)

EMAIL:tkeerthanashreeeie@gmail.com

bhuvanas111@gmail.com

ABSTRACT: The energy crisis has spread widely in the world. Everyone is looking for an alternative source of energy or an effective energy conservation method. In this paper we have said about one of the energy conservation methods from the exhaust gases using modern electrical technology. Generally the efficiency of an automobile engine is around 35-40%. Losses due to exhaust gases will account to about 35%. The other losses are due to friction and in the cylinder walls. The loss due to exhaust gas cannot be reduced, but can be utilized usefully. The production of electricity is possible from the waste exhaust gas heat. This paper describes the first approach modelling another concept of low-emission vehicles, i.e. a vehicle powered by an internal combustion engine (ICE), where a waste-heat-utilisation–unit in the exhaust system is applied. The recovery and conversion of waste heat in the exhaust system of ICE-powered vehicles is very attractive, because as a rule of thumb about one third of the fuel energy is emitted with the exhaust to the ambient. There are several approaches to convert exhaust energy into useful energy, e. g. applying a thermal engine, which is powered by the exhaust. Another possibility are thermoelectric devices, which convert heat into electricity. However, in almost all concepts for waste heat utilisation a heat exchanger in the exhaust system has to be integrated and additionally a heat sink has to be provided to run the complete process. As an example for a waste-heat-utilisation-unit a thermoelectric device is considered in this paper. Almost 65% of exhaust gas in automobiles has been sent through exhaust pipe. The exhaust gas temperature will be around 2000C to 5000C. This exhaust heat is wasted and cause pollution to atmosphere. This heat

can be used efficiently using the basic law of physics. Seebeck effect states that “Two dissimilar metals maintained at different temperature, when connected by a wire, will produce potential difference “. This is called seebeck voltage. Peltier-Effect and the heat release caused by electricresistances in the thermoelectric material are taken into account as proposed in . Traditional thermoelectric materials are PbTe, Bi2Te3, SiGe, BiSb or FeSi2.Thermoelectric devices based on these materials allow efficiencies – i.e. electrical power divided by the heat flow dragged through the thermoelectric material - of around 5%, whereby temperature differences around 200 K are required.Due to recent innovations in thermoelectric materials,which promise higher efficiencies – higher than 10% - their application for waste heat conversion is gaining more interest .

INTRODUCTION:

Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating. These materials can be competitive with fluid-based systems, such as two-phase air conditioning compressors or heat pumps, or used in smaller-scale applications such as in automobile seats, night-vision systems, and electrical-enclosure cooling. More widespread use of thermoelectrics requires not only improving the intrinsic energy-conversion efficiency of the materials but also implementing recent advancements in system

architecture. These principles are illustrated with several proven andpotential applications of thermoelectrics.

FORMS OF ENERGY IN AUTOMOBILE FUEL:

EXHAUST GAS:

The IC engine emits the exhaust gas at the end of exhaust stroke at very high temperature. The temperature of the exhaust gas will e around 700-8000C. The gas is to be sent through the exhaust gas pipe to the atmosphere. The exhaust pipe has many chambers and it will purify the gas and sent into the atmosphere. The exhaust gas will emit its heat to the inner walls of the exhaust pipe and so the exhaust pipe will also be in the same temperature around 7000C. Thus the

pipe is unable for us to touch. The gas coming out from the gas outlet will not be as hot as inside the engine due the radiation.

Thermoelectrics As Heat Engines: TE devices are solid-state heat engines. Unlike today’s air conditioners, which use two-phase fluids such as the standard refrigerant R-134A, TE devices use electrons as their working fluid. Figure 1 demonstrates the principal effects that govern their performance. In 1834, Peltier observed that if a current is applied across a junction of dissimilar electrically conductive materials, either heating or cooling can occur at the junction. When the current is reversed, the opposite effect is observed. Electric current is propagated by electrons in n-type materials and by holes (traveling in the opposite direction) in p-type materials, be they semiconductors,metals, or semimetals. If voltage is applied in the right direction across a p-n junction, electron/hole pairs are created in the vicinity of the junction. Electrons will flow away from the junction in the n-type material, and holes will flow away in the p-type material. The energy to form them comes from the junction region, cooling it. On the opposite end, electrons and holes stream toward junctions where pairs recombine. This process releases energy and heats the junctions. At the bottom of Fig. 1 is a typical TE module, configured so that all junctions on one side heat andthose on the other side cool.

SEEBECK EFFECT:

The discovery of thermoelectricity dates back to Seebeck. In 1821 he discovered that a compass needle deflected when placed in the vicinity of a closed loop formed from two dissimilar metal conductors if the

junctions were maintained at different temperatures. He also observed that the magnitude of the deflection was proportional to the temperature difference and depended on the type of conducting material, and does not depend on the temperature distribution along the conductors.

Seebeck effect states that “Two dissimilar metals maintained at different temperature, when connected by a wire, will produce potential difference“. Seebeck tested a wide range of materials, including the naturally found semiconductors ZnSb and PbS. The Seebeck coefficient is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1K exists between those points.

For a given combination of materials, the voltage difference varies in direct proportion to the temperature difference. This phenomenon can be utilized for the accurate measurement of temperature by means of a thermocouple in which one wire junction is maintained at a known reference temperature (for example, in an ice bath) and the other at the location where the temperature is to be measured. At moderate temperatures (up to about 260° C/500 F), wire combinations of iron and copper, iron and constantan (a copper-nickel alloy), and copper and constantan are frequently used. At high temperatures (up to 1649° C/3000° F), wires made from platinum and a platinum-rhodium alloy are employed. Because thermocouple wires can be made very small, they also provide a means for the accurate measurement of local spot temperatures. The current can be increased by using semiconductors instead of metals, and a few watts of power can be produced at efficiencies of up to 6 percent (Transistor). Such

thermoelectric converters, powered by kerosene lamps, are widely used in Russia and other republics of the Commonwealth of Independent States to provide power for radio receivers in remote areas.

V= a(Th-Tc)

The voltage difference, v, produced across the terminals of an open circuit made from a pair of dissimilar metals, A&B, whose two junctions are held at different temperature, is directly proportional to the difference between the hot and cold junctions temperature (Th-Tc).

THERMOELECTRIC GENERATOR(TEG):

Thermoelectric generators are solid-state devices that convert heat into electricity. TEG devices generate electricity via the Seebeck Effect, where electricity is produced from a temperature differential applied across the device. The temperature difference (∆T) between the hot (Th)and the cold (Tc) sources leads to a difference in the Fermi energy (∆EF) across the thermoelectric material yielding a potential difference, which drives a current.

The simplest thermoelectric 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 basic advantage of the thermoelectric generator contemplated by this invention is a direct consequence of its solid state construction. The electric energy is created without rotary or moving machinery and thus the power may be produced over an extended period of time and at an extremely reduced cost due to limited wear and maintenance. By utilizing a modular configuration, generators having various power output ranges may be assembled utilizing similar elements. By employing mass production techniques for the Modular structures the generation of thermoelectric energy becomes economically feasible.

For more electrical energy output the TEG is connected together. This is called thermoelectric generator. A typical thermoelectric module consists of pairs of p-type and n-type semiconductor thermo elements forming thermocouples which are connected electrically in series and thermally in parallel.

A thermoelectric generator module which is formed with a hot side heat

exchanger having extruded fins on one surface and in contact with a series of individual thermoelectric semiconductor modules on the opposite side of the exchanger. A cold side heat exchanger attached to the opposite side of the semiconductor modules from the hot side heat exchanger, producing a thermal gradient across the semiconductor modules. The semiconductor modules are placed in an arranged pattern so that a maximum of heat flow through the modules is produced. Each semiconductor module is connected electrically to each other so that their output may be combined to produce a large quantity of electric power. A series of generator modules may be interconnected in a series or parallel combination to form a thermoelectric generator of varying power output.

PRODUCTION OF ELECTRICITY FROM EXHAUST GAS:

The electricity can be produced from the exhaust gas heat using the above said theory. The thermoelectric generator/module is connected to the exhaust gas pipe. The electricity

produced is directly connected to the battery in the automobile. The electricity produced is according to the amount of heat in the exhaust gas pipe. Normally the temperatures will be around 500-7000c.

TEG has demonstrated output power levels of >300mW at ∆T of 70°K and >500mW at ∆T of 120°K. With modules measuring just 3.5mm x 3.5mm, the TEG has corresponding output power densities of ~ 1 – 3W/cm2.The thermoelectric generator will extract waste heat from the exhaust that will deliver DC electrical power to recharge the battery. By reducing or even Eliminating the need for the alternator, the load on the engine is reduced thereby improving fuel efficiency by as much as 10%. Instead of recovering Waste heat, Co-generation recovers some of the useful work wasted on heat. Often a high energy content fuel with a high flame temperature (such as natural gas) is used for low ΔT heating (e.g. home heating or hot water). In electricity–heat cogeneration, electricity is produced with nearly 100% efficiency (as opposed to ~40% for power plants) because the remaining energy is used for heating instead of being wasted. In applications such as home co-generation, the desire for silent, vibration, and maintenance free operation will favor thermoelectrics.

The amount of electricity will be around 25w for 400-5000c and this will increase according to the increase in temperature of the exhaust heat. The thermoelectric module up to 50w is also available. But the material should withstand the temperature.

To produce the electricity as above said Bismuth Telluride is used. The

number of thermoelectric module used is 24.The power production calculation is done and the graph is drawn between the temperature drop across the module and the power produced. From the graph we can infer that the power increases as the temperature

ADVANTAGES:

Charging of the battery is enhanced through this process.

The efficiency of the engine is increased up to 10-50%.

The process is safe and decreases the temperature of the exhaust gases.

The installation cost is very cheap.

The energy is conserved in a very effective way.

CONCLUSION:

Though the exhaust gases can be used in many ways this method of producing the current will benefit us in charging the battery.