Waste Heat from Exhaust Gases Generated from Automobiles Applications - Coursework Example

Table of Contents Table of Contents 1.0Introduction 2 2.0Review of the scientific literature 3 2.1Waste Heat from Exhaust Gases Generated from Automobiles Applications 5 3.0Application of heat transfer 7 3.1Model Description 7 3.1.1 Thermoelectric electric generator Modeling 8 3.1.2 Thermal Resistance Network 9 3.1.3 Solution Method 11 3.1.4 Varying Inlet Conditions 11 3.1.5 Average Inlet Conditions 13 3.1.6 System Efficiency 14 Conclusion 15 Work Cited 17 Abstract A numerical model has been developed to model coupled electric and thermal energy transfer processes in a thermoelectric electric generator designed for vehicles waste-heat recovery systems. The model presented has the capability to compute the overall heat transferred and the electric power output. However, the thermoelectric electric generator offers a possible application in the direct conversion of vehicle waste-heat energy. The application of this thermoelectric electric generator in converting waste heat energy directly into electric power also improves the overall productivities of energy conversion systems. This research paper presents an analysis of the basic concept of thermoelectric electric generation with its relevant and important application to vehicle waste-heat energy. Keywords: Thermoelectric electric generation, direct energy conversion, waste-heat recovery, thermoelectric materials, electric power. 1.0 Introduction A thermoelectric electric generator is a solid state device responsible for providing direct energy conversion from thermal energy (heat) as a result of a temperature gradient into electrical energy established in ‘Seebeck effect’. However, the thermoelectric electric cycle, with electrons (charge carriers) function as the working fluid, follows as the central laws of thermodynamics closely bear a resemblance to the electric cycle of a conventional heat engine. Nonetheless, thermoelectric electric generators offer numerous distinctive advantages as compared to other technologies (Goldsmid 35); They are safe, compact, and safe. They are tremendously reliable and silent in operation. This is because they do not have mechanical moving parts. They are environmental friendly. They can operate at elevated temperatures. Considerable thermal energy is available from the exhaust gas in the modern vehicle engines. Notably, two-thirds of the energy produced from combustion are lost as waste heat whereby, of this, 40% is in the form of heat exhaust gas (Goldsmid 43). The use of the thermoelectric electric has the capability to recover some of the waste heat available in the exhaust stream. This will potentially improve fuel economy by as much as 4%. 2.0 Review of the scientific literature A conclusive study concluded that a thermoelectric generator powered by exhaust heat could meet the electrical necessities of a medium-sized vehicle. For the past decades, alloy-based thermoelectric materials including Si-Ge, Bi2Te3-Sb2Te3 systems have been lengthily studied for use in their different temperature ranges (Umeda 42). Since the temperatures of vehicle exhaust gasses are in the range of 4000C to 8000C, high-temperature thermoelectric generator devices are needed for at least a part of the flow path. Reputable thermoelectric semiconductors exhibit poor figures of importance when operating temperatures that exceed 5000C (Umeda 50). However, Thomas Bauer employed a two-stage segmented thermoelectric element based on Bi2Te3-Sb2Te3 elements near the exit and half-Heusler alloy (Zr, Hf) near the hot gas inlet of a thermoelectric generator for an inline six-cylinder engine prototype. Seung Ko and Grigoropoulos Costas have employed high-temperature skutterudites for a prototype intended for general motors suburban. Increased Expanded thermoelectric productivity has been acknowledged by exploiting electronic band structure engineering and phonon engineering. Multiple- filled skutterudites have guaranteed higher ZT (>1) values in the temperature scope of 3000C to 6000C and show predominant mechanical quality, and are in this way of essential enthusiasm for the present study. Thermoelectric electric generators have verifiably been utilized in particular military and space applications. Thermoelectric converters have been utilized to power profound space tests subsequent to the 1950s because of the simplicity of versatility and the general effortlessness as contrasted and options approaches. In any case, late changes in energy change efficiencies of thermoelectric materials, joined with expanded enthusiasm for energy proficiency and mileage, have prompted an extraordinary increment in exploration into their potential arrangement in situations where thermal energy is basically free, for example, sun based radiation, car fumes, and gas turbine and diesel cycle cogeneration frameworks. Scientists in Japan have been working on oxide thermoelectric generators as a garnish cycle to expel a percentage of the heat from the steam in incinerators to abridge utilization of lavish turbines. While current anticipated thermoelectric generator efficiencies are low (regularly under 4%), the way that the energy accessible is basically free and that the units are mechanically straightforward has cultivated restored investment. Thomas Bauer evaluated vital issues to be considered for a fumes gas generator outline, for example, area, heat exchange from fumes gas, generator mass, thermoelectric material dependability, and the general natural agreeableness. This work demonstrated that the inward finning and diffuser course of action in the thermoelectric generator system were critical configuration contemplations for minimizing the temperature distinction between the hot gas and the hot side of the thermoelectric components. The first thermoelectric electric generators models were developed in the late 1960s utilizing Pb-Te- and Ge-Bi-Te-based amalgams. In the second 50% of the most recent century, models were created by Porsche, Hi-Z, Nissan Motors, and Clarkson University in a joint effort with General Motors (British 56). These thermoelectric electric generators utilized fumes gasses and motor coolant as the heat source and sink, separately. Thomas Bauer highlighted the utilization of a thermoelectric generator set in the fumes stream of games utility vehicle (SUV) and a stationary, packed regular gas (CNG)-filled motor generator set. However, a number of models have been applied to thermoelectric electric generators analysis, with fluctuating levels of sophistication. Thermal resistances over the different material interfaces, electrical burden impedance adjusting, and pivotal angles in temperature because of heat extraction are essential contemplations. Huge changes in mass gas temperature in the hub bearing are a test for keeping up the thermoelectric material execution more than an extensive variety of working conditions. As the motor execution is firmly coupled to the general weight degree, heat exchanger finning courses of action must not make an undue weight drop in the thermoelectric electric generators, or execution picks up from electrical power will be balanced by relating misfortunes in the fundamental Otto cycle. In numerous auto applications, the Reynolds numbers inside the stream way put the stream in a move locale in the middle of the laminar and turbulent conduct, consequently obscuring flow analysis. 2.1 Waste Heat from Exhaust Gases Generated from Automobiles Applications The use of waste heat energy from fumes gasses in responding inside burning motors (e.g. autos) is an alternate novel utilization of power era utilizing thermoelectric power generators. Despite the fact that a responding cylinder motor changes the substance energy accessible in fossil powers productively into mechanical work a generous measure of warm energy is scattered to the earth through fumes gas, radiation, cooling water and greasing up oils. Case in point, in a gas controlled motor, roughly 30% of the essential fuel energy is scattered as waste heat energy in the fumes gasses; waste heat energy released in the fumes gasses from an ordinary traveler auto going at a standard rate is 20-30 kW (Umeda 84). A thorough hypothetical study inferred that a thermoelectric generator fueled by fumes heat could meet the electrical necessities of a medium estimated vehicles (Umeda 90). It was accounted for by David Rowe that among the built thermoelectric materials, those modules in view of PbTe innovation were the most suitable for changing over waste heat energy from vehicles into electrical power. Widescale uses of thermoelectrics in the auto business would prompt a few diminishments in fuel utilization, and subsequently natural an unnatural weather change, however, this innovation is not yet demonstrated (British 79). Since 1914 the likelihood of utilizing thermoelectric power era to recoup some of waste heat energy from responding motors has been investigated and licensed (Ko and Costas 63). A schematic graph is demonstrating this patent of changing over waste heat into electrical power connected to an interior ignition motor utilizing a thermoelectric power generator. In this creation, the fumes gasses in the channel give the heat source to the thermoelectric power generator while the heat sink (cool side) is proposed to be given by the flow of cooling water. All the more as of late, David Rowe imagined gasses gas based thermoelectric power generator for an auto application. A schematic graph is demonstrating this late patent connected to an auto for changing over waste heat accessible in fumes gasses specifically into electrical power utilizing a thermoelectric power generator. In this patent, a pump supplies cooling water through each of cooling water course ways. The cooling watercourse way incorporates a cooling water channel orchestrated along the fumes funnel to pass the cooling water. At stacks, a majority of thermoelectric era components are joined to the fumes funnel and the cooling water pipe progressively in a course from the upstream toward downstream of the fumes gas. The cooling water funnel and the fumes channel pass the cooling water and the fumes gas, individually, in inverse headings so that the downstream stack has an expanded contrast in temperature between the fumes funnel and the cooling water funnel, and the stacks give power yields having a lessened distinction, and subsequently an expanded aggregate power yield. This development is proposed to give expanded thermoelectric change effectiveness without entangled channeling. 3.0 Application of heat transfer 3.1 Model Description The model description of thermoelectric electric generator is rectangular in shape is represented by Fig.1. Fig. 1. Schematic of a rectangular thermoelectric electric generator model (Ko and Costas 78) The thermoelectric modules (TEMs) are mounted on both the top side and the bottom side, and they are arranged uniformly on the surface as shown. However, the TEMs cover only 80% or the surface. The remaining 20% and the lateral walls are insulated thermally so as to minimize any heat leakage. Fig. 2 shows a plate-fin heat exchanger with fins running along the thermoelectric generator length. Notably, the cold-side temperature of the modules is sustained by the engine coolant system. The exit and entry of the thermoelectric electric generator are connected to the exhaust pipe of the vehicle. Fig. 2. Front view (bottom) showing the integrated plate-fin heat exchanger and Side view (top) showing the control volume (CV) in the dashed box (Ko and Costas 82) 3.1.1 Thermoelectric electric generator Modeling In the above model, the variation in the fluid properties and their moelectric properties with temperature is conceived along the fluid flow direction. Because the thermoelectric electric generator is symmetric considering its height, only half of the total domain is simulated. The thermoelectric electric generator domain is divided into a small control volumes which along the length. This is evident from fig. 2. However, the gas temperature is assumed to be uniform inside the small control volumes. The available hot-side surface area is indicated as 80% of the total base area A Base in a control volume. This is presumed to be covered by the uniform distribution of thermoelectric modules which are represented as Amodule. However, the number density of thermoelectric couples and leg dimensions (p- and n-legs) may vary with the cost, temperature range operation, and thermoelectric materials. 3.1.2 Thermal Resistance Network A typical thermoelectric electric generator can be assumed as a system that is composed of thermoelectric couple (one p- and one n-legs), the engine coolant system mounted closer to the cold junction, and the plate-fin heat exchanger located at the hot-side junction as shown in fig.3. A Control Volume can be modeled as a parallel combustion of plentiful such systems. This can be calculated using nCV1TEC = NumberDensity * AModule. An equal thermal resistance network for a control volume is represented in fig. 4. Figure 3. A representation of a thermoelectric couple (p- and n-legs). The representation has the coolant on the cool side and the fins at the hot side. The thermoelectric couple is attached to an external electric load for electric power generation (Ko and Costas 85). Figure 4. An equivalent thermal resistance network for a control volume. The dashed boxes hem in the thermoelectric components. The block arrows indicate electrical and thermal energy flows through the circuit (Ko and Costas 89) The hot-side heat exchanger for the generator can be modeled as an operative thermal resistance. This is given by Rfin,eq = 1/( n0hgAt). Fin resistance modeling items for a plate-fin heat exchanger assembly. Here, n0 refers to the overall fin effectiveness whereas, At represents the total area of the heat exchanger. In other words, the unfinned base area and the fin surface area in a control volume. hg refers to the average heat transfer coefficient. This coefficient is based on the fin channel Reynolds number. However, the thermal resistance of the top surface of the generator and thermal insulation can be given by the below equations; RTEG,base = tbase / (kbaseAbase), Rgrease = tgrease / (kgreaseAmodule), Rceramic = tceramic / (kceramicAmodule), RIns = tIns / (kInsAIns) The radiation heat transfer is looked at for hot exposed surfaces. In other words, insulated top surfaces and the part of the thermoelectric modules hot surface not covered by the thermoelectric legs. The thermoelectric properties of p- and n-legs are the functions of temperature. The properties are averaged out over the junction temperature. The thermal conductance (K), internal electrical conductance (rel), and Seebeck coefficient (x) can compute for a thermoelectric couple. 3.1.3 Solution Method Since the nonlinear thermal resistances rely on upon the terminal temperatures and its thermoelectric material properties, the temperatures in the thermal circuit must be tackled in an iterative way. The thermoelectric properties and thermal resistances are redesigned at every emphasis venture until the temperatures dont change past a resistance esteem (10-6). The arrangement comprises of an internal and van external emphasis circle, such that the external cycle circle runs until the gas mass temperature units for every control volume. The internal circles run until the temperature, and resistance qualities focalize inside a control volume taking into account the mean mass gas temperature supplied by the external cycle circle. 3.1.4 Varying Inlet Conditions An increased flow rate builds the heat exchange rate through the thermoelectric modules with expanded power generation; notwithstanding, the power yield immerses at higher stream rates because of heat exchange restrictions of the heat exchanger. Then again, mass fluxes inside the heat exchanger expand the contact drag drives on the balances and consequently build weight drops as shown in figure 5 (Aparicio, Andrei and Lisa 47). The accumulated weight drops were discovered to be not exactly as far as possible. Be that as it may, the current examination does not represent recirculation impacts close to the delta and way out ports emerging because of high territory proportions. Figure 5. Electric power output and associated pressure drops for fluctuating rates with Tin = 5000C (Aparicio, Andrei and Lisa 47). The spatial variety in stream administrations along the width and stature of the thermoelectric generator is likewise disregarded, thus this may not be a genuine measure of real gadget weight drop (Aparicio, Andrei and Lisa 53). The electrical power generation rate increments with expanding the gulf temperature. The moderately more sizzling temperatures in the stream area raise the hot-side temperature of the thermoelectric modules, and consequently a higher Seebeck voltage is created by the intersections. The variety in weight drop with the fluctuating delta temperature. An increment noticeable all around thickness with higher gulf temperatures tends build the channel speeds. This clarifies the slight increment in the weight drop with delta deplete temperature. 3.1.5 Average Inlet Conditions Figure 6 below represents the temperature drop that is across various materials along the length of the in the thermoelectric power generator It is momentous to note that there is a difference of more than 1000C between the gas mass temperature and the hot side of the thermoelectric module. The temperature drop over the hot-side contact by warm oil is of the request of 300C. Be that as it may, the current investigation does not consider the blade contact resistances, despicable surface contacts because of thermally instigated disfigurements, nonuniformity of warm oil thickness, and so on. Thus, the real temperature drop is required to be much higher than expressed here. The temperature drop over the intersections diminishes from 3000C to 1200C. Figure 6. Variation in temperature drops across materials in the thermoelectric generator’s flow direction (Aparicio, Andrei and Lisa 59). For the skutterudites, ZT qualities diminish with abatement in temperature, so the modules close to the gulf create more electrical power than those close to the backside. This demonstrates that the electrical power generation is very reliant on the genuine temperature distinction over the intersections. The power fluxes were ascertained as the power exchange rate every unit zone from the top surface of the generator. The requests of extent of the thermal spillage because of radiation and the warm protection are low as contrasted and conduction misfortunes, subsequently the vast majority of the thermal exchanged by the thermal exchanger courses through the thermoelectric modules. 3.1.6 System Efficiency The pie chart in Figure. 7 introduces the energy dispersion for the benchmark model. The yield productivity of the benchmark display regarding electrical power generation is discovered to be 3.33% of occurrence energy. About 36% of episode energy leaves the generator to the earth as fumes gas. Figure 7. Presents the distribution of energy at average inlet conditions (Bauer 64). Of the episode energy rate, 58% is rejected to the motor coolant framework at normal delta conditions. The expanded load on the coolant framework suggests a requirement for bigger motor radiators to reject more heat to nature. The thermoelectric productivity of the TEMs was discovered to be 5.5% though the heat exchanger exchange effectiveness was figured to be 64% (Priya, Anke, Linan and David 51). Conclusion The model presented has the capability to compute the overall heat transferred and the electric power output. However, the thermoelectric electric generator offers a possible application in the direct conversion of vehicle waste-heat energy. The application of this thermoelectric electric generator in converting waste heat energy directly into electric power also improves the overall productivities of energy conversion systems. A thermoelectric electric generator is a solid state device responsible for providing direct energy conversion from thermal energy (heat) as a result of a temperature gradient into electrical energy established in ‘Seebeck effect’. Presently, waste heat powered thermoelectric electric generators are utilized in different ways apart from vehicles. However, this application can be categorized as a macro and micro-scale applications. This will depend on the volume of waste energy that is available for direct conversion into electric power. The electrical power generation is seen to be an in number capacity of stream rate, and delta debilitate temperature. The ramifications of changing gulf conditions could be extremely serious if fitting molding of yield power is not did. The ZT estimation of high-temperature skutterudites diminishes impressively along the stream course because of diminishing DT and temperatures at the hot-side intersection. The thermoelectric modules near to the gulf are presented too much higher gas temperatures and henceforth produce higher electrical power yield every unit region. By upgrading the balance dividing and thickness, the heat exchange rate can be improved extensively. Work Cited Aparicio, Mario, Andrei Jitianu, and Lisa C. Klein. Sol-gel Processing for Conventional and Alternative Energy. New York: Springer, 2012. Bauer, Thomas. Thermophotovoltaics: Basic Principles and Critical Aspects of System Design. Berlin: Springer, 2011. Print. British, Gas. Combustion Engineering and Gas Utilisation. Hoboken: Taylor and Francis, 2014. Goldsmid, H J. Introduction to Thermoelectricity. Heidelberg: Springer, 2010. Print. Ko, Seung H, and Costas P. Grigoropoulos. Hierarchical Nanostructures for Energy Devices. , 2015. Print. Priya, Shashank, Anke Weidenkaff, Linan An, and David P. Norton. Advances in Electronic Ceramics Ii. Hoboken: John Wiley & Sons, Incorporated, 2009. Rowe, David M. Thermoelectrics and Its Energy Harvesting: [volume 2]. Boca Raton, FL: CRC Press, 2012. Umeda, Yasushi. Design for Innovative Value Towards a Sustainable Society: Proceedings of Ecodesign 2011: 7th International Symposium on Environmentally Conscious Design and Inverse Manufacturing. Dordrecht: Springer, 2012. Read More