Development of Thermoelectric Generator

Transcription

1 IJIRST International Journal for Innovative Research in Science & Technology Volume 2 Issue 11 April 2016 ISSN (online): Development of Thermoelectric Generator Anand P N Aswin Joseph Anshad A Geo Eucharist James Tobin Thomas Assistant Professor Abstract Thermoelectric generators (TEG) in internal combustion engine (ICE) powered vehicles has been focused as a green energy technology to improve fuel economy and consequently to reduce emission of the greenhouse gas of CO 2.Only small amount of the fuel combustion energy is converted to mechanical energy, while about a large amount is wasted through exhaust gas. A recent study revealed that the fuel economy of ICE vehicles can be increased by up to 20% simply by capturing the waste heat of gas and converting about 10% of it to electricity. The possibility of energy conversion using a thermoelectric generator (TEG) to tap the exhaust heat energy. TEG is like a heat engine which converts the heat energy into electric energy and it works on the principle of Seebeck effect. The seebeck effect is the conversion of temperature directly into electricity. As this exhaust heat recovery system reduces the wastage of energy, it improves the overall efficiency of vehicle. This system is profitable in the automobile industry. Generated useful power is used to charge the vehicle battery, to power auxiliary systems and minor electronics in vehicles. Harvesting that waste heat energy with a thermoelectric generator decreases the electric generator load on the engine which in turn can increase the fuel efficiency while lowering its emissions. Keywords: exhaust gas, greenhouse gas, internal combustion engine, seeback principle, thermoelectric generator I. INTRODUCTION Thermoelectric generators (TEG) in internal combustion engine (ICE) powered vehicles has been focused as a green energy technology to improve fuel economy and consequently to reduce emission of the greenhouse gas of CO 2. Only 30% of the fuel combustion energy is converted to mechanical energy, while about 40% is wasted through exhaust gas, and waste heat of engine coolant constitutes up to 30% of the fuel consumption energy. Only about 25% of the fuel consumption energy is used for vehicle operations due to frictional losses. A recent study revealed that the fuel economy of ICE vehicles can be increased by up to 20% simply by capturing the waste heat of gas and converting about 10% of it to electricity. Every major automobile manufacturer is working with waste heat recovery system for improving the fuel economy of ICEV, and some of them are trying to develop Thermoelectric Hybrid Electric vehicles. In order to meet the increasing electrical demands of modern automobiles, bigger and bulkier alternators are connected to engines. Bigger and bulkier alternators which operate at an efficiency of 50 to 62% consume around 1 to 5% of the rated engine output. However, due to the expansion of the passenger room and to improve the vehicle aerodynamics, the space for the alternator in engine room is becoming smaller. If approximately 6% heat can be recovered from the engine exhaust, it can meet the electrical requirements of an automobile and it would be possible to reduce the fuel consumption around 10%. Heat is rejected through exhaust gases at high temperature when compared to heat rejected through coolant and lubricating oil. This shows the possibility of energy conversion using a thermoelectric generator (TEG) to tap the exhaust heat energy. TEG is like a heat engine which converts the heat energy into electric energy and it works on the principle of Seebeck effect. Moreover, TEG are highly reliable, operate quietly and are usually environmentally friendly. Semi-conducting materials (in conjunction with copper inter- connecting pads), were found to offer the best combination of Seebeck coefficient, electrical resistivity, and thermal conductivity. Semi-conducting materials provide another benefit ability to use electrons or holes (the absence of an electron in a crystal matrix) to conduct, current. Thermoelectric module (TEM) has a cold side and a hot side. At the hot side, heat is absorbed by electrons as they pass from a high energy level in the n-type semiconductor element, to a lower energy level in the p-type semiconductor element. The power supply provides the energy to move the electrons through the system. At the cold side, energy is expelled to a heat sink as electrons move from a high energy level element (n-type) to a lower energy level element (p-type). Bismuth Telluride-based TEM are designed All rights reserved by 346

2 primarily for cooling or combined cooling and heating applications where electrical power creates a temperature difference across the TEM. By using the modules in reverse, where a temperature differential is applied across the faces of the module, it is possible to generate electrical power. Although power output and generation efficiency are presently low, useful power often may be obtained where a source of heat is available. II. BASICS Thermoelectric devices use the principle of Seeback effect. When two sides of the thermocouple is maintained at a temperature difference electric current is produced The Seebeck coefficient (frequently measured in microvolts/k) is defined as the open circuit voltage produced between two points on a conductor when a uniform temperature difference of 1K is applied between those points. The simplest TEG consists of a thermocouple consisting of n type (materials with deficit of electrons) elements connected electrically in series and thermally in parallel. Heat is input on one side and rejected from the other side, generating a voltage across the TE couple. The magnitude of the voltage produced is proportional to the temperature gradient. Fig. 1: TEG III. THERMOELECTRIC SYSTEM DESIGN IN EXHAUST MANIFOLD Increasing Operating Range of Thermoelectric Materials Systems: Thermoelectric assemblies are made up of multiple components: thermoelectric material, electrical insulators, connectors, heat sinks, interface expansion/compression materials, and interface thermal transfer materials. Each of these items has environmental limits (temperature, pressure, chemical exposure structural stress, shock and vibration, and cycling) within which they can operate without deterioration. The TEG system combines these components into a system that has its own environmental limits, which in many cases will be more restrictive than any one of the individual component that make up the assembly. Currently, TEG devices are limited to niche applications where low performance and high cost are not show-stopping barriers. These applications have included remote power and cooling applications in military and spacecraft systems, and medical applications where a premium is placed on high reliability and/or human comfort. Bi 2 Te 3 and its alloys have been used extensively in TE refrigeration applications, such as medical cooling, laser diode cooling, and infrared detector cooling, and some niche low-power generation applications. They generally have a useful temperature range of 180 K to 473 K (93 C to 200 C). PbTe and SiGe materials have been used extensively in higher temperature power generation applications, particularly spacecraft power generation, and have a useful temperature range of 500 K to 900 K (227 C to 627 C) and 800 K to 1300 K (527 C to 1027 C) respectively. There is just as strong a need for performance improvement in the applications mentioned above as there is for performance improvements to enable new applications in waste energy recovery. Therefore devices need to be able to operate in the same environments as current conventional systems, as well as new industrial process and automotive waste energy environments. The new industrial process environments will require advanced TE systems to operate at exhaust temperatures of 1033 K (760 C) or higher, while automotive exhaust environments will require operation at exhaust temperatures of 670 K to 970 K (400 C to 700 C). Maintaining High T Across a Thermoelectric Device: The heat/energy flowing through the device is constrained by the hot-side and cold-side heat sink technology. The overall performance (i.e., efficiency-power characteristic) of a thermoelectric generator is a function of the heat sink technology as much as it is of the material s ZT.Although the heat flow through the system via the heat exchangers is critically important for the efficiency of a thermoelectric device, it is a direct function of the temperature difference between the hot side and the cold side, T, the highest efficiency being obtained with the largest T across the TE device. This requires All rights reserved by 347

3 establishing and maintaining a high heat flux rate across the TE device. In addition, waste flue gas flow rates are typically relatively slow, creating a dually difficult challenge to obtain high convective heat transfer coefficients. In any waste flue gas heat application, the cold side will likely be a liquid- cooled heat sink, while the hot side is the waste flue gas. R&D is required in this area to fully enable high-performance TEGs and exploit the most recent advanced TE material gains in performance. Exhaust Heat Exchanger: Exhaust gas flows through the exhaust heat exchanger. Hexagonal heat exchanger has been chosen to accommodate six modules to get the required output to charge the battery. Coolant Heat Exchanger: Fig. 2: Exhaust Heat Exchanger Coolant heat exchanger is the heat sink where the heat is rejected from the hot side and cold water is circulated through the rectangular chamber six coolant chambers are used to cool six thermoelectric modules. TEG Installation in Exhaust Manifold: Engine coolant was chosen to cool the TEG instead of air cooling because the larger heat transfer coefficient available with liquid cooling allows smaller heat exchangers to be used and does not require fans at low or zero vehicle speed. As the TEG coolant loop was connected to the engine coolant system through the cabin heat hoses, the temperature of the coolant supplied to the TEG was actually highest in the coolant system However, the operation of the by-pass and thermostat valves could then prevent continuous coolant flow to the TEG. The PCU was essentially a direct-current-to-direct- current converter functioning as a buck regulator. It matched the generator's output voltage to the approximately 14.5 V potential of the vehicle's direct current bus and kept the TEG operating at its maximum power point. Parts IV. COMPONENTS AND FUNCTIONS Thermoelectric Module: The thermoelectric module chosen was the TEG supplied by Thermal Electronics Corp, Canada. The output power from the thermoelectric modules was close to the specifications quoted by the manufacturer and could withstand temperatures of up to 320 C intermittently. The 40 mm x 40 mm TEG is composed of 126 Bismuth Telluride p n junctions. While TEGs with larger area specify higher output power per degree temperature difference, a greater heat flux is required to maintain the same DTTEG. According to the manufacturer s specification at temperature difference of TEG = 210 C, TEG is capable of producing 5.9W of electrical power at matched load. All rights reserved by 348

4 Fig. 3: TEG Module Hexagonal Hot Side Heat Exchanger: Hexagonal heat exchanger was made to accommodate 6 modules of dimension 40x40mm. Material chosen was aluminium as it was easily available, lighter in weight than other metals, cheaper, easily machinable. The hexagonal heat exchanger is of 100mm length maximum diagonal 134 mm, flat face length. Fig. 4: Hexagonal Heat Exchanger Coolant Heat Exchanger: Coolant heat exchanger was made of stainless steel as aluminium welding was costlier and was not available at every welding shops. Dimensions are 50x50x27 mm. 10mm steel pipe was welded to it. Leak proof welding was done. Fig. 5: Coolant Heat Exchanger Hot Side Fins: Hot side fins are made of aluminium and tapings for allen screw was also made on each fin. All rights reserved by 349

5 Fig. 6: Hot Side Fins Assembly: Fig. 7: Assembled views V. RESULTS AND DISCUSSIONS It consists of thermoelectric module, hexagonal hot side heat exchanger, hot side fins, fins for air cooling. The thermoelectric exhaust system is connected between the exhaust and the muffler of the automobile. As the engine starts, hot exhaust gas will pass through the system. These is conducted to one side of thermoelectric module. The other side of the thermoelectric module is made cooled by providing coolant. According to the principle of Seebeck principle, electric current is produced by the thermoelectric module which is placed between the hot and cold side. The energy generated by six modules are connected in series to get the common output. Fig. 8: Thermoelectric Exhaust System In Exhaust Of An Engine Test was conducted in Greaves single cylinder diesel 395 CC engine. Temperatures were measured using thermocouples. Engine rpm was measured by a non-contact type tachometer. Voltage and Current were measured using digital multimeter. All rights reserved by 350

6 The output terminals of thermoelectric modules were connected in series.the rpm is made constant, then the readings such as temperature, voltage, current, engine rpm were measured. This was repeated for various engine rpm. Observation Table 1 Overall Summary SL.NO SPEED INLET TEMP OUTLET TEMP VOLTAGE CURRENT POWER (rpm) ( C) ( C) (V) (A) (Watts) ` Fig. 9: Inlet temperature Vs Current Fig. 10: Inlet temperature Vs Voltage VI. CONCLUSION As this exhaust heat recovery system reduces the wastage of energy, it improves the overall efficiency of vehicle. This system is profitable in the automobile industry. Generated useful power is used to charge the vehicle battery, to power auxiliary systems and minor electronics in vehicles. Harvesting that waste heat energy with a thermoelectric generator decreases the electric generator load on the engine which in turn can increase the fuel efficiency while lowering its emissions. Integrating this into vehicles improves gas mileage and reduces carbon dioxide and other greenhouse gas emissions. Further improvements in thermoelectric generators can be develop as follows, All rights reserved by 351

7 - It was found that to get improved efficiency of this system, thermal management is very important. - More power could also be extracted by improving the exhaust gas heat exchanger. - TE materials must provide high thermoelectric power conversion and be mechanically and thermally stable to insure the TEG will last for the life of the vehicle with little or no maintenance impact. ACKNOWLEDGEMENT First and foremost, we express our heartfelt gratitude to God almighty for being the guiding light throughout our project, without whose intercession this project would not have been a successful one. We thank our parents for being a guiding light and supporting me all throughout our life. We would like to extend our sincere thanks to the Principal Dr. M C Philipose and our Head of Department Dr. Sreejith C.C for rendering all the facilities and help for the successful completion of our project. We take this opportunity to express our sincere profund obligation to our guide Er. Tobin Thomas, Assistant Professor, for his helpful suggestions and overall guidance throughout this project. We are thankful to Er. Vineeth V K, Assistant Professor (Project coordinator), Er. Jenny John Mattom, Er. Bibin Varkey Assistant Professors, who gave us an opportunity to present the project successfully. We would like to extend our gratitude to our friends who have encouraged and supported me for the successful presentation of our project. REFERENCES [1] Prathamesh Ramade., Automobile Exhaust Thermo-Electric Generator Design & Performance Analysis, International Journal of Emerging Technology and Advanced Engineering Journal, 2014, Vol. 4, Issue 5, pp [2] Nadaf S L., A Review on Waste Heat Recovery and Utilization from Diesel Engines, International Journal of Advanced Engineering Technology, 2014, Vol. 5, Issue 4, pp All rights reserved by 352