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inventions Article Design, Manufacture and Performance Test of the Thermoelectric Generator System for Waste Heat Recovery of Engine Exhaust Tzer-Ming Jeng † , Sheng-Chung Tzeng *,† , Bo-Jun Yang and Yi-Chun Li Received: 26 November 2015; Accepted: 5 January 2016; Published: 11 January 2016 Academic Editor: Chien-Hung Liu Department of Mechanical Engineering, Chienkuo Technology University, Changhua City 500, Taiwan; [email protected] (T.-M.J.); [email protected] (B.-J.Y.); [email protected] (Y.-C.L.) * Correspondence: [email protected]; Tel.: +886-4-711-1111 (ext. 3190); Fax: +886-4-713-5677 † These authors contributed equally to this work. Abstract: This study integrated the techniques of the high-performance heat transfer and thermoelectric conversion to build a thermoelectric generator system installed at the exhaust pipe of a real single-cylinder and four-stroke engine of 35.8 c.c. displacement. This system was made of the heat absorber, thermoelectric generator modules and the external heat sink. The pin-fin array was inserted into the circuitous duct to form the heat absorber, which could increase the heat-exchange surface area and the heat-exchange time to reach the object of absorbing heat and increasing thermoelectric conversion effectively. Keywords: heat recovery; thermoelectric generator system; heat absorber of pin-fin array 1. Introduction Thermoelectric conversion was discovered towards the end of the 19th century. Electrons are capable of carrying heat, as well as electricity. When a temperature difference exists between the two end faces of a thermoelectric material, many electrons will travel from the hot end face to the cold one. This phenomenon is known as the Seebeck effect and forms the basis on which the thermoelectric generation module was developed. As we are all aware, the global energy crisis is becoming worse and much emphasis is being placed on environment protection and the recovery of energy resources. These include waste heat from cooling water, exhaust gas, and steam power plants, amongst others. TEG modules are light and silent, have no moving parts and can convert recycled heat directly into electricity. Champier et al. [1] pointed out that developing countries are currently using massive bio-fuel furnaces to mitigate global warming. The efficiency of these bio-fuel furnaces has been significantly improved in laboratory prototypes which give better combustion efficiency and produce less pollution. TEGs have been installed in them and the electricity generated can be used for lighting or to power circulation fans to improve the fuel-air ratio which, in turn, results in more complete combustion. Experiments were first carried out to verify the thermoelectric conversion ratio of a Bi2 Te3 TEG module and then the feasibility of combining the TEG module with a bio-fuel furnace and determination of the optimal position for installation was studied. Finally it was demonstrated that 6 W could be generated by thermoelectric conversion. Karri et al. [2] studied the efficiency of waste heat electricity generation using thermoelectric conversion in two practical cases: a sport utility vehicle (SUV) and a compressed natural gas (CNG)-fueled generator engine. Two different thermoelectric modules (available on the market) were used: (1) bismuth telluride (Bi2 Te3 ), and (2) quantum-well (QW) device. Tests of the CNG engine and the SUV under the same conditions and the same generated AC output, showed the Inventions 2016, 1, 2; doi:10.3390/inventions1010002 www.mdpi.com/journal/inventions Inventions 2016, 1, 2 2 of 16 QW device to be more efficient than the Bi2 Te3 TEG. In other words, more electricity was generated from the same amount of heat and more fuel was saved. Zhou et al. [3] pointed out that because water is a good heat conductor, combining commercial water heaters with thermoelectric modules in a thermoelectric conversion generator system was a good idea. They employed Carnot efficiency and the Coupled Physical Energy Balance Method that combines Joule heating, with the Seebeck, Peltier, and Thomson effects to predict the efficiency of a thermoelectric conversion generator system. Thirty different configurations involving a number of parameters were investigated, these included: the water flow model, hot water inlet temperature, pressure drop, cross-sectional area of the duct, duct length, the number of ducts, and so on. Systematic analysis showed that the upper modules would rival the upstream module in heat absorption, resulting in a lower temperature difference between the two faces of the thermoelectric module. They also pointed out that reducing coverage by modules in the downstream of an exhaust duct might result in its wall temperature being higher than the temperature of the hot-face of the upstream modules, which offers an opportunity to maximize overall power generation. Wang and Huang [4] investigated the conversion of vehicle engine exhaust heat to electrical power using thermoelectric generator modules. They employed numerical simulation to analyze multiple thermoelectric modules attached to heat exchanger surfaces in a series arrangement. They found that using more modules did not necessarily generate more power, and that the average generation ratio of each module actually decreased as the number of modules rose. The reason for this was that the temperature of the exhaust duct wall decreased rapidly along the direction of flow and this resulted in poor performance by the downstream TEGs. Jang and Tsai [5] considered attaching multiple TEG modules on stacked wall faces and found the amount of unit area generation was strongly affected by the spacing of the modules. Their thermoelectric generator configuration consisted of thermal plates (a stack wall), heat distribution plates, TEG modules, and water-cooled cooling plates. They used limited element analysis to simulate the overall thermoelectric conversion of the modules with different spacing and also with different thickness of heat distribution plates. The variable operation parameters included the temperature difference between the hot exhaust gas and the cooling water (∆T = 200–800 K) and heat conductance coefficient of the heat distribution plate (h = 20–80 W/m2 /K). The aim of the simulation was determination of the optimal spacing between modules and the ideal thickness of heat distribution plates that gave the maximum generation per unit area. It has become clear that as the conversion efficiency of TEG material has increased so has the attention being paid to studies and applications involving combinations of TEG modules and waste heat sources from both domestic and industrial activity. These include heat from vehicle engine exhausts, cooling water circulation systems in vehicle engines, generators using internal combustion engines, waste heat gas stacks, bio-fuel furnaces, ground heat from hot springs, etc. These many heat sources can provide suitable environments for thermoelectric modules that generate power using differential temperatures. With proper cooling, the temperature difference between the cold and hot sides of TEG modules can be raised to achieve ideal power generation. Many current studies have addressed waste heat recovery from engines using TEG devices and many have claimed patents. Most studies on thermoelectric generation by the recovery of engine waste heat emphasize integrated thermo-electric conversion devices and performance simulations involving vehicle systems. A patent filed by Cumming [6] claimed that power can be generated and saved in batteries using a thermoelectric semi-conductor installed on the outside of an engine exhaust pipe used for driving a hybrid vehicle. A patent filed by Handa and Nagoya [7] involved the adding of an insulator on both the heated and cooled faces of a TEG module to widen the temperature difference between the faces and give better generation efficiency. Shinohara et al. [8] invented an electronic power supply control system that utilizes a thermoelectric device to generate electric power from exhaust gas as a secondary system generator. After a summary of applications, Rowe [9] pointed out that thermoelectric conversion systems using temperatures above 140 ˝ C are commercially competitive. Hendricks and Lustbader [10] analyzed systems using waste heat thermoelectric conversion systems on medium and high power vehicles and found that they were capable of generating 5–6 kW power. Inventions 2016, 1, 2 3 of 16 However, there are three technical impediments to the achievement of a high-performance system: the thermoelectric conversion system itself, optimal integration with the vehicle, and design of the thermoelectric interface. A study team at the Ohio State University accomplished waste heat recovery using a thallium-doped lead telluride thermoelectric module [11], which characterizes a thermoelectric effect. Heating up one end of the material will cause the electrons to move to the colder end, therefore generating electrical current. If used on vehicles, waste heat from the vehicle can be converted into electricity and fed back to the vehicle for re-use. Jeng et al. [12] investigated the performance of a heat absorber made of packed brass beads in a thermoelectric conversion system. This packed bead heat absorber was installed in a square channel and various flow orientations, both straight and jet, were investigated as well as the ratio of channel width to bead diameter. Their studies showed the local and average heat transfer characteristics for several parameters and can be used as a basis for the design of a novel porous heat absorber for thermoelectric conversion systems. This study establishes an initial performance test platform for thermoelectric modules, examines the relationships between the external load resistance and the output voltage/current of the TEG and investigates the influence of temperature difference between the hot and cold faces and their average against maximum power generation. Using high-temperature exhaust gas from an actual 35.8 c.c. engine as the heat source and a patent applied for by the authors [13] as the blueprint, a complete thermoelectric conversion system (including heat absorber, thermoelectric generation module and heat sink) was constructed. The experiments were conducted under controlled laboratory conditions and careful measurements of the displacement of the engine, operation speed, temperature, and composition of the high temperature exhaust gas were made. The effects of the installation position of the thermoelectric conversion system and the corresponding external cooling airflow were studied and the results may be used as important references for the future design of vehicle thermoelectric conversion systems. 2. Experimental Method 2.1. Design of Waste Heat Recovery Thermoelectric Conversion System A thermoelectric conversion system normally consists of heat absorbers, TEG modules, and heat sinks, where heat absorbers function as heat collectors to heat up the TEG, and the heat sink functions to dissipate heat from the cold end of the TEG module as rapidly and efficiently as possible to widen the temperature difference between the hot and cold faces, to enhance performance of the TEG module. The thermoelectric conversion heat recovery device constructed for this study is illustrated in Figure 1. The pin-fin heat absorber is situated inside the duct and the exhaust gases are guided along a circuitous path from the inlet across them as can be seen in Figure 1. The absorbed heat is conducted to the hot face of the TEG modules which are bolted to the surface of the heat absorbers. The exhaust gases are forced past the pin-finned heat absorbers and the resulting turbulence enhances heat exchange between the hot exhaust gases and the extensive surface of the heat exchangers. The device also acts as a silencer and reduces the noise of the engine. The parts are anodized to protect them from corrosion and this also improves their aesthetic appearance. The fin pins are all the same length and make contact with the baffle box in the center of the chamber. This arrangement guides the flow so that it is impossible for the exhaust gasses to get to the outlet without the development of turbulence and close contact with the large heat exchange surfaces on all four sides of the chamber, see Figure 1. The bases of the finned heat exchange units are fastened to the body of the device with machine screws. The TEG modules are clamped between the top of the heat exchangers and the cooling heat sinks, also with machine screws. This arrangement effectively stays the hot waste gas and achieves massive heat exchange. Figure 1 also shows an assembled heat exchange device with one heat exchanger block, TEG and cooling fin assembly removed for clarity. In use, the outer housing has four heat exchange blocks bolted to it and the ends of the heater exchange pins make contact with the central baffle box which acts to guide the hot gasses on the proper path across all the heat exchange surfaces to the outlet. Inventions 2016, 1, 2 4 of 16 All the components, the heat absorbers, heat sinks, aluminum housing, and central baffle, are made of aluminum alloy Inventions 2016, 1, 2and have been anodized. 4 of 16 Inventions 2016, 1, 2 4 of 16 (a) (b) Figure 1. The thermoelectric conversion device used herein. (a) Cross-sectional view; (b) Photo of (a) (b) Figure 1. The thermoelectric conversion device used herein. (a) Cross-sectional view; (b) Photo prototype. of prototype. Figure 1. The thermoelectric conversion device used herein. (a) Cross-sectional view; (b) Photo of prototype. 2.2. Generation Performance Test of a Single-Chip TEG Module 2.2. Generation Performance Test of a Single-Chip TEG Module 2.2. Performance a Single-Chipperformance TEG Module test of a single chip TEG module was TheGeneration experimental setupTest forof generation The experimental setup for generation performance test of a single chip TEG module was shown shown inThe Figure 2. The setup (1) an air supply system, a TEGchip module, a testwas section experimental setupincludes for generation performance test of(2) a single TEG (3) module in Figure 2. The setup includes (1) an air supply system; (2) a TEG module; (3) a test section (heating (heating and cooling2. devices, (4) the(1)data acquisition equipment. The air supply shown in Figure The setupand includes an air supply system, (2) a TEG module, (3) a for test cooling section the and cooling devices); anddevices, (4) theand data(4)acquisition equipment. The air The supply for cooling the cold side cooling the data equipment. air supply for the in cold(heating side of and the TEG module was provided byacquisition an air compressor. The compressed aircooling was stored of thecold TEG module was provided by an air compressor. The compressed air was stored in an 800 TEG module by an air compressor. Thewas compressed air was stored in andL an 800 Lside tankoftothe ensure a steadywas andprovided pulse-free supply. The air flow passed through a dryer tank to ensure a steady and pulse-free supply. The air flow was passed through a dryer and filter to L tankwater to ensure steady and pulse-free supply. The air flow (SIN-DP) was passed through and the filteranto800 remove anda impurities and an electronic controller was used atodryer regulate remove water and impurities and electronic controller (SIN-DP) was used to regulate the the air flow filter to remove water andto impurities and device an electronic (SIN-DP) was rates used to air flow which was guided thean cooling of thecontroller test section. Air flow ofregulate 350–450 L/min airwas flow which to was guided to the cooling device of the testAir section. Air flow rates of 350–450 L/min which guided the cooling device of the test section. flow rates of 350–450 L/min were used were used for cooling the heat sink of cooling device, and then the temperature differences between were used for cooling thecooling heat sink of cooling and then the temperature differences for the heat sinkofof device, and device, then the temperature differences betweenbetween the hot and thecooling hot and cold faces the TEG module were controlled to be 80, 100 and 120 °C. ˝ the hot and cold faces of the TEG module were controlled to be 80, 100 and 120 °C. cold faces of the TEG module were controlled to be 80, 100 and 120 C. Figure 2. Experimental setup for the single-chip thermoelectric module experiment. Figure 2. Experimental setup for the single-chip thermoelectric module experiment. Figure 2. Experimental setup for the single-chip thermoelectric module experiment. As shown in Figure 3, the TEG used in this experiment was manufactured by Russian Kryotherm As ininFigure TEG in was manufactured by Russian Russian Kryotherm (Model: TGM-287-1.0-1.5; dimensions: mm ×experiment 40 mm × 3.8 mm). Figure 4 shows the present test Asshown shown Figure3,3,the the TEGused used40 inthis thisexperiment was manufactured by Kryotherm section. In this experiment, the hot side of the TEG module was fastened to the heater using thermal (Model: TGM-287-1.0-1.5; dimensions: 40 mm ˆ 40 mm ˆ 3.8 mm). Figure 4 shows the present (Model: TGM-287-1.0-1.5; dimensions: 40 mm × 40 mm × 3.8 mm). Figure 4 shows the present test grease. The heater was made of Bakelite (85 mm × 85 mm × 65 mm) with a stainless steel heating test section. In this experiment, theside hotofside themodule TEG module was fastened to theusing heater using section. In this experiment, the hot the of TEG was fastened to the heater thermal element (40 The mm heater × 40 mm) was of powered by a DC power supply. The coldwith side aofstainless the TEG steel thermal wasand Bakelite ˆ ×8565mm ˆ with 65 mm) grease. grease. The heater was made ofmade Bakelite (85 mm(85 × 85mm mm mm) a stainless steel heating module was fastened by thermal grease to an aluminum alloy heat sink with an array of round pin heating element (40×mm ˆ 40 mm) and powered was powered a DC power supply. The coldside sideofofthe theTEG TEG element (40 mm 40 mm) and was by aby DC power supply. The cold fins (see Figure 5). The heat sink, with an acrylic duct and cool air source, formed the cooling device module was fastened by thermal grease to with an an array array of of round roundpin pin module fastened byof thermal toan an aluminum aluminum alloy heat sink with used was for the cold side the TEGgrease module. fins (see Figure 5). The heat sink, with an acrylic duct and cool air source, formed the cooling device used for the cold side of the TEG module. Inventions 2016, 1, 2 5 of 16 fins (see Figure 5). The heat sink, with an acrylic duct and cool air source, formed the cooling device 1, 2 5 of 16 usedInventions for the 2016, cold of the TEG module. Inventions 2016, 1,side 2 5 of 16 Inventions 2016, 1, 2 5 of 16 Figure 3. Photo TEGmodule. module. (Russian Kryotherm: TGM-287-1.0-1.5). Figure 3. Photo ofof TEG Kryotherm: TGM-287-1.0-1.5). Figure 3. Photo of TEG module.(Russian (Russian Kryotherm: TGM-287-1.0-1.5). Figure 3. Photo of TEG module. (Russian Kryotherm: TGM-287-1.0-1.5). Figure4.4.Exploded Exploded view ofoftest Figure testsection. section. Figure 4. Exploded view view of test section. Figure 4. Exploded view of test section. Figure 5. Configuration and dimensions of heat sink for cooling device. (Unit: mm). Figure 5. Configuration and dimensionsof of heat heat sink for cooling device. (Unit: mm). Figure 5. Configuration and sinkfor forcooling cooling device. (Unit: Figure 5. Configuration anddimensions dimensions heat sink device. (Unit: mm).mm). Inventions 2016, 1, 2 6 of 16 TT-T-30SLE T-Type thermocouples were embedded in the center of both the cold and hot faces of TEG module. Differential potential signals were retrieved by a YOKOGAWA MX-100 data recorder and converted into temperature readings. The TEG was connected to an external resistor load and the output voltage was measured by a multi-meter in parallel with the circuit. Power generated by the TEG module can be calculated using the voltage and resistor values. All data readings for computer processing were made at thermal equilibrium condition, when temperature fluctuation was less than 0.2 ˝ C over 15 min. The main target parameter of the experiment was generated power (P) which was measured using voltage values with resistors of different values connected in series to the external circuit. The formula for the calculation of TEG power generation is: P “ V 2 {R (1) where V stands for voltage, R for load resistance. 2.3. Generation Performance Test of the Thermoelectric Conversion System for a Real Engine This experiment investigated power generation by a TEG system installed on the exhaust pipe of a 38.5 c.c. four-stroke engine. Figure 6 shows the layout used in this experiment, the equipment used includes (1) the engine; (2) the TEG conversion system; (3) an external air source (fan); and (4) the data acquisition system. The high-temperature exhaust gases from engine operation were guided through the thermoelectric conversion device in a way that ensures that maximum heat exchange took place. The thermoelectric conversion system, as shown in Figure 7, includes (1) the heat absorber; (2) the TEG modules; (3) the heat sinks; (4) the aluminum-alloy duct and other fittings. As the engine exhaust gases flowed through the duct, the internal heat absorber of pin-fin arrays conveyed heat rapidly to their external surface to which the TEG modules were fixed by machine screws and thermal grease. The other side (cold side) of each TEG was fastened in the same way to an outer heat sink. The substantial temperature difference between heat absorber and heat sink resulted in the efficient conversion of thermal energy into electricity. Figure 8 shows the assembled thermoelectric conversion system, an exploded diagram of the components, and the dimensions of the aluminum alloy duct. The heat absorber was made of 6061 aluminum alloy, and the configuration is shown in Figure 9. The heat sinks were made by extrusion from 6061 aluminum alloy and had evenly spaced rows of pin fins on the surface as shown in Figure 10. The four TEG modules used on the device were connected in series. The thermal grease was applied to the contact surfaces to ensure good heat conduction between the TEG surfaces and the heat absorber as well as heat sink. A fan drove air across the front of the engine, to simulate the oncoming air flow of a moving vehicle, and cooled the thermoelectric conversion system. The data acquisition system monitored and recorded the relevant temperatures, rotation speed, air flow rate, and the voltage and power outputs in real time. A K-type thermocouple was located at the inlet and outlet of the aluminum-alloy duct with built-in heat absorber. Thermocouples were also embedded between the TEG modules and heat-absorber plates as well as the TEG modules and the heat sinks. A one-point T-type thermocouple was embedded at the fan outlet. Measurements of the voltage generated by the TEGs were monitored by a micro-voltmeter in real time. The output of the thermocouples was measured through a data recorder and sent to the PC for real time analysis and recording. The speed of cooling air delivered by the fan was measured at the fan outlet and downstream of the test section using the airflow meter. The mean velocity (U) of air passing through the heat sink was estimated based on the average value of the air speeds measured at these two positions. Engine rotation speed, controlled by the throttle valve, was measured by a tachometer. The accuracy of measurements made during engineering tests of this kind is always affected by unpredictable factors. These may be physical and connected with the equipment, or may be caused by the environment. Inaccuracy of some degree is inevitable in all engineering measurements and control. Once encountered, steps can be taken to reduce the sources of error and predictions can be Inventions 2016, 1, 2 7 of 16 made of the likelihood of future occurrence. Measures can be taken to reduce and control inaccuracy of measurements test procedures to eliminate the source, or at least reduce the degree of1616error. Inventions 7 7ofof Inventions2016, 2016,1,during 1,2 2 The effects of inaccuracy in measurement should be estimated in advance, before the collection of data, error. ininexperimental measurement before the error. The The effects effects of inaccuracy inaccuracy measurement should should be be estimated estimated advance, before the and modifications can of be made to the conditions to reduceinin oradvance, eliminate the problem. collection of data, and modifications can be made to the experimental conditions to reduce oror collection ofto data, and modifications caninclude be made to the experimentaland conditions to reduce The likely errors be found in test results both measurement calculation parameters. eliminate problem. The errors be inintest include both and eliminatethe the Thelikely likely errorstotocan befound found testresults results bothmeasurement measurement The uncertainty of problem. measurement parameter be attributed to theinclude instrument system itself and and/or calculation parameters. The uncertainty of measurement parameter can be attributed to calculation parameters. The uncertainty of measurement parameter can be attributed to the the human error, and the uncertainty of calculation parameters results from both parameters. We used instrument instrumentsystem systemitself itselfand/or and/orhuman humanerror, error,and andthe theuncertainty uncertaintyofofcalculation calculationparameters parametersresults results Moffat’s [14] uncertainty analysis to assess[14] our experiment and the uncertainty of power generation from fromboth bothparameters. parameters.We Weused usedMoffat’s Moffat’s [14]uncertainty uncertaintyanalysis analysistotoassess assessour ourexperiment experimentand andthe the (P) was determined to be ˘2.1%. uncertainty uncertaintyofofpower powergeneration generation(P) (P)was wasdetermined determinedtotobe be±2.1%. ±2.1%. (a) (a) (b) (b) Figure 6.6.The experimental setup system installed atataareal Figure The experimentalsetup setupof ofthermoelectric thermoelectricconversion conversion system installed real engine. (a) Figure 6. The experimental of thermoelectric conversion system installed atengine. a real (a) engine. (b) The 4-stroke engine and the waste heat recovery Layout ofofthe experimental equipment; and (b) The 4-stroke engine and the waste heat recovery Layout the experimental equipment; and (a) Layout of the experimental equipment; and (b) The 4-stroke engine and the waste heat device. device. recovery device. Figure the thermoelectric Figure 7.Diagram Diagram thethermoelectric thermoelectricconversion conversionsystem. system. Figure 7. 7. Diagram ofofof the conversion system. Inventions 2016, 1, 2 8 of 16 Inventions 2016, 1, 2 8 of 16 Inventions 2016, 1, 2 8 of 16 (a) (b) (a) (b) (c) (c) Figure 8. Configuration and dimensions of thermoelectric conversion system. (Unit: mm). (a) The Figure 8. Configuration and dimensions of thermoelectric conversion system. (Unit: mm). (a) The assembled thermoelectric conversion device; (b) Exploded conversion view of thesystem. thermoelectric conversion Figure 8. Configuration and dimensions of thermoelectric (Unit: mm). (a) The assembled thermoelectric conversion device; (b) Exploded view of the thermoelectric conversion device; (c) Configuration and dimensions of aluminum-alloy duct. assembled thermoelectric conversion device; (b) Exploded view of the thermoelectric conversion device; (c) Configuration and dimensions of aluminum-alloy duct. device; (c) Configuration and dimensions of aluminum-alloy duct. Figure 9. Pin-fin array used in heat absorber. (Unit: mm). Figure 9. Pin-fin arrayused usedin in heat heat absorber. Figure 9. Pin-fin array absorber.(Unit: (Unit:mm). mm). Inventions 2016, 1, 2 Inventions 2016, 1, 2 9 of 16 9 of 16 Figure 10. 10. Pin-fin Pin-fin array array used used in in heat heat sink. sink. (Unit: (Unit: mm). Figure mm). 3. Results 3. Resultsand andDiscussions Discussions 3.1. Generation TEG 3.1. Generation Results Results of of Single-Chip Single-Chip TEG Figure 11 11 shows shows relationship relationship between between external Figure external load load resistor resistor (R) (R) and and TEG TEG output output voltage voltage (V). (V). The results temperature difference between hot and the TEG is TEG ΔT = Tish The results show showthat thatwhen when temperature difference between hot cold and faces cold of faces of the − Tc,=a T larger load resistor (R) corresponds to a greater TEG output voltage (V). TEG voltage resulting ∆T h ´ Tc , a larger load resistor (R) corresponds to a greater TEG output voltage (V). TEG from Seebeck effect is Seebeck the product ofisSeebeck coefficient multiplied by ΔT. Therefore, of voltage resulting from effect the product of Seebeck coefficient multiplied by an ∆T.increase Therefore, the external load resistor (R) may also increase the Seebeck coefficient of the TEG. Figure 12 shows an increase of the external load resistor (R) may also increase the Seebeck coefficient of the TEG. the relationship between TEG output voltage (V) andvoltage output(V) current (I). When ΔT is Figure 12 shows the relationship between TEG output and output current (I).constant, When ∆Tthe is output current (I) has an almost linear tendency to decrease with TEG voltage (V). The constant, the output current (I) has an almost linear tendency to decrease withoutput TEG output voltage (V). relationship between the the TEGTEG generation power (P)(P) and external load The relationship between generation power and external loadresistor resistor(R) (R)(see (seeFigure Figure 13) 13) shows that, that, at at the the load load range rangeof of2–20 2–20Ω, Ω,the themaximum maximumTEG TEGgeneration generation power power output output (P) (P) is is achieved achieved shows when RR==6.8 6.8Ω. Ω.Based Basedonon maximum power transfer theory, the maximum conversion when thethe maximum power transfer theory, the maximum powerpower conversion occurs occurs when the external load resistor (R) equals the internal resistance (r). The internal resistance of when the external load resistor (R) equals the internal resistance (r). The internal resistance of the TEG the TEG(TGM-287-1.0-1.5 module (TGM-287-1.0-1.5 KRYOTHERM) about 7 Ω,with which present module KRYOTHERM) is about 7 Ω, is which agrees the agrees present with result.the Figures 14 result. Figures 14 and 15 show the relationships between TEG output voltage (V) and TEG and 15 show the relationships between TEG output voltage (V) and TEG generation Power (P), as well generation Power (P), (V) as well as TEG output voltage and generation efficiency (η). The results as TEG output voltage and generation efficiency (η).(V) The results show that there exists an optimal show that there that exists an yield optimal voltage that will power yield the generation power (P) output voltage will theoutput maximum generation (P)maximum and efficiency (η); this voltage and efficiency (η); this voltage corresponds to an external load resistance of 6.8 Ω, which is closest to corresponds to an external load resistance of 6.8 Ω, which is closest to the internal resistance of the the internal of the selected TEG.WAwas maximum W was achieved the selected TEG.resistance A maximum output of 2.716 achievedoutput when of the2.716 temperature differencewhen between temperature difference between the ∆T hot=and cold TEG ΔT = generation Th − Tc = 120 °C, where the hot and cold faces of the TEG was Th ´ Tc =faces 120 ˝of C,the where thewas optimal efficiency (η) the optimal generation efficiency (η) is 3.61%. Figure 16 shows the relationship between temperature is 3.61%. Figure 16 shows the relationship between temperature difference (∆T = Th ´ Tc ) and optimal difference (ΔT = Th(P −max Tc))and optimal generation power (P ) when the average ofalso the generation power when the average temperature ofmax the TEG was 89–117 ˝temperature C. This figure TEG was 89–117 °C. This figure also shows that maximum power (P max ) increases as the temperature shows that maximum power (Pmax ) increases as the temperature difference between the hot and cold difference thethe hot and cold of the TEG rises;difference the reason is that the a larger temperature faces of thebetween TEG rises; reason is thatfaces a larger temperature between hot and cold faces difference between hot and results in voltage. higher heat rate and results in higher heatthe transfer ratecold andfaces output electric Aftertransfer summarizing all output the test electric results, voltage. After summarizing all the test results, we could write a correlation regarding maximum we could write a correlation regarding maximum generation power (Pmax ) and temperature difference generation power (Pmax) and temperature difference (ΔT = Th − Tc) between the hot2 and cold faces of (∆T = Th ´ T c ) between the hot and cold faces of the TEG: Pmax “ 0.0002 ˆ pTh ´ Tc q . Figure 17 shows the relationship TEG: Pmax between . Figure 17 difference shows the(∆T relationship between the = Th ´ Tc ) and V/(Ththe ´ Tcaverage ) when (Thaverage = 0.0002 × − Tc ) 2 temperature ˝ ∆T = T ´ T = 80–120 C. This figure shows that the difference of the Seebeck coefficient (S ´ SA) c B h temperature difference (ΔT = Th − Tc) and V/(Th − Tc) when ΔT = Th − Tc = 80–120 °C. This figure shows of the TEG decreases with its average temperature ((T ´ T )/2). This must be caused by the smaller that the difference of the Seebeck coefficient (SB −h SA) cof the TEG decreases with its average difference of ((T theh TEG Seebeck ) when its average temperature also B ´ SA temperature − Tc)/2). Thiscoefficient must be (S caused by the smaller difference of the rises. TEG We Seebeck arrived at a correlation regarding the (S ´ S ) of the single-chip TEG used in the experiment and B A coefficient (SB − SA) when its average temperature rises. We also arrived at a correlation regarding ´0.142 q {2s the average temperature of the hot and cold faces: S ´ S “ 0.0708 ˆ rpT ` T , within the c B A and the average h the (SB − SA) of the single-chip TEG used in the experiment temperature of the hot current range of test (SB ´ the S A ) current is seen, at an approximately constanta and cold faces: S B −temperature, S A = 0.0708 × a(minor Th + Tc )variation / 2 −0.142 ,ofwithin range of test temperature, value of 0.0368. minor variation of (SB − SA) is seen, at an approximately constant value of 0.0368. Inventions 2016, 1, 2 10 of 16 Inventions 2016, 1, 2 Inventions 2016, 1, 2 Inventions 2016, 1, 2 10 of 16 10 of 16 10 of 16 Figure 11. External resistance (R) vs. TEG output voltage (V). Figure 11.11.External vs. TEG TEGoutput outputvoltage voltage (V). Figure Externalresistance resistance (R) (R) vs. (V). Figure 11. External resistance (R) vs. TEG output voltage (V). Figure 12. TEG output voltage (V) vs output current (I). Figure 12. TEG output voltage (V) vs output current (I). Figure 12.12.TEG vs.output output current Figure TEGoutput outputvoltage voltage (V) (V) vs current (I).(I). Figure 13. External load resistance (R) vs. TEG power generation (P). Figure 13. External load resistance (R) vs. TEG power generation (P). Figure 13. External load resistance (R) vs. TEG power generation (P). Figure 13. External load resistance (R) vs. TEG power generation (P). Inventions 2016, 1, 2 11 of 16 Inventions 2016, 1, 2 Inventions 2016, 1, 2 Inventions 2016, 1, 2 11 of 16 11 of 16 11 of 16 Figure 14. TEG output voltage (V) vs. generation power (P). Figure 14.14.TEG vs.generation generationpower power Figure TEGoutput outputvoltage voltage (V) (V) vs. (P).(P). Figure 14. TEG output voltage (V) vs. generation power (P). Figure 15. TEG output voltage (V) vs. generation efficiency (η). Figure 15. TEG output voltage (V) vs. generation efficiency (η). Figure 15.15. TEG vs.generation generationefficiency efficiency Figure TEGoutput outputvoltage voltage (V) (V) vs. (η).(η). Figure 16. Temperature difference between TEG hot/cold faces (Th − Tc) vs. maximum generation power (Pmax). Figure 16. Temperature difference between TEG hot/cold faces (Th − Tc) vs. maximum generation power (Pmax). Figure 16. Temperature difference between TEG hot/cold faces (Th − Tc) vs. maximum generation power (Pmax). Figure 16. Temperature difference between TEG hot/cold faces (Th ´ Tc ) vs. maximum generation power (Pmax ). Inventions 2016, 1, 2 Inventions 2016, 1, 2 12 of 16 12 of 16 Figure 17. Average TEG temperature (Th − Tc)/2 vs. Seebeck-coefficient difference V/(Th − Tc). Figure 17. Average TEG temperature (Th ´ Tc )/2 vs. Seebeck-coefficient difference V/(Th ´ Tc ). 3.2. Generation Results of Thermoelectric Conversion System on a Real Engine 3.2. Generation Results of Thermoelectric Conversion System on a Real Engine Based on the power transfer theory, the resistance of the load must equal the internal resistance on thesource powerfor transfer theory, thetransference resistance of equalathe internal resistance ofBased the voltage maximum power tothe the load load.must Therefore, variable resistance of the voltage for maximum poweroftransference to thethe load. Therefore, variable resistance load systemsource was connected to the output the TEG to adjust resistance of theaload. The output voltage different loads was recorded. loadcurrent systemand wasoutput connected to under the output of the TEG to measured adjust theand resistance of We the calculated load. The the output maximum power voltage under different andloads foundwas that measured the load resistance that gaveWe thecalculated maximum the current and output under loads, different and recorded. power complied with the internalloads, resistance the TEG was taken as the maximum power under different and of found thatand thethis load resistance thatmaximum gave thepower. maximum The main target parameter of the experiment was generation power (P). The variable power complied with the internal resistance of the TEG and this was taken as the maximum power. parameters included engine rotation speed (Ω [rpm]) and mean velocity (U) of air passing through The main target parameter of the experiment was generation power (P). The variable parameters the heat sink, where (U) is achieved by adjusting the switch to control the speed of the fan’s motor. included engine rotation speed (Ω [rpm]) and mean velocity (U) of air passing through the heat sink, The engine speed was controlled by the throttle valve and change in engine speed (Ω) caused the where (U) is achieved by adjusting the switch to control the speed of the fan’s motor. The engine change in the quantity (Qflow) of exhaust gas and the temperature (Ti) of the exhaust gasses entering speed controlled by the throttle valveIncreasing and change engine caused change in thewas thermoelectric conversion system. Qflowin and Ti speed would (Ω) increase thethehot-side the temperature quantity (Q fof ) of exhaust gas and the temperature (T ) of the exhaust gasses entering i low TEG, promoting the generating performance of TEG. Figure 18 shows the the thermoelectric conversion system. Increasing Q and T would increase hot-side temperature f low relationship between engine rotation (Ω) and quantity of iexhaust gas (Qflow).the From the near idle (Ω = of TEG,2700 promoting Figure 18the shows the relationship between engine rpm) to the thegenerating maximum performance torque outputof(ΩTEG. = 5400 rpm), exhaust gas discharge (Qflow) was rotation (Ω) and quantity of exhaust gasincreases (Q f low ).asFrom the nearasidle (Ω = rpm)(2): to the maximum estimated from 500 to 1350 c.c./s. Qflow Ω increases, shown in2700 Equation torque output (Ω = 5400 rpm), the exhaust gas 1 1discharge (Q f low ) was estimated from 500 to 1350 c.c./s. × × (Total(2): displacement) × ϕ (2) flow = Ω ×in Equation Q f low increases as Ω increases, asQshown 60 2 The total displacement is 35.8 c.c. and 1 the 1 volume efficiency φ is between 0 and 1 and increases Q “ Ω ˆ ˆ ˆ pTotal displacementq ˆ ϕ (2) f low with Ω. Altering engine rotation speed 60(Ω) 2also affects the temperature (Ti) of the exhaust gasses entering the thermoelectric conversion system. Since combustion is more efficient at higher engine The total displacement is 35.8 c.c. and the volume efficiencyof ϕ the is between and 1 and increases speed, more combustion heat is generated and the temperature exhaust 0 gas goes up when withengine Ω. Altering engine rotation speed (Ω) also affects the temperature (T ) of the exhaust gasses speed (Ω) is raised. From Figure 19 it can be seen that an increase in engine speed (Ω) from i entering thermoelectric conversion system. Since combustion is more at to higher engine 2700 the to 5400 rpm, results in a change of exhaust gas temperature (Ti) efficient from 230 325 °C. Additionally, no significant was seen thetemperature exhaust gas temperature (Ti) gas withgoes the up mean speed, more combustion heat change is generated andinthe of the exhaust when cooling air velocity (U) used in the experiments. The range of U may be too small to reveal the engine speed (Ω) is raised. From Figure 19 it can be seen that an increase in engine speed (Ω) from ˝ C.the in Ti.results Furthermore, the 10ofcm distance between the engine exhaust and inlet of 2700influence to 5400 rpm, in a change exhaust gas temperature (Ti ) from 230outlet to 325 Additionally, the thermoelectric conversion system accounts for a drop of 25–50 °C at the inlet of velocity the no significant change was seen in the exhaust gas temperature (Ti ) with the mean cooling air thermoelectric conversion system due to heat loss. (U) used in the experiments. The range of U may be too small to reveal the influence in Ti . Furthermore, During the generation power (P) test a voltmeter and a variable resistor are connected in series the 10 cm distance between the engine exhaust outlet and the inlet of the thermoelectric conversion in the TEG circuit as an external load. The value of the external load resistor (R) must be equal to the system accounts for a drop of 25–50 ˝ C at the inlet of the thermoelectric conversion system due to internal resistance of the TEG to give the maximum output power. The equation for calculating heatgeneration loss. power (P) is the same as Equation (1) in Section 2.2. During the generation power (P) test a voltmeter and a variable resistor are connected in series in the TEG circuit as an external load. The value of the external load resistor (R) must be equal to Inventions 2016, 1, 2 13 of 16 the internal resistance of the TEG to give the maximum output power. The equation for calculating generation Inventionspower 2016, 1, 2(P) is the same as Equation (1) in Section 2.2. 13 of 16 Inventions 2016, 1, 2 13 of 16 3000 3000 Qflow [c.c./s] Qflow [c.c./s] 2500 2500 2000 2000 1500 1500 1000 1000 500 500 0 0 0 0 2000 2000 4000 6000 4000Ω [rpm] 6000 Ω [rpm] 8000 8000 10000 10000 Figure Engine rotationspeed speed (Ω)vs. vs. exhaust gas flow). Figure 18. 18. Engine rotation gasdischarge discharge(Q low ). Figure 18. Engine rotation speed(Ω) (Ω) vs. exhaust exhaust gas discharge (Q(Q flowf). Figure 19. Mean air velocity vs. exhaust temperature. Figure 19. Mean air velocity vs. exhaust temperature. Figure 19. Mean air velocity vs. exhaust temperature. In this study we used an actual engine to investigate waste heat recovery and the test results are In this study we used an actual engine to investigate waste heat recovery and the test results are from a specially designed thermoelectric conversion system installed on the exhaust. The variables In this study we used anthermoelectric actual engineconversion to investigate waste heat recovery and the test results are from a specially designed system installed on the exhaust. The variables discussed include the mean velocity (U) of air driven by an external fan, the temperature difference discussed include the mean velocity (U) of air driven system by an external fan,on thethe temperature difference from(ΔT) a specially designed thermoelectric conversion installed exhaust. The variables between hot/cold faces of the TEGs, the engine rotation speed (Ω) and the quantity (Qflow ) of (ΔT) between hot/cold faces of the TEGs, the engine rotation speed (Ω) and the quantity (Qflow) of(∆T) discussed include the mean velocity (U) of air driven by an external fan, the temperature difference exhaust Gas. Figure 20 shows the relationship between the mean air velocity (U) and the mean exhaust Gas. Figure 20 the shows the relationship between the mean airthe velocity (U) (Q and the between hot/cold of TEGs, rotation speed (Ω) and quantity ofmean exhaust temperature of faces the TEGs and it canthe be engine seen that the mean temperature of the TEGs goes up )with an f low temperature of the TEGs and it can be seen that the mean temperature of the TEGs goes up with an Gas.increase Figure 20 relationship between the mean air velocity (U) and mean temperature in shows engine the rotation speed (Ω). However, no significant change wasthe seen in the mean increase in engine rotation speed (Ω). However, no significant change was seen in the mean temperature TEGs with that the mean air velocity (U) usedof in the the experiments. This means the in of the TEGs andofitthe can be seen the mean temperature TEGs goes up with an that increase temperature of the TEGs with the mean air velocity (U) used in the experiments. This means that the mean air velocity (U) mayHowever, have beenno setsignificant too low andchange coolingwas was seen insufficient. Furthermore, since the engine rotation speed (Ω). in the mean temperature of the mean air velocity (U) may have been set too low and cooling was insufficient. Furthermore, since the operation of the TEG cannot 200 °C, the engine rotation speed (Ω) in air thevelocity current (U) TEGs with thetemperature mean air velocity (U) used inexceed the experiments. This means that the mean operation temperature of the TEG cannot exceed 200 °C, the engine rotation speed (Ω) in the current nottoo cause the TEGs. Further studies with since higher engine rotation speed maystudy have could been set lowoverheating and coolingof operation temperature study could not cause overheating ofwas the insufficient. TEGs. FurtherFurthermore, studies with higherthe engine rotation speed (Ω) can be conducted in the future. Figure 21 shows the relationship between the mean air velocity ˝ of the exceedin200 C, the Figure engine21rotation speed (Ω) in the currentthe study not cause (Ω)TEG can cannot be conducted the future. shows the relationship between meancould air velocity (U) and the temperature difference (ΔT) of TEGs. Within the current range of the mean velocity (U) overheating of the TEGs. Further studies higher engine rotationrange speed can bevelocity conducted (U) and the temperature difference (ΔT) with of TEGs. Within the current of (Ω) the mean (U) in of air driven by an external fan, the trend was towards a significant connection between ΔT of TEGs the future. Figure the relationship between mean air connection velocity (U) and the of air driven by21 anshows external fan, the trend was towardsthe a significant between ΔTtemperature of TEGs and engine rotation speed (Ω). When engine rotation speed (Ω) was increased from 2700 to 5400 and engine rotation speed (Ω). When engine rotation speed velocity (Ω) was (U) increased from 2700 to 5400 difference (∆T) of TEGs. Within the current range of the mean of air driven by an external rpm, ΔT of TEGs increased from 20 to 58 °C. rpm, ΔT of TEGs increased from 20 to 58 °C. Inventions 2016, 1, 2 14 of 16 fan, the trend was towards a significant connection between ∆T of TEGs and engine rotation speed (Ω). When engine rotation speed (Ω) was increased from 2700 to 5400 rpm, ∆T of TEGs increased from 20 toInventions 58 ˝ C. 2016, 1, 2 14 of 16 Inventions 2016, 1, 2 14 of 16 Figure 20. Mean air velocity vs. mean TEG temperature. Figure 20. Mean air velocity vs. mean TEG temperature. Figure 20. Mean air velocity vs. mean TEG temperature. Figure 21. Mean air velocity (U) vs. ΔT of the TEGs. Figure 21. Mean air velocity (U) vs. ΔT of the TEGs. Figure 21. Mean air velocity (U) vs. ∆T of the TEGs. From Figure 22 it can be seen that engine rotation speed (Ω) has a significant effect on power From Figure 22 itgoes can up be with seen an that engineofrotation (Ω) has a(Ω). significant effect on power generation (P) which increase engine speed rotation speed Within the current range generation (P) which goes up with an increase of engine rotation speed (Ω). Within the current range From Figure 22 it can be seen that engine rotation speed (Ω) has a significant effect on power of mean air velocity (U) used in our experiments, an increase of mean air velocity (U) did not of mean air velocity (U) used in our experiments, an increase of mean air velocity (U) did not generation which goes up(P). with increase of engine rotation speed (Ω). current range increase(P) generated power At an engine rotation speed (Ω) of 5400 rpm, theWithin output the power (P) was increase generated power At an engine generation rotation (Ω)of of 5400 rpm, the output (P) increase was 2.5 W. on(U) Seebeck thermoelectric theory, the TEG voltage (V)not satisfies of mean air Based velocity used(P). in our experiments, an speed increase mean airoutput velocity (U)power did 2.5 W. power Based thermoelectric TEG Equation (3): on generated (P).Seebeck At an engine rotationgeneration speed (Ω)theory, of 5400the rpm, theoutput outputvoltage power(V) (P)satisfies was 2.5 W. Equation (3): Based on Seebeck thermoelectric generation theory, V = ΔSthe ⋅ ΔTTEG output voltage (V) satisfies Equation (3) (3): V = ΔS ⋅ ΔT (3) where ΔS is the difference value of the Seebeck V “ coefficient ∆S ¨ ∆T of the semiconductor material used in the (3) where ΔS isvalue the difference value of the Seebeck of the semiconductor material used in the TEGs. The of ΔS is not a constant, it varies coefficient with temperature of TEGs. However this temperature TEGs. The value of ΔS is not a constant, it mean varies value with temperature ofand TEGs. However this temperature range is limited as is variation of ΔS. A may be used the generated power (P) will where ∆S is the difference value of the Seebeck coefficient of the semiconductor material used in the range limited as isofvariation of ΔS. Avoltage mean value may be used and external the generated power(R). (P) will equal isthe square the produced (V) divided by the resistance Our TEGs. The value of ∆S is not a constant, it varies with temperature of TEGs. However this temperature equal the square the produced voltage (V) Furthermore, divided by itthe external that resistance (R). Our experimental resultsofcomply with this assumption. is predicted by extending the range is limited asresults is variation ofwith ∆S. this A mean value may be used and the generated power (P) willthe equal experimental comply assumption. Furthermore, it is predicted that by extending experimental data to set the engine rotation speed at 6000 rpm, the output power (P) will be 3.8 W. the square of the produced voltage (V) divided by the external resistance (R). Our experimental results experimental to set engine rotation speed atby6000 thethermoelectric output powerconversion (P) will be system 3.8 W. The generateddata power (P)the is far below that achieved the rpm, similar comply with this assumption. Furthermore, it is predicted that by extending the experimental data to The generated power (P) is far below that achieved by the similar thermoelectric conversion system previously designed by the authors where the heat absorber had arrays of straight fins with copper previously designed by the authors where the heat absorber had arrays of straight fins with copper foams in the gaps between fins, and were capable of generating 4.2 W at an engine rotation speed of foams in the gaps between fins, and were capable of generating 4.2 W at an engine rotation speed of Inventions 2016, 1, 2 15 of 16 set the engine rotation speed at 6000 rpm, the output power (P) will be 3.8 W. The generated power (P) is far below that achieved by the similar thermoelectric conversion system previously designed by the Inventions 2016,the 1, 2heat absorber had arrays of straight fins with copper foams in the gaps between 15 of 16 fins, authors where and were capable of generating 4.2 W at an engine rotation speed of 6000 rpm. However, the pin-fin 6000 rpm. However, the pin-fin array heat absorber used here has lower flow resistance, and the array heat absorber used here has lower flow resistance, and the back pressure is not so severe that it back pressure is not so severe that it would impede normal operation of the engine. Finally, would impede normal operation of the engine. Finally, according to the Figure 22, the present system according to the Figure 22, the present system may generate over 5 W when the present small mayengine generate W when theispresent small engine (35.8 c.c. displacement) is operating at the (35.8over c.c. 5displacement) operating at the maximum power output condition (Ω = 7000 maximum rpm). power output condition (Ω = 7000 rpm). Figure 22. Engine rotation speed (Ω) vs. output power (P). Figure 22. Engine rotation speed (Ω) vs. output power (P). 4. Conclusions 4. Conclusions An actual four-stroke engine (35.8 c.c.) was used for these experiments on a test platform An actualfor four-stroke engine (35.8 c.c.) was used for these using experiments on a recovered test platform designed designed the investigation of thermoelectric generation waste heat from the engine exhaust gases. The study successfully investigated theheat effects of the engine rotation speed for the investigation of thermoelectric generation using waste recovered from the engine exhaust and thestudy meansuccessfully velocity of external coolingthe aireffects on theof temperature discharged quantity of engine gases. The investigated the engine and rotation speed and the mean velocity exhaustcooling gas. Experiments were also performed to measure the relationship betweengas. theExperiments external of external air on the temperature and discharged quantity of engine exhaust andtothe TEG output voltage/current, as well betweenload TEGresistance output voltage andTEG wereload alsoresistance performed measure the relationship between theasexternal and the generated power/efficiency. A correlation was derived for calculating the maximum generation output voltage/current, as well as between TEG output voltage and generated power/efficiency. power of a single-chip TEG and the temperature difference between the hot and cold faces of TEG. A correlation was derived for calculating the maximum generation power of a single-chip TEG and the The study also examined the influence of engine rotation speed and the velocity of external cooling temperature difference between the hot and cold faces of TEG. The study also examined the influence air against power generation of the thermoelectric conversion system, and found that power of engine rotation speed andengine the velocity external cooling air against power of the generation increased with rotation of speed. Within the current range of mean generation velocity of the thermoelectric conversion system,ofand found that power withpower. engineIt rotation external cooling air, an increase cooling-air speed wouldgeneration not increaseincreased the generated was speed. Within the current range of mean velocity of the external cooling air, an increase cooling-air found that four TEG modules in series could generate 2.5 W power at an engine rotationofspeed of speed would thetorque generated power. It was found that four TEG modules series could 5400 rpm not (theincrease maximum output). If the present small engine operates at theinmaximum generate W power at an(Ωengine ofsystem 5400 rpm maximum torque output). If the power2.5 output condition = 7000 rotation rpm), thespeed present will (the generate over 5 W. present small engineThe operates the maximum output condition (Ω = 7000ofrpm), the present Acknowledgment: authors at would like to thank power the Ministry of Science and Technology the Republic of system will over 5 W. this research under Contract Nos. MOST 104-2221-E-270-005 and MOST China forgenerate financially supporting 104-2622-E-270-005-CC3. Acknowledgments: The authors would like to thank the Ministry of Science and Technology of the Republic AuthorforContributions: Tzer-Ming Jeng conductedunder the experimental measurements and data analysis and of China financially supporting this research Contract Nos. MOST 104-2221-E-270-005 Sheng-Chung Tzeng led this work and participated discussions; and Bo-Jun Yang and Yi-Chun Li assisted in the MOST 104-2622-E-270-005-CC3. implementation of the experimental measurements. Author Contributions: Tzer-Ming Jeng conducted the experimental measurements and data analysis Conflicts ofTzeng Interest: authors no conflict ofdiscussions; interest. Sheng-Chung ledThe this work declare and participated and Bo-Jun Yang and Yi-Chun Li assisted in the implementation of the experimental measurements. References Conflicts of Interest: The authors declare no conflict of interest. 1. Champier, D.; Bedecarrats, J.P.; Rivaletto, M.; Strub, F. Thermoelectric power generation from biomass cook stoves. Energy 2010, 35, 935–942. Inventions 2016, 1, 2 16 of 16 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Champier, D.; Bedecarrats, J.P.; Rivaletto, M.; Strub, F. Thermoelectric power generation from biomass cook stoves. Energy 2010, 35, 935–942. [CrossRef] Karri, M.A.; Thacher, E.F.; Helenbrook, B.T. Clarkson Exhaust energy conversion by thermoelectric generator: Two case studies. Energy Convers. Manag. 2011, 52, 1596–1611. 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