Rozprawy doktorskie na temat „Thermoelectric System”

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 152-155).
Recent years have witnessed a trend of rising electricity costs and an emphasis on energy efficiency. Thermoelectric (TE) devices can be used either as heat pumps for localized environmental control or heat engines to convert heat into electricity. Thermoelectrics are appealing because they have no moving parts, are highly reliable, have high power densities, and are scalable in size. They can be used to improve the overall efficiency of many systems including vehicle waste heat, solar thermal, HVAC, industrial waste heat, and remote power for sensor applications. For thermoelectric generators to be successful, research progress at the device level must be made to validate materials and to guide system design. The focus of this thesis is thermoelectric device testing and system modeling. A novel device testing method is developed between room temperature range and 230°C. The experimental technique is capable of directly measuring an energy balance over a single leg, with a large temperature of 2-160°C. The technique measures all three TE properties of a single leg, in the same direction, with significantly less uncertainty than other methods. The measurements include the effects of temperature dependent properties, side wall radiation loss, and contact resistance. The power and efficiency were directly measured and are within 0.4 % and 2 % of the values calculated from the property measurements. The device property measurement was extended to higher temperatures up to 600°C. The experimental system uses an inline unicouple orientation to minimize radiation losses and thermal stress. Two major experimental challenges were the construction of a high temperature calibrated heater and a thermocouple attachment technique. We investigated skutterudite materials which are of interest to many research groups due to their high thermoelectric figure-of-merit (ZT), and good thermomechanical properties. Unlike room temperature Bi2Te 3 devices, skutterudite module construction techniques are not well established and were a major challenge in this work. Skutterudite device samples were fabricated by a direct bonding method in which a rigid electrode is sintered directly to the TE powder during press. Compatible electrode materials were identified and evaluated based on thermal stress, parasitic electrical/thermal resistance, chemical stability and ease of prototype fabrication. The final electrodes solutions were Co2 Si with the P-type and CoSi2 with the N-type. The direct hot press process was modified into what we call a hybrid hot press to produce device samples with strong bonds and no cracks. Preliminary accelerated aging tests were conducted to evaluate the long term chemical stability of the TE-electrode contacts. We demonstrated ZTff = 0.74 for the N-type between 52°C and 595°C corresponding to 11.7% conversion efficiency and Zlff = 0.51 for the P-type between 77°C and 600°C corresponding to 8.5% efficiency. The maximum efficiency of the NP unicouple was measured to be 9.1% at ~550°C. The effective ZT and efficiency measurement includes electrical contact resistance, and parasitic thermal/electrical resistance in the electrodes, and heat losses at the sides of the legs. Thus we have included all the parasitic loss effects that are present in a real unicouple. The efficiency values measured in this work are among the highest recorded for a skutterudite unicouple. The TE-electrode combinations meet all the criteria for device testing and offer a practical, manufacturable solution for module construction. Solar thermal power generation is fast becoming cost competitive for utility scale electricity with 380 MW electric currently installed. Parabolic trough concentrators have proven economical and reliable but their efficiency is limited by the maximum temperature of the heated fluid. We explored the idea of a solar thermoelectric topping cycle (STET) in which a thermoelectric generator (TEG) is added at high temperature to increase the overall efficiency of the solar Rankine cycle. In this design the perimeter of the receiver tube is covered with thermoelectrics so that the absorber temperature is raised and the energy rejected from the TEG is used to heat the fluid at its originally specified temperature. A heat transfer analysis was carried out to determine the overall system efficiency. A parametric study was performed to identity design constraints and put bounds on the total system efficiency. The system performance was simulated for all conceivable concentrations and fluid temperatures of a solar thermal trough. As the absorber temperature increases more power is generated by the TEG but is offset by a rapidly decreasing absorber efficiency which results in only a marginal increase in net power. It was concluded that for the proposed STET to increase the system efficiency of a state of the art trough system by 10% requires a ZI =3 TEG, which is well beyond the state-of-the-art thermoelectric materials.
by Andrew Muto.
Ph.D.

An efficient thermoelectric distillation system was designed, constructed and tested. The unique aspect of this design is to use the waste heat from the hot side of thermoelectric module for heating of the feed water, to improve the evaporation while using the cold side of the module to cool the condenser and improve the condensation process. The developed thermoelectric distillation system produces 28.5 mL of distilled water (equivalent to 678 mL/m2) over a period of 1 hour. The corresponding electrical energy required for the water production is 0.0324 kWh, which gives a specific energy consumption of 0.00114 kWh/mL. The developed system in this research has significantly lower energy consumption than the existing thermoelectric distillation systems. The transient to steady state behaviour of the developed thermoelectric distillation system was investigated. It was found that the system reaches steady state after approximately three hours of the system operation. The water temperature in evaporation chamber was increased from 22.3 oC to 47.8 oC. Similarly, the vapour temperature was increased moderately from 20.3 oC to 30.4 oC. The steady state water production, humidity, energy consumption and COP of the thermoelectric distillation system were 15.3 mL/h, 81%, 0.0324 kWh and 1.04, respectively. Thermal models have been developed through water-vapour phase-change theory to interpret the evaporation and condensation processes involved in the fresh water production of the thermoelectric distillation system. The first model was related to the evaporation process to determine the vapour production in the system. A theoretical distillation ratio of 12% was obtained, with a predicted water temperature of 42.7 oC. This is in reasonable agreement with the 9.5% value experimentally obtained. The second ii model has been developed for the water condensation process. The developed model can be used for determining the key parameters that control the condensation processes and the system thermal performance. This model shows that the rate of water condensation is dependent upon the convection heat transfer coefficient of the cold-side heat exchanger. The fitted value of the convection heat transfer coefficient in the thermoelectric distillation system is 8 W/m2.K. Key factors that influence the total water production and water production rate have been investigated, including sample water temperature, vapour volume at sample water level, Peltier current and thermoelectric input power. The experimental data shows that an increase in sample water temperature from 30 oC to 60 oC gives a 47 % increase in total water production. Peltier current is demonstrated as a control factor in the design of an effective thermoelectric distillation system. The results show that the total water production increases by 61%, when the volume occupied by the vapour is reduced from 600 cm3 to 400 cm3 by increasing the sample water level from 10 mm to 30 mm in the system. The maximum water production is achieved by increasing sample water temperature and the corresponding optimised input power. Measurements of the distilled water show that it has similar quality to drinkable tap water in terms of pH, total dissolved solids and electrical conductivity values. Photovoltaic Geographical Information System was used to estimate the global irradiation per square meter and the solar electricity generation in kWh received by a solar panel in a specific region. Using the experimental prototype, the maximum monthly average water production is 4023.3 mL when using 8.52 kWh of electricity produced during March at the University of Kufa. The minimum average monthly water production is 2970.3 mL using 6.29 kWh of electricity produced during November.

Thermoelectric devices are useful for a variety of applications due to their ability to either convert heat directly into electricity, or to generate a temperature gradient from an electric current. These devices offer several attractive features including compact size, no moving parts, limited maintenance requirements, and high reliability. Thus thermoelectric devices are used for temperature-control, cooling, or power generation in various industrial systems such as automobiles, avionics, refrigerators, chillers, laser diodes, dehumidifiers, and a variety of sensors. In order to improve the efficiency of thermoelectric devices, many endeavors have been made to design and fabricate materials with a higher dimensionless thermoelectric figure of merit (ZT), as well as to optimize the device structure and packaging to manage heat more effectively. When evaluating candidate thermoelectric materials, one must accurately characterize the electrical conductivity, thermal conductivity, and the Seebeck coefficient over the temperature range of potential use. However, despite considerable research on thermoelectric materials for decades, there is still significant scatter and disagreement in the literature regarding accurate characterization of these properties due to inherent difficulties in the measurements such as requirements for precise control of temperature, simultaneous evaluation of voltage and temperature, etc. Thus, a well-designed and well-calibrated thermoelectric measurement system that can meet the requirements needed for multiple kinds of thermoelectric materials is an essential tool for the development of advanced thermoelectric devices. In this dissertation, I discuss the design, fabrication, and validation of a measurement system that can rapidly and accurately evaluate the Seebeck coefficient and electrical resistivity of thermoelectric materials of various shapes and sizes from room temperature up to 600 K. The methodology for the Seebeck coefficient and electrical resistivity measurements is examined along with the optimization and application of both in the measurement system. The calibration process is completed by a standard thermoelectric material and several other materials, which demonstrates the accuracy and reliability of the system. While a great deal of prior research has focused on low temperature thermoelectric materials for cooling, such as Bi2Te3, high temperature thermoelectric materials are receiving increasing attention for power generation. With the addition of commercial systems for the Seebeck coefficient, electrical resistivity, and thermal conductivity measurements to expand the temperature range for evaluation, a wide range of materials can be studied and characterized. Chapter Two of this dissertation describes the physical properties characterization of a variety of thermoelectric materials, including room temperature materials such as Bi0.5Sb1.5Te3, medium temperature level materials such as skutterudites, and materials for high temperature applications such as half-Heusler alloys. In addition, I discuss the characterization of unique oxide thermoelectric materials, which are Al doped ZnO and Ca-Co-O systems for high temperature applications. Chapter Four of this dissertation addresses the use of GaSn alloys as a thermal interface material (TIM), to improve thermal transport between thermoelectric devices and heat sinks for power generation applications at high temperature. I discuss the mechanical and thermal behavior of GaSn as an interface material between electrically insulating AlN and Inconel heat exchangers at temperatures up to 600 °C. Additionally, a theoretical model for the experimental thermal performances of the GaSn interface layer is also examined.
Ph. D.

Thermoelectric generators (TEGs) are currently a topic of interest in the field of energy harvesting for automobiles. In applying TEGs to the outside of the exhaust tailpipe of a vehicle, the difference in temperature between the hot exhaust gases and the automobile coolant can be used to generate a small amount of electrical power to be used in the vehicle. The amount of power is anticipated to be a few hundred watts based on the temperatures expected and the properties of the materials for the TEG.
This study focuses on developing efficient heat exchanger modules for the cold side of the TEG through the analysis of experimental data. The experimental set up mimics conditions that were previously used in a computational fluid dynamics (CFD) model. This model tested several different geometries of cold side sections for the heat exchanger at standard coolant and exhaust temperatures for a typical car. The test section uses the same temperatures as the CFD model, but the geometry is a 1/5th scaled down model compared to an full-size engine and was fabricated using a metal-based rapid prototyping process. The temperatures from the CFD model are validated through thermocouple measurements, which provide the distribution of the temperatures across the TEG. All of these measurements are compared to the CFD model for trends and temperatures to ensure that the model is accurate. Two cold side geometries, a baseline geometry and an impingement geometry, are compared to determine which will produce the greater temperature gradient across the TEG.
Master of Science

Thesis: S.M. in Engineering and Management, Massachusetts Institute of Technology, System Design and Management Program, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 69-70).
Thermoelectric power generators (TEGs) convert a temperature difference into electricity. This temperature difference can be created from waste heat. Since up to 50% [1] of US industrial energy input is lost as waste heat, an economical means of recovering waste heat and converting it into useful electricity could represent significant energy savings. Coupled with our integrative system design which involves creating application specific thermoelectric arrays, this technology can also help enable low power generation for off-grid needs in the developing world. Although conversion efficiencies as high as 20.9% [2] (heat to electrical energy) have been predicted from simulations of TEGs systems, in practice the efficiencies are typically only a few percent. Moreover, conventional systems often require expensive components to manage heat flow through the system.
As a result of the low efficiency and high system cost, electricity generated by thermoelectric energy harvesting from waste heat is currently not competitive with conventional electricity generation on a dollars-per-watt basis. This realization has led researchers to not only focus on increasing TEG device efficiency limits but to devise cheaper manufacturing processes and methods. A system design constraint that has not been fully investigated is the coupling of thermal and mechanical properties in thermoelectric materials. The extent to which this coupling affects the performance of the TEGs will be studied. This thesis develops an approach for decoupling the thermal and mechanical properties and tests it through a variety of simulations. We propose a mechanically compliant attachment strategy which could be integrated in various waste heat recovery applications.
The strategy involves breaking the thermal and mechanical bond formed by the brittle thermoelectric elements and its substrate. Copper wire, which is more pliable, is then used to connect the thermoelectric element to the substrate. A system analysis was performed for waste heat recovery from a vehicles exhaust pipe. We found that utilizing the proposed strategy should not only lead to increased mechanical compliance but can also lead to cost savings on a dollars-per-watt basis. We found that 84% power retention could be obtained when up to 16x less material is used under most apparent conditions¹.
by Corey D. Christian.
S.M. in Engineering and Management
S.M.inEngineeringandManagement Massachusetts Institute of Technology, System Design and Management Program

This thesis is concerned with the design and evaluation of a small scale solarthermoelectric power generation system. The system is intended for electricity generation and thermal energy supply to small scale applications in developing countries of the sunny equatorial regions. Detailed design methodologies and evaluations of both the thermoelectric device and the solar energy collector, which are parts of the combined system, are presented. In addition to experimental evaluations, three theoretical models are presented which allow the design and evaluation of both the thermoelectric module and the solar energy collector. One of the models (a unified thermoelectric device model) concerns the geometrical optimization and performance prediction of a thermoelectric module in power generation mode. The model is unified in the sense that it accounts for the effect of all the parameters that contribute to the performance of the thermoelectric module, a number of which are ignored by the available design models. The unified model is used for a comparative evaluation of five thermoelectric modules. One of these is commercially available and the others are assumed to have optimum geometry but with different design parameters (thermal and electrical contact layer properties). The model has been validated using data from an experimental investigation undertaken to evaluate the commercial thermoelectric module in power generation mode. Results showed that though the commercially available thermoelectric cooling devices can be used for electricity generation, it is appropriate to have modules optimized specifically for power generation, and to improve the contact layers of thermoelement accordingly. Attempts have also been made to produce and evaluate thermoelectric materials using a simple melt-qucnching technique which produces materials with properties similar to those of the more expensive crystalline materials.

There is growing interest in the use of cryosurgical treatment for the ablation of cancerous and diseased tissue. This thesis describes experimental and numerical investigation of the thermoelectric devices to be utilized in development of the cryosurgical probe for generating freezing and rewarming temperature required for tissue ablation. Thermoelectric cooling devices were used in this research due to being compact, noiseless with no moving parts and no circulating refrigerant. A novel three-dimensional model of human living tissue including metabolic heat, perfusion of blood and variation of tissue properties with temperature has been developed to determine thermal behaviour of tissue during cryosurgery process and predict the cooling requirement of the cryosurgical probe using COMSOL Multiphysics 5.2 software. COMSOL Multiphysics was used for the first time to develop three dimensional model of single stage and multistage thermoelectric devices and to predict the temperature difference across the thermoelectric modules at different input of electrical power. It is concluded that three stage thermoelectric module is capable of generating the temperature of the 213 K for cancer tissue ablation. The laboratory prototype of the cryosurgical probe was developed to investigate the performance of three stage thermoelectric device and the minimum temperature of the approximately 240 K were achieved in the experimental test. A circular hollow pin fin with lower thermal resistance was developed in SolidWorks flow simulation 2015 software and introduced as a suitable heat exchanger to be used in the laboratory prototype.

Due to quiet operation, no moving parts, long lifespan and compact structure, the thermoelectric application has become a potential green technology which has been used in different areas in the efforts of contributing to achieve simplified and compact system structures and environmental friendliness. Its applications cover a wide range from the earliest application on kerosene lamp to aerospace applications, transportation tools, industrial utilities, medical services, electronic devices and temperature detecting & measuring facilities. Its disadvantage lies in the low conversion efficiency which only converts small amount (for Bi2Te3, up to 5%) of harvested energy to electrical power. It makes the use of the TEG system far from being economically feasible due to long cost recovery period. Consequently, its use is limited to specialised area where it is unnecessary to consider the cost of the thermal energy input and system cost recovery. This research aims to explore a way of widening the application range of thermoelectric generation based on introducing a potential direction of improving energy utilisation efficiency to a higher level by adopting thermoelectric cogeneration concept in residential house. It focuses on investigating the practicality of using thermoelectric applications in domestic sectors where the large amount of heat is exhausted to environment without being used and developing thermoelectric cogeneration system to generate electricity and produce pre-heated water for domestic use by recovering the waste heat from the domestic boiler and utilising the on-site solar energy. With the conversion efficiency given by the current commercially available thermoelectric modules, the optimised heat exchanging regimes and systems for thermoelectric applications have been comprehensively studied from the aspects of system design, integration, experimental study, numerical simulation and modification. The importance and necessity of effective heat exchanging methods have been emphasised by the experimental and numerical proofs for the development of a domestic thermoelectric cogeneration system with higher thermal efficiency. The impacts of this domestic energy solution have been evaluated from the aspects of the improvement for outdoor environment and indoor energy profile, as well as economic benefit. For the flue gas heat exchanger, the model with sudden expansion and gradual constriction has been identified gives in terms of overall performance. The model with sudden expansion, gradual constriction and staggered pipe layout and the one with sudden expansion, gradual constriction and inline pipe layout show better overall performance than other models. Among these two models, the one with staggered pipe layout shows better performance than the one with inline pipe layout in the velocity range of 3.6m/s-5m/s, whilst the one with inline pipe layout shows better performance between 0-3.6m/s. For the cold side heat exchanger, the one with four ø5mm branch channel angled at 90 against the main channel delivers the best overall performance out of 9 cooling plates built according to three variables. Experimental studies show the one-stage TCS produces more power than the two-stage TCS does when the heat input is supplied at 47W and 60W. As the heat input increases, the power output of two-stage TCS gets closer to that of one-stage TCS. In the system construction and assembly, uneven assembly can lead to a 20% drop in conversion efficiency. The pressure load at 18lpsi gives the highest power output out of five load values, which are 136psi, 159psi, 181psi, 204psi and 227psi, respectively. In comparison with individual assembly, module thickness difference in whole assembly degrades the system conversion efficiency. The cost recovery period of deploying this system in a residential house installed with a 24kW boiler and a 1 m2 solar collector has been evaluated. Based on the conversion efficiency and thermal efficiency that is 4% and 67% at 130C temperature difference. the house can produce 98W electricity and 1640W useable heat when the boiler is running and the contribution from the solar energy is included. It takes less than 4.2 years to recover the system cost.

Increasing fuel prices, higher demands on "greener" transports and tougher international emission regulations puts requirements on companies in the automotive industry in improving their vehicle fuel efficiency. On a typical heavy duty Scania truck around 30% of the total fuel energy is wasted through the exhaust system in terms of heat dissipated to the environment. Hence, several investigations and experiments are conducted trying to find ways to utilize this wasted heat in what is called a waste heat recovery (WHR) system. At Scania several techniques within the field of WHR are explored to find the profits that could be made. This report will cover a WHR-system based on thermoelectricity, where several new thermoelectric (TE) materials will be investigated to explore their performance. A reference material which is built into modules will be mounted in the exhaust gas stream on a truck to allow for measurements in a dyno cell. To analyze new materials a Simulink model of the WHR-system is established and validated using the dyno cell measurements. By adjusting the model to other thermoelectric material properties and data, the performance of new TE materials can be investigated and compared with today’s reference material. From the results of the simulations it was found that most of the investigated TE materials do not show any increased performance compared to the reference material in operating points of daily truck driving. This is due to dominance of relatively low exhaust gas temperatures in average, while most advantages in new high performing TE-materials are seen in higher temperature regions. Still, there are candidates that will be of high interest in the future if nanotechnology manufacturing process is enhanced. By using nanostructures, a quantum well based BiTe material would be capable of recovering 5-6 times more net heat power compared to the reference BiTe material. Another material group that could be of interest are TAGS which in terms of daily driving will increase the power output with pending values between 40-80 %. It is clear that for a diesel truck application, materials with high ZT-values in the lower temperature region (100-350°C) must be developed, and with focus put on exhibiting low thermal conductivity for a wide temperature span.

This research entails the first comprehensive and systematic study on a heat-driven, self-cooling application based on the thermoelectric generation effect. The system was studied using the first and second laws of thermodynamics to provide a solid and basic understanding of the physical principles governing the system. Multiphysics equations that relate heat transfer, fluid dynamics and thermoelectric generation are derived. The equations are developed with increasing complexity, from the basic Carnot heat engine to externally and internally irreversible engines. A computational algorithm to systematically use the fundamental equations has been presented and computer code is implemented based on the algorithm. Experiments were conducted to analyze the geometric and system parameters affecting the application of thermoelectric based self-cooling in devices. Experimental results show that for the highest heat input studied, the temperature of the device has been reduced by 20-40% as compared to the natural convection case. In addition, it has been found that in the self-cooling cases studied, convection thermal resistance could account for up to 60% of the total thermal resistance. A general numerical methodology was developed to predict steady as well as transient thermal and electrical behavior of a thermoelectric generation-based self-cooling system. The methodology is implemented by using equation modeling capabilities to capture the thermo-electric coupled interaction in TEG elements, enabling the simulation of major heating effects as well as temperature and spatial dependent properties. An alternative methodology was also presented, which integrates specialized ANSI-C code to integrate thermoelectric effects, temperature-dependent properties and transient boundary conditions. It has been shown that the computational model is able to predict the experimental data with good accuracy (within 5% error). A parametric study has been done using the model to study the effect of heat sink geometry on device temperature and power produced by TEG arrays. In addition, a dynamic model suited for integration in control systems is developed. Therefore, the study has shown the potential for a heat driven self-cooling system and provides a comprehensive set of tools for analysis and design of thermoelectric generation.

Il presente lavoro di tesi è stato svolto presso la DTU, Technical University of Denmark, nel Department of Energy Conversion and Storage, Riso Campus. Lo scopo del periodo di soggiorno estero è stato quello di caratterizzare appropriati moduli termoelettrici forniti da aziende del settore, utilizzando un opportuno apparato di caratterizzazione. Quest’ultimo è noto come “module test system” e, nello specifico, è stato fornito dalla PANCO GmbH, azienda anch’essa attiva nel campo delle tecnologie termoelettriche. Partendo da uno studio teorico dei fenomeni fisici interessati (effetto Seebeck per la produzione di potenza termoelettrica), si è cercato in seguito di analizzare le principali caratteristiche, ed elementi, del “module test system”. Successivamente a questa prima fase di analisi, sono stati condotti esperimenti che, con l’aiuto di modelli computazionali implementati attraverso il software Comsol Multiphysics, hanno permesso di studiare l’affidabilità del sistema di caratterizzazione. Infine, una volta acquisite le basi necessarie ad una corretta comprensione dei fenomeni fisici e delle caratteristiche relative alla strumentazione, sono stati analizzati moduli termoelettrici di tipo commerciale. In particolare, sono stati estrapolati dati quali correnti, tensioni, gradienti di temperatura, che hanno permesso di ricavare flussi termici, efficienze, e potenze che caratterizzano il modulo in questione durante le condizioni di funzionamento. I risultati ottenuti sono stati successivamente comparati con dati forniti dal produttore, presenti sul catalogo.

PV systems in practice experience excessive thermal energy dissipation that is inseparable from the photo-electric conversion process. The temperature of PV cells under continuous illumination can approach 40°C above ambient, causing a drop in the electrical performance of about 30%. The significance of elevated temperature on PV cells inspired various thermal management techniques to improve the operating temperature of the cells and hence their conversion efficiency. Hybrid PV/Thermal (PV/T) collectors that can supply both electrical and thermal energy are attractive twofold solution, being able to cool the PV cells and thus improving the electrical power output as well as collecting the thermal energy by-product for practical utilization. The challenges present on the performance of PV systems due to elevated operating temperature is considered the research problem within this work. In this research, an integrated hybrid heat pipe-based PV/Thermoelectric (PV/TEG) collector is proposed and investigated theoretically and experimentally. The hybrid collector considers modular integration of a PV absorber rated at 170W with surface area of 1.3 m2 serving as power generator as well as thermal absorber. Five heat pipes serving as the heat transport mediums were attached to the rear of the module to extract excessive heat accumulating on the PV cells. The extracted heat is transferred via boiling-condensation cycle within the heat pipe to a bank of TEG modules consisting of five 40 mm x 40 mm modules, each attached to the condenser section of each heat pipe. In principle, the incorporation of heat pipe-TEG thermal waste recovery assembly allow further power generation adopting the Seebeck phenomena of Thermoelectric modules. A theoretical numerical analysis of the collector proposed is conducted through derivation of differential equations for the energy exchange within the system components based on energy balance concepts while applying explicit finite difference numerical approach for solutions. The models developed are integrated into MATLAB/SIMULINK environment to assess the cooling capability of the integrated collector as well as the addition power generation through thermal waste heat recovery. The practical performance of the collector proposed is determined experimentally allowing for validation of the simulation model, hence, a testing rig is constructed based on the system requirements and operating principles. Reduction in the PV cell temperature of about 8°C, which account for about 16% reduction in the PV cell temperature response compared to a conventional PV module under identical conditions is attained. In terms of the power output available from the PV cells, enhanced power performance of additional 5.8W is observed, contributing to an increase of 4% when compared with a PV module. The overall energy conversion efficiency of the integrated collector was observed to be steady at about 11% compared to that of the conventional PV module (9.5%) even at high ambient temperature and low wind speeds. Parametric analysis to assess the performance enhancements associated to the number of heat pipes attached to the PV module is conducted. Increasing the number of heat pipes attached to 15 pipes permits improved thermal management of the PV cells realised by further 7.5% reduction in the PV module temperature in addition to electrical output power improvement of 5%.

In the text, the investigastion and optimization of a Thermoelectric generator-based system, under different conditions, has been studied. The investigation aim is to obtain the characteristic parameters of the system: the Seebeck coeff.(α), the Thomson coeff. (τ), the Heat transferred to the Thermoelectric generators (Qh) and the electrical conversion (η). The system is built using four resistive elements as heating system, fed by a controllable generator, and forced circulating air as heat sink. The heat flow, from the source to the sink, passes through a hot heat exchanger, the thermoelectric generators, and the cold heat exchanger. The two heat exchangers are designed in order to have the best heat exchange at each surface; this is obtained by creating a planar and compact are where the heat flow is primarly conduction-type, and providing a finned and wide surface where the heat flow is primarly convection-type. The circulation of air at the heat sink is obtained through the utilization of two fans. Utilizing the open-circuit data, the Seebeck coefficient, the Peltier coefficient, and the Thomson coefficient have been calculated, obtaining values of 187.7 μV/K,0.065 W/A,-0.043 V/K, respectively, at an average temperature of 345.731 K and temperature difference of 72.486 K. It’s been shown that the temperature range, explored with the system built, does cover the point in which the Seebeck effect has its max-value and, following, that such point does not match with the max efficiency point. Utilizing the controllable load-test data, macroscopic thermoelectric parameters as heat flows, electrical otuput power and electrical conversion efficiency, have been calculated. In the best condition occured, results as 144.538 W, 1.829 W,1.265 % have been measured for Qh, Pout, η, respectively, at an average temperature of 367.078 K and temperature difference of 103.418 K. All graphs show that the results are in strong agreement with the reported literature.

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This study sought to identify the feasibility of utilizing a radioisotope thermal (thermoelectric/stirling) generator to provide power to a deployed USMC Expeditionary Force. The conceptual system architecture was constructed through use of the systems engineering process, identifying necessary subsystems and integration boundaries. Radioisotope comparison was then performed, utilizing weighted design factors. It was determined that Sr-90, Cs-137, and Cm-244 would be the most effective fuel sources for this mission area. By analyzing current thermoelectric technology, it was determined that maximum system efficiency is limited to 1015 percent when utilizing available lead telluride thermoelectrics. Barriers to development of identified physical subsystem components were then identified, including health and environmental hazards of potential isotopes, as well as shielding criteria. The system development was found to be feasible and additional design work and development work is proposed.

As the importance of water use in the power generation sector increases across the nation, the ability to obtain and analyze real power plant data is pivotal in understanding the water energy nexus. The Navajo Generating Station in Arizona and the Browns Ferry Nuclear Plant in Alabama are examples of where water shortages have threatened the operation of power generators. The availability of freshwater in the United States is beginning to dictate how and where new power plants are constructed. The purpose of this study is to provide and analyze cooling system parameters using 2008 data provided by the Energy Information Administration. Additionally, the cost of water saved among different categories of power plants is calculated. In general, the conditions which cause cooling systems to withdraw less water are not necessarily the more expensive conditions, and vice versa. While not all the variability in the cost of cooling systems is being accounted for, the results from this study prove that nameplate capacity, capacity factor, age of power plant, and region affect the costs of installed cooling systems. This study also indicates that it would be most cost effective for once-through cooling systems to be replaced with recirculating- pond instead of recirculating- tower systems. The implications of this study are that as power plant owner's struggle in balancing cost with water dependence, several parameters must first be considered in the decision-making process.
Master of Science

Carbon emissions, climate change and the finite resource of fossil fuels are driving an increasing need for renewable energy, and in particular, an interest in photovoltaic (PV) cells. Most PV cells operate in temperatures above 25 oC, and the performance of PV cells reduces with increased operation temperature. This research aims to resolve some engineering issues by integrating PV cells with a thermoelectric generator (TEG). Integrating TEG with PV cells helps to transfer heat from the PV through the TEG to an actively or passively cooled heat sink. The temperature difference established across the TEG can generates additional electrical power by the Seebeck effect. The main objective of this research is to investigate the feasibility of developing a PV/TEG hybrid system that can offer better performance than that can be obtained from each individual system. The key parameters, which are crucial to the development of efficient hybrid system, were investigated. These include the temperature coefficient of PV cells, geometry of TEGs and thermal coupling between the PV and TEG. It was found that the dye sensitised solar cells (DSC) has a preferred temperature coefficient that are the most suitable for use in a PV/TEG system. In this work, a theoretical model was also developed for determination of the optimal geometry of the TEG for PV/TEG hybrid systems. A special type of DSCs was designed and fabricated which employ titanium as the counter electrode (other than conventional FTO-glass) to improve the thermal coupling between the PV and TEG. A unique DSC/TEG hybrid system was constructed using this special type of DSC and its generating performance was studied in comparison with a similar system that uses conventional FTO-glass counter electrode. The experimental results show that the power output and efficiency of the hybrid PV/TEG system with Ti counter electrode is significantly higher than the similar system with a conventional FTO-glass electrode due to an improved thermal coupling between the DSC and TEG. It is concluded that a hybrid PV/TEG system can provide improved performance beyond that of each individual system. However, the improvement can only be achieved with appropriate type of PV cells, optimised TEGs and advanced structures for integration, such as Ti counter electrode.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
PROGRAMA DE EXCELENCIA ACADEMICA
Este trabalho tem por objetivo principal a avaliação de desempenho de um sistema piloto de preaquecimento dos motores da central termelétrica Gera Maranhão, via energia solar térmica, em Miranda do Norte, Maranhão, através de uma simulação numérica. Cinco subsistemas independentes, cada um responsável pelo preaquecimento de um motor Wartsila 20V32 de 8,73 MW, foram construídos, somando um total de 500 coletores solares instalados e uma superfície de captação solar total de 1000 metros quadrados. Uma estação meteorológica com sensores de radiação solar global, difusa, direta e temperatura ambiente foi posicionada do lado dos sistemas para medir as condições ambientais na região. A simulação do desempenho do sistema solar foi efetuada ao longo de um ano com dados de radiação solar da estação meteorológica de Buriticupu, no Maranhão, dados que mais se aproximam dos dados disponíveis de Miranda do Norte. Correlações para transformar a radiação global medida numa superfície horizontal para uma superfície inclinada foram selecionadas após uma revisão bibliográfica dentre as disponíveis na literatura. Diferentes cenários de controle do acionamento das bombas de água foram comparados a fim de determinar a melhor configuração de operação. A influência da temperatura de preaquecimento dos motores no desempenho do sistema solar foi avaliada também. Os resultados da simulação foram comparados com os resultados obtidos via o método F-CHART. Uma participação média anual da energia solar de 11,5 por cento foi encontrada para o preaquecimento dos motores levando a uma redução de 24693 kg/ano de óleo combustível usado na caldeira do sistema de preaquecimento dos motores da usina termelétrica.
The present work has as main objective the performance evaluation of a pilot system for preheating the engines of Gera Maranhão power plant, in Miranda do Norte, state of Maranhão, via thermal solar energy using a numerical simulation. Five independent subsystems, each one responsible for the preheating of a Wartsila 20V32 internal combustion engine of 8.73 MW, were installed. These systems amount five hundred solar collectors, with a total solar collecting area of 1000 square meters. A meteorological station with sensors for global, diffusive and beam solar radiation, as well as ambient temperature recorders, was placed by the side of the system in mode to measure ambient condition in the area. The simulation of the solar system performance was processed over a year with data of solar radiation for a meteorological station of Buriticupu, state of Maranhão, Brazil. Correlations to transform the global radiation measured on a horizontal plane to a sloped plane were selected, following a selection from a literature review. For the control of the water pumps, different scenarios were compared in order to determine the best operational configuration. The influence of engine preheating temperature in the performance of the solar system was also evaluated. Simulation results were compared with results obtained with the F-CHART method. An annual average solar energy contribution of 11.5 percent was found for the preheating of the engines. This resulted in a reduction of 24693 kg per year of fuel oil used in the boiler of the traditional preheating system of the power plant.

Analytical, closed-form solutions governing thermoelectric behavior are derived. An analytical model utilizing a thermal circuit is presented involving heat transfer into, through, out of, and around a thermoelectric device. A nondimensionalization of the model is presented. Linear heat transfer theory is applied to the model to obtain a series of closed form equations predicting net power output for the thermoelectric device. Fluid streams flowing through shrouded heat sinks with square pin fins are considered for the thermal pathways to and from the device. Heat transfer and pressure drop are characterized in a manner conducive to an analytical model using previously published experimental results. Experimental data is presented which validates and demonstrates the usefulness of the model in predicting power output for commercially available thermoelectric generators. A specific design for a thermoelectric power harvester is suggested consisting of a pattern of thermoelectric generators. An economic model for calculating payback time is developed. An optimization process is demonstrated that allows for the payback time of such a system to be minimized through optimization of the physical design of the system. It is shown that optimization of the thermal pathways dramatically reduces payback time. Optimized design of a system is discussed in light of theoretical cases with feasible payback times.

Thesis (M.S.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In this work, a thermoelectric energy harvester system aimed at harvesting energy for locally powering sensor nodes in nuclear power plant coolant loops has been designed, fabricated and tested. Different mathematical modeling methods have been validated by comparing with experimental results. The model developed by this work has the best accuracy in low temperature range and can be adapted and used with any heat sink, heat pipe, or thermoelectric system, and have proven to provide results closely matching experimental data. Using the models, an optimization of the thermoelectric energy harvesting system has been performed which is applicable to any energy harvester of this variety. With experimental validation, the system is capable of generating sufficient energy to power all the sensors and electronical circuits designed for this application. The effect of gamma radiation on this thermoelectric harvester has also been proved to be small enough through radiation experiment.
Master of Science

The performance of a novel micro-thermodielectric power generation device (MTDPG) was investigated in order to determine if thermodielectric power generation can compete with current portable power generation technologies. Thermodielectric power generation is a direct energy conversion technology that converts heat directly into high voltage direct current. It requires dielectric (i.e., capacitive) materials whose charge storing capabilities are a function of temperature. This property is exploited by heating these materials after they are charged; as their temperature increases, their charge storage capability decreases, forcing them to eject a portion of their surface charge to an appropriate electronic storage device. Previously, predicting the performance of a thermodielectric power generator was hindered by a poor understanding of the materials thermodynamic properties and the affect unsteady heat transfer losses have on system performance. In order to improve predictive capabilities in this study, a thermodielectric equation of state was developed that describes the relationship between the applied electric field, the surface charge stored by the thermodielectric material, and its temperature. This state equation was then used to derive expressions for the material's thermodynamic states (internal energy, entropy), which were subsequently used to determine the optimum material properties for power generation. Next, a numerical simulation code was developed to determine the heat transfer capabilities of a micro-scale parallel plate heat recuperator (MPPHR), a device designed specifically to a) provide the unsteady heating and cooling necessary for thermodielectric power generation and b) minimize the unsteady heat transfer losses of the system. The previously derived thermodynamic equations were then incorporated into the numerical simulation code, creating a tool capable of determining the thermodynamic performance of an MTDPG, in terms of the thermal efficiency, percent Carnot efficiency, and energy/power density, when the material properties and the operating regime of the MPPHR were varied. The performance of the MTDPG was optimized for an operating temperature range of 300 500 K. The optimization predicted that the MTDPG could provide a thermal efficiency of 29.7 percent. This corresponds to 74.2 percent of the Carnot efficiency. The power density of this MTDPG depends on the operating frequency and can exceed 1,000,000 W/m3.

The process of laser-induced brazing constitutes a potential option for connecting several ceramic components (n- and p-type ceramic bars and ceramic substrate) of a thermoelectric generator (TEG) unit. For the construction of the TEGs, TiOₓ and BₓC were used as thermoelectric bars and AlN was used as substrate material. The required process time for joining is well below that of conventional furnace brazing processes and, furthermore, establishes the possibility of using a uniform filler system for all contacting points within the thermoelectric unit. In the work reported here, the application-specific optimization of the laser-joining process is presented as well as the adapted design of the thermoelectric modules. The properties of the produced bonding were characterized by using fatigue strength and microstructural investigations. Furthermore, the operational reliability of the modules was verified.

Freshwater scarcity is a major obstacle of growth and prosperity for many nations in the world. Conventional centralised freshwater supply options in general are depleting and the unanticipated social and environmental costs of alternative solutions are emerging. Similar to energy, water sector may also need to explore renewable decentralised freshwater alternatives such as atmospheric moisture as discussed in this thesis. For hot and humid regions, condensed water is unwillingly discharged out of air-conditioning systems and the energy consumed for condensation to full humidity comfort level is wasted. Only a few limited small-scale experimental studies and no systematic modelling have been found in the literature on atmospheric water capture. This thesis works to fill some of this gap by developing an understanding of the fundamental factors that have and continue to challenge the development of technologies for atmospheric water capture. In this thesis, a framework is developed encompassing several modelling elements for assessment of feasibilities of moist air dehumidification technologies for atmospheric water capture. This framework integrates technical, meteorological and economic modelling elements. In the technosphere, detailed models of thermoelectric and absorption cooling are developed as potential dehumidification technologies. These models are interfaced to renewable energy input algorithms, namely solar photo-voltaic (PV) and solar-thermal. Solar energy collection technologies are also part of this framework which includes models of solar PV systems and evacuated tube collectors (ETCs). Studies of such integration of solar-assisted dehumidification and associated analysis for atmospheric water capture are limited in the literature. Fundamental solar energy input models are developed and interfaced to meteorological data to provide geographical location specific analysis. In this way the model framework is generic and applicable to any location on Earth where meteorological data is available. Finally, an economic modelling component completes the framework to provide comprehensive techno-economic assessments of different technologies for atmospheric water capture. This framework therefore provides a tool to support decision making related to feasibilities of different technologies associated with water capture from atmosphere. Along the way to developing the modelling framework, a detailed categorisation of dehumidification systems is established and a model to estimate condensation rates based on local climate data is built. The hurdle of condensation energy requirement is highlighted through simulation results. To alleviate this energy burden, an assessment of renewable solar energy input is then made. Techno-economic challenges for two different climates, Sydney and Abu Dhabi are examined and compared throughout this thesis providing comparisons for water and energy profiles. Several modelling components are developed and presented f or this purpose, requiring implementations in different modelling environments including Matlab, Trnsys, Homer and VBA. Based on the operation principles, dehumidification techniques are categorised into three categories in this thesis (Fig. 2.2). Gas separation membrane technologies were modelled but are not included in this thesis presentation because initial analysis showed they suffer from several key technical drawbacks primarily associated with the sensitivity to fluctuations in feed air temperature and humidity. Technologies in the cooling surfaces category in general use electrical or mechanical power to circulate and compress a refrigerant and cooling down conductive surfaces or coils. This process aims to decrease the temperature of moist air stream below dew point where water vapour molecules start to bond and settle forming the condensation stream. Amongst a wide range of cooling surface techniques, thermoelectric cooler (TEC) devices are attachable to cooling surfaces without using a refrigerant medium. A conceptual TEC dehumidification system is modelled in this thesis targeted at moist air streams with ambient temperature ranges (10-50) C and relative humidity ranges (10-100) %. For large-scale water production, the energy cost is calculated and found to be the major factor contributing to more than 95% of the total cost of generated water. This model is implemented for Sydney and Abu Dhabi case studies by using their annual typical meteorological weather data. This shows the generic nature of the applicability of the model and in this specific comparison confirms the influence of energy consumption over the cost of generated water in those two very different regions. However, lower local utility rates and favourable climatic conditions for dehumidification in Abu Dhabi show significant differentiation in water cost over Sydney. To confront excessive energy demands for atmospheric water capture, the idea of facilitating solar energy via PV panels is examined in this thesis. A comprehensive solar algorithm is developed and implemented to optimise solar collector positioning and for calculating solar penetration ratios for Sydney and Abu Dhabi. As far as the author is aware, this is the first time such optimal position calculation for Sydney and Abu Dhabi is done. It is found that optimal surface tilt angles for Sydney and Abu Dhabi are 32 and 22 respectively, while optimal surface azimuth angles for Sydney and Abu Dhabi are 195 and 16 respectively. This algorithm is generic in its structure allowing such calculation to be executed for any city in the world and is later used in this thesis for calculations associated with a new ETC diffuse at reflector (DFR) model. This thesis also presents a detailed economic model for prediction of utility costs with consideration for CAPEX, OPEX, subsidies and carbon taxation. It is found that investing a $338,000 on a PV array of 100 kW at current utility rates can meet 53% of energy demand of proposed dehumidification system and reduce LCOE by 6 c/kWh in Sydney. Solar PV array at current utility rates to feed proposed dehumidification system is found to be uneconomical for Abu Dhabi. Solar-thermal collectors represent an attractive option for driving refrigeration techniques. Evacuated tube collection technology has progressed significantly over the last few years and this technology is assessed in this thesis as a heat collector for absorption chillers. The role of DFR to improve the performance of ETC is highlighted and modelled. Results showed that DFR can significantly improve ETC performance by an average of 24.1% for Sydney and 22.9% for Abu Dhabi respectively. The optimisation of DFR is therefore an important factor for the enhancement of this solar energy collection technology and the algorithm developed in this thesis is generically applicable across geographical locations. The concept of solar refrigeration is reviewed and investigated for the implementation of sorption refrigeration. Sorption techniques use low-grade heat sources such as solar energy to convert thermal heat into chilling effect. This function is investigated for dehumidification of a moist air stream via cooling coils. A conceptual absorption model is developed in TRNSYS to calculate overall energy demand and water productivity. An ASHRAE algorithm is developed and implemented to cross validate the TRNSYS model. This absorption model was used in an optimisation analysis and showed water productivity improvement of 29% for Sydney and 34% for Abu Dhabi, while energy demand can be reduced by 22% for Sydney and 55% for Abu Dhabi. Unlike Sydney, the cumulative cost of generated water is declining over time in Abu Dhabi reaching $15 /kL. If this system is projected to work during the day only, solar penetration ratio will substantially increase and could meet the entire diurnal load for dehumidification in Abu Dhabi. If the capital cost of developing such system is affordable, absorption model can be further optimised to specifically match local conditions in respect to solar radiation and energy sources where the cost of generated water can economically compete with other conventional sources. In regions such as Abu Dhabi, the idea of having small-scale dehumidification system where the energy demand is mostly met by solar radiation and the volume of generated water is freely controlled and managed by household seems appealing.

Performance of two different waste heat recovery systems (one based on Rankine cycle and the other one using thermoelectricity) combined with non-hybrid, mild-hybrid and full hybrid systems are investigated. The vehicle under investigation was a 440hp Scania truck, loaded by 40 tons. Input data included logged data from a long haulage drive test in Sweden.All systems (waste heat recovery as well as hybrid) are implemented and simulated in Matlab/Simulink. Almost all systems are modeled using measured data or performance curves provided by one manufacturer. For Rankine system results from another investigation were used.Regardless of practical issues in implementing systems, reduction in fuel consumption for six different combination of waste heat recovery systems and hybrid systems with different degrees of hybridization are calculated. In general Rankine cycle shows a better performance. However, due to improvements achieved in laboratories, thermoelectricity could also be an option in future.This study focuses on “system” point of view and therefore high precision calculations is not included. However it can be useful in making decisions for further investigations.

The effects of greenhouse gases have seen a significant rise in recent years due to the use of fossil fuels like gasoline and diesel. Conversion of the energy stored in these fossil fuels to mechanical work is an extremely inefficient process which results in a high amount of energy rejected in the form of waste heat. Thermoelectric materials are able to harness this waste heat energy and convert it to electrical power. Thermoelectric devices work on the principle of the Seebeck effect, which states that if two junctions of dissimilar materials are at different temperatures, an electrical potential is developed across them. Even though these devices have small efficiencies, they are still an extremely effective way of converting low grade waste heat to usable electrical power. These devices have the added advantage of having no moving parts (solid state) which contributes to a long life of the device without needing much maintenance. The performance of thermoelectric generators is dependent on a non-dimensional figure of merit, ZT. Extensive research, both past and ongoing, is focused on improving the thermoelectric generator's (TEG's) performance by improving this figure of merit, ZT, by way of controlling the material properties. This research is usually incremental and the high performance materials developed can be cost prohibitive. The focus of this study has been to improve the performance of thermoelectric generator by way of improving the heat transfer from the exhaust gases to the TEG and also the heat transfer from TEG to the coolant. Apart from the figure of merit ZT, the performance of the TEG is also a function of the temperature difference across it, By improving the heat transfer between the TEG and the working fluid, a higher temperature gradient can be achieved across it, resulting in higher heat flux and improved efficiency from the system. This area has been largely neglected as a source of improvement in past research and has immense potential to be a low cost performance enhancer in such systems. Improvements made through this avenue, also have the advantage of being applicable regardless of the material in the system. Thus these high performance heat exchangers can be coupled with high performance materials to supplement the gains made by improved figure of merits. The heat exchanger designs developed and studied in this work have taken into account several considerations, like pressure drop, varying engine speeds, location of the system along the fuel path, system stability etc. A comprehensive treatment is presented here which includes 3D conjugate heat transfer modeling with RANS based turbulence models on such a system. Various heat transfer enhancement features are implemented in the system and studied numerically as well as experimentally. The entire system is also studied experimentally in a scaled down setup which provided data for validation of numerical studies. With the help of measured and calculated data like temperature, ZT etc, predictions are also presented about key metrics of system performance.
Ph. D.

Electrical Engineering
M.S.E.E.
According to the U.S. energy consumption survey in 2012, about 25% of the commercial and 42% of the residential building energy were used for heating. Despite the development of new and more efficient Heating, Ventilation, and Air Conditioning (HVAC) systems over the years, the high energy consumption in heating is still one of the major energy efficiency issues. Studies showed that decreasing HVAC operating temperature set points by 4°F will result in energy savings of 15% or more. Thus, the smart localized heating control (SLHC) system was designed and prototyped to provide localized heat directly to a person so that HVAC can run at a lower temperature set point. SLHC detects human movement and delivers the heat based on the result of the target location estimation and temperature measurement feedback. To detect the human movement, image processing techniques were used; image segmentation, mass center detection, background subtraction using the Mixture of Gaussian model, and human feature detection. In SLHC, a near-infrared heater and a tracking function were used to provide an instant and a direct heat to the person in order to minimize wasting energy. The SLHC system is divided into the sensing and processing (SP) and the heating and regulating (HR) subsystem. The SP’s primary function is to process captured video images and measured temperature data. SP also generates and sends the heater operating signal to HR. HR purposes to control the heater’s direction and power based on the signal. The communication between SP and HR was established through Wi-Fi enabled development platform. The SLHC prototype successfully processed the sensing data and transmitted the control signal. The result shows that it detected human movement and estimated the person’s location in 3D space within 10% margin of error. Also, it delivered the focused heat to the surface of the human body and increased the temperature by 10.0°F in 3 minutes at the distance of 1.5m away from the heater. This cost-effective, wireless, and localized heating system demonstrates the potential to improve energy efficiency in buildings.
Temple University--Theses

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.
Cataloged from PDF version of thesis. In title ((Biy̳Sb1̳-̳y̳)2̳Te3̳) on title page, double-underscored characters appear as subscript.
Includes bibliographical references (p. 185-188).
This thesis looks at (BiySb1-y)2Te3 nanocomposites as an example of the currently available nano systems. In this thesis, (BiySb-y)2Te3 nanocomposites are characterized from ~325K down to ~3K. Advantages of this low temperature regime include the minimization of lattice vibrations and the decreasing of ke with decreasing temperature. As a result, nano effects on IL could be better observed and characterized in this low temperature regime. We are also interested in studying the effect of an applied magnetic field on the conduction carriers in this low temperature regime. We like to find out whether an applied magnetic field could impede the carriers' heat conducting ability more than their current conducting ability. Therefore, a magnetic field effect study is also carried out to see whether any improvement in ZT could be achieved by the applying of a magnetic field. The measurement system used in this thesis is QD PPMS. Only the ACT and TTO options of the QD PPMS apparatus are used for measurements in this thesis. Under the ACT option, Hall and 4-pt p measurements on the same sample are performed. On the other hand, Kti, S, and 2-pt p measurements are performed simultaneously on the same sample under the TTO option. Both the ACT and TTO options use an AC current instead of a DC current during p measurement to eliminate any unwanted Seebeck voltage. Since the ability to perform correct measurements on thermoelectric samples is not a trivial task, benchmarking with known results is a must. In this thesis, I calibrate our QD PPMS against both the manufacturer's results and the published data, and demonstrate that our measurement system gives accurate results. I also benchmark our nt, results under a magnetic field using a pyrex sample. Our results confirm that the QD PPMS apparatus does not introduce artifacts under an applied magnetic field. Thus, any changes observed under the QD PPMS apparatus measurements in an applied magnetic field would be expected to be solely due to the sample. Lastly, no measurable difference is found between our 2-pt p (TTO) and 4-pt p (ACT) measurements. A total of eight (BiySb1-y) 2Te3 samples are measured in this thesis. The sample set includes: (a) one bulk ingot sample manufactured by Marlow (Ingot), (b) four nanocomposite samples (XY21, XY146, XY144, and GJ99) made by collaborators from Boston College (BC) where the letters simply indicate the sample maker's initials, and (c) three nanocomposite samples (0%, 40%, and 100%) made by collaborators from Nanyang Technological University (NTU) in Singapore, where the % denotes the weight % of the nanoinclusions prepared via melt spinning [1] in the sample. All the nanocomposite samples in this thesis are made solely for research uses and are purposely fabricated under conditions different than those used for the best samples previously reported [2, 1]. Although BC and NTU use different starting materials, different fabrication machines, and different fabrication parameters, the resulting densities of the nanocomposites from the ball-milled nanopowders alone (XY21, XY146, XY144, GJ99, and 0%) are almost identical. Moreover, the addition of nanoinclusions prepared via melt spinning decreases the sample mass density somewhat.
(cont.) From the XRD measurement results, we notice that (a) both the NTU and BC samples have the same XRD peak locations, (b) the NTU samples have a lower intensity for peaks (1 0 10) and (1 1 0), and (c) the NTU samples have a higher intensity for peaks (0 0 g) where g is an integer. Comparing the XRD patterns with the reference database, the difference in peak intensities is a good indication that the NTU samples are not completely randomized and have internal preferred orientations. From the SEM images, we notice that the NTU samples and the BC samples are markedly different. For example, the BC samples are shown to have grains in the pm range with a small grain size distribution. On the other hand, the grain sizes of the NTU samples decreases with the addition of nanoinclusions prepared via melt spinning. Moreover, the NTU samples have a wider grain size distribution that ranges from nm to pum. This observed difference is believed to arise from the difference in the fabrication techniques used by the BC and NTJ teams. Temperature-dependent hta, S, and p measurements, along with the carrier concentration measurements, are performed on all samples. All samples are found to be p-type materials. Transport measurements are performed both // and I to the press direction for the nanocomposite samples, and only // to the growth direction for the Ingot sample. Anisotropic behavior is observed in ~tha nd p for all the nanocomposite samples investigated, with the anisotropy being always higher in p than in th . On the other hand, S is found to be isotropic. Thus, care needs be taken during the fabrication process to ensure that no unwanted anisotropic behavior is introduced. Common p and S features among all samples include: (a) a dramatic decrease in the peak value of sth for the nanocomposite samples when compared with the Ingot //'s value, (b) constant slope &S/&T for T < 20K, (c) constant slope &S/ln(T) of ~ 130-140pV/ln(K) for 200K < T < 300K, (d) close-to-zero slope in p for T < 20K, and (e) p cx T'.1 0 for 200K < T < 300K. From the measured stl, S, and p data, the mobility pp, hole mean free path e, and phonon mean free path Eph are computed. It is found that nanocomposite approach decreases lp, fe, and Eph. Moreover, the pp, fe, and E values are always lower in the // direction for the nanocomposite samples than in the I direction. Furthermore, 4e in general is in the nm range while eph ranges from pm to nmn as the temperature increases. Therefore, if one wants to decrease the sl,, a possible solution is to decrease f further. However, in order not to affect the p too much, the lower limit for f should be in the nm range. As a result, decreasing f would have the biggest effect on Kth in the low temperature regime. Using the Kth and - data, KL is extracted through the intercept method (see Section 3.5.3). This method only makes sense if all the samples have similar f. Since pressure is coining from top and bottom during the fabrication process for the nanocomposite samples, my samples are expected not to behave as the same materials system along the // direction. However, for the I direction, they can be considered as a same materials system since no pressure is applied. The 40% and 100% samples are believed to deviate from the results for the 0% sample because of the presence of nanoinclusions in them. rlLL is found to be 0.76W/mn-K for the 40% sample at 297K. When I compare this value with previously determined values for Bi 2Te3 (1.4W/in-K [3]) and (Bio.3Sbo. 7)2Te3 (0.9W/m-K [4]) alloy at 300K, these results confirm that the nanocomposite approach does indeed lower the lattice thermal conductivity. The semi-classical model is then used to interpret the various transport coefficients (o- = 1/p, S, and t;e) and is based on the Boltzmann Transport Equation (BTE) under the relaxation time approximation (RTA). Acoustic phonon scattering, ionized impurity atom scattering, neutral impurity atom scattering, alloy scattering for a 3-atom II"_1II system, point defect scattering, grain boundary scattering, and polar optical phonon scattering are considered for the electrons. On the other hand, boundary scattering, point defect / alloy scattering, and Umklapp scattering are considered for the phonons. We find that for holes, point defect scattering dominates at low T, while acoustic phonon scattering dominates around 300K. As for phonons, boundary and point defect scattering mechanisms dominate at low T, while point defect and Umklapp scattering mechanisms dominate at high T (~300K). We also find that the nano approach increases the crossover temperature Tcross. For the electron model, we observe that the deformation potential (DA) seems to be both process dependent and materials dependent. We see that DA changes from the BC samples to the NTU samples (process dependent). Moreover, DA changes in the NTU samples when going from 0% to 100% (materials dependent). From the electron model, the ionized impurity atom concentration Ni and the neutral impurity atom concentration No reflect the somewhat anisotropic behavior of all the samples investigated. Lastly, f seems to play little role in the determination of p. For the phonon model, we observe that C plays a rather important role in the determination of r1L, especially at low temperatures. The value of C seems to be consistently lower for the BC samples than for the NTU sample (0%) for the nanocomposite samples made solely from ball-milled nanoparticles. We also see that the Umklapp scattering contribution (B') has a materials dependent factor, where B' decreases from 10x101'8s/K for the nanoparticle nanocomposite samples to e 4x1018 s/K for (cont.) the nanocomposite sample made using 100% nanoinclusions prepared via melt spinning.
Furthermore, we see that the point defect contribution (A') reaches the highest value when both the nanoparticles and nanoinclusions prepared via melt spinning are present in the nanocomposite samples (e.g. the 40% sample), similar to the alloying effect on the thermal conductivity. In general, it is desirable to increase the values of A' and B', resulting in a decrease in the KL values. However, care needs to be taken to ensure that the phonon parameters are independent of the electron parameters so that no adverse effect on ZT would result. The determination of L is also carried out based on my electron model findings. We observe that L is isotropic. Moreover, L for each sample investigated reaches the same value of 2.44x10-8 W-Ohm/K2 as T -> OK (completely degenerate limit of (+_ )2). Furthermore, the higher the hole concentration the sample has, the higher its C value is at a given temperature. Lastly, I find that a lower f leads to higher ZT values at 297K for the BC nanocomposite samples measured in the _L direction. On the other hand, a lower f leads to lower ZT values at 297K for the NTU nanocomposite samples measured in both the // and _L directions. From the magnetic field studies on the Ingot and on the 40% samples, few important facts are demonstrated. First, an applied magnetic field can be used to effectively increase the ZT of (BiySbpy)2Te3 , especially at temperatures below 200K. Use of a magnetic field might theoretically extend the effective temperature ranges over which (BiySb-y)2Te3 materials can be used for refrigeration. Second, the data under various applied B fields allow me to confidently calculate the C value below the temperature ranges where a plateau has occurred. Third, the data under various applied B fields serve as an important guideline for both validating any electron model and extrapolating values for L above the plateau occurrence temperatures. As a result, this allows me to get some insights into the temperature dependence of L (see Figure 4-14). Fourth, from the magnetic field dependent transport studies on our samples, we observe that the applied B field pushes away the holes more effectively in the Ingot // than the holes in the nanocomposite samples. We also find that the VvtIplateau values obtained under the magnetic field study serve as a more realistic and practical limit for KL. Lastly, from the magnetic field-dependent studies, we find that having point defects as the dominant scattering mechanism for the carriers results in an increase in ZT under an applied magnetic field. It would be extremely useful if one can make a sample such that the point defect dominant regime is extended to higher temperatures, resulting in a shift of the increase in the ZT ratio regime to a temperature range closer to room temperature (300K). However, care needs to be taken to ensure that such modifications would result in an increase in the ZT values under an applied magnetic field.
by Ming Y. Tang.
Ph.D.

In order to replace a conventional air-conditioner (AC) based on vapour compression technology that directly has high global warming potential and also currently consumes the most fossil fuel primary energy in building sector of tropical countries for generating thermal comfort on sleeping purpose, other alternative green space cooling technologies, as thermoelectric cooling (TEC), has to be improved to have same performance with AC. This research aims to develop and investigate a performance of Solar-powered Thermoelectric Radiant Cooling (STRC) system, as the combination of TEC and radiant cooling (RC) that is well known in its low energy consumption advantage. The studies were conducted through calculations, CFD simulations, system performance simulations and experiments. The results of optimum STRC system design was proved to provide better thermal and air quality performances, while the result in energy performance was depended on the TEC’s COP and vapour condensation prevention. After novel developing of TEC’s cooling channel with combined helical and an oblique fin to induce effective secondary flows that highly reduced the TEC’s hot side temperature in this research, the COP was able to increase up to 175%. Meanwhile, a novel bio-inspired combined superhydrophobic and hydrophobic coating on RC panel were able to competently repel most condensed water droplets, leaving just tiny droplets that was hard to be seen by naked eye. Finally, the COP of STRC system from house model experiment in 1:100 scales under hot and high humid climate was as high as 2.1 that helped STRC to consume electricity 34% less than AC system. Along with other benefits, as no working fluid, noise-free and low maintenance needs, the return of investment (ROI) was studied to be only 5-6 years when being operated with grid electricity and 17-18 years with PV panel generated electricity.

Wastewater treatment has developed as a mature technology over time. However, conventional wastewater treatment is a very energy-intensive process. Bioelectrochemical system (BES) is an emerging technology that can treat wastewater and also recover resources such as energy in the form of electricity/hydrogen gas and nutrients such as nitrogen and phosphorus compounds. Microbial electrolysis cell (MEC) is a type of BES that, in the presence of an additional voltage, can treat wastewater and generate hydrogen gas. This is a promising approach for wastewater treatment and value-added product generation, though it may not be sustainable in the long run, as it relies on fossil fuels to provide that additional energy. Thus, it is important to explore alternative renewable resources that can provide energy to power MEC. Waste heat is one such resource that has not been researched extensively, particularly at the low-temperature spectrum. This was utilized as a renewable resource by converting waste heat to electricity using a device called thermoelectric generator (TEG). TEG converted simulated waste heat from an anaerobic digester to power an MEC. The feasibility of TEG to act as a power source for an MEC was investigated and its performance compared to the external power source. Various cold sources were analyzed to characterize TEG performance. To explore this integrated TEG-MEC system further, a hydraulic connection was added between the two systems. Wastewater was used as a cold source for TEG and it was recirculated to the anode of the MEC. This system showed improved performance with both systems mutually benefitting each other. The operational parameters were analyzed for the optimization of the system. The integrated system could generate hydrogen at a rate of 0.36 ± 0.05 m3 m-3 d-1 for synthetic domestic wastewater treatment. For the practical application, it is necessary to estimate the cost and narrow the focus on the functions of the system. Techno-economic analysis was performed for MEC with cost estimation and net present value model to understand the economic viability of the technology. The application niche of the BES was described and directions for addressing the challenges towards a full-scale operation were discussed. The present system provides a sustainable method for wastewater treatment and resource recovery which can play an important role in human health, social and economic development and a strong ecosystem.
Doctor of Philosophy
An average person produces about 50-75 gallons of wastewater every day. In addition to the households, wastewater is generated from industries and agricultural practices. As the population increases, the quantity of wastewater production will inevitably increase. To keep our rivers and oceans clean and safe, it is essential to treat the wastewater before it is discharged to the water bodies. However, the conventional wastewater treatment is a very energy (and thus cost) intensive process. For low-income and developing parts of the world, it is difficult to adapt the technology everywhere in its present form. Furthermore, as the energy is provided mostly by fossil fuels, their limited reserves and harmful environmental effects make it critical to find alternative methods that can treat the wastewater at a much lower energy input. For a circular and sustainable economy, it is important to realize wastewater as a resource which can provide us energy, nutrients, and water, rather than discard it as a waste. Bioelectrochemical systems (BES) is an emerging technology that can simultaneously treat wastewater and recover resources in the form of electricity/hydrogen gas, and nitrogen and phosphorus compounds. Microbial electrolysis cell (MEC) is a type of BES that is used to treat wastewater and generate hydrogen gas. An additional voltage is supplied to the MEC for producing hydrogen. In the long run, this may not be sustainable as it relies on fossil fuels to provide that additional energy. Thus, it is important to explore alternative renewable resources that can provide energy to power MEC. Waste heat is a byproduct of many industrial processes and widely available. This was utilized as a renewable resource by converting waste heat to electricity using a device called thermoelectric generator (TEG). TEG converted simulated waste heat from an anaerobic digester to power an MEC. The mutual benefit for MEC and TEG was also explored by connecting the system electrically and hydraulically. Cost-estimation of the system was performed to understand the economic viability and functions of the system were developed. The present system provides a sustainable method for wastewater treatment and resource recovery which can play an important role in human health, social and economic development and a strong ecosystem.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 107-111).
Great interest exists for and progress has be made in the effective utilization of the human body as a possible power supply in hopes of powering such applications as sensors and continuously monitoring medical devices [1]. This report furthers into the area of thermal energy harvesting, which focuses on using the temperature differential generated between the human body and the ambient environment to generate power. More specifically, a body-powered, thermoelectric-based power supply and system will be introduced and examined, with hopes that this technology will be utilized alongside low-power, medical monitoring applications in order to achieve self-sufficiency. This report also analyzes the performance of existing thermoelectric-based body-powered energy harvesting applications and compares that with the new design introduced in this work. The new designs were able to output upwards of 25[mu]W/cm2 or, equivalently, 280µW for the entire heat sink system. Additionally, this report details the physics associated with thermoelectric modules, addresses the issues with modern thermoelectric heat-sinks, introduces two new types of wearable, conformal heat sinks, quantifies the performance of the body-powered thermoelectric supply, tests a flexible EKG processing board, and analyzes future paths for this project.
by Krishna Tej Settaluri.
M.Eng.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, May, 2020
Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 99-103).
Thermoelectric devices present unique opportunities for sustainable energy conversion. While research efforts have remarkably improved material capabilities over the past several decades, material advancement alone is insufficient to realize the full potential of thermoelectric technology [25, 24, 39, 14]. Here, an integrated perspective is applied to thermoelectric technology to identify potential system improvements. The traditional thermoelectric architecture is dissected to identify limitations. It is found that the coupling of the device height to the thermoelectric element height imposed by the architecture can significantly hinder performance. A novel distributed architecture, which de-couples the device and element heights, is theorized to address these limitations. A modeling program incorporating device parameters and external conditions is developed to simulate and optimize the system architecture. The new architecture is shown to out-perform the traditional architecture in both a broad range of general generation and refrigeration conditions and the specific application of a phase-change material thermoelectric generator. The results signal the importance and potential value of an integrated approach to thermoelectric system design.
by Aaron Paul Schlenker.
S.M.
S.M. Massachusetts Institute of Technology, Department of Mechanical Engineering

碩士
國立臺灣大學
機械工程研究所
82
The purpose of present study is to develop the design technique thermoelectric cooler system.The present study provides the optimal design analysis and establishes a design tool to manufacture the thermoelectric cooler system. In order to investigate the characteristics of thermoelectric module, the auto-measurement performance equipment has been set up. The characteristic equation derived from measured data can serve as the fundemental of developing system analysis with comparing the experimental testification of present module and the pratical application of cooling linear compressor ,the results are quite satisfatory both in direction of precision and practicality.

碩士
國立中央大學
電機工程研究所
98
The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant.

碩士
國立臺灣大學
機械工程學研究所
104
In recent years, the problem of shortage of energy is growing up, therefore, waste heat recovery have become a very important issue. And the thermoelectric material plays an important role in this issue. A geothermal plant, since the heat is lost in the pipeline, we can use the thermoelectric material to recover the waste heat, and utilize dissipated heat as a stable heat source. The aim of this study is to simulate the situation of the waste heat recovery with the geothermal module. In this thesis, the experiment and simulation are discussed. In the experiment, we investigate the case of waste heat recovery with single thermoelectric generator, followed by the geothermal module. In the simulation, we simulated the single thermoelectric generator by the Thermoelectric module in the Ansys Workbench, followed by the geothermal module which was simulated by the Fluent and the Thermoelectric module in the Ansys Workbench. Finally, we compared and discussed the experimental and simulated data. The result shows that the thermoelectric generator TEG616-6 provided by ITRI is a high-performance thermoelectric generator,. The open circuit voltage, output power, and the internal resistance of the thermoelectric generator all increase with the elevating temperature of the high-temperature side,. When the high-temperature side was at 200℃,and the low temperature side was at 30℃, the output power can reach 5.42W, the power density was 3387.5W⁄m^2 , and the efficiency was about 5%. In terms of geothermal module, when the hot water side was at 90 ℃, and the cold water side was at 25℃, the output power can reach 2.6529W, and the power density was 414.515W⁄m^2 ,. In the simulation of the single thermoelectric generator, according to the verification process,the electrical resistivity can be predict by the first order regression analysis with intermetallic layer modification,. The seebeck coefficient and thermal conductivity can be predict by the secondary order regression analysis. In the simulation of the geothermal module, we utilized the turbulence model and the steady energy model to approach the practical situation. After comparing the experimental data with the simulated data, it shows the error is very small. In the end of the thesis, we can predict the performance of the geothermal module with the hot water temperature exceeding 100℃,. The simulation result shows that when the hot water side was at 100℃, 125℃, 150℃, 175℃ and 200℃as well as the cold water side was at 25℃, the maximum output power can reach 3.68W,5.55W, 8.68W, 13.92W and 18.66W.

碩士
國立臺北科技大學
電機工程系所
103
In this thesis, a step-up converter is presented, which possesses a high voltage gain, continuous input current and galvanic isolation. After this, the proposed converter is applied to a thermoelectric conversion system along with stable tracking based on three-point-weighting method, so as to upgrade output power of the thermoelectric module. For LED dimming to be considered, a switched LED module is used to adjust the number of LEDs in the LED string so as to achieve a dimmable function. Moreover, a field programmable gate array (FPGA) chip, named EP1C3T100C8N made by Altera Co., is used as a control kernel, so as to realize the maximum power tracking and LED dimming. Finally, several thermoelectric modules, manufactured by Kryotherm Co., are used to verify the effectiveness of the proposed electric thermoelectric conversion system.

碩士
臺灣大學
機械工程學研究所
98
This thesis aims to design-ThermoElectric Generator System. The ThermoElectric Generator System generates power by temperature difference. The hot side uses a heater to simulate exhaust heat. The cooling side is particularly designed utilizing the passive heat sink tech, Loop Heat Pipe. As a result, the system achieves the goal of electricity generation. The study discusses the Maximum Power Point Trick controller in order to maintain the maximum power exportation. Then, the final step is about to research the design of ThermoElectric Generator System with near Maximum Power Point Operation (nMPPO), According to testing result of the study, As long as the temperature of hot side of ThermoElectric Generator System(TEGS) stays within 150℃~210℃, the transformative efficiency of nMPPO will higher than the one of MPPT. Finally, the study analyses the proper usage of the MPPT and the nMPPO. First, the usage of MPPT is suitable for using when the temperature range of hot side is higher than 150℃~210℃. In addition, the MPPT is not affected by systematic ageing effect. It can be used for a long time. Second, the usage of nMPPO is suitably used when the hot side is within 150℃~210℃. Moreover, the time which it is able to be used is much shorter.

碩士
國立中央大學
化學工程與材料工程學系
101
This study investigates electromigration in Bi2Te3 thermoelectric (TE) material systems and the effectiveness of the diffusion barrier under current. The influences of the interfacial reaction by electromigration and Peltier effect were decoupled in this research. n- and p-type Bi2Te3 were connected to SAC305 solders and different current densities at various temperatures were applied. The Bi2Te3 samples were fabricated by the Spark Plasma Sintering (SPS) technique and electroless Ni-P was deposited at the solder/TE interfaces. The results elucidate the importance of the Ni diffusion barrier to the joint reliability. Different intermetallic compound (IMC) layers including (Cu, Ni)6Sn5 and NiTe formed at solder/Ni-P and Ni-P/substrate interfaces, respectively. The results show that the mechanism of compound growth was dominated by electromigration at low current density and dominated by Peltier effect at high current density.

碩士
國立虎尾科技大學
光電與材料科技研究所
101
This thesis indicates those based on the current household dish dryer in the market and modify it. We change its heater from current general circuit heater to thermoelectric cooling module heat surface. We discuss the possibilities to apply thermoelectric cooling module on the dish dryer. This experiment is that we use single TEC1-12706 thermoelectric cooling module and two pieces of thermoelectric cooling modules as heat source, and assist with a cross flow fan as adjustment the direction of air flow out, which makes the dish dryer air flow circulation smoothly, and has well-distributed effect. According to the experiment data, 2 pieces of thermoelectric cooling modules do not have better dryer performance than single thermoelectric cooling module. If it requests better dryer speed, it requires thermoelectric cooling module with higher watt. In half-closed space, adjusting the angle of air flow out of cross flow fan cannot make the dryer speed quicker, but it can maintain the temperature in the space averagely. Hot air recycle system can make hot air to re-use and recycle again which makes the temperature of the space to keep in a certain range. It does not have impact on the dryer efficiency due to heat spreading which causes temperature continuously going down.

博士
國立清華大學
化學工程學系
101
Thermoelectric material has been recognized as a promising candidate in the application of sustainable energy. The quaternary Pb-Ag-Sb-Te system has attracted much attention because it contains high-zT AgPb18SbTe20 (zT>2 at 800K), PbTe and AgSbTe2. The phase stability, microstructural evolution and resultant electrical/thermal transport properties of quaternary Pb-Ag-Sb-Te and the constituent systems (Ag-Sb-Te, Pb-Sb-Te and Ag-Pb-Te) are investigated. The liquidus projections of both ternary Ag-Pb-Te and Pb-Sb-Te systems are constructed. In particular, the Ag-Pb-Te ternary-eutectic alloy, with composition of Ag-4.3at%Pb-62.6at%Te, forms a partially aligned nano-sized lamellar microstructure, which comprises both the PbTe and Ag5Te3 phases, and an additional dotted PbTe of 200-600nm. This particular ternary-eutectic alloy is unidirectionally solidified using the Bridgman method, resulted in a nanostructured composite with an extremely low thermal conductivity(κ) of 0.3 W/mK and a zT peak of 0.41 at 400K. The phase diagrams of ternary Ag-Sb-Te system are constructed as well, including the 400℃ and 250℃ isothermal sections and the liquidus projection. The ternary AgSbTe2 is stabilized at 400℃ but not at 250℃, with homogeneity region of 49.0-53.0at%Te and 28.0-30.0at%Sb. The nano-scaled microstrucutre and crystal structures of the non-stoimeteric AgSbTe2 are analyzed by the transmission electron microscope (TEM). In particular, an ordered array of nano-wire microstructure, comprising a 200nm Ag2Te and a matrix of AgSbTe2+δ-Sb2Te, was resulted from a Class I reaction: L=AgSbTe2+Ag2Te+δ-Sb2Te with liquid composition of Ag-40at%Sb-36.0at%Te at 496.5℃. To understand and guide production of uniform bulk samples of this composite, the liquidus projection of quaternary Pb-Ag-Sb-Te system at 36.0at%Te isoplethal section is constructed experimentally using quenched samples. High-resolution transmission electron microscopy (TEM) confirms that these three phases are simultaneously present at the nanometer scale. Furthermore, unidirectional solidification experiments of the ternary eutectic alloy using the Bridgman method are carried out to examine the alloy's solidification behaviors. Pb-alloyed AgSbTe2 (PbxAg20Sb30-xTe50, x=3,4,5 and 6) are also unidirectionally solidified using the modified Bridgman method. The as-solidified 5at%Pb and 6at%Pb alloys, which exhibit high phase purity of AgSbTe2, contain grain-boundary inhomogeneity and nano-precipitates of δ-Sb2Te, leading to an extremely low thermal conductivity (κ) of 0.3-0.4 W/mK. A peak zT of 0.7-0.8 is found in as-solidified 5at% specimen at 425K. However, after annealing at 673K, the zT peak of 5at%Pb(annealed) decreases to 0.4, presumably due to increase in grain size and decrease in inhomogeneity.

博士
國立清華大學
工程與系統科學系
95
This work presented a novel method based on genetic algorithms (GAs) to optimize thermoelectric energy systems. The objective of the optimization is on maximizing the cooling capacity or maximizing the coefficient of performance (COP) of thermoelectric cooling (TEC) systems. Two kinds of arrangements, including single- and two-stage TEC systems, have been studied. While optimizing a single-stage TEC system, structural parameters – including the thermocouple length, the thermocouple cross-section area and the number of thermocouple – were taken as the variables. While optimizing a two-stage TEC system, parameters – including the applied electrical current, the thermocouple length, the thermocouple cross-section area and the number of thermocouples – were considered. Two-stage TEC systems can be further categorized into three types, which are with two stages electrically connected in parallel, in series and in separate. A new mathematical modelling was also proposed to deal with the temperature-dependent material properties and to include the effects of contact and spreading thermal resistances between the two stages. For both single- and two-stage TEC systems, this study developed the design flowchart and programs that combine the mathematical modelling with GAs’ technique. All kinds of design constraints–space constraints and all others–can be considered and modeled during the optimization. The results indicate that the cooling capacity or COP can be increased by optimizing the parameters of TEC systems. This study also demonstrates that the new approach based on GAs can be used effectively to optimize the thermoelectric energy systems, and this method exhibits highly potential in handling complex design problems.

博士
國立臺灣大學
機械工程學研究所
105
The aim of this study is to investigate the use of waste heat that is recovered from a multifuel biomass (Japanese cedar and rice husk) gasifier. In the gasification process, the low heating value of the biomass can be transferred to a high heating value for combustible gaseous fuel, which is a form that is widely used in industry and power plants. Conventionally, some cleaning processes must be conducted under higher operating temperatures than the low temperatures typically used to burn biomass. Thus, a catalytic reactor was designed before installing the scrubber in the downdraft gasifier system to effectively utilize the waste heat. The experimental results show that the temperature of the gasifier outlet is approximately 250-400℃; dolomite is used for tar removal in the catalytic reactor. To further improve the use of waste heat, a thermoelectric generator is added to recover waste heat. The thermoelectric generator system is manufactured using a Bi2Te3-based material and is composed of eight thermoelectric modules on the surface of the catalytic reactor. The measured surface temperature of the catalytic reactor is 100-250℃, which is the correct temperature for using Bi2Te3 as a thermoelectric generator. The results of this study show that the maximum hydrogen concentration is approximately 17.82vol% from pure Japanese cedar gasification and approximately 15.89vol% from pure rice husk gasification. The optimal operation of the gasification performance was found when the fuel is pure Japanese cedar at Φ = 0.4, and the obtained cold gas efficiency (CGE) at these conditions is approximately 63.59%. However, the optimal available biogas condition was found when 50% Japanese cedar and rice husk are mixed at Φ = 0.2, with a biogas high heating value (HHV) of approximately 5.75 MJ/m3. The performance of the thermoelectric generation system (TEG) that is used for waste heat recovery shows that the maximum power output of the thermoelectric generator system is 5.8 W and that the thermoelectric generator power density is approximately 180.6 W/m2. In this study, the total efficiency of a multifuel gasifier with a waste heat recovery system from thermoelectric generation modules was also investigated. The maximum exergetic efficiencies for 100% and 75% Japanese cedar are 55.46% and 54.49%, respectively. As the rice husk ratio increased, the maximum exergetic efficiency decreased to 44.2%~46.7%.