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Environmental challenges, such as global warming, growing demand on energy, and diminishing oil sources have accelerated research on alternative energy conversion methods. Thermoelectric power generation is a promising method to convert wasted heat energy into useful electrical energy form. A temperature gradient imposed on a thermoelectric device produces a Seebeck potential. However, this temperature gradient causes thermal stresses due to differential thermal expansions and mismatching of the bonded components of the device. Thermal stresses are critical for thermoelectric devices since they can generate failures, including dislocations, cracks, fatigue fractures, and even breakdown of the entire device. Decreases in power-generation performance and operation lifetime are major consequences of these failures. In order to minimize thermal stresses in the legs without affecting power-generation capabilities, this study concentrates on structural solutions. Thermoelectric devices with non-segmented and segmented legs were modeled. Specifically, the possible effect of various leg geometries, configurations, and dimensions were evaluated using finite-element and statistical methods. Significant changes in the magnitudes and distributions of thermal stresses occurred. Specifically, the maximum equivalent stresses in the rectangular-prism and cylindrical legs were 49.9 MPa and 43.3 MPa, respectively for the temperature gradient of 100ºC. By using cylindrical legs with modified dimensions, decreases in the maximum stresses in legs reached 21.2% without affecting power-generation performance. Moreover, the effect of leg dimensions and coaxial-leg configurations on power generation was significant; in contrast, various leg geometries and rotated-leg configurations had very limited affect. In particular, it was possible to increase power output from 20 mW to 65 mW by simply modifying leg widths and heights within the defined range. It should be noted, however, this modification also increased stress levels. It is concluded that leg geometries, configurations, and dimensions can be redesigned for improved durability and overall performance of thermoelectric devices.

Thermoelectric (TE) module integration into a notebook computer is experimentally investigated in this thesis for its energy harvesting opportunities. A detailed Finite Element (FE) model was constructed first for thermal simulations. The model outputs were then correlated with the thermal validation results of the selected system. In parallel, a commercial TE micro-module was experimentally characterized to quantify maximum power generation opportunity from the combined system and component data set. Next, suitable &ldquo / warm spots&rdquo / were identified within the mobile computer to extract TE power with minimum or no notable impact to system performance, as measured by thermal changes in the system, in order to avoid unacceptable performance degradation. The prediction was validated by integrating a TE micro-module to the mobile system under test. Measured TE power generation power density in the carefully selected vicinity of the heat pipe was around 1.26 mW/cm3 with high CPU load. The generated power scales down with lower CPU activity and scales up in proportion to the utilized opportunistic space within the system. The technical feasibility of TE energy harvesting in mobile computers was hence experimentally shown for the first time in this thesis.

Thermoelectric devices are capable of providing both localized active cooling and waste heat power generation. This work will explore the possibility of embedding thermoelectric devices within electronic packaging in order to achieve better system performance. Intel and Nextreme, Inc. have produced thin-film superlattice thermoelectric devices that have above average performance for thermoelectrics and are much thinner than most devices on the market currently. This allows them to be packaged inside of the electronic package where the thermoelectric devices can take advantage of the increased temperatures and decreased thermal lag as compared to the devices being planted on the outside of the package. This work uses the numerical CFD solver FLUENT and the analog electronic circuit simulator SPICE to simulate activity of thermoelectric devices within an electronics package.

The present study aims to increase understanding of the physical processes that govern thermoelectric efficiency in Na-containing group 14 type II clathrates. This has been achieved through structural characterization and physical property measurements. Local and electronic structures of Si clathrates with the formula NaxSi136, where x = 0, 1.3, 5.5, 7.2, 8.8, 14.1, 20 and 21.5 were studied using x-ray absorption spectroscopy. Thermoelectric properties, namely Seebeck coefficient, electrical conductivity and thermal conductivity were measured from 2.5 K to 400 K. Low Na content samples, x < 8, showed reduced thermal conductivity compared to the empty clathrate, x = 0. For x > 8, increased Na content led to increased charge transfer, increased thermal conductivity and decreased magnitude of Seebeck voltages. The heat capacities of the NaxSi136 materials were measured from 2.5 K to 300 K. Analysis of the heat capacity data showed that the vibrational modes associated with Na in the Si28 cages are of sufficiently low energies to interact with heat transporting acoustic phonons, leading to reduced thermal conductivity as x is increased up to ~ 8. Increasing Na content beyond x = 8 introduces Na into the Si20 cages. This stiffens the lattice, increasing (or maintaining) phononic contributions to thermal conductivity, and increasing electronic contributions. Electronic thermal conductivity is responsible for upwards of 50 % of heat conduction when x = 21.5. Na containing type II Ge clathrates were produced using an ionic liquid reaction medium. Seebeck coefficients observed in Na9Ge136 materials, were negative but larger in magnitude than those of the NaxSi136 materials and thermal conductivities of Na9Ge136 were lower than those of the NaxSi136 materials. While both Si and Ge type II clathrates showed modest figures of merit, with maximum ZT values of 2.5 × 10-6 and 2.8 × 10-5 observed in Na20Si136 and Na¬9Ge136, of the two framework elements, type II Ge clathrates have been shown to have more favourable thermoelectric properties.

Thesis (MDiploma (Mechanical Engineering))--Cape Technikon, 1992. / An investigation into the use of thermoelectric cooling energised by photovoltaic (PV) panels for removing sensible heat from electronic telecommunications equipment. The thermoelectric cooler consists of a solid-state heat pump which operates on the principle of the Peltier effect. The thermoelectric device transfers heat through a cold sink to ambient outside air via a hot sink. A major prerequisite was that the system should be selfsufficient in terms of power because the sites for the microwave transmitters are often remote. Solar power was the only alternative source of energy and the cooler was designed to accept direct current from PV panels which are usually used to power transmitters on distant locations. The cooling device had to be reliable, virtually maintenance-free and simple to repair.

Today many processes generate a lot of waste heat, for example industries or cars. One way to make this thermal energy useful is to transform it into electrical energy with a thermoelectric generator (TEG) or thermoelectric cooler (TEC). This technology is not used in any large scale today, but a lot of research is being done on the subject. The technology is based on the Seebeck effect and uses a temperature difference between two sides of an element to generate an electrical current. The reason that the research has gained more attention in recent years is because of the increasing electricity prices and the diminishing natural resources. Other benefits are that they run quietly and do not demand much maintenance.Another area where this technology could be useful is in off-grid cabins where it is easy to generate a lot of thermal energy by burning wood, but electrical energy is in high demand.In this thesis two different types of TEGs and one type of TEC are tested to investigate how much power they generate at different temperature differences, how well they meet the specified values in their respective data sheet and what their power per euro value is. For this, an experimental setup was made with:- An induction plate to increase the temperature on the hot side.- A CPU-fan, to reduce the temperature on the cold side.- Two temperature sensors (one for measuring the hot temperature and one for the cold one).- An electric circuit featuring a voltmeter, an amperemeter and an adjustable resistor (rheostat).The results show that, for this experiment the highest received power (6,38 W) comes from the medium-priced element but the highest power per euro comes from the lowest priced element (1,16 W/€). A quality problem for the lowest priced element was that parts of the solder melted when the temperature exceeded 225 °C. Another problem was that the induction plate was unable to provide enough heat for the most expensive of the elements to reach the temperature for which the retailer supplies their measured data.

In recent decades the development of technologies capable to offer highly localized and precise temperature control has received increasing attention due to their relevance and applicability in numerous engineering fields. Multiple scientific papers have been written that focus on the enhancement of the performance of thermoelectric materials and micro-devices. This Ph.D. thesis in the field of Mechanical Engineering pursues three main research goals regarding electrothermal phenomena: (1) To provide an optimization design strategy for micro-thermoelectric coolers by analyzing the interplay between electrical and thermal fluxes during device operation. (2) To analyze the suitability of a device, based on micro-thermoelectric coolers, for controlling the thermal environment in microbiological systems. (3) To develop an experimental technique, based on optical pump-probe thermal imaging, to extract the thermal conductivity anisotropy of thin films. For this purpose, numerical simulations and experiments were carried out. The results show, that the design of micro-thermoelectric devices must take into account the impact of parameters that are typically neglected in the construction of macro scale devices. Poorly designed parameters, such as the metallic contacts, the distance between thermoelectric elements and their interaction with the substrate, carry severe reductions of the performance of micro-thermoelectric devices. It is demonstrated that the optimal performance is achieved when the thermoelectric legs are properly dimensioned, so that a balance of the Fourier and Joule fluxes is reached. Numerical analyses prove that micro-thermoelectric coolers offer a feasible alternative to overcome the current spatial and temperature limitations of conventional technologies and therefore enable to investigate the thermal environment of biological systems at the micro-scale. Guidelines for the implementation of the experimental platform are provided. The evaluation of the numerical and experimental data proves that optical pump-probe thermal imaging is suitable to characterize both the in-plane and the through-plane thermal conductivity of thin films. The experimental conditions to extract the anisotropy of the sample under study are determined. The outcome of this work yields new insights into electrothermal phenomena at the micro-scale and thus creates new routes in the design, fabrication and characterization of micro- thermoelectric materials and devices.:Acknowledgements IV Erklärung der Urheberschaft VI Summary VII Zusammenfassung VIII Table of content IX List of figures XI List of tables XIV Abbreviations and symbols XV 1 Introduction 1 1.1 Motivation 1 1.2 Outline of the thesis 4 1.2.1 Chapter 2 - Fundamentals 4 1.2.2 Chapter 3 - Design guidelines of micro-thermoelectric coolers 4 1.2.3 Chapter 4 - Development of a platform for biological systems experimentation 4 1.2.4 Chapter 5 - Development of a technique for thermal transport characterization in thin films 5 1.2.5 Chapter 6 - Main conclusion and future research 5 1.3 Main research objectives 5 2 Fundamentals 7 2.1 Thermoelectric phenomena 7 2.2 Performance estimation of micro-thermoelectric coolers 10 2.3 Finite element modelling 12 2.3.1 Introduction to finite element modelling 12 2.3.2 Finite element modelling of thermoelectric phenomena 17 2.4 Thermoreflectance imaging microscopy 19 3 Design guidelines of micro-thermoelectric coolers 26 3.1 Introduction 26 3.2 Micro-thermoelectric coolers: an alternative for thermal management 28 3.3 Analysis approach 29 3.3.1 Input current optimization 31 3.3.2 Metallic contacts 32 3.3.3 Leg pair geometry 35 3.3.4 Fill factor 38 3.3.5 Experimental characterization of µTECs 41 3.4 Summary 44 4 Development of a platform for biological systems experimentation 46 4.1 Introduction 46 4.2 Thermal analysis on biological systems 48 4.3 Platform conceptual proposal 50 4.4 Analysis approach 52 4.4.1 Input current optimization 52 4.4.2 Fill material 54 4.4.3 Thermotaxis 55 4.4.4 Top material 56 4.4.5 Cold spot optimization 58 4.5 Experimental platform construction 59 4.6 Summary 62 5 Development of a technique for thermal transport characterization in thin films 64 5.1 Introduction 64 5.2 Thermal anisotropy characterization in thin films 65 5.3 Experimental apparatus 66 5.4 Experimental measurements 69 5.5 Analysis approach 72 5.5.1 Thermal conductivity anisotropy analysis 76 5.5.2 Effect of the laser power on the temperature distribution 79 5.5.3 Enhancement of the system sensitivity 80 5.6 Summary 83 6 Main conclusion and future research 85 6.1 Main conclusion 85 6.2 Outlook 88 7 References 89 8 Scientific output 97 8.1 Publications in peer review journals 97 8.2 Selected conference abstracts 98 9 Curriculum vitae 99

The thermoelectric properties of bismuth telluride based thermoelectric (TE) materials are well-characterized, but comparatively little has been published on the thermomechanical properties. In this paper, dynamic mechanical analysis (DMA) and differential scanning calorimetry data for bismuth telluride based TE materials is presented. The TE materials' tan delta values, indicative of viscoelastic energy dissipation modes, approached that of glassy or crystalline polymers, were greater than ten times the tan delta of structural metals, and reflected the anisotropic nature of TE materials. DMA thermal scans showed changes in mechanical properties versus temperature with clear hysteresis effects. These results showed that the application of DMA techniques are useful for evaluation of thermophysical and thermomechanical properties of these TE materials.

With growing world population and decreasing fossil fuel reserves we need to explore and utilize variety of renewable and clean energy sources to meet the imminent challenge of energy crisis. Solar energy is considered as the leading promising alternate energy source with the pertinent challenge of off sunshine period and uneven worldwide distribution of usable sun light. Although thermoelectricity is considered as a reasonable energy harvester from wasted heat, its mass scale usage is yet to be developed. By transforming window glasses into generators of thermoelectricity, this doctoral work explores engineering aspects of using the temperature gradient between the hot outdoor heated by the sun and the relatively cold indoor of a building for mass scale energy generation. In order to utilize the two counter temperature environments simultaneously, variety of techniques, including: a) insertion of basic metals like copper and nickel wire, b) sputtering of thermoelectric films on side walls of individual glass strips to form the thickness depth of the glass on subsequent curing of the strips, and c) embedding nano-manufactured thermoelectric pillars, have been implemented for innovative integration of thermoelectric materials into window glasses. The practical demonstration of thermoelectric windows has been validated using a finite element model to predict the behavior of thermoelectric window under variety of varying conditions. MEMS based characterization platform has been fabricated for thermoelectric characterization of thin films employing van der Pauw and four probe modules. Enhancement of thermoelectric properties of the nano- manufactured pillars due to nano-structuring, achieved through mechanical alloying of micro-sized thermoelectric powders, has been explored. Modulation of thermoelectric properties of the nano-structured thermoelectric pillars by addition of sulfur to nano-powder matrix has also been investigated in detail. Using the best possible p and n type thermoelectric materials, this novel energy generation technique promises 304 watts of thermoelectricity from a 9 m2 glass window utilizing temperature difference of 20 OC. In addition to be useful even during off sunshine hours of the day, these energy harvesting windows will be capable of power generation even in the absence of a cooling systems inside the building as long as a natural temperature gradient exists between the two counter environments. With an increasing trend of having the exterior of buildings and high rises entirely made up of glass, this work offers an innovative transformation of these building exteriors into mass scale energy harvesters capable of running average lighting loads inside the building hence providing a complimentary source of electricity to the main power grid.