Analysis and design of a silicide-tetrahedrite thermoelectric generator concept suitable for large-scale industrial waste heat recovery

Industrial Waste Heat Recovery (IWHR) is one of the areas with strong potential for energy efficiency and emissions reductions in industry. Thermoelectric (TE) generators (TEGs) are among the few technologies that are intrinsically modular and can convert heat directly into electricity without moving parts, so they are nearly maintenance-free and can work unattended for long periods of time. However, most existing TEGs are only suitable for small-scale niche applications because they typically display a cost per unit power and a conversion efficiency that is not competitive with competing technologies, and they also tend to rely on rare and/or toxic materials. Moreover, their geometric configuration, manufacturing methods and heat exchangers are often not suitable for large-scale applications. The present analysis aims to tackle several of these challenges. A module incorporating constructive solutions suitable for upscaling, namely, using larger than usual TE elements (up to 24 mm in diameter) made from affordable p-tetrahedrite and n-magnesium silicide materials, was assessed with a multiphysics tool for conditions typical of IWHR. Geometric configurations optimized for efficiency, power per pair and power density, as well as an efficiency/power balanced solution, were extracted from these simulations. A balanced solution provided 0.62 kWe/m2 with a 3.9% efficiency. Good prospects for large-scale IWHR with TEGs are anticipated if these figures could be replicated in a real-world application and implemented with constructive solutions suitable for large-scale systems.Fundação para a Ciência e a Tecnologia, European Regional Development Fund (ERDF), P.O.F.C.—COMPETE, European and National Funds: M-ERA.net Project THERMOSS (M-ERANET2/0011/2016), MEtRICs—Mechanical Engineering and Resource Sustainability Centre (UIDB/ 04077/2020), C2TN—Center for Nuclear Sciences and Technologies (UID/Multi/04349/2019), Project Exhaust2Energy (PTDC/EMS-ENE/3009/2014)

Meta-study: Analysis of thermoelectric figure of merit parameters for silicides with various doping agents

Thermoelectric (TE) materials are increasingly promising for power generation in medium to high-temperature environments. Recent research on thermoelectric generators (TEGs) has determined the thermodynamic properties which impact the total efficiency and figure of merit (ZT) of these materials. A large Seebeck coefficient, high electrical conductivity and low thermal conductivity optimise ZT. This meta-study investigates silicides for potential TEG applications due to their high chemical stability and higher natural abundance than other TE materials. Data on the thermoelectric properties of CrSi2, FeSi2, Mg2Si and MnSi2, with a range of dopants, was sourced from a wide scope of literature and is analysed. The above thermodynamic properties contributing to ZT for each of these materials are graphed between 300 and 1000 K. It was found that chromium silicides are most effective at a temperature range of 600-800 K, and undoped magnesium silicides are most effective around 900 K. Oxide addition to β-FeSi2 produced relatively high ZT scores (ZT ≈ 0.5) among iron silicides. Rhenium substitution in manganese silicides produced a maximum figure of merit (1.05) at 900 K. Supersaturation via liquid quenching was determined to maintain high rhenium substitution and this technique may be the key to further improving the thermoelectric properties of other silicides

Long term stability of silicide based thermoelectric materials and modules

Silicide-based thermoelectric generators are potential candidates for waste heat recovery at temperatures below 500  C. For the last two decades, the conversion efficiency of modules based on n-type magnesium silicides and p-type higher manganese silicide has improved significantly. However, the conditions in which thermoelectric generators operate (for example, remote areas in the oil, gas, and telecommunication industries, in automobiles, etc.) are harsh (corrosive, for example) and hostile (due to thermal instability). In this project, there was much focus on the stability of the thermoelectric modules, with special interest given to oxidation of the thermoelectric materials and module stability. The thermal oxidation studies were conducted on higher manganese silicide alloys; the studies mainly investigated the effect of the alloys’ composition, consolidation techniques and the operational atmosphere’s effect on their oxidation potential. Moreover, the choice of matching electrodes and good bonding technology for the module assembly was the ultimate step before finally testing the actual performance and stability of the module over an extended period. The thorough oxidation studies conducted in this thesis revealed the importance of different production processes for the higher manganese silicide thermoelectric materials on the oxidation robustness of the alloys. The study showed that the purity (fewer impurities) of the raw elements and optimal doping level are among the key factors for the alloys to resist oxidation by growing a protective SiO2 protective oxide layer. Moreover, it was also shown that powder consolidation by spark plasma sintering produced stronger bulk pellets, and mechanical strength played a key role in passive oxidation. During the module’s contacts design, silver electrodes and solid liquid interdiffusion bonding technology were used. The contact resistance of the assembled modules were measured using an automated point contact measurement test rig. On the magnesium silicide the specific contact resistance was on average 0.17 m cm2 with 2.1% standard deviation. The higher manganese silicide’s contact interface, on the other hand, the results were dispersed along the bond, where 0.07 m cm2 was the lowest value and 1.12 m cm2 the highest (81.3% standard deviation). Finally, the module stability was investigated by testing the performance of the assembled modules. The tested modules produced up to 7.4mW/cm2 power density at 400  C and sustained more than 300 thermal cycles. The gradual degradation was found to mainly originate from the mechanical failure of the contact interfaces and oxidation of the n-type magnesium silicide relative to the p-type material. Applying a high-temperature coating did not reduce the degradation rate, which showed that it would be better to encapsulate the modules to count-act the effect of oxidation.publishedVersio

Production of Functional Materials for Advanced Thermoelectric Applications

With ever increasing energy costs, climate change and energy supply concerns there has been a drive towards sustainable and renewable energy sources. There are many industries which currently produce excess waste heat such as reactors, motorised transport, metal production, and gas turbines to name a few. These industries can reduce their carbon footprint with successful heat scavenging. A much-overlooked technology is thermoelectric generators. These are solid-state devices which can convert heat directly into electricity using the Seebeck effect. There are number of advantages of thermoelectrics over conventional renewables including no moving parts, maintenance-free functionality in extreme environments, high-temperature resistance and long-life span. Thermoelectrics can function as both primary generators or as thermal scavengers, however they are not yet suitable for mass market applications. This thesis will investigate the entire thermoelectric device and develop scalable alternatives to current technologies. In this the production of earth abundant thermoelectrics, focusing on transition metal silicides, was investigated using a novel pack cementation technique to produce high quality materials that are affordable and require only low-cost equipment to produce. This technique was shown to produce high purity materials; however, production rates were limited due to the diffusion rates. The second part of this thesis investigated the soldered contacts for device construction; as current soldered contacts are subject to fatigue, can be costly and at times toxic. Molten liquid electrical contacts were developed and whilst promising are limited by their compatibility with current thermoelectric materials. The most successful work was that of printable, conductive polymers. These were developed to be stable at high temperatures, bind well with current thermoelectric materials and provide a new contact material allowing fully automated production. To ascertain the viability of the developed conductive polymer contacts, further work was undertaken to prototype functional devices which led to promising results for future upscaling

New oxidation protective coatings for thermoelectric materials

L'abstract è presente nell'allegato / the abstract is in the attachmen

Thermoelectric generators for long duration lunar missions

The number of lunar missions involving the deployment of probes, rovers and other equipment is expected to raise significantly. Some of these missions will face the challenge to survive the night on the Moon, with very low temperatures on the surface. Solar thermal energy can be stored during the day and transformed into electricity at night. We present a study of the performance of thermoelectric modules. Some of the modules are used in simulations of a thermal energy storage system, which includes the test of different materials as a thermal mass. The power generated in all the cases is obtained.Peer ReviewedPostprint (published version

Performance evaluation and stability of silicide-based thermoelectric modules

Long-term studies on thermoelectric generators based on N-type magnesium silicide (Mg2.01Si0.49Sn0.5Sb0.01) and P-type higher manganese silicide (Mn0.98Mo0.02Si1.73Ge0.02) materials are presented, in the operating temperature range of 200 °C–400 °C. Emphasis is put on the performance and reliability of the current collector configuration, especially on the hot side of the module, and on the thermomechanical stresses that are created during operation and lifetime testing as a result of large temperature gradients experienced across the thermoelectric legs. With silver (Ag) paste as contact material, the long term-stability of the uni-couples was carried out on non-metalized legs and gold metalized legs under ambient conditions. Under isothermal and thermocycling tests, the non-metalized legs showed a gradual decrease in open circuit voltage (after a period of 200 h) and increase in internal resistance. Conversely, the module made of metalized legs was robust and stable for the same isothermal period. However, after 300cycles the n-type material showed mechanical failure (cracks) but the p-type resisted. Post-operation analysis by SEM/EDS and mechanical testing revealed that oxidation, adherence of the contact material and diffusion of the bonding material were the cause of performance degradation of the unicouples.publishedVersio

Synthesis of thermoelectric magnesium-silicide pastes for 3D printing, electrospinning and low-pressure spray

In this work, eco-friendly magnesium-silicide (Mg2Si) semiconducting (n-type) thermoelectric pastes for building components concerning energy-harvesting devices through 3D printing, spray and electrospinning were synthetized and tested for the frst time. The Mg2Si fne powders were obtained through the combination of ball milling and thermal annealing under Ar atmosphere. While the latter process was crucial for obtaining the desired Mg2Si phase, the ball milling was indispensable for homogenizing and reducing the grain size of the powders. The synthetized Mg2Si powders exhibited a large Seebeck coeffcient of~487 µV/K and were blended with a polymeric solution in diferent mass ratios to adjust the paste viscosity to the diferent requirements of 3D printing, electrospinning and low-pressure spray. The materials produced in every single stage of the paste synthesis were characterized by a variety of techniques that unequivocally prove their viability for producing thermoelectric parts and components. These can certainly trigger further research and development in green thermoelectric generators (TEGs) capable of adopting any form or shape with enhanced thermoelectric properties. These green TEGs are meant to compete with common toxic materials such as Bi2Te3, PbTe and CoSb that have Seebeck coefcients in the range of ~ 290–700 μV/K, similar to that of the produced Mg2Si powders and lower than that of 3D printed bulk Mg2Si pieces, measured to be ~ 4866 μV/K. Also, their measured thermal conductivities proved to be signifcantly lower (~ 0.2 W/mK) than that reported for Mg2Si (≥4 W/mK). However, t is herein demonstrated that such thermoelectric properties are not stable over time. Pressureless sintering proved to be indispensable, but difcultly achievable by long thermal annealing (even above 32 h) in inert atmosphere at 400 °C, at least for bulk Mg2Si pieces constituted by a mean grain size of 2–3 μm. Hence, for overcoming this sintering challenge and become the silicide’s extrusion viable in the production of bulk thermoelectric parts, alternative pressureless sintering methods will have to be further explored

Thermoelectrics: Ecological Profile and Fundamental Principles for Sustainability

The need for efficient energy conversion and utilization has magnified on account of global environmental concerns, leading to a dramatic rise in focus on technologies that can accomplish such enhanced efficiencies. In this context, thermoelectrics (TEs) have emerged as a prominent platform on account of rigorous research that has enabled a significant leap in their conversion efficiencies, which enhances their potential to lower fossil fuel consumption. However, the advent of novel TEs has been accompanied by growing concerns about the use of scarce and toxic constituent elements in most of these materials/systems, raising questions about their eco-friendliness. While these concerns must be suitably addressed, the very nature of looking at TEs solely in terms of either benefits during their usage, or at issues with their constituents, confines the notion of sustainability to one or few stages of their life cycle. This creates doubts about the traditional claims of TEs being ecofriendly, since other environmental issues associated with their life cycle, such as impacts caused by their production or end-of-life treatment, remain neglected. These gaps hinder a true assessment of ecological credentials of TEs as an energy harvesting platform, and also make it difficult to provide adequate directions to policymakers and other stakeholders on the nature of steps required to make this platform ecologically suitable and economically viable. To ameliorate these gaps, this work explores the environmental profile of TEs using life cycle assessment (LCA). TE devices – modules and generators – were evaluated for environmental performance across their life cycle for three applications differing in their nature of waste heat emission and mobility. These were: (a) baseload coal-based power plant (static, constant emission); (b) peak load natural gas-based power plants (static, periodic emission); and (c) automobiles (mobile, intermittent emission). For all end-uses, TEs were assessed on various impacts. The first-ever exhaustive inventory analysis to date was conducted for production of TE devices, while three end-of-life (EOL) scenarios were considered to determine the benefits and pitfalls of recycling TEs as these use scarce constituents. Subsequently, the results from these LCA analyses were used to distill key findings and postulate principles for developing sustainable thermoelectrics. LCA analysis of TEs showed that both high electricity consumption for TE processing and use of constituent elements that emit toxic waste during their extraction and refining, caused the bulk of their production-related impacts. Further, while TE devices were observed to be environmentally sound for applications involving continuous waste heat emission (coal-based power), they showed ineffectiveness for periodic (gas-based electricity) and intermittent waste heat emission (automobiles) to varying degrees. In addition, recycling of TEs was seen to have moderate influence on their ecological output, with heat exchanger-based components playing a more significant role. Lastly, using the results from LCA analyses, eight sustainability principles were postulated for TEs encompassing their entire life cycle, that can guide policymakers to work with other stakeholders on enhancing overall eco-friendliness of this platform

Thermal durability of novel thermoelectric materials for waste heat recovery

DoktorgradsavhandlingThermoelectric Generators (TEG) are a potential technology for waste heat recovery. At their hearts, thermoelectric materials convert a heat flux into electric current. By placing TEGs on the surface of a waste heat source, some of the heat can be converted into valuable electric energy. However, todays state-of-the art materials have low efficiencies, are limited to low temperature operation, contains toxic and rare elements and are in general not cost-effective for such applications. New thermoelectric materials are needed that can overcome these barriers. In this thesis, two classes of materials have been investigated; skutterudites and silicides. Both have relatively high efficiencies, can be used at high temperatures (400-500°C) and contain cheaper and less toxic compounds than state-of-the-art materials. To move these materials from lab-scale testing into real-life applications, the focus is shifted from optimizing for thermoelectric efficiency to long-term stability. As these materials will be used at higher temperatures, different temperature activated processes leading to degradation of the materials needs to be accounted for and controlled, such as oxidation and interdiffusion. Adoption into industrial mass production requires simple and cheap synthesis methods that produces homogenous materials. Homogenous materials are very important for achieving stable thermoelectric and mechanical properties over time. Secondary phases can easily form during synthesis of skutterudites, and knowledge about the formation and annihilation of such phases during synthesis was therefore studied thoroughly as these can have unbeneficial effect on both thermoelectric properties and thermal durability. Understanding the mechanisms and kinetics governing skutterudite formation is of great importance for optimizing synthesis parameters leading to faster and more homogenous production of skutterudites. P-type skutterudites with composition Fi0.6Co2Fe2Sb12 (Fi = In, Ca, Ce and Yb) were synthesized to look at the effect of filler element and microstructure. Ce-filled sample was furthermore synthesized using four different methods to assess the effect of synthesis method on secondary phase formation and thermoelectric properties. A precursor method was found to not only be the fastest method, but also produce the most homogenous material with highest zT of 0.7. An important contribution of this thesis is the results and discussion around peritectic formation and transformation in the skutterudite system. In addition to grain size, the diffusivity of the filler element in skutterudite was found to be the main factor governing transformation time into phase pure skutterudite. High temperature oxidation of skutterudites as a function of filler atoms and iron content were furthermore investigated. Residual secondary phases resulting from incomplete transformation into the phase pure skutterudite, such as FeSb2 and CeSb2, were found to have severe effects on the oxidation rate. High iron content was also found to lower the onset temperature of oxidation in accordance with previous studies. In addition, the effect of In as a filler element was found to form an intermediate InSb layer that actually caused a reduction in oxidation rate. Also the high temperature oxidation of Mg2(Si-Sn) was studied as a function of the Sn content. All these materials formed a passivating outer layer of MgO. Above a certain ignition temperature, the MgO layer broke down and the oxidation proceeded exponentially. An increasing Sn content was found to significantly lower the ignition temperature due to formation of liquid Sn below the MgO layer. Similarly, the Pilling-Bedworth ratio (ratio of volume of oxide to volume of underlying alloy) was found to affect the ignition temperature. In both these oxidation studies, the common theme is that variations in elemental composition affect the resistance towards oxidation significantly. Clearly, these materials need to be optimized not only in terms of thermoelectric performance, but also stability towards oxidation. First of all, oxidizing resistant materials can be employed in application without the need of protective environments such as coatings or encapsulations. Secondly, control of the oxidation mechanisms is import in a mass production process both to ease the production process and to avoid accumulation of oxides in the final material. An alternative to oxidation resistant materials is the application of coatings on the outer surfaces. Several commercially available coatings were tested on both skutterudite and silicide. An aluminum-based coating was found most promising and could protect both material classes from oxidation during thermal cycling up to temperatures of 500-550°C. It is believed that further optimization of this type of coating has a good future for use in protection of thermoelectric materials at high temperatures. To assess how different thermally activated degradation process affect the performance of thermoelectric materials and modules, long term testing of both single functionalized legs and prototype modules were conducted. A new test method was developed, where several single legs were electrically characterized separately throughout thermal cycling. This made it possible to compare different types of coatings with uncoated material, thereby identify the effect of the degradation processes upon performance. Both diffusion of metal contact into thermoelectric leg, oxidation in from sides, and crack formation, were found to significantly affect the performance of the legs over time, by reducing open circuit voltage and increasing inner resistance. Finally, several silicide based modules were assembled and tested. The n-type and p-type material was Mg2(Si-Sn) and MnSi1.75 respectively. The highest power of 3 W was measured for one of these modules at hot side temperatures of 700°C and an estimated efficiency of 5.3% – the highest efficiency recorded to date for modules made purely of silicides. However, long term testing of one of these modules revealed gradual reduction in performance. Post-characterization showed how particularly oxidation and interdiffusion between thermoelectric leg and metal contact played a major role in the decline in power output