Roadmap on thermoelectricity

The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices

Development of MEMS Piezoelectric Vibration Energy Harvesters with Wafer-Level Integrated Tungsten Proof-Mass for Ultra Low Power Autonomous Wireless Sensors

La génération d’énergie localisée et à petite échelle, par transformation de l’énergie vibratoire disponible dans l’environnement, est une solution attrayante pour améliorer l’autonomie de certains noeuds de capteurs sans-fil pour l’Internet des objets (IoT). Grâce à des microdispositifs inertiels résonants piézoélectriques, il est possible de transformer l’énergie mécanique en électricité. Cette thèse présente une étude exhaustive de cette technologie et propose un procédé pour fabriquer des microgénérateurs MEMS offrant des performances surpassant l’état de l’art. On présente d’abord une revue complète des limites physiques et technologiques pour identifier le meilleur chemin d’amélioration. En évaluant les approches proposées dans la littérature (géométrie, architecture, matériaux, circuits, etc.), nous suggérons des métriques pour comparer l’état de l’art. Ces analyses démontrent que la limite fondamentale est l’énergie absorbée par le dispositif, car plusieurs des solutions existantes répondent déjà aux autres limites. Pour un générateur linéaire résonant, l’absorption d’énergie dépend donc des vibrations disponibles, mais aussi de la masse du dispositif et de son facteur de qualité. Pour orienter la conception de prototypes, nous avons réalisé une étude sur le potentiel des capteurs autonomes dans une automobile. Nous avons évalué une liste des capteurs présents sur un véhicule pour leur compatibilité avec cette technologie. Nos mesures de vibrations sur un véhicule en marche aux emplacements retenus révèlent que l’énergie disponible pour un dispositif linéaire résonant MEMS se situe entre 30 à 150 Hz. Celui-ci pourrait produire autour de 1 à 10 μW par gramme. Pour limiter la taille d’un générateur MEMS pouvant produire 10 μW, il faut une densité supérieure à celle du silicium, ce qui motive l’intégration du tungstène. L’effet du tungstène sur la sensibilité du dispositif est évident, mais nous démontrons également que l’usage de ce matériau permet de réduire l’impact de l’amortissement fluidique sur le facteur de qualité mécanique Qm. En fait, lorsque l’amortissement fluidique domine, ce changement peut améliorer Qm d’un ordre de grandeur, passant de 103 à 104 dans l’air ambiant. Par conséquent, le rendement du dispositif est amélioré sans utiliser un boîtier sous vide. Nous proposons ensuite un procédé de fabrication qui intègre au niveau de la tranche des masses de tungstène de 500 μm d’épais. Ce procédé utilise des approches de collage de tranches et de gravure humide du métal en deux étapes. Nous présentons chaque bloc de fabrication réalisé pour démontrer la faisabilité du procédé, lequel a permis de fabriquer plusieurs prototypes. Ces dispositifs ont été testés en laboratoire, certains démontrant des performances records en terme de densité de puissance normalisée. Notre meilleur design se démarque par une métrique de 2.5 mW-s-1/(mm3(m/s2)2), soit le meilleur résultat répertorié dans l’état de l’art. Avec un volume de 3.5 mm3, il opère à 552.7 Hz et produit 2.7 μW à 1.6 V RMS à partir d’une accélération de 1 m/s2. Ces résultats démontrent que l’intégration du tungstène dans les microgénérateurs MEMS est très avantageuse et permet de s’approcher davantage des requis des applications réelles.Small scale and localized power generation, using vibration energy harvesting, is considered as an attractive solution to enhance the autonomy of some wireless sensor nodes used in the Internet of Things (IoT). Conversion of the ambient mechanical energy into electricity is most often done through inertial resonant piezoelectric microdevices. This thesis presents an extensive study of this technology and proposes a process to fabricate MEMS microgenerators with record performances compared to the state of the art. We first present a complete review of the physical and technological limits of this technology to asses the best path of improvement. Reported approaches (geometries, architectures, materials, circuits) are evaluated and figures of merit are proposed to compare the state of the art. These analyses show that the fundamental limit is the absorbed energy, as most proposals to date partially address the other limits. The absorbed energy depends on the level of vibrations available, but also on the mass of the device and its quality factor for a linear resonant generator. To guide design of prototypes, we conducted a study on the potential of autonomous sensors in vehicles. A survey of sensors present on a car was realized to estimate their compatibility with energy harvesting technologies. Vibration measurements done on a running vehicle at relevant locations showed that the energy available for MEMS devices is mostly located in a frequency range of 30 to 150 Hz and could generate power in the range of 1-10 μW per gram from a linear resonator. To limit the size of a MEMS generator capable of producing 10 μW, a higher mass density compared to silicon is needed, which motivates the development of a process that incorporates tungsten. Although the effect of tungsten on the device sensitivity is well known, we also demonstrate that it reduces the impact of the fluidic damping on the mechanical quality factor Qm. If fluidic damping is dominant, switching to tungsten can improve Qm by an order of magnitude, going from 103 to 104 in ambient air. As a result, the device efficiency is improved despite the lack of a vacuum package. We then propose a fabrication process flow to integrate 500 μm thick tungsten masses at the wafer level. This process combines wafer bonding with a 2-step wet metal etching approach. We present each of the fabrication nodes realized to demonstrate the feasibility of the process, which led to the fabrication of several prototypes. These devices are tested in the lab, with some designs demonstrating record breaking performances in term of normalized power density. Our best design is noteworthy for its figure of merit that is around 2.5 mW-s-1/(mm3(m/s2)2), which is the best reported in the state of the art. With a volume of 3.5 mm3, it operates at 552.7 Hz and produces 2.7 μW at 1.6 V RMS from an acceleration of 1 m/s2. These results therefore show that tungsten integration in MEMS microgenerators is very advantageous, allowing to reduce the gap with needs of current applications

Energy: A special bibliography with indexes, April 1974

This literature survey of special energy and energy related documents lists 1708 reports, articles, and other documents introduced into the NASA scientific and technical information system between January 1, 1968, and December 31, 1973. Citations from International Aerospace Abstracts (IAA) and Scientific and Technical Aerospace Reports (STAR) are grouped according to the following subject categories: energy systems; solar energy; primary energy sources; secondary energy sources; energy conversion; energy transport, transmission, and distribution; and energy storage. The index section includes the subject, personal author, corporate source, contract, report, and accession indexes

Electrocaloric coolers and pyroelectric energy harvesters based on multilayer capacitors of Pb(Sc0.5Ta0.5)O3

The following work investigates the development of heat pumps that exploit electrocaloric effects in Pb(Sc,Ta)03 (PST) multilayer capacitors (MLCs). The electrocaloric effect refers to reversible thermal changes in a material upon application (and removal) of an electric field. Electrocaloric cooling is interesting because 1) it has the potential to be more efficient than competing technologies, such as vapour-compression systems, and 2) it does not compel the use of greenhouse gases, which is crucial in order to slow down global warming and mitigate the effects of climate change. The continuous progress in the field of electrocalorics has promoted the creation of several electrocaloric based heat pump prototypes. Despite the different designs and working principles utilized, these prototypes have struggled to maintain temperature variations as large as 10 K, discouraging their industrial development. In this work, bespoke PST-MLCs exhibiting large electrocaloric effects near room temperature were embodied in a novel heat pump with the motivation to surpass the 10 K-barrier. The experimental design of the heat pump was based on the outcome of a numerical model. After implementing some of the modifications suggested by the latter, consistent temperature spans of 13 K at 30 °C were reported, with cooling powers of 12 W / kg. Additional simulations predicted temperature spans as large as 50 K and cooling powers in the order of 1000 W / kg, if a new set of plausible modifications were to be put in place. Similarly, these very same PST-MLCs samples were implemented into pyroelectric harvesters revisiting Olsen's pioneering work from 1980. The harvested energies were found to be as large as 11.2 J, with energy densities reaching up to 4.4 J / cm3 of active material, when undergoing temperature oscillations of 100 K under electric fields applied of 140-200 kV / cm. These findings are two and four times, respectively, larger than the best reported values in the literature. The results obtained in this dissertation are beyond the state-of-the-art and show that 1) electrocaloric heat pumps can indeed achieve temperature spans larger than 10 K, and 2) pyroelectric harvesters can generate electrical energy in the Joule-range. Moreover, numerical models indicated that there is still room for improvement, especially when it comes to the power of these devices. This should encourage the development of these kinds of electrocaloric- and pyroelectric-based applications in the near future

Microturbopompe avec isolation thermique pour cycle Rankine sur puce

Les micromoteurs thermiques (Power-MEMS) pourraient offrir une alternative aux batteries pour répondre aux besoins d’énergie compacte et distribuée pour des applications telles que l'électronique portable, les robots, les drones et les systèmes embarqués, les capteurs et les actionneurs. La microturbine à vapeur de cycle thermodynamique de Rankine fait partie de ce domaine de micromoteurs. Ce dispositif est destiné à la génération d’électricité à petite échelle à partir de la récupération de la chaleur perdue. Dans ce contexte, l’objectif de ce travail est la fabrication et la démonstration expérimentale d’une microturbopompe à haute température pour implémenter le cycle de Rankine. Une configuration originale qui intègre l’isolation thermique est, tout d’abord, proposée. Cette configuration est constituée d’un empilement de cinq tranches (silicium et verre) pour enfermer un rotor hybride (silicium et verre) supporté par des paliers hydrostatiques. Le rotor est un disque de 4 mm de diamètre et de 400 µm d’épaisseur avec des pales de turbine sur le dessus et une pompe visqueuse à rainures en spirale sur le dessous. Une technique de micromoulage de verre a été développée dans ce travail pour intégrer du verre dans le rotor comme un matériau isolant thermiquement. La microturbopompe est fabriquée avec succès en utilisant les méthodes de microfabrication des MEMS. Tout d'abord, les paliers hydrostatiques, la turbine et le fonctionnement de la pompe sont caractérisés, jusqu'à une vitesse de rotation de 100 kRPM. La turbine a fourni 0,16 W de puissance mécanique et le débit de la pompe était supérieur à 2.55 mg/s. Ensuite, la première démonstration d'une turbopompe MEMS fonctionnant à des températures élevées a été réalisée. Une comparaison a été faite avec un rotor non isolé pour prouver l'efficacité des stratégies d'isolation thermique. La turbopompe MEMS isolée a été démontrée à 160°C du côté de la turbine. Par extrapolation, la microturbopompe devrait fonctionner jusqu'à une température de 400°C avant que la température dans la pompe n'atteigne 100°C. Pour la première fois, une microturbopompe pour un fonctionnement à haute température est fabriquée et caractérisée

SYSTEM MODELING AND MATERIAL DEVELOPMENT FOR STANDALONE THERMOELECTRIC POWER GENERATORS

This dissertation addresses the need to develop a scalable and standalone power generator for personal, commercial, and military transportation and communication systems. The standalone thermoelectric power generator (TPG) converts heat to electrical power in a unique way that does not draw on conventional power sources like batteries. A TPG is comprised of four main components: a heat source, thermoelectric modules, a heat sink, and thermal insulation. For system modeling and materials development purposes, the dissertation invented the first pyrophoric heated standalone TPG, solid-state renewable heat source, and two-component nanocomposite thermoelectric power generation material. In this work, the first pyrophoric heated standalone thermoelectric power generator was designed, fabricated, and tested. The bases of the system were four porous silicon carbide combustors for the exothermic reaction of pyrophoric iron powder with oxygen. These combustors provided a heat source of 2,800 to 5,600 W to the heat sinks (through TE modules) at conditions suitable for a standalone, pyrophoric iron fueled TE power generator. The system integrated with 16 commercial bismuth telluride thermoelectric modules to produce 140 to 280 W of electrical power with a TE power conversion efficiency of ~5%. This demonstration represents an order-of-magnitude improvement in portable electrical power from thermoelectrics and hydrocarbon fuel, and a notable increase in the conversion efficiency compared with other published works. To optimize the TE heat-to-power conversion performance of the TPG, numerical simulations were performed with computational fluid dynamics (CFD) using FLUENT. The temperature dependent material properties of bismuth telluride, effects of air flow rate (6 – 14 m/s) at 300 K, and effects of thermoelectric element thickness (4 – 8 mm) on temperature gradient generated across the module are investigated under constant power input (7.5 W). The obtained results reveal that all geometric parameters have important effect on the thermal performance of thermoelectric power generation module. The optimized single TE element thickness is 7 mm for electrical power generation of 0.47 W at temperature difference of 138 K. The TE heat-to-power conversion efficiency is 6.3%. The first solid-state renewable heat source (without the use of hydrocarbons) were created with porous silicon carbide combustors coated with pyrophoric 1-3 micron-sized iron particles mixture. The thermal behavior and ignition characteristics of iron particles and mixtures were investigated. The mixture include activate carbon and sodium chloride, in which iron is the main ingredient used as fuel. The final mixture composition is determined to consist of iron powder, activate carbon, and sodium chloride with a weight ratio of approximately 5/1/1. The mixture generated two-peak DSC curves featured higher ignition temperatures of 431.53°C and 554.85°C with a higher heat generation of 9366 J/g than single iron particles. The enhancement of figure-of-merit ZT or efficiency of thermoelectric materials is dependent on reducing the thermal conductivity. This dissertation synthesized and characterized the advanced two-component Si-Ge nanocomposites with a focus on lowering the thermal conductivity. The ball-milled two-component Si-Ge material demonstrated 50% reduction in thermal conductivity than the single component material used in the radioisotope thermoelectric generators and 10% reduction than the p-type SiGe alloy

THERMAL ENERGY HARVESTING IN WIRELESS SENSOR NODES USED FOR CONDITION MONITORING

Presently, wireless sensor notes (WSN) are widely investigated and used in condition monitoring on industrial process monitoring and control, based on their inherent advantages of lower maintenance cost, easy installation and the ability to be installed in places not reached easily. However, current WSN based monitoring system still need dedicated power line or regular charging / replacing the batteries, which not only makes it difficult to deploy it in the fields but also degrades the operational reliability. This PhD research focuses on an investigation into energy harvesting approaches for powering WSN so as to develop a cost-effective, easy installation and reliable wireless measurement system for monitoring mission critical machinery such as multistage gearbox. Among various emerging energy harvest approaches such as vibrations, inductions, solar panels, thermal energy harvesting is deemed in this thesis to be the most promising one as almost all machines have frictional losses which manifest in terms of temperature changes and more convenient for integration as the heat sources can be close to wireless nodes. In the meantime, temperature based monitoring is adopted as its changes can be more sensitive to early health conditions of a machine when its tribological behaviour is starting to be degraded. Moreover, it has much less data output and more suitable for WSN application compared the mainstream vibration based monitoring techniques. Based on these two fundamental hypothesis, the research has been carried out according to two main milestones: the development of a thermoelectric harvesting (TEH) module and the evaluation of temperature based monitoring performances based on an industrial gearbox system. The first one involves the designing, fabricating and optimising the thermal EH module along with a WSN based temperature node and the second investigates the analysis methods to detect the temperature changes due to various faults associated with tribological mechanisms in the gearbox. In completing the first milestone, it has successfully developed a TEH module using cost-effective thermoelectric generator (TEG) devices and temperature gradient enhancement modules (heat sinks). Especially, the parameters such as their sizes and integration boundary conditions have been configured optimally by a proposed procedure based on the fine element (FE) analysis and the heat generation characteristics of machines to be monitored. The developed TEG analytic models and, FE models along with simulation study show that three different specifications of heat sinks with a Peltier TEG module are able to produce power that are consistently about 85% of the experimental values from offline tests, showing the good accuracy in predicting power output based on different applications and thus the reliability of the models proposed. And further investigation shows that a Peltier TEG module based that the thermal energy harvesting system produces is nearly 10 mW electricity from the monitored gearbox. This power is demonstrated sufficient to drive the WSN temperature node fabricated with low power consumption BLE microcontroller CC2650 sensor tags for monitoring continuously the temperature changes of the gearbox. Moreover, it has developed model based monitoring using multiple temperature measurements. The monitoring system allows two common faults oil shortages and mechanical misalignment to be detected and diagnosed, which demonstrates the specified performance of the self-power wireless temperature system for the purpose of condition monitoring

Assessment of novel utilisation pathways for biogas and nitrogenous waste streams at wastewater treatment plants

A combination of process modelling, numerical modelling, economic analysis and experimental techniques have been used to analyse novel utilisation pathways for biogas and nitrogenous waste streams at wastewater treatment plants. An assessment of a large wastewater treatment plant serving a population equivalent of 750,000 people was carried out including compositional analysis of various streams at the facility. This facilitated three key findings that function as the bedrock for the rest of study: the facility’s greenhouse gas footprint, its energy balance and its digestate liquor ammonia concentration. Aspen Plus process modelling software was used to develop a system that recovers ammonia in a way that prepares it for thermochemical decomposition to hydrogen and nitrogen. Sensitivity analysis showed that air stripping was energetically preferable to steam stripping as the base recovery technology. This was proceeded by an absorption step that uses a water-only solvent and finally a distillation step that was found to be energetically preferential to flash separation. The modelling showcased an ability to recover 91% of ammonia contained in the digestate liquor. Ordinarily, the wastewater treatment plant would recycle the liquor back into its conventional process. By recovering the ammonia, and diverting it away from conventional treatment, it is proposed that the plant will experience significant reductions in energy consumption and GHG emissions. Aspen Plus was used to develop a process model that combines the recovery of ammonia with the operation of an internally reforming solid oxide fuel cell stack, which uses a blend of biomethane and ammonia as its fuel. A numerical model was developed that precisely calculates its power production potential, based on a commercially available solid oxide fuel cell stack. It was found to operate at a net electrical efficiency of 48% and if implemented at the referenced wastewater treatment plant, would increase the site’s power production by 45%. It was also proposed that the site’s lifecycle GHG emissions would reduce by 7.7% due to a combination of ammonia diversion and reduced grid electricity consumption. An economic study showed that it would be financially viable to implement this technology at the site with a positive net present value facilitated after eight years of operation. A process model was developed which utilises recovered ammonia and biomethane as feedstock for a thermochemical H2 production system. Steam to carbon ratios of 2, 3 and 4 were analysed to assess their impact on H2 production, energetics and financial viability. The scenario with a steam to carbon ratio of 3 showcased the best economic potential with net present value becoming positive during its 14th year of operation. It was proposed that if the H2 produced was used as a vehicle fuel for bus transportation the process implementation would reduce the facility’s lifecycle GHG emissions by 25%. An H2-rich syngas was generated experimentally using ammonia, methane and steam feeds in a fixed-bed reactor holding a conventional Ni-Al catalyst. Ammonia, methane and carbon monoxide conversions were less than predicted via equilibrium calculations. However, the general selectivity of products closely resembled that of equilibrium equivalents – showcasing an ability to combine the steam reforming of methane and the decomposition of ammonia in a single reactor