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Thermoelectric technology has key benefits and strengths in many terrestrial energy recovery applications. Thermoelectric system cost is a key factor governing final decisions on the use of thermoelectric energy recovery systems in all terrestrial applications; thus cost being just as important as power density or efficiency for the adoption of waste energy recovery (WER) thermoelectric generators (TEG). New integrated cost analysis / thermoelectric analysis approaches have now shown key relationships and interdependencies between overall TEG system costs, including TE material costs, manufacturing costs, and specifically heat exchanger costs; and the TE performance design metrics such as TE material properties, TE device design parameters, heat exchanger performance metrics such as hot-side and cold-side conductances and UA values, and hot side heat flux in achieving optimal TEG WER designs. These new approaches have led to a new thermoelectric system economics paradigm that strongly influences TEG cost and performance decisions. While prior work provided foundations for the latest cost scaling analysis / TE performance analysis, this new work offers new insights and understandings and provides the basis for new thermoelectric system economics. Optimum TEG system cost conditions can now be tied directly to the TE materials, TEG design parameters, and heat exchanger design parameters through critical non-dimensional analysis. The non-dimensional analysis and metrics show the TEG system cost and performance interdependencies and intercouplings in one unifying and cohesive relationship. Prior work by T.J. Hendricks, S.K. Yee, and S. LeBlanc, J of Electronic Mater, 45, (3), 1751-1761, 2015 has shown that the system design that minimizes cost (e.g., the G [$/W] value) can be close to designs that maximize power, but these design regimes are not necessarily aligned with high system conversion efficiency or high specific power. Key sensitivities and interrelationships between critical cost metrics and critical TE performance and design metrics in the new thermoelectric system economics paradigm are explored. Quantitative data showing these sensitivities and their implications on TEG system design in terrestrial WER applications are presented. Critical non-dimensional parameter mapping has shown where heat exchanger cost- dominated conditions, TE material or manufacturing cost-dominated conditions, and combinations of cost conditions control and drive the overall TEG cost and performance. This new cost-performance paradigm shows the required pathways and challenges to achieving TEG system costs of $1 -$3/Welec.
Thermoelectric energy recovery is an important technology for recovering waste thermal energy in high-temperature industrial, transportation and military energy systems. Thermoelectric (TE) power systems in these applications require high performance hot-side and cold-side heat exchangers to provide the critical temperature differential and transfer the required thermal energy to create the power output. Hot-side and cold-side heat exchanger performance is typically characterized by hot-side and cold-side thermal resistances, Rh,th and Rc,th, respectively. Heat exchanger performance determines the hot-side temperature, Th, and cold-side temperature, Tc, conditions when operating in energy recovery environments with available temperature differentials characterized by exhaust temperatures, Texh, and ambient temperature, Tamb. This work analytically defined a crucially important design relationship between (P/Pmax) and (Rh,th / Rc,th) in TE power generation systems to determine the optimum ratio of (Rh,th / Rc,th) maximizing TE system power. A sophisticated integrated TE device / heat exchanger analysis was used, which simultaneously integrates hot- and cold-side heat exchanger models with TE device optimization models incorporating temperature-dependent TE material properties for p-type and n-type materials, thermal and electrical contact resistances, and hot side and cold side heat loss factors. This work examined the (P/Pmax) - (Rh,th / Rc,th) relationship for system designs employing single-material and segmented-material TE couple legs with various TE material combinations, including bismuth telluride alloys, skutterudite compounds, and skutterudite / bismuth telluride segmented combinations. This work defined the non-dimensional functional relationships and found the optimum thermal resistance condition: (Rh,th / Rc,th)opt > 10 to 30 created the maximum power output in TE optimized designs for various TE material combinations investigated. The non-dimensional relationships were investigated for various electrical contact resistances, differing thermal loss factors, and at various hot-side/cold-side temperature conditions. This work showed that the non-dimensional functional relationships were invariant under these differing conditions. It was determined that a condition of (Rh,th / Rc,th) = 1 creates power output far below maximum power conditions. The (P/Pmax) - (Rh,th / Rc,th) relationship also dictated certain temperature profile conditions, defined by the parameter, (Th – Tc) / (Texh – Tamb), which were directly associated with design points in this relationship including maximum power points. The value of (Th – Tc) / (Texh – Tamb) was generally less than 0.5 at maximum power conditions in TE energy recovery designs using TE materials investigated here. The wide-ranging ramifications on TE energy recovery systems and their design optimization for industrial and transportation-related applications are discussed.
The temperature-dependent thermoelectric (TE) and structural properties of n-type filled skutterudites were measured from 300–625 K. In0.2Co4Sb12, and In0.2Ce0.05Yb0.1Co4Sb12 exhibited figure of merit (ZT) values as high as 1.2 at 625 K and In0.2Ce0.15Co4Sb12 showed ZT values of ∼1.4 at 625 K. The room temperature Young’s modulus, Poisson’s ratio, and coefficient of thermal expansion (at 298–673 K) of In0.2Ce0.15Co4Sb12, In0.2Co4Sb12, and In0.2Ce0.05Yb0.1Co4Sb12 compositions were found to be lower than that for the unfilled Co4Sb12 skutterudite material. It was discovered that thermal cycling of n-type In0.15Ce0.1Co4Sb12 and In0.2Ce0.17Co4Sb12 materials from 323–673 K (200 cycles) actually increased their power factors by 13.6–36% at 510–525 K without appreciably changing the Young’s modulus or the Poisson’s ratio. The transport and structural properties characterized in this work are critical to transitioning these materials into operating TE devices and systems.
The U.S. military uses large amounts of fuel during deployments and battlefield operations. Consequently, the U.S. military has a strong need to develop technologies that increase fuel efficiency and minimize fuel requirements all along the logistics trail and in all battlefield operations. There are additional requirements to reduce and minimize the environmental footprint of various military equipment and operations and reduce the need for batteries (non-rechargeable) in battlefield operations. The tri-agency SERDP (Strategic Environmental Research and Development Program) office is sponsoring a challenging, high-payoff project to develop a lightweight, small form-factor, soldier-portable advanced thermoelectric generator (TEG) system prototype to recover and convert waste heat from a variety of deployed equipment with the ultimate purpose of obtaining additional power for soldier battery charging, advanced capacitor charging, and other battlefield power applications. The project seeks to achieve power conversion efficiencies of 10% (double current commercial TE conversion efficiencies) in a system with ˜1.6-kW power output for a spectrum of battlefield power applications. In order to meet this objective, the project is taking on the multi-faceted challenges of tailoring LAST/LASTT-based thermoelectric (TE) materials for the proper temperature ranges (300 K – 700 K), fabricating these materials with cost-effective hot-pressed and sintered processes while maintaining their TE properties, measuring and characterizing their thermal fatigue and structural properties, developing the proper manufacturing processes for the TE materials and modules, designing and fabricating the necessary microtechnology heat exchangers, and fabricating and testing the final TEG system. The ultimate goal is to provide an opportunity to deploy these TEG systems in a wide variety of current military equipment. This would help the Army in achieving one of the Office of Secretary of Defense’s major strategic objectives to maintain and enhance operational effectiveness while reducing total force energy demands. The presentation will review the progress made on 1) the performance of LAST / LASTT TE materials and tailoring their temperature dependency; 2) evaluating the structural (Elastic modulus, Poisson’s ratio and mechanical strength) properties of these materials, 3) development of the necessary LAST/LASTT-based TE modules, 4) development of the required hot- and cold-side microtechnology heat exchangers, and 5) the overall system designs for 30 kW and 60 kW TQG applications and potential performance pathways/differences for these two TQG cases. This work leverages critical fundamental research performed by the Office of Naval Research in developing LAST/LASTT materials.
Advanced, direct thermal energy conversion technologies are receiving increased research attention in order to recover waste thermal energy in advanced vehicles and industrial processes. Advanced thermoelectric (TE) systems necessarily require integrated system-level analyses to establish accurate optimum system designs. Past system-level design and analysis has relied on well-defined deterministic input parameters even though many critically important environmental and system design parameters in the above mentioned applications are often randomly variable, sometimes according to complex relationships, rather than discrete, well-known deterministic variables. This work describes new research and development creating techniques and capabilities for probabilistic design and analysis of advanced TE power generation systems to quantify the effects of randomly uncertain design inputs in determining more robust optimum TE system designs and expected outputs. Selected case studies involving stochastic TE .material properties demonstrate key stochastic material impacts on power, optimum TE area, specific power, and power flux in the TE design optimization process. Magnitudes and directions of these design modifications are quantified for selected TE system design analysis cases.
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