Solar-Combined Thermoelectric Power Generation Simulator

Photovoltaic (PV) devices are gaining popularity in harnessing solar energy as a form of sustainable energy source to generate electricity. However, these devices including tandem PV cells are limited to utilizing only high energy photons from the solar spectrum. This curtails their efficiency restricting them from being employed in mega Watts scale power generation. This study develops a software tool that allows engineers to tap into the wasted wavelengths of the spectrum by adding a thermoelectric (TE) module and a bottoming steam turbine cycle thus spreading the use of the spectrum. The tool allows investigating how power output and thus overall efficiency can be enhanced by combining these systems. In the TE device, solar heat develops a temperature gradient to generate electricity via the Seebeck effect. A steam-driven Rankine cycle through a heat exchanger connects to thermal storage at the bottom side of the TE. This storage allows dispatchability for off-sunlight power demand at a modest cost. The simulation tool built computes expected power output and efficiency at each individual stage of the combined system. The user is at liberty to manipulate material properties such as the band gap of PV materials which is a key parameter to optimize the PV efficiency. Test runs indicate that overall efficiency of power generation has increased up to 50% by the combined system for 1000 suns using optimized band gap and TE module design. This system can be used as a basis for future models in high efficiency distributed energy production

Excluded Volume Effect on the Power Factor of Carbon Nanotube based Polymer Composites

Investigations into polymeric materials as flexible thermoelectric (TE) materials have encountered issues, such as conflicting thermoelectric property behaviors that result in a low power factor. To tackle these issues, we propose the use of two unique sorts of fillers - carbon nanotubes (CNT) and silica particles -- embedded in polymer matrix for enhanced TE properties. Embedding micro-scale segregated structures as the secondary fillers creates an excluded volume within CNT networks, which leads to the simultaneous increase in the electrical conductivity and Seebeck coefficient of the composite. Polydimethylsiloxane (PDMS) is used as a suitable matrix because of its merits such as solution processability, light weight, low thermal conductivity and an internet of things. As silica content increased up to 40 wt%, electrical conductivity and Seebeck coefficient increases in the segregated composite framework, resulting in maximum power factor of approximately 25.96 and 42.89 microW/mK2 for 1 and 3 micrometer size silica particles, respectively. Moreover, using much lower CNT content, such as 10 wt% CNT stacking, results at a desired level of electrical conductivity and Seebeck coefficient. This study ultimately develops the hypothesis that the network topology of CNT-based polymer composites depends on the size and characteristics of the secondary fillers

Enhanced Thermoelectric Properties in Bulk Nanowire Heterostructure-Based Nanocomposites through Minority Carrier Blocking

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride–silver telluride (PbTe–Ag_2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe–Ag_2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag_2Te nanowire-based nanocomposite produced in similar method, the PbTe–Ag_2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe–Ag_2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting

Composition Modulation of Ag_2Te Nanowires for Tunable Electrical and Thermal Properties

In this article, we demonstrated that composition modulation of Ag_2Te nanowires can be achieved during the self-templated transformation of Te nanowires into Ag_2Te nanowires during solution phase synthesis, which provides a mean to tune the carrier density of the Ag_2Te nanowires. Both nearly stoichiometric and Ag-rich nanowires have been synthesized, which give rise to p-type and n-type Ag_2Te nanocomposites after hot press, respectively. The electrical and thermal properties of the two kinds of samples have been measured. Theoretical modeling based on the near-equilibrium Boltzmann transport equations has been used to understand the experimental results. We found that ZT of the heavily doped n-type sample reaches 0.55 at 400 K, which is the highest ZT value reported for Ag_2Te at the same temperature mainly due to the reduced thermal conductivity by the nanostructures. Theoretical analysis on the carrier transport shows that the power factor is also very well optimized in the doped Ag_2Te sample considering the reduced carrier mobility by the nanostructures

High temperature thermoreflectance imaging and transient Harman characterization of thermoelectric energy conversion devices

Advances in thin film growth technology have enabled the selective engineering of material properties to improve the thermoelectric figure of merit and thus the efficiency of energy conversion devices. Precise characterization at the operational temperature of novel thermoelectric materials is crucial to evaluate their performance and optimize their behavior. However, measurements on thin film devices are subject to complications from the growth substrate, non-ideal contacts, and other thermal and electrical parasitic effects. In this manuscript, we determine the cross-plane thermoelectric material properties in a single measurement of a 25 mu m InGaAs thin film with embedded ErAs (0.2%) nanoparticles using the bipolar transient Harman method in conjunction with thermoreflectance thermal imaging at temperatures up to 550K. This approach eliminates discrepancies and potential device degradation from the multiple measurements necessary to obtain individual material parameters. In addition, we present a strategy for optimizing device geometry to mitigate the effect of both electrical and thermal parasitics during the measurement. Finite element method simulations are utilized to analyze non-uniform current and temperature distributions over the device area as well as the three dimensional current path for accurate extraction of material properties from the thermal images. Results are compared with independent in-plane and 3 omega measurements of thermoelectric material properties for the same material composition and are found to match reasonably well; the obtained figure of merit matches within 15% at room and elevated temperatures. (C) 2014 AIP Publishing LLC

Ion Write Microthermotics: Programing Thermal Metamaterials at the Microscale.

Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10-100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale

Right sizes of nano- and microstructures for high-performance and rigid bulk thermoelectrics

In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to similar to 2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2 similar to 6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to similar to 0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano-and microstructures of the material

Pathogen-induced binding of the soybean zinc finger homeodomain proteins GmZF-HD1 and GmZF-HD2 to two repeats of ATTA homeodomain binding site in the calmodulin isoform 4 (GmCaM4) promoter

Calmodulin (CaM) is involved in defense responses in plants. In soybean (Glycine max), transcription of calmodulin isoform 4 (GmCaM4) is rapidly induced within 30 min after pathogen stimulation, but regulation of the GmCaM4 gene in response to pathogen is poorly understood. Here, we used the yeast one-hybrid system to isolate two cDNA clones encoding proteins that bind to a 30-nt A/T-rich sequence in the GmCaM4 promoter, a region that contains two repeats of a conserved homeodomain binding site, ATTA. The two proteins, GmZF-HD1 and GmZF-HD2, belong to the zinc finger homeodomain (ZF-HD) transcription factor family. Domain deletion analysis showed that a homeodomain motif can bind to the 30-nt GmCaM4 promoter sequence, whereas the two zinc finger domains cannot. Critically, the formation of super-shifted complexes by an anti-GmZF-HD1 antibody incubated with nuclear extracts from pathogen-treated cells suggests that the interaction between GmZF-HD1 and two homeodomain binding site repeats is regulated by pathogen stimulation. Finally, a transient expression assay with Arabidopsis protoplasts confirmed that GmZF-HD1 can activate the expression of GmCaM4 by specifically interacting with the two repeats. These results suggest that the GmZF-HD1 and –2 proteins function as ZF-HD transcription factors to activate GmCaM4 gene expression in response to pathogen