Thermal performance of two heat exchangers for thermoelectric generators

Thermal performance of heat exchanger is important for potential application in integrated solar cell/module and thermoelectric generator (TEG) system. Usually, thermal performance of a heat exchanger for TEGs is analysed by using a 1D heat conduction theory which ignores the detailed phenomena associated with thermo-hydraulics. In this paper, thermal and mass transports in two different exchangers are simulated by means of a steady-state, 3D turbulent flow k -e model with a heat conduction module under various flow rates. In order to simulate an actual working situation of the heat exchangers, hot block with an electric heater is included in the model. TEG model is simplified by using a 1D heat conduction theory, so its thermal performance is equivalent to a real TEG. Natural convection effect on the outside surfaces of the computational model is considered. Computational models and methods used are validated under transient thermal and electrical experimental conditions of a TEG. It is turned out that the two heat exchangers designed have a better thermal performance compared with an existing heat exchanger for TEGs, and more importantly, the fin heat exchanger is more compact and has nearly half temperature rise compared with the tube heat exchanger

Enhancement of thermoelectric properties of CuFeS2 through formation of spinel-type microprecipitates

CuFeS2 (chalcopyrite) is a promising n-type thermoelectric candidate for low-grade waste heat recovery. In this work, chromium-containing CuFeS2 materials of general formula Cu1 xCrxFeS2 (0.0 ≤ x ≤ 0.1) were prepared via solid-state synthesis. Efforts to substitute chromium in CuFeS2 leads to the preferential formation of a composite, in which lamellar precipitates of a Cr-rich, spinel-type [Cu,Fe,Cr]3S4 phase, are embedded in the unsubstituted CuFeS2 matrix. X-ray absorption near-edge spectroscopy (XANES) reveals that the electronic structure of copper, iron and sulfur in the principal CuFeS2 phase remains unaltered by chromium incorporation. However, the formation of [Cu,Fe,Cr]3S4 precipitates alters the Cu:Fe ratio of the CuFeS2 phase, producing a change in the net carrier concentration through reduction of a portion of Fe3+ ions to Fe2+. The chromium content of the spinel precipitates determines the extent of the change in the Cu:Fe ratio of the main CuFeS2 phase, and hence, indirectly affects the electrical properties. The micro/nanometre-sized [Cu,Fe,Cr]3S4 precipitates and nanoscale dislocations enable a broad spectrum of heat-carrying acoustic phonons to be scattered, resulting in a significantly reduced lattice thermal conductivity. Combined with an enhanced power factor, a maximum thermoelectric figure-of-merit, zT of 0.31 at 673 K is achieved for the x = 0.08 sample; a three-fold increase over that of the pristine phase

Prospects for Engineering Thermoelectric Properties in La1/3NbO3 Ceramics Revealed via Atomic-Level Characterization and Modeling

A combination of experimental and computational techniques has been employed to explore the crystal structure and thermoelectric properties of A-site-deficient perovskite La1/3NbO3 ceramics. Crystallographic data from X-ray and electron diffraction confirmed that the room temperature structure is orthorhombic with Cmmm as a space group. Atomically resolved imaging and analysis showed that there are two distinct A sites: one is occupied with La and vacancies, and the second site is fully unoccupied. The diffuse superstructure reflections observed through diffraction techniques are shown to originate from La vacancy ordering. La1/3NbO3 ceramics sintered in air showed promising high-temperature thermoelectric properties with a high Seebeck coefficient of S1 = −650 to −700 μV/K and a low and temperature-stable thermal conductivity of k = 2–2.2 W/m·K in the temperature range of 300–1000 K. First-principles electronic structure calculations are used to link the temperature dependence of the Seebeck coefficient measured experimentally to the evolution of the density of states with temperature and indicate possible avenues for further optimization through electron doping and control of the A-site occupancies. Moreover, lattice thermal conductivity calculations give insights into the dependence of the thermal conductivity on specific crystallographic directions of the material, which could be exploited via nanostructuring to create high-efficiency compound thermoelectrics

The Structure and Thermoelectric Properties of Tungsten Bronze Ba6Ti2Nb8O30

Tungsten bronze structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN) was prepared by the conventional mixed oxide (MO) route with some samples processed by Spark Plasma Sintering (SPS). Additions of MnO enabled fabrication of high density BTN ceramics at the low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallises with tetragonal symmetry (P4bm space group). HAADF-EELS analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimisation of the processing parameters the ceramics achieved a high power factor of 280 μW/m∙K2 at 873 K. The BTN ceramics showed phonon glass type thermal conduction behaviour with low thermal conductivity of 1.7 to 1.65 W/m.K at 300 to 873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films : Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures

Donor-doped TiO 2-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO 2 thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10 -4 W m -1 K -2 at 673 K, the highest value for TiO 2-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials

Concurrent La and A-site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3. Experimental and Computational Evidence

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1–xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration

A novel absorptive/reflective solar concentrator for heat and electricity generation: an optical and thermal analysis.

The crossed compound parabolic concentrator (CCPC) is one of the most efficient non-imaging solar concentrators used as a stationary solar concentrator or as a second stage solar concentrator. In this study, the CCPC is modified to demonstrate for the first time a new generation of solar concentrators working simultaneously as an electricity generator and thermal collector. The CCPC is designed to have two complementary surfaces, one reflective and one absorptive, and is named as an absorptive/reflective CCPC (AR-CCPC). Usually, the height of the CCPC is truncated with a minor sacrifice of the geometric concentration. These truncated surfaces rather than being eliminated are instead replaced with absorbent surfaces to collect heat from solar radiation. The optical efficiency including absorptive/reflective part of the AR-CCPC was simulated and compared for different geometric concentration ratios varying from 3.6× to 4×. It was found that the combined optical efficiency of the AR-CCPC 3.6×/4× remained constant and high all day long and that it had the highest total optical efficiency compared to other concentrators. In addition, the temperature distributions of AR-CCPC surfaces and the assembled solar cell were simulated based on those heat flux boundary conditions. It was shown that the addition of a thermal absorbent surface can increase the wall temperature. The maximum value reached 321.5 K at the front wall under 50° incidence. The experimental verification was also adopted to show the benefits of using absorbent surfaces. The initial results are very promising and significant for the enhancement of solar concentrator systems with lower concentrations