Ofer Beeri

Hybrid photovoltaic-thermoelectric system for concentrated solar energy conversion: Experimental realization and modeling

An experimental demonstration of the combined photovoltaic (PV) and thermoelectric conversion of concentrated sunlight (with concentration factor, X, up to ∼300) into electricity is presented. The hybrid system is based on a multi-junction PV cell and a thermoelectric generator (TEG). The latter increases the electric power of the system and dissipates some of the excessive heat. For X ≤ 200, the systems maximal efficiency, ∼32%, was mostly due to the contribution from the PV cell. With increasing X and system temperature, the PV cells efficiency decreased while that of the TEG increased. Accordingly, the direct electrical contribution of the TEG started to dominate in the total system power, reaching ∼20% at X ≈ 290. Using a simple steady state finite element modeling, the cooling effect of the TEG on the hybrid systems efficiency was proved to be even more significant than its direct electrical contribution for high solar concentrations. As a result, the total efficiency contribution of the TEG reach...

High-pressure studies of the TiCr1.8–H2 system Statistical thermodynamics above the critical temperature

Abstract Pressure–composition isotherms of the TiCr 1.8 –H 2 system were measured within the temperature range 298–433°K and over a wide pressure range up to 1000 atm H 2 . The above temperature range is well above the critical temperature, T C , of the system. Hence, partial molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H/M atomic composition ratio (with M=Ti+Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg–Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H–H interaction parameter, η , was evaluated for each isotherm. For both approximations a similar temperature dependence of η ( T ) was obtained, with η changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T C values (derived from the η ( T ) parameters) and the experimental observations.

Morphological effects on the thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1+δSn alloys following phase separation

Thermoelectrics are known as one of the emerging renewable power generation technologies. Half-Heusler based semiconducting intermetallic compounds show high potential as thermoelectric materials due to the abundance of their elements in nature, their high mechanical and chemical stability, and their favorable electronic properties. Their main limitation lies in their high lattice thermal conductivity, κl. In the current research, the potential of κl reduction due to generation of composites based on phase separation of off-stoichiometric Ti0.3Zr0.35Hf0.35Ni1+δSn alloys into half- and full-Heusler composites was investigated. Due to the strong metallic nature of the full-Heusler phase, its electronic effect on the thermoelectric transport properties under various morphological and compositional conditions was analyzed by the general effective media (GEM) approach. It was shown that although a major κl reduction of up to ∼37% was associated with phonon scattering by the embedded full Heusler phase, the electronic properties for the parallel-like alignment of this phase, following arc melting and spark plasma sintering, deteriorated with increase of the relative amount of this phase. Therefore a maximal ZT enhancement of ∼41%, compared to the uni-phase HH stoichiometric composition, was obtained for the minimal (δ = 0.01) deviation of the stoichiometry examined, corresponding to the minimal relative amount of the scattered FH phase inside the HH matrix.

Sites occupation and thermodynamic properties of the TiCr2-xMnx-H2 (0≤x≤1) system: statistical thermodynamics analysis

Pressure–composition isotherms of the ternary (pseudobinary) Laves phases TiCr2−xMnx–H2(D2) (0≤x≤1) systems were measured over a wide range of temperatures and pressures (up to 1000 atm H2). The lower composition hydride phase, TiB2H∼3 (where B=Cr+Mn), is formed for all these compound series (i.e. for 0≤x≤1) with a similar stability (i.e. independent of x). On the other hand, the high-composition hydride phase, TiB2H∼4, is apparent only for lower concentrations of manganese substitution (x<0.75). A generalized statistical thermodynamics treatment, which considers the possible simultaneous occupation of two interstitial sites (namely the Ti2B2 and the TiB3 tetrahedral sites) was applied, leading to calculated isotherms with two plateau regions. The fit of the calculated isotherms to the experimental ones yielded the microscopic energy-related parameters utilized in the statistical thermodynamics models (i.e. the H-lattice and the H–H nearest neighbors interaction parameters) as well as the two sites occupation fraction and its hydrogen composition variation. It turned out that an almost step-like sequential occupation of the above two sites is indicated by the above analysis, consistent with experimental structural data in the literature. A correlation between the interstitial sites volume and the pairwise H–H interaction parameter was found, with an opposite trend displayed for the two types of sites. The disappearance of the higher composition hydride phase at higher manganese composition is accounted for by the above correlations.

Thermodynamic characterization and statistical thermodynamics of the TiCrMn–H2(D2) system

Pressure–composition (p–c) isotherms of the TiCrMn–H2(D2) system were measured over a wide temperature and pressure range (up to 1000 atm H2/D2) which included the super-critical range (above TC). The macroscopic thermodynamic parameters of the system and their isotope effects were evaluated. Also, hysteresis phenomenon between absorption and desorption isotherms were obtained as a function of temperature for the two hydrogen isotopes. The experimental p–c isotherms were compared to model calculated ones derived by simplified statistical thermodynamic treatments. From this comparison microscopic energy-related parameters were evaluated. These parameters included the average hydrogen–lattice interaction, the pairwise nearest-neighbor hydrogen–hydrogen interaction and the average vibrational frequency of H(D) at the interstitial sites.

High-pressure studies of Laves phase intermetallic hydrides–Adaptation of statistical thermodynamic models

Abstract Pressure–composition isotherms of unstable intermetallic hydrides of some Laves phases (TiCr ∼2 , TiCrMn) were measured over a wide pressure range up to 1000 atm H 2 . These measurements enabled the evaluation of the critical temperatures, T c , of the respective systems as well as the derivation of their thermodynamic characteristics above T c . For this one-phase high-temperature range, simplified statistical-thermodynamics models can be adapted to calculate analytical forms of the corresponding isotherms. A comparison between the model-derived and the experimental isotherms then yields the average pairwise nearest neighbor H–H interaction parameter, η , and its temperature dependence. In the present study, a rigid-metal sublattice model was utilized and solved employing the conventional Bragg–Williams (BW) and Quasi-Chemical (QC) approximations. In fact, both approximations resulted in similar η ( T ) values, as well as close estimates of T c . For the TiCr ∼2 –H 2 system the above analysis indicated that η changes from attractive (i.e., negative) to repulsive (i.e., positive) with increasing isotherm temperatures. This trend was qualitatively interpreted as resulting from the net interplay of two energy terms, the elastic strain contribution, which induces an effective attractive interaction, and the electrostatic contribution which adds a repulsive term. For the TiCrMn–H 2 system, it turned out that the partial substitution of chromium by manganese had only a minor effect on the stability of the hydride, however, it pronouncedly increased the critical temperature ( T c ) of the system. This observation can be accounted for by the simultaneous electronic and structural effects of manganese in this compound.

Interstitial sites in the hydrogen-inactive intermetallic compounds ZrPd2 and UPd3

Abstract The possible interstitial sites for hydrogen occupation in the intermetallic lattices of ZrPd 2 (MoSi 2 tetragonal structure type) and UPd 3 (Ni 3 Ti hexagonal structure type) were identified by their Wyckoff notation and characterized by the metal atoms on the polyhedron vertices. Interstitial hole sizes and intersite distances were also calculated utilizing lattice constants and estimated atomic metal radii. It is concluded that hydrogen absorption is not restricted by geometrical factors imposed by empirical and theoretical criteria for the minimum hole size and intersite distance. The hydrogen inertness of the ZrPd 2 and UPd 3 compounds is discussed from the point of view of their large stabilities and the recently found correlations between bonding strengths and hydrogen absorption.