Integrated photovoltaic-thermal solar energy conversion systems

A combined photovoltaic/thermal collector has been built and is now being tested. Initial tests have concentrated on evaluating the thermal efficiency of the collector before and after the silicon cells are mounted. With likely improvements in bonding between cells and receiver and in the absorptivity of the cells, thermal efficiencies greater than 50% can be expected for the combined receiver operating at 100 C

Evaluation of on-board hydrogen storage methods for hypersonic vehicles

Hydrogen is the foremost candidate as a fuel for use in high speed transport. Since any aircraft moving at hypersonic speeds must have a very slender body, means of decreasing the storage volume requirements below that for liquid hydrogen are needed. The total performance of the hypersonic plane needs to be considered for the evaluation of candidate fuel and storage systems. To accomplish this, a simple model for the performance of a hypersonic plane is presented. To allow for the use of different engines and fuels during different phases of flight, the total trajectory is divided into three phases: subsonic-supersonic, hypersonic and rocket propulsion phase. The fuel fraction for the first phase is found be a simple energy balance using an average thrust to drag ratio for this phase. The hypersonic flight phase is investigated in more detail by taking small altitude increments. This approach allowed the use of flight profiles other than the constant dynamic pressure flight. The effect of fuel volume on drag, structural mass and tankage mass was introduced through simplified equations involving the characteristic dimension of the plane. The propellant requirement for the last phase is found by employing the basic rocket equations. The candidate fuel systems such as the cryogenic fuel combinations and solid and liquid endothermic hydrogen generators are first screened thermodynamically with respect to their energy densities and cooling capacities and then evaluated using the above model

Tailorable, visible light emission from silicon nanocrystals

J. P. Wilcoxon and G. A. Samara Crystalline, size-selected Si nanocrystals in the size range 1.8-10 nm grown in inverse micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across the visible range of the spectrum. The most intense PL for the smallest nanocrystals produced This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. to induce a useful level of visible photoluminescence (PL) from silicon (Si). The approaches understood. Visible PL has been observed from Si nanocrystals, or quantum dots, produced by a variety of techniques including aerosols,2 colloids,3 and ion implantation.4 However, all of The optical absorption spectra of our nanocrystals are much richer in spectral features spectrum of bulk Si where the spectral features reflect the details of the band structure shown in nanocrystals estimated to have a Si core diameter of 1-2 nm. These measured quantum those in the spectrum of bulk Si in Fig. 1 are striking indicating that nanocrystals of this size 8-Room temperature PL results on an HPLC size-selected, purified 2 nm nanocrystals but blue shifted by -0.4 eV due to quantum confinement. Excitation at 245 nm yields the PL shows the PL spectrum for a similar sample excited at 490 nm (2.53 eV) trapped excitons at the surface of Si nanocrystals. The excitons are obtained for dimer bonds 1.8- 10 nm. These nanocrystals retain bulk-like optical absorption and an indirect bandgap Figure 1. The absorption spectrum of d = 2 nm Si nanocrystals compared to that of bulk7 Si. Figure 2. The extinction and PL (excitation at 490 nm) spectra ford= 8-10 nm Si nanocrystals

Random local strain effects in homovalent-substituted relaxor ferroelectrics: a first-principles study of BaTi0.74Zr0.26O3

We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a prototype material for relaxors with a homovalent substitution. From a statistical analysis of relaxed structures, we give evidence for four types of Ti-atom polar displacements: along the , , or directions of the cubic unit cell, or almost cancelled. The type of a Ti displacement is entirely determined by the Ti/Zr distribution in the adjacent unit cells. The underlying mechanism involves local strain effects that ensue from the difference in size between the Ti4+ and Zr4+ cations. These results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi(1-x)Zr(x)O3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution.Comment: 5 pages, 4 figure

Results of the post flash-flood disaster investigations in the Transylvanian Depression (Romania) during the last decade (2001–2010)

Flash-flood disasters are very rare in the Transylvanian Depression. In the last decades just three events were signalled in the study area, all of them during the last 10 years. The flash floods occurring in the study area during the last decade had a significant impact on several localities situated at the Transylvanian Depression border. Based on the post flash-flood investigation, the present study intends to find out the main characteristics of the flash floods and the causes that have led to disasters in a region rarely affected by such kinds of events. Analyzing the hydrological data, it has been seen that the maximum intensity of the flash floods was observed in the upper and middle basins. By comparing the unit peak discharges from the studied region with other specific peak discharges related to the significant flash floods from Romania, it was noticed that the events from the Transylvanian Depression have moderate to low intensity. On the other hand, the results showed that besides high stream power and unexpected character common to flash floods, the inappropriate flood risk management measures increased the dimension of the negative effects, leading to tens of lives lost and economical damages of tens of millions of dollars

Very low cost thin film CdS-Cu2S solar cell development using chemical spraying

A chemical spray process for the production of thin film CdS-Cu2S solar cells is discussed that is projected to cost less than $60/kW in very large scale production. The average efficiency of these cells has been improved from less than 0.3% in 1971 about 4.5% at present. Further developments for the process are considered to raise the efficiency, and to attain long life stability

Poling effect on distribution of quenched random fields in a uniaxial relaxor ferroelectric

The frequency dependence of the dielectric permitivity's maximum has been studied for poled and unpoled doped relaxor strontium barium niobate $Sr_{0.61}Ba_{0.39}Nb_{2}O_{6}:Cr^{3+}$ (SBN-61:Cr). In both cases the maximum found is broad and the frequency dispersion is strong. The present view of random fields compensation in the unpoled sample is not suitable for explaining this experimental result. We propose a new mechanism where the dispersion of quenched random electric fields, affecting the nanodomains, is minimized after poling. We test our proposal by numerical simulations on a random field Ising model. Results obtained are in agreement with the polarization's measurements presented by Granzow et al. [Phys. Rev. Lett {\bf 92}, 065701 (2004)].Comment: 7 pages, 4 figure

A combined theoretical and experimental study of the low temperature properties of BaZrO3

Low temperature properties of BaZrO3 are revealed by combining experimental techniques (X-ray diffraction, neutron scattering and dielectric measurements) with theoretical first-principles-based methods (total energy and linear response calculations within density functional theory, and effective Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations). Unlike most of the perovskite systems, BaZrO3 does not undergo any (long-range-order) structural phase transition and thus remains cubic and paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On the other hand, these latter pure quantum effects lead to a negligible thermal dependency of the cubic lattice parameter below ~ 40 K. They also affect the dielectricity of BaZrO3 by inducing an overall saturation of the real part of the dielectric response, for temperatures below ~ 40 K. Two fine structures in the real part, as well as in the imaginary part, of dielectric response are further observed around 50-65 K and 15 K, respectively. Microscopic origins (e.g., unavoidable defects and oxygen octahedra rotation occurring at a local scale) of such anomalies are suggested. Finally, possible reasons for the facts that some of these dielectric anomalies have not been previously reported in the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also discussed.Comment: 8 pages, 5 figures, submitted to Physical Review