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At least 145 records · Page 8

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NASA NTRS2014

Development and Experimental Evaluation of Passive Fuel Cell Thermal Control

To provide uniform cooling for a fuel cell stack, a cooling plate concept was evaluated. This concept utilized thin cooling plates to extract heat from the interior of a fuel cell stack and move this heat to a cooling manifold where it can be transferred to an external cooling fluid. The advantages of this cooling approach include a reduced number of ancillary components and the ability to directly utilize an external cooling fluid loop for cooling the fuel cell stack. A number of different types of cooling plates and manifolds were developed. The cooling plates consisted of two main types; a plate based on thermopyrolytic graphite (TPG) and a planar (or flat plate) heat pipe. The plates, along with solid metal control samples, were tested for both thermal and electrical conductivity. To transfer heat from the cooling plates to the cooling fluid, a number of manifold designs utilizing various materials were devised, constructed, and tested. A key aspect of the manifold was that it had to be electrically nonconductive so it would not short out the fuel cell stack during operation. Different manifold and cooling plate configurations were tested in a vacuum chamber to minimize convective heat losses. Cooling plates were placed in the grooves within the manifolds and heated with surface-mounted electric pad heaters. The plate temperature and its thermal distribution were recorded for all tested combinations of manifold cooling flow rates and heater power loads. This testing simulated the performance of the cooling plates and manifold within an operational fuel cell stack. Different types of control valves and control schemes were tested and evaluated based on their ability to maintain a constant temperature of the cooling plates. The control valves regulated the cooling fluid flow through the manifold, thereby controlling the heat flow to the cooling fluid. Through this work, a cooling plate and manifold system was developed that could maintain the cooling plates within a minimal temperature band with negligible thermal gradients over power profiles that would be experienced within an operating fuel cell stack.

Fuel Cells↗

NASA NTRS1984

Multi-100kW: Planar low cost solar array development

Seven low cost multi-100 kW planar solar array modules were fabricated and tested. Two different designs were used, demonstrating advanced solar array construction practices. Both module types utilized second generation gridded back cells featuring high efficiency and IR transparency. A silicon dioxide AR coating optimized for transmission at gamma = 1.7 microns was applied to the back surface. Two interconnect types, a single sheet printed circuit and a roll type, with alternate approaches to increasing transparency and reducing cost were designed and fabricated. Hinge stress and electrical power optimization were also examined. Two point designs were studied. The first design used a coilage longeron mast and is autonomously deployable. The second design used a Stac Beam for high natural frequency response and required astronaut assistance and assembly on orbit. It was conclusively demonstrated that planar arrays are the most cost effective design for use on the space station or other high power applications.

Source record↗

NASA NTRS1990

Wide-bandgap epitaxial heterojunction windows for silicon solar cells

It is shown that the efficiency of a solar cell can be improved if minority carriers are confined by use of a wide-bandgap heterojunction window. For silicon (lattice constant a = 5.43 A), nearly lattice-matched wide-bandgap materials are ZnS (a = 5.41 A) and GaP (a = 5.45 A). Isotype n-n heterojuntions of both ZnS/Si and GaP/Si were grown on silicon n-p homojunction solar cells. Successful deposition processes used were metalorganic chemical vapor deposition (MO-CVD) for GaP and ZnS, and vacuum evaporation of ZnS. Planar (100) and (111) and texture-etched - (111)-faceted - surfaces were used. A decrease in minority-carrier surface recombination compared to a bare surface was seen from increased short-wavelength spectral response, increased open-circuit voltage, and reduced dark saturation current, with no degradation of the minority carrier diffusion length.

Landis, Geoffrey A.↗

NASA NTRS2009

Gamma ray detector modules

A radiation detector assembly has a semiconductor detector array substrate of CdZnTe or CdTe, having a plurality of detector cell pads on a first surface thereof, the pads having a contact metallization and a solder barrier metallization. An interposer card has planar dimensions no larger than planar dimensions of the semiconductor detector array substrate, a plurality of interconnect pads on a first surface thereof, at least one readout semiconductor chip and at least one connector on a second surface thereof, each having planar dimensions no larger than the planar dimensions of the interposer card. Solder columns extend from contacts on the interposer first surface to the plurality of pads on the semiconductor detector array substrate first surface, the solder columns having at least one solder having a melting point or liquidus less than 120 degrees C. An encapsulant is disposed between the interposer circuit card first surface and the semiconductor detector array substrate first surface, encapsulating the solder columns, the encapsulant curing at a temperature no greater than 120 degrees C.

Capote, M. Albert↗

NASA NTRS2002

Probability of cell hits in selected organs and tissues by high-LET particles at the ISS orbit

The fluence of high-LET particles (HLP) with LET infinity H2O greater than 15 keV micrometers-1 in selected organs and tissues were measured with plastic nuclear track detectors using a life-size human phantom on the 9th Shuttle-Mir Mission (STS-91). The planar-track fluence of HLP during the 9.8-day mission ranged from 1.9 x 10(3) n cm-2 (bladder) to 5.1 x 10(3) n cm-2 (brain) by a factor of 2.7. Based on these data, a probability of HLP hits to a matured cell of each organ or tissue was roughly estimated for a 90-day ISS mission. In the calculation, all cells were assumed to be spheres with a geometric cross-sectional area of 500 micrometers2 and the cell-hit frequency from isotropic space radiation can be described by the Poisson-distribution function. As results, the probability of one or more than 1 hit to a single cell by HLP for 90 days ranged from 17% to 38%; that of two or more than 2 hits was estimated to be 1.3-8.2%. c2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

NASA Discipline Radiation Health↗

NASA NTRS1986

Exact solution of three-dimensional transport problems using one-dimensional models

Several parameters of certain three-dimensional semiconductor devices including diodes, transistors, and solar cells can be determined without solving the actual boundary-value problem. The recombination current, transit time, and open-circuit voltage of planar diodes are emphasized here. The resulting analytical expressions enable determination of the surface recombination velocity of shallow planar diodes. The method involves introducing corresponding one-dimensional models having the same values of these parameters.

Misiakos, K.↗

NASA NTRS1982

Solar array switching power management

Solar array power switching concepts are explored for a 250 kWe manned LEO platform, a 50-250 kWe load for an orbit transfer vehicle (OTV), and an unmanned platform with a 50 kWe load in GEO. A solar array switching power management (SASPM) system is under study to satisfy the switching demands. Direct connections to arrays would be implemented for voltage regulations, power distribution, and the capability of reconfiguring the arrays to meet requirements. Mission characteristics that would require the power sources were explored. The LEO platform was projected to use a concentrator, have no reconfigurability, use 250 NiH2 batteries, supply 80-0 Vdc to an ion drive, and have a 20-30 yr life. Both GEO and OTV arrays were planar, would feature reconfigurability, and supply 800 Vdc to an ion drive. NiH2 batteries would be on the OTV, while the GEO spacecraft would use AgH2 cells. A block diagram of the basic switching configuration is presented.

Cassinelli, J. E.↗

NASA NTRS1982

Multi-100 kW: Planar low cost solar array development

The applicability of selected low cost options to solar array blanket design was studied by fabricating representative modules and submitting them to thermal cycle environment. Large area (5.9 x 5.9 cm) solar cells of 3 varieties were purchased: (1) Standard wraparound, (2) Copper contacts substituted for the conventional Titanium-Palladium-Silver, and (3) Standard wraparound except with gridded back contact instead of continuous metallization. The baseline cell was purchased to compare fabrication cost and to serve as a control cell during test evaluation of the other two cells. All cells were assembled into either substrate modules where the cell is individually filtered and welded to an integrated Kapton-copper circuit or into a superstrate configuration with 4 cells jointly adhered to a single sheet of microsheet and then welded to the integrated Kapton-copper circuit. Cell quality, particularly in the metallization of contacts, was less than desired. Problems were encountered with copper metallization in laying down a barrier metal which would ohmically bond to the silicon. The cells received were shunted (sintered) or with low contact pull strength (non-sintered), thus leading to the decision to solder rather than weld the copper cells to the Kapton substrate.

Source record↗

NASA NTRS1999

Studies of Premixed Laminar and Turbulent Flames at Microgravity

Several topics relating to combustion limits in premixed flames at reduced gravity have been studied. These topics include: (1) flame balls; (2) numerical simulation of flame ball and planar flame structure and stability; (3) experimental simulation of buoyancy effects in premixed flames using aqueous autocatalytic reactions; and (4) premixed flame propagation in Hele-Shaw cells.

Abid, M.↗

NASA NTRS1999

Studies of Premixed Laminar and Turbulent Flames at Microgravity

Abid, M.↗

NASA NTRS2003

Coupling the Solar-Wind/IMF to the Ionosphere through the High Latitude Cusps

Magnetic merging is a primary means for coupling energy from the solar wind into the magnetosphere-ionosphere system. The location and nature of the process remain as open questions. By correlating measurements form diverse locations and using large-scale MHD models to put the measurements in context, it is possible to constrain out interpretations of the global and meso-scale dynamics of magnetic merging. Recent evidence demonstrates that merging often occurs at high latitudes in the vicinity of the cusps. The location is in part controlled by the clock angle in the interplanetary magnetic field (IMF) Y-Z plane. In fact, B(sub Y) bifurcated the cusp relative to source regions. The newly opened field lines may couple to the ionosphere at MLT locations of as much as 3 hr away from local noon. On the other side of noon the cusp may be connected to merging sites in the opposite hemisphere. In face, the small convection cell is generally driven by opposite hemisphere merging. B(sub X) controls the timing of the interaction and merging sites in each hemisphere, which may respond to planar features in the IMF at different times. Correlation times are variable and are controlled by the dynamics of the tilt of the interplanetary electric field phase plane. The orientation of the phase plane may change significantly on time scales of tens of minutes. Merging is temporally variable and may be occurring at multiple sites simultaneously. Accelerated electrons from the merging process excite optical signatures at the foot of the newly opened field lines. All-sky photometer observations of 557.7 nm emissions in the cusp region provide a "television picture" of the merging process and may be used to infer the temporal and spatial variability of merging, tied to variations in the IMF.

Maynard, Nelson C.↗

NASA NTRS2003

A Comparison between Growth Morphology of "Eutectic" Cells/Dendrites and Single-Phase Cells/Dendrites

Directionally solidified (DS) intermetallic and ceramic-based eutectic alloys with an in-situ composite microstructure containing finely distributed, long aspect ratio, fiber, or plate reinforcements are being seriously examined for several advanced aero-propulsion applications. In designing these alloys, additional solutes need to be added to the base eutectic composition in order to improve heir high-temperature strength, and provide for adequate toughness and resistance to environmental degradation. Solute addition, however, promotes instability at the planar liquid-solid interface resulting in the formation of two-phase eutectic "colonies." Because morphology of eutectic colonies is very similar to the single-phase cells and dendrites, the stability analysis of Mullins and Sekerka has been extended to describe their formation. Onset of their formation shows a good agreement with this approach; however, unlike the single-phase cells and dendrites, there is limited examination of their growth speed dependence of spacing, morphology, and spatial distribution. The purpose of this study is to compare the growth speed dependence of the morphology, spacing, and spatial distribution of eutectic cells and dendrites with that for the single-phase cells and dendrites.

Tewari, S. N.↗

NASA NTRS1972

Feasibility study of a 110 watt per kilogram lightweight solar array system

An investigation of the feasibility of a solar array panel subsystem which will produce 10,000 watts of electrical output at 1 A.U. with an overall beginning-of-life power-to-weight ratio of at least 110 watt/kg is reported. A description of the current baseline configuration which meets these requirements is presented. A parametric analysis of the single boom, two blanket planar solar array system was performed to arrive at the optimum system aspect ratio. A novel concept for the stiffening of a lightweight solar array by canting the solar cell blankets at a small angle to take advantage of the inherent in-plane stiffness to increase the symmetric out-of-plane frequency is introduced along with a preliminary analysis of the stiffening effect. A comparison of welded and soldered solar cell interconnections leads to the conclusion that welding is required on this ultralightweight solar array. The use of a boron/aluminum composite material in a BI-STEM type deployable boom is investigated as a possible advancement in the state-of-the-art.

Shepard, N. F.↗

NASA NTRS1982

Multi-100 kW: Planar low cost solar array development

The development of a 100 kW planar low cost solar array is presented. Tasks are defined, objectives are stated, and test results are given. A study of alternate contact configuration concluded that gridded back contact cells sharply reduced solar absorptance, that copper contacts will require a developmental program before interface can be achieved, and that gridded back contact cells using Ti-Pd-Ag as the contact materials appear to be the most cost-effective design.

Source record↗

NASA NTRS1981

Topographic forcing of supercritical convection in a porous medium such as the oceanic crust

Experiments carried out with a Hele-Shaw cell indicate that above the critical Rayleigh number, topography does not control the convection pattern, except when the topographic wavelength is comparable to the depth of water penetration. Scaled to the depth of the layer, the convective wavenumbers are restricted to values between 2.5 and 4.8 - a range that brackets pi, the natural wavenumber for convection in a porous slab with planar, isothermal, impermeable boundaries. Topographies within this range are found to control the circulation pattern perfectly, with downwelling under valleys and upwelling aligned with topographic height. Other topographies do not force the pattern, although it is noted that in some cases the convection wavenumber may be a harmonic of the topographic wavenumber. It is pointed out that unforced circulation cells wander and vary in size because they are not locked to the topography.

Hartline, B. K.↗

NASA NTRS1993

Grid-independent upwind scheme for multidimensional flow

Recent advances in the development of a grid-independent finite volume scheme for the Euler equations of gas dynamics are described. In the proposed method, flowfield gradient data are reconstructed locally (on a triangle) using five elementary planar waves, and an upwind numerical flux function for grid-oblique waves is developed to model the effect of the passage of these waves on the data in a cell. Numerical examples for several two-dimensional test problems are included. These results show high wave resolution and nearly monotone strong-wave transitions.

Parpia, Ijaz H.↗

NASA NTRS1997

Study of Injection of Helium into Supersonic Air Flow Using Rayleigh Scattering

A study of the transverse injection of helium into a Mach 3 crossflow is presented. Filtered Rayleigh scattering is used to measure penetration and helium mole fraction in the mixing region. The method is based on planar molecular Rayleigh scattering using an injection-seeded, frequency-doubled ND:YAG pulsed laser and a cooled CCD camera. The scattered light is filtered with an iodine absorption cell to suppress stray laser light. Preliminary data are presented for helium mole fraction and penetration. Flow visualization images obtained with a shadowgraph and wall static pressure data in the vicinity of the injection are also presented.

Seaholtz, Richard G.↗

NASA NTRS1984

Recent developments in high performance planar solar array technology

The NASA-OAST high performance solar array program is described. The rationale for this effort, its objectives and strategy, as well as progress made during the past 5 years, are discussed. It is shown that welded, ultrathin silicon solar cell array blankets are on the verge of technical readiness. It is argued that the most reasonable approach to achieving more significant performance improvements (to about 300 W/kg) involves the development of a higher efficiency (16-18 percent AM0) solar cell and a lightweight, efficient structure.

Scott-Monck, J.↗