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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.


SNOWDON, LLOYD R. January 1978 (has links)
No description available.


WHITE, MARK GILMORE January 1978 (has links)
No description available.


WELCH, MARGARET OGDEN January 1980 (has links)
Primary productivity was investigated for 24 months in a 404 ha cooling reservoir of a 530 MW electrical generating plant located on the Texas coastal plain. Carbon fixation was estimated in situ at 0.5 meters from the surface and in the laboratory by carbon-14 techniques. For 18 months phytoplankton community structure was analyzed for biomass, density and species diversity. Increased temperature stimulated the rate of carbon fixation. Average overall carbon fixation for the two-year study for Station 1 was 34 mg m('-3) hr('-1) and 24 mg m('-3) hr('-1) for Station 2. There was no significant difference between these means but for 66% of the sampling dates, carbon fixation was significantly higher at the heated station than at the control. Four out of seven in situ experiments in which water samples from Station 1 and Station 2 were reversed and incubated at the opposite station indicated that the productivity difference was due to temperature. In laboratory experiments where natural phytoplankton from each station were incubated at different temperatures, the phytoplankton community of each station responded to temperature similarly. However, the difference in primary productivity ((DELTA)P) between stations was not a simple function of the difference in temperature ((DELTA)T) between stations. Nutrients were low, perhaps due to removal from the water column by the primary producers, but did not appear to be limiting. Values for primary productivity were independent of light intensity and photosynthesis was probably light saturated at the 0.5 m incubation depth. Community structure was similar at both stations. The phytoplankton at both stations also responded similarly to temperature, increasing in activity up to an optimum range of about 24-28(DEGREES)C and decreasing in activity above 30(DEGREES)C. Community structure changed from greens to bluegreens when the temperature optimum was exceeded. Peaks of primary productivity occurred in the spring and in the fall and coincided with the occurrence of optimum water temperatures. Hence, it is suggested that temperature was responsible for the seasonal variation in carbon fixation.


ASGARPOUR, SOHEIL January 1981 (has links)
The heat transfer characteristics of a viscous, low Peclet number fluid entering a circular duct from an isothermal reservoir is considered. A technique for realistically modeling the constant temperature entrance condition in the presence of axial conduction is described. Graphs of the local Nusselt number and the local temperature profile are given for each case of constant temperature and constant heat flux at the duct wall. The energy equation is solved numerically using a finite difference scheme; Langhaar's solution for developing flow is used to describe the fluid dynamics. The results were tested by assuming either uniform or parabolic velocity profile. The results compared favorably with those reported in the literature. The classical Graetz problem was also solved, and the numerical result agreed to within 1% with the known analytical solution. An analysis of heat transfer in a shell and tube energy device having a phase change material (PCM) on the shell side is presented for the case of energy recovery from the PCM. The PCM is initially liquid, and at its freezing point. The working fluid originates from an isothermal reservoir. Developing, laminar Newtonian flow is assumed. The coupled energy equations written for the fluid and the PCM fully account for the effects of axial conduction. Finite difference methods are employed in solving the equations. Alternating direction procedures are applied to the energy equations, and the nonlinear heat balance equation written for the interface is solved by the Newton-Raphson method. The numerical results were obtained by assuming the ratio of the conductivity of the PCM to that of the fluid to be 4.0, and the ratio of the diffusivities to be 2.5. Physically, this could represent an n-octadecan wax water system. Certain assumptions that have been made in the literature have been checked for validity. This work supports the assumption that axial conduction can be neglected in the PCM. However, the heat capacity of the PCM can not be neglected even for small Stefan numbers. In addition the bulk temperature of the fluid is shown to be a function of the Stefan number and therefore can not be imposed a priori until the general form of the problem (coupled energy equation for PCM and fluid) has been solved. Graphs of the temperature distribution for the fluid and the PCM, the bulk temperature of the fluid, the interface position and the Biot number are presented.


HWU, FRANK SHAW-SHYONG January 1981 (has links)
Reactions of methanol and some light olefins (including ethylene, propylene, butenes, and 3-methylbutene-1) were studied over Ni-ZSM-5 zoelite catalyst. The product distribution over a broad range of space velocities indicates the reaction sequence: dimethyl paraffins methanol (,ether) C(,2)-C(,5) olefins (,aromatics) Monomethyl aliphatics predominate over both straight-chain and dimethyl hydrocarbons. Aromatics consist mainly of toluene and xylene mixtures. At 368(DEGREES)C, compositions of butene isomers are in thermodynamic equilibrium, over the range of residence times investigated (i.e. (tau) = 22 to 648 mg-sec/cc). Equilibrium composition of xylenes is not established until (tau) = 1440 mg-sec/cc; p-xylene is kinetically favored at short residence times. The effect of temperature on the reaction of propylene shows that a maximum in conversion is achieved at 310(DEGREES)C. As the temperature goes above 310(DEGREES)C, conversion decreases, indicating the occurrence of reversible cracking reactions. Temperature is also a crucial factor in determining the product distribution, for no aromatics are observed for reaction temperatures below 310(DEGREES)C. In both propylene and cis-butene-2 conversion experiments, the effect of temperature on isomer compositions (for butenes and pentenes) suggests that positional and/or configurational isomerization is an easier reaction than skeletal isomerization whose rate increases rapidly at higher temperatures. Comparative sorption studies for a number of hydrocarbons show the following order of rate of sorption: monomethyl dimethyl n-paraffins (,paraffins) aromatics (,paraffins) Isosteric heats of sorption obtained from adsorption isotherms are 11.5 kcal/mole for n-butane and 13.6 kcal/mole for n-pentane. The activation energy for diffusion of benzene in the Ni-ZSM-5 catalyst is 7 kcal/mole. All isotherms investigated can be represented by the Langmuir equation. ('14)C-labeled tracer studies confirm the important role that ethylene, propylene and butenes play as active intermediates in the over all methanol-to-gasoline reaction, although ethylene shows less reactivity than propylene and butenes. Isobutane, n-hexane and n-heptane are relatively stable final products, but the latter two paraffins can undergo some cracking reactions. Benzene and toluene can alkylate with methanol to form toluene and to a lesser extent the xylenes. No cracking of the benzene ring is observed at 368(DEGREES)C.


SIMONS, SEDGWICK LEWIS, JR. January 1981 (has links)
The phototron, originally known as the photoklystron, is a vacuum tube device invented by Dr. John Freeman at Rice University for use on the solar power satellite. The design objective called for a device which would convert sunlight to microwaves in one simple direct step. Several test model tubes were constructed by ITT for concept evaluation at frequencies of 2 MHz. to 200 MHz. The operation of these test model phototrons has been found to be distinctly different from the operation of conventional klystron tubes. Past work by the author identified a multi-pass mode in which the electrons in the phototron exhibited a period of motion equal to a multiple of the RF period. In this mode improperly phased electrons are selectively removed from the tube by collision with the photocathode. Recent work has revealed a new and potentially more efficient mode of operation in which the electron period of motion is approximately half of the RF period. Computer plots of the electron trajectories reveal that unlike the multi-pass mode, this mode exhibits a genuine electron bunching process. Since these "half-cycle" modes operate at low bias voltages an investigation of space charge effects and electron emission energy has been made. Theoretical results from a computer program based on an analysis by Guernsey and Fu show that potentials within the phototron can be dramatically reshaped by space charge. These potentials will alter the electron motion in the phototron and also filter slow electrons out of the beam. Experimental measurements of the electron energy spectra have documented this filtering effect. Efficiency calculations based on a theoretical analysis and illustrated by experimental observations show that the greatest restriction on overall efficiency is due to the photocathode itself. The test model tubes show a cathode efficiency of only about 1%, but better materials may improve this value to about 10%. If the efficiency of the conversion of DC to RF in the phototron can also be improved an overall device efficiency of 1% for the conversion of sunlight into RF may be realized in the near future. Further improvements of the basic design may ultimately improve the overall efficiency to near 3%. In principle such a device could easily be scaled to produce microwaves instead of RF. At this efficiency the phototron could fulfill many applications, most notably in satellite communications. Its output is easily modulated in response to light or voltage, so it may also be useful in some transducer applications. If the efficiency can be improved the phototron may offer cost and weight advantages over other energy conversion systems.

Using Thermodynamics and Mechanism to Understand and Improve Dioxygen Reduction Electro- and Photoelectrocatalysts

Pegis, Michael Lee 21 August 2018 (has links)
<p> The dioxygen reduction reaction (ORR) is a key process for renewable energy technologies such as batteries and fuel cells. Reliance on dioxygen as the terminal oxidant in fuel cell technologies requires the development of electrocatalysts that proceed at low overpotentials with fast rates, while maintaining adequate selectivity for H<sub>2</sub>O production over thousands of hours of operation. While many advances have been made, the discovery of efficient and inexpensive ORR electrocatalytic materials remains a holy-grail of energy science. The study of soluble, molecular electrocatalysts allow for more detailed structure : activity analyses to be made than for electrocatalytic materials, providing an atomic level understanding of catalytic barriers and opportunities for improvement.</p><p> Chapters 1-5 of this thesis develop a rational approach for analyzing, comparing, and improving homogeneous and molecular ORR electrocatalysts in non-aqueous solvents, and use this approach to study the reduction of dioxygen to water using iron porphyrin electrocatalysts. Quantification of the equilibrium potential for O<sub>2</sub>/H<sub>2</sub>O under the conditions of study allowed, for the first time, accurate estimations of the reaction overpotential in non-aqueous solvents. Knowledge of the reaction overpotential proved critical for comparing electrocatalysts under the somewhat diverse conditions encountered for homogeneous catalysts with varying solubilities in organic solvents.</p><p> A detailed kinetic and mechanistic study was then conducted on iron tetraphenyl porphyrin, Fe(TPP), which revealed that the turnover limiting step is protonation of the iron(III) superoxo adduct, formed via pre-equilibrium dioxygen binding to Fe<sup>II</sup>(TPP). The protonation step was found to have a large kinetic barrier, suggesting that targeting proton delivery to the active site may improve the ORR activity of iron porphyrin electrocatalysts.</p><p> Studies of eleven substituted iron porphyrin ORR electrocatalysts all showed high selectivity for the 4H<sup>+</sup>/4<i>e</i><sup> -</sup> reduction to water. The turnover frequencies (TOFs) were found to correlate with the reduction potential required to initiate electrocatalysis, in two log(TOF):overpotential linear free energy relationships (LFERs). The iron porphyrin electrocatalysts with well-positioned proton donors above the active site fell upon the same LFER as those without such proton relays, suggesting that the second coordination sphere does not directly participate in the rate-limiting proton transfer. These results contradict the general sense that well-positioned proton relays should decrease kinetic barriers. However, some iron porphyrin catalysts in the series can break the LFER, leading to more efficient catalysis. Computational studies suggest that, rather than directly participating in an intramolecular proton transfer, the second coordination sphere of some iron porphyrins can hydrogen bond with the O<sub>2</sub> adduct to influence the thermochemistry for proton transfer. Importantly, the presence of these LFERs was shown to stem from the electrocatalyst <i>E</i><sub>1/2 </sub> influencing the thermodynamics for O<sub>2</sub> binding and proton transfer. Analogies are drawn between these linear free energy relationships and the scaling relationship analyses used for electrocatalytic materials for the ORR.</p><p> Using the mechanism, rate law, and thermochemistry, the log(TOF) : overpotential correlations were then derived for ORR catalyzed by iron porphyrins. Given that the TOF is a function of the catalytic rate law (TOF = <i>k</i><sub> cat</sub>[O<sub>2</sub>][HA]) and the overpotential is a function of the reaction conditions, the predicted correlation between log(TOF) and effective overpotential is independently. derived for changes in the reaction conditions or for changes to the catalyst <i>E</i><sub>1/2</sub>. For each parameter varied, a unique correlation coefficient was identified and shown to agree with experimental data. The very shallow dependence between log(TOF) and the p<i>K</i><sub> a</sub> of the acid used was used to enable Fe(TPP) catalyzed ORR to break the prior LFERs by 10<sup>4</sup> s<sup>-1</sup> in TOF. These scaling relations highlight how decoupling the ET, PT and substrate binding events can lead to diverse scaling relationships, providing opportunities for improving the activity of a <i>catalytic system</i> by targeting the medium, as opposed to the catalyst.</p><p> In chapter 6, an exploratory research project on driving the ORR using sunlight to produce hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) is discussed. H<sub>2</sub>O<sub>2</sub> is a commodity chemical with diverse applications in water purification, as an oxidant, and as a liquid fuel. Preparation of nickel(II) oxide photocathodes sensitized with simple dyes revealed that these photocathodes are surprisingly active for H<sub>2</sub>O<sub>2</sub> production, proceeding to produce H<sub>2</sub>O<sub>2</sub> with unity faradaic efficiency at low overpotentials (&lt;20 mV). The reaction is found to proceed via outer sphere electron transfer from reduced dyes to O<sub>2</sub>, forming superoxide, which disproportionates in solution, forming H<sub>2</sub>O<sub>2</sub>. Remarkably, these unoptimized systems are among the most active photocathodes for H<sub> 2</sub>O<sub>2</sub> production. These results are promising for developing the delocalized production of H<sub>2</sub>O<sub>2</sub> using dye-sensitized photoelectrosynthesis cells.</p><p>

Optimizing the Process Parameters for Electrochemical Reduction of Carbon Dioxide

January 2017 (has links)
abstract: One the major problems of this modern industrialized world is its dependence on fossil fuels for its energy needs. Burning of fossils fuels generates green-house gases which have adverse effects on global climate contributing to global warming. According to Environmental Protection Agency (EPA), carbon dioxide makes up 80 percent of green-house gases emitted in USA. Electrochemical reduction of carbon dioxide is an approach which uses CO2 emissions to produce other useful hydrocarbons which can be used in many ways. In this study, primary focus was on optimizing the operating conditions, determining the better catalyst material, and analyzing the reaction products for the process of electrochemical reduction of carbon dioxide (ERC). Membrane electrode assemblies (MEA’s) are developed by air bushing the metal particles with a spray gun on to Nafion-212 which is a solid polymer based electrolyte (SPE), to support the electrodes in the electrochemical reactor gas diffusion layers (GDL) are developed using porous carbon paper. Anode was always made using the same material which is platinum but cathode material was changed as it is the working electrode. The membrane electrode assembly (MEA) is then placed into the electrochemical reactor along with gas diffusion layer (GDL) to assess the performance of the catalyst material by techniques like linear sweep voltammetry and chronoamperometry. Performance of MEA was analyzed at 4 different potentials, 2 different temperatures and for 2 different cathode catalyst materials. The reaction products of the process are analyzed using gas chromatography (GC) which has thermal conductivity detector (TCD) used for detecting hydrogen (H2), carbon monoxide (CO) and flame ionization detector (FID) used for detecting hydrocarbons. The experiments performed at 40o C gave the better results when compared with the experiments performed at ambient temperature. Also results suggested that copper oxide cathode catalyst has better durability than platinum-carbon. Maximum faradaic efficiency for methane was 5.3% it was obtained at 2.25V using copper oxide catalyst. Furthermore, experiments must be carried out to make the electrochemical reactor more robust to withstand all the operating conditions like higher potentials and to make it a solar powered reactor. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2017

Design, Simulation, and Analysis of Domestic Solar Water Heating Systems in Phoenix, Arizona

January 2012 (has links)
abstract: Research was conducted to quantify the energy and cost savings of two different domestic solar water heating systems compared to an all-electric water heater for a four-person household in Phoenix, Arizona. The knowledge gained from this research will enable utilities to better align incentives and consumers to make more informed decisions prior to purchasing a solar water heater. Daily energy and temperature data were collected in a controlled, closed environment lab. Three mathematical models were designed in TRNSYS 17, a transient system simulation tool. The data from the lab were used to validate the TRNSYS models, and the TRNSYS results were used to project annual cost and energy savings for the solar water heaters. The projected energy savings for a four-person household in Phoenix, Arizona are 80% when using the SunEarth® system with an insulated and glazed flat-plate collector, and 49% when using the FAFCO® system with unglazed, non-insulated flat-plate collectors. Utilizing all available federal, state, and utility incentives, a consumer could expect to recoup his or her investment after the fifth year if purchasing a SunEarth® system, and after the eighth year if purchasing a FAFCO® system. Over the 20-year analysis period, a consumer could expect to save $2,519 with the SunEarth® system, and $971 with the FAFCO® system. / Dissertation/Thesis / M.S.Tech Mechanical Engineering 2012

The Profitability of an Investment in Photovoltaics in South Carolina

Welsh, Thomas McClain 28 July 2017 (has links)
<p> As renewable energy becomes more prevalent across the United States and the world, solar energy investment has also grown. There have been many studies done on photovoltaic (PV) systems in terms of energy payback and efficiency, but little research done to understand a PV system as a financial investment specific to South Carolina. This study aims to understand the return on investment that a PV system can achieve. More specifically whether PV systems in areas of South Carolina that uses Duke Energy achieve a favorable return on investment and what affects the profitability. This study uses the PVwatts calculator provided by NREL as well as an investment simulation to calculate the Internal Rate of Return (IRR) and Net Present Value on 1024 simulated 5kW PV arrays and evaluates their profitability. It then uses this information to apply it to real case studies for houses in South Carolina. This study found that shade has a significant impact on profitability of investment. At 30% shading, profitability drops near 0% IRR or below. Orientation impacts profitability significantly as well. Panels that are facing south, southeast, and southwest yielded the best return. While north, northeast and northwest orientations yielded very low or negative IRR. East and west facing panels can yield positive financial return, but this return is lower than panels orientated to the south. PV systems oriented towards the east or west must have optimal conditions to remain efficient. This study found that tilt had minimal impact on financially return. Incentives also significantly impacted profitability of investment. For a PV system to be profitable, federal, state, and Duke Energy incentives needed to be applied to the investment. When homes with PV systems are sold also has a great impact on profitability. Research has shown that there is a housing premium for homes with PV systems (Adomatis, 2015). This premium is highest when first installed and declines as the PV systems age. People also associate premiums with houses with PV systems even if the system is not adding much value to the home. This study has also found that the price of the PV system impacts investment. Premium grade panels had significantly less return compared to standard grade panels because prices per watt were higher.</p><p>

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