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Real-Time Load-Side Control of Electric Power Systems
Get PDFTwo trends are emerging from modern electric power systems: the growth of renewable (e.g., solar and wind) generation, and the integration of information technologies and advanced power electronics. The former introduces large, rapid, and random fluctuations in power supply, demand, frequency, and voltage, which become a major challenge for real-time operation of power systems. The latter creates a tremendous number of controllable intelligent endpoints such as smart buildings and appliances, electric vehicles, energy storage devices, and power electronic devices that can sense, compute, communicate, and actuate. Most of these endpoints are distributed on the load side of power systems, in contrast to traditional control resources such as centralized bulk generators. This thesis focuses on controlling power systems in real time, using these load side resources. Specifically, it studies two problems.
(1) Distributed load-side frequency control: We establish a mathematical framework to design distributed frequency control algorithms for flexible electric loads. In this framework, we formulate a category of optimization problems, called optimal load control (OLC), to incorporate the goals of frequency control, such as balancing power supply and demand, restoring frequency to its nominal value, restoring inter-area power flows, etc., in a way that minimizes total disutility for the loads to participate in frequency control by deviating from their nominal power usage. By exploiting distributed algorithms to solve OLC and analyzing convergence of these algorithms, we design distributed load-side controllers and prove stability of closed-loop power systems governed by these controllers. This general framework is adapted and applied to different types of power systems described by different models, or to achieve different levels of control goals under different operation scenarios. We first consider a dynamically coherent power system which can be equivalently modeled with a single synchronous machine. We then extend our framework to a multi-machine power network, where we consider primary and secondary frequency controls, linear and nonlinear power flow models, and the interactions between generator dynamics and load control.
(2) Two-timescale voltage control: The voltage of a power distribution system must be maintained closely around its nominal value in real time, even in the presence of highly volatile power supply or demand. For this purpose, we jointly control two types of reactive power sources: a capacitor operating at a slow timescale, and a power electronic device, such as a smart inverter or a D-STATCOM, operating at a fast timescale. Their control actions are solved from optimal power flow problems at two timescales. Specifically, the slow-timescale problem is a chance-constrained optimization, which minimizes power loss and regulates the voltage at the current time instant while limiting the probability of future voltage violations due to stochastic changes in power supply or demand. This control framework forms the basis of an optimal sizing problem, which determines the installation capacities of the control devices by minimizing the sum of power loss and capital cost. We develop computationally efficient heuristics to solve the optimal sizing problem and implement real-time control. Numerical experiments show that the proposed sizing and control schemes significantly improve the reliability of voltage control with a moderate increase in cost.</p
Atomic-Level Structure and Deformation in Metallic Glasses
Get PDFMetallic glasses (MGs) are a relatively new class of materials discovered in 1960
and lauded for its high strengths and superior elastic properties. Three major obstacles
prevent their widespread use as engineering materials for nanotechnology and
industry: 1) their lack of plasticity mechanisms for deformation beyond the elastic
limit, 2) their disordered atomic structure, which prevents effective study of their
structure-to-property relationships, and 3) their poor glass forming ability, which
limits bulk metallic glasses to sizes on the order of centimeters. We focused on
understanding the first two major challenges by observing the mechanical properties
of nanoscale metallic glasses in order to gain insight into its atomic-level structure
and deformation mechanisms. We found that anomalous stable plastic flow emerges
in room-temperature MGs at the nanoscale in wires as little as ~100 nanometers
wide regardless of fabrication route (ion-irradiated or not). To circumvent experimental
challenges in characterizing the atomic-level structure, extensive molecular
dynamics simulations were conducted using approximated (embedded atom
method) potentials to probe the underlying processes that give rise to plasticity in
nanowires. Simulated results showed that mechanisms of relaxation via the sample
free surfaces contribute to tensile ductility in these nanowires. Continuing with characterizing
nanoscale properties, we studied the fracture properties of nano-notched
MGnanowires and the compressive response of MG nanolattices at cryogenic (~130
K) temperatures. We learned from these experiments that nanowires are sensitive
to flaws when the (amorphous) microstructure does not contribute stress concentrations,
and that nano-architected structures with MG nanoribbons are brittle at low
temperatures except when elastic shell buckling mechanisms dominate at low ribbon
thicknesses (~20 nm), which instead gives rise to fully recoverable nanostructures regardless
of temperature. Finally, motivated by understanding structure-to-property
relationships in MGs, we studied the disordered atomic structure using a combination
of in-situ X-ray tomography and X-ray diffraction in a diamond anvil cell
and molecular dynamics simulations. Synchrotron X-ray experiments showed the
progression of the atomic-level structure (in momentum space) and macroscale volume
under increasing hydrostatic pressures. Corresponding simulations provided
information on the real space structure, and we found that the samples displayed
fractal scaling (rd ∝ V, d < 3) at short length scales (< ~8 Å), and exhibited a
crossover to a homogeneous scaling (d = 3) at long length scales. We examined
this underlying fractal structure of MGs with parallels to percolation clusters and
discuss the implications of this structural analogy to MG properties and the glass
transition phenomenon
An Experimental and Technoeconomic Study of Silicon Microwire Arrays for Fuel Production Using Solar Energy
Get PDFDirect solar energy conversion is one of few sustainable energy resources able to wholly satisfy global energy demand; however, utility scale adoption and reliance are currently limited by the lack of a cost effective energy storage technology. The production of fuel from sunlight (solar fuels) enables solar energy storage in chemical bonds, a volumetrically and gravimetrically dense form compatible with current infrastructure worldwide. Hydrogen production via water splitting is a first generation solar fuel targeted herein that is currently used for hydrocarbon up-grading and fertilizer production and could further be utilized in combustion cycles and/or fuel cells for electricity and heat production and transportation.
This thesis presents achievements that form the foundation for Si microwire array based solar water splitting devices beginning with a tandem junction device design using Si microwire arrays as the architectural motif and one of many active components. Si microwire arrays have potential advantages over two dimensional planar device architectures such as minimized resistance losses, lower semiconductor material usage, and embedment in a polymeric membrane enabling a flexible device.
Experimental fabrication and characterization of this tandem junction device design was realized in the form of a np+-Si microwire array coated by either tungsten oxide (WO3) or titanium dioxide (TiO2) as the second tandem semiconductor. The Si/TiO2 device demonstrated the highest performance with an expected solar-to-hydrogen efficiency of 0.39%. To achieve these demonstrations new processing methods were needed and developed for formation of the np+-Si microwire array homojunction and formation of a low resistance contact between the p+-Si and second semiconductor using sputtered tin- doped indium oxide (ITO) and spray pyrolyzed fluorine-doped tin oxide (FTO).
Another achievement includes demonstration of the longest known (>2200 hours) photoanode stability for water oxidation using a np+-Si microwire array coated with an in-house developed amorphous TiO2 protection layer and NiCrOx electrocatalyst. Additionally, the Si microwire array architecture was used to enable decoupling of semiconductor light absorption and catalytic activity, two performance metrics that ideally are maximized simultaneously. However, all previous demonstrations have shown anti-correlation between these performance metrics because planar architectures are subject to a trade-off where adding electrocatalyst increases catalytic activity, but decreases semiconductor light absorption and vice versa.
Finally, a techno-economic analysis of solar water splitting production facilities was performed to assess economic competitiveness because this is the ultimate metric by which all energy production technologies are currently evaluated. This analysis suggests that a hydrogen production facility that is cosmetically similar to current solar panel installations with hydrogen collection from distributed tilted panels is unlikely to achieve cost competitiveness with fossil fuel derived hydrogen due to the balance of systems costs alone. A cost of CO2 greater than ~3 (MM BTU)-1 ($1.39 (kg H2)-1). Direct CO2 reduction systems were also explored and resulted in even larger challenges than hydrogen production. Accordingly, major facility wide breakthroughs are required to obtain viable economic costs for solar hydrogen production, but the barriers to achieve cost-competitiveness with existing large-scale thermochemical processes for CO2 reduction are even greater.</p
Thermal Ignition Using Moving Hot Particles
Get PDFIn this work, ignition of n-hexane-air mixtures was investigated using moving hot spheres of various diameters and surface temperatures. Alumina spheres of 1.8-6 mm diameter were heated using a high power CO2 laser and injected with an average velocity of 2.4 m/s into a premixed n-hexane-air mixture at a nominal initial temperature and pressure of 298 K and 100 kPa, respectively. The 90% probability of ignition using a 6 mm diameter sphere was 1224 K. High-speed experimental visualizations using interferometry indicated that ignition occurred in the vicinity of the separation point in the boundary layer of the sphere when the sphere surface temperature was near the ignition threshold. Additionally, the ignition threshold was found to be insensitive to the mixture composition and showed little variation with sphere diameter.
Numerical simulations of a transient one-dimensional boundary layer using detailed chemistry in a gas a layer adjacent to a hot wall indicated that ignition takes place away from the hot surface; the igniting gas that is a distance away from the surface can overcome diffusive heat losses back to the wall when there is heat release due to chemical activity. Finally, a simple approximation of the thermal and momentum boundary layer profiles indicated that the residence time within a boundary layer varies drastically, for example, a fluid parcel originating at very close to the wall has a residence time that is 65x longer than the residence time of a fluid parcel traveling along the edge of the momentum boundary layer.
A non-linear methodology was developed for the extraction of laminar flame properties from synthetic spherically expanding flames. The results indicated that for accurate measurements of the flame speed and Markstein length, a minimum of 50 points is needed in the data set (flame radius vs. time) and a minimum range of 48 mm in the flame radius. The non-linear methodology was applied to experimental n-hexane-air spherically expanding flames. The measured flame speed was insensitive to the mixture initial pressure from 50 to 100 kPa and increased with increasing mixture initial temperature. One-dimensional freely-propagating flame calculations showed excellent agreement with the experimental flame speeds using the JetSurF and CaltechMech chemical mechanisms.</p
Electrochemical Methods to Study Iron-Sulfur Cluster Proteins
Get PDFElectron transfer between proteins is an important mechanism in multiple biological processes. In this thesis, methods were developed to study electron transfer in two biological contexts: 1) DNA-mediated signaling between DNA binding proteins with 4Fe-4S clusters and 2) nitrogenase.
The first portion of this thesis focuses on the spectroscopic and electrochemical characterization of the iron-sulfur cluster in Dna2. Dna2 is a helicase-nuclease that is involved in Okazaki fragment maturation, double strand break repair, mitochondrial genome maintenance, and telomere maintenance. Dna2 is one of multiple DNA repair and replication proteins that contain a 4Fe-4S cluster, a cofactor that generally participates in electron transfer processes. It has been proposed that these enzymes may use their 4Fe-4S clusters to signal one another over large molecular distances to coordinate their activity on biological time scales through DNA-mediated redox chemistry. A combination of EPR and UV-visible absorption spectroscopy along with electrochemistry studies on DNA-modified gold electrodes was performed to provide insight into the chemical characteristics of the 4Fe-4S cluster in Dna2. These studies also provide a foundation for how DNA charge transport might coordinate the action of eukaryotic DNA repair and replication proteins with 4Fe-4S clusters.
The second portion of this thesis describes the development of electrochemical methods to study nitrogenase, the enzyme that catalyzes the reduction of atmospheric dinitrogen to bioavailable ammonia. First, flavodoxin II, the biological reductant of the Fe-protein of nitrogenase, was characterized using a combination of electrochemical and structural methods to determine the molecular interactions that facilitate reduction of the nitrogenase iron protein. Second, two electrochemical methods, edge-plane pyrolytic graphite electrodes and single crystal gold electrodes modified with ω-functionalized alkane-thiols, were adapted to study the redox chemistry at the iron-sulfur cluster of the Fe-protein. These studies provided insight into both the fundamental characteristics of electron transfer reactions involving nitrogenase, as well as insight into how to better study this enzyme using electrochemical methods.</p
Observations and Modeling of Tropical Planetary Atmospheres
Get PDFThis thesis is a comprised of three different projects within the topic of tropical atmospheric dynamics. First, I analyze observations of thermal radiation from Saturn’s atmosphere and from them, determine the latitudinal distribution of ammonia vapor near the 1.5-bar pressure level. The most prominent feature of the observations is the high brightness temperature of Saturn’s subtropical latitudes on either side of the equator. After comparing the observations to a microwave radiative transfer model, I find that these subtropical bands require very low ammonia relative humidity below the ammonia cloud layer in order to achieve the high brightness temperatures observed. We suggest that these bright subtropical bands represent dry zones created by a meridionally overturning circulation.
Second, I use a dry atmospheric general circulation model to study equatorial superrotation in terrestrial atmospheres. A wide range of atmospheres are simulated by varying three parameters: the pole-equator radiative equilibrium temperature contrast, the convective lapse rate, and the planetary rotation rate. A scaling theory is developed that establishes conditions under which superrotation occurs in terrestrial atmospheres. The scaling arguments show that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global-warming simulations.
Third, I use a moist slab-ocean general circulation model to study the impact of a zonally-symmetric continent on the distribution of monsoonal precipitation. I show that adding a hemispheric asymmetry in surface heat capacity is sufficient to cause symmetry breaking in both the spatial and temporal distribution of precipitation. This spatial symmetry breaking can be understood from a large-scale energetic perspective, while the temporal symmetry breaking requires consideration of the dynamical response to the heat capacity asymmetry and the seasonal cycle of insolation. Interestingly, the idealized monsoonal precipitation bears resemblance to precipitation in the Indian monsoon sector, suggesting that this work may provide insight into the causes of the temporally asymmetric distribution of precipitation over southeast Asia.</p
Metaconcrete: Engineered Aggregates for Enhanced Dynamic Performance
Get PDFThis work presents the development and investigation of a new type of concrete for the attenuation of waves induced by dynamic excitation. Recent progress in the field of metamaterials science has led to a range of novel composites which display unusual properties when interacting with electromagnetic, acoustic, and elastic waves. A new structural metamaterial with enhanced properties for dynamic loading applications is presented, which is named metaconcrete. In this new composite material the standard stone and gravel aggregates of regular concrete are replaced with spherical engineered inclusions. Each metaconcrete aggregate has a layered structure, consisting of a heavy core and a thin compliant outer coating. This structure allows for resonance at or near the eigenfrequencies of the inclusions, and the aggregates can be tuned so that resonant oscillations will be activated by particular frequencies of an applied dynamic loading. The activation of resonance within the aggregates causes the overall system to exhibit negative effective mass, which leads to attenuation of the applied wave motion. To investigate the behavior of metaconcrete slabs under a variety of different loading conditions a finite element slab model containing a periodic array of aggregates is utilized. The frequency dependent nature of metaconcrete is investigated by considering the transmission of wave energy through a slab, which indicates the presence of large attenuation bands near the resonant frequencies of the aggregates. Applying a blast wave loading to both an elastic slab and a slab model that incorporates the fracture characteristics of the mortar matrix reveals that a significant portion of the supplied energy can be absorbed by aggregates which are activated by the chosen blast wave profile. The transfer of energy from the mortar matrix to the metaconcrete aggregates leads to a significant reduction in the maximum longitudinal stress, greatly improving the ability of the material to resist damage induced by a propagating shock wave. The various analyses presented in this work provide the theoretical and numerical background necessary for the informed design and development of metaconcrete aggregates for dynamic loading applications, such as blast shielding, impact protection, and seismic mitigation
Polaron Hopping in Olivine Phosphates Studied by Nuclear Resonant Scattering
Get PDFValence fluctuations of Fe2+ and Fe3+ were studied in a solid solution of LixFePO4 by nuclear resonant forward scattering of synchrotron x rays while the sample was heated in a diamond-anvil pressure cell. The spectra acquired at different temperatures and pressures were analyzed for the frequencies of valence changes using the Blume-Tjon model of a system with a fluctuating Hamil- tonian. These frequencies were analyzed to obtain activation energies and an activation volume for polaron hopping. There was a large suppression of hopping frequency with pressure, giving an anomalously large activation volume. This large, positive value is typical of ion diffusion, which indicates correlated motions of polarons, and Li+ ions that alter the dynamics of both.
In a parallel study of NaxFePO4, the interplay between sodium ordering and electron mobility was investigated using a combination of synchrotron x-ray diffraction and nuclear resonant scattering. Conventional Mossbauer spectra were collected while the sample was heated in a resistive furnace. An analysis of the temperature evolution of the spectral shapes was used to identify the onset of fast electron hopping and determine the polaron hopping rate. Synchrotron x-ray diffraction measurements were carried out in the same temperature range. Reitveld analysis of the diffraction patterns was used to determine the temperature of sodium redistribution on the lattice. The diffraction analysis also provides new information about the phase stability of the system. The temperature evolution of the iron site occupancies from the Mossbauer measurements, combined with the synchrotron diffraction results give strong evidence for a relationship between the onset of fast electron dynamics and the redistribution of sodium in the lattice.
Measurements of activation barriers for polaron hopping gave fundamental insights about the correlation between electronic carriers and mobile ions. This work established that polaron-ion interactions can alter the local dynamics of electron and ion transport. These types of coupled processes may be common in many materials used for battery electrodes, and new details concerning the influence of polaron-ion interactions on the charge dynamics are relevant to optimizing their electrochemical performance.</p
Mechanics of Sediment Transport and Bedrock Erosion in Steep Landscapes
Get PDFErosion is concentrated in steep landscapes such that, despite accounting for only a small fraction of Earth’s total surface area, these areas regulate the flux of sediment to downstream basins, and their rugged morphology records transient changes (or lack thereof) in geologic and climatic forcing. Steep landscapes are geomorphically active; large sediment fluxes and rapid landscape evolution rates can create or destroy habitat for humans and wildlife alike, and landslides, debris flows, and floods common in mountainous areas represent a persistent natural and structural hazard. Despite the central role that steep landscapes play in the geosciences and in landscape management, the processes controlling their evolution have been poorly studied compared to lower-gradient areas. This thesis focuses on the basic mechanics of sediment transport and bedrock incision in steep landscapes, as these are the fundamental processes which set the pace and style of landscape evolution. Chapter 1 examines the spatial distribution of slow-moving landslides; these landslides can dominate sediment fluxes to river networks, but the controls on their occurrence are poorly understood. Using a case-study along the San Andreas Fault, California, I show that slow-moving landslides preferentially occur near the fault, suggesting a rock-strength control on landslide distribution. Chapter 2 provides the first field-measurements of incipient sediment motion in streams steeper than 14% and shows a large influence of slope-dependent flow hydraulics and grain-scale roughness on particle motion. Chapter 3 presents experimental evidence for bedrock erosion by suspended sediment, suggesting that, in contrast to prevailing theoretical predictions, suspension-regime transport in steep streams can be the dominant erosion agent. Steep streams are often characterized by the presence of waterfalls and bedrock steps which can have locally high rates of erosion; Chapters 4 and 5 present newly developed, experimentally validated theory on sediment transport through and bedrock erosion in waterfall plunge pools. Finally, Chapter 6 explores the formation of a bedrock slot canyon where interactions between sediment transport and bedrock incision lead to the formation of upstream-propagating bedrock step-pools and waterfalls
Proton-Coupled Reduction of N₂ Facilitated by Molecular Fe Complexes
Get PDFThe activation of Fe-coordinated N2 via the formal addition of hydrogen atom equivalents is explored in this thesis. These reactions may occur in nitrogenase enzymes during the biological conversion of N2 to NH3. To understand these reactions, the N2 reactivity of a series of molecular Fe(N2) platforms is investigated. A trigonal pyramidal, carbon-ligated FeI complex was prepared that displays a similar geometry to that of the resting state 'belt' Fe atoms of nitrogenase. Upon reduction, this species was shown to coordinate N2, concomitant with significant weakening of the C-Fe interaction. This hemilability of the axial ligand may play a critical role in mediating the interconversion of Fe(NxHy) species during N2 conversion to NH3. In fact, a trigonal pyramidal borane-ligated Fe complex was shown to catalyze this transformation, generating up to 8.49 equivalents of NH3. To shed light on the mechanistic details of this reaction, protonation of a borane-ligated Fe(N2) complex was investigated and found to give rise to a mixture of species that contains an iron hydrazido(2-) [Fe(NNH2)] complex. The identification of this species is suggestive of an early N-N bond cleavage event en route to NH3 production, but the highly-reactive nature of this complex frustrated direct attempts to probe this possibility. A structurally-analogous silyl-ligated Fe(N2) complex was found to react productively with hydrogen atom equivalents, giving rise to an isolable Fe(NNH2) species. Spectroscopic and crystallographic studies benefited from the enhanced stability of this complex relative to the borane analogue. One-electron reduction of this species initiates a spontaneous disproportionation reaction with an iron hydrazine [Fe(NH2NH2)] complex as the predominant reaction product. This transformation provides support for an Fe-mediated N2 activation mechanism that proceeds via a late N-N bond cleavage. In hopes of gaining more fundamental insight into these reactions, a series of Fe(CN) complexes were prepared and reacted with hydrogen-atom equivalents. Significant quantities of CH4 and NH3 are generated in these reactions as a result of complete C-N bond activation. A series of Fe(CNHx) were found to be exceptionally stable and may be intermediates in these reactions. The stability of these compounds permitted collection of thermodynamic parameters pertinent to the unique N-H bonds. This data is comparatively discussed with the theoretically-predicted data of the N2-derived Fe(NNHx) species. Exceptionally-weak N-H bond enthalpies are found for many of these compounds, and sheds light on their short-lived nature and tendency to evolve H2. As a whole, these works both establish and provide a means to understand Fe-mediated N2 activation via the addition of hydrogen atom equivalents