Browsing by Subject "Alternative energy"

In this dissertation, we develop a novel methodology for characterizing and simulating nonstationary, full-field, stochastic turbulent wind fields.

In this new method, nonstationarity is characterized and modeled via temporal coherence, which is quantified in the discrete frequency domain by probability distributions of the differences in phase between adjacent Fourier components.

The empirical distributions of the phase differences can also be extracted from measured data, and the resulting temporal coherence parameters can quantify the occurrence of nonstationarity in empirical wind data.

This dissertation (1) implements temporal coherence in a desktop turbulence simulator, (2) calibrates empirical temporal coherence models for four wind datasets, and (3) quantifies the increase in lifetime wind turbine loads caused by temporal coherence.

The four wind datasets were intentionally chosen from locations around the world so that they had significantly different ambient atmospheric conditions.

The prevalence of temporal coherence and its relationship to other standard wind parameters was modeled through empirical joint distributions (EJDs), which involved fitting marginal distributions and calculating correlations.

EJDs have the added benefit of being able to generate samples of wind parameters that reflect the characteristics of a particular site.

Lastly, to characterize the effect of temporal coherence on design loads, we created four models in the open-source wind turbine simulator FAST based on the \windpact turbines, fit response surfaces to them, and used the response surfaces to calculate lifetime turbine responses to wind fields simulated with and without temporal coherence.

The training data for the response surfaces was generated from exhaustive FAST simulations that were run on the high-performance computing (HPC) facilities at the National Renewable Energy Laboratory.

This process was repeated for wind field parameters drawn from the empirical distributions and for wind samples drawn using the recommended procedure in the wind turbine design standard \iec.

The effect of temporal coherence was calculated as a percent increase in the lifetime load over the base value with no temporal coherence.

This dissertation shows the use of Constructal law to find the relation between the morphing of the system configuration and the improvements in the global performance of the complex flow system. It shows that the better features of both flow and heat transfer architecture can be found and predicted by using the constructal law in energy systems. Chapter 2 shows the effect of flow configuration on the heat transfer performance of a spiral shaped pipe embedded in a cylindrical conducting volume. Several configurations were considered. The optimal spacings between the spiral turns and spire planes exist, such that the volumetric heat transfer rate is maximal. The optimized features of the heat transfer architecture are robust. Chapter 3 shows the heat transfer performance of a helically shaped pipe embedded in a cylindrical conducting volume. It shows that the optimized features of the heat transfer architecture are robust with respect to changes in several physical parameters. Chapter 4 reports analytically the formulas for effective permeability in several configurations of fissured systems, using the closed-form description of tree networks designed to provide flow access. The permeability formulas do not vary much from one tree design to the next, suggesting that similar formulas may apply to naturally fissured porous media with unknown precise details, which occur in natural reservoirs. Chapter 5 illustrates a counterflow heat exchanger consists of two plenums with a core. The results show that the overall flow and thermal resistance are lowest when the core is absent. Overall, the constructal design governs the evolution of flow configuration in nature and energy systems.

Here we document and explain a general trend in the performance of refrigeration and heat pump systems: larger installations are more efficient. We show analytically why the performance of the system must increase with the size of the installation. The second law efficiency of refrigeration systems must increase with their size. We also show that the power requirement a ground-coupled heat pump system must decrease as the size of the ground heat exchanger increases. From these two trends emerges the trade off between the size of the heat pump and the size of the ground heat exchanger. The challenge is to find the optimum size of the ground-coupled heat pump. We show numerically the optimum heat pump size and the ground heat exchanger size that correspond to minimum total power requirement subject to a cost constraint.

The variability of renewable sources, such as wind and solar, when integrated into the electrical system must be compensated by traditional generation sources in-order to maintain the constant balance of supply and demand required for grid stability. The goal of this study is to analyze the effects of increasing large levels of solar Photovoltaic (PV) penetration (in terms of a percentage of annual energy production) on a test grid with similar characteristics to the Duke Energy Carolinas (DEC) and Progress Energy Carolinas (PEC) regions of North and South Carolina. PV production is modeled entering the system at the distribution level and regional PV capacity is based on household density. A gridded hourly global horizontal irradiance (GHI) dataset is used to capture the variable nature of PV generation. A unit commitment model (UCM) is then used determine the hourly dispatch of generators based on generator parameters and costs to supply generation to meet demand. Annual modeled results for six different scenarios are evaluated to determine technical, environmental and economic effects of varying levels of distributed PV penetration on the system.

This study finds that the main limiting factor for PV integration in the DEC and PEC balancing authority regions is defined by the large generating capacity of base-load nuclear plants within the system. This threshold starts to affect system stability at integration levels of 5.7%. System errors, defined by imbalances caused by over or under generation with respect to demand, are identified in the model however the validity of these errors in real world context needs further examination due to the lack of high frequency irradiance data and modeling limitations. Operational system costs decreased as expected with PV integration although further research is needed to explore the impacts of the capital costs required to achieve the penetration levels found in this study. PV system generation was found to mainly displace coal generation creating a loss of revenue for generator owners. In all scenarios, CO2 emissions were reduced with PV integration. This reduction could be used to meet impending EPA state-specific CO2 emissions targets.

The demand for water and energy increases as the world's population grows. Unfortunately, many people have limited access to these two very necessary resources. In order to solve this dire issue, industry leaders, governments, scientists, and researchers have been developing and improving various technologies that show promise for being able to sustain the demand for energy and water of a growing population. One method that has proven effective in supplying water to regions that lack potable water is desalination. However, this process requires a great amount of energy to produce enough clean drinking water for a given population. The research described in this work focuses on improving the energy efficiency and productivity of desalination through cogeneration, e.g. combined heat and power (CHP). The cogeneration system consists of a polymer electrolyte membrane (PEM) fuel cell coupled with a direct contact membrane distillation (DCMD) desalination system in a bottoming cycle. For this study, the cogeneration system was modeled mathematically and computationally using the quasi-2-D PEM fuel cell model described in Hotz et al., 2006, Int. J. Heat Mass Transfer [1]. The computational model will be used to create a physical prototype for testing. The computational model has shown that using a PEM fuel cell is an effective system for cogeneration with a DCMD desalination unit. However, the low operating temperatures of the device limit the overall performance and efficiency of the cogeneration system.

Halide semiconductors have recently emerged as a class of materials that unite outstanding optoelectronic properties with the ability to process device-quality thin films at low or even room temperature. Successful adoption of halide semiconductor-based technologies will, however, be contingent on the development of device architectures and film processing approaches that enable efficient, low-cost devices with stable performance and rigorous study of these materials’ photophysical properties. Herein, the goals are twofold: first, to develop low-cost device processing methods that deliver efficient solar cell performance while managing sources of instability; second, to extend existing thin film processing techniques to novel materials, enabling investigation of their optoelectronic properties.

After general introduction to halide perovskite materials, films and devices in Chapters 1 and 2, Chapter 3 confronts the first device challenge—i.e., reducing solar cell cost—by investigating cheap electron and hole transport layers (ETL and HTL) for halide perovskite solar cells. Efficient CH3NH3PbI3 perovskite solar cells are constructed using earth-abundant ETL CdS and HTL CuCrO2 that are deposited at low temperature (<100 °C). Although CuCrO2 appears to yield an inert interface with CH3NH3PbI3, X-ray photoelectron spectroscopy reveals that CdS can easily release Cd into the CH3NH3PbI3 film. X-ray diffraction (XRD) measurements show that excessive amounts of Cd cause phase segregation of insulating compounds in the perovskite, compromising solar cell performance. Nevertheless, careful optimization of device fabrication avoids this detrimental interaction, leading to solar cells with power conversion efficiency of over 15%. In addition to demonstrating efficient devices using low-cost materials, this work emphasizes the importance of managing interfacial as well as bulk stability.

Chapter 4 focuses on the second device challenge—i.e., managing instability—by developing inherently robust architectures via lamination and hot pressing. This technique circumvents the intrinsic thermal instability of perovskite thin films during processing and forms a self-encapsulating device architecture. Annealing MAPbI3 films under pressure in a specially-constructed tool allows significant grain growth at temperatures that would ordinarily decompose them rapidly to PbI2. However, these temperatures can also activate unexpected reactions with carrier transport materials previously thought to be inert, such as nickel oxide. Applying this knowledge, techniques are developed that avoid reactivity-related problems and recover the targeted solar cell performance.

Chapters 5 and 6 of this dissertation focus on developing deposition methods for new halide semiconductor films, with emphasis in this case on exploration of fundamental physical properties rather than device fabrication. In Chapter 5, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is first used to deposit films of the archetypal halide perovskite CH3NH3PbI3, which possesses properties comparable to those prepared by more conventional methods such as spin coating, as determined by XRD, electron microscopy and optical spectroscopy. CH3NH3PbI3 solar cells fabricated using RIR-MAPLE reach power conversion efficiency of over 12%. RIR-MAPLE is then extended to the deposition of layered lead halide perovskite films incorporating oligothiophene-derived molecular cations, which cannot be controllably deposited by other methods. By varying the number of rings in the thiophene chain and the halide component of the inorganic layers, the photoluminescence emission from these films can be tuned to originate from either the inorganic or the organic component or be quenched altogether, supporting prior computational predictions of the tunable quantum well nature of these types of perovskite structures. Carrier transfer between the inorganic and organic moieties can synergistically populate triplet states in the organic, showcasing the unique physical properties attainable in complex-organic perovskites.

Chapter 6 focuses on the halide semiconductor indium(I) iodide, which possesses elements of its electronic structure like those of halide perovskites, that are often invoked as an explanation for these materials’ remarkable defect tolerance. Indium(I) iodide is prepared in thin-film form by thermal evaporation. A photovoltaic effect is demonstrated for the first time in this material, with solar cells demonstrating ~0.4% power conversion efficiency. Overall, the results advance our scientific understanding of halide semiconductors, and provide crucial pathways by which they can be made more technologically effective, and be studied in greater depth.

In applications of ocean wave energy conversion, it is well known that feedback control can be used to achieve favorable performance. Current techniques include methods such as tuning a device to harvest energy at a narrow band of frequencies, which leads to suboptimal performance, or methods that are anticausal and require the future wave excitation to be known. This thesis demonstrates how to determine the maximum-attainable power generation and corresponding controller for a buoy type wave energy converter with multiple generators in a stochastic sea environment using a causal dynamic controller. This is accomplished by solving a nonstandard H2 optimal control problem. The performance of the causal controller is compared to the noncausal controller for various cases. This work provides a significant improvement over current control techniques because it involves a causal controller that can absorb a large amount of power over a broader bandwidth than other control techniques, including absorbing power across multiple modes of resonance. The importance of an adaptive control algorithm is also demonstrated.

Intermittent energy resources, including wind and solar power, continue to be rapidly added to the generation fleet domestically and abroad. The variable power of these resources introduces new levels of stochasticity into electric interconnections that must be continuously balanced in order to maintain system reliability. Energy storage systems (ESSs) offer one potential option to compensate for the intermittency of renewables. ESSs for long-term storage (1-hour or greater), aside from a few pumped hydroelectric installations, are not presently in widespread use in the U.S. The deployment of ESSs would be most likely driven by either the potential for a strong internal rate of return (IRR) on investment and through significant benefits to system reliability that independent system operators (ISOs) could incentivize.

To assess the potential of ESSs three objectives are addressed. (1) Evaluate the economic viability of energy storage for price arbitrage in real-time energy markets and determine system cost improvements for ESSs to become attractive investments. (2) Estimate the reliability impact of energy storage systems on the large-scale integration of intermittent generation. (3) Analyze the economic, environmental, and reliability tradeoffs associated with using energy storage in conjunction with stochastic generation.

First, using real-time energy market price data from seven markets across the U.S. and the physical parameters of fourteen ESS technologies, the maximum potential IRR of each technology from price arbitrage was evaluated in each market, along with the optimal ESS system size. Additionally, the reductions in capital cost needed to achieve a 10% IRR were estimated for each ESS. The results indicate that the profit-maximizing size of an ESS is primarily determined by its technological characteristics (round-trip charge/discharge efficiency and self-discharge) and not market price volatility, which instead increases IRR. This analysis demonstrates that few ESS technologies are likely to be implemented by investors alone.

Next, the effects of ESSs on system reliability are quantified. Using historic data for wind, solar, and conventional generation, a correlation-preserving, copula-transform model was implemented in conjunction with Markov chain Monte Carlo framework for estimating system reliability indices. Systems with significant wind and solar penetration (25% or greater), even with added energy storage capacity, resulted in considerable decreases in generation adequacy.

Lastly, rather than analyzing the reliability and costs in isolation of one another, system reliability, cost, and emissions were analyzed in 3-space to quantify and visualize the system tradeoffs. The modeling results implied that ESSs perform similarly to natural gas combined cycle (NGCC) systems with respect to generation adequacy and system cost, with the primary difference being that the generation adequacy improvements are less for ESSs than that of NGCC systems and the increase in LCOE is greater for ESSs than NGCC systems.

Although ESSs do not appear to offer greater benefits than NGCC systems for managing energy on time intervals of 1-hour or more, we conclude that future research into short-term power balancing applications of ESSs, in particular for frequency regulation, is necessary to understand the full potential of ESSs in modern electric interconnections.

This PhD dissertation presents three studies that individually aim to increase understanding of various aspects of solar and wind energy and the challenge of integrating them in the electricity power system. The first study explores the opportunity to improve the technical and economic performance of concentrated solar thermal power plants (CSPs) through joint installation and operation with fossil-fuel combined cycle power plants (CCPPs). The second study presents an analysis of the feasibility and technical, environmental, and economic effects of integrating large levels of distributed solar Photovoltaic (PV) into a rather inflexible (nuclear-heavy generating base) power system. The last study evaluates the potential of off-grid distributed generation (DG) technologies including solar PV and wind systems to provide a cost-effective solution to supply electricity to isolated loads in Saudi Arabia.

Several tools and models have been developed to accomplish these studies including a thermodynamic model developed in MATLAB environment to simulate ISCC plant operations, a PV production model that estimates hourly PV power output, a unit commitment and real-time economic dispatch (UC−ED) model that simulates hourly system operations and a mixed integer linear programing that determines the optimal off-grid energy mix and capacity.

Although two of three main studies presented do not focus specifically on Saudi Arabia, they provide valuable insights for a transition of its electricity sector towards less dependence on fossil fuels and increased use of renewables.

Saudi Arabia, the world’s largest oil producer, relies on fossil fuels as the primary energy source to meet its electricity needs. Its existing electricity generation fleet consists of a large number of old and inefficient gas combustion and steam turbines, several gas and oil-fired combined cycle power plants, and many diesel combustion engines located in non-interconnected areas. The recently launched governmental plan Saudi Arabia Vision 2030 intends to enhance the resiliency of the Saudi economy by diversifying the electricity generation portfolio through the inclusion of renewable and nuclear energy.

Pertinent to the Kingdom of Saudi Arabia (KSA), the first study shows that Integrated Solar Combined Cycle Power Plants (ISCCs) offer an opportunity to reduce fossil-fuel consumption while reducing the levelized cost of solar thermal energy (LCOE) by 35-40%. The third study shows that the in three non-interconnected regions of KSA, off-grid distributed generation including more than 300 MW of solar PV and wind energy is a cost-effective alternative assuming plausible scenarios for fuel prices and electricity demand. In addition, the results reveal that the local excellent solar resources and the high efficiency of the wind turbine technologies that could be installed make the LCOE of solar PV and wind lower than the LCOE of highly efficient oil-fired combined cycle power plants (CCPP) under moderate and high oil price scenarios.

Finally, the second study illustrates a potential barrier to the integration of a high share of distributed intermittent energy sources into a power network that operates large base-load thermal generation units and rather inflexible nuclear power plants.