Browsing by Subject "Energy storage"

Now showing 1 - 7 of 7
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    Compression/expansion within a cylindrical chamber: Application of a liquid piston and various porous inserts
    (2013-08) Yan, Bo
    Efficiency of high pressure air compressors/expanders is critically important to the economic viability of Compressed Air Energy storage (CAES) systems, where air is compressed to a high pressure, stored and expanded to output work when needed. Any rise in internal energy of air during compression is wasted as the compressed air cools back to ambient temperature. Similarly, a drop in temperature of the air during the expansion process would reduce the work output. Therefore, the amount of heat transfer between air and surrounding heat sink/source surfaces determines the compression/expansion efficiency. Slowing down the compression/expansion process would give more time for heat transfer, thus increasing the efficiency. However, it reduces the power of the compressor/expander which is undesirable for a CAES system. A porous medium inside the compression chamber and a liquid piston is an ideal candidate for effectively increasing the heat transfer area and, consequently, thermal efficiency of the compression/expansion process, without sacrificing power.The present study focuses on experimentally testing and evaluating the effectiveness of various types of porous media, including two types of metal foam and two types of plastic interrupted plates of different pore size and porosities inside of a liquid-piston compression/expansion chamber. The liquid piston compression system is a cylindrical cavity first filled with air. As water is pumped into the bottom of the cavity, the air inside is compressed. Flow meters and pressure transducers are used to measure the volume and pressure changes during compression. Porous inserts of various designs are placed inside the chamber to reduce the rise in temperature as the air is compressed and to reduce the temperature drop as the air is expanded. Compression and expansion efficiencies are investigated with and without the porous inserts. For the compression experiments, all the experiments are conducted with constant volume trajectories. However, in expansion experiments, the volume trajectories are determined by constant orifice opening area. The study shows that compression efficiency is increased from 77% without a porous insert to 94% with the best-performing, 40ppi metal foam insert at a compression ratio of 10 and compression time of 2s. Due to the significant amount of water trapping inside the metal foam, in the expansion tests, only interrupted plates are tested. The expansion efficiency is increased from 80% to 90% with the 2.5 mm characteristic size interrupted plate insert for expansion process at expansion ratio of 6 and expansion time of 2s. Normalized pressure volume trajectories and dimensionless temperature profiles are calculated and compared for different types of inserts. It is concluded that adding a porous insert into the air space of a liquid piston compressor/expander is an effective means of boosting heat transfer rate and increase compression/expansion efficiency. It is recommended that future work is needed to optimize the pore size and layout of the porous insert and to couple other heat transfer augmentation schemes, including spray cooling trajectory optimization and control. Meanwhile, in order to investigate the compression/expansion process at a higher pressure and wider pressure ratios, a high compression/expansion setup is designed and being fabricated in order to have a better control the pressure-volume trajectory, and improve ease of operation. Detailed design requirements, specifications, system schematic, 3-D models and drawings are presented and discussed.
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    Increasing efficiency and power density of a liquid piston air compressor / expander with porous media heat transfer elements
    (2014-12) Wieberdink, Jacob Henry
    In this thesis, a power dense and efficient air compressor/ expander is investigated experimentally. High power density and high efficiency are realized with a quasi-isothermal process, made possible by a liquid piston compressor/ expander and the addition of porous media heat transfer elements. Uniform and non-uniform distributions of porous media are considered and compared with a baseline case.Experiments are conducted using a 2.2 L displacement compressor/ expander. Air is compressed from 7 bar to 210 bar in compression tests and expanded from 210 bar to 7 bar in expansion tests. Baseline compression times vary from 2s to 400s and compression power density varies from 4 kW/m3 to 600 kW/m3. Baseline expansion times vary from 1s to 400s and expansion power density varies from 4 kW/m3 to 2 MW/m3. The baseline compression experiments covered. This study finds that as power density increases, efficiency decreases. At 90% efficiency, a moderate amount of porous media (uniform distribution of 76% porosity) improves compression power density by a factor of 10 and expansion efficiency by a factor of 17. Further improvements are possible with an optimized porous medium geometry.These results have implications for many applications where efficient gas compression/expansion is required including: compressed air for energy storage at scales that range from residential-scale to grid-scale, pneumatics, compressed industrial gasses, and compressed gaseous fuels like hydrogen and natural gas. Quasi-isothermal compression and expansion also enables the realization of thermodynamic cycles that require isothermal compression or expansion.
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    Market Rules in Transition: Energy Storage Value and the U.S. Electric Grid
    (2019-05) Forsberg, Lindsey
    A comparative analysis of proposals filed by six U.S. RTO/ISOs in order to comply with the requirements of FERC Order 841. This work examines the participation models proposed for energy storage resources, and includes a case study of each RTO/ISOs treatment of state of charge management. The proposals are analyzed in the context of private and social value optimization, with the concept of "value stacking" at the forefront of the analysis.
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    Microscopic theory of supercapacitors.
    (2011-08) Skinner, Brian
    As new energy technologies are designed and implemented, there is a rising demand for improved energy storage devices. At present the most promising class of these devices is the electric double-layer capacitor (EDLC), also known as the supercapacitor. A number of recently created supercapacitors have been shown to produce remarkably large capacitance, but the microscopic mechanisms that underlie their operation remain largely mysterious. In this thesis we present an analytical, microscopic-level theory of supercapacitors, and we explain how such large capacitance can result. Specifically, we focus on four types of devices that have been shown to produce large capacitance. The first is a capacitor composed of a clean, low-temperature two-dimensional electron gas adjacent to a metal gate electrode. Recent experiments have shown that such a device can produce capacitance as much as 40% larger than that of a conventional plane capacitor. We show that this enhanced capacitance can be understood as the result of positional correlations between electrons and screening by the gate electrode in the form of image charges. Thus, the enhancement of the capacitance can be understood primarily as a classical, electrostatic phenomenon. Accounting for the quantum mechanical properties of the electron gas provides corrections to the classical theory, and these are discussed. We also present a detailed numerical calculation of the capacitance of the system based on a calculation of the system's ground state energy using the variational principle. The variational technique that we develop is broadly applicable, and we use it here to make an accurate comparison to experiment and to discuss quantitatively the behavior of the electrons' correlation function. The second device discussed in this thesis is a simple EDLC composed of an ionic liquid between two metal electrodes. We adopt a simple description of the ionic liquid and show that for realistic parameter values the capacitance can be as much as three times larger than that of a plane capacitor with thickness equal to the ion diameter. As in the previous system, this large capacitance is the result of image charge formation in the metal electrode and positional correlations between discrete ions that comprise the electric double-layer. We show that the maximum capacitance scales with the temperature to the power -1/3, and that at moderately large voltage the capacitance also decays as the inverse one third power of voltage. These results are confirmed by a Monte Carlo simulation. The third type of device we consider is that of a porous supercapacitor, where the electrode is made from a conducting material with a dense arrangement of narrow, planar pores into which ionic liquid can enter when a voltage is applied. In this case we show that when the electrode is metallic the narrow pores aggressively screen the interaction between neighboring ions in a pore, leading to an interaction energy between ions that decays exponentially. This exponential interaction between ions allows the capacitance to be nearly an order of magnitude larger than what is predicted by mean-field theories. This result is confirmed by a Monte Carlo simulation. We also present a theory for the capacitance when the electrode is not a perfect metal, but has a finite electronic screening radius. When this screening radius is larger than the distance between pores, ions begin to interact across multiple pores and the capacitance is determined by the Yukawa-like interaction of a three-dimensional, correlated arrangement of ions. Finally, we consider the case of supercapacitor electrodes made from a stack of graphene sheets with randomly-inserted "spacer" molecules. For such devices, experiments have produced very large capacitance despite the small density of states of the electrode material, which would seem to imply poor screening of the ionic charge. We show that these large capacitance values can be understood as the result of collective entrance of ions into the graphene stack (GS) and the renormalization of the ionic charge produced by nonlinear screening. The collective behavior of ions results from the strong elastic energy associated with intercalated ions deforming the GS, which creates an effective attraction between them. The result is the formation of "disks" of charge that enter the electrode collectively and have their charge renormalized by the strong, nonlinear screening of the surrounding graphene layers. This renormalization leads to a capacitance that at small voltages increases linearly with voltage and is enhanced over mean-field predictions by a large factor proportional to the number of ions within the disk to the power 9/4. At large voltages, the capacitance is dictated by the physics of graphite intercalation compounds and is proportional to the voltage raised to the power -4/5. We also examine theoretically the case where the effective fine structure constant of the GS is a small parameter, and we uncover a wealth of scaling regimes.
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    Pumped Hydro Energy Storage (PHES) Using Abandoned Mine Pits on the Mesabi Iron Range of Minnesota – Final Report
    (University of Minnesota Duluth, 2011) Fosnacht, Donald R
    This project focuses on developing an energy storage capability within Minnesota that will enable a larger build‐out of variable renewable generation sources. Currently, a significant challenge associated with the predominant renewable resource in our region (wind) is the variable and off‐peak nature of the energy generated. This feature of some renewable generation systems can, unfortunately, cause: (1) the need to build new fossil fuel generating facilities; (2) operation of existing fossil fuel generating facilities at inefficient levels; (3) transmission grid instability and unreliability; and (4) higher electricity rates. Energy storage is key to overcoming these problems. Currently, the only viable means of storing energy on a large scale are through pumped hydro energy storage (PHES), compressed air storage systems or liquid sodium sulfide battery systems. Fortunately, Minnesota has a unique and largely untapped resource for PHES in the form of idled taconite mines on the Mesabi Iron Range. The goal of this research project was to determine the potential viability, environmental sustainability and societal benefits of PHES as a vital, enabling technology for wind turbine‐based power generation. The intent of this research is to provide a clear roadmap for PHES development in Minnesota. The project is multifaceted and draws resources across the University System and from key industrial partners: Great River Energy and Minnesota Power. The results from the project provide vital information to decision makers on the potential of PHES and give guidance on how the technology can be implemented using the unique assets of the Minnesota Iron Ranges so that renewable mandates and green house gas reduction can be effectively accomplished. The results show that the topography and water resources exist at various sites that could allow a 100 to 200 MW facility to be constructed if the overall economic, mineral rights, and environmental issues associated with a given site can be properly managed. The report delves into the possibilities and outlines selection criteria that can be used for site selection. Other information is developed to compare the potential economic impact of implementation of the project within the constraints of the factors that can be monetized using the current policy environment. Finally, potential life cycle, regulatory, environmental, and permitting issues that are associated with implementation of the concept are discussed.
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    Renewable ammonia for sustainable agriculture and energy: Process, system, and enterprise optimization
    (2021-08) Palys, Matthew
    Synthetic ammonia (NH3) used as fertilizer is essential for modern agriculture, but its production at present is fossil energy and emissions intensive. A more sustainable NH3 production alternative is to use renewable-derived electricity to obtain its precursors, specifically hydrogen (H2) from water electrolysis and nitrogen (N2) separated from air. The transformative impact of renewable NH3 is not limited to agriculture alone. Energy storage costs using NH3 are considerably lower than with H2or batteries, making it an ideal candidate for the high capacity, long duration seasonal energy storage necessitated by high fractions of renewables in the power generation mix. Internal combustion technologies which are well-developed for fossil fuel feedstocks can be easily modified to be fueled with NH3, allowing its use for controllable power generation or as a carbon neutral liquid fuel. Despite its promise, a number of challenges remain in realizing the full potential of this alternative paradigm. This thesis aims to collectively address some of these challenges through the use of mathematical optimization. The economics of small-scale renewable NH3 production for agriculture are analyzed and optimized at both the synthesis process and supply chain level. A flowsheet model is developed for optimal design and technoeconomic analysis of an absorbent-enhanced NH3 synthesis process which can reduce pressure and increase separation temperature, the main drivers of capital cost in traditional condenser-based synthesis process. Absorbent-enhanced process design optimization gives 30% lower capital costs and comparable energy efficiencies to the condenser-based process at production scales smaller than conventional by one to two orders of magnitude. Optimal deployment of this absorbent-enhanced process via wind-powered NH3 production modules in fertilizer supply chains makes renewable NH3 economically viable at approximately 25% lower conventional NH3 prices than if the traditional synthesis process is simply scaled down. The economic competitiveness of synergistic renewable NH3 production and utilization systems is maximized through combined optimal design and scheduling (CODS). These CODS models select and size the best technologies for given applications while simultaneously scheduling their operation to accommodate renewable intermittency. Performing CODS for wind-powered production of NH3 for use as fertilizer, agricultural fuel, and energy storage enabled 95% emissions reduction at a cost less than $20/tonCO2. Then, the optimal economics of H2- and NH3-based electrical energy storage were investigated for 15 locations throughout the continental U.S. which comprehensively represent its different climate-demand regions. Lowest cost systems in every location included both H2 and NH3 storage pathways and optimized the trade-off between H2's higher overall efficiency and NH3's lower storage cost. This hybrid energy storage concept was extended to combined heat and power systems in remote locations as a potential market for early adoption. Low cost, long term NH3 storage and subsequent power and heat cogeneration enabled fully renewable systems to be economically competitive with those that could purchase power and heat from conventional sources. Overall, the results of this thesis demonstrate the promise of renewable NH3 and the power of mathematical optimization in achieving its full potential.
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    Two-dimensional clay and graphene nanosheets for polymer nanocomposites and energy storage applications
    (2013-08) Qian, Yuqiang
    Clay and graphene nanosheets are attractive to materials scientists due to their unique structural and physical properties and potentially low cost. This thesis focuses on the surface modification and structure design of clay and graphene nanosheets, targeting special requirements in polymer nanocomposites and energy storage applications. The high aspect ratio and stiffness of clay and graphene nanosheets make them promising candidates to reinforce polymers. However, it is challenging to achieve a good dispersion of the nanosheets in a polymer matrix. It is demonstrated in this study that organic modifications of clay and graphene nanosheets lead to better filler dispersion in polymer matrices. A prepolymer route was developed to achieve clay exfoliation in a polyurethane-vermiculite system. However, the phase-separated structure of the polyurethane matrix was disrupted. Intragallery catalysis was adopted to promote the clay exfoliation during polymerization. With both catalytic and reactive groups on the clay modifier, the polyurethane-vermiculite nanocomposites showed a significant increase in modulus and improved barrier performance, compared to neat polyurethane. The toughening effect of graphene on thermosetting epoxies and unsaturated polyesters (UPs) was also investigated. Various types of graphene with different structures and surface functionalities were incorporated into the thermosetting resin by in situ polymerization. The toughening effect was observed for epoxy nanocomposites at loading levels of less than 0.1 wt%, and a peak of fracture toughness was observed at 0.02 or 0.04 wt% of graphene loadings for all epoxy-graphene systems. A microcrack-crazing mechanism was proposed to explain the fracture behavior of epoxy-graphene systems based on fractography observations. Similar peak behavior of fracture toughness was not observed in UP system. UP nanocomposites with modified graphene oxide showed better mechanical performance than those with unmodified graphene oxide, which was attributed to better graphene dispersion and a stronger UP-graphene interface. Graphene has also been extensively studied in energy storage applications, due to its high conductivity and surface area. In order to utilize the benefits of graphene, macroscopic graphene/V2O5 films and graphene aerogels were fabricated from the self-assembly of graphene materials. The unique 2D structure of graphene helped to maintain the integrated film morphology in graphene/V2O5 composites and the monolithic macroporous structure in graphene aerogels. Good conductivity was obtained by incorporation of graphene sheets in the structure, which results in good electrochemical performance as electrode materials for batteries or supercapacitors. The facile preparation methods allow good control of the composition and thus the properties of the macroscopic graphene nanostructures.