Browsing by Subject "Renewable Energy"

Now showing 1 - 17 of 17
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    Application of Power System Economic Metrics to a Microgrid with Storage and Renewables
    (2017-12) Hunt, Michelle
    Interest in multisource microgrids is becoming more prevalent in areas without access to grid power. Many of these areas are currently served by groups of diesel generators. However, the high capital cost of storage and renewable sources makes the initial investment in multisource microgrids frightening. This work is interested in examining the factors that can make a difference in whether or not these additional investments will reduce the total cost of ownership. To do so, this work uses the load and generation profiles of the MEHPS microgrid - a well-defined remote microgrid - and adds in the renewable resources of interest, solar, and storage in order to analyze the cost of the additions. This work takes multiple variables that could have an effect on the final total cost of ownership into consideration, including the price of fuel, price of PV panels, price of batteries, location of installation of the microgrid, cost of pollution, duration of installation, and cost of capital. All of these factors are considered in order to suggest what will make a difference to whether or not additional investments will reduce the total cost of ownership. The results of this work offer suggestions to those planning to deploy multisource microgrid systems in remote areas around the world.
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    Assembly and characterization of quantum-dot solar cells
    (2009-09) Leschkies, Kurtis Siegfried
    Environmentally clean renewable energy resources such as solar energy have gained significant attention due to a continual increase in worldwide energy demand. A variety of technologies have been developed to harness solar energy. For example, photovoltaic (or solar) cells based on silicon wafers can convert solar energy directly into electricity with high efficiency, however they are expensive to manufacture, and thus unattractive for widespread use. As the need for low-cost, solar-derived energy becomes more dire, strategies are underway to identify materials and photovoltaic device architectures that are inexpensive yet efficient compared to traditional silicon solar cells. Nanotechnology enables novel approaches to solar-to-electric energy conversion that may provide both high efficiencies and simpler manufacturing methods. For example, nanometer-size semiconductor crystallites, or semiconductor quantum dots (QDs), can be used as photoactive materials in solar cells to potentially achieve a maximum theoretical power conversion efficiency which exceeds that of current mainstay solar technology at a much lower cost. However, the novel concepts of quantum dot solar cells and their energy conversion designs are still very much in their infancy, as a general understanding of their assembly and operation is limited. This thesis introduces various innovative and novel solar cell architectures based on semiconductor QDs and provides a fundamental understanding of the operating principles that govern the performance of these solar cells. Such effort may lead to the advancement of current nanotechnology-based solar power technologies and perhaps new initiatives in nextgeneration solar energy conversion devices. We assemble QD-based solar cells by depositing photoactive QDs directly onto thin ZnO films or ZnO nanowires. In one scheme, we combine CdSe QDs and singlecrystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell (QDSSC). An array of ZnO nanowires was grown vertically from a fluorine-doped-tinoxide conducting substrate and decorated with an ensemble of CdSe QDs, capped with mercaptopropionic acid. When illuminated with visible light, the CdSe QDs absorb photons and inject electrons into the ZnO nanowires. The morphology of the nanowires then provided these photoinjected electrons with a direct and efficient electrical pathway to the photoanode. When using a liquid electrolyte as the hole transport medium, our quantum-dot-sensitized nanowire solar cells exhibited short-circuit current densities up to 2.1 mA/cm2 and open-circuit voltages between 0.6–0.65 V when illuminated with 100 mW/cm2 of simulated AM1.5 light. Our QDSSCs also demonstrated internal quantum efficiencies as high as 50–60%, comparable to those reported for dye-sensitized solar cells made using similar nanowires. We found that the overall power conversion efficiency of these QDSSCs is largely limited by the surface area of the nanowires available for QD adsorption. Unfortunately, the QDs used to make these devices corrode in the presence of the liquid electrolyte and QDSSC performance degrades after several hours. Consequently, further improvements on the efficiency and stability of these QDSSCs required development of an optimal hole transport medium and a transition away from the liquid electrolyte. Towards improving the reliability of semiconductor QDs in solar cells, we developed a new type of all-solid-based solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We found that the photovoltage obtained in these devices depends on QD size and increases linearly with the QD effective bandgap energy. Thus, these solar cells resemble traditional photovoltaic devices based on a semiconductor– semiconductor heterojunction but with the important difference that the bandgap energy of one of the semiconductors, and consequently the cell’s photovoltage, can be varied by changing the size of the QDs. Under simulated 100 mW/cm2 AM1.5 illumination, these QD-based solar cells exhibit short-circuit current densities as high as 15 mA/cm2 and open-circuit voltages up to 0.45 V, larger than that achieved with solar cells based on junctions between PbSe QDs and metal films. Moreover, we found that incident-photonto- current-conversion efficiency in these solar cells can be increased by replacing the ZnO films with a vertically-oriented array of single crystal ZnO nanowires, separated by distances comparable to the exciton diffusion length, and infiltrating this array with colloidal PbSe QDs. In this scheme, photogenerated excitons can encounter a donor– acceptor junction before they recombine. Thus, we were able to construct solar cells with thick QD absorber layers that were still capable of efficiently extracting charge despite short exciton or charge carrier diffusion lengths. When illuminated with the AM1.5 spectrum, these nanowire-based quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin film ZnO devices constructed with the same amount of QDs. Supporting experiments using field-effect transistors made from the PbSe QDs as well as the sensitivity of these transistors to nitrogen and oxygen gas show that the solar cells described above are unlikely to be operating like traditional p–n heterojunction solar cells. All data, including significant improvements in both photocurrent and power conversion efficiency with increasing nanowire length, suggest that these photovoltaic devices operate as excitonic solar cells.
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    City of Forest Lake Sustainability Action Plan
    (Minneapolis: Center for Urban and Regional Affairs, 2008) Breakiron-Aultman, Sara; Birkeland, Brant; Vardhan Das, Kirti; Flannerty, Sean; Meyer, Kate; Quinn, Julie
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    City of Prior Lake Sustainability Implementation Plan
    (Minneapolis: Center for Urban and Regional Affairs, 2009) Finis, Abby; Schaum, Jessica; Torres, Angela; Shrestha, Grishma; Cannon, John
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    Design and Control of Hydrostatic Continuous Variable Transmission for a Community Wind Turbine
    (2019-10) Mohanty, Biswaranjan
    This research aims to develop an efficient and reliable hydrostatic drive train (HST) for community wind turbines by implementing an advanced controller to maximize energy capture and stabilize the power grid. An HST is a continuously variable transmission (CVT) consisting of a hydraulic pump driving a variable displacement motor. HSTs are simple, light, cost-effective, and offer high power density. An HST drive train was designed for application in community wind turbines. To validate the performance of the HST, a novel power regenerative test platform was successfully designed, constructed and commissioned. It has two hydrostatic closed loops, coupled to each other. The research platform is capable of generating 100 kW output with only 55 kW of electrical input by taking advantage of power regeneration. The performance of each hydraulic component was measured on the test platform. The test platform is a multi-domain system, consisting of electrical, mechanical and hydraulic components. Using a bond graph-based method, a dynamic model of the system was developed. All possible pairings between inputs and outputs were studied in this multi input and multi output system to select the pair with the strongest coupling. A decentralized controller was later designed to control the rotor speed and pressure of the system. The start-up and shut down algorithms developed, enabled smooth operation of the testbed without a cavitation and pressure spikes. The performance of the HST was validated under various wind profile inputs. A pressure control strategy was developed to maximize power capture. To implement the above controller, one only needs measurement of the rotor speed for the reference command and pressure for tracking. The control law automatically drives the turbine to the optimum point, since the optimal parameters of the turbine are included in the control gain. A hardware-in-the-loop simulation was implemented to mimic the wind turbine environment. The turbine rotor dynamics are emulated on the test platform by implementing a decoupling controller. We examined the results of the HST drive train in transient and steady conditions. The efficiency of the HST wind turbine estimated from these experiments was comparable with that of a conventional turbine. Finally, the feasibility of connecting an HST wind turbine to the grid via a synchronous generator is studied. Furthermore, we investigated the electromechanical dynamics of the synchronous generator and performance of the system under large disturbances in incident wind, through detailed time-domain simulations. We found that the generator terminal voltage and frequency comply with the grid regulation band under all operating conditions. This strategy circumvents the need of expensive power electronics. This research have offered a novel insides of the HST for wind turbine applications. The outcomes of the project will stimulate industry to develop more efficient hydraulic components, system and control for wind applications and contribute to our green economy.
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    The dynamic relationship between firm capabilities, regulatory policy, and environmental performance: renewable energy policy and investment in the U.S. electric utility sector.
    (2009-09) Fremeth, Adam Ryan
    The choice by a firm to improve its environmental performance is a result of both characteristics that distinguish firms from one another and the public policy that compels such action. This dissertation examines how policy outcomes and subsequent firm responses are contingent upon the capabilities of the firms to respond to such policy. I present a theory of compliance specificity that ties firms and regulators together based on the heterogeneity of capabilities that firms hold with regards to a pending public policy of variable stringency. I test this theory within the context of the regulation and investment of renewable power in the U.S. electric utility sector and the growth of a utility scale renewable energy industry. I have identified that firms are able to shape the stringency of an environmental policy in the electric utility sector as the heterogeneity among firms can impact the potential costs that a regulator would face in the case that a policy is set that would leave firms out-of-compliance. Further, the choices that firms make with regards to their use of renewable power is conditioned on the contingent relationships of the capabilities that they possess and these same policies that they have influence over. My theoretical approach and empirical analyses provides a more sophisticated depiction of the interrelationships between firms and regulators within this industry context.
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    Enabling Distributed Renewable Energy and Chemical Production through Process Systems Engineering
    (2018-12) Allman, William
    New renewable energy technologies offer the promise of preserving a sustainable energy supply for future generations. Coupling chemical production with renewable energy production can help to address many of the challenges associated with the large-scale implementation of renewable energy, including short time scale variability of electricity production, potential mismatch of supply and demand, and energy stranded in areas of low population density. However, such co-production systems necessitate considering chemical production at scales smaller than what is typically seen in today's infrastructure due to the geospatially dispersed nature of renewable energy resources. This small scale production is economically prohibitive due to economies of scale, which promote building large scale facilities whenever possible. These economic challenges motivate the use of process systems engineering to develop decision making frameworks which minimize the costs of building and operating new renewable energy and chemical production systems. The application of process systems engineering methods to systems producing renewable energy and chemicals presented in this thesis centers around three major themes. First, decomposition, an approach which breaks down large optimization problems into smaller, easier to solve subproblems, is used to solve a challenging problem which finds the optimal design of a combined biorefinery and microgrid system. By doing so, hydrogen production is identified as a critical cost bottleneck in the combined system design. A method for automatically finding subproblems for decomposition for a broad class of optimization problems is also presented. Second, a framework is proposed for considering where new facilities should be built within an existing chemical supply chain. Here, policies and market conditions, such as a carbon tax, are identified that can have a strong effect on reducing emissions from ammonia production. Finally, the connection between the optimal design and operation of renewable energy and chemical systems is analyzed. Here, a framework which determines operating strategies which minimize cost is developed, and the optimal operation of a wind-powered ammonia system in different electricity market structures is analyzed. This framework is used to generate correlations between system design and operating cost which are embedded in a design optimization problem to improve solution efficiency.
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    Environmental Sustainability Policies and Resources
    (Minneapolis: Center for Urban and Regional Affairs, 2010) Shively, Emily
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    Finding the Chemistry in Biomass Pyrolysis: Millisecond Chemical Kinetics and Visualization
    (2016-06) Krumm, Christoph
    Biomass pyrolysis is a promising thermochemical method for producing fuels and chemicals from renewable sources. Development of a fundamental understanding of biomass pyrolysis chemistry is difficult due to the multi-scale and multi-phase nature of the process; biomass length scales span 11 orders of magnitude and pyrolysis phenomena include solid, liquid, and gas phase chemistry in addition to heat and mass transfer. These complexities have a significant effect on chemical product distributions and lead to variability between reactor technologies. A major challenge in the study of biomass pyrolysis is the development of kinetic models capable of describing hundreds of millisecond-scale reactions of biomass into lower molecular weight products. In this work, a novel technique for studying biomass pyrolysis provides the first- ever experimental determination of kinetics and rates of formation of the primary products from cellulose pyrolysis, providing insight into the millisecond-scale chemical reaction mechanisms. These findings highlight the importance of heat and mass transport limitations for cellulose pyrolysis chemistry and are used to identify the length scales at which transport limitations become relevant during pyrolysis. Through this technique, a transition is identified, known as the reactive melting point, between low and high temperature depolymerization. The transition between two mechanisms of cellulose decompositions unifies the mechanisms that govern low temperature char formation, intermediate pyrolysis conditions, and high temperature gas formation. The conditions under which biomass undergoes pyrolysis, including modes of heat transfer, have been shown to significantly affect the distribution of biorenewable chemical and fuel products. High-speed photography is used to observe the liftoff of initially crystalline cellulose particles when impinged on a heated surface, known as the Leidenfrost effect for room-temperature liquids. Order-of-magnitude changes in the lifetime of cellulose particles are observed as a result of changing modes in heat transfer as cellulose intermediate liquid droplets wet and de-wet polished ceramic surfaces. Introduction of surface macroporosity is shown to completely inhibit the cellulose Leidenfrost effect, providing avenues for surface modification and reactor design to control particle heat transfer in industrial pyrolysis applications. Cellulosic particles on surfaces consisting of microstructured, asymmetric ratchets were observed to spontaneously move orthogonal to ratchet wells above the cellulose reactive Leidenfrost temperature (>750 °C). Evaluation of the accelerating particles supported the mechanism of propelling viscous forces (50-200 nN) from rectified pyrolysis vapors, thus providing the first example of biomass conveyors with no moving parts driven by high temperature for biofuel reactors. Combined knowledge of pyrolysis chemistry, kinetics, and heat and mass transport effects direct the design of the next generation pyrolysis reactors for tuning bio- oil quality and design of improved catalytic upgrading technology.
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    Local and Non-local Geomorphic Effects of Hydrokinetic Turbines: Bridging Renewable Energy and River Morphodynamics
    (2019-06) Musa, Mirko
    Marine and Hydrokinetic (MHK) energy is an emerging renewable and sustainable technology which harnesses kinetic energy of natural water flows such as tides, rivers and ocean currents. In particular, rivers are currently an overlooked source of local and continuous kinetic energy that can be exploited using the available in-stream converters technology. The uncertainties regarding the interaction between these devices and the surrounding environment complicate the regulatory permitting processes, slowing down the expansion of MHK industry. A crucial issue that needs further attention is the interaction between these devices and the physical fluvial environment such as the bathymetry, sediment transport, and the associated morphodynamic processes. Analytical and experimental research conducted at Saint Anthony Falls Laboratory (SAFL) addressed this topic, unveiling the local and non-local (far from the device location) effects of hydrokinetic turbines on channel bathymetry and morphology. A theoretical model framework based on the phenomenology of turbulence was derived to predict the scour at the base of MHK device. Asymmetric installations of turbine array models within multi-scale laboratory channels were observed to trigger river instabilities known as forced-bars. Results suggest that the amplitude of these instabilities might be reduced by limiting the power plant lateral obstruction within the channel cross-section. A 12-turbine staggered array also proved to be resilient to intense flooding conditions, encouraging the expansion of this technology to large sandy rivers. Current research is investigating how hydrokinetic technology can be synergistically integrated in rivers, not only minimizing the environmental costs but also providing a positive feedback on the channel. Experiments suggest that turbines strategically installed in the river (i.e. at the side bank in yawed condition or in a vane-shaped array) could be used as stream bank protection systems and, eventually, be integrated in stream restoration projects.
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    Minutes: Senate Committee on Social Conerns: April 4, 2005
    (2005-04-04) University of Minnesota: Senate Committee on Social Concerns
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    Minutes: Senate Committee on Social Conerns: September 13, 2004
    (2004-09-13) University of Minnesota: Senate Committee on Social Concerns
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    On the Power Performance and Integration of Carbon-dioxide Plume Geothermal (CPG) Electrical Energy Production
    (2015-05) Adams, Benjamin
    CO2 Plume Geothermal (CPG) energy is a method for producing electricity from heat extracted from hot rock layers or reservoirs deep within the earth's crust. CPG is differentiated from other geothermal technologies by several factors: 1) CPG uses CO2 as the primary geologic working fluid instead of brine, 2) CPG utilizes naturally permeable porous reservoirs to extract heat, such as saline aquifers or depleted hydrocarbon reservoirs, 3) CPG is deep--a CPG reservoir must have a depth of 1 km to maintain CO2 in its supercritical state; though depths of 2 to 5 km are more common, and 4) CPG utilizes reservoirs at common geologic temperature gradients, unlike traditional hydrothermal which utilizes shallow reservoirs of unusually high temperature. Thus, CPG is intended to be integrated into an existing CO2 sequestration site affording an economic return on CO2 capture expenses by providing carbon-neutral, dispatchable electricity. Even when CPG is used as a base-load power source, it correlates well with electrical demand, unlike wind and solar (Chapter 5). Typically, CPG configurations consist of one or more injection and production wells. These wells link the surface plant with the porous reservoir to create a fluid circuit. Cooled fluid is injected at the surface, heated within the reservoir, and then returned to the surface at higher temperature and pressure which can then be used to create electricity. The variation in CO2 density between injection and production wells creates a thermosiphon which can drive circulation of CO2 without the use of pumps (Chapter 2). The geologic CO2 can be passed directly through a turbine, called a direct system, or heat can be extracted and used to power an Organic Rankine Cycle, called an indirect system. Either system may be used to generate electricity, although a direct system will nearly always produce more electricity than the indirect system. With reservoirs at moderate depth and temperature, these direct systems will also produce more electricity than comparable brine hydrothermal systems (Chapter 3). The reservoir well spacing and diameter affect the average power and longevity of a CPG system. For every combination of well diameter and reservoir depth, temperature, permeability, and thickness, an optimum spacing between the central injection well and a circumferential collection well will provide the greatest power output over time; placing the collection well too close to the injection well depletes the reservoir too quickly while spacing it too far away increases pressure losses, decreasing the overall power (Chapter 4). Likewise, the selection of too small a well diameter will limit mass flowrate, and thus power, while an oversized well diameter may quickly deplete the reservoir and provide no additional benefit (Chapters 3 & 4). This research has provided a significantly deeper understanding of CPG power systems and their operation. The impact of this work is to establish a basis of CPG research to be used in several ways. It can directly inform industrial developments, such as a green-field implementation of CPG or the long-term planning of a CPG-ready Carbon Capture and Storage site. This work may also be the basis for future economic or policy analyses that can further argue for the development of CPG. Thus, this work will help enable CPG as part of the 21st century energy portfolio.
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    Optimization of Sediment Microbial Fuel Cell for Power Generation
    (2015-08-28) Lehman, Jack
    Microbes are able to create an electric potential as they undergo oxidation and reduction reactions. The optimal configuration that a Sediment Microbial Fuel Cell may be arranged in is therefore desirable. Three Sediment Microbial Fuel Cells were built in order to determine the ideal configuration for power generation. The configurations all consisted of: sediment contained within a reactor at a depth of 6 cm, a carbon cloth anode buried 1.5 cm beneath the sediment and a medium consisting of water for an overall reactor depth of 17.7 cm. Configurations differed in that: one reactor utilized a floating cathode coated in an activated carbon, isopropyl and nafion solution on the side in contact with water and polyvinylidene fluoride (PVDF) on the surface in contact with air, while the remaining reactors consisted of submerged cathodes coated in the same activated carbon, isopropyl and nafion solution on each side. These two reactors had their cathodes located at different depths in order to monitor how factors such as internal resistance and dissolved oxygen levels would affect power output. After monitoring the three reactors for two months it was seen that the two reactors with submerged cathodes consistently generated more power than the reactor with a floating cathode; furthermore there was not a significant difference in the power generated by the reactors with submerged cathodes regardless of the distance of the cathode in relation to the anode. From the results seen in this study the superior configuration of a Sediment Microbial Fuel Cell is one in which the cathode is completely submerged in the medium, but the depth of the cathode in the medium makes no substantial difference to power generation.
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    The Performance of a Carbon-Dioxide Plume Geothermal Energy Storage System
    (2019-02) Fleming, Mark
    CO2-Plume Geothermal (CPG) is a system that can produce electricity from low-temperature heat from the subsurface of the earth, effectively combining geothermal energy and carbon capture and geologic storage; two technologies that have the potential to significantly reduce the amount of CO2 emitted into the atmosphere and limit the impacts of climate change. This system is different from other geothermal concepts as 1) the system uses CO2 as the heat extraction fluid in the subsurface reservoir, 2) the system does not rely shallow-natural hydrothermal locations or engineered (i.e. enhanced or fractured) reservoirs, instead using naturally permeably sedimentary basins, and 3) CPG systems utilize low-temperature resources which are currently undeveloped for geothermal energy. Therefore, CPG has significant potential to expand the geographic region where geothermal energy can operate, while providing an end used for captured CO2. This research demonstrates how the unique properties of the CPG system allow the system to be modified to operate as an energy storage system, which can increase the penetration of variable wind and solar resources on the grid, by using an additional shallow reservoir to separate the components that generate and consume power. To operate, the system generates power by extracting CO2 from the deeper-hotter reservoir and generates power in the turbine before the CO2 is slightly cooled and injected into the shallow reservoir, making use of the thermosiphon effect, where the thermal expansion of CO2 results in a density difference in each vertical well that can circulate CO2 without the need for pumps. To store power, the CO2 can be produced from the shallow reservoir, cooled and compressed, and then reinjected into the deep reservoir where it is heated. This research began by establishing the feasibility of the CPGES cycle for a single reservoir configuration and a mass flow rate near the optimum energy generation condition, demonstrating the effects of the intermittent injection and production of CO2 on the transient reservoir pressures and the power generated and consumed by the system over the first 10 years of operation (Chapter 2). The results demonstrated that the system was at a quasi-steady state condition at 10 years, and that the system could generate more energy to the grid than it consumed, providing both net energy generation of and energy storage. Using historical electrical price data, it was found that the CPGES system could use price arbitrage to be competitive with a CPG system, for the same geothermal heat extraction rate. Work was then expanded to illustrate how the CPGES system can operate over a range of time scales, with the cycle duration ranging from diurnal to seasonal (Chapter 3), and over a range of duty cycles (Chapter 5), demonstrating the versatility of this system. The CPGES system was compared to the CPG system for a range of geologic conditions, and it was determined that the trade-off of the flexible energy storage system was a reduction in the net energy generated per cycle (Chapter 4 & 5). However, these energy losses could be alleviated by operating the CPG and CPGES systems concurrently in the CPG+CPGES system. The addition of the second reservoir required for the energy storage operation increases the capital cost of the system, however, the increased cost of this flexible system could be alleviated by the value that the system adds to the grid as the amount of variable renewable energy increases (Chapter 5). Lastly, the effect of the co-production of water in solution with the CO2 is considered and found to increase the generation capacity of the CPG system, a result of the higher production temperature despite the reduced CO2 mass flow rate (Chapter 6). Overall, this research has demonstrated how the CPG system can be modified to operate as an energy storage system. The impact of this work is to establish the flexibility of the CPG technology and demonstrate that captured carbon can be used to increase the penetration of renewable energy technologies onto the grid, thereby further mitigating the emission of CO2 into the atmosphere. This will enable CPG to be integrated into future renewable energy portfolios.
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    Tidal Turbine Rotor Spacing Influence On Power Performance: Simulating A Scaled Dual-Rotor Axial Flow Turbine
    (2023) Guzman de la Rosa, Javier
    Power performance and turbulent wake characteristics of a scaled current-driven marine turbine were simulated using unsteady 3D RANS with the k-ω SST turbulence model and sliding mesh technique. The turbine is an axial flow, dual rotor tidal turbine with counter-rotating rotors, each with two blades and a diameter of d_T = 0.5 m, representing an approximately 1:40 scale system based on the U.S. Department of Energy’s Reference Model 1 (RM1) tidal turbine. Validation of numerical results for three tip speed ratios was performed by comparison with experimental data. The influence of rotor cross-stream spacing on power production was also studied by modeling three distinct lateral rotor separations, equal to 1.2d_T, 1.4d_T, and 1.6d_T. Numerical results showed a good correlation ranging within ±3.8% of turbine performance to experimental measurements for all tip-speed ratios studied, validating the numerical results for power estimation and demonstrating the advantages of this model when dealing with high-flow detachment. Inflow dynamics were captured well, exhibiting a difference of less than 5% compared to experimental data. However, wake dynamics showed a significant difference between numerical results and experimental data, ranging from 16% error at approximately 〖X/d〗_T=4, up to 170% error at 〖X/d〗_T=8. Finally, numerical results indicated a tendency for higher power production as the rotors are spaced farther apart, with the resulting power coefficient values of C_p = 0.449, 0.461, and 0483 for lateral rotor spacings of 1.2d_T, 1.4d_T, and 1.6d_T, respectively. This behavior was accredited to the reduction of the flow through the swept area of the rotors, causing what is known as 'choking effect’.
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    Trajectory-based Combustion Control Enabled by Free Piston Engine
    (2017-08) ZHANG, CHEN
    Abstract The free piston engine (FPE), considered as a promising alternative to the conventional internal combustion engine, has received more and more attention due to its great potential for efficiency improvement and emission reduction. Such a potential arises from its unique characteristic that the piston motion of the FPE is ultimately free due to the absence of the mechanical crankshaft. With the capability of employing variable piston trajectories, the FPE enables real-time control of the combustion chamber volume and therefore can adjust the in-cylinder gas pressure-temperature history and species concentration prior, during and after the combustion event. Enlightened by this capability, a new control method, namely piston trajectory-based combustion control, is proposed. The objective of this research is to investigate the feasibility and advantages of this advanced control method and realize the fuel-engine co-optimization in real-time. In order to achieve this objective, the entire research is separated into three phases. The first phase of the research focuses on the modeling and analysis of the trajectory-based combustion control in the FPE. A comprehensive model, representing the HCCI combustion process in the FPE along various piston trajectories, is developed, which includes the geometric structure of the FPE, the physics-based model of the FPE operation, and the detailed chemical kinetics of the utilized fuel. Extensive simulation results and the corresponding analysis clearly show that the FPE is able to adjust the entire combustion process by varying the volume of the combustion chamber and therefore altering the in-cylinder gas temperature and pressure traces to increase the indicated output work. In addition, the trajectory-based combustion control can also influence the chemical kinetics of the combustion via manipulating the in-cylinder temperature-pressure history. Specifically, unique asymmetric trajectories are designed that decreases the amount of NOx emission and increases the engine thermal efficiency simultaneously. At last, the analysis of the trajectory-based combustion control is also extended to multiple renewable fuels, e.g. hydrogen, biogas, syngas, ethanol, DME (dimethyl ether), biodiesel, and F-T (Fisher-Tropsch) fuels. It shows that an optimal asymmetric piston trajectory can be designed for each specific renewable fuel, which enables a significant reduction in the NOx emission and an improvement in the thermal efficiency simultaneously. In this way, the trajectory-based combustion control realizes the co-optimization of fuels and engine operation. The second phase of the research is aimed to develop a novel control-oriented model to realize the trajectory-based HCCI combustion control in practice. Intuitively, the comprehensive model from the first phase is not suitable for the control purpose, since the detailed reaction mechanisms usually generate heavy computational burdens. In order to reduce the computational burden and keep sufficient chemical kinetics information for HCCI combustion simulation, in the new control-oriented model, the engine cycle is separated into multiple phases and in each phase, a specific reaction mechanism with the minimal size is applied. With this unique phase separation method, the proposed control-oriented model not only shows a good agreement with the detailed physics-based model but also reduces the computational time significantly. In addition, such a good agreement is sustained at various working conditions, including different CRs, multiple AFRs and various piston motion patterns Ω. The last phase of this dissertation discusses systematic approaches to optimize piston trajectory for the trajectory-based HCCI combustion control. As claimed by the concept of trajectory-based combustion control, the derived optimal piston trajectory is considered as the optimal control signal to the FPE, which provides ultimate engine performance, in terms of maximal engine thermal efficiency and minimal emissions production. In this part, both offline and online optimizations are investigated. For the offline optimization, two approaches are proposed and implemented into the proposed control-oriented model: The first approach represents the piston trajectory as a function of parameter Ω and converts the original problem to a parameters optimization problem. Both optimal symmetric trajectories and asymmetric trajectories are derived at given CR. The advantages of this optimization approach lie on its much lighter computational burden; the second method transforms the trajectory optimization problem into a constrained nonlinear programming and then solves it via the SQP algorithm. By removing the constraints placed by piston motion patterns, this approach enlarges the candidate pool of various piston trajectories. Hence, the derived optimal trajectory further increases the engine output work and sustains the NOx emissions at the same level. For the online optimization, a searching process aimed to determine the optimal piston motion pattern Ω according to variable working conditions is developed. By using the proposed control-oriented model, the designed piston trajectory can be achieved within 0.4s under different working conditions, which enables real time optimal control of HCCI combustion phasing.