Browsing by Subject "Availability"
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Item Comparison of the efficiency of a thermo-chemical process to that of a fuel cell process when both involve the same chemical reaction(2009-05-15) Bulusu, Seshu PeriahThis work assesses if a plausible theoretical thermo-chemical scheme can be conceived of, that is capable of extracting work from chemical reactants which can be compared with work produced by a fuel cell, when both processes are supplied with the same reactants. A theoretical process is developed to convert heat liberated from a chemical reaction to work. The hypothetical process is carried over a series of isothermal chemical reactor - heat engine combinations. Conducting the chemical reaction and work extraction over a series of temperature steps minimizes irreversibilities that result from the chemical reaction and heat transfer. Results obtained from the numerical calculations on the scheme confirm that when a large number of reactors-engine combinations are used, irreversibility of the proposed hypothetical reactor-engine combination can be reduced to zero. It is concluded from the results, that the theoretical model is as efficient as a fuel cell when both have the same chemical reaction under identical conditions. The effect of inert gas chemistry on the process has also been observed. It is determined from the results that the chemistry of the inert gas does not affect the proposed process. It is determined from results of a parametric study on the composition of inert gas, that the reduction of inert gas does not significantly improve the efficiency of the proposed process.Item Effects of EGR, water/N2/CO2 injection and oxygen enrichment on the availability destroyed due to combustion for a range of conditions and fuels(2009-06-02) Sivadas, Hari ShankerThis study was directed at examining the effects of exhaust gas recirculation (EGR), water/N2/CO2 injections and oxygen enrichment on availability destroyed because of combustion in simple systems like those of constant pressure and constant volume. Higher cooled EGR fractions lead to higher availability destruction for reactant temperatures less than 2000 K. The availability destroyed for 40% EGR at 300 K for constant pressure and constant volume combustion was 36% and 33%, respectively. Neglecting the chemical availability in the products, the equivalence ratio and reactant temperature that corresponded to the lowest availability destruction varied from 0.8 to 1.0 and 800 K to 1300 K, respectively, depending on the EGR fraction. The fraction of the reactant availability destroyed increased with the complexity of the fuel. The trends stayed the same for the different EGR fractions for the eight fuels that were analyzed. Higher injected water fractions lead to higher availability destruction for reactant temperatures less than 1000 K. The availability destroyed for a 40% injected water fraction at 300 K for constant pressure combustion was 36%. The product temperature ranged from 2300 K to 450 K at a reactant temperature of 300 K for injected fractions from 0% to 90%. For a 40% injected fraction at a reactant temperature of 300 K, water injection and cooled EGR resulted in the greatest destruction of availability (about 36%) with CO2 injection leading to the least destruction (about 32%). Constant volume combustion destroyed less availability compared to constant pressure combustion at a reactant pressure of 50 kPa. At a higher reactant pressure of 5000 kPa, constant pressure combustion destroyed less availability compared to constant volume combustion for reactant temperatures past 1000 K. Higher fractions of oxygen in the inlet lead to higher product temperatures that lead to lower availability destruction. For 40% oxygen in inlet, the product temperature increased to 2900 K and the availability destroyed dropped to 25% at a reactant temperature of 300 K for constant pressure combustion.Item Energy storage sizing for improved power supply availability during extreme events of a microgrid with renewable energy sources(2012-08) Song, Junseok; Kwasinski, Alexis; Grady, William M.; Driga, Mircea; Hallock, Gary; Son, Yoo SeokA new Markov chain based energy storage model to evaluate the power supply availability of microgrids with renewable energy generation for critical loads is proposed. Since critical loads require above-average availability to ensure reliable operation during extreme events, e.g., natural disasters, using renewable energy generation has been considered to diversify sources. However, the low availability and high variability of renewable energy sources bring a challenge in achieving the required availability for critical loads. Hence, adding energy storage systems to renewable energy generation becomes vital for ensuring the generation of enough power during natural disasters. Although adding energy storage systems would instantaneously increase power supply availability, there is another critical aspect that should be carefully considered; energy storage sizing to meet certain availability must be taken into account in order to avoid oversizing or undersizing capacity, which are two undesirable conditions leading to inadequate availability or increased system cost, respectively. This dissertation proposes to develop a power supply availability framework for renewable energy generation in a given location and to suggest the optimal size of energy storage for the required availability to power critical loads. In particular, a new Markov chain based energy storage model is presented in order to model energy states in energy storage system, which provides an understanding of the nature of charge and discharge rates for energy storage that affect the system's power output. Practical applications of the model are exemplified using electrical vehicles with photovoltaic roofs. Moreover, the minimal cut sets method is used to analyze the effects of microgrid architectures on availability characteristics of the microgrid power supply in the presence of renewable energy sources and energy storage. In addition, design considerations for energy storage power electronics interfaces and a comparison of various energy storage methods are also presented.Item Microgrid availability during natural disasters(2014-08) Krishnamurthy, Vaidyanathan; Kwasinski, AlexisA common issue with the power grid during natural disasters is low availability. Many critical applications that are required during and after natural disasters, for rescue and logistical operations require highly available power supplies. Microgrids with distributed generation resources along with the grid provide promising solutions in order to improve the availability of power supply during natural disasters. However, distributed generators (DGs) such as diesel gensets depend on lifelines such as transportation networks whose behavior during disasters affects the genset fuel delivery systems and as a result affect the availability. Renewable sources depend on natural phenomena that have both deterministic as well as stochastic aspects to their behavior, which usually results in high variability in the output. Therefore DGs require energy storage in order to make them dispatchable sources. The microgrids availability depends on the availability characteristics of its distributed generators and energy storage and their dependent infrastructure, the distribution architecture and the power electronic interfaces. This dissertation presents models to evaluate the availability of power supply from the various distributed energy resources of a microgrid during natural disasters. The stochastic behavior of the distributed generators, storage and interfaces are modeled using Markov processes and the effect of the distribution network on availability is also considered. The presented models supported by empirical data can be hence used for microgrid planning.Item RAS enhancements for RDMA communications(2010-12) Cardona, Omar; Nettles, Scott M.; Bard, WilliamEthernet as the communication medium in the enterprise data center has outlived all competing mediums and resisted the test of time with regards to speed and costs. The future is also poised for growth with 40 and 100Gps speeds just over horizon. The current state of the technology is being enhanced and extended with lossless features to allow for fabric convergence of Storage and Inter Process Communication (IPC) Networks. It is under this medium that an increase in the adoption of Remote Direct Memory Access (RDMA) over Ethernet using offloaded TCP/IP (iWARP) and Infiniband over Ethernet (RoCE) communication stacks to RDMA capable NIC adapter s (RNIC) is observed. RDMA enables direct application to application communication over the network resulting in numerous and significant benefits such as reduced CPU utilization, lower latency communications, increased energy efficiency, and reduced overall system requirements. However, with said benefits also comes increased software complexity in how RDMA interface users communicate. The RDMA communication semantics, which originate from the HPC domain, are heavily biased towards Low-Latency and High-Bandwidth communications rather than Reliability, Availability, and Serviceability (RAS). As adoption increases, and enterprise data centers begins to leverage RDMA over Ethernet, enhancements to the OS stack software architecture and design of the components involved is required to address these deficiencies. Operating system interfaces, device drivers, adapter hardware design, and embedded firmware features must be viewed from a high-availability and maintainability point of view. RAS enhancements for RDMA communications proposes the software architectural tradeoffs for enhancing the iWARP and RoCE RDMA implementations for communications in the enterprise data center, with new and traditional RAS features for existing communications stacks and devices. The architecture leverages software enhancements in traceability, availability, maintainability, serviceability, fault-isolation and resource management; such that in the advent of errors, the probability that the forensics data points to identify root cause are immediately and automatically available is increased.Item Utilizing a cycle simulation to examine the use of exhaust gas recirculation (EGR) for a spark-ignition engine: including the second law of thermodynamics(Texas A&M University, 2008-10-10) Shyani, Rajeshkumar GhanshyambhaiThe exhaust gas recirculation (EGR) system has been widely used to reduce nitrogen oxide (NOx) emission, improve fuel economy and suppress knock by using the characteristics of charge dilution. However, previous studies have shown that as the EGR rate at a given engine operating condition increases, the combustion instability increases. The combustion instability increases cyclic variations resulting in the deterioration of engine performance and increasing hydrocarbon emissions. Therefore, the optimum EGR rate should be carefully determined in order to obtain the better engine performance and emissions. A thermodynamic cycle simulation of the four-stroke spark-ignition engine was used to determine the effects of EGR on engine performance, emission characteristics and second law parameters, considering combustion instability issues as EGR level increases. A parameter, called 'Fuel Fraction Burned,' was introduced as a function of the EGR percentage and used in the simulation to incorporate the combustion instability effects. A comprehensive parametric investigation was conducted to examine the effects of variations in EGR, load and speed for a 5.7 liter spark-ignition automotive engine. Variations in the thermal efficiencies, brake specific NOx emissions, average combustion temperature, mean exhaust temperature, maximum temperature and relative heat transfer as functions of exhaust gas recycle were determined for both cooled and adiabatic EGR configurations. Also effects of variations in the load and speed on thermal efficiencies, relative heat transfers and destruction of availability due to combustion were determined for 0% EGR and 20% EGR cases with both cooled and adiabatic configurations. For both EGR configurations, thermal efficiencies first increase, reach a maximum at about 16% EGR and then decrease as the EGR level increases. Thermal efficiencies are slightly higher for cooled EGR configuration than that for adiabatic configuration. Concentration of nitric oxide emissions decreases from about 2950 ppm to 200 ppm as EGR level increases from 0% to 20% for cooled EGR configuration. The cooled EGR configuration results in lower nitric oxide emissions relative to the adiabatic EGR configuration. Also second law parameters show the expected trends as functions of EGR. Brake thermal efficiency is higher for the 20% EGR case than that for the no EGR case over the range of load (0 to WOT) and speed (600 rpm to 6000 rpm). Predictions made from the simulation were compared with some of the available experimental results. Predicted thermal efficiencies showed a similar trend when compared to the available experimental data. Also, percentage of unused fuel availability increases as the EGR level increases, and it can be seen as one of the effects of deteriorating combustion quality as the EGR level increases.