Scott Samuelsen

Dynamic Simulation of an Integrated Solid Oxide Fuel Cell System Including Current-Based Fuel Flow Control

A two-dimensional dynamic model was created for a Siemens Westinghouse type tubular solid oxide fuel cell (SOFC). This SOFC model was integrated with simulation modules for other system components (e.g., reformer, combustion chamber, and dissipater) to comprise a system model that can simulate an integrated 25 kilowatt SOFC system located at the University of California, Irvine. A comparison of steady-state model results to data suggests that the integrated model can well predict actual system power performance to within 3 percent, and temperature to within 5 percent. In addition, the model predictions well characterize observed voltage and temperature transients that are representative of tubular SOFC system performance. The characteristic voltage transient due to changes in SOFC hydrogen concentration has a time scale that is shown to be on the order of seconds while the characteristic temperature transient is on the order of hours. Voltage transients due to hydrogen concentration change are investigated in detail. Particularly, the results reinforce the importance of maintaining fuel utilization during transient operation. The model is shown to be a useful tool for investigating the impacts of component response characteristics on overall system dynamic performance. Current-based flow control (CBFC), a control strategy of changing the fuel flow rate in proportion to the fuel cell current is tested and shown to be highly effective. The results further demonstrate the impact of fuel flow delay that may result from slow dynamic responses of control valves, and that such flow delays impose major limitations on the system transient response capability.Copyright

Experimental Study of a Model Gas Turbine Combustor Swirl Cup, Part 11: Droplet Dynamics

As the second part of a study to characterize the performance of a 3 x scale gas turbine combustor swirl cup, the focus of the present article addresses the droplet dynamics. Droplet axial, radial, and tangential velocities as well as size were measured using phase Doppler interferometry. Bimodal droplet axial and tangential velocities distributions were observed in the shear layer close to the exit plane of the swirl cup. Bimodal droplet radial velocity distributions around the closing point of the on-axis recirculation zone and at the periphery of the spray were observed as well. Droplet velocity histograms, based on droplet size classes, reveal how these velocity bimodal distributions are formed and explain why velocity fluctuations for larger droplets are greater than those of the smaller droplets in some regions. Correlations between size and velocity, individual velocity components, and size and flow angle provide evidence of intermittent flowfield structures superimposed on the local and global turbulence. Specific flowfield structures and flowfield intermittencies are found to give rise to these bimodal distributions. Overall, these data add measurably to the understanding of the two-phase transport that can occur in practical systems. 2 refs.

Gas and drop behavior in reacting and non-reacting air-blast atomizer sprays

A detailed study of the two-phase flow produced by a gas-turbine air-blast atomizer is performed with the goal of identifying the interaction between the two phases for both nonreacting and reacting conditions. A two-component phase Doppler interferometry is utilized to characterize three flowfields produced by the atomizer: (1) the single-phase flow, (2) the two-phase nonreacting spray, and (3) the two-phase reacting spray. Measurements of the mean and fluctuating axial and azimuthal velocities for each phase are obtained. In addition, the droplet size distribution, volume flux, and concentration are measured. The results reveal the strong influence of the dispersed phase on the gas, and the influence of reaction on both the gas and the droplet field. The presence of the spray significantly alters the inlet condition of the atomizer. With this alteration quantified, it is possible to deduce that the inertia associated with the dispersed phase damps the fluctuating velocities of the gas. Reaction reduces the volume flux of the droplets, broadens the local volume distribution of the droplets in the region of the reaction zone, increases the axial velocities and radial spread of the gas, and increases the anisotropy in the region of the reaction zone. 20 refs.

Experimental Study of a Model Gas Turbine Combustor Swirl Cup, Part I: Two-Phase Characterization

The behavior of droplets and the continuous phase (i.e., gas in the presence of the droplets) downstream of a 3 x model gas turbine combustor dome swirl cup is characterized via phase Doppler interferometry in the absence of reaction. The goal is to improve the understanding of droplet-gas interaction in a complex flow typical of that produced by engine hardware. Three components of continuous phase and droplet velocities were measured along with droplet size. The measurements reveal that (1) at the exit plane of the swirl cup, more uniform and finer droplets are produced relative to the atomizer alone, (2) both the continuous phase and droplets recirculate, (3) the region downstream of the swirl cup into which droplets join the recirculation is correlated with droplet size, and (4) significant slip velocities exist between the continuous phase and the droplets which are also correlated with droplet size and reflect a strong momentum exchange between the phases. 11 refs.

Design, Simulation and control of a 100 MW-class solid oxide fuel cell gas turbine hybrid system

A 100 MW-class planar solid oxide fuel cell, synchronous gas turbine hybrid system has been designed, modeled and controlled. The system is built of 70 functional fuel cell modules each containing 10 fuel cell stacks, a blower to recirculate depleted cathode air, a depleted fuel oxidizer and a cathode inlet air recuperator with bypass. The recuperator bypass serves to control the cathode inlet air temperature while the variable speed cathode blower recirculates air to control the cathode air inlet temperature. This allows for excellent fuel cell thermal management without independent control of the gas turbine, which at this scale will most likely be a synchronous generator. In concept the demonstrated modular design makes it possible to vary the number of cells controlled by each fuel valve, power electronics module, and recirculation blower, so that actuators can adjust to variations in the hundreds of thousands of fuel cells contained within the 100 megawatt hybrid system for improved control and reliability. In addition, the modular design makes it possible to take individual fuel cell modules offline for maintenance while the overall system continues to operate. Parametric steady state design analyses conducted on the system reveal that the overall fuel-to-electricity conversion efficiency of the current system increases with increased cathode exhaust recirculation. To evaluate and demonstrate the conceptualized design, the fully integrated system was modeled dynamically in Matlab–Simulink®. Simple proportional feedback with steady state feed-forward controls for power tracking, thermal management, and stable gas turbine operation were developed for the system. Simulations of the fully controlled system indicate that the system has a high efficiency over a large range of operating conditions, decent transient load following capability, fuel and ambient temperature disturbance rejection as well as the capability to operate with a varying number of fuel cell modules. The efforts here build upon prior work and combine the efforts of system design, system operation, component performance characterization and control to demonstrate hybrid transient capability in large-scale coal synthesis gas-based applications through simulation. Furthermore, the use of a modular fuel cell system design, the use of blower recirculation, and the need for integrated system controls are verified.Copyright

Evaluating options for Balancing the Water-Electricity Nexus in California: Part 1 - Securing Water Availability

The technical potential and effectiveness of different water supply options for securing water availability in a large-scale, interconnected water supply system under historical and climate-change augmented inflow and demand conditions were compared. Part 1 of the study focused on determining the scale of the options required to secure water availability and compared the effectiveness of different options. A spatially and temporally resolved model of Californias major surface reservoirs was developed, and its sensitivity to urban water conservation, desalination, and water reuse was examined. Potential capacities of the different options were determined. Under historical (baseline) hydrology conditions, many individual options were found to be capable of securing water availability alone. Under climate change augment conditions, a portfolio approach was necessary. The water savings from many individual options other than desalination were insufficient in the latter, however, relying on seawater desalination alone requires extreme capacity installations which have energy, brine disposal, management, and cost implications. The importance of identifying and utilizing points of leverage in the system for choosing where to deploy different options is also demonstrated.

Gas turbine assessment for air management of pressurized SOFC/GT hybrid systems

This paper analyzes and compares transient and steady-state performance characteristics of different types of single-shaft turbo-machinery for controlling the air through a pressurized solid oxide fuel cell (SOFC) stack that is integrated into a SOFC/GT pressurized hybrid system. Analyses are focused on the bottoming part of the cycle, where the gas turbine (GT) has the role of properly managing airfiow to the SOFC stack for various loads and at different ambient conditions. Analyses were accomplished using two disparate computer programs, which each modeled a similar SOFC/GT cycle using identical generic gas turbine performance maps. The models are shown to provide consistent results, and they are used to assess: (1) the influence of SOFC exhaust composition on expander behavior for on-design conditions, (2) the off-design performance of the bypass, bleed, and variable speed controls for various part-load conditions and for different ambient conditions; (3) the features of such controls during abrupt transients such as load trip and bypass/bleed valve failure. The results show that a variable speed micro-turbine is the best option for off-design operation of a SOFC/GT hybrid system. For safety measures a bleed valve provides adequate control of the system during load trip. General specifications for a radial GT engine for integration with a 550 kW pressurized SOFC stack are identified, which allow operation under a wide range of ambient conditions as well as several different cycle configurations. Copyright

Impact of Biodiesel on Fuel Preparation and Emissions for a Liquid Fired Gas Turbine Engine

This paper reports the fuel injection, vaporization, and emissions characteristics when running a 30 kW gas turbine engine on biodiesel (B99), and diesel fuel distillate # 2. Compositional analysis is used to assess the distillation of these fuels and a comparison is made with ethanol. The role of the liquid properties on fuel preparation and the subsequent engine performance is also assessed. The results show that while compositionally simple, biodiesel features some properties that result in inferior atomization and longer evaporation times compared to DF2. In addition, the overall spray behavior differs substantially in terms of density and width. The measured NO and CO emissions levels produced by the engine reveal significant increases with the use of biodiesel which is expected given the inferior fuel preparation characteristics. It appears reasonable to modify the fuel injector in order to overcome the deleterious effects observed for biodiesel. The benefits of blending biodiesel and ethanol in order to eliminate duel fuel operation during cold startup are discussed.Copyright

Hybrid Fuel Cell Gas Turbine System Design and Optimization

Ultrahigh efficiency, ultralow emission fuel cell gas turbine (FC/GT) hybrid technology represents a significant breakthrough in electric power generation. FC/GT hybrid designs are potentially fuel flexible, dynamically responsive, scalable, low-emission generators. The current work develops a library of dynamic component models and system design tools that are used to conceptualize and evaluate hybrid cycle configurations. The physical models developed for the design analysis are capable of off-design simulation, perturbation analysis, dispatch evaluation, and control development. A parametric variation of seven fundamental design parameters provides insights into design and development requirements of FC/GT hybrids. As the primary generator in most configurations, the FC design choices dominate the system performance, but the optimal design space may be substantially different from a stand-alone FC system. FC operating voltage, fuel utilization, and balance of plant component sizing has large impacts on cost, performance, and functionality. Analysis shows that hybridization of existing fuel cell and gas turbine technology can approach 75% fuel-to-electricity conversion efficiency. [DOI: 10.1115/1.4024569]

The Effect of Discrete Pilot Hydrogen Dopant Injection on the Lean Blowout Performance of a Model Gas Turbine Combustor

In view of increasingly stringent NOx emissions regulations on stationary gas turbines, lean combustion offers an attractive option to reduce reaction temperatures and thereby decrease NOx production. Under lean operation, however, the reaction is vulnerable to blowout. It is herein postulated that pilot hydrogen dopant injection, discretely located, can enhance the lean blowout performance without sacrificing overall performance. The present study addresses this hypothesis in a research combustor assembly, operated at atmospheric pressure, and fired on natural gas using rapid mixing injection, typical of commercial units. Five hydrogen injector scenarios are investigated. The results show that (1) pilot hydrogen dopant injection, discretely located, leads to improved lean blowout performance and (2) the location of discrete injection has a significant impact on the effectiveness of the doping strategy.Copyright