Hybrid Fuel Cell Gas Turbine System Design and Optimization for Sofc

Fuel Cell–Gas Turbine (FC-GT) hybrid technology portends a significant breakthrough in electrical generation. Hybrid systems reach unprecedented high efficiencies, above 70% LHV in some instances, with little to no pollution, and great scalability. This work investigates two high temperature fuel cell types with potential for hybrid application ranging from distributed generation to central plant scales; sub MW to 100MW. A new library of dynamic model components was developed and used to conceptualize and test several hybrid cycle configurations. This paper outlines a methodology for optimal scaling of balance of plant components used in any particular hybrid system configuration to meet specified design conditions. The optimization strategy is constrained to meet component performance limitations and incorporates dynamic testing and controllability analysis. This study investigates seven different design parameters and confirms that systems requiring less cathode recirculation and producing a greater portion of the total power in the fuel cell achieve higher efficiencies. Design choices that develop operation of the fuel cell at higher voltages increase efficiency, often at the cost of lower power density and greater stack size and cost. This work finds existing SOFC technology can be integrated with existing gas turbine and steam turbine technology in a hybrid system approaching 75% fuel to electricity conversion efficiency in optimized FC-GT hybrid configurations. INTRODUCTION The United States faces an impending energy revolution. The current means by which our transportation, residential and industrial energy needs are met will not sustainably power our economy into the future. The evolving solution to our energy demands shifts due to rising fuel costs, environmental concerns, foreign oil dependence, and public policy. Three desirable features for future energy solutions are: diversity in primary energy sources and generation technology, improved efficiency in energy conversion and use, and optimally matching energy technologies and resources to specific uses. A new power generation technology that should represent a portion of any diverse energy portfolio is fuel cell-gas turbine hybrid, FC-GT, technology. FC-GT technology significantly improves power generation efficiency, and can use a variety of light hydrocarbon fuels [1]. Integration of fuel cell and gas turbine technologies into a single symbiotic system represents a breakthrough in electricity production technology. Gas turbine performance limitations result from the Carnot Limit and the entropy generated through combustion of fuel. Conversion of fuel to heat reduces the potential for useful work, and the Carnot principle limits the subsequent conversion of heat into mechanical energy. A fuel cell extracts work directly from the available Gibbs free energy. However, a fuel cell cannot produce power while fully utilizing the fuel and the wasted fuel hampers efficiency. A delicate balance of power and efficiency ensues without the implementation of a means to fully utilize the fuel in the anode off-gas. Hybrid FC-GT systems can effectively use fuel cell heat and anode off-gas fuel to produce electricity and compressor power. Molten carbonate and solid oxide fuel cells are two fuel cells types that are well-suited for hybridization with a gas 1 Copyright © 2010 by ASME Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 08/31/2014 Terms of Use: http://asme.org/terms turbine generator [2]. Both types operate at high temperatures, and thus produce high quality heat that is ideal for producing additional power in a turbine. These fuel cell hybrids have a range of applications from the distributed generation, submegawatt, and scale to large central power plants producing hundreds of megawatts to gigawatts. Accurate simulation of FC-GT behavior can be achieved only by a methodology meeting the following guidelines. • Physical and chemical behaviors of each component must be resolved from first principles with the exception of compressor and turbine components with well defined empirical maps. The fuel cell must calculate Nernst potential and loss parameters from governing equations of electrochemistry and can not be based on a single V-I curve. The V-I curve methodology is insufficient to capture the changes when temperature, oxidant concentration, or fuel utilization changes [3]. • Dimensional models are superior to bulk models for their ability to capture detailed spatial information and accurate temperature and concentration profiles. Bulk models are computationally efficient and useful for first approximations, but only estimate average heat transfer. Nodal fuel cell models capture local conditions for accurate calculation of Nernst potential and loss terms. • Careful consideration must be taken when approximating the heat transfer between nodes, as certain assumptions can have a large impact on temperature profiles and performance in a nodal model. Physical parameters including wall thickness and channel dimensions are critical in determining the convective and conductive surface areas between nodes. The principle means of heat mobility throughout the stack is conduction through the solid materials and must be determined as accurately as possible. Material properties specifications are important to approximate conduction as more conductive materials move heat quicker and reduce thermal gradients. Simulating a specific FC-GT hybrid for a design study, and simulating a generic FC-GT hybrid for dynamics and control studies require different parameter specifications. Designing a scalable model that is based on the use of several dimensionless parameters leads to robustness and versatility. Determining the turbomachinery size to meet requirements at optimal fuel cell conditions is a great starting point for sizing all balance of plant components and selecting the hybrid system operating point. SOFC stacks are characterized by several key parameters, notably average operating temperature, operating pressure, operating power density, fuel utilization, and maximum temperature rise across cell. This sufficiently constrains the balance of plant design; however, with the addition of cathode recirculation and bypass flows additional parameters can be varied to achieve optimal performance under a range of conditions. The current study developed the following methodology for sizing the balance of plant components. 1. Simulate the fuel cell and determine the inlet, outlet temperatures that satisfy temperature constraints over a range of operating conditions. Nomenclature A Area C Thermal Capacitance cp Constant Pressure Specific Heat cv Constant Volume Specific Heat Flow Turbomachinery Flow Rate h Enthalpy hc Convection Coefficient kc Conduction Coefficient m Mass M Mach Number N Normalized Turbomachinery Parameter Flow Rate P Pressure PR Pressure Ratio Q Sensible Enthalpy of Transfer Ions RPM Shaft Speed Ru Universal Gas Constant T Temperature V Velocity