Fuel Cells
Hybrids
NETL, in partnership with private industries and others, is leading the development and demonstration of high efficiency solid oxide fuel cells (SOFCs) and fuel cell/turbine (FCT) hybrid power generation systems. These are promising systems offering possibly the only option for meeting the DOE’s efficiency goal for advanced coal based power systems of 60 percent (HHV) for fuel-to-electricity, with near zero emissions and competitive costs for multi-MW class central power plants in a 2020 time frame. Realizing this goal supports the Presidential initiatives of Clear Skies and Climate Change that aim to reduce CO2, NOX, and SO2 emissions. Furthermore, FCT hybrid technology will provide one of the most efficient power islands for the FutureGen Power Plant Project. The FutureGen project offers coal-based, zero-emissions electricity and hydrogen production and options for CO2 capture and sequestration.
Fuel Cell/Turbine Hybrid Systems
A fuel cell is a device that uses hydrogen (or hydrogen-rich fuel) and oxygen from air to create electricity by an electrochemical process without combustion. The absence of the combustion process eliminates the formation of pollutants including NOx, SOx, hydrocarbons and particulates, and significantly improves electrical power generation efficiency. Further efficiency gains are realizable by integration of a turbine with the fuel cell. Figure 1 shows t he FCT hybrid concept in simple form to provide some understanding of the synergy offered and the basic relationships of components.
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Figure 1: Direct FCT Hybrid
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In this direct operating mode, the fuel cell serves as the combustor for the gas turbine. Residual fuel in the already high temperature fuel cell exhaust mixes with the residual oxygen in an exothermic oxidation reaction to further raise the temperature. Both the fuel cell and the gas turbine generate electricity, and the gas turbine provides some balance-of plant functions for the fuel cell, such as supplying air under pressure and preheating the fuel and air in a heat exchanger called a recuperator. In the indirect mode, the recuperator transfers fuel cell exhaust energy to the compressed air supply, which in turn drives the turbine. The expanded air is supplied to the fuel cell. The indirect mode uncouples the turbine compressor pressure and the fuel cell operating pressure, which increases flexibility in turbine selection. Critical issues are the integration of pressure ratios and mass flows and the dynamic control through start-up, shutdown, emergency, and load-following operating scenarios.
Current Projects in Support of FCT Hybrids
Solid-State Energy Conversion Alliance (SECA)
The application of fuel cell systems and ultimately FCT hybrids is limited by the high cost of the fuel cell. To address the cost issue, the DOE is implementing the SECA program. The SECA program is dedicated to developing innovative, effective, low-cost ways to commercialize SOFCs. NETL is partnering with Pacific Northwest National Laboratory in developing new directions in advanced materials, processing and system integration research under the SECA initiative for the development and commercialization of modular, low cost, and fuel flexible 3- to 10-kWe SOFC systems by 2010.
DOE has estimated that a 5-kWe planar SOFC system can reach $400/kW at reasonable manufacturing rates. With this low-cost, the SOFC has the potential to move out of limited niche markets into widespread market applications. SECA developed technologies will provide the basis for the development of SECA-based SOFC FCT hybrid systems that achieve 60 percent electrical efficiency and near zero emissions when integrated in FutureGen power plants. Click here for more information on the SECA program and participants.
Solid Oxide Fuel Cell Hybrid System for Distributed Power
The Hybrid Power Generation Systems Division of General Electric is collaborating with NETL to develop SOFC/gas turbine hybrid systems for distributed power generation applications. The objectives for this project are to analyze and evaluate SOFC/gas turbine system concepts. Technical barriers in pressurization and scale-up of preliminary design concepts will be resolved for both the feasibility demonstration system and the conceptual system. A preliminary design for high-temperature heat exchangers for hybrid system applications has been developed, and pressurized operation of SECA-type planar SOFC stacks has been demonstrated. The SOFC is based on thin-film electrolyte technology fabricated with the tape calendaring method and thin-foil metallic interconnects leading to a low-cost, high-performance, compact planar SOFC. The gas turbine is based on commercial products. The proposed hybrid system has a potential for efficiency greater than 65 percent.
220-kWe Pressurized Hybrid Demonstration
A 220-kWe FCT hybrid demonstration was recently completed through a multi-year collaborative effort led by Southern California Edison in partnership with Siemens-Westinghouse Power Corporation (SWPC), NETL, and the University of California at Irvine’s National Fuel Cell Research Center. The project was the world’s first demonstration of a pressurized SOFC generator, and the world’s first demonstration of a SOFC coupled with a microturbine generator (MTG). The SOFC stack was contained in a pressure vessel and operated at 3 atmospheres (absolute) pressure and a temperature of 1000°C. The hot, high-pressure exhaust gas from the SOFC generator drove an Ingersoll-Rand 70-kWe MTG. The SOFC stack produced 170-kW DC and the MTG produced 20-kW net AC. The system accumulated more than 3,200 hours of run-time, while operating at a calculated net AC electrical efficiency of 53 percent. Pre-commercialization efforts by SWPC are being redirected to smaller sizes for combined heat and power applications.
300-kWe Atmospheric Hybrid Demonstration
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| Figure 2: FCE SubMW Hybrid
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DOE/NETL and FuelCell Energy (FCE) are working collaboratively to develop and demonstrate an atmospheric molten carbonate Direct FuelCell/Turbine (DFC/T) hybrid system. To date, the R&D efforts have resulted in significant progress in validating the DFC/T cycle concept. FCE has completed successful proof-of-concept testing of a DFC/T power plant based on a 250-kWe DFC integrated initially with a Capstone 30-kWe and then a 60-kWe modified MTG (see Figure 2). The subMW system tests have accumulated over 6,800hrs of successful operation with efficiency of 52 percent. This proof-of-concept demonstration has provided information for the continued design of a 40-MWe DFC/T power plant that is expected to approach 75 percent efficiency (LHV natural gas), as well as to serve as a platform for optimization of subMW class DFC/T hybrid systems. One of the significant challenges for this technology is the development of high temperature heat exchangers that offer differential pressure operation. Pre-commercial subMW alpha and beta units will be demonstrated over the next two years.
Hybrid Performance Simulation Facility
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Figure 3: The Hybrid Performance Project Facility |
Researchers at NETL have completed shakedown of an experimental facility capable of physically simulating the dynamic operation of a FCT hybrid system. The objective of the Hybrid Performance (Hyper) project at NETL is to conceptualize, simulate, analyze and demonstrate critical operability issues inherent in hybrid fuel cell systems. The hardware-in-the-loop simulation facility enables researchers to identify dynamic issues related to the interdependencies of fuel cell and turbine technology integration without risk to expensive fuel cell stacks. This is accomplished by operating a burner with a real-time control algorithm that mimics the expected dynamic behavior of an SOFC stack. In this manner, the remaining integration/control functions can be optimized before operating with a fuel cell. This approach will allow the development of validated models and control architectures that can avoid potential issues with load following and load shedding scenarios. The facility will ultimately accommodate a variety of fuel cell gas turbine configurations, but will initially focus on a direct-fired solid oxide fuel cell gas turbine configuration. A more detailed description of the facility has been presented previously. The facility is open to researchers for collaboration and is illustrated in Figure 3.
The planned experiments for the facility are being carried out in the following four phases: Phase One: Speed Control and System Characterization; Phase Two: Fuel Cell Simulation; Phase Three: Independent APU Speed Control and Load Following; and Phase Four: Integration of a Commercial Fuel Cell.
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