Understanding Fuel Cells and the Solid Oxide Advantage
"New Developments in Creating Cost-Effective Solid Oxide Fuel Cells"
GRID Magazine
September 1997
New Developments in Creating Cost-Effective Solid Oxide Fuel Cells
Affordable fuel cells are far less speculative as Phase II of a GRI solid oxide fuel cell (SOFC) research program gets under way. The goals of Phase II are to establish the SOFC potential for long life and reliability, reduced size and weight, and most importantly, low cost.
The Basic Research program is targeting distributed and on-site power generation applications in populated as well as remote areas that require one or more of a fuel cell's key advantages-pollution-free operation, high efficiency, quiet operation, modularity, and low maintenance. GRI does not expect commercial products from its research for the next three to six years, but many advances look promising in both planar and tubular SOFC designs.
Planar and tubular SOFCs stack fuel cells in an electrical series through interconnects to produce practical amounts of power. In a planar design, fuel and air flow across opposite sides of thin, flat cells. In the tubular design, fuel and air flow on the outside and inside of closed-end, cylindrical cells. Planar designs have advantages in power density and compactness, and tubular designs have strong reliability advantages.
"In Phase II, we're developing better stacks for the planar cells developed under the Phase I program and are looking to reduce costs for tubular SOFCs," says Kevin Krist, GRI Principal Technology Manager, Basic Research Business Unit.
During Phase I, researchers sponsored by both GRI and the Electric Power Research Institute produced inexpensively fabricated, single planar cells with power densities that are two to three times higher than those in commercial phosphoric acid cells. Researchers achieved these striking results at operating temperatures reduced from 1000°C to 700°C. At 700°C, low-cost metallic interconnect materials can be used and cell reliability is increased. This unconventional result was obtained by designing cells with thinner electrolytes and new electrode structures. These experimental cells have been scaled-up to 4" x 4" and have operated without significant performance changes for more than 1000 hours.
Patented mixed-conducting (ion and electronic conducting) electrode cells at the University of Pennsylvania could further improve elec-trode kinetics and greatly simplify cell design by making the electrode and electrolyte components out of similar materials.
Research at the University of Pennsylvania is also beginning to identify materials for direct oxidation of natural gas at the fuel electrode. GRI's thin electrolyte planar cells are dense, yet less than 20 microns thick. GRI-developed methods for this process-including tape-calendering, dip-coating, spray coating, and spin coating-continue to be improved. GRI has also licensed the spin coating method for a non-fuel cell application.
Under Phase II, AlliedSignal and the University of Utah are attempting to retain high cell performance when the cells are incorporated into metal-interconnected stacks. Small stacks (on the order of five cells) are already achieving half of the high power densities of single cells, and their power densities are rapidly improving. The research is also beginning to meet planar technology challenges concerning sealing, joining, minimizing thermal stress, and metallic- interconnect oxidation resistance. These advances are gradually increasing thermal cycling capability.
Tubular SOFCs are closer to commercialization (approximately three years) because of their seal-less design and tolerance to thermal stress, which enhances reliability and thermal cycling capability. Recent advances in reducing costs and the promise for further reductions have encouraged the Basic Research Business Unit to join the extensive effort of Westinghouse Electric Corporation and the U.S. Department of Energy to commercialize this simple, yet revolutionary, technology. A Westinghouse tubular cell unit has completed almost eight years of continuous operation-the longest running fuel cell of any type.
Because of their high temperature and pressurized operation, tubular cells are well-suited for integration with gas turbines. In this combined cycle, the SOFC essentially replaces the combustor for the gas turbine. The additional electricity generated by the SOFC for the same fuel consumption leads to very high electrical efficiencies (lower heating value, LHV) of 70% or higher. Westinghouse's initial SOFC products are integrated SOFC/gas turbine power systems for the distributed generation market in the 0.5 to 5 MW range.
In Phase II, GRI will help Westinghouse develop tubular fuel cells that have lower costs. The fabrication of tubular cells has traditionally involved three electrochemical vapor deposition (EVD) steps, a capital-intensive process. Westinghouse recently replaced two EVD steps by much cheaper processes- plasma spraying for the interconnection deposition and sintering for the fuel electrode fabrication. GRI is working with both the company and subcontractors at the University of Utah, and Lawrence Berkeley Laboratory to eliminate the last EVD step, involving deposition of the electrolyte, while also working to improve the cell power density.
"We're confident that our tubular SOFCs have overcome major technical challenges," says S.C. Singhal, Manager of Fuel Cell Technology/SOFC Power Generation for Westinghouse Electric Corporation. "The final challenge is to reduce cost, and GRI's Phase II program is a step in this direction."
Understanding Fuel Cells and the Solid Oxide Advantage
A fuel cell system initially reforms natural gas fuel into more reactive fuels such as hydrogen (H2) and carbon monoxide (CO). The fuel cell then converts chemical energy into power by electrochemically combining the reformed products and air across a dense, ion-conducting electrolyte layer sandwiched between porous fuel and air electrodes. High temperature operation and corrosion resistant conditions enable SOFC systems to closely couple reforming and power production to manage heat transfer more effectively and reduce the system size relative to other fuel cells. Though technically challenging, SOFCs may also be able to convert natural gas directly without a reforming step.
griweb.gastechnology.org
Basic Research for Low-Cost, Reduced-Temperature, Solid Oxide Fuel Cells (1998)
Dr. Kevin Krist
Gas Research Institute
Chicago IL 60631
Summary
Introduction
Fuel cells may provide significant benefits for gas customers. Many distributed power and cogeneration applications in populated as well as remote areas need one or more of the efficiency, emissions, low maintenance, modularity, and quiet operation advantages of fuel cells. High cost is limiting the rate of introduction of fuel cells, although low-temperature fuel cell costs are gradually decreasing. GRI’s Basic Research Business Unit is supporting efforts to develop solid oxide fuel cells (SOFCs) that have reduced cost and size as well as the potential to be used in combination with turbines. Both planar and tubular SOFC options are being pursued. Sealless design, tolerance to thermal stress, and recent technical improvements are enabling tubular SOFCs to approach commercialization relatively closely. This paper describes significant progress that is also beginning to occur in planar SOFC research.
The research approach includes: (1) evaluation of the cost of SOFC stacks and systems (stack + balance-of-plant) for different materials, fabrication methods, and system designs; (2) developing SOFCs that operate at temperatures reduced from 1000oC to <700oC and are fabricated at the lowest possible sintering temperatures; and (3) defining the science underlying reliable, SOFC stacks that do not crack, warp, leak, or delaminate when operating at high power density and under thermal cycling conditions.
Projected SOFC Cost
Evaluations by TDA Research, Inc. for GRI (Figure 1) indicated the potential to manufacture SOFC stacks for <$300/kW at production levels of 200 MW/yr. The figure’s three horizontal lines also give the stack cost needed to use a fuel cell system with a standard balance-of-plant in applications for a 50 MW turbine, a 200 kW reciprocating engine, and a 225 MW SOFC-turbine combined cycle, respectively. The projected SOFC stack manufacturing costs appear to meet the requirements for many SOFC-turbine and small-scale SOFC-only applications. The low cost required for the SOFC stack to compete directly with a large turbine suggests that the preferred way to use SOFCs in large-scale applications would be as a SOFC-turbine combined cycle.
Figure 1: SOFC stack manufacturing cost.
Because of the high cost of the ceramic interconnect in the planar design, a stack manufacturing cost of <$300/kW requires operation at reduced-temperatures where relatively inexpensive metallic interconnects can be used. In addition, certain types of fabrication such as wet processing or atmospheric plasma spraying are required. We are currently evaluating the sensitivity of manufacturing cost to production level because commercialization may require starting at low production levels.
Analysis of a preliminary total planar system designed by Bechtel suggested that the system could cost <$700/kW and produce electricity at 5 cents/kwh (Figure 2) based on the TDA Research stack cost estimates, a four year pay back, a gas cost of $4/MMBtu, and assuming the stack can achieve the performance currently available in the GRI program single cells. Further electrical performance improvements will lead to further improvements in cost.
Reduced-Temperature Cell Performance
In addition to lowering manufacturing cost, reduced-temperature operation lessens: thermal stress, morphological changes, sealing difficulty, insulation requirements, and air preheating. However, as the operating temperature of a SOFC decreases from 1000oC, the rates of the fuel and air electrode reactions tend to slow markedly and the ohmic loss in the standard SOFC electrolyte, yttria stabilized zirconia (YSZ) increases sharply. The research has shown that it is possible to address both of these issues. Effective electrodes have been designed for reduced-temperature operation. Ohmic losses have also been greatly reduced by supporting thin (<20 micron), dense, YSZ electrolyte films which have minimal resistance on thick porous electrode supports that provide strength. To date, this approach has been more successful than attempts to design more conductive, thick, electrolyte materials that - like YSZ - are stable over the wide range of oxygen partial pressures and exhibit no electronic conductivity.
AlliedSignal, University of Utah, and Lawrence Berkeley Laboratory are now obtaining very high maximum power densities (up to 1500 mW/cm2 at 800oC and 800 mW/cm2 at 700oC) in fuel-electrode-supported, thin YSZ layer cells, operating at low fuel utilization on H2 and air. The cells were scaled-up to areas of 4"x 4" and greater, thermally cycled repeatedly, and operated without performance change for over 1000 hours. The research is also beginning to show ways to reduce the fabrication (sintering) temperature in order to reduce the tendency for diffusion to produce compounds such as lanthanum zirconate which increase interfacial resistance.
Better SOFC cell materials and fabrication methods will improve SOFC performance further. A few examples are described below:
The dissimilarity of the fuel and air electrodes - Ni/YSZ cermet and lanthanum manganite, respectively - and the YSZ electrolyte tends to increase interfacial diffusion, structural instability, interfacial contact resistance, and sealing difficulty. The inability of the dissimilar electrode materials to conduct both electrons and oxygen ions (be mixed-conducting) also limits the electrode reaction to the three-phase boundary of YSZ, the electrode, and the reacting gas. The three phase boundary requirement reduces the electrode reaction rate and increases fabrication cost. The University of Pennsylvania has patented an approach whereby the fuel cell is essentially a single YSZ material. The electrodes consist of YSZ, homogeneously doped to produce mixed-conducting fuel and air electrode behavior with minimal changes in material properties. Metal-doped YSZ anodes and cathodes have produced overpotentials 3-5 times lower than current nickel cermet and lanthanum strontium manganite electrodes in comparative laboratory tests. Research is continuing to design electrodes which improve upon the best available electrode performance by developers and take full advantage of the mixed-conductive properties.
Electrode-supported thin-layer cells required new methods for depositing thin, dense, ceramic electrolyte films on porous electrode supports. The methods developed include tape-calendering, dip-coating, spray coating, and spin-coating. These methods may also be applicable to other technologies such as ceramic air separation membranes and advanced ceramic syngas production membranes. Research is continuing to perfect the deposition processes in order to minimize interfacial resistance and cover large areas without introducing defects.
Reliable, Reduced-Temperature Stacks
Planar stacks (groups of cells electrically connected in series by interconnects to increase the voltage) are capable of high power densities due to the short path which the current takes through the cell. GRI recently started a second phase of research with AlliedSignal, University of Utah, and University of Pennsylvania to develop light-weight, low-cost, metal-interconnected stacks which produce high power densities at reduced-temperatures. Figure 3 compares the power density at a 0.7 reference operating voltage for a self-supporting (thick) electrolyte planar stack (Siemens), and small electrode-supported, thin-film stacks by AlliedSignal and the University of Utah. The figure indicates that encouraging power densities are being realized and the thin-film geometry is enabling higher power densities at lower temperatures. Figure 4 shows the expected effect of increasing power density on the cost per kW for the lowest manufacturing cost estimated in figure 1.
To maintain stack integrity and reduce leaks, the researchers are pursuing technical solutions to problems of sealing, joining, scale-up of cell areas, interconnect stability, and thermal stress distribution. The basic design in terms of such quantities as stack dimensions and flow distribution is also being investigated. Recent improvements have occurred in developing suitable interconnect materials and reducing sealing problems.
Conclusions
SOFC technology is potentially low in cost because: (1) natural gas can be processed directly within the stack by internal reforming or possibly direct oxidation (system integration); (2) the system can be one, compact unit with effective heat transfer from the stack to the reforming step and the incoming air so that external air preheating and air/fuel ratios are minimized (thermal management), (3) all solid-state construction (long life); and (4) combined high power density and efficiency are possible. The improving cell and stack performance of reduced-temperature, planar SOFCs may lead to a very cost-effective, light-weight, compact option for fuel cell technology.
Acknowledgments: The author expresses his appreciation to TDA Research, Inc. (J. Wright) and Bechtel (T-P. Chen) for providing the figures used in this paper. The paper discusses results from AlliedSignal (N. Minh), University of Utah (A Virkar), University of Pennsylvania (W. Worrell), Lawrence Berkeley Laboratory (S. Visco) and University of Missouri-Rolla (H. Anderson).
We are evaluating a new, "one-box" design by Bechtel for the complete SOFC system (<$800/kW).
Background
Increased distributed electric power generation by fuel cells would significantly improve the way energy is provided to customers. Energy consumers could avoid electrical transmission and distribution (T&D) losses and costs, environmental concerns, power outages, poor signal quality, and use waste heat more effectively. Because natural gas may be the most suitable fuel for distributed fuel cells, natural gas use for electric power generation might increase greatly. The requirements for distributed power include high efficiency on a small scale (up to 10MW), low pollutant emissions, low maintenance, quiet operation (for urban areas), and modularity. Although technically challenging, solid oxide fuel cells (SOFCs) may provide these requirements better than other technologies (engines, turbines, other fuel cells, photovoltaics). SOFCs may also greatly reduce cost because they can consume natural gas directly without external reforming, integrate thermal management, high efficiency and power density, have extended life, and produce high quality heat for use directly or in hybrid cycles. A SOFC-gas turbine cycle would provide the most efficient power (70% HHV). A SOFC-absorption cooling cycle would provide electrical, heating, and cooling needs with equipment using minimal moving parts. GRI’s basic research is focusing on reducing the operating temperature of SOFCs to (500-800 oC) and sintering temperatures used in fabrication. Lower temperatures would lead to cheaper materials and fabrication, as well as longer life and increased reliability.
Results
We have developed planar, single cells for reduced-temperature operation.
A very low-cost SOFC was also investigated where the cell layers are simply mechanically assembled rather than sintered together.
Future Plans
The current RFP responses will focus on further optimization and scale-up of cells, the science underlying durable, efficient, high-power density, thermally cyclable, metal-ceramic planar or tubular stack structures, and designing stacks to improve system thermal integration.
griweb.gastechnology.org ... original with graphs, and charts
Re:
Supervalue posted ... " I believe reading from GLE news that ceramics were costly when GLE was at the extreme temperatures the prototype used alot of ceramics due to high heat, lower heat made stainless steel more the cost cutting material of choice " ...
F.Y.I.
GLE has never used extreme temperatures, and has always used ferrous metals interconnects, according to their website they started their SOFC development program back in 1997 ...
globalte.com
Global Thermoelectric
Based in Alberta, Canada, Global Thermoelectric launched its Fuel Cell Division in 1997. The division will aid in the further enhancement of next generation power products that will provide Global customers additional alternatives. The Solid Oxide Fuel Cell program is based on the development and commercialization of one of the leading solid oxide technologies from the Julich Research Institute in Germany. more information on Global Thermoelectric
nfcrc.uci.edu
Here's a rough German to English translation of "In The Beginning" as reported by an eyewitness present at Global Thermoelectric's laboratory back in the spring of '97 ...
Juelicher gas cells for of Canada the high north
Date of the report: 15.05.1998
Sender: Peter Schaefer
Mechanism: Research center Juelich GmbH
Category: supraregional
Mechanical engineering and process engineering, electrical and energy engineering, materials sciences, ecology
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Juelicher gas cells for of Canada the high north
Successful transfer of technology from Juelich to Calgary
New technologies (they may be also still so ingenious and pfiffig (to fail often because of the demonstrating effect. Which functions in the inventor workshop or in the developer laboratory outstanding, wants not to run other place one or before spectators simply. The fact that it goes also differently proves the project "gas cells" of the research center Juelich: For few weeks a complete unit resource stands in Juelich of developed and manufactured high temperature gas cells in the Canadian Calgary and produces on a test status "global of the Thermoelectric Inc.." electric current. Already the first measuring data convinced the Canadians so much that beyond the Atlantiks already from the "first milestone on the way to the marketing of the new technology" the speech is.
Gas cells rank because of their high efficiency (up to 70 per cent) and their extremely small pollutant emissions for many energy experts among the most important generators of the future. In contrast to the conventional burn of gases an electro-chemical conversion takes place in the gas cell. A three-layer system from two gas-permeable electrodes and intermediate gas densities electrolytes produces an electric current directly in form of a reverse electrolysis made of atmospheric oxygen and gaseous fuel.
The high temperature gas cell contains solid electrolytes from yttrium-stabilized zircon oxide - oxide ceramics, to which the cell owes also its English contraction SOFC (SOFC = solvently oxides Fuel Cell). A group of scientists, engineers and technicians from Institut for energy process engineering, Institut for materials of energy engineering and the central department technology works in the context of the Juelicher of gas cell project for approximately two and a half years on the development of new SOFC concepts.
A crucial progress succeeded to the gas cell specialists with a changed SOFC Design: The electrolyte (in earlier SOFCs the basic thing element of the cell construction (from the layer thickness on approximately 20 micrometers (thousandth millimeter)"one abgespeckt" and is now as thin layer on the anode carrying now. Most important advantage of this alternative "anode substrate concept" is the operating temperature lowered from formerly 1000 °C on now about 750 °C. An obstacle for the field use of the SOFC is eliminated thereby: Instead of the heatproof and thus expensive special materials needed so far usual materials of the lying close and mechanical engineering are sufficient now completely in the cell periphery.
The Canadian company "global Thermoelectric", world-wide market leader in the area "thermoelectric current generators", reacted promptly and announced its interest to the Juelicher SOFC and a transfer of technology. In the development department of the enterprise one looks particularly for systems, which can supply river reliably practically without each maintenance and servicing during long periods. Conceivable places of work of such generators are for example unmanned relay stations for radio traffic somewhere in the widths of the arctic. The gas cell, which does not possess any wear-sensitive mechanical parts, would come for it straight quite.
Naturally ready for the market one is the SOFC still some development necessary up to. So that this can orient itself as exactly as possible at the goals of "global Thermoelectric", the Canadians in their development lab would install their own SOFC test status after Juelicher model. To Germany one transferred the task to manufacture the first SOFC stack for the new plant. It acts thereby around a unit resource out of five square gas cells switched into series with in each case 10 centimeters of edge length. In the spring the individual components went to this yearly from Juelich to Calgary on the journey. Dr. Uwe Diekmann, central department technology, and Dr. Izaak Vinke, Institut for energy process engineering, supervised the assembly and the start-up of the stack locally.
Already the first measurements offered cause for the shoulder knocking: "with a current density of over 300 milliamperes per square centimeter we obtained immediately a value, which we measured, report in this height otherwise at smaller cells" Vinke. The electrical achievement of the stack amounted to with an operating temperature of 750 °C approximately 90 Watts. "with increasing size and number of cells normally rises the danger of defects, which lead to an achievement loss. The measurements in Canada show however clearly that the step from smaller experimental models is quite possible for Diekmann to technically relevant yardsticks ", describes. And still it learned something in Calgary: "the SOFC technology is transferable. The Canadian colleague succeeded in on the basis our documents and information well establishing a functioning test status including stack."
Hermann Kabs, project manager gas cell, sees in the transfer of technology on "global Thermoelectric" still another advantage: "the more groups at the development of the gas cell, is the larger the probability works that the technology of a daily is actually used. Also niche segments could open the entrance to a broad market."
Further information about the research center Juelich in the InterNet under kfa juelich.de
idw-online.de ... original German version
As the saying goes the rest is history ... GLE has progressed rapidly to the point where they were able to successfully be awarded the first of many patents all on their own. |
