This paper discusses the development of high-temperature fuel cells for stationary industrial and residential power generation applications. The system can operate on hydrogen, extracted by an internal reformer, and on a fuel comprising carbon monoxide. The technology enables fuel flexibility and, in addition, the high temperature provides high-quality co-generation of a thermal product and an ultimate overall efficiency exceeding 80%. Alone, high-temperature fuel cells show tremendous promise. Through hybridization, however, high-temperature fuel cells have a novel capability to achieve a quantum jump in fuel-to-electricity efficiency. In a hybrid configuration, high-temperature fuel cell technology promises new means to provide hoteling or propulsive power for ships, locomotives, long-distance trucks, and civil aircraft.
These flat metal and ceramic plates form the heart of a solid oxide fuel cell, a technology that promises to provide clean and efficient electricity.
Fuel cells are a technology that has sat on the cusp of mainstream applications for some time. It hasn’t yet reached the point of cliché, as has nuclear fusion, but it is wearisome nonetheless.
But while it has been slow, progress is being made. Indeed, stationary fuel cells are now emerging as a true alternative to combustion heat engines for the production of electrical power and the co-generation of a thermal product. We are, in fact, at the beginning of two paradigm shifts for the generation of electricity. The first is a new transforming technology; the second is the quiet generation of electricity and a thermal product at the point of use with all the potential attendant attributes: reliability, power quality, lower operating costs, remarkably higher system efficiency, and the production and utilization of direct current.
The commercial deployment of stationary fuel cells has been led by United Technologies Corp. of Hartford, Conn. Powered by natural gas, the phosphoric acid fuel cell has a fuel-to-electrical conversion efficiency of 34 percent and an operating temperature of 200°C. Combined with the use of waste heat for preheating boiler feedwater, the PAFC can reach overall efficiencies that approach 80 percent.
Today, however, a new product line of fuel cells is emerging that operates at relatively high temperatures— between 600 and 1,000°C. Through support from the U.S. Department of Energy, two technologies are moving from concept to commercialization: molten carbonate fuel cells and solid oxide fuel cells. Both operate at fuel-to-electrical efficiencies approximating 50 percent.
High-temperature fuel cells hold promise for stationary industrial and residential power generation applications, and for myriad military applications. The system can operate on hydrogen, extracted by an internal reformer, and on a fuel comprising carbon monoxide. The technology enables fuel flexibility and, in addition, the high temperature provides high-quality co-generation of a thermal product and an ultimate overall efficiency exceeding 80 percent. When integrated with a gas turbine in a hybrid configuration, the waste heat can effectively be converted to electricity with the potential, in the future, of achieving fuel-to-electrical efficiencies exceeding 70 percent.
Called hybridization, this use of fuel cells provides a particularly remarkable opportunity and portends a revolution in the means by which power will be generated in the future.
The deployment of stationary fuel cells requires a new perspective on the generation and use of electricity, and the co-generation and use of a thermal product. Stationary fuel cells, for example, are well suited for the emerging distributed generation market. Located at the point of use—say, a commercial building, hospital, or factory—stationary fuel cells add to reliability and quality of power, and take advantage of heat that would otherwise be exhausted and wasted. The low acoustic signature and the “near zero” emission of criteria pollutants make the stationary fuel cell well suited for this application.
In spite of those advantages, the new paradigms face challenges in a well-established market for the generation, transmission, and distribution of electricity. To overcome that, state programs are being established to facilitate the burgeoning new market for stationary fuel cells. California and Ohio are providing especially proactive leadership.
Highly Efficient, Ultra-Clean
It has been a long road to get to this point. Highly efficient and ultra-clean molten carbonate fuel cell technology, for instance, has matured from promising laboratory experiments 30 years ago. The leading U.S. manufacturer, FuelCell Energy, has pioneered the market with the direct fuel cell 250-kilowatt product that was developed in a private-public sector partnership with the U.S. Department of Energy. The high-performance components and stack technology were developed in the 1980s, and the scale-up and proof-of-concept pilot systems were tested in the 1990s. (To get a sense of how far things have come, the proof-of-concept was a single 3-square-centimeter cell. Today’s commercial-design power plants use 9,000-square-centimeter cells.)
The molten carbonate fuel cell operates at a temperature of approximately 650°C. The bipolar plate and the corrugated current collectors are made of 300-series stainless steel, and the electrodes are constructed from porous nickel-based materials. The fuel-and-air isolating membrane is a porous ceramic lithium aluminate that holds an electrolyte mixture of lithium and potassium salts.
The fuel cell reacts with both hydrogen and carbon monoxide at the anode. Fuels such as natural gas are converted to hydrogen and carbon monoxide by steam reformation within the fuel cell. Water is available as a product of the fuel cell reaction. The steam reforming reaction is highly endothermic and the fuel cell anode reaction is exothermic. As a result, the molten carbonate fuel cell is able to reform natural gas and other light hydrocarbons within the stack. The internal reformation leads to an enhanced efficiency and facilitates thermal management of the system.
Mark Williams, who served as the fuel cell technology manager for the U.S. Department of Energy for over 10 years, reentered the private sector in December 2005. Scott Samuelsen is director of the National Fuel Cell Research Center at the University of California. Irvine.
FuelCell Energy began a field deployment program in 2003. To date, more than 40 of the company’s model DFC300A 250 kW units have been deployed throughout the world at industrial and municipal wastewater treatment facilities, hotels, universities, manufacturing plants, data communication centers, hospitals, and prisons.
A 1 MW installation was recently commissioned at the Sierra Nevada Brewery in Chico, Calif., and a 1 MW power plant is in operation at the King County wastewater treatment facility in Washington. The cost of the DFC300A unit has declined from approximately $8,000 per kilowatt in 2004 to $4,800 per kilowatt today.
Solid oxide fuel cells operate at higher temperatures, from 700 to 1,000°C, depending on the design and application. The name derives from the electrolyte, a solid oxide ceramic, typically perovskite. Yttrium-stabilized zirconia (abbreviated to YSZ) is today the most common electrolyte for SOFC. Challenges of corrosion and management associated with other electrolytes are removed, but the high operating temperature places stringent demands on the solid oxide electrolyte.
Although a wide range of materials has been considered for the anode of the SOFC, most developers today use YSZ bonded to nickel. The composition of the anode, particle sizes of the powders, and the manufacturing method are keys to achieving high electrical conductivity, adequate ionic conductivity, and high activity for electrochemical reactions and reforming and shift reactions. Reduction of the nickel oxide powder in the virgin anode mixture to Ni results in the desired porosity. For the more recent anode-supported cells, it also achieves good mechanical properties and maintains geometric stability during manufacture and operation—for example, by also achieving the desired contact between the Ni phase and the YSZ phase.
Most cathode materials used in SOFCs today are lanthanum-based perovskite materials. In high-temperature SOFCs, a strontium-doped compound of lanthanum, manganese, and oxygen (called LSM) is used. For operation below 700°C, the use of LSM as the cathode material represents significant potential loss in the form of resistance to ion flow and other materials are being pursued.
There is substantial, ongoing research worldwide to establish SOFC operating conditions and material sets that could enable both ease of manufacturing and relatively low-cost mass production. For example, SOFCs with high power densities operating at lower temperatures— 700°C instead of 1,000—recently have been developed and operated. The lower operating temperature will, in principle, lessen the demand on the interconnect and other potentially metallic components in the stack, and may thereby reduce the cost of the SOFC.
A solid oxide fuel cell system has not yet been commercialized, but the excitement over the technology has substantial market interest and a relatively large number of companies working to establish systems for the distributed generation market. The range of companies extends to major participants in the power generation market, such as General Electric, Siemens Power Corp., and Rolls-Royce, and to new contributors to the field, such as Acumentrics and IonAmerica.
The excitement is predicated upon the potential high reliability of the relatively robust solid oxide technology. An example fueling this excitement is the enduring operation of an initial demonstration of SOFC technology, a 25 kW Siemens Power Corp. system located at the National Fuel Cell Research Center. More than 10 years old, the system has operated on natural gas and reformate gases derived from both diesel and jet fuels, and is today exploring SOFC performance of coal-derived reformate gas.
Alone, high-temperature fuel cells show tremendous promise. Through hybridization, however, high-temperature fuel cells have a novel capability to achieve a “quantum jump” in fuel-to-electricity efficiency. Hybridization occurs by combining a high-temperature fuel cell with a traditional heat engine such as a gas turbine. The fuel cell and gas turbine can be configured in several different fashions. For example, the air stream can be first pressurized through the compressor of the turbine. The pressurized air stream is then fed to the high-temperature fuel cell where fuel (typically natural gas) is added, and the resultant electrochemical reactions lead to the direct production of electrical energy. The high-pressure, high-temperature fuel cell effluent can then be expanded in the turbine to provide both the compressor work and additional electrical energy.
The resulting system exhibits a synergism in which the combination performs with an efficiency that far exceeds that which can be provided by either system alone. Combined with an inherent low level of pollutant emission, hybrid configurations are likely to make up a major percentage of the next-generation advanced power generation systems across a wide scale of applications from distributed generation to central power plants.
A prototype SOFC 220 kW hybrid for distributed power applications has been extensively analyzed and studied over the past five years with support from a variety of sources, including the U.S. Department of Energy, Southern California Edison, Siemens Power Corp., the National Fuel Cell Research Center, the Electric Power Research Institute, and the South Coast Air Quality Management District. In addition to a variety of steady state thermodynamic and dynamic modeling and analyses, the prototype operated for 3,000 hours (as designed) and demonstrated that the predicted synergistic performance can in fact be achieved.
One can also integrate an intercooled gas turbine with a pressurized tubular SOFC and humidification of the air as part of a central power application. The humidified air is preheated in a turbine exhaust recuperator before it is fed to the SOFC. The air leaving the high-pressure compressor is cooled in an aftercooler and then introduced into the humidifier column, where it comes into counter-current contact with hot water. Some of the water is evaporated into the air stream, with the heat required for the humidification operation being recovered from the intercooler and the stack gas by circulating water leaving the humidifier. The desulfurized fuel is also humidified in a similar manner. The optimum efficiency of the cycle occurs at a pressure ratio of approximately 20 and a gas turbine firing temperature at a modest value, less than 1,200°C.
The effort to reach high efficiencies will likely be worthwhile. Escalating fossil fuel prices are putting an unprecedented premium on system efficiency. Inadequate grid reliability is inhibiting growth in all economic sectors, including information technology, and the baseload profiles of these industrial sectors favor baseload fuel cells. The potential is enormous for high-temperature fuel cells in the distributed generation market. To enable the market, key policy governing the generation, transmission, and distribution of electricity must be addressed, codes and standards for interconnecting to the electrical utility grid must evolve, and the community of architects and developers must adapt to the new paradigms.
In light of that huge potential, some states have taken aggressive steps to be the manufacturing and employment base for fuel cell technology.
“Through hybridization, high-temperature fuel cells can achieve a ‘quantum jump’ in efficiency... The combination performs with an efficiency that far exceeds what can be provided by either system alone.”
California, for example, has established the Stationary Fuel Cell Collaborative, with a core group composed of state, federal, and non-government agencies that carry responsibility for some aspect of the fuel cell market— technology development, purchase authority, and regulation. The goal is to enable the fuel cell market by a coordinated strategy. Industry is engaged through an advisory panel that includes manufacturers, end users, utilities, and energy providers.
In Ohio, the state government introduced a $103 million, three-year initiative to establish a manufacturing and employment base for fuel cell technology. This commitment includes $75 million in financing to make strategic capital investments that will create and retain jobs; $25 million for fuel cell research, development, and demonstration, and $3 million for worker training. In addition, the Ohio Department of Development has set aside $60 million in federal volume cap for tax-exempt financing of qualified projects.
Both of these examples are remarkable and reflect the growing attention to the two paradigm shifts associated with high-temperature fuel cells: the technology itself for the generation of electric and thermal product, and the initiation of a vibrant distributed generation market. Further in the future, high-temperature fuel cell technology will likely become an integral strategy for central power production of electricity and transportation fuels. In a hybrid configuration, high-temperature fuel cell technology promises new means to provide hoteling or propulsive power for ships, locomotives, long-distance trucks, and civil aircraft.
In all their potential applications—residential, commercial, industrial, or institutional, in distributed generation or in central power plants—high-temperature fuel cells indeed portend a profound change in the manner by which power is generated in the decades to come. The transformation has begun.