Engineers and environmentalists have long dreamed of being able to obtain the benefits of clean electric power without pollution-producing engines or heavy batteries. Solar panels and wind farms are familiar images of alternative energy technologies. While they are effective sources of electrical energy, there are problems with the stability of their energy source as, for example, on a cloudy or windless day. Their applications are somewhat limited due to lack of portability; a windmill is not much help to the power plant of a diesel truck, a solar panel cannot provide power at night, etc.
In 1962 a revolution in energy research occurred. Scientists at Westinghouse Electric Corporation (now Siemens Westinghouse) demonstrated for the first time the feasibility of extracting electricity from a device they called a "solid electrolyte fuel cell" [Weissbart].
Since then there has been an intense research and development effort to develop the alternative energy technology known as fuel cells. Now, as energy issues are at the forefront of current events, fuel cell technology is ripening and on the verge of being ready for large scale commercial implementation.
The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces.
A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.
They operate at very high temperatures, typically between 500 and 1,000 °C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.
Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. In 2009, Australian company, Ceramic Fuel Cells Ltd successfully achieved an efficiency of a SOFC device up to the previously theoretical mark of 60 percent.
The higher operating temperature make SOFCs suitable candidates for application with heat engine energy recovery devices or combined heat and power, which further increases overall fuel efficiency.
Because of these high temperatures, light hydrocarbon fuels, such as methane, propane and butane can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat given off by the exothermic electrochemical oxidation within the fuel cell for endothermic steam reforming process.
These fuel cells are best suited for large-scale stationary power generators that could provide electricity for factories or towns. This type of fuel cell operates at very high temperatures (between 700 and 1,000 degrees Celsius). This high temperature makes reliability a problem, because parts of the fuel cell can break down after cycling on and off repeatedly. However, solid oxide fuel cells are very stable when in continuous use. In fact, the SOFC has demonstrated the longest operating life of any fuel cell under certain operating conditions. The high temperature also has an advantage: the steam produced by the fuel cell can be channeled into turbines to generate more electricity. This process is called co-generation of heat and power (CHP) and it improves the overall efficiency of the system.
SOFC-GTAn SOFC-GT system is one which comprises a solid oxide fuel cell combined with a gas turbine. Such systems have been evaluated by Siemens Westinghouse and Rolls-Royce as a means to achieve higher operating efficiencies by running the SOFC under pressure. SOFC-GT systems typically include anodic and/or cathodic atmosphere recirculation, thus increasing efficiency.
Theoretically, the combination of the SOFC and gas turbine can give result in high overall (electrical and thermal) efficiency. Further combination of the SOFC-GT in a combined heat and power configuration (via HVAC) also has the potential to yield even higher thermal efficiencies in some cases.
Where are we to find its Applications? and potential Markets?
The United States government is taking a proactive role in expediting the technology through the Solid State Energy Conversion Alliance (SECA), which is coordinated by the Department of Energy and Pacific Northwest National Laboratory. The technical goal is to develop mass producible, modular SOFC units capable of 3-10 kW at a price of $400/kW. SECAs approach is to develop industrial collaborations and to extend financial support of technical research [SECA].
There seems, therefore, to be little doubt that SOFC technology will be implemented. Analysts expect that the overall market for fuel cell technology could reach $95 billion by the year 2010 [ceramic]. The market share that will belong to SOFCs is unclear but will surely be significant, as SOFCs are targeted for use in three energy applications: stationary energy sources, transportation, and military applications.
Stationary installations would be the primary or auxiliary power sources for such facilities as homes, office buildings, industrial sites, ports, and military installations. They are well suited for mini-power-grid applications at places like universities and military bases. According to the SECA, worldwide demand for electricity is expected to double in the next 20 years. SOFC technology is ideal for such an expansion, since much of the anticipated demand is expected to come from growing economies with minimal infrastructure. SOFCs can be positioned on-site, even in remote areas; on-site location makes it possible to match power generation to the electrical demands of the site.
Stationary SOFC power generation is no longer just a hope for the future.
Siemens Westinghouse has tested several prototype tubular systems, with excellent results. A plant in the Netherlands has been operational for two years and an earlier prototype installation has been operating for 8 years.
The fuel cells have been through over 100 thermal cycles and the voltage degradation during the test time has been minimal less than 0.1%/thousand hours.
In the transportation sector, SOFCs are likely to find applications in both trucks and automobiles. In diesel trucks, they will probably be used as auxiliary power units to run electrical systems like air conditioning and on-board electronics. Such units would preclude the need to leave diesel trucks running at rest stops, thereby leading to a savings in diesel fuel expenditures and a significant reduction in both diesel exhaust and truck noise. Meanwhile, automobile manufacturers have invested at least $4.5 billion in fuel cell research (not all SOFC) [ceramic].
There are an estimated 600 million vehicles worldwide, 75% of which are personal automobiles, and the number is expected to grow by 30% in the next 10 years [SECA]. With more stringent environmental restrictions in the United States and European Union, automobile manufacturers are under growing time pressure to bring non-polluting cars to the marketplace. SOFCs are attractive prospects because of their ability to use readily available, inexpensive fuels.
Finally, SOFCs are of high interest to the military because they can be established on-site in remote locations, are quiet, and non-polluting. Moreover, the use of fuel cells could significantly reduce deployment costs: 70% by weight of the material that the military moves is nothing but fuel [SECA].
Where are we now?
Forty years have passed since the first successful demonstration of a solid oxide fuel cell. Through ingenuity, materials science, extensive research, and commitment to developing alternative energy sources, that seed of an idea has germinated and is about to bloom into a viable, robust energy alternative. Materials development will certainly continue to make SOFCs increasingly affordable, efficient, and reliable.