Fuel cells have emerged in the last decade as one of the most promising new technologies for meeting the Nation's energy needs well into the 21st century.
Unlike power plants that use conventional technologies, fuel cell plants that generate electricity and usable heat can be built in a wide range of sizes - from 200-kilowatt units suitable for powering commercial buildings, to 100-megawatt plants that can add baseload capacity to utility power plants.
Fuel cells are similar to batteries in that both produce a direct current by using an electrochemical process. Two electrodes, an anode and a cathode, are separated by an electrolyte. Like batteries, fuel cells are combined into groups, called stacks, to obtain a usable voltage and power output.
Unlike batteries, however, fuel cells do not release energy stored in the cell or run down when the energy is gone. Instead, they convert the energy in a hydrogen-rich fuel directly into electricity and operate as long as they are supplied with fuel.
Fuel cells emit almost none of the sulfur and nitrogen compounds released by conventional generating methods, and can utilize a wide variety of fuels: natural gas, coal-derived gas, landfill gas, biogas, or alcohols.
Three different fuel cell technologies are being developed by the Department of Energy and the power industry for larger-scale stationary power generation (The Proton Exchange Membrane technology, see above, is primarily suited for residential/business and transportation applications). They differ in the composition of the electrolyte and are in different stages of development.
Phosphoric Acid Fuel Cells (PAFCs) are the most mature fuel cell technology and are already in the first stages of commercialization. Turnkey 200-kilowatt plants are now available and have been installed at more than 70 sites in the United States, Europe, and Japan. Operating at about 200oC (400oF), the PAFC plant also produces heat for domestic hot water and space heating, and its electrical efficiency exceeds 40 percent.
Molten Carbonate Fuel Cells (MCFCs) - now being tested in full-scale demonstration plants - offer higher fuel-to-electricity efficiencies, approaching 60 percent.
MCFCs operate at higher temperatures, around 650oC (1,200oF), making them candidates for combined-cycle applications, in which the exhaust heat is used to generate additional electricity. When the waste heat is used, total thermal efficiencies can approach 85 percent.
Solid Oxide Fuel Cells (SOFCs) - currently being demonstrated in a 160-kilowatt plant - are state-of-the-art fuel cell technology and offer the stability and reliability of all-solid-state ceramic construction.
operation. up to 1,000oC (1,800oF), allows more flexibility in the choice of fuels and can produce better performance in combined-cycle applications. Adjusting air and fuel flows allows the SOFC to easily follow changing load requirements. Like MCFCs, SOFCs approach 60 percent electrical efficiency, and 85 percent total thermal efficiency.
Fuel Cells: The Best Electric Power Plants Yet:
Fuel cells are on the verge of revolutionizing the electric power industry by offering a better way to produce electricity and better ways to deliver it to the consumer. DOE and its industry partners are demonstrating the many advantages of fuel cell power plants.
Clean Power for a Clean Environment:
Increasing powergeneration without increasing emissions is the challenge facing power producers today, and fuel cells are a key approach to balancing our energy needs with our desire for a cleaner, healthier environment.
Fuel cell power plants produce dramatically fewer emissions, and their byproducts, primarily water and carbon dioxide, are so environmentally friendly that natural-gas fuel cell power plants have a blanket exemption from regulations in California's South Coast Air Quality Management District, possibly the strictest in the Nation.
Saving Fuel with Energy-Efficient Technology:
Fuel cells convert a remarkably high proportion of the chemical energy in fuel to electricity. With efficiencies approaching 60 percent, even without cogeneration, fuel cell power plants are nearly twice as efficient as conventional power plants.
And efficiency is not a function of plant size or load, either: small-scale fuel cell plants are just as efficient as large ones, and operation at partial load is as efficient as at full load. Higher efficiencies mean fuel savings for the producer and cost savings for the consumer.
Making a Good Thing Better - Thermal Recovery:
High-grade waste heat from fuel cell systems is perfect for use in commercial, industrial, and residential applications, including cogeneration, heating, and air-conditioning. When by-product heat is used, the total energy efficiency of fuel cell systems approaches 85 percent.
Sizing Plants to Meet the Load:
The fuel cell stack is the basic component of a fuel cell power plant. Stacks are combined into modules, and plant capacity is determined by the number of modules. Individual modules can go from idle to full load in minutes.
Modular plants can help planners overcome many difficult expansion problems. Mass-assembly construction techniques and shorter lead times for installation reduce the capital risk in adding generating capacity. Capacity can be better matched to load, and the high costs of large new plants with underutilized capacity can be avoided.
Modularity also produces a flat economy of scale: the cost per kilowatt is about the same in small plants as in large ones.
And because electrical efficiency is determined by individual cell performance, the number of modules in the power plant has little or no effect on overall efficiency. As a result, fuel cell power plants offer the same advantages at 25 kilowatts as they do at 50 megawatts.
Fuel cells promise to be one of the most reliable, if not the most reliable, power generation technology. They are now being used by hospitals, hotels, and telephone companies as part of critical uninterruptible power systems.
Putting Power Where It's Needed:
The modular nature of fuel cells allows power capacity to be added wherever it's needed. In the typical central power configuration, additional capacity is sited at the central plant or at substations.
In a distributed power configuration, capacity is placed close to the demand. In high-growth or remote areas, distributed placement offsets the high costs of acquiring rights-of-way and installing transmission and distribution lines. A distributed configuration also eases public concerns about exposure to electromagnetic fields from high-voltage lines.
Smaller scale distributed configuration power plants are perfect for commercial buildings, prisons, factories, hospitals, telephone switching facilities, hotels, schools, and other facilities. In these applications, consumers get the best of all worlds - high-quality power that is economical and reliable.
On-site power conditioning eliminates the voltage spikes and harmonic distortion typical of utility grid power, making fuel cell power plants suitable even for sensitive electronic loads like computers and hospital equipment. And in many cases, utility grid backup reduces the need for expensive uninterruptible power supply systems.
Power Plants as Good Neighbors:
Having few moving parts and requiring little on-site maintenance, fuel cell power plants are reliable and safe, and can be sited in environmentally sensitive areas.
They are relatively small -- a 2-megawatt demonstration plant in Santa Clara, California, is the size of a tennis court -- and produce negligible noise. A 200-kilowatt plant is about as noisy as an ordinary air conditioner. No fuel has to be stored on site. The "good neighbor" character of fuel cell power results in short permitting and licensing schedules for both indoor and outdoor installations.
Choosing From a Variety of Fuels:
Fuel cells need hydrogen, which can be generated internally from natural gas, coal gas, methanol, landfill gas, or other fuels containing hydrocarbons.
Although most market-entry fuel cell plants are fueled by natural gas, fuel flexibility means that power generation can be assured even when the primary fuel source is unavailable.
Solutions that Meet Public Needs:
As the most environmentally friendly source of fossil-fueled energy, fuel cell technology meets public demand for clean, quiet, and efficient power. Fuel cell exhaust is cleaner than the air of some cities.
The low level of noise at a fuel cell installation makes it possible to hold a normal conversation while standing right next to the power plant. And safe, reliable operation reduces public concerns about siting, even in densely populated areas.
A New Force in Energy Markets:
The Changing Face of Electricity Generation
Fuel cell power plants will provide a significant share of our electrical power in this decade and well into the next century.
They are set to play a major role in a deregulated power industry. Large-scale plants will compete in the baseload power generation market while smaller plants will penetrate the distributed power and cogeneration markets.
Baseload generation currently relies on coal-fired, nuclear, or natural-gas-fired technologies. The natural-gas-powered fuel cell is more efficient, more environmentally friendly, and potentially more cost-effective than the current technologies in the baseload market segment.
Technologies for the distributed power and cogeneration market segment include gas turbines, diesel engines, hydroelectric plants, solar and wind generation, and the already commercialized PAFC.
In this market, MCFC and SOFC plants also hold distinct advantages: the smaller applications favor fuel cells for their high-efficiency, low-emission, and load-following capabilities. In addition, the attractiveness of economical and reliable on-site power generation may significantly expand the market for small-scale commercial and industrial power plants.
By the year 2010, it is estimated that approximately 130 gigawatts of new generating capacity will be installed in the United States while, in world markets and within a much closer time frame, nearly 550 gigawatts of generating capacity will be added.
Fuel cell commercialization opportunities in the U.S. market are focused in several areas: repowering, central power plants, industrial generators, and commercial/residential generators.
Estimates of plant repowering installations between 1999 and 2010 range from 15 percent to approximately 65 percent of the installed generating capacity. Most repowering will occur in central power plants: fuel cell installations of 100 megawatts or more are targeted to this market, powered initially by natural gas and later by coal gas.
Central Power Plants:
New generating capacity of approximately 100 gigawatts will be required in the central powering market by 2010. Coal gas-powered fuel cell power plants are targeted to this market, with plants sized at 100 megawatts or more.
Industrial Generators and Commercial/Residential Generators:
The market for additional industrial capacity by 2010 is estimated at 3 gigawatts, and the market for additional commercial/residential capacity at 6 gigawatts. These markets are targeted for early entry and will be a proving ground for natural-gas fuel cell power plants sized from 500 kilowatts to 20 megawatts.
Source:United states Energy Information Administration
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