Fuel cells

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 base load capacity to utility power plants.

Fuel cells produce direct current power from hydrogen-rich fuel gas and air that flow over two cell electrodes. The principal byproducts are water, carbon dioxide, and heat.

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.

Types of Fuel Cells

	Fuel Cell Type
	Polymer Electrolyte Membrane	Phosphoric Acid	Carbonate	Solid Oxide
Electrolyte	Ion Exchange Membrane	Phosphoric Acid	Alkali Carbonates Mixture	Yttria Stabilized Zirconia
Operating Temp., °C	80	200	650	1,000
Charge Carrier	H+	H+	co3	0
Electrolyte State	Solid	Immobilized Liquid	Immobilized Liquid	Solid
Cell Hardware	Carbon- or Metal-Based	Graphite-Based	Stainless Steel	Ceramic
Catalyst	Platinum	Platinum	Nickel i	Perovskites
Cogeneration Heat	None	Low Quality	High	High
Fuel Cell Efficiency, % LHV	<40	40-45	50-60	50-60

Three different fuel cell technologies are being developed by the Department of Energy and the power industry for larger-scale stationary power generation

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. Turnke 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 200°C (400°F), 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 650°C (1,200°F), 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 1,000°C (1,800°F), 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 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 power generation 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,