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Sustainable Energy Systems Engineering Peter Gevorkian. Brevard Community College ETP1401 Bruce Hesher. Ch 9: Fuel Cell Technologies. Introduction. Fuel cells are battery like devices that make electrical energy by means of electromechanical reactions.
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Sustainable Energy Systems EngineeringPeter Gevorkian Brevard Community College ETP1401 Bruce Hesher Ch 9: Fuel Cell Technologies
Introduction Fuel cells are battery like devices that make electrical energy by means of electromechanical reactions. They keep making electricity as long as hydrogen gas continues to flow. They degrade 1-4% every 1000 hours of operation. So, they need to be rebuilt or replaced after 5-7 years.
Fuel Cell Technology Technical definition: “an energy conversion device that generates electricity and heat by electromechanically blending a gaseous fuel (ex: hydrogen) and oxidizing gas using an ion-conducting electrolyte”. The only by-product is water. A fuel cell converts chemical to electrical energy without combustion and is therefore more efficient.
Short History The fuel cell was discovered in 1839 by Sir William Grove, a Welsh judge and scientist. The large boost in FC technology came from NASA. In the late 1950's, NASA needed a compact way to generate electricity for space missions. Nuclear was too dangerous, batteries too heavy, and solar power too cumbersome. The answer was FCs. NASA went on to fund research contracts for FC technology. The Gemini, Apollo, and Space Shuttles programs all used fuel cells.
Research Hydrogen and fuel cell research at the National Renewable Energy Laboratory (NREL) contributes to the growing role that advanced technologies play in addressing the nation's energy challenges. R & D is currently being done by many companies in many nations.
Basic Operation Principles A fuel cell consists of two electrodes that sandwich an electrolytic membrane. The hydrogen atom consists of only an electron and a proton, there is no neutron. The proton is much larger than the electron, but it passes through the Proton Exchange Membrane (PEM).
Chemistry PEM Electrolyzer: 2H20 + energy 2H2 + 02 PEM Fuel Cell: 2H2 + 02 2H20 + energy The electrolyzer produces hydrogen and oxygen gas. The water must be distilled (pure) or else the PEM in the electrolyzer will get contaminated! The energy can be from a Photovoltaic module.
Fuel Reformers Some fuel cell systems also include a preprocessing mechanism known a fuel reformer, which enables the hydrogen from any hydro-carbon fuel such as natural gas, methanol, land fill methane gasses, or gasoline to be separated from the main molecules and be used. Depending on the type of fuel being reformed and how it is being reformed, the efficiency, by-products, temperatures of the process, and etc. vary greatly.
Type of Fuel Cells There are many types of fuel cells that use various chemicals. See http://www.fuelcells.org/basics/types.html The dividing line between fuel cells and batteries blurs with some types of fuel cells. The following slides detail many of them for the purpose of completeness and as a reference.
Phosphoric Acid Fuel Cells (PAFC)p235 Commercially available. Hundreds have been installed in 19 nations - in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, landfills and waste water treatment plants. PAFCs generate electricity at more than 40% efficiency - and nearly 85% of the steam this fuel cell produces is used for cogeneration - this compares to about 35% for the utility power grid in the United States. Phosphoric acid fuel cells use liquid phosphoric acid as the electrolyte and operate at about 450°F. One of the main advantages to this type of fuel cell, besides the nearly 85% cogeneration efficiency, is that it can use impure hydrogen as fuel. PAFCs can tolerate a CO concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed.
Proton Exchange Membrane (PEM) Fuel Cells p236 These fuel cells operate at relatively low temperatures (about 175°F), have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications, such as in automobiles, where quick startup is required. According to the U.S. Department of Energy (DOE), "they are the primary candidates for light-duty vehicles, for buildings, and potentially for much smaller applications such as replacements for rechargeable batteries." This type of fuel cell is sensitive to fuel impurities. Cell outputs generally range from 50 watts to 75 kW.
Molten Carbonate Fuel Cells (MCFC)p237 Use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert matrix, and operate at high temperatures - approximatelly 1,200ºF. They require carbon dioxide and oxygen to be delivered to the cathode. To date, MCFCs have been operated on hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products. 10 kW to 2 MW MCFCs have been tested on a variety of fuels and are primarily targeted to electric utility applications.
Solid Oxide Fuel Cells (SOFC)p237 Use a hard, non-porous ceramic compound as the electrolyte, and operate at very high temperatures - around 1800°F. One type of SOFC uses an array of meter-long tubes, and other variations include a compressed disc that resembles the top of a soup can. Tubular SOFC designs are closer to commercialization and are being produced by several companies around the world. SOFCs are suitable for stationary applications as well as for auxiliary power units (APUs) used in vehicles to power electronics.
High Temperature Proton Exchange Membrane Fuel Cell (HT-PEM) Similar to PEM fuel cells as they both include Membrane Electrode Assemblies (MEAs); however, HT-PEM fuel cells operate at higher temperatures (250°F - 390°F) than PEM fuel cells. The MEAs of HT-PEM fuel cells can have a membrane that either consists of a proton conductive polymer or a polymer doped with a proton conductive compound. A common example of the latter is an MEA with a phosphoric acid doped polybenzimidazole (PBI) membrane. Since HT-PEM fuel cells have been proven to tolerate up to 3% CO, they are a preferred fuel cell technology for integration with fuel reformers. Typical applications for HT-PEM fuel cells include stationary and mobile applications, such as range extenders for battery electric vehicles.
Alkaline Fuel Cells Long used by NASA on space missions, alkaline fuel cells can achieve power generating efficiencies of up to 70 percent. They were used on the Apollo spacecraft to provide both electricity and drinking water. Alkaline fuel cells use potassium hydroxide as the electrolyte and operate at 160°F. However, they are very susceptible to carbon contamination, so require pure hydrogen and oxygen.
Direct Methanol Fuel Cells (DMFC) Similar to the PEM cells in that they both use a polymer membrane as the electrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the need for a fuel reformer. Efficiencies of about 40% are expected with this type of fuel cell, which would typically operate at a temperature between 120-190°F. This is a relatively low range, making this fuel cell attractive for tiny to mid-sized applications, to power cellular phones and laptops. Higher efficiencies are achieved at higher temperatures. Companies are also working on DMFC prototypes to be used by the military for powering electronic equipment in the field.
Regenerative Fuel Cells Attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyzer. The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar-powered electrolyzer and the process begins again. These types of fuel cells are currently being researched by NASA and others worldwide.
Zinc Air Fuel Cell (ZAFC) Use a gas diffusion electrode (GDE), a zinc anode separated by electrolyte, and some form of mechanical separators. The GDE is a permeable membrane that allows atmospheric oxygen to pass through. After the oxygen has converted into hydroxyl ions and water, the hydroxyl ions travel through an electrolyte, and reach the zinc anode. Here, it reacts with the zinc, and forms zinc oxide. This process creates an electrical potential; when a set of ZAFC cells are connected, the combined electrical potential of these cells can be used as a source of electric power. This electrochemical process is very similar to that of a PEM fuel cell, but the refueling is very different and shares characteristics with batteries. ZAFCs contain a zinc "fuel tank" and a zinc refrigerator that automatically and silently regenerate the fuel. In this closed-loop system, electricity is created as zinc and oxygen are mixed in the presence of an electrolyte (like a PEMFC), creating zinc oxide. Once fuel is used up, the system is connected to the grid and the process is reversed, leaving once again pure zinc fuel pellets. The key is that this reversing process takes only about 5 minutes to complete, so the battery recharging time is not an issue.The chief advantage zinc-air technology has over other battery technologies is its high specific energy, which is a key factor that determines the running duration of a battery relative to its weight.
Protonic Ceramic fuel cell (PCFC) This new type of fuel cell is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures. PCFCs share the thermal and kinetic advantages of high temperature operation at 700 degrees Celsius with molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in PEM and phosphoric acid fuel cells. The high operating temperature is necessary to achieve very high electrical fuel efficiency with hydrocarbon fuels. PCFCs can operate at high temperatures and electrochemically oxidize fossil fuels directly to the anode. This eliminates the intermediate step of producing hydrogen through the costly reforming process. Gaseous molecules of the hydrocarbon fuel are absorbed on the surface of the anode in the presence of water vapor, and hydrogen atoms are efficiently stripped off to be absorbed into the electrolyte, with carbon dioxide as the primary reaction product. Additionally, PCFCs have a solid electrolyte so the membrane cannot dry out as with PEM fuel cells, or liquid can't leak out as with PAFCs.
Microbial fuel cell (MFC) Use the catalytic reaction of microorganisms such as bacteria to convert virtually any organic material into fuel. Some common compounds include glucose, acetate, and wastewater. Enclosed in oxygen-free anodes, the organic compounds are consumed (oxidized) by the bacteria or other microbes. As part of the digestive process, electrons are pulled from the compound and conducted into a circuit with the help of an inorganic mediator. MFCs operate well in mild conditions relative to other types of fuel cells, such as 20-40 degrees Celsius, and could be capable of producing over 50% efficiency. These cells are suitable for small scale applications such as potential medical devices fueled by glucose in the blood, or larger such as water treatment plants or breweries producing organic waste that could then be used to fuel the MFCs.
Benefits of Fuel Cell Technology Estimated $20 Billion market worldwide in next decade. Fuel cells can make hydrogen gas whenever there is an energy source and water (or a hydro-carbon fuel), it can be stored for later use. Also, in hydrogen form the energy can be transported with little losses! Passenger cars account for 6 million barrels of oil every day which is 85% of our oil imports. If 25% of the cars on the U.S. roads where fuel cell hybrids, oil imports could be reduced by 1.8 million barrels daily. Electric fuel cell cars are expected to increase in demand by $15 billion annually in the U.S.
Fuel Cell Research at NREL Generator H2 Fuel pump Storage
Impact of Fuel Cells on the Global Economy One of the biggest limiters to the advancement of alternative energy is the ability to store the energy. Since hydrogen fuel cells work with gasses or liquids that can be easily stored and transported, they may help solve the storage issue. If the U.S. maintains its leadership in fuel cell technology, it could have a very positive impact on our trade deficit! All nations need energy. The U.S. in particular. Fuel cells could have a significant impact on the global flow of money by having the U.S. buy less energy.
