Fuel
Cells Fuel Cell TypesPhosphoric
Acid
Phosphoric AcidUnder Construction
Proton Exchange Membrane (PEM)The operating principle of the PEM fuel cell (PEM stands for proton exchange membrane) is as simple as it is promising. The loud pop used to demonstrate the explosive reaction of hydrogen with atmospheric oxygen is a familiar feature in school chemistry lessons, and provides a graphic illustration of the amount of energy released. This reaction also takes place in fuel cells, but under "cold" conditions, without the generation of flames. The core of the PEM cell is a special plastic membrane that is only permeable to hydrogen ions (protons). On one side, hydrogen streams past platinum catalysts, while air or pure oxygen is present on the other side. The protons pass through the membrane and combine with the oxygen to form water, which is the only "waste" product of the reaction. The electron flow forming part of this exchange takes place via an external electrical circuit—the hydrogen supplies the electrons to one electrode while the oxygen receives them from the other electrode. This flow of electrons can power the electric motor of an automobile or other electrical consumer. Replacing the existing network of gas stations with hydrogen outlets would be too expensive, so PEM pioneers plan to generate hydrogen on board vehicles. Their preferred fuel is methanol, which can be easily obtained from natural gas or even renewable raw materials, and dispensed just like gasoline. A reformer—a small chemical plant—is required on board to split the methanol. Development of a fuel cell that can run directly using methanol, without the need for chemical reforming, is also underway. The main problem here is that existing membranes are methanol permeable to a certain extent, which sharply reduces power density and requires operation with a large excess of air, i.e. with a powerful blower. A fuel cell's energy conversion factor of 60 to 70 % is about twice that of an internal combustion engine. However, so much energy is lost in the system itself (during air compression, for example) and particularly during conversion from natural gas to methanol to hydrogen and electricity that overall efficiency, considered over the entire process from primary energy source to wheels, is only slightly higher than that of a turbo-diesel direct-injection engine. Nevertheless, the major advantage of such an engine is low emissions, since a methanol-powered fuel cell only produces half as much carbon dioxide and many times less pollutants than a gasoline engine. For stationary industrial applications PEM cells are at a disadvantage in that their operating temperature of 60 to 70 °C—otherwise considered advantageous for reasons of safety and efficiency—is too low for a useful supply of thermal energy. Source: Siemens web site 8/01
Molten CarbonateWith the closing of MC Power in the spring of 2000, FuelCell Energy is the only company focusing exclusively on molten carbonate fuel cell technology. Interestingly, the demise of MC Power didn’t occur because of inferior technology. According to former MC Power employees, the failure occurred because the company was unable to wean itself from government research and development money, and it could not raise cost-sharing funds to support its development work.
Solid Oxide
Solid oxide fuel cell (SOFC ) advocates frequently point out that SOFCs have a higher conversion efficiency than PEMs for turning fuel to electricity. For PEMs, the range of conversion efficiency is typically 30–40 percent, but solid oxide could exceed 50 percent. In fact, one early field test unit (built by Siemens-Westinghouse) exceeded 46 percent in its test run. Previously, solid oxide fuel cell technology has been seen as lagging behind PEM technology in terms of development. However, given the current timing of commercial-ization efforts, solid oxide companies may be coming to market only two or three years after PEM developers do. Operating at a temperature of 1,000 °C, this type of cell can produce electric power from natural gas or other fossil fuels while also supplying process heat or hot water. Compact SOFC systems with a power output of between 1 and 100 MW are therefore an ideal solution for distributed power plants that supply industrial facilities, shopping centers, hospitals, office parks, airports or entire sections of cities. A solid oxide fuel cell (SOFC) converts energy carriers such as natural gas into electric current directly and at extremely high rates of efficiency. First, the gas is desulfurized. Within the fuel cell module at 1,000 °C, the natural gas is converted into hydrogen and carbon monoxide, which flow along the outer walls of the ceramic tubes. In the next step, air is pumped into the interior of the tubes. The oxygen component of the air, in the form of oxygen ions, permeates the zirconia electrolyte layer of the tube. There, it combines with the hydrogen to form water, and with the carbon monoxide to form carbon dioxide. The two "waste products" of the SOFC are therefore water vapor and carbon dioxide. The process also generates DC electric power, which is converted to AC power in an inverter and fed into the network. The high temperature of the resulting exhaust air can be converted into additional electric power in an associated gas turbine. This boosts the overall efficiency of the system to about 60 %. In more advanced plants it can be raised to 70 % or more.
AlkalineBecause of problems with carbon dioxide (CO 2 ) poisoning of the stack, alkaline fuel cells are typically only considered usable under conditions where pure oxygen is provided (such as in the space shuttle) and not under ambient air conditions on Earth. However, one company reports that the concern about carbon dioxide poisoning is being overcome by scrubbing CO 2 out of the oxygen air stream prior to pumping it into the fuel cell stack.
Direct MethanolUnder Construction
ManufacturersA list of Manufacturers and Vendors of Fuel Cells is located within the Applications Guide, Manufacturers Section.
Source: TechPro DTE Energy Bob Fegan 2002 |
