The fuel cell: A new process to control
The fuel cell is like a battery, except that it never needs recharging because the heat and electricity produced by it are made from the inexhaustible and clean sources of water and air.
AS PROCESS control and the world at large forge ahead in this new millennium, we face new challenges. We all know that the reign of the internal combustion engine should end, and that we should start harnessing the chemical energy of hydrogen to generate the heat and electricity we need without causing pollution.
In order to do that, we have to learn how to control the fuel cell. The fuel cell is like a battery, except that it never needs recharging because the heat and electricity produced by it are made from the inexhaustible and clean sources of water and air.
The fuel cell consists of an electrolyte sandwiched between two electrodes. The hydrogen fuel can be obtained from a hydrocarbon "reformer" or from hydrogen directly. If it is from a reformer, the initial fuel can be a hydrocarbon such as ethane or methane. These hydrocarbons can be obtained from renewable agricultural sources, such as corn, or from the abundant coal reserves that the United States possesses. We could supply all our energy needs for 200-400 years from coal.
In an earlier column I reported on a fluidized bed-type coal-gasification technology, which I helped develop a quarter century ago as a synthetic fuel source [CONTROL—March ’02, p14]. At its heart, it reduced the residence time of the batch-type Lurgi process from two hours to a few seconds. Today, the project is gathering dust, because it does not serve the financial interests of the oil companies.
Eventually, the reformer will be dropped and hydrogen will be made directly from sea water by the use of solar energy. But no matter where the hydrogen comes from, it enters the fuel cell at the anode, where, with the help of a catalyst, it splits into an electron and a positive hydrogen ion (a proton). This proton travels to the cathode through the electrolyte, while the electron is available for use. It can be used to make electricity or can be returned to the cathode, where it is reunited with the hydrogen ion and oxygen to form clean water, which is exhausted.
Over the last century, we have learned how to control the internal combustion engine. Now we have to meet the challenge of not only controlling the electrolytic process in the fuel cell, but also the generation, transportation, storage, and distribution of liquid hydrogen. These are topics I plan to discuss in future columns.
The tank-to-wheel efficiency of the present internal combustion engine is 16%, while that of the fuel cell is three times that, or about 48%. Today's internal combustion engines are responsible for 65% of the oil consumption and 78% of the carbon dioxide generation in the United States. Every gallon of gasoline used for transportation releases 25 lb. of carbon dioxide, which is one of the greenhouse gases responsible for global warming. In addition, oil, the fuel supply of the internal combustion engine, is exhaustible and is a cause of international tensions.
FIGURE 1: HYDROGEN IN, POWER OUT
Hydrogen fuel enters the fuel cell, where an anode with a catalyst removes the electron to create electricity. The proton is combined with oxygen from the air for an exhaust product of water.
In contrast, the use of fuel cells would reduce, if not eliminate, international tensions. It would also allow us to convert to not only an inexhaustible energy source but also to a sustainable life style, which does not cause pollution. On top of that, because fuel cell vehicles operate with electric motors, vehicle noise and vibration would be reduced while eliminating the need for oil changes or spark plug replacements.
Fuel Cell Designs
Several fuel cell designs have evolved to date. Each requires a different control package. They are:
- Phosphoric Acid Fuel Cell -- Phosphoric acid electrolyte, platinum catalyst; can use hydrocarbon fuel. It is suited for stationary applications and generates both electricity and steam. There are 200 units in operation, in sizes ranging from 200 kW to 1 MW.
- Proton Exchange Membrane -- Solid organic polymer electrolyte, platinum catalyst; requires hydrogen fuel. It is suited for automobiles.
- Molten Carbonate Fuel Cell -- Carbonate electrolyte, conventional metal catalyst; can use coal gas or natural gas fuel. It’s best suited for 10kW to 2 MW power plants.
- Solid Oxide Fuel Cell -- Solid zirconium oxide electrolyte. Suitable for large-scale central electric power plants.
- Alkaline-Electrolyte -- An aqueous solution of alkaline potassium hydroxide, which is soaked in a matrix. It’s been used by NASA on space missions to generate both electricity and water.
- Direct Methanol Fuel Cell -- Polymer membrane electrolyte, no fuel reformer is needed because the catalyst draws the hydrogen directly from the liquid methanol. Usable for small power users as cellular phones or laptops.
- Regenerative Fuel Cell -- Solar-powered electrolyser separates water into hydrogen and oxygen to produce heat, electricity and water. It is regenerative, because the water is recirculated to make more heat and electricity.
- Zinc-Air Fuel Cell -- Electricity is produced as zinc and oxygen are mixed in the presence of an electrolyte to make zinc oxide. Can be a substitute for batteries.
- Protonic Ceramic Fuel Cell -- Eliminates the need for fuel reformers by using a ceramic electrolyte, which electrochemically oxidizes fossil fuels.
Many fuel cells are already in use. In the Space Shuttle, both electricity and water are provided by fuel cells. In 1993, the first fuel cell-powered bus was introduced. The prototype of a fuel cell-powered car was introduced in 1997 by Daimler Benz and Toyota. In 1999, Daimler Chrysler unveiled Necar 4, a liquid hydrogen vehicle having a top speed of 90 mph and a 280-mi. tank capacity.
It is up to all of us to speed this progress, so that hydrogen-based technology can mature before we run out of oil or before its shortage causes a tragedy.
After a century or so of transition, during which we've mixed agricultural and coal-gas based fuels, we should reach the final goal, which is a completely solar-based economy. This economy will probably depend on man-made solar islands distributed around the equator and used to collect the solar energy needed to make liquid hydrogen from sea water and thereby supply the total energy needs of the globe by a completely clean and inexhaustible energy source.
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