H2 and FC Properties
Hydrogen is the simplest of all elements with one electron and one proton. Two hydrogen atoms form one hydrogen gas molecule, or H2, but this gas is rarely found in large quantities in nature. Hydrogen’s chemical properties allow it to combine easily with other elements to form other molecules. The simplest example is hydrogen’s presence in water, or H2O. As water makes up 70 percent of the Earth’s surface, hydrogen is in abundant supply. Moreover, hydrogen can be extracted from fossil fuels through reformation. Similarly, hydrogen can be extracted from organic materials such as bio-waste, solid waste, landfill gases or biomass (agricultural products specially grown for fuel or parts of agricultural products, such as stalks and stems, not used for human or animal consumption).
Hydrogen has the highest energy content by weight of any fuel – 52,000 Btu per pound . Hydrogen gas is nontoxic with no color, odor or taste; a pure hydrogen flame is invisible without special glasses. Like gasoline, hydrogen ignites easily. Hydrogen compared to other gases has a high diffusion rate, the process by which the gas molecules spread out and interact as a result of energy and random motion. This requires that hydrogen be stored in ways to ensure the gas has a reasonable density for applications.
Hydrogen can be used to increase efficiency in internal combustion engines (ICEs). It is estimated that a direct-injected hydrogen ICE could have 20-25 percent greater efficiency than a similar gasoline ICE . However, most proponents of hydrogen envision its use to generate electricity when powering a fuel cell. In a fuel cell, the theoretical efficiency can reach 83 percent; in practice 60 percent of hydrogen’s energy is converted to electricity with the rest generating heat energy that can be used in combined heat and power (CHP) applications. Comparing gasoline to hydrogen, the energy in one gallon of gasoline is roughly the equivalent to 1 kg of hydrogen . By weight, hydrogen has about three times the amount of energy as gasoline.
Most hydrogen today is not used as a fuel source, but rather as a chemical for oil refining and ammonia production . About two-thirds of industrial hydrogen is used in ammonia production for fertilizer . Hydrogen can also be used in fat hydrogenation, methanol production, welding, and the production of hydrochloric acid. To give an idea of the amount of hydrogen in use in today’s economy, the small amount of merchant hydrogen produced in the United States in 2002, according to one estimate, could suffice to support a fleet of 20-30 million fuel cell cars .
The technical understanding of fuel cells has existed since the 19th century. Fuel cells were first created in 1839 by Sir William Grove and refined in 1932 by Francis Bacon. The most well known application of fuel cells was aboard NASA space shuttles to provide electricity to various systems. A fuel cell provides electricity in a manner similar to a battery. Like a battery, a fuel cell produces direct current (DC) power, not alternating current (AC) power. However, the fuel cell can continue to provide energy so long as a fuel is present. A battery, in contrast, has a finite storage of energy before it needs to be recharged.
All fuel cells contain an anode, cathode and electrolyte. The hydrogen fuel is broken into electrons and protons by virtue of a catalyst, and combines with oxygen supplied to the fuel cell to create electricity, water and heat. The hydrogen fuel is fed into the anode (a negative electrode that repels electrons) of the fuel cell. Oxygen enters through the cathode (a positive electrode that attracts electrons). Encouraged by a catalyst, such as platinum, the hydrogen atom splits into a proton and an electron. The electrons cannot permeate the electrolyte and therefore are released through an external current to produce electricity. The hydrogen protons filter through the electrolyte to the cathode. The electrons provide an electrical current before returning to the cathode to be reunited with the hydrogen and oxygen (usually coming from ambient air, but sometimes pure oxygen) in a molecule of water.
There are different types of fuel cells that can be used to generate energy. The properties of each fuel cell provide the basis for deciding their most suitable application. The top fuel cell designs are Polymer Electrolyte also known as Proton Exchange Membrane (PEM), Phosphoric Acid, Molten Carbonate, and SolidOxide. There are a few other types of fuel cells, but these are the models being developed and marketed by manufacturers for commercial applications.
Polymer Electrolyte/Proton Exchange Membrane Fuel Cell (PEM) - The PEM fuel cell uses an advanced plastic electrolyte to move protons from the anode to the cathode. The PEM uses a solid electrolyte and operates at a low temperature. The PEM uses a thin platinum catalyst to split the electrons from the hydrogen protons. PEM fuel cells are best suited for 1kW to 100kW applications.
Phosphoric Acid Fuel Cell (PAFC) - This fuel cell has been commercially available since 1992. The PAFC is suited for small Distributed Generation (DG) units. They are highly reliable, quiet to operate, and highly efficient. The PAFC runs at a medium temperature range and uses impure hydrogen, which makes them more flexible with multiple sources of hydrogen and production methodologies.
Molten Carbonate Fuel Cell (MCFC) - MCFCs use a ceramic electrolyte filled with carbon and salt. MCFCs operate at high temperatures (800°F), which best suits them for large stationary applications. These fuel cells operate at 85 percent efficiency when operated in conjunction with traditional energy grids. MCFCs are currently used in many demonstration projects, and are expected to be market ready in 2004. Large buildings like hospitals, hotels, or other industrial facilities that require electricity and heating (or cooling) around the clock would be likely applications for the MCFC.
Solid Oxide Fuel Cell (SOFC) - These fuel cells are considered utility grade and are well suited for largescale stationary power generators that could provide electricity for factories or towns. SOFCs use a ceramic oxide electrolyte. Like MCFCs, they operate at higher temperatures (about 1,000°F) and work best as co-generation devices for industrial applications where high temperature steam is required. These should be commercially competitive in the 2005 to 2007 timeframe. SOFCs are also being developed for residential CHP applications.
1 United States Department of Energy, Energy Efficiency and Renewable Energy. "Hydrogen Quick Facts." [online] <http://www.eere.energy.gov/hydrogenandfuelcelss/hydrogen/hydrogen_feature.html>.
2 Ogden, Joan M. "Hydrogen: The Fuel of the Future?" Physics Today. April 2002: 55 (4) 69-75.
3 Ogden, Joan M. "Prospects for Building a Hydrogen Energy Infrastructure." Annual Review of Energy and the Environment. 1999: 24 227-79.
4 National Hydrogen Association. "Hydrogen FAQs." [online] <http://www.hydrogenus.com/h2-FAQ.asp>.
6 United States Department of Energy, Energy Efficiency and Renewable Energy. "Hydrogen Quick Facts." [online] <http://www.eere.energy.gov/hydrogenandfuelcelss/hydrogen/hydrogen_feature.html>
7 The text that follows is adapted from the U.S. Department of Defense's online Fuel Cell Information Guide: United States Department of Defense. "Fuel Cell Information Guide." [online] <http://www.dodfuelcell.com/fcdescriptions.html>.
Amory Lovins Hydrogen Primer, Rocky Mountain Institute, click here