Hydrogen fuel cells are one of the most promising up-and-coming clean power sources today. Fuel cells have been used for decades on NASA spacecraft, and other types of fuel cells are currently in use for power generation at a variety of commercial and industrial sites. Use of fuel cells in small system/home power applications has so far been limited by cost considerations, but prices are falling and fuel cells should emerge as a viable home power source within the near future.
A fuel cell consists of a central electrolyte layer, sandwiched between two catalyst layers. Various materials for these layers are used, but the basic process is the same. When a hydrogen atom contacts the negative anode catalyst layer, it splits into a proton and an electron. The proton passes straight through the central electrolyte layer, while the electron produces electricity as it passes through an external circuit. The circuit returns the electrons to the positive side of the electrolyte layer, where they bond again with the protons and join with an oxygen molecule, creating water in the positive cathode catalyst layer.
The fuel cell itself can be roughly correlated to the alternator in a wind, hydro or engine generator. The fuel cell itself is the mechanism that actually produces the electricity. However, in order for a wind, water or engine generator to produce electricity, the propeller or engine must turn the alternator. In order for a fuel cell to produce power, something must supply it with hydrogen and oxygen.
Various methods are used to supply the fuel cell with the necessary hydrogen and oxygen. Some systems use a "fuel reformer" to extract hydrogen from another fuel source such as propane, and can extract oxygen from the surrounding air. Some systems (in laboratory or industrial settings) are designed to be attached to tanks of pure hydrogen and oxygen.
The most interesting method of obtaining hydrogen, from a renewable energy standpoint, is to use an "electrolyser" to separate water into hydrogen and oxygen, which is then stored in tanks and fed into either end of the fuel cell. The "waste" water produced at the end of the fuel cell process is then fed back into the initial water source. A fuel cell generator set up to electrolyze and re-use water is known as a regenerative fuel cell. Any type of fuel cell could be used in a regenerative system, and the water electrolyser could be powered with wind, solar or hydro energy, resulting in a truly clean power system.
Proton Exchange Membrane (PEM) fuel cells are currently being considered for development of fuel cell powered cars, home power generators, and other small applications. Instead of using a liquid electrolyte, they use a thin polymer membrane. They operate in the range of 200º Fahrenheit, and can quickly vary power output depending on current demand. Many companies are currently working to develop commercially available, mass-produced PEM fuel cells.
Alkaline fuel cells have been used by NASA to provide power to spacecraft since the 1960s. They use alkaline potassium chloride as their electrolyte. Alkaline fuel cells can reach power generating efficiency of 70%, although their production costs have long rendered them out of range for mass production. A few companies are currently working on mass production techniques for these cells that would reduce their price within range of commercial use.
Phosphoric Acid fuel cells are by far the most widely used type of fuel cell today. They are primarily used for large back-up and remote power applications in hospitals, schools and other locations where an engine generator would traditionally be used. They operate in the 400ºF range, and can reach 40% power generation efficiency (much higher if byproduct heat and steam are used for other purposes). Phosphoric acid cells can also be used in large vehicles, such as buses and train engines.
Solid Oxide fuel cells are currently being refined for optimum operation in high-power industrial and utility applications. Operating efficiency could reach 60%, and the use of a hard ceramic electrolyte allows operating temperatures to run as high as 1800ºF.
Molten Carbonate fuel cells operate in the range of 1200ºF, and show promise for high power generation efficiency. They have the ability to use coal-based fuels, making them easy to integrate into the existing fuel supply system.
Direct Methanol fuel cells are a newer sub-type of the PEM cells. Rather than using a fuel reformer to extract hydrogen from an external fuel source or a electrolyser to break down water molecules, the anode catalyst extracts hydrogen directly from liquid methanol. These cells are expected to reach operating efficiencies around 40%.
A fuel cell would be of most use in one of two ways. For individuals with an existing solar, wind or hydro power system, the fuel cell could be used for backup power in place of an engine generator. Given that an engine generator operates at approximately 30% efficiency and the least efficient fuel cell currently offers 40% efficiency (up to 80-90% if byproduct heat and/or steam are used for other heating needs), the advantage is clear. When you also consider that the fuel cell will operate silently, with no waste products in regenerative systems and minimal waste in others, the fuel cell comes out a clear winner.
For individuals without an existing alternative energy system, a larger capacity fuel cell could comprise their primary power system. Since a fuel cell can produce power on demand, as long as hydrogen is available, there is no need for storage batteries if the fuel cell generator is large enough to support the electrical system in question. A wind turbine and/or solar panels could be added to power the water electrolyser or fuel reformer, and the entire power system would be virtually self contained.