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Fuel Cells

While the concept of fuel cells has been around for more than 100 years, the first practical fuel cells were developed for the U.S. space program in the 1960s. The space program required an efficient, reliable, and compact energy source for the Gemini and Apollo spacecraft, and the fuel cell was a good fit. Today, NASA continues its reliance on fuel cells to power space shuttle vehicles. Because of technology improvements in recent years and significant investment by auto companies, utilities, NASA, and the military, fuel cells are now expected to have applications for distributed power generation within the next few years.

 

A fuel cell is similar to a battery in that an electro-chemical reaction is used to create electric current. The charge carriers can be released through an external circuit via wire connections to anode and cathode plates of the battery or the fuel cell. The major difference between fuel cells and batteries is that batteries carry a limited supply of fuel internally as an electrolytic solution and solid materials (such as the lead acid battery that contains sulfuric acid and lead plates) or as solid dry reactants such as zinc carbon powders found in a flashlight battery. Fuel cells have similar reactions; however, the reactants are gases (hydrogen and oxygen) that are combined in a catalytic process. Since the gas reactants can be fed into the fuel cell and constantly replenished, the unit will never run down like a battery.

Fuel cells are named based on the type of electrolyte and materials used. The fuel cell electrolyte is sandwiched between a positive and a negative electrode. Because individual fuel cells produce low voltages, fuel cells are stacked together to generate the desired output for DG applications. The fuel cell stack is integrated into a fuel cell system with other components, including a fuel reformer, power electronics, and controls. Fuel cell systems convert chemical energy from fossil fuels directly into electricity. The image below shows the basic components of a generic fuel cell.

The fuel (hydrogen) enters the fuel cell and is mixed with air, which causes the fuel to be oxidized. As the hydrogen enters the fuel cell, it is broken down into protons and electrons. In the case of PEMFC and PAFC, positively charged ions move through the electrolyte across a voltage to produce electric power. The protons and electrons are then recombined with oxygen to make water, and as this water is removed, more protons are pulled through the electrolyte to continue driving the reaction and resulting in further power production. In the case of SOFC, it is not protons that move through the electrolyte but oxygen radicals. In MCFC, carbon dioxide is required to combine with the oxygen and electrons to form carbonate ions, which are transmitted through the electrolyte.

Types of Fuel Cells
There are four fuel cell technologies currently under development. These include phosphoric acid fuel cells (PAFC), which are the only fuel cells commecially available, although at a price not competitive with alternative technologies. Molten carbonate fuel cells (MCFC) are best suited for large power plants and can use natural gas directly without the need for an external fuel processor. MCFC developers project commercialization within five years. Solid oxide fuel cells (SOFC) have a comparable efficiency and the same power applications as MCFCs. Proton exchange membrane fuel cells (PEMFC) are considered a promising technology in the transportation and small stationary power markets. The technologies are at varying states of development or commercialization.

Natural gas (methane) is considered to be the most readily available and the cleanest fuel (next to hydrogen) for distributed generation applications, so most work is focused on natural-gas-powered fuel cells. However, fuel cells need hydrogen gas to operate; so the key is converting natural gas into a hydrogen-rich gas.

The low-temperature fuel cell technologies such as the phosphoric acid fuel cell (PAFC) and proton exchange membrane fuel cell (PEMFC) require a fuel processing unit (reformer) to convert the natural gas into a hydrogen-rich mixture suitable for use in the fuel cell. The reformer uses a catalytic reaction process that creates the hydrogen-rich mixture. High temperature fuel cells such as the molten carbonate fuel cell or the solid oxide fuel cell do not require a reformer since the high operating temperature of the fuel cell allows for the direct conversion of natural gas to hydrogen.