FUEL CELLS

The fuel cell is an electrochemical device, closely related to the battery, which harnesses a chemical reaction between two reactants to produce electricity. A battery is usually intended as a portable or self-contained source of electricity, and it must carry the reactants it needs to generate electricity within it. Once they are exhausted the battery can no longer supply any power. A fuel cell, by contrast, does not contain any chemical reactants itself but is supplied with them from an external source. So long as these reactants are made available, the cell will continue to provide power.

Batteries come in many different forms, each exploiting reactions between a variety of different chemicals to generate electricity. Fuel cells are unique in that almost all of them exploit a single reaction—that between hydrogen and oxygen to produce water. (There is one exception, the direct methanol fuel cell, which utilizes methanol and oxygen and produces carbon dioxide and water as its by-products.) The simplicity of the basic fuel cell reaction makes it both technically and environmentally attractive—the latter since the only by-products are water and some heat. However, in practice, fuel cells are not quite so simple.

The oxygen for a fuel cell can be supplied from air and air is normally pro- vided to one of the cells electrodes. However, there is no ready source of hydro- gen available today. To overcome this, most fuel cells utilize natural gas that they convert to hydrogen in a process known as reforming. This generates car- bon dioxide as a by-product, somewhat tarnishing the otherwise spotless environmental credentials of the fuel cell. Even so fuel cells are relatively benign, environmentally, compared to other fossil fuel–generating technologies.

Aside from its environmental performance, one of the most attractive features of the fuel cell is the fact that its energy conversion process is not limited by Carnot cycle thermodynamics that restrict the ultimate efficiency at which any type of heat engine can operate. The best practical simple cycle heat engine efficiency is 50%, achieved in a diesel engine. By contrast, the best fuel cell— an alkaline fuel cell provided with hydrogen reactant—can achieve 60% efficiency today.

The theoretical maximum efficiency for a hydrogen fuel cell operating at room temperature is 83%, but in practice most fuel cells operate at elevated temperatures, reducing efficiency. However, some high-temperature fuel cells can operate in a variety of combined cycle configurations allowing additional gains in efficiency. Meanwhile, overall cycle efficiency is reduced when natural gas must be reformed since this is an energy-intensive reaction. Even so, practical cells are competitive on an efficiency basis with other fossil fuel technologies. If a hydrogen economy ever develops, with hydrogen replacing natural gas as the primary gaseous fuel, then fuel cells should be able to outperform the competing combustion technologies with ease.

The fuel cell has other advantages too. The cell itself has no moving parts and can operate for long periods without maintenance—far longer than any tur- bine or engine-based generating system. The absence of moving parts makes them inherently quiet too (although this is limited by the use of mechanical pumps, which generate noise), and they emit relatively low levels of pollution compared to other types of generating system based on fossil fuel.

With so much going for them, why are there no fleets of fuel cell power plants today? The answer is cost. While the fuel cell principle has been known since the first half of the 19th century, development of a cheap version of the device has proved extremely challenging. As a result, the first commercial fuel cells only appeared in the early 1990s. Since then research into a wide range of fuel cells has advanced and a variety of commercial units are available. Cost still remains a challenge, but interest in fuel cells as potential power units for vehicles has spurred research that could see some types eventually achieve competitively low prices. Meanwhile, domestic heat and power units based on fuel cells are opening potential new markets for these devices.

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