Fuel cells

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The fuel cell is an electrochemical device, closely related to the battery, which harnesses a chemical reaction between two reagents to produce electricity. A battery is usually intended as a portable or self-contained source of electricity and it must carry the reagents it needs with it. Once they are exhausted the battery can no longer supply electricity. A fuel cell, by contrast, is supplied with reagents externally. So long as these reagents are made available, the cell will continue to provide power.

In addition to this difference between the fuel cell and a battery, there is something special about the chemical process the fuel cell harnesses to generate electricity. It consumes hydrogen and oxygen (usually supplied as air) and the only product of the reaction is water. The simplicity of this energy producing reaction and its inherent cleanliness makes the fuel cell an extremely attractive proposition from an environmental perspective.

Of course there is no large-scale source of hydrogen today, so fuel cells have to make do with hydrogen generated from natural gas in a chemical-reforming process. For now, this somewhat tarnishes the environmental credentials of the system. Nevertheless it can still provide an environmentally attractive source of electricity.

There are already a multitude of ways of generating electricity from a fossil fuel such as natural gas; why develop another? The answer is that there is a major difference between a fuel cell and these other electricity generating plants. Fossil fuel power stations which employ gas turbines, steam turbines or piston engines are all reliant on the thermodynamics of a heat engine. This limits the maximum theoretical efficiency that such devices can achieve.1 The fuel cell, by contrast, is limited by electrochemical conversion efficiency. Thus, while the highest efficiency a modern simple cycle heat engine can achieve is around 50%, the best fuel cell can convert 70% of the fuel energy into electricity.2

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 turbine- 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 do 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 nineteenth 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 and these were never competitive. Much research and investment has taken place since then and new generations of fuel cells are expected in the next 4 or 5 years which will be much more competitive.

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