Fuel cells

It has taken since 1839, when Sir William Grove invented the technology, for the fuel cell to be recognized as the likely principal power source of the future. It is the fuel cell that will be the bridge between the hydrocarbon economy and hydrogen-based society. David Hart, who is head of fuel cells and hydrogen research at Imperial College London, has no doubt about the possibilities for fuel cells:

If fuel cells fulfil their potential, there's no reason why they shouldn't replace almost every battery and combustion engine in the world.1

There is still considerable potential for improvements in the efficiency of fuel cells since they are not dependent upon heat per se but on electrochemical conversion which means they are not limited by the second law of thermodynamics.

Until recently one reason for scepticism about the technology was the cost. However, since 1989 there has been a dramatic fall in cost per kilowatt of output (see Fig. 7.1).

System developers are confident that cost will ultimately fall to $300-500 per kW installed capacity for stationary application, mostly due to economies of scale, but they are unable to predict a date.

In the USA there is considerable activity in fuel cell development, not least because of the DOE's (Department of Energy) upbeat stance over the technology.

The vision is staggering: a society powered almost entirely by hydrogen, the most abundant element in the universe. . . The overall goal of the DOE is to replace two to four quads of conventional energy with hydrogen by the year 2010, and replace 10 quads per year by 2030. A quad is the amount of energy consumed by one million households.2

So, what is it about the fuel cell that gets people so excited?

Fuel cells are electrochemical devices that generate direct current (DC) electricity similar to batteries. Unlike batteries they require a continual input of a hydrogen-rich fuel. They have been described as electrochemical internal combustion engines. In essence the fuel cell is a reactor which combines hydrogen and oxygen to produce electricity, heat and water. It is a robust technology with no moving parts. It is clean, quiet and emits no pollution when fed directly with hydrogen.

At the outset it should be useful to provide a glossary of terms associated with this technology.

12,000

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Figure 7.1 Improving economics of fuel cells (based on data from New Scientist, 25 November 2000)

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Figure 7.1 Improving economics of fuel cells (based on data from New Scientist, 25 November 2000)

Anode. Electrode at which an oxidation reaction takes place.

Bipolar plates. Plates used to connect fuel cells in series to form a stack and build up voltage. They can be made of steel, graphite or conducting polymer. The plates are designed to facilitate gas distribution to each cell of the stack; to assist cooling; fluids separation and distribution; electrical conduction and physical support. Flow channels are carved into the plates to allow an even distribution of hydrogen and oxygen to the cells (see Fig. 7.3). Catalyst. Molecule, metal or other chemical substance used to increase the rate of a reaction. The catalyst takes part in the reaction mechanism without being consumed by the reactants. Cathode. Electrode at which a reduction reaction takes place.

Cogeneration. The utilization of both electrical and thermal energy from a power plant. Electrode. Electric conductor through which a flow of electrons is created (an electrical current). An electrochemical system has a minimum of two: an anode and a cathode that are in direct contact with the electrolyte.

Electrolyte. A substance, solid or liquid, that conducts ions between electrodes in an electrochemical cell. The electrolyte is in direct contact with the electrodes.

Energy density. The amount of available energy per unit weight or volume of the power plant. Exothermic reaction. A chemical reaction that releases heat.

Ion exchange membrane. A thin film which allows ion conduction and separation of fuel (e.g. hydrogen) at the anode and oxidant (air) at the cathode. Another term for electrolyte. Matrix. The electrolyte-containing layer between the anode and cathode of the fuel cell.

Hydrogen

Oxygen Water and heat Figure 7.2 Polymer electrolyte or proton exchange membrane fuel cell

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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