Fuel
cells convert hydrogen or fuels containing hydrogen (like natural gas) to electricity,
heat, and water. Their efficiency of 75% to 80% has made them the target of much
research.
Fuel cells are catorgized by the type of electrolyte
that they utilize. The five main types are:
1. Alkaline
2. Solid polymer (proton exchange membrane)
3. Phosphoric acid
4. Molten carbonate
5. Solid oxide
Alkaline and solid polymer fuel cells operate at lower temperatures
(50-260 0C) and are best suited for transportation applications. The
other three operate at temperatures up to 1000 0C and are mainly used for
large, central power plants.
Since pure hydrogen and oxygen are expensive, substitutions are usually made. Air is
used as a source of oxygen. Hydrogen is provided in the form of gaseous mixtures
from natural gas, biomasses, or heavy hydrocarbon liquids such as fuel oil.
Obviously this is where current fuel cell use needs more work.
A possible solution to the problem may be to utilize the sun. For
some time, we've known that the sun is a great source of energy, but there has been a
significant problem. HOW DO WE STORE THE SUN'S ENERGY? Sure, you can use
batteries, but you need either several thousand batteries or some very large ones to store
an appreciable amount of energy. But, a fuel cell, combined with a photovoltaic cell
and some heat exchanging equipment can provide a very good way to store the sun's energy
for later use. Before you become skeptical and raise the issue of cost, keep in mind
that someday it will be cost effective (especially when hydrocarbons become
scarce). The key being the poor efficiency (around 30%) of the photovoltaic cells.
With a photovoltaic cell that performs better, the setup below may work very well.
By storing the sun's energy in the form of hydrogen and
oxygen (both pure gases), a fuel cell can be used to supplement a photovoltaic source.
Going one step further and utilizing the heat produced by the fuel cell to generate
electricity in a turbine could result in an overall efficiency of 85-90% for the
fuel cell system. The bottom line being that electricity is produced in an extremely
efficient manner, with zero emissions! In the future, we may someday store sunlight
in a bottle.
