SUNLIGHT IN A BOTTLE

    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.