Introduction
Conclusions
Renewable Sources of Hydrogen
Many of the effects of using hydrogen fuel will depend on the way hydrogen is produced. A sustainable and environmentally friendly source of hydrogen is water electrolysis. If a renewable source of energy could be used for water electrolysis, then there will be more environmental benefits to a hydrogen economy than if fossil fuel energy has to be used.
>>Solar Energy
One of the possible energy sources to produce hydrogen gas is solar energy. The sun produces enough energy for water electrolysis, and this energy can be harnessed more efficiently using metal catalysts. One suggested catalyst is zinc oxide [1]. Hydrogen would be produced in two steps using this catalyst. The first step would require solar energy:
ZnO = Zn + 0.5 O2
This step is endothermic, but at 2235K, delG°= 0. The activation energy of this reaction is 310-350 kJ/mol. The second step would not require solar energy:
Zn + H2O = ZnO + H2
This step would take place at 700K. The zinc oxide produced in this step could be reused in the first step. This process has the benefit that hydrogen and oxygen are not produced in the same step, so do not have to be separated.
The equipment used for this technique would concentrate the sun’s energy to an intensity of 5000 suns, which corresponds to an ideal temperature of 3000K. This means there would be enough energy to overcome the activation barrier in step one. The efficiency of this method would be around 29%. The estimated cost of hydrogen produced in this way is $0.15/kWh. This is more expensive than hydrogen produced from steam-reformation of natural gas, which costs $0.03-0.04/kWh, but hydrogen production from solar energy has more environmental benefits, such as no release of carbon dioxide.
Solar energy could also be used for water photolysis, which is like electrolysis without the extra step of generating electricity [2]. Electrolyzers run at around 1.7-1.9 V, which corresponds to a wavelength of 650 nm, which is in the red region of light. Therefore, sunlight has enough energy to directly split water, but a catalyst needs to be found to make the energy go into splitting the water.
>>Wind Energy
Another proposal is to use wind energy to produce hydrogen [3]. There would be an initial high cost of implementation, but once windmills are set up, the wind energy that is not needed can be used to electrolyze water, so the energy will be stored as hydrogen. This method would be more useful in some places than others, depending on wind conditions. Ireland, for example, has a large potential to use wind energy. It is estimated that wind power in Ireland could produce 344 TWh/y in energy, and that in 2010 the electricity demand will be 33.5 TWh. Although Ireland would not be able to produce the maximum estimated wind power, there still could be much more power than needed, and the extra energy could be used for water electrolysis.
Using renewable energy sources to produce hydrogen makes the energy sources more practical. Excess energy from the sun or wind will not be wasted; instead it will be stored. Also, since hydrogen is more transportable than electricity, solar or wind power will benefit people who do not live near places with high amounts of sunlight or wind if the energy can be stored as hydrogen.
References:
1. Steinfeld, A., Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. International Journal of Hydrogen Energy, 2002. 27: p. 611-619.
2. Turner, J.A., A Realizable Renewable Energy Future. Science, 1999. 285: p. 687-689.
3. Gonzalez, A., E. McKeogh, and B.O. Gallachoir, The role of hydrogen in high wind energy penetration electricity systems: The Irish case. Renewable Energy, 2003. 29: p. 471-489.