The world energy demand depends mainly on fossil fuels with respective shares of petroleum, coal and natural gas at 38, 30 and 20%, respectively. The remaining 12% are filled by the non-conventional energy alternatives, which are 7% hydropower and 5% nuclear energy shares. It is expected that the world oil and natural gas reserves will last for about several decades, but the coal will sustain the energy requirements for few centuries. This means that the fossil fuel amount is currently limited and even though new reserves might be explored in the future, they will still remain limited and the rate of energy demand increased will require exploitation of other renewable alternatives at ever increasing rates. The desire to use renewable energy sources is not only due to their availability in many parts of the world, but more empathetically as a result of the fossil fuel damage to environmental and atmospheric cleanness issues. The search for new alternative energy systems has increased vigorously in the last few decades because of the following reasons:
1. the extra demand on energy within the next five decades is expected to continue and increase in such a manner that the use of fossil fuels will not be sufficient. Therefore, the deficit in the energy supply will be covered by additional energy productions and discoveries,
2. fossil fuels are not available in each country because they are unevenly distributed over the world, but renewable sources, and especially, the solar radiation is more evenly distributed, and consequently, each country can make the best to search and develop her own energy harvest, and finally,
3. fossil fuel combustion leads to some undesirable effects such as the atmospheric pollution because of the CO2 emissions and environmental problems including the air pollution, acid rains, greenhouse effect, climate changes, oil spills, etc. It is understood by now that even with refined precautions and present technology level, these undesirable effects cannot be avoided completely but minimized. One way of such minimization is to substitute at least some important part of the fossil fuel usage by solar energy.
The world wide environmental problems resulting from the use of fossil fuels are the most compelling reasons for the present vigorous search for future alternative energy options that are renewable and environmentally friendly. The renewable sources have also some disadvantages as being available intermittently as in the case of solar and wind sources or fixed to certain locations including hydropower, geothermal and biomass alternatives. Another shortcoming, for the time being, is their transportation directly as a fuel. These shortcomings point to the need for an intermediate energy systems to form the link between their production sites and the consumer locations. If, for example, heat and electricity from solar power plants are to be made available at all times to meet the demand profile for useful energy, then an energy carrier is necessary with storage capabilities over long periods of time for use when solar radiation is not available .
The solar radiation is abundant and becoming more economical, it is not harvested on large scales. This is due to the fact that it is difficult to store and move energy from ephemeral and intermittent sources such as the sun. However, fossil fuels can be transported easily from remote areas to consumption sites. For the transportation of electric power it is necessary to invest and currently spend money in large amounts. Under these circumstances of economic limitations, it is more rational to convert solar power to a gaseous form than is far cheaper to transport and easy to store. For this purpose, hydrogen is an almost completely clean-burning gas that can be used in place of petroleum, coal or natural gas. It does not release the carbon compounds that lead to global warming. In order to produce hydrogen, it is possible to run an electric current through water and this conversion process is known as electrolysis. After the production of hydrogen it can be transported to any distance with virtually no energy loss. Transportation of gases such as hydrogen is less risky than any other form of energy such as oil which is frequently spilled in tanker accidents, or during routine handling .
On the other hand, storage of hydrogen is much easier than electricity, especially in pressurized tanks or in metal hydrides, metal powders that naturally absorb gaseous hydrogen and release it when heated. Moreover, hydrogen can provide the concentrated energy needed by factories and homes. It can be used instead of natural gas in many human activities, such as restaurants, heat warehouses, and a wide range of industrial processes. It is also possible to develop appliances as hydrogen-powered furnaces, stove burners, water heaters, etc. Hydrogen can be used to run automobiles through either an internal combustion engine or more efficiently a fuel cell .
In all countries, automobiles consume petroleum as a chief fuel that causes air and atmospheric pollution in many cities. For sustainable energy future, non-polluting solar (renewable) energy sources are essential to power transportation systems. Prototype electric automobiles are now available which do not consume air-polluting fossil fuels. These environmentally friendly automobile developments and wide uses are restricted due to the size of their batteries, and the need for frequent recharging. Tightening air quality standards led many companies to produce electric cars and vans during the 1990s.
Automakers now consider vehicles fueled with hydrogen. It is the cleanest burning fuel, which produces only water vapor and small amounts of nitrogen oxides. Minor modifications are needed to make a gasoline-powered engine run on hydrogen although storing hydrogen remains a problem.
The ideal intermediary energy carrier should be storable, transportable, pollution-free, independent of primary resources, renewable and applicable in many ways. These properties may be met by hydrogen when produced electrolytically using solar radiation, and hence, such a combination is referred to as the solar-hydrogen. This is to say that transformation to hydrogen is one of the most promising methods of storing and transporting solar energy in large quantities and over long distances.
Water can be split electrolytically into its components, hydrogen and oxygen by using thermal and/or photovoltaic electricity from solar power plants. Hydrogen can then be used similar to natural gas for heating and as gasoline fuel for transportation. If electrochemical fuel cell conversions are used then hydrogen can be converted directly for electricity production. Among the many renewable energy alternatives, the solar-hydrogen energy is regarded as the most ideal energy resource that can be exploited in the foreseeable future in large quantities. On the other hand, where conventional fuel sources are not available, especially in rural areas, solar energy can be used directly or indirectly by its transformation into hydrogen gas. The most important property of hydrogen is that, it is the cleanest fuel, non-toxic with virtually no environmental problems during its production, storage and transportation. Combustion of hydrogen with oxygen produces absolutely no pollution, except its combustion in air produces small amounts of nitrogen oxides. Solar-hydrogen energy through the use of hydrogen does not give rise to acid rains, greenhouse effects, ozone layer depletions, leaks or spillages. It is possible to regard hydrogen after the treatment of water by solar energy as a synthetic fuel. Hydrogen can be produced by steam reforming of natural gas or coal gasification. In order to benefit from the unique properties of hydrogen, it must be produced by the use of a renewable source so that there will not be any limitation or environmental pollution in the long run.
Different methods help to produce hydrogen from direct or indirect forms of solar energy. These methods can be viewed under four different processes, namely, direct thermal decomposition or thermolysis, thermochemical processes, electrolysis and photolysis. Big scale hydrogen production has been obtained so far from the water electrolysis method which can be used effectively in combination with photovoltaic cells. Hydrogen can be extracted directly from water by photolysis using solar radiation. Photolysis can be accomplished by photobiological systems, photochemical assemblies or photo-electrochemical cells.