Renewable and green sources of energy are essential replacements for fossil fuel because of two reasons:
Worldwide fossil fuel reserves are limited.
While they are available, they give rise to greenhouse gas emissions thus threatening human life on earth.
Unfortunately, all renewable sources are intermittent. Their integration into the existing power network creates problems unless we can store the spare available energy in a quickly retrievable form. Further, electricity forms only about 30% of the total energy expense. The remaining 70 percent expense demands fuels, e.g. for transport sector. Therefore, solar fuels are important to us.
Parameters of a Good solar fuel
A good solar fuel will have the following characteristics.
Low Ultimate Cost- the overall cost per unit of energy as used by the consumer must be low. This includes costs at all the various stages:
o Cost of feedstock
o Cost ofrenewable energy going in
o Cost of conversion
o Conversion efficiency
o Storage cost including cost of leakage, losses.
o Cost of transportation, if any
o Cost of recovery to energy
o Cost of losses during recovery.
GHG Foot Print- Renewable sources like the sun, wind, sea, etc. are by themselves totally green. But when we want to convert them into another form, eg, solar to photovoltaic, the equipment and the processes used do have a green-house foot print because fossil fuel indirectly used. Similarly, storage of energy in any form will need some equipment and some processes, and hence, will have a carbon foot print. The same is true of energy recovery and utilization process. It is important therefore, that the process of generating a solar fuel, its storage, and re-utilization, must have a low carbon foot-print.
Energy Efficiency- The two-way conversion process must be efficient. That means collection from the renewable source, generation of the fuel, storage, and recovery from the fuel should have the best overall energy efficiency.
Energy Density-The generated solar fuel must not demand too much storage volume on a per Meg-Joule basis.
Storage at or Near Ambient Pressure- Gases can be stored at normal pressure but need excessively large volumes. While compression can give lower volumes, special design will be needed for high pressure storage.
Fast Recovery- We should be able to fast and easily. This is particularly important when integration with power generation network is required.
Integration With the Existing Distribution Network- The fuel should be easily distributable through the existing distribution and utilization networks minimal adjustment and modifications. For example, if we are able to generate a solid fuel which can be distributed through the existing coal network that will be one advantage. A gas could be merged into the natural gas distribution. Thus, the form of solar fuel should pose minimal distribution problems.
Solar Fuel Feed-stock
Current research is directed towards preparation of solar fuel from two natural feed-stocks. These are water, and carbon. Both are abundantly available and inexpensive. Both have merits and demerits. It is still not clear whether any one technology will ultimately dominate, and which one. It is also possible that altogether different feedstocks may be identified in the future.