Sun is a major and sustainable source of energy for the planet earth. Every being on earth is dependent on sun in one way or other, except for the rare bacterial species that can survive solely on chemicals present on earth. As explained during our Physics tuition classes, the law of conservation of energy states that energy can neither be created nor destroyed but can be converted from one form to another. As per the law, solar energy can be converted to electric energy but the ways employed to generate electricity vary. Concentrated solar heating, solar water splitting are the alternatives to photovoltaics where solar energy is converted into electricity.
Concentrated solar power (CSP) was discussed in the earlier article and this article deals with solar water splitting. Energy is released when strong chemical bonds are formed and a chemical bond demands energy to be broken. The energy released is generally in the form of heat. This heat energy can be used to convert water to steam and subsequently run turbines. The chemical energy released in the formation of bonds can also be directly used to generate power like it is done in fuel cells (with no turbines used in the process). Oxygen and hydrogen gases form a water molecule with a release of large amounts of energy. Hydrogen based cars are already mainstream. These chemicals can be mixed in a certain way in a fuel cell to generate large amounts of electricity. Alternatively, the heat energy released in the process of water molecule formation can be used to run turbines for large-scale centralised power generation. But how do we get oxygen and hydrogen and how is it connected to Sun?
Solar water splitting cells are used to generate oxygen and hydrogen from water with the help of sunlight. In a typical solar water splitting cell, there are two terminals immersed into water. At one terminal, electrons are released in the presence of sunlight and they move to the other terminal where they are consumed. Hydrogen ion is a positively charged species that consumes the electrons to produce hydrogen gas. At the oxygen releasing terminal, electrons are released and also oxygen gas. Semiconducting materials are used at terminals where the process of release or consumption electrons takes place. Oxygen and hydrogen thus formed are trapped and contained separately. These gases can be used to generate power as mentioned above or can also be used for other industrial chemical purposes. Thus solar energy can harvested and stored in the form of chemical energy that can readily generate power when needed.