Hydrogen: A Clean Energy Source
Hydrogen is a colourless, odourless, tasteless and flammable gas. Hydrogen has highest energy content per unit mass of 120.7 MJ/kg among known fuels. But, its energy content per unit volume is low. So, storage of Hydrogen is challenging compared to storage of fossil fuels. Hydrogen, when burnt, produces water as a by-product. So it is a clean and environment friendly fuel.
Hydrogen is found only in combined form on earth, and has to be isolated from its compounds, which requires energy. Globally about 96 % Hydrogen is produced using hydrocarbons (48 % from natural gas, 30 % from oil, 18 % from coal) through steam reformation, partial oxidation and gasification processes. About 4 % Hydrogen is produced by electrolysis of water.
Three general categories of processes used in production of Hydrogen are: thermal processes, electrolytic processes and photolytic processes. Some thermal processes use the energy of resource such as natural gas, coal or biomass to release hydrogen, which is part of molecular structure of resource. In other processes, heat in combination with closed-chemical cycles, produces hydrogen from feed-stocks such as water. These are known as 'thermo-chemical' processes. However, thermo-chemical process is in early stage of development.
Steam Methane reformation process is most widely used for production of hydrogen for industrial application. The hydrogen is also produced by gasification of coal and biomass, which are locally available and biomass is renewable. Electrolytic processes use electricity to split water into hydrogen and oxygen.
Refineries and fertiliser plants are major in-situ producers and consumers of hydrogen in India. Hydrogen is also produced as a by-product in choler-alkali industry.
Storage is the major barrier in commercialisation of hydrogen-fueled vehicles. The most common method of storage of hydrogen is in gaseous form in highly pressurised cylinders/tanks. High pressure is required as hydrogen is the lightest element. Hydrogen can also be stored in liquid form, which requires low temperatures with cryogenic storage systems. This form of storage of hydrogen consumes higher amount of energy.
It is also possible to store hydrogen in certain solid-state materials like metal hydrides, complex hydrides, liquid organic hydrides, carbon nanostructure and in chemicals. Solid state storage materials require moderate temperatures and pressures for absorption and desorption of hydrogen. Storage of hydrogen in solid state materials is considered to be safe in comparison in gaseous and liquid form storage.
Hydrogen Energy Technologies
The current technologies for use of hydrogen for power generation and in automobiles are yet to be optimised and commercialised. Efforts have already started for development of hydrogen-CNG blend as fuel for vehicles. Both IC engine and fuel cells technologies will be of interest.
Applications of Hydrogen Energy
In the area of utilisation of hydrogen in IC engines, pilot projects for using hydrogen blends with CNG as well as diesel, development of hydrogen fuelled three wheelers with hydrogen stored in metal hydride canister are underway. Catalytic combustion cookers using hydrogen as fuel have also been developed.
A fuel cell is a electrochemical device that converts chemical energy of hydrogen directly into electricity and heat without combustion. Fuel cell systems generally operate on pure hydrogen and air/oxygen to produce electricity with water and heat as by-products. Fuel cell technology is important, because it is cleaner process and produces energy ia an more-efficient manner than combustion of fossil fuels. Fuel cells have the potential to replace the IC engine in vehicles and provide power in stationery and portable power applications as they are energy efficient, clean and fuel flexible. Fuel cells power system can be used as UPS systems, replacing batteries and diesel generators.
In view of these, developments of Polymer Electrolyte Membrane Fuel Cells (PEMFC) is underway. A number of automobile companies have developed PEMFC based cars and buses. Among different types of fuel cells, while Direct Methanol Fuel Cells (DMFC), Direct Ethanol Fuel Cells (DEFC) are being developed for portable applications, high temperature fuel cells like Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC) are being developed for stationery applications. Phosphoric Acid Fuel Cells (PAFC) have been developed and commercialised. The present effort in these fuel cells are focussed on reducing its cost and improving its durability.