One of the biggest challenges facing us today is to produce ever increasing amounts of energy while mitigating the human impact on the Earth’s environment. This challenge can be met by promoting all forms of clean energy technologies. While the ultimate source of energy is the Sun, the energy is stored in various forms. Currently the primary reservoirs of energy are fossil fuels like petroleum and coal which represent energy from the Sun that was collected over millennia. The usage of fossil fuels adversely impacts our environment through the emission of green house gases such as carbon dioxide and nitrous oxide. Hence, humanity must decrease the emission of these gases into the atmosphere. However, there is a growing need for energy that must be fulfilled for improving the quality of lives of all human beings. It is clear that major advances in alternative energy technologies are required at the earliest.
Alternatives to fossil fuels are solar energy and nuclear energy. The challenge in using solar energy effectively mainly consists of distributing the energy among the various users. Homes need to be supplied with electricity and vehicles also need to be powered. Many believe that hydrogen will be the means of distributing solar energy, especially for transportation and distributed power generation. The idea is to use solar power to dissociate water into hydrogen and oxygen gases. The hydrogen can then be distributed, much like fossil fuels are distributed at present. Other ways of distributing solar energy are rechargeable batteries and solar thermal power.
The biggest challenge in nuclear energy technology is safety. The nuclear reactor must be operated in a fail safe manner, and the radioactive materials must be isolated from all life forms. Radioactive waste management technology is of paramount importance in this regard.
While we focus on developing clean energy technology based on solar and nuclear sources, can we afford to ignore the vast deposits of fossil fuels that are still accessible to us? The answer is a definite “No!” What is required is a clean, environmentally friendly and responsible way of using the fossil fuels. The efficiency of fossil fuel energy harvesting must be enhanced significantly. This means our vehicles must return many more kilometres to the litre. Our thermal power stations must consume less coal per megawatt hour of electricity produced. We must use more of public transport or choose highly fuel efficient vehicles, and switch off gadgets that are not in use.
Widespread use of clean energy technology is hindered by political, social and technological challenges. In this article, I shall focus on the technological challenges. More often than not, the success of energy technology hinges on the materials used. Development of better materials is one of the most important parts of progress in clean energy technology. Materials mainly fall into the categories of metallic, polymeric and ceramic. Of course, there are composite materials which combine two or more of these basic material types. All these material types play their roles in energy technology. In this article, the focus is on ceramic materials.
The major source of energy continues to be the fossil fuel deposits. Gas turbines in combined cycle plant enhance the overall efficiency of power generation. Ceramic coatings play an important role in enhancing the efficiency of power generation using coal, natural gas or petroleum-derived fuels. The purpose of ceramic coatings is to provide protection to metallic components against corrosion, oxidation, wear, erosion and excessive heat. A well-established application is that of porcelain enamels on steel components of heat exchangers. Specially developed compositions are coated using frits in the form of slurries. Enamelling is completed in furnaces at temperatures of ~425C and above. Steam turbines are coated with wear resistant materials, often ceramics. On the high end, stabilised zirconia coatings are deposited on the high pressure turbines and combustor liners of gas turbines used for power generation as well as aircraft engines. These ‘thermal barrier coatings’ or TBC’s represent one of the most critical applications of ceramic coatings. They allow the gas turbine to be maintained at high temperatures, which enhances efficiency. Moreover, the lifetime of the metallic components is increased as the TBC’s keep them below their maximum permitted temperature. Air plasma spray or electron beam physical vapour deposition are used for depositing TBC’s with thickness in the range of about 150 to 250 micrometres. Usually, a temperature difference of ~100C is maintained across the TBC. Another application of ceramics in gas turbines is in the form of abradable coatings which maintain a tight seal between the rotating turbine and its casing.
We are dependent on fossil fuels for transportation, mainly in the form of petrol and diesel. The efficiency of internal combustion engines, especially diesel engines can be enhanced by using ceramics in certain places. For instance, a thick coating of a zirconia ceramic on the piston head can increase performance or lifetime. Similarly, ceramic turbochargers can operate at high temperatures and increase the power delivered by the diesel engine for the same amount of fuel consumed. Many automobile components are coated with wear resistant ceramics such as nitrides to enhance wear resistance.
Ceramic materials have a major role in alternative energy technologies. Focussing on solar energy harvesting, the main photovoltaic (PV) material continues to be silicon. Ceramic materials, particularly silicate glasses, are used as substrates on which the various layers of a photovoltaic cell, i.e. a solar cell, are deposited. Also, transparent but electrically conducting ceramics such as oxides of indium, tin, zinc and their combinations act as contacts in the PV cell. As remarked earlier, the electrical energy generated in a solar cell can be stored in rechargeable batteries, or it can be used for dissociating water into hydrogen and oxygen gases. Splitting of water can also be achieved by directly using sunlight in the presence of a catalyst. Right now, titanium dioxide based ceramic photocatalysts are being developed for this purpose.
The idea of hydrogen as the medium of distributing energy for transportation and power generation has fascinated many of us for several decades. The product of oxidation of hydrogen is water. It is environmentally friendly. The most important energy conversion device for power generation from hydrogen is the solid oxide fuel cell. This device is almost entirely made out of ceramics. Stabilised zirconia ceramics can conduct electricity through the diffusion of oxygen ions at temperatures above ~600C. This property is exploited for building SOFC’s. The chemical energy of the fuel-oxygen reaction is released directly as electricity. While hydrogen is the most common fuel, hydrocarbon fuels like methane and combustion by-product gases like producer gas can also be used as fuels in an SOFC. This is not possible in other types of fuel cells based on polymers and expensive platinum. Such fuel flexibility of SOFC’s is perhaps its strongest point. Apart from stabilised zirconia, SOFC’s make use of ceramics such as strontium doped lanthanum manganite (LSM) in cathodes, lanthanum chromite in interconnects, and glass-ceramic sealants. The only metallic material required is nickel catalyst in the anode. Research efforts are in progress to develop better materials for SOFC’s.
Nuclear energy is the major alternative to solar energy. Current nuclear power plants are of fission types, i.e. large atoms emit radiation and become smaller atoms. These power plants also have steam turbines and heat exchangers which are coated with ceramics as mentioned above. However, the biggest contribution of ceramics is in the form of radioactive waste immobilisers. Glass ceramics and crystalline ceramics with the ability to hold radioactive atoms within their structural cages are being developed so that the waste can be prevented from entering our ecosystems for centuries to come. Only ceramic materials offer this possibility of managing radioactive waste.
The second type of nuclear power plant uses a fusion reactor. Such reactors are still in the experimental stage. In a fusion reactor, small atoms like deuterium and tritium, which are isotopes of hydrogen, are made to collide with each other to produce larger atoms like helium. Similar reaction occurs in the Sun. The primary challenge in fusion reactors is to develop the materials that can face plasma and withstand bombardment of neutrons. Ceramic materials like SiC/SiC composites are candidate materials for confining plasma. In magnetically confined fusion reactors like Tokamaks, superconducting ceramics are used for generating very strong magnetic fields.
So far, I have described a variety of energy conversion technologies. In the Indian context, and in the context of manufacturing technologies, there are ample opportunities in each of these areas. Surface engineering of power plant components is fairly well established in India. Various coating deposition techniques like plasma spraying, high velocity oxyfuel (HVOF), detonation gun, cold spray, and chemical vapour deposition can be carried out for depositing ceramic coatings for wear resistance, corrosion and oxidation resistance, and thermal insulation (TBC’s). However, the raw materials for these deposition techniques are almost always imported. Indian manufacturing enterprises have the opportunity to not only carry out the deposition jobs, but also develop the raw materials and further improve the coating techniques.
Alternative energy tehnologies have tremendous opportunities for getting ahead of the competition. In the area of solar cells, cost reduction is the greatest challenge in the Indian context, and indeed for the whole world. Innovative manufacturing techniques can achieve this goal. Ceramics processing technologies are put to the test when developing solid oxide fuel cells. Any advancement in this area will prove crucial for the profitability of this technology. I hope that the Indian entrepreneur will find exciting opportunities in the area of ceramic materials for energy technology. A large amount of research efforts have been put in by academic institutions and government research laboratories. Now the time has come for the entrepreneurs to grab these opportunities to become global leaders in energy technology.