Hydrogen fuel car: Visions and challenges
The never ending debate on energy supply for a cleaner environment, recently associated with the worldwide effort to decrease global CO2 (carbon dioxide) emissions, has been revived by the rapid growth of energy demand and gradual depletion of energy resources. An assessment of the Intergovernmental Panel on Climate Change (IPCC) draws attention to the need for a change in a current energy policy. According to the report, world energy consumption grew by three times in 2008 compared to the year 1965 áhttp://www.bp.comñ. More than 86% of all energy production (heat, electricity or movement) is based on fossil fuels, such as oil, gas, coal, etc., which cause CO2 emissions to the atmosphere equivalent to 7 Gton (gega ton) of carbon per year and this value is expected to be doubled by 2050 (International Energy Agency, IEA) áhttp://www.energybulletin.net/3998.htmlñ.
Current CO2 concentration in the atmosphere, 30% above the level of the pre-industrial era, remains the major factor for coming trends in global warming.
It has been reported and now widely accepted that the increase in CO2 concentration is significantly due to human activities. According to some energy experts, about half of the world's oil production is consumed by automobiles (Conference on the ecological dimensions of biofuels, Washington DC, 2008). Oil, nowadays constituting around 33% of primary world energy, is produced in a small number of countries characterized by political instability. As a result, the price of petroleum is subjected to dramatic fluctuations due to economic and political issues. Moreover, considering the linear extrapolations of the growth rate of oil consumption and the increasing rate of known oil reserves, it can be deduced that the end of the petroleum supply will probably take place around 2050 áhttp://www.worldenergy.org/wecgeis/publications/default/speeches/spc040819hk.pdfñ.
Natural gas appears as an alternative in the medium term, although a similar method of calculation predicts its total consumption will take place in 70100 years. Of the fossil fuels used at present, only coal may retain its level of availability, considering its increasing rate of consumption, for another couple of centuries. As a result, it's time to think of alternative to fossil fuels.
Albert Einstein (18791955) said “Visions are more important than knowledge since knowledge is finite”. One version of the vision for a sustainable energy system that has been able to unite economic growth and environmental concerns is the vision of the hydrogen economy. The hydrogen economy has the potential to provide a sustainable and secure energy system and there is a wide and growing perspective of promoting and exploring different possible hydrogen futures. Visions of the future select, combine and reconfigure individual hydrogen generation, storage, transport and end-use technologies into more or less mutually compatible energy and transportation systems, which embody deeply contested and conflicting views of sustainability.
Hydrogen storage is a real challenge for realizing hydrogen economy that will solve the critical issues of humanity such as energy depletion, air pollution, greenhouse emission and climate change. The traditional hydrogen storage facilities, both for stationary and mobile applications, are complicated due to its very low boiling point (20.2 K) and very low density both as a gas (0.09 kg/NAm3) (kilogram per normal cubic meter) and a liquid (70.9 kg/NAm3). Moreover, hydrogen proton exchange membrane (PEM) fuel cells need improved approaches for the high-capacity storage of hydrogen at temperatures ranging from near ambient to about 373 K and at pressure below 10 MPa (mega pascal).
In every stage of the hydrogen production to utilization in fuel cells, materials play crucial roles in achieving high conversion efficiency, safety and robustness of the technologies involved. Among the most pressing issues, a major challenge to realize the hydrogen economy is the development of efficient and safe materials for practical hydrogen storage.
Hydrogen can be made available on-board in several ways: compression, as a liquid, metal hydrides, chemical storage or gas-on-solid adsorption. Although each method possesses desirable characteristics, currently no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. For example, liquid hydrogen storage systems lose up to 1% a day by boiling and up to 30% during filling, as well as requiring good (bulky) insulation to keep the hydrogen at 20 K.
Physical adsorption of hydrogen onto an adsorbent is too unstable, generally requiring cryoadsorption, where the adsorbent is cooled to very low temperatures, to store large amounts of hydrogen. Although not as severe as liquid hydrogen, cryoadsorption suffers as an economic proposition because of the need to maintain such a low temperature.
Hydrogen storage in solid-state matrix, e.g., hydrogen storage materials such as nanostructural carbon, metal organic frameworks (MOFs), metal hydrides, alanates, amides, ammonia and ammonia borane, have not only their own advantages but also limitations to hinder their applications in some niche areas, in particular, for portable power facilities and in fuel cell electric vehicles (FCEV). Metal hydrides and complex hydrides have high hydrogen storage capacity but, unfortunately, are too stable, which requires a high temperature to decompose. Although this kind of materials is generally of high capacity but suffers from the critical drawbacks of irreversibility and sluggish kinetics.
Furthermore, a key technical impediment to the deployment of hydrogen as a transportation fuel is relatively low energy density for on-board hydrogen storage systems. Detailed technical targets for on-board hydrogen storage system performance were developed through the FreedomCAR, a partnership between the US Department of Energy (DOE) and the US Council for Automotive Research (USCAR). These targets address gravimetric and volumetric densities, cost, cycle life, full-flow rate, delivery and loss of usable hydrogen.
Up-to-date no ideal hydrogen storage system meets the targets of DOE. A number of materials are awfully promising in terms of hydrogen storage capacity; however, their kinetics and reversibility performances suffer from critical drawbacks. On the contrary, some materials with intrinsic microporosity favours the operating properties, nevertheless, they have low storage capacity. Nowadays, the best compromise between all hydrogen storage parameters is given by metal hydrides. At present, the USA and Germany have set up hydrogen fuel pumps in cities for buses and cars on experimental basis. Some other countries like Italy, France, Japan and Canada are also going on in parallel.
The USA uses about a quarter of the world's energy, almost twice more than Japan on a per capita basis and many times higher per capita than in less-developed countries (LDC). However, energy consumption in LDC is projected to rise the times as fast as in developed countries. As energy use increases, so does pollution. Hydrogen fuel car in Dhaka is a dream to us up to now! of petroleum fuels emits carbon monoxide (CO), nitrogen oxides (NOx), particulates containing lead compounds and unreacted hydrocarbons; the major air pollutants- a threat to the green world.
Recently, rapid expansion of research efforts in this field has brought the talents of a wide range of researchers to bear in solving the grand challenges of hydrogen storage. There is a need for state-of-the-art hydrogen storage properties of new materials that are being developed towards practical application. A clear and comprehensive resource that will provide strategy to hydrogen storage systems is critical to the success of green hydrogen en route to 'zero-emission' vehicles.
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