Technical, environmental and exergetic aspects of hydrogen energy systems
Introduction
Energy is a key element of the interactions between nature and society and is considered a key input for economic development. Environmental issues span a continuously growing range of pollutants, hazards and eco-system degradation factors that affect areas ranging from local through regional to global. Some of these concerns arise from observable, chronic effects on, for instance, human health, while others stem from actual or perceived environmental risks such as possible accidental releases of hazardous materials. Many environmental issues are caused by or relate to the production, transformation and use of energy, for example, acid rain, stratospheric ozone depletion and global climate change [1].
The impact of energy resource utilization on the environment and the achievement of increased resource-utilization efficiency are best addressed by considering exergy. The exergy of an energy form or a substance is a measure of its usefulness or quality or potential to cause change and provide the basis for an effective measure of the potential of a substance or energy form to impact the environment. It is important to mention that in practice a thorough understanding of exergy and the insights it can provide into the efficiency, environmental impact and sustainability of energy systems, are required for the engineer or scientist working in the area of energy systems and the environment. During the past decade, the need to understand the linkages between exergy and energy, and environmental impact has become increasingly significant [2], [3].
Hydrogen is one of the most promising energy carriers for the future. It is an energy-efficient, low-polluting fuel. When hydrogen is used in a fuel cell to generate electricity or is combusted with air, the only products are water and a small amount of NOx. Hydrogen is renewable and found in many compounds such as water, fossil fuels, and biomass. Hydrogen typically makes up about 6% by weight of dry biomass. Using biomass for energy results in lower emissions than using fossil fuels. CO2 is continuously recycled as biomass in the form of trees and other plants that use it to regenerate, and lower emissions of sulfur and NOx can be expected when converting woody biomass in comparison to coal. To obtain hydrogen from biomass, pyrolysis or gasification must be applied, which typically produces a gas containing 20% hydrogen by volume, which can be further steam-reformed to make higher-purity streams for various fuel cells. The challenge is to overcome the economic barriers that current technology presents for converting biomass to hydrogen for use in clean, efficient energy conversion devices [4].
Hydrogen that is manufactured from renewable resources and used in fuel cells can provide sustainable energy to power electric vehicles. The total system, including distribution, refueling and on-board storage of hydrogen may prove superior to batteries recharged with grid power. A hydrogen-powered electric vehicle may offer a market entry for hydrogen and renewable resources in transportation. Attractive transitional applications of hydrogen include use in combustion engine vehicles and production from natural gas. In either case, the environmental and energy policy consequences are significantly less than continued use of oil-derived fuels in conventional combustion engine vehicles.
Fuel cells, which employ hydrogen to produce electricity, can be used to power a wide variety of applications. This is especially true in transportation, where there are several options for providing hydrogen for the fuel cells.
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One option for obtaining the hydrogen is to use an on-board reformer to extract it from the gasoline in our gas tanks. (Reformers break down hydrogen–carbon bonds to produce a mixed gas from which pure hydrogen is derived.) This approach could also be applied to other hydrocarbons.
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A second option is to use methanol as the hydrogen carrier. Methanol is easier to reform than gasoline and can be produced from natural gas, solid fossil fuels, or renewable biomass resources.
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A third option is to develop a fuel cell that uses methanol directly, eliminating the need for a separate reformer. Instead, a catalyst on the fuel–cell membrane would chemically break the methanol into hydrogen and carbon dioxide [4].
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A fourth option is to produce the hydrogen at central locations and then store it on board the vehicle as a gas, as a cryogenic liquid, or in a solid. With this option, the hydrogen could be produced via steam reforming of natural gas, via pyrolysis or gasification of biomass or fossil fuels, or via electrolysis of water.
To date, the principal niche application for power generation with fuel cells has been in spacecraft. Recently, however, there has been increased interest in their application for both stationary and mobile power generation. This interest has been motivated by the fuel cells’ high efficiency, even in small-scale installations, and their low waste emissions. Recent legislative initiatives in California, USA aimed at mandating the introduction of zero-emission vehicles, and the failings of other technologies (e.g., the limited range and long refueling times of battery-powered vehicles) have further promoted the investigation of fuel cells in mobile applications [5].
The primary objective of the present paper is to discuss technical, environmental and exergetic aspects of hydrogen and hydrogen energy systems. In order to highlight the importance of the exergy analysis, some illustrative examples are also presented.
Section snippets
Environmental issues
During the past two decades the risk and reality of environmental degradation have become more apparent. Growing evidence of environmental problems is due to a combination of several factors since the environmental impact of human activities has grown dramatically because of the sheer increase of world population, consumption, industrial activity, etc. Throughout the 1970s most environmental analysis and legal control instruments concentrated on conventional pollutants such as ,
Comparison of possible fuels
Since we need to manufacture a fuel for the post-fossil fuel era, we are in a position to select the best possible fuel. There are many candidates, such as synthetic gasoline, synthetic natural gas (methane), methanol, ethanol and hydrogen. The fuel of choice must satisfy the following conditions [7]:
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It must be a convenient fuel for transportation.
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It must be versatile or convert with ease to other energy forms at the user end.
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It must have high utilization efficiency.
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It must be safe to use.
In
Hydrogen
Unlike most other fuels, hydrogen cannot be produced directly by digging a mine or drilling a well. It must be extracted chemically from hydrogen-rich materials such as natural gas, water, coal, or plant matter. Accounting for the energy required for the extraction process is critical in evaluating any hydrogen use option. The current hydrogen production techniques include steam reforming of natural gas, cleanup of industrial by-product gases, and electrolysis of water. A number of other
Hydrogen energy production
Although hydrogen is the universe's most abundant element, it is present in the atmosphere only in concentrations of less than one part per million. Most of the Earth's hydrogen is bound up in chemical compounds. Hydrogen for large-scale use should therefore be extracted from a source such as water, coal, natural gas, or plant matter. It cannot simply be produced from a mine or a well. Since considerable energy is consumed in the extraction process, hydrogen should properly be considered an
Environmental aspects of hydrogen energy
The use of hydrogen as a fuel is inherently very clean. Hydrogen consumed by either combustion or a fuel cell produces only water as an end product. The high temperatures involved in combustion may stimulate some NOx production from nitrogen and oxygen in the air, but this problem is familiar from other fuels and can be controlled. Unlike other fuels, hydrogen contains no other pollutant-producing elements, so it has no potential to produce SO2, CO, CO2, volatile organic chemicals, etc.
The
Fuel cells as hydrogen energy systems
Fuel cells power generation is not a new idea. The principle was discovered over 160 years ago by a Welsh judge, Sir William Grove. Until recently, their use was confined to the laboratory and to space applications where they provide electricity, heat and water, and have done so since the 1960s when they were chosen over riskier, less reliable options. But the technology was immature and far too expensive for terrestrial applications.
Recently, interest in fuel cells has increased sharply and
Exergetics
In a broader perspective (except for the zeroth and third law of thermodynamics), we can define the thermodynamics as a science of energy and exergy including a number of concepts of temperature, pressure, enthalpy, heat, work, energy, as well as entropy. Apparently, the first law of thermodynamics refers to the energy analysis which only identifies losses of work and potential improvements or the effective use of resources, e.g., in an adiabatic throttling process. However, the second law of
Exergy and energy
Exergy is defined as the maximum amount of work which can be produced by a system or a flow of matter or energy as it comes to equilibrium with a reference environment. Unlike energy, exergy is not subject to a conservation law (except for ideal, or reversible, processes). Rather exergy is consumed or destroyed, due to irreversibilities in any real process. The exergy consumption during a process is proportional to the entropy created due to irreversibilities associated with the process. Here,
Exergy analysis of fuel cell systems
A hydrogen fuel cell is a device that converts hydrogen and oxygen directly into electricity, water and waste heat while producing none of the noxious by-products typical of combustion processes. A single fuel cell is connected in series with other cells in a stack to provide a higher voltage. A basic hydrogen fuel cell power system is comprised of this stack together with the required ancillary components to provide the stack with the necessary reactants as well as to remove the wastes.
The
Conclusions
The present study has discussed technical, environmental and exergetic aspects of hydrogen and hydrogen energy systems and presented some illustrative examples in order to highlight the importance of the exergy analysis of hydrogen energy systems. The following key concluding remarks can be drawn from this study:
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An enhanced understanding of the environmental problems relating to energy use presents a high-priority need and urgent challenge, both to allow the problems to be addressed and to
Acknowledgements
The support for this work provided by KFUPM is gratefully acknowledged.
References (45)
Renewable energy and sustainable development: a crucial review
Renew and Sustainable Energy Rev
(2000)- et al.
Exergy analysis of a fuel cell power system for transportation applications.
Exergy — An Int J
(2001) - et al.
Exergy, waste accounting, and life-cycle analysis
Energy
(1998) - et al.
Exergy analysis of energy conversion during the thermal interaction between hot particles and water
Energy
(1998) - et al.
Comparison of exergy of emissions from two energy conversion technologies, considering potential for environmental impact
Int J Hydrogen Energy
(1992) Availability: the concept and associated terminology
Energy
(1980)Energy and environmental impacts: present and future perspectives
Energy Sources
(1998)- Dincer I. Thermodynamics, exergy and environmental impact. Proceedings of the ISTP-11, the 11th International Symposium...
- et al.
The intimate connection between exergy and the environment
- Schmidt DD, Gunderson JR. Opportunities for hydrogen: an analysis of the application of biomass gasification to farming...
Hydrogen: the wonder fuel
Int J Hydrogen Energy
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