A common feature of the hydrogen and carbon cycles is that both are inaccurate descriptions of the phenomena they claim to depict. In neither case does elemental carbon or hydrogen play a major role in the underlying mechanisms of the cycle. In the case of the carbon cycle the predominant forms of carbon are carbon dioxide and complex petrochemicals. For the hydrogen cycle, the major engine of the cycle activity is provided by water rather than elemental hydrogen. A full understanding of the phenomena involved begins, therefore, with the hydrological cycle.
The hydrological cycle, as the name implies, describes the circulation of water between terrestrial and atmospheric systems. The key elements are shown in Fig. 3.5. Water enters the atmosphere by evaporation, predominantly from the ocean surfaces. Smaller degrees of evaporation occur via landlocked bodies of water and evapotranspiration of plant life across the continental land masses. The return of water from the atmosphere occurs in various forms of precipitation. Typical fluxes participating in the global water budget are also shown in Fig. 3.5.
The use of the hydrogen cycle as a practical source of energy is shown schematically in Fig. 3.6. Production of hydrogen by the electrolysis of water using renewable energy sources, such as solar, wind, biomass or hydro-electricity, allows the derived hydrogen to be stored, distributed and converted into useful energy. Hydrogen may be used directly in combustion processes in a similar manner to petrol or natural gas. However, as will be shown later, hydrogen fuel cells represent a carbon-free method of heat and electricity generation compatible with many domestic and industrial applications. During all these end of cycle conversion processes, hydrogen reacts with oxygen leading to water or steam, which is returned to the atmosphere or hydrological system. At the front end of the cycle, using renewable energy sources for hydrogen production would eliminate the carbon dioxide emissions associated
with the currently used reformation technologies for hydrogen manufacture, thus reducing global warming. A further benefit of the hydrogen energy cycle is that, in contrast with electricity, it allows the possibility of large-scale storage. This facet of the cycle is of particular importance where hydrogen is derived from intermittent energy sources, such as solar or wind.
Thus, the hydrogen cycle may be regarded as a perturbation or subset of the larger scale hydrological cycle, in that hydrogen may be extracted from water by a variety of processes (these are described later in this chapter) and burned as a fuel from which the combustion product is water.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.