Water, energy, and environmental issues are closely related. New water techniques consume energy, and innovative renewable energy techniques using biofuels and biodiesel consume an incredible amount of water. Different desalination techniques that consume different energy levels from different sources are in use today. Some people, environmentalists, decision makers, and even scientists, mainly in nonscientific publications, consider energy consumption in desalination to be too high and are seeking new ways of reducing it, which often involves increasing capital investment. Efforts should be directed at reducing not only energy consumption but also total water cost. A competent grasp of thermodynamics and heat and mass transfer theory, as well as a proper understanding of current desalination processes, is essential for ensuring beneficial improvements in desalination processes. Thermodynamics sets the absolute minimum limit of the work energy required to separate water from a salt solution. Unavoidable irreversibilities augment the actual energy consumption, yet modern desalination techniques have succeeded in considerably narrowing the gap between actual and limiting energy levels. The implication of this smaller gap is that only marginal energy reductions are possible. The current energy consumption of different desalination processes is reviewed in this paper. A comparison with other common energy-consuming ventures leads to some interesting conclusions.

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V Wf Z' Z z o ff" €0 € cd Po Pc P W P l'ij x 5 2 P:= burn velocity (m/s) = mass fraction of pyrolysis gas remaining in coal = bulk oxygen mole fraction = fixed distance coordinate = moving coordinate = thickness of dry zone = reactivity factor = porosity of wet coal = porosity of dry coal = initial porosity of dry coal at Z = 0 = initial density of dry coal at Z = 0 = density of wet coal = density of dry coal (variable) = viscosity of water = viscosity of gas = stoichiometric coefficient of ith species in jth reaction

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