It is useful to compare the merits of the various processes for seawater desalination, as described in the previous sections. Although the comparison will be primarily qualitative at this stage, it should be helpful in providing a deeper insight into the strengths and weaknesses of each process.

Foremost among the aspects of comparison is the energy consumption of each process. With the known process specification, it is theoretically possible to calculate the minimum work or energy needed for separation of pure water from brine. For the real process, however, the actual work required is likely to be many times the theoretically possible minimum. This is because the bulk of the work is required to keep the process going at a finite rate rather than to achieve the separation. The minimum work needed is equal to the difference in free energy between the incoming feed (i.e. seawater) and outgoing streams (i.e. product water and discharge brine). For the normal seawater (3.45 per cent salt) at a temperature of 25_C, for usual recoveries the minimum work has been calculated as equal to about 0.86 kWh m^-3. Table 1 makes the desired comparison.

Table 1. Energy requirements of four industrial desalination processes.

Source: International Atomic Energy Agency 1992.

There are no major technical obstacles to desalination as a means of providing an unlimited supply of fresh water, but the high energy requirements of this process pose a major challenge. Theoretically, about 0.86 kWh of energy is needed to desalinate 1 m³ of salt water (34 500 ppm). This is equivalent to 3 kJ kg^-1. The present day desalination plants use 5 to 26 times as much as this theoretical minimum depending on the type of process used. Clearly, it is necessary to make desalination processes as energy-efficient as possible through improvements in technology and economies of scale.

Desalination as currently practiced is driven almost entirely by the combustion of fossil fuels. These fuels are in finite supply; they also pollute the air and contribute to global climate change. The whole character of human society in the 20th century in terms of its history, economics and politics has been shaped by energy obtained mostly from oil. Almost all oil produced to date is what is called conventional oil, which can be made to flow freely from wells (i.e. excluding oil from tar sands and shale). Of this vast resource, about 1600 billion barrels have so far been discovered, and just over 800 billion barrels had been used by the end of 1997. It is estimated that there may be a further 400 billion barrels of conventional oil yet to be found. With current annual global consumption of oil being approximately 25 billion barrels, and rising at 2 per cent per annum, the "business as usual" scenario would suggest that the remaining oil will be exhausted by 2050.

The supply of oil will undoubtedly be boosted by an increase of supplies from unconventional sources, notably the tar sands and shale of Canada and the "Orinoco sludge" of Venezuela. This oil can only be extracted using high energy inputs, and at very high environmental costs. There will be strong political and international pressure against development of these resources, but, when world oil prices are high enough, production will inevitably increase. In theory, unconventional oil could stretch the world's oil supply by another 30 years. In practice, of course, the rate of consumption of oil will be heavily influenced by economic and many other factors, so that prediction in this area is very difficult. The political situation of two of the world's largest potential producers, Iran and Iraq, could be highly relevant to supplies as well as to the global political economy. It is clear, however, that one of the most important of the influencing factors will be the relative cost of renewable energy and how quickly the world can switch to sustainable technologies. There is nothing to gain by deferring investment in this area, and everything to lose by postponing it any longer.

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