The second law of thermodynamics

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This has been discussed in Chapter 1 but a brief restatement of its implications in the desalination context is worthwhile. It is obvious that any desalting process consumes energy well in excess of the theoretical minimum. Some desalination processes use mechanical energy or electrical energy whereas distillation uses thermal energy. The maximum conversion of thermal to mechanical energy is given by the Carnot factor (J1! - T2)IT1, in which T\ and T2 are the absolute temperatures of heat source and sink respectively. In practice this is typically 35 per cent for a modern power station. However, while mechanical and electrical energy can be completely transformed these forms of energy usually have their origins in a source of thermal energy, i.e. the use of mechanical or electrical energy is therefore inherently more expensive than thermal energy.

It is not often relevant to talk of the quantity of energy consumed per unit of water produced as a basis for process comparison. What must be compared is the energy cost per unit of water, e.g. reverse osmosis will consume electrical power whereas distillation uses thermal, but one kWh of electrical energy is much more expensive than one kWh of thermal energy (approximate ratio of 4 : 1). A distillation plant may therefore have a specific energy consumption of three times a reverse osmosis plant and still be competitive on energy cost terms.

When comparing plants of the same type, comparisons can of course be made in terms of energy consumption but sight must never be lost of the fact that it is product cost which matters and the lowest product cost for any given condition is a function of both energy and capital costs. Energy costs can be reduced in distillation plants at the expense of increased capital expenditure and a trade-off is made between the two - a theme which subsequent chapters will expand.

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