Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 201

expensive than flat plate collectors per unit area installed and

per unit of thermal energy delivered. Solar ponds, being low-

grade heat source, can be competitive with convectional heat

sources in many applications.

Flat-plate collectors, evacuated tube collectors and focus-

sing collectors are used to produce hot water or steam as the

heat medium for the distillation units. For reverse osmosis or

electrodialysis units, photovoltaic devices are used or ther-

mal conversion systems, e.g., central receivers, to drive the

turbine generator.

A very important aspect of the solar assisted desalination

process is the cost of energy and water produced. However,

experience has shown that cost estimates are different every-

where. Labour, material cost, etc. depend on local circum-

stances, so the cost of water is not the same at all places.

Solar assisted desalination capacity is only a very

small percentage, about 0.80%, of the total world capacity

of convectional-fossil fuel fired desalination plants. A part,

0.60% is coupled to collectors or photovoltaic devices and

0.13% are wind-driven plants. The total capacity of worldwide

solar-driven desalination plants is only about 15,250 m^3 /d, and

wind driven as low as 2,530 m^3 /d.^5

Figure 4 presents the first known sketch for solar distil-

lation equipment.^18

Distillation process, operated with conventional energy

sources, i.e., low pressure steam, are applied up to very large

capacities by using various types of evaporators and are clas-

sified accordingly as follow:

Multiple-effect evaporator (ME)

Vertical tube evaporators VTE, falling or climbing

type

Horizontal tube evaporators HTE

Multi-stage-flash evaporator (MSF)

Vapor compression evaporator (VC)

Thermal vapor compression TVC

Vacuum vapor compression VVC

Mechanical vapor compression MVC

The term “evaporation” in the desalination refers espe-

cially to the vaporization of water from an aqueous saline

solution, as brackish or seawater, where the solid constitu-

ents are practically nonvolatile, in the range of working

FIGURE 3 Photograph of the Solar distillation plant in Porto Santo, Madeira, Portugal. It is the only solar plant in

operation in Europe. Has a total evaporating area of 1,200 m^2 , and consists from two different kinds of solar stills, of

the assymetrical type. The Greek design developed at the T.U. of Athens and the design developed by the university

of Berlin.

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