Thermal desalination to obtain drinking waterDuring the 1960’s, before the emergence and industrialization of reverse osmosis membranes, the method to desalinate seawater and make it safe was only through evaporation equipment that consumed significant amounts of energy.

Currently, there are small desalination plants evaporating seawater or brackish well water. These plants consume small amounts of electrical energy, which can be obtained by windmills, solar panels or other renewable electrical energy sources.

Basically, there are two technologies:

Thermal vapor compression obtains distilled water from the same process as multiple effect distillation (MED), however utilizing a different thermal energy source: thermal compressors.  Thermal compressors use medium pressure steam from the electricity production plant (if we have a dual plant, if not, it would be steam process obtained specifically for this purpose). The compressor sucks part of the generated vapor in the last stage at a very low pressure, compressing and giving rise to an intermediate pressure steam suitable for the above furnished to stage 1, which is the only energy consumed in the process.

The yield for this type of plant is similar to MED plants (multiple effect distillation).   However its desalination capacity can be much greater by allowing a broader adaptability by taking vapor from similar steam making plants.  Many times they are considered the same process, but here they will be addressed individually because power consumption of the plant is done by different equipment.

On the other hand, we find vacuum evaporators with mechanical vapor compression that evaporate liquid. In the case of salt water, it is evaporated on one side of the exchange surface and is sufficiently compressed to condense on the other side. The distillation cycle is maintained, saving water lost in the process and raising the boiling temperature of salt water with respect to purity.

This equipment is small and is much more reliable and simple to operate than reverse osmosis equipment, requiring almost no maintenance. This makes it ideal for fresh water supply to small towns, remote areas, insular areas, etc.

The specific consumption of these facilities is lower than that of the other distillation processes: usually the equivalent power consumption is about 10 kWh/m3.

OTHER ALTERNATIVES

Another way to obtain drinking water from seawater or brackish sources is by water vacuum evaporators. These evaporators use residual heat sources from cooling circuits of cogeneration motors. This increases the percentage of energy recovery and reaches the minimum goals in order to collect bonuses from energy sold to the grid.

Solar heat concentrators also make it possible to produce potable water using flash type evaporators. In fact, you can find fields that combine solar electricity production with the production of drinking water.