OPINION PIECE: How to get fresh water in drought-stricken Southern Europe

9. august 2022 kl. 14:54
OPINION PIECE: How to get fresh water in drought-stricken Southern Europe
Illustration: AlexLMX/Bigstock.
Artiklen er ældre end 30 dage

Most people have probably noticed the unusually large shortage of fresh water along with heat waves in large parts of Europe this summer. Forest fires are raging in Spain, and Italian rivers such as the Po and the Tiber have almost dried up.

One of the hardest hit areas is Andalusia in southern Spain. There is usually only rain in the winter there, and water is stored in reservoirs for use during summer. However, the water levels are catastrophically low at the moment. On 19 July, the water level was only 30% (see www.embalses.net). A look into the climate crystal ball indicates that the issues will probably get even worse in the future.

What is the solution to the lack of fresh water in one of Europe’s most popular holiday destinations? The obvious solution would be to increase the production of fresh water through seawater desalination. There are two common methods to achieve this.

High pressure requires a lot of power

One of them is reverse osmosis. In this method, saltwater is forced with great force through a membrane that only allows water molecules to pass through it. The Na+ and Cl- ions are retained by the membrane. This method requires powerful pumps and is energy-intensive, as the water must be forced through the membrane against the osmotic pressure. Since the energy consumption depends on the salinity, this method is very power-intensive when desalinating Mediterranean seawater due to its high salinity.

Artiklen fortsætter efter annoncen

The second method involves boiling the water and then condensing the steam, which does not contain salt. This method requires large amounts of heat due to water’s enormous heat of vaporization. However, the heat is returned when the steam condenses. So if this heat is used to heat saltwater before it evaporates, heat consumption can be kept low. This is best done by allowing the vaporization to take place in a series of tanks with gradually decreasing pressure and temperature (the boiling point of water decreases with decreasing pressure). This method is called multi-stage flash distillation (MSFD).

Since MSFD plants, as mentioned, require heat, the plants are usually placed close to a thermal power plant or some other industry installation, which receives excess heat. The heat would otherwise just go to waste.

Sun and excess heat

With a climate as sunny as in southern Spain, heat can also be created relatively cheaply with solar collectors. Finally, surplus heat from large air conditioning systems and other cooling or freezing facilities can also be used. MSFD is used almost exclusively in Middle Eastern countries, especially the UAE, Kuwait, and Saudi Arabia. The MSFD plants are built together with gas-fired power plants and utilize their surplus heat.

There is reason to believe that the MSFD plants can be optimized so that even more fresh water can be produced per unit of energy. The very cheap natural gas in Middle Eastern countries means that it has not been necessary to limit energy consumption in the MSFD plants as much as possible. It has been cheaper to simply burn some more gas than to invest in more stone wool insulation and better heat exchangers in the MSFD plants.

Artiklen fortsætter efter annoncen

Illustration: HOW.

Hans Ole Wanner
MSc in geophysics, specialization in meteorology
Formerly associated with the DMI research department in connection with, among other things, climate modelling, now meteorologist at DMI

It has also not been beneficial to utilize solar energy on a larger scale. In Europe, where, as we know, gas availability has decreased and prices have gone up, the situation is of course completely different. Is it realistic to produce that much fresh water in southern Spain from, for example, solar energy?

A calculation

According to Wikipedia, MSDF uses 90 MJ to produce one cubic meter of water. Let us assume that we want to produce 100 m3/s. This is slightly more than the mean water flow in Denmark’s two largest rivers, Gudenå and Skjern River combined. That would require 9000 MW of power. The daily average solar radiation in southern Spain in the summer is 100–300 W/m2 (compare with the solar constant; 1.36 kW/m2). Let us start from approx. 200 W/m2.

Therefore, 45 km2 is required to produce 9000 MW. This corresponds to approx. 1/2000th of Andalusia’s surface area, and the plants can be built without major expropriations and without destroying plant life, as the region has many barren and uninhabited areas. Add to that the fact that the existing MSFD plants are not fully optimized in relation to energy consumption due to the huge quantities of cheap natural gas in the countries where MSFD is used.

It could be a good amount of work for Danish companies to build MSFD plants and optimize them so that the biggest possible amount of fresh water is produced from the smallest possible amount of energy. Grundfos could supply pumps. Danfoss could supply thermostats and heat exchangers, and Rockwool must of course be used for insulation.

Ingen kommentarer endnu.  Start debatten
Log ind eller opret en bruger for at deltage i debatten.