Desalination of Water for Irrigation with Solar Energy

Nowadays there are at least 2 million people without access to sanitary water. Due to several causes like drought, pollution and saline water not suitable for human consumption; world’s population growing and the decrease of foods, need the spreading of agriculture in arid zones.

Arid zone have the lack of water and the huge amount of run radiation as their main feature. The presence of saline water is uprising in our geography because of the over-exploitation of our aquiferous.

Solar Energy may be the key to desalinate water for human consumption and for its use in agriculture.

The use of solar energy to distillate water is an ancient practice. The first document about it comes from 1551 and it is written by Arabic Alchemists. In 1589, Della Porta describes a solar energy water distillation system. In 1862, Lavoisier experiments this technique with the use of big lenses concentrating solar energy.

In Greece, there are many solar distillation facilities working for the supply of sanitary water. According to the kind of construction and the climate conditions, with an average condensation time of 2.450m2, you may obtain from 7.5 to 15 m3 of sanitary water per day.

In Freeport, at the Gulf of Mexico, 4 millions of water are obtained daily by the LTV (Long Tube Vertical Multiple Effect Distillation). It is a system of multiple phases, through a progressive steaming process and a constantly decreasing pressure guarantees a relatively favorable energetic balance. The water pumped from the Gulf of Mexico undergoes to a total of 12 condensations. The ratio of fresh sea water obtained is 4:3. A system that competes against this process is the MFP (Multiple Flash Process). Through this system is working a 10 phases distilling facility in San Diego, producing daily an approximate of 300,000 water liters of fresh water.

Description of the open air systems.

The simplest systems are by now the more developed; this is the case of the one named: “deposit”. That system has a series of essential elements: water deposit, sheets of translucent crystal (sometimes plastic), channels and a collector for distilled water, kind of rudimentary but enough to caught solar energy.

The layout of those elements varies according to the models: on the ground the container is used, it may be back to absorb a better amount of solar radiation; the water steams (separated from salts) and condenses over the sheet placed on the top. This sheet is usually inclined, at the end the distilled water is picked up in the sides.

Most used models in Greenhouses

They built a conventional greenhouse, but in its interior, at the very top they have a water container made of semitransparent plastic. This plastic was chosen because it was capable of retain the thermic solar radiation and because it let the Photosyntetic Active Radiation (P.A.R.). This team of researchers prove material such as polyethylene and metacrilate, red and blue, for its construction, and they observed that the P.A.R. radiation transmitted by the red metacrilate doubled the ones transmitted by the other materials.

Luft and Froechtenight designed two garments that raised the effectivity of previously developed models. In this model the water runs between a couple of crystal sheets, the interior sheet only lets trough the P.A.R. radiation, keeping the thermic radiation in between the crystals. That is how water steams and condense quickly, because between the crystals a raise of the temperature is happening.

There is an investigation going on other model that has a solar energy collector, a steaming tower and a condensation tower.

The water raises to the solar collector on microtubes where the water heats and goes to the steaming tower. Now the steam goes through the condensation chamber where the condensation happens when the water steam contacts the cold surface of such chamber.

Sometimes saline water is pre-heated using a plate changer, placed in the condensation chamber. That is how water heating inside the solar collector will be more quickly, because the water will already be at an elevated temperature.

Non evaporated water is picked up in a deposit where saline water is, and when the salt concentration is to high, this water is thrown away.

The cost of the installation is not too high, neither its maintenance, it is estimated that this kind of installations has an useful life of 20 years.

Design considerations

Needed water

The quantity of water is determined by the ratio of daily consume, adding a percentage considering loses. You also may want to keep in mind the quality of the water needed by the exploitation, sometimes as it happens in the focused risk, more concentrated saline waters may be used than the ones used in traditional land watering. We also have to keep in mind the volume that we are able to store, keeping in mind the weather and previewing possible emergencies.

Productivity by Area Unit

This datum is the amount of water that a facility can desalinate per solar energy catching area unit. This productivity will be calculated with a series of empiric formulas, obtained when investigating this type of systems.

Solar Surface

In function of the productivity per area unit and the water needs, the necessary solar surface for interception is calculated.

Solar Radiation

Availability of solar radiation in times when the needs of water is bigger is a requirement to project a facility. You need to know the distribution of solar radiation in different time of the year. There are some formulas in this case, that allow us to calculate the affecting solar radiation, keeping in mind the latitude, altitude and other parameters.

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