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Water desalination using solar energy

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shadi barakat

on 26 December 2010

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Transcript of Water desalination using solar energy

Water Desalination using Solar Energy Solar Energy Its an important source of energy because enormous amounts of energy is emitted into the space and only a small amount of it reaches the earth.
The solar radiation is being collected by means of solar collectors and then changed into another form of energy.
Jordan location in the so-called earth-sun belt area and has a high potential of solar energy with a radiation of 6 kwh/m2day.

Solar energy in Jordan is mainly used for domestic solar water heating (about 30% of the houses in the country).
The main draw backs of solar energy is that the suns radiation could be interfered by clouds, dust or pollution, and that its only available during daylight.

Low-temperature solar collectors also absorb the sun's heat energy to be directly used for hot water or space heating at a small scale.
Solar thermal systems They are some innovative designs which objective is to apply the suns power to fulfill energy demands.

They are basically designed to produce electrical energy.

1-Parabolic trough 2-Solar Dish 3-Solar Power Tower
The three main types of solar thermal power are: Water Desalination Water Desalination The importance of water Water is the most essential material in life. And has the highest demand growth.

The reasons of this highest demand is; the rapidly growing world population at the same time with decreasing in dwindling resources mainly at the developing countries .

About one third of the world’s population has no direct access to clean, potable drinking water; that’s roughly 2 Billion people.

Solution strategies for the upcoming drinking water Problems 1-Water management ,as in increased prudence in water consumption.

2-The exploration of new sources of water supply ,one of them being the desalination of abundant saltwater supplies.
Water pollution is on of the most important problem in the globe. More than 14,000 people deaths daily because of pollution.

Water is typically referred to as polluted when it is impaired by anthropogenic contaminants and either does not support a human use, and/or undergoes a marked shift in its ability to support its constituent biotic communities, such as fish.

Natural phenomena such as volcanoes, algae blooms, storms, and earthquakes also cause major changes in water quality and the ecological status of water.
Saline water Saline water is water that contains significant amounts or concentrations of dissolved salts.

The concentration is defined by the amount by weight of salt in water, it can be expressed in part per million (ppm).
The parameters for saline water The parameters for saline water:

Fresh water: Less than 1,000 ppm.
Slightly saline water: From 1,000 ppm to 3,000 ppm.
Moderately saline water: From 3,000 ppm to 10,000 ppm.
Highly saline water: From 10,000 ppm to 35,000 ppm.
Categorizations of water in Jordan according to their salinity Water in Jordan The drinking water standard in Jordan is based on the guidelines of the World Health Organization as well as in any other countries.

Jordan's standards took place in 2001, after a major drinking water pollution outbreak occurred in Amman in the summer of 1998 because of malfunction of the capital’s major drinking water treatment plant.
Water problems in Jordan The most important problems are :

1- the lack of rainfall.
2- high rate of evaporation.
3- the increasing of population.
4- increasing demand of water.
5- inconsistencies external shared waters.
The ratio of the deficit in drinking water estimated for current years to 27% while seemed more acute in the irrigation water that they had been to be 50%, In the general situation, characterized by Jordan as a lack of desert rainfall and high evaporation rates; and roughly 94% of the area of the country hit by torrential rain less than 200 mm per annum, which leads to the scarcity of water.
Losses percentage in Jordan Techniques Using solar stills Thermal
processes Membrane processes Thermal desalination includes:

A- MSF evaporators
B-MED evaporators
C-MED-TVC evaporators
D-MED-MVC evaporators
A- MSF evaporators 1-the once through system: An evaporator consists of several evaporating chambers maintained at decreasing pressures.
2-The brine recirculation system B- MED evaporators Multiple-effect distillation (MED) is a distillation process used for sea water desalination.
The MED process is widely combined with thermal vapor compressor (MED-TVC), or with mechanical vapor compressor (MED-MVC).
C- MED-TVC evaporators The MED-TVC evaporator is basically an MED evaporator fitted with a thermo compressor. D- MED-MVC evaporators MED-MVC evaporator: is an MED evaporator using only electrical energy and its combined with a mechanical compressor . Energy Consumption Desalination is a process that removes dissolved minerals (including but no limited to salt) from feed water sources such as seawater, brackish water or treated wastewater.

Almost half of the world’s capacity is used to desalt seawater in the Middle East and North Africa for municipal water supplies. Saudi Arabia ranks first in total capacity installed (approximately 24 percent of total world capacity).
Membrane process:

1-reverse osmosis (RO) .
2-electro dialysis (ED) .
Reverse osmosis The osmosis phenomenon in nature is one where a dilute solution is transported across a semi permeable membrane toward a concentrated solution on the other side. The process of RO is just the opposite of osmosis and is illustrated in Figure 1.
As you can see the below picture shows how contaminated water passes through reverse osmosis membrane and purification takes place. The pore size of R.O membrane is 0.0001 micron.

When water is forced through this, only pure water can pass through it. All contaminants, impurities, dissolved salts, etc... Cannot pass through the membrane and are flushed out through the reject water. Even harmful bacteria and virus cannot pass. The R.O element is smaller than the typical virus by 500 times and bacteria by 10000 times
System Design Consideration When designing a seawater RO system, many factors must be considered. First and extremely important is the feed water source , seawater well systems are preferred because they provide a low turbidity feed water requiring less pretreatment, However, even well systems produce varying seawater quality depending on the origin of the strata they are constructed in .

Once the best feed water source is selected and characterized, both chemically and physically, the pretreatment system must be designed to create the optimum conditions for membrane operation and performance .

With a high quality feed water (one with low colloidal, microorganism, organics and iron content), a seawater RO system needs only five micron cartridge filtration for pretreatment. Lower quality feed water may require much more pretreatment including any and all combinations of the following:

• Infection coagulation

• Clarification

• Multi-media filtration

• Sequestering and dispersant chemical feeds

• Five micron cartridge filtration Membrane in RO Membranes are the largest single consumable cost factor in RO desalination.
Therefore; increasing membrane life will contribute significantly to lowering operating cost. Membranes most often require replacement because of reduced capacity, which in most cases is attributable to colloidal and/or biological fouling .

Common membrane materials include:
1- polyamide thin film composites (TFC).
2-cellulose acetate (CA).
3-cellulose triacetate (CTA)

with the membrane material being spiral wound around a tube, or hollow fibers bundled together. Hollow fiber membranes have a greater surface area and hence capacity but are more easily blocked than spiral wound membranes.
Electro dialysis Is a technique based in the transport of ions through selective membranes under the influence of an electrical field.

This technique has proved its feasibility and high performance in the desalination of brackish water, the desalting of amino acids and other organic solutions, effluent treatment and or recycling of industrial process streams and salt production. Our design Solar stills What Is a Solar Still? A solar still is a green energy product that uses the natural energy of the sun to purify water.

Some solar stills are used in homes to help reduce energy costs and pollution, while others are used in poverty-stricken areas around the world where there are no other sources of clean drinking water.
Basic design of a solar still -A solar still works on two scientific principles: evaporation and condensation.

-Because none of the minerals, bacteria or other substances are able to evaporate with the pure H20, the water droplets that end up in the second trough are simply purified, and are now safe for drinking and cooking.
Energy demand of the still The energy required to evaporate water is 2260 kilojoules per kilogram (kJ/kg), which is the latent heat of vaporization of water. This means that to produce 1 liter (i.e. 1kg since the density of water is 1kg/liter) of pure water by distilling brackish water requires a heat input of 2260kJ.
Efficiency of a solar still is an important parameter that distinguishes a still from another. The highest efficiency achieved by simple still reached around sixty percent with basic adjustments on the still.

Efficiency = Energy in the sun's radiation that falls on the still.
Integration of still design The selection of material of the still affects its performance and efficiency, the cover of the basin; glass or plastic, the material of the basin; sand concrete or Ferro-concrete are choices for the still determined based on our goal or demand.
The design objectives to achieve a higher efficiency operating still include:

Higher feed water temperature.
Increase the temperature difference between the feed water and the condensing cover.
Minimize the vapor leakage.
Solar Still Design Variations Almost 95 percent of all available solar stills are of the basin type because of the simplicity of the design and good efficiency but in general there`s 4 main categories of the solar stills:
1-CONCENTRATING COLLECTOR STILL 2-MULTIPLE TRAY TILTED STILL 3-TILTED WICK SOLAR STILL 4-BASIN STILL Operating Requirements of Basic Stills 1-Protecting Distilled Water from Contamination.

2-Filling and Cleaning a Basin Still.

3-Feeding Hot Water to a Basin Still.
Factors Influencing Solar Still
Operating Performance 1- Climate factors.

2-Condensing surface temperature.

3-Thermal loss factors.

4-Solar still design factors.
Maintenance of Basin Stills 1-Handling the buildup of mineral deposits.

2-Accumlation of Dust on the surface of the condensing surface.

3-Repair and Replacement of Basin Still Components.
Our solar desalination system is constituted from:
solar collector
feed tank
and condenser
The solar collector that had been used is a plate with 0.624m 1.43m holding hollow tubes made from galvanized iron aligned over it, They are painted black to absorb sun irradiation and they also surrounded by a glass tube to prevent heat losses to the surrounding.
The solar collector was tilted with a (45o) angle form the horizontal which corresponds to the solar radiation here in Jordan.
The evaporator is an aluminum coil.

We use coil to increase the interfacial area between the coil and the water .

Cooling the cover of the condensing surface is achieved by using an air vent in the apparatus.
How does it work Design of evaporator Step 1: select the material of coil and tank.
Aluminum is selected for the evaporator coil.
The material of the tank which contains the water is galvanized iron.
Step 2: determine the heat transfer rate.
We take the value of heat transfer rate q from the worse case which is equal to 300W

The yield:
m = 1 kg/hr
Step3: select the inlet and outlet temperatures.
From the previous study we can assume :
Tin=60 C
Tout=50 C
Tbalk= 40 C. Step 4: calculate the overall heat transfer coefficient.
we take:
di = 1.27 cm and do = 1.57cm
And we used this correlation:

after trail & error we find :
Uo = 142.328 W/m2.K Step 5: calculate the length required.
We use this correlation:
q = Ao Tm Uo

And we find that :
L = 2.962 m 3 m. Step 6: required thickness of insulation.
Considering the worst case for surrounding (air) to be at 0.0oC and 14 m/s air velocity (u).

We find that the thickness of the insulation :
x = 0.024m =2.4cm
Conclusions The goal of our research was to provide same new reliable source for clean water Because of increasing demand on water supplies .
Using solar energy maybe the key of this problem specially in the area that have high solar intensity like Middle East.
Two major parameters contribute water situation in the world are water management and exploration of new sources of water supplying.
Desalination increases at dramatic rates due to fresh water demand and decreasing production cost. In Jordan the current prices for desalination makes it competitive to produce desalinated water only in remote locations.
After comparing the energy consumptions for the desalination processes we found that the reverse osmosis is the most energy consuming technique.

The solar stills designs variations makes a decision on choosing a suitable still is somewhat unclear. The basin still or the conventional still is chosen for its simplicity and easy maintenance.
There are many other modification that increase the efficiency of the still.
Finally we find that Under the conditions of 10 degrees difference between the two ends of coil and a heat flux of 300 W, 3 meters of the coil tube were needed to produce 10L/day of desalinated water.
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