Solar Powered Desalination

A shortage of water is problem that affects many regions in the world. On the other side, there is a huge amount of salty water on our planet. Desalinization is a process through which the salt is removed from the saline water, so it can be used for drinking and other purposes where fresh water is needed.
Process of desalinization consumes certain amount of energy which is currently obtained mostly from the fossil fuels. Solar energy can be used to replace fossil fuels, and make this process less costly and less polluting.


There are two types of desalination systems, based upon either thermal distillation or membrane separation.
  • In a solar context, the thermal systems will heat saline water and separate the relatively pure vapor for subsequent condensation and use. These systems utilize energy recovery in the vapor condensation processes and the recovered heat either drives additional evaporation at a lower pressure or preheats the water. The energy required for thermal distillation is essentially the same irrespective of salinity, so these systems are mainly used for seawater desalination.
  • Membrane systems use process where saline water is mechanically pressured on one side of a membrane. The membrane has static charge groups on its surface which inhibit the absorption of ion into the membrane. Water molecules are soluble in the membrane and diffuse through it from the high pressure saline side to the low-pressure pure water side. These systems usually rely on solar-generated electricity either to drive high pressure pumps that overcome osmotic pressure differentials, or to create electric fields that drive electromigration of ions in solution.
Some methods of desalination require greater energy per unit mass as the salinity rises. Further, saline waters may contain a considerable variety of dissolved ions, and the proportions of ions found in low-salinity ground waters are typically quite different than those in high salinity seawater or those found in waste waters. Thus, the composition of a raw water source is very important for the selection of the treatment technology to be used.

Thermal distillation systems

Active solar stills have an external thermal energy source added to the unit, to increase heating of the salty water. Additional heat could be provided by a concentrating solar panel. Some models include a vacuum pump to enhance evaporation rate.
Furthermore, adding energy storage units to the solar still leads to a significant increase in productivity.
Solar stills generally integrate the functions of solar collection, water heating, evaporation, and condensation into a single volume, and this configuration results in considerable thermal inefficiency.




Humidification–dehumidification (HDH) desalination systems use separate components for each of the thermal processes, allowing each component to be independently designed. It allows much greater flexibility in the design of the thermodynamic cycle for vaporizing water into air, and subsequently condensing the vapor.
HDH cycles may be classified according to whether air or water is heated and according to whether the air or water circuit is open or closed loop.


Water-heated system with open-air
and closed-water loops
 

 Air-heated cycle with closed-air loop
Air-heated cycle with both air and
water streams open

Air-heated cycle with closed-water loop
                                                                                                   
Solar driven membrane distillation is a thermally driven separation process of aqueous solutions that involves the transport of vapor molecules through a hydrophobic microporous membrane. The membrane supports a vapor–liquid interface at the pores. The surface tension forces of the hydrophobic membrane prevent liquid molecules from entering the pores, while vapor passes due to a difference in vapor pressure at both sides of the membrane. The main advantage of MD is that it operates at lower pressures than other separation processes based on membranes. Typical operating pressures are on the order of zero to a few hundred kPa. Feed temperatures typically ranges from 60 C to 90 C, so low energy heat sources, like solar energy, are suitable for the process.

DCMD - direct contact membrane distillation; AGMD - air gap membrane distillation; SGMD - sweeping gas membrane distillation; VMD vacuum membrane distillation.
DCMD - direct contact membrane distillation; AGMD - air gap membrane distillation; SGMD - sweeping gas membrane distillation; VMD vacuum membrane distillation.

Photovoltaic powered reverse osmosis
This system converts solar radiation into electricity using photovoltaic, and electricity is then used for both seawater and brackish water desalination. Reverse osmosis (RO) is a membrane separation process that recovers water from a saline solution pressurized to a point greater than the osmotic pressure of the solution. In essence, membrane filters hold back the salt ions from the pressurized solution, allowing only the water to pass. RO membranes are sensitive to pH, oxidizers, a wide range of organics, algae, bacteria, depositions of particulates and fouling. Therefore, pre-treatment of the feed water is an important process step.

Concentrating Solar Power Plants (CSP)
Concentrating solar thermal power technologies are based on the concept of concentrating solar radiation to provide high temperature heat for electricity generation within conventional power cycles using steam turbines, gas turbines or Stirling engines. For concentration, most systems use glass mirrors that continuously track the position of the sun.
The sun light is focused on a receiver that is specially designed to reduce heat losses. A fluid flowing through the receiver takes the heat away towards a thermal power cycle, where high pressure, high temperature steam is generated to drive a turbine. Air, water, oil and molten salt can be used as heat transfer fluids.
  Due to the high energy consumption of desalination processes, large desalination plants are typically associated with or close to power production facilities. In the case of thermal desalination processes, the desalination plants are normally integrated into electricity production plants using steam extracted from a conventional Rankine cycle. This is very useful as solar radiation is usually abundant in places where fresh water is scarce. With the current commercial development of CSP technology all over the sunny areas of the world, solar power-water cogeneration plants appear as a possible means of sustainably reducing power and water problems in many arid and semiarid areas of the world.





Why is this important?

In many regions where freshwater resources or water supply infrastructure are inadequate, fossil energy costs may be high, whereas solar energy is abundant. Further, in the industrialized world, government policies increasingly emphasize the replacement of fossil energy by renewable, low-carbon energy. So the regions with water shortage are considering solar-driven desalination systems as a supplement to existing freshwater supplies. Even in regions where petroleum resources are abundant, solar-driven desalination is attractive as a means of conserving fossil fuel resources and limiting the carbon footprint of desalination. Finally, in remote regions, a solar driven desalination system may be more economical than alternatives such as trucked-in water or desalination driven by diesel-generated electricity.

How it was before?

The first man-made large-scale water desalination system, which dates back to the nineteenth century, is the solar still. A solar still is made of an airtight insulated basin that is covered with a tilted glass sheet. Solar radiation passes through the transparent glass or plastic cover and is absorbed by salty water in the basin so that water is heated and causes evaporation. The water vapor condenses at the inner side of the glazing and the liquid flows by gravity into a trough, where it is collected. Basins are painted black to increase solar absorption.




Advantages of CSP

  • Due to energy storage and hybrid operation with (bio) fuel, concentrating solar power plants can provide around-the-clock firm capacity that is suitable for large scale desalination either by thermal or membrane processes
  • CSP desalination plants can be realized in very large units up to 100,000 m³/day
  • Huge solar energy potentials
  • Within two decades, energy from solar thermal power plants will become the least cost option for electricity (below 4 ct/kWh) and desalted water (below 0.4 €/m³)
  • Management and efficient use of water, enhanced distribution and irrigation systems, reuse of waste water and better accountability are important measures for sustainability
  • Advanced solar powered desalination with horizontal drain seabed intake and nano filtration will avoid most environmental impacts from desalination occurring today

How to build it?

Here are links to some examples of how to build a simple solar powered desalinization device:



http://www.sciencebuddies.org/science-fair-projects/project_ideas/EnvEng_p022.shtml#procedure







http://www.instructables.com/id/plastic-bottle-desalination/?ALLSTEPS


 


http://www.wikihow.com/Desalinate-Water


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