**2. Desalination technologies**

Future water supply is a major concern in developed and developing countries of the world, such as in Middle East and North Africa (MENA region). In these countries conventional water resources are limited and cannot sustain the growing demand where population growth is increasing dramatically. Hence, since 1970 other non conventional methods have been adopted on a large scale to satisfy this growing demand. One of the most promising technologies is water desalination whom the total world capacity increased from 100.000 m3/day in 1970 to 6.800.000 m3/day in 2007 (GWI, 2010).

By looking at the total desalination plants installed in the world we simply realize that three major desalination technologies are used:


MED and MSF are classified us thermal desalination technologies, however RO desalination is considered as membrane technology. Besides these technologies other techniques can be used (i.e. Mechanical Vapor Compression, MVC; Electrodialysis, ED; Humidification and Dehumidification of Air, HD; etc.). However their application still limited for specific context.

#### **2.1 MSF desalination**

An MSF distillation plant consists of several consecutive stages (evaporating chambers) maintained at decreasing pressures from the first stage (hot) to the last stage (cold). The vapor condenses to form fresh water. At vacuum conditions the boiling point of water is low requiring less energy. Before entering the first cell, seawater sweeps all cells from the last one to the first by flowing through the tubes of the heat exchangers where it is warmed by condensation of the vapor produced in each stage (Fig. 1). Its temperature increases from sea temperature to inlet temperature of the brine heater.

The seawater then flows through the brine heater where it receives the heat necessary for the process (generally by condensing steam). At the outlet of the brine heater, when entering the first cell, seawater is overheated exceeding boiling point compared to the temperature and pressure of stage 1.

Therefore, it is the abrupt introduction of this sea water into a lower pressure "stage" that makes it boil so quickly as to "flash" into steam to reach equilibrium with stage conditions. The produced vapor is condensed into fresh water on the tubular exchanger at the top of the stage. The process takes place again once the water is introduced into the following stage, and so on until the last and coldest stage. The cumulated fresh water builds up the distillate production which is extracted from the coldest stage. Seawater slightly concentrates from stage to stage and builds up the brine flow which is extracted from the last stage.

Typically a number of units are constructed alongside a combined cycle power plant and utilize low-grade steam (semi waste heat) from the power plant to produce the desalinated water. An MSF plant performance is selected to ensure the overall optimization of the plant power and steam cycles.

Fig. 1. MSF Desalination Process

#### **2.2 MED desalination**

90 Modeling and Optimization of Renewable Energy Systems

The present methodology has the advantage to take into account all the critical functioning parameters that have an influence on the electricity and desalinated water productions and

The minimization of the function total cost was implemented by using Genetic algorithms (GA), that have the capacity to reach the solution corresponding to the global optimum with a relative simple calculation. The benefit of using Genetic algorithms in the proposed methodology is the calculation of the optimal solution in the global space of feasible solutions of desalination systems (individuals). These later are obtained by different

Future water supply is a major concern in developed and developing countries of the world, such as in Middle East and North Africa (MENA region). In these countries conventional water resources are limited and cannot sustain the growing demand where population growth is increasing dramatically. Hence, since 1970 other non conventional methods have been adopted on a large scale to satisfy this growing demand. One of the most promising technologies is water desalination whom the total world capacity increased from

By looking at the total desalination plants installed in the world we simply realize that three

Multi effect distillation (MED), which has increased dramatically in the world during

MED and MSF are classified us thermal desalination technologies, however RO desalination is considered as membrane technology. Besides these technologies other techniques can be used (i.e. Mechanical Vapor Compression, MVC; Electrodialysis, ED; Humidification and Dehumidification of Air, HD; etc.). However their application still limited for specific

An MSF distillation plant consists of several consecutive stages (evaporating chambers) maintained at decreasing pressures from the first stage (hot) to the last stage (cold). The vapor condenses to form fresh water. At vacuum conditions the boiling point of water is low requiring less energy. Before entering the first cell, seawater sweeps all cells from the last one to the first by flowing through the tubes of the heat exchangers where it is warmed by condensation of the vapor produced in each stage (Fig. 1). Its temperature increases from sea

The seawater then flows through the brine heater where it receives the heat necessary for the process (generally by condensing steam). At the outlet of the brine heater, when entering the first cell, seawater is overheated exceeding boiling point compared to the temperature

100.000 m3/day in 1970 to 6.800.000 m3/day in 2007 (GWI, 2010).

 Multi stage flash process (MSF) — 43.5% of world production, Reverse osmosis (RO) — 43.5% of world production, and

the investment and operational costs.

simulation during all over a year.

**2. Desalination technologies** 

major desalination technologies are used:

temperature to inlet temperature of the brine heater.

the last years.

**2.1 MSF desalination** 

and pressure of stage 1.

context.

MED, like MSF, takes place in successive effects and uses the principle of reducing the ambient pressure in the various effects. This permits the seawater feed to undergo multiple boiling without supplying additional heat after the first effect. In a MED plant, the seawater enters the first effect and is raised to the boiling point after being preheated in tubes. The seawater is either sprayed or distributed onto the surface of evaporator tubes in a thin film to promote rapid boiling and evaporation. The tubes are heated by steam from a boiler or other source, which is condensed on the inside of the tubes. The condensate from the boiler steam is recycled to the boiler for reuse (Fig. 2).

In MED the maximum temperature is now limited to 80°C to reduce the scale deposition, which limit the gain output ratio (GOR) to a maximum level of 12 kg distillate/kg of steam. However, with the introduction of a compression technology plant (hybrid) to the MED process the performance has been radically improved to GOR of 15. The compression is provided by electric compressors or thermo-compressors, which utilize motive steam.

Thermal desalination (MED and MSF) produce very low TDS production (50 mg/l), and does not depend on feed quality, as is the case with the Reverse Osmosis technology.

Optimization of Renewable Energy Systems: The Case of Desalination 93

Membrane properties and feed water salinity are the two major factors controlling the energy requirements of an RO system. Higher water salinity requires more energy to

Pre-treatment of seawater feeding RO membranes is recognized as a key in designing RO desalination plants (Gaid and Treal, 2007). The use of an adapted pre-treatment minimizes the fouling problems and can provide good protection of the membranes and a longer lifetime.

Solar and Wind systems can be used to provide heat required to produce steam for the thermal desalination plants and electricity to drive high pressure pumps in RO units and

Different solar energy collectors may be used in order to convert solar energy to thermal energy. In most of them, a fluid is heated by the solar radiation as it circulates along the solar collector through an absorber pipe. This heat transfer fluid is usually water or synthetic oil. The fluid heated at the solar collector field may be either stored at an insulated

The solar collector may be a static or suntracking device. The second ones may have one or two axes of sun tracking. Otherwise, with respect to solar concentration, solar collectors are already commercially available; nevertheless, many collector improvements and advanced solar technologies are being developed. The main solar collectors suitable for seawater

Flat-plate collectors (FPCs) are used as heat transfer fluid, which circulates through absorber pipes made of either metal or plastic. The absorber selective coatings are used to reduce heat losses and to increase radiation absorption. Thus the thermal efficiency increases although

A typical flat-plate collector is an insulated metal box with a glass or plastic cover and a darkcolored absorber plate. The flow tubes can be routed in parallel or in a serpentine pattern. Flat plate collectors have not been found as a useful technology for desalination (Belessiotis and Delyannis, 2001; Gracia-Rodriguez, 2002). Although they have been used for relatively small desalinated water production volumes, production of large volumes of

A parabolic trough is a linear collector with a parabolic cross-section. Its reflective surface concentrates sunlight onto a receiver tube located along the trough's focal line, heating the heat transfer fluid in the tube. Parabolic troughs typically have concentration ratios of 10 to

Parabolic trough collectors (PTCs) require sun tracking along one axis only. In this way, the receiver tube can achieve a much higher temperature than flat-plate or evacuated-tube

overcome the osmotic pressure.

**3.1 Solar technologies** 

distillation are as follow.

**3.1.1 Flat-plate collector** 

the collector cost also increase.

**3.1.2 Parabolic trough collector** 

**3. Renewable energy systems for desalination** 

tank or used to heat another thermal storage medium.

water would require an additional energy source.

100, leading to operating temperatures of 100–400°C.

auxiliary components in the different desalination technologies.

Fig. 2. MED Desalination Process

#### **2.3 Reverse osmosis (RO)**

RO is a pressure-driven process that separates two solutions with different concentrations across a semi-permeable membrane. The fresh water flow rate through the membrane is proportional to the pressure differential that exceeds the natural osmotic pressure differential. The membrane itself represents a major pressure differential to the flow of fresh water. For brackish water desalination the operating pressures range from 15 to 30 bar, and for seawater desalination from 55 to 70 bar (Abdallah et al., 2005). The initial pressurization of the feed water represents the major energy requirement. As fresh water permeates across the membrane, the feed water becomes more and more concentrated. There is a limit to the amount of fresh water that can be recovered from the feed without causing fouling. Seawater RO plants have recoveries from 25 to 45%, while brackish water RO plants have recovery rates as high as 90%. RO system major components include membrane modules, high-pressure pumps, power plant, and energy recovery devices as needed (Fig 3).

Fig. 3. Schematic diagram of one RO Desalination process with two stages

Membrane properties and feed water salinity are the two major factors controlling the energy requirements of an RO system. Higher water salinity requires more energy to overcome the osmotic pressure.

Pre-treatment of seawater feeding RO membranes is recognized as a key in designing RO desalination plants (Gaid and Treal, 2007). The use of an adapted pre-treatment minimizes the fouling problems and can provide good protection of the membranes and a longer lifetime.
