**1. Introduction**

88 Modeling and Optimization of Renewable Energy Systems

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The application of renewable energies (RE) for driving desalination units (DES) is very promising in isolated areas (i.e. islands, villages in the desert, etc.), where the electricity production is very expensive, potable water resources are inexistent and the potential of renewable energies (solar and wind) is very important (Koroneos et al., 2007; Kalogirou, 2005). The application of renewable energies in the desalination industry does not face the same barriers as in the case of RES for electricity power production (expensive storage systems to compensate the stochastic characteristics of the renewable energies). In the case of RES/DES coupling, the energy is consumed directly for water production, the water can be stored, cheaply in large quantities and for long periods (Koroneos et al., 2007).

The operational performances, the cost and the reliability of RES/DES units depend on the design and calibration of such systems. Optimal use of RES potential is necessary in order to reduce the cost of produced water. In this frame considering hybrid configurations (PV and Wind) is the best way to optimize the use of RE.

On the other hand, the design of such systems is complex because of uncertain renewable energy supplies, load demands and the non-linear characteristics of some components.

Despite the great effort done to improve the efficiency of RES/DES systems the desalinated water cost still relatively high compared to conventional desalination units and more effort should be done to optimize this kind of systems.

In this chapter we present different methods to optimize renewable energy systems driving desalination unit, with a particular interest to the RO driven by hybrid PV/Wind systems. For this last configuration we will present a new methodology based on Genetic Algorithms.

The objective function used in this optimization is the unit cost of desalinated water during the life cycle of the plant (20 years). The presented methodology consists in selecting from available components in the market, the optimal number and the type of each unit (PV panels, wind turbines, membranes, etc.) in such way that the water needs are satisfied and the production cost is minimized. The total water cost for the life cycle of the plant is equal to the sum of the capital and maintenance costs.

Optimization of Renewable Energy Systems: The Case of Desalination 91

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

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

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

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.

stage to stage and builds up the brine flow which is extracted from the last stage.

power and steam cycles.

Fig. 1. MSF Desalination Process

steam is recycled to the boiler for reuse (Fig. 2).

**2.2 MED desalination** 

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 investment and operational costs.

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 simulation during all over a year.
