**3. Renewable energy systems for desalination**

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 auxiliary components in the different desalination technologies.

#### **3.1 Solar technologies**

92 Modeling and Optimization of Renewable Energy Systems

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

Fig. 2. MED Desalination Process

**2.3 Reverse osmosis (RO)** 

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 tank or used to heat another thermal storage medium.

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 distillation are as follow.

#### **3.1.1 Flat-plate collector**

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 the collector cost also increase.

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 water would require an additional energy source.

#### **3.1.2 Parabolic trough collector**

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

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

Optimization of Renewable Energy Systems: The Case of Desalination 95

Interface

Electricity

Thermal Energy

Gravitational Potential Energy

Wind Energy Desalination Unit

Kinematical Power

Direct conversion from wind energy to thermal energy to drive thermal desalination units (distiller) has been studied since the efficiency of direct wind-thermal conversion is higher than that of wind-electricity conversion and their structures are simpler (Nakatake and Tanaka, 2005). The proposed distiller could produce 1.5 kg/d or more when a 6 m/s wind

To reduce the energy loss caused by the wind-electricity conversion, gravitational energy has also been used as the interface between wind energy and desalination process. Fadigas and Dias (2009) designed an alternative configuration to conventional RO desalination systems by incorporating the use of gravitational potential energy, without using either electricity or fossil fuels. The gravitational potential energy, presented by water stored in a reservoir above a certain height, was converted by wind energy from windmills (or wind

Many different renewable energy desalination systems are technically feasible (Kalogirou, 2005). Fig.5 presents the possible combinations between desalination processes and RE

A methodology for selecting the most appropriate combination between desalination technologies and renewable energies for a given site based on different criteria was developed (Setiawan et al., 2009). Desalination systems are energy intensive, and their energy consumption is a driving factor in determining their economic feasibility when they are coupled to RES. Typical energy consumptions for different desalination processes are

Fig. 4. Existing interfaces between wind energy and desalination unit.

blew steadily all day on a sunny or cloudy day.

turbines).

technologies.

shown in Table 1.

**4. RE/DES systems** 

collectors. The parabolic trough collector systems usually include a mechanical control system that keeps the trough reflector pointed at the sun throughout the day. Parabolictrough concentrating systems can provide hot water and steam, and are generally used in commercial and industrial applications.

Due to the high temperatures parabolic troughs are capable of producing high-grade thermal energy that is generally used for electricity generation (Belessiotis and Delyannis, 2001). Parabolic troughs could be a suitable energy supply for most desalination methods, but in practice, they have mainly been used for thermal distillation as these methods can take advantage of both the heat and electricity troughs produce. Parabolic Trough Collectors can also drive RO units by using Rankine Organic Cycle.

#### **3.1.3 Photovoltaic systems**

Photovoltaic systems consist of a number of PV modules, which convert solar radiation into direct-current (DC) electricity. The voltage and current of the system can be increased by connecting multiple cells in series and parallel, respectively. The other system equipment includes a charge controller, batteries, inverter, and other components needed to provide the output electric power suitable to operate the systems coupled with the PV system. PV is

a rapidly developing technology, with costs falling dramatically with time, and this will lead to its broad application in all types of systems. Today, however, it is clear that PV-RO and PV-ED will initially be most cost-competitive for small-scale systems where other technologies are less competitive.

The electricity form PV systems can be used to drive high-pressure pumps in RO desalination plants. The main advantage of PV/desalination systems is their ability to develop small size desalination plants. The energy production unit consists of a number of photovoltaic modules, which convert solar radiation into direct electric current (DC). DC/AC inverters have to be used because RO uses alternating current (AC) for the pumps.

Energy storage (batteries) is required for PV output power smoothing or for sustaining system operation when insufficient solar energy is available.

#### **3.2 Wind systems**

Wind energy and desalination plants can be coupled in various ways (Ma and Lu, 2011). Currently, wind energy can power desalination plants directly or indirectly through four types of energy media (Fig. 4): electricity, thermal energy, gravitational potential energy and kinematical power (shaft power).

Electricity is the most commonly used energy form as the interface between wind energy and desalination process. After having converted into electricity, the energy from wind plant can be employed to drive desalination processes such as RO, ED and MVC (Kalogirou, 2005). The wind plant can be on or off the grid. Due to the intermittent characteristic of wind power, usually backup facilities like battery, water tank, flywheel system might be integrated into the system to store or release energy when the wind speed exceeds or cannot achieve the required level.

collectors. The parabolic trough collector systems usually include a mechanical control system that keeps the trough reflector pointed at the sun throughout the day. Parabolictrough concentrating systems can provide hot water and steam, and are generally used in

Due to the high temperatures parabolic troughs are capable of producing high-grade thermal energy that is generally used for electricity generation (Belessiotis and Delyannis, 2001). Parabolic troughs could be a suitable energy supply for most desalination methods, but in practice, they have mainly been used for thermal distillation as these methods can take advantage of both the heat and electricity troughs produce. Parabolic Trough Collectors

Photovoltaic systems consist of a number of PV modules, which convert solar radiation into direct-current (DC) electricity. The voltage and current of the system can be increased by connecting multiple cells in series and parallel, respectively. The other system equipment includes a charge controller, batteries, inverter, and other components needed to provide the output electric power suitable to operate the systems coupled with the PV

a rapidly developing technology, with costs falling dramatically with time, and this will lead to its broad application in all types of systems. Today, however, it is clear that PV-RO and PV-ED will initially be most cost-competitive for small-scale systems where other

The electricity form PV systems can be used to drive high-pressure pumps in RO desalination plants. The main advantage of PV/desalination systems is their ability to develop small size desalination plants. The energy production unit consists of a number of photovoltaic modules, which convert solar radiation into direct electric current (DC). DC/AC inverters have to be used because RO uses alternating current (AC) for the

Energy storage (batteries) is required for PV output power smoothing or for sustaining

Wind energy and desalination plants can be coupled in various ways (Ma and Lu, 2011). Currently, wind energy can power desalination plants directly or indirectly through four types of energy media (Fig. 4): electricity, thermal energy, gravitational potential energy and

Electricity is the most commonly used energy form as the interface between wind energy and desalination process. After having converted into electricity, the energy from wind plant can be employed to drive desalination processes such as RO, ED and MVC (Kalogirou, 2005). The wind plant can be on or off the grid. Due to the intermittent characteristic of wind power, usually backup facilities like battery, water tank, flywheel system might be integrated into the system to store or release energy when the wind speed exceeds or cannot

commercial and industrial applications.

**3.1.3 Photovoltaic systems** 

technologies are less competitive.

kinematical power (shaft power).

achieve the required level.

system. PV is

pumps.

**3.2 Wind systems** 

can also drive RO units by using Rankine Organic Cycle.

system operation when insufficient solar energy is available.

Fig. 4. Existing interfaces between wind energy and desalination unit.

Direct conversion from wind energy to thermal energy to drive thermal desalination units (distiller) has been studied since the efficiency of direct wind-thermal conversion is higher than that of wind-electricity conversion and their structures are simpler (Nakatake and Tanaka, 2005). The proposed distiller could produce 1.5 kg/d or more when a 6 m/s wind blew steadily all day on a sunny or cloudy day.

To reduce the energy loss caused by the wind-electricity conversion, gravitational energy has also been used as the interface between wind energy and desalination process. Fadigas and Dias (2009) designed an alternative configuration to conventional RO desalination systems by incorporating the use of gravitational potential energy, without using either electricity or fossil fuels. The gravitational potential energy, presented by water stored in a reservoir above a certain height, was converted by wind energy from windmills (or wind turbines).
