Volume 2 wind energy 2 22 – special wind power applications

Volume 2 wind energy 2 22 – special wind power applications Volume 2 wind energy 2 22 – special wind power applications Volume 2 wind energy 2 22 – special wind power applications Volume 2 wind energy 2 22 – special wind power applications Volume 2 wind energy 2 22 – special wind power applications Volume 2 wind energy 2 22 – special wind power applications

E Kondili, Technological Education Institute of Piraeus, Athens, Greece

© 2012 Elsevier Ltd All rights reserved

2.22.1 Introduction – The Water Demand Problem

2.22.2 Desalination Processes and Plants

2.22.2.2 Membrane/RO Desalination Processes

2.22.3 Energy Requirements of Desalination Processes

2.22.3.2 Utilizing RESs in Desalination

2.22.4 Integrated Systems of RES with Desalination Plants

2.22.5.1 Basic Characteristics

2.22.5.3 Operational Issues – Technical Difficulties

2.22.6 Wind–RO Configuration Possibilities

2.22.6.1 Systems with Backup (Diesel/Grid)

2.22.6.3 Near-Constant Operating Conditions

2.22.6.7 Variable Operating Conditions

2.22.8 Implementation of Projects with Hybrid Energy Systems

2.22.9 Economic Considerations in RES-Based Desalination

2.22.9.2 Parameters Affecting Economics of Desalination

2.22.10 Examples of Wind-Based Desalination Applications – Case Studies

2.22.10.1 General Issues for the Case Studies Analysis

2.22.10.5 Milos Island, Greece

2.22.11 Technological Developments and Future Trends in Hybrid Desalination Systems

2.22.12.1 General Considerations

2.22.13 The Wind Power-Based T/C Station

2.22.13.1 Configuration Options Overview

2.22.14 Applications of Wind Energy in T/C Stations

2.22.16 Water Pumping System Applications

References

Further Reading

Desalination The process of removing salt from saline of water through a membrane against the natural osmotic water and producing fresh potable water pressure to separate ions

RES based desalination Desalination processes that cover Wind pumping The exploitation of wind power systems their energy needs from Renewable Energy Sources to pump water mechanically

2.22.1 Introduction – The Water Demand Problem

Wind based desalination is the first of the special applications that are dealt with in this chapter This topic is very crucial as today about 3 billion people around the world have no access to clean drinking water Moreover, about 1.76 billion people live in areas already facing a high degree of water shortage [1] Water shortage is one of the most critical global problems As a result, various solutions for the security of water supply are investigated and desalination is considered as one of the most promising ones [2] To that effect, much attention is being paid in research and technological development fields in desalination issues

The specific objectives of this section are to analyze and describe the use of wind energy in desalination processes More specifically, the main directions are

• to highlight the critical water shortage problems being faced by various areas of the planet;

• to focus on the energy aspects of desalination and emphasize the use of renewable energy sources (RESs) and, more specifically, wind energy in desalination processes and plants;

• to identify the critical parameters and provide guidelines for the successful design and operation of a wind-based desalination system;

• to provide an insight into the future prospects of wind-based desalination systems

As a matter of fact, water is a valuable natural resource and access to freshwater is considered as a basic human right Water shortage

is considered as one of the most serious social and environmental problems to be faced in the next years in many areas of the planet Water scarcity implies not only the lack of water in arid regions but also the mismatch between water supply and demand, a problem with very strong spatial and temporal characteristics Even in cases of a positive total water balance, there may be periods of time or specific areas when and where water is not available

The water shortage problem is being solved with various methods, depending on the specific case, such as the construction and operation of infrastructure projects like desalination plants, dams, and groundwater reservoirs As almost 97% of the water on earth

is seawater, desalination, that is, the removal of salt from the virtually unlimited supply of seawater or brackish water, is considered

as a very promising method to meet the water demand and it is today widely applied in areas with limited water resources Wind energy is used for solving the water shortage problem because of the fact that desalination is an energy-intensive process and RES, more specifically wind energy, is a very promising solution for supplying energy to these units

2.22.2 Desalination Processes and Plants

2.22.2.1 General Considerations

Desalination is the process of removing salt from saline water and producing fresh potable water Seawater desalination separates saline water into two streams: a freshwater stream containing a low concentration of dissolved salts and a concentrated brine stream

A large number of desalination plants have been installed throughout the world, the majority of which can be found in the Middle East and the Caribbean islands, with very good prospects for the coming years in China Desalination is still considered more expensive than other methods, mainly due to its intensive use of energy, but this picture is continuously changing as R&D efforts and technological advancements have reduced the cost of the produced freshwater Today desalination has proved to be more reliable and an economically cheaper solution in various cases, compared with other solutions such as dam construction or transportation of water by marine vessels The new amount added each year to total desalination capacity is shown in Figure 1 Demand in desalination capacity is predicted to grow rapidly and is taking place not only in the Middle East, led by the Gulf Cooperation Council countries, but also in other countries led by Algeria, Australia, and Spain New markets are opening in China, India, and the United States [3]

The currently available desalination technologies can be categorized as follows:

1 Phase change processes that involve heating the feed (seawater or brackish water) to ‘boiling point’ at the operating pressure to produce ‘steam’ and condensing the steam in a condenser unit to produce freshwater Applications of this principle include solar distillation (SD), multieffect distillation (MED), multistage flash distillation (MSF), mechanical vapor compression (MVC), and thermal vapor compression (TVC)

2 Nonphase change processes that involve separation of dissolved salts from the feed water by mechanical or chemical/electrical means using a membrane barrier between the feed (seawater or brackish water) and the product (potable water) Applications of this principle include electrodialysis (ED) and reverse osmosis (RO)

3 Hybrid processes that involve a combination of phase change and separation techniques (as in the case of nonphase change processes) in a single unit or in sequential steps to produce pure or potable water Examples include membrane distillation (MD) and RO combined with MSF or MED processes The most common desalination processes being implemented today are distillation and membrane processes (Figure 2), each accounting for about half of the installed global desalination capacity Today, most of the R&D efforts and the technological innovations are oriented toward membrane processes and, more specifically, toward RO processes As in any type of separation, the critical issue in water desalination is the high energy demand Many countries

in the world that lack freshwater sources are also deficient in energy sources, making the problem even more difficult to solve With

New desalination capacity 1980−2009

Seawater desalination processes

Thermal processes (Phase change)

Multistage flash evaporation (MSF)

Multieffect distillation (MED)

Vapor compression (VC)

mechanical (MVC) and thermal (TVC)

Membrane processes (Single phase)

Reverse osmosis (RO)

Electrodialysis (ED)

the world’s freshwater demand increasing, much research has been directed at addressing the challenges in using renewable and environmentally friendly energy to meet the power needs for desalination plants

Typically, desalination processes are powered by energy derived from combustion of fossil fuels, which contribute to acid rain and climate change by releasing greenhouse gases (GHGs) as well as several other harmful emissions Therefore, the environmental impacts of the energy use in desalination plants are also a very significant problem that needs to be considered Table 1 presents the

Table 1 World population, desalination capacity, oil requirements, and GHG emissions over the past five decades [5]

World population World desalination capacity Oil required GHG emissions

Year (billions) (million m3 day−1) (million metric tons day−1) (tons CO2 day−1)a

a Basis: 1 m3 of water generated from a desalination plant using fossil fuel (oil) contributes to 3 tons CO2

world population growth with increased desalination capacity and the oil requirements to produce freshwater through desalination technologies and associated GHG emissions over the past five decades

Therefore, it is necessary to develop alternatives to replace conventional energy sources used in the desalination process with renewable ones and reduce the energy requirements for desalination by developing innovative low-cost, low-energy technologies and processes The driving forces for such an increase are the rising water shortage and the technology-driven cost reductions Although desalination has been considered as a very expensive water supply method, the technological advancements (mainly focused on improved energy utilization) have allowed it to really become a competitive method against other water supply approaches

2.22.2.2 Membrane/RO Desalination Processes

Most new desalination plants now use membrane technologies Membrane processes have considerable advantages in desalting water and are now being widely applied in this market More specifically, the most widely applied membrane process, RO, represents more than 88% of membrane processes [6]

RO process involves the forced passage of water through a membrane against the natural osmotic pressure to separate water and ions

In these high pressures, the water molecules can pass through the membranes and the salts are left behind as a briny concentrate

A typical RO system consists of four major subsystems (Figure 3):

Pretreatment

Reverse osmosis unit

Energy recovery

RO operating pressure varies from 17 to 27 bar for brackish water and from 55 to 82 bar for seawater Part of the feed water passes through the membranes, removing from it the majority of the dissolved solids resulting in the so-called product or permeate water The remaining water together with the rejected salts emerges from the membrane modules at high pressure as a concentrated reject stream (brine)

In large plants, the reject brine pressure energy is recovered by a turbine, recovering from 20% up to 40% of the consumed energy

In fact this is one of the most significant issues in RO technological development and innovation The energy saving, that is, the percentage of the mechanical energy that can be recovered pressurizing the feed water, and the water recovery ratio – the ratio of the desalinated water output volume to the seawater input volume used to produce it – are the critical parameters in the RO process

RO processes have been characterized by a significant reduction in energy consumption Apart from its need for an elaborate pretreatment plant, the RO process has many advantages such as the following:

• The modular structure of the process makes it flexible enough to handle different plant capacities

• The process is conducted at ambient temperature, which minimizes corrosion hazard

• There is an embedded potential of water–power cogeneration and coupling with energy recovery systems

• The rate of development in RO technology is high compared with other desalination processes and this fact promises for more cost reduction of desalted water produced by RO in the near future

• Desalination by RO results in high salt rejection (up to 99%) and high recovery ratios (up to 40%)

• Seawater RO (SWRO) can produce potable water with salt content of about 500 ppm

The energy issues of desalination processes and plants are discussed in the following sections of the chapter

2.22.3 Energy Requirements of Desalination Processes

2.22.3.1 General Issues

All desalination processes use energy, which is the largest cost component in the operation of a desalination plant and offers the greatest potential for further efficiency improvement and cost reduction In fact, energy consumption is considered as the main reason that desalination has not yet been widely applied The share of energy in overall cost varies with the plant, its operation parameters, and location, as shown in Figures 4 and 5 for thermal and membrane processes, respectively

Typical cost structure for a typical thermal

desalination of SW

Thermal energy, 50%

Electrical energy, 9%

Personnel, Chemicals,

3%

Capital, 32%

6%

Typical cost structure for SWRO unit

Maintenance Labor, 4%

Membrane replacement,

Figure 6 Distribution of power usage in an RO plant [7]

Therefore, it is necessary to develop alternatives to replace conventional energy sources used in the desalination process with renewable ones and reduce the energy requirements for desalination by developing innovative low-cost, low-energy technologies and processes There are various possible combinations of RESs with well-established desalination technologies with different suitability and cost requirements of such desalination processes for domestic, small-scale, and large-scale applications

Furthermore, the distribution of power usage in a two-stage SWRO system is shown in Figure 6 More than 80% of the power is required for the primary feed pumps [7]

In any desalination process, the energy consumption depends on a variety of factors, including

• seawater salinity,

• the technology being used,

• the ability of the system for energy recovery,

• the temperature of operation for membrane processes,

• performance ratio,

• heat losses, and

• temperature difference for thermal processes

In Table 2 the major power requirements of desalination processes are shown [6, 8]

The development of RO and more recently the improvements in energy recovery devices have changed that situation With energy consumption on Mediterranean SWRO plants down to 3 kWh m−3, seawater desalination is now feasible for many communities In practice, much higher energy is required by the currently available desalination technologies

In countries making significant desalination investments, energy policies and energy investment planning should possibly be revised

to provide the right incentives for appropriate desalination processes and to decide whether cogeneration of water and power is a suitable option under particular circumstances This has become more significant for reasons ranging from integration of policies, the demand for water growing at a different rate than the demand for power, and seasonal variations between power and water demands [9, 10] However, thermal processes (MSF, MED) operating with steam supplied by the exhaust and steam bleeding from backpressure

or extraction steam turbines are economically attractive and comparable with RO energy cost [11]

2.22.3.2 Utilizing RESs in Desalination

The use of RESs for the operation of desalination plants is a feasible and environmentally compatible solution in areas with significant RES potential The main driving forces for applying RES in desalination plants are

• the continuous technological advancements in RES systems and their cost reduction;

• the seasonal variability in water (and energy) demand, usually occurring in areas with high renewable energy availability, for example, islands;

Process Gain output ratio

Electrical energy consumption (kWh m−3)

Thermal energy consumption (kWh m−3)

MVC BWRO SWRO

N/A N/A N/A

9.5–17 1.0–2.5 4.5–8.5

N/A N/A N/A N/A, not available

Other Feed water supply

Primary feed pumps

Electricity

Heat

Solar thermal

SD

HD

HD

MD MED MSF TVC

Electricity

• the limited availability of a conventional energy supply in remote areas;

• the technological advancements being achieved in desalination systems;

• the limitation of environmental impacts of conventional desalination systems; and

• the relative ease of the plant’s operation and maintenance compared with conventional energy ones

To that end, a lot of research and development work has been carried out and the problem of the optimal configuration/ combination of an RES energy source with a desalination plant attracts the interest of many researchers and construction and engineering companies

Figure 7 shows potential pathways by which common RESs can be utilized to drive the different desalination processes Each pathway involves different technologies, each with its own yield and efficiency

The best coupling of RES to desalination systems is a complicated and interesting problem and its solution is not always obvious and unique In fact, this is a major decision-making issue, part of the wider problem of infrastructure planning Various criteria should be taken into account, including among others

• the renewable energy availability,

• the investment and operational cost and the availability of financial resources,

• the system’s efficiency,

• the availability of operational personnel,

• the suitability of the system to the characteristics of the location, and

• the possibility for future increase of the system capacity

Matching renewable energies with desalination units, however, requires a number of important factors to be considered Not all the combinations of RES-driven desalination systems are practicable, as many of these possible combinations may not be viable under certain circumstances The optimum or just simple specific technology combination must be studied in connection with various local parameters such as geographical conditions, topography of the site, capacity and type of energy available at low cost, availability of local infrastructures (including electricity grid), plant size, and feed water salinity

More specifically, the factors to be considered for selecting desalination process suitable for a particular application include the following:

• the amount of freshwater required in a particular application (i.e., the plant’s capacity) combined with the applicability of the various desalination processes;

• the seawater treatment requirements, that is, the feed water’s salinity;

• the technical infrastructure of the area (e.g., road access, network) and the local regulations concerning the land use and the land area required, or that could be made available, for the installation of the integrated energy and desalination unit;

• the remoteness of the area and the availability of grid electricity;

• the suitability and effectiveness of the process with respect to energy consumption;

• the capital cost of the equipment;

• robustness criteria and simplicity of operation;

• low maintenance, compact size, and easy transportation to site;

Table 3 Evaluation of various RESs in desalination applications [12]

Suitability for Well suited for desalination Well suited for desalination Well suited for desalination Well suited for desalination powering plants requiring thermal plants requiring electrical plants requiring electrical plants requiring thermal

plants

Site requirements Typically good match with Typically good match with Resources are location Resources are limited to and resource need for desalinationa need for desalinationa dependentb certain locationc availability

Continuity of power Output is intermittent Output is intermittent Output is intermittent Continuous power outputa

Predictability of Output is relatively Output is relatively Output is very stochastic/ Output is predictablea power output unpredictableb unpredictableb fluctuatesc

a Excellent compliance with criterion

b Good compliance with criterion

c Poor compliance with criterion

• acceptance and support by the local community; and

• organization at local level with relatively simple training

Table 3 evaluates the combinations of desalination and RES according to certain energy-related criteria

2.22.4 Integrated Systems of RES with Desalination Plants

Desalination using renewable energy is undergoing a rapid development nowadays The most likely market for coupling renewable energy with desalination is small communities in remote locations where there is no power grid connection or where energy is expensive In the context of the utilization of the more established RESs, that is, the sun (thermal and photovoltaic (PV)) and the wind, stand-alone desalination systems have been widely discussed Even if one focuses on one particular renewable source and a specific desalination method, there may still be many options available in terms of the final system configuration

There is very strong research interest in this specific area Many research teams work in specific technical issues or in integration and optimization aspects of the combination between RES and desalination However, as far as implementation is concerned, many small-scale and rather experimental projects have been installed but there is no serious experience from industrial-scale projects The Red-Dead project, aiming at linking the Red Sea with the Dead Sea, might be the first large-scale renewable energy-driven desalination scheme It would have a potential of producing up to 850 million m3 yr−1 of potable water

2.22.5 RO–Wind Desalination

2.22.5.1 Basic Characteristics

Desalination systems driven by wind power are the most frequent renewable energy desalination plants (Figure 8) Wind-powered desalination represents one of the most promising renewable energy options for desalination, especially in coastal areas with high availability of wind energy resources In fact, after solar energy, wind energy is the most widely used RES for low-capacity desalination plants The two most common approaches for wind-powered desalination systems include connecting both the wind turbines and the desalination system to the grid and direct coupling of the wind turbines with the desalination system

Also a primary concern with the use of wind energy for desalination is that wind speed is very variable Another option is likely to

be a stand-alone system at remote locations which have no electricity grid In this case, the desalination system may be affected by power variations and interruptions caused by the wind Hence, the stand-alone wind desalination systems are often hybrid systems combined with another type of RES (e.g., solar) or a backup system such as batteries or diesel generators For stand-alone wind energy-driven desalination units, the reported cost of freshwater produced ranges from 1.5 to 3.5 € m−3 [9]

More specifically, wind energy can be used efficiently on condition that the average wind velocity is above 5 m s−1 This makes wind-powered desalination a particularly interesting option for windy islands, both for the solution of their energy supply problem and for the operation of seawater desalination plants

The main design variables that affect the design of a wind–RO system are

• the water demand and, therefore, the RO plant’s capacity,

• the location where the wind turbine and the desalination plant will be installed (required siting, altitude, etc.),

• the feed water salinity,

• the wind speed distribution,

Sea water

Battery bank

Fresh water storage

Post treatment

Wind energy unit

Energy manag ement

• the configuration of the energy system,

• the water storage capacity,

• the available power distribution,

• the feed water source, that is, seawater, brackish water,

• desalination unit energy consumption,

• the salt rejection,

• the forecasted environmental impacts,

• the operating pressure, and

• the permeate flux, in terms of both overall product rate and specific rate (per unit membrane area)

Desalination plants using membrane technologies are available in a wide range of capacities As far as the recommended RES–desalination combinations are concerned, it is considered that wind desalination is suitable for a wide spectrum of desalina­tion capacities (50–2000 m3

day−1), resulting in a cost of desalinated water of 1.5–4 € m−3 [14] Recent developments in wind turbine technology imply that wind power can now be regarded as a reliable and cost-effective power source for many areas of the world Wind turbines may be classified depending on their nominal power ‘No ’ as very small (No < 10 kW), small (No < 100 kW), medium sized (No < 1.0 MW), and large (No > 1.0 MW) All are based on mature technol­ogies and they are commercially available except for the very large power systems (> 5 MW), which still require several adjustments

2.22.5.2 Design Issues

The basic assumptions for the required calculations concerning the energy efficiency of the wind turbines with or without an energy storage system may be considered as below

For a wind turbine with a nominal power of No kW, we expect an energy production ‘E’ in the order of magnitude of

‘E = CF � No � 8760’ kWh yr−1 Note that the installation capacity factor ‘CF’ usually varies between 20% and 30% Depending on the type of desalination plant, the required amount of energy per cubic meter of potable water will also be given Therefore, we may have a series of alternatives concerning the installed power of the wind turbine and the combined capacity of the desalination plant Many other parameters should be taken into account in this design issue, such as the possible losses of an energy storage system and the availability of a water storage system [15]

The variable nature of wind power is not a problem as far as water availability is concerned, because water can be stored inexpensively With a plant that is dimensioned according to the local wind conditions, water becomes available any time However, the serious problem of this type of installations is that variable wind power may cause operational problems in the system’s operation and this is one of the most critical issues to be resolved in the design and implementation of an RES–wind-based desalination project

One common way of storing the surplus energy is by using batteries [10] or water pumping systems Storage size should be considered in the design stage In addition, capital and maintenance costs should carefully be assessed

2.22.5.3 Operational Issues – Technical Difficulties

RESs are characterized by intermittent and variable intensity, whereas desalination processes are designed for continuous steady-state operation One of the problems of utilizing wind power in process applications is the variable nature of the resource While the wind is relatively predictable, it is seldom constant and there will be periods of calm spells

The storage of wind energy in the form of electrical power is really practical only when small amounts are involved Storage batteries increase the total investment cost; therefore, running a process of any magnitude on stored electrical energy is not a practical proposition However, if the product of the process can be stored inexpensively, then it may be practical to oversize the process equipment to allow for downtime Water can be stored for long periods of time without deterioration and the storage vessels are relatively cheap Variable power input forces the desalination plant to operate in nonoptimal conditions, which may cause operational problems

To avoid the fluctuations inherent in renewable energies, different energy storage systems may be used The only areas that would require some careful design would be the relative sizes of the wind turbine and the RO plant and the cut-in and cut-out criteria for the RO plant to avoid excessive start-up and shutdown cycles

For the operation of a wind-powered desalination plant, it is most important to have a plant that is insensitive to repeated start-up and shutdown cycles caused by sometimes rapidly changing wind conditions RO is, with regard to pretreatment, membrane fouling, after-treatment, and efficiency of the high-pressure pumps, a process that is rather sensitive to a stop and start operation

2.22.6 Wind–RO Configuration Possibilities

Different wind-powered RO systems found in the literature have been classified, also taking into account some of the points previously discussed [16]:

• the existence of an alternative electrical supply (weak grid or diesel generator);

• the matching of the available wind energy to the load; and

• the operational characteristics of RO membranes

2.22.6.1 Systems with Backup (Diesel/Grid)

In these systems, an additional energy source is provided (a diesel-powered generator or even the local grid) so that the power supplied to the RO is constant The backup generation complements the power production from the wind turbine to match the RO unit power consumption The main benefit of these systems, as in any hybrid wind–diesel configuration, is fuel savings, which may increase the generator availability and reduce overall energy costs On the other hand, problems such as fuel shortages, diesel generator maintenance, and interruptions or power cuts in the supply may lead to unavailability of the RO system as it cannot be powered by the wind turbine alone for a long period of time including calm spells

2.22.6.2 Systems without Backup

Systems without an external energy source can be divided into two categories, with emphasis on the RO unit operation: systems which run under approximately constant operating conditions and those that experience variable operational conditions

2.22.6.3 Near-Constant Operating Conditions

This first type of operation can be implemented by three different means: on/off switching of the RO units, usage of storage devices, and derating the wind turbine In all three cases, an attempt is made to supply the individual RO modules with approximately constant power

2.22.6.4 Storage Devices

In this strategy, storage devices are employed to accumulate energy surplus during periods when the power generated by the wind turbine is greater than the load demand from the desalination unit This surplus would then be used later when the generated power is insufficient to meet the load demand One common way of storing the surplus energy is by using batteries In this case, the relation between operational pressure, storage sizing, and average wind speed should be considered in the design stage In addition, capital and maintenance costs should be carefully assessed A disadvantage of this approach to the system design is the rating of the energy storage system, as this can make it economically unattractive at higher power levels due to the sizing of the battery bank

2.22.6.5 RO Unit Switching

This strategy is based on the use of a higher power wind turbine connected with multiple smaller RO units The power control is achieved by switching the units on and off so as to match the demand to the total power generated instantaneously by the turbine There is no limitation concerning the system power rating, and this approach is feasible up to power levels of hundreds of kilowatts

Although frequent cycling of RO units is not usually recommended, this problem can be overcome by implementing different types of configurations Higher power wind turbines operating at near-constant speed connected to many equally smaller RO units switching on/off (load management) may be employed To smooth out the fluctuations, short-term energy storage (a flywheel in this instance) may be used

2.22.6.6 Wind Turbine Derating

This approach consists of making use of the flat end of a pitch-controlled wind turbine power curve to operate the RO unit at approximately constant power An implication of this configuration is that, as the turbine rated power is only achieved at high wind speeds, it would have to be derated by changing the settings of the pitching mechanism This will cause the generated power to be flattened at lower wind speeds and consequently to have lower values Therefore, the original rating of the turbine rotor should be considerably higher than the RO unit rated power, making the system more expensive

2.22.6.7 Variable Operating Conditions

In contrast to systems that operate under constant conditions, another operational strategy is based on the establishment and imposition of certain operational limits This means that, based on the input power to the RO unit (flow times pressure), a control strategy is determined which imposes a fixed operating point on the system that lies within the allowed region (i.e., the operational window of the RO unit)

By doing this, an attempt is made to operate the system autonomously over a wider power range, without the need to use a backup unit or storage device The overall effect is to reduce capital and operating costs One aspect that should be emphasized is that very little is known about the consequences of variable operation of RO membranes It is recognized that mechanical fatigue can occur and that the lifetime of the RO elements may be shortened and performance impaired

2.22.7 Implementation of Projects

The practical experience regarding wind-powered RO systems has been with relatively low-capacity systems There have been a number of attempts to combine wind energy with RO A number of plants have actually been operated However, most of them are

of small size, mainly for research purposes, as previously mentioned

Therefore, not many conclusions have been reached in terms of expertise and know-how It is still difficult to control the usage of wind in a cost-effective way Coupling of a variable energy supply system, as mentioned earlier, to a desalination unit requires either power or demand management, and there is not much experience on it However, the prospects of this combination are high mainly due to the low cost of wind energy The operational experience from early demonstration units is expected to contribute to improved designs, and a large number of commercial systems are expected to be implemented

A number of units have been designed and tested; however, most of them are in the demonstration and experimental scale [17–19]

As early as 1982, a small system was set at Ile du Planier, France: a 4 kW turbine coupled with a 0.5 m3 h−1 RO desalination unit The system was designed to operate via direct coupling or batteries

Another case where wind energy has been combined with RO is that at the Island of Drenec in France, in 1990 The wind turbine

in this case was rated at 10 kW and was used to drive an SWRO unit

More recently, some R&D projects have been carried out, such as the wind desalination system built at Drepanon on a cement plant, near Patras, Greece The project, including a 35 kW wind turbine, was initiated in 1992 and was completed in 1995 The project called for full design and construction of the wind generator turbine (blades, etc.) plus installation of two RO units with a production capacity of 5 and 22 m3 day−1 Unfortunately, since 1995, operational results have been poor due to the low wind regime

A very interesting experiment has been carried out at a test facility in Lastours, France, where a 5 kW wind turbine provides energy

to a number of batteries (1500 Ah, 24 V) and via an inverter to an RO unit with a nominal power of 1.8 kW Furthermore, great work

on wind RO systems has been carried out by the Instituto Tecnologico de Canarias (ITC) in several projects such as AERODESA, SDAWES, and AEROGEDESA

An energy optimization model which simulates hourly power production from RESs has been applied using the wind and solar radiation conditions for Eritrea, East Africa, for the computation of the hourly water production for a two-stage SWRO system with a capacity of 35 m3 day−1 According to the results obtained, specific energy consumption is about 2.33 kWh m−3, which is a lower value than that achieved in most of the previous designs The use of a booster pump, energy recovery turbine, and an appropriate membrane allows the specific energy consumption to be decreased by about 70% compared with less efficient designs without these features The energy recovery turbine results in a reduction in the water cost of about 41% The results show that a wind-powered system is the least expensive and a PV-powered system the most expensive, with water costs of about 0.50 and 1.00 $ m−3, respectively By international standards, for example, in China, these values are considered economically feasible [1]

2.22.8 Implementation of Projects with Hybrid Energy Systems

Due to the intermittent production of wind energy, suitable combinations of other RESs can be employed to provide smooth operating conditions Autonomous hybrid systems are independent and incorporate more than one power source