Wind Powered Water Desalination Youssef Dahioui

*, Khalid Loudiyi#

"'School o/Science and Engineering

Al Akhawayn University in Ifrane

/frane, Morocco

ly.dahioui@aui.ma 2k.loudiyi@aui.ma

Abstract- Renewable energy, and more specifically, wind

energy powered desalination, has been going through an

upwards trend, especially during the last decade. Still, there is a

domain that has not been much researched 111; hence, this paper

tries to address wind powered independent desalination systems.

With its lower energy consumption and portability, Reverse

Osmosis (RO) method has been chosen as the desalination

technology that will be integrated with wind energy. In this work

a MA TLAB simulation is used to find out the effects of

fluctuating wind energy on a system that is designed to operate

under steady conditions. The results show that varying electrical

power leads to extreme fluctuation in feed water pressure,

beyond the operational range of the RO membranes. Still, this

does not draw a cross on the wind and desalination combo; in

fact, several methods exist to diminish the pressure variation,

including wind turbine de-rating, or use of pressure stabilizers.

K�words-Desalination, wind energy, reverse osmosis,

MA TLAB, simulation

I. INTRODUCTION

Without water, nothing alive on this planet would have existed. 75% of the surface of the earth is covered by water; nevertheless, only 3% of that water is available as fresh water, and only 13% of those 3% are directly available for drinking and any other domestic, industrial or agricultural uses [2]. Just a few decades ago, fresh water was viewed as an eternal, renewable and easily accessed resource; however, nowadays, water shortage has become a serious issue that may be the main cause of conflicts in the near future. One of the most widely used indicators for defining water stress is the Falkenmark indicator, illustrated in Table I and defined as the entire annual water available for human use [3].

TABLE I WATER SHORTAGE CATEGORIES

Index (m3 per capita) Category

>1700 No stress 1000 - 17000 Stress 500 - 1000 Scarcity < 500 Absolute Scarcity

To tackle this water shortage issue, water desalination has represented for years, an effective, yet an energy consuming method [4]. Thus, and taking into account the current energy world market, with increasing fossil fuel prices, renewable energy powered desalination is taking an interesting trend.

II. DESALINATION TECHNOLOGIES

A. Thermal Technologies

1) Multi-stage Flash Distillation: Multi-stage Flash (MSF) is the most used thermal desalination technology, worldwide; it represents about 50% of the installed capacity [5]. Basically, this process is about evaporating feed water in a group of chambers, each having a lower pressure than the previous one. When getting into one stage, water flashes, or evaporates instantaneously due to the low pressure implying a lower evaporation point.

2) Multi-Effect Distillation: The Multi-Effect Distillation (ME) was the first process used for desalination of seawater [6]. A quantity of water is heated up till becoming vapor then goes through a heat exchanger. Feed water that is going to be desalinated is sprayed in the heat exchanger condensing vapor flowing through it. Latent heat released due to condensation causes some of the feed water to evaporate and flows to another heat exchanger for the process to take place again until a significant quantity of condensate water has been collected [6].

3) Vapor Compression: Vapor compression (VC) process is a more recent method for water desalination; still, it is based on a simple principle. As shown in Fig.l, it is usually composed of three parts, a compressor, a heat exchanger and an evaporator. Some of the vapor produced within the evaporator is sent to the compressor, increasing its pressure, and its temperature as well. The superheated vapor that leaves the compressor, gets into the heat exchanger submerged within the feed water, and causes its evaporation [7].

� --=====::== Compressor L�I �

- {} . r l�W'·� ===:===i[:;;:;:::::�� -I. -= Feed Waler

/=,-esh Water

Fig. 1 Vapor compression cycle

B. Membranes Technologies

978-1-4673-6374-7/13/$31.00 ©2013 IEEE

1) Electrodialysis: Electrodialysis (ED) is the oldest desalination membrane-based technology and has been used all around the world for more than 40 years. More than 10 million m3 of water are produced on a daily basis using this technology [8]. From Fig.2, it is a process in which ions are attracted to their respective electrically charged electrode through ion-selective semi-permeable membranes. Positively and negatively charged dissolved salts in the aqueous solution are attracted to the electrode with the opposite charge, and membranes, that allow either cations or anions to pass through, are installed in an alternative way, thus creating concentrated and purified streams.

Concentrate Oiruate Concentrate

� + � � +

0 0 0 0 0 0 cathode

0 0 0 0

Feed Water

Fig. 2 Electro dialysis process

2) Reverse Osmosis: The movement of solvent molecules from one space with low solute concentration to another space through a semi-permeable membrane with a higher solute concentration is what is called osmosis, observed in 1748 [9]. What causes this movement is the difference in chemical potential between the two solutions, affected in its turn by three different factors: Salt concentration, the higher the concentration the lower the chemical potential; temperature, the higher the temperature, the higher the chemical potential; pressure, the higher the pressure the higher the chemical potential This movement, or osmosis process, would carry on until equilibrium in chemical potential between the two sides is reached; this is the "osmosis equilibrium."

Therefore, in order to desalinate water, it is the reverse process that has to be done, called "reverse osmosis." As pointed up in Fig.3, some pressure has to be applied at the beginning to initiate the flow from the saline side to the fresh side; this is the "osmotic pressure."

- r T:U Osmotic PresS(Jre

Fresh Water Saline Water'

D' Semi-permeable Membrane

Fig. 3 Reverse osmosis process

III. W[ND POWERED DESALlNAT[ON

One problem that has been facing water desalination is its significant energy needs. Consequently, to incorporate renewable energy and more specifically wind energy that can be considered nowadays as a mature technology, there are two main aspects or factors that should be taken into consideration:

• Specific energy consumption (Table II) or ability to produce as much water as possible from the available energy during any period

• Operability under variable conditions and this is what actually keeps the thermal technologies away; they usually require a long start-up time and significant energy waste could result from frequent stops.

TABLE II

SPECIFIC ENERGY PER DESALINATION TECHNOLOGY [15]

Desalination Technology Specific

Energy

(kWh/m3)

Multi-stage Flash 6-9 Multiple Effect 10 - 14.5 Vapor Compression 7 - 15 Electrodialysis 0.7 - 2.2 Reverse Osmosis 3 - 13

Other factors could be taken into account as well such as ease of maintenance and portability. At the end, RO seems to be the technology with the highest potential.

In the literature review we have gone through [10], [11], [12] very few implemented projects dealt with this problematic. Amongst these, a prototype system using mechanical energy transmission between the wind turbine and the water pump. This type of desalination using mechanical energy instead of electrical faces several difficulties related to more frequent failures due to the use of mechanical bearings, and most importantly, the end result was not really satisfying, since the water product quality was not high enough to be drinkable even though it still could be used for irrigation. Another prototype implemented in Coconut Island, Hawaii uses electrical energy of the wind turbine, and a feedback system that enables the control of water flow, thus stabilizes its pressure. [t was able to achieve a cost of $5.4 per m3 [10].

A. Wind & RO

Reverse Osmosis has been continuously improved over the years making it one of the most energy efficient, and representing more than half of the new desalination plants that are being installed every year. This is mainly due to the development of the main part of a RO system, its membranes. Lots of efforts have been put into improving their

performance, their resistance to pressure fluctuations. Eventually, to optimize their performance, they have to undergo a specific flow rate and pressure, and this is the main challenge with fluctuating wind energy [13], [14], [7].

Thus, as demonstrated in Fig.4, that was made arbitrarily, when designing the system, there are several conditions that have to be respected to maximize the lifetime of the membranes and avoid any significant deterioration:

• Maximum feed Pressure (membrane resistance) • Maximum brine flow rate (membrane resistance) • Minimum brine flow rate (fouling problem) • Maximum product concentration (depends of osmotic

and applied pressure)

Pressure

Minimum

Brine Flow

Rate

Maximu m

Al lowe d

Co n c entrati on

Maxim um Pressure

RO membrane

O p erational

Region Maxi mum

Brin e Flow

Rate

Flow Rat·e

Fig. 4 RO membrane operating region under specific conditions

1) System with Backup: Basically, an additional energy source (diesel generator, grid connection) will be used besides wind energy. This will compensate for electricity coming from the wind generator during low or no wind. This represents an easy solution for the wind fluctuations; however, this makes our system completely dependent on the additional source. [n the case of energy shortage, or power cuts, the system will simply stop operating.

2) Independent System (No Backup): To keep the system operating at nearly steady conditions and without any backup generators, there are three main ways:

• Storage: either electrical storage with batteries or water storage, by pumping water into a tanl<- when surplus of electrical energy is produced

• Switching ON/OFF modules: this requires the availability of several independent reverse osmosis systems all connected to the same wind energy generator. Systems will be turned on or off depending on the available energy [11], [\5].

• Wind Turbine Power limitation: this consists in applying a pitch mechanism that would put a boundary on the power produced from the wind turbine generator, thus, limiting the power fluctuation [11], [15].

In the case of variable conditions, no major efforts are taken into stabilizing the electrical power, flow rate or pressure of the feed water going into the RO membranes. Since, these filters have been designed to operate optimally

under specific conditions; any extreme variation is expected to cause mechanical fatigue and impact significantly the lifetime of the membranes [11], [[6].

B. MATLAB Model

Since, there has not been much major research regarding RO membranes operating under variable conditions, we designed a MA TLAB model simulation (Fig.S) in order to investigate the effects of fluctuating wind energy on the overall system.

1) Wind Turbine

Funct i on

Consta nt

Fig. 5 Wind turbine MATLAB model

The wind turbine model has been designed in a way to gather wind speed data over a fixed period of time from a spreadsheet file.

A saturation model was added, to limit the wind speed input, assuming that a pitch mechanism would starts operating when wind speed reaches 12 ms-1. In order to compute the electrical power, we used the relation shown: P ='h p A v3 Cp

Where the different parameters in the equation are:

• Cp: Power coefficient (given a value of 0.4)

(1)

• A: rotor area (computed for a turbine with 5.2m diameter)

• p: air density (l.225 kgm-3) • v: wind speed (taken from an excel file)

2) Whole System

Fig. 6 Wind Powered Desalination System Model

This model (Fig.6) calculates, depending on the power coming from the wind turbine generator, the feed water flow rate and its pressure. The main purpose of this example is to

illustrate some of the effects of variable wind conditions on our desalination system.

For the flow rate calculation we have used [17]:

With P representing the electrical power ( in k W), H the head (given a value of 1m), g the gravitational acceleration (9.81 ms-\ and 11 the pump efficiency.

For the pressure calculation we used [18]:

QF being the feed flow rate calculated in the previous equation, R the recovery rate (given a value of 0.5), p the membrane permeability (given a value of 0.2464m3/m2h/bar), Am the membrane area (given a value of 1 m2), and n the osmotic pressure

As it can be noticed in the graph resulting from the simulation (Fig.7), wind speed variation leads to significant pressure changes. Thus, from this simulation, a simple wind powered desalination RO system may be regarded as not feasible.

Fig. 7 Wind speed impact on pressure and flow rate

p, • -\\lndSpOIQd

rIawR:tc:

The remedy for this situation could be de-rating the wind turbine. Thus, by using the same model, but flattening the electrical power coming from the wind turbine generator we get the following results of de-rating shown in Fig.8.

"" ,

Fig. 8 De-rating impact on pressure variation

-Pr.uW'_ - ..... nllSp.ad

'1ooI!'.,.

By reducing the maximum power produced by the wind turbine, we simply diminish the pressure fluctuation; therefore, we may reduce it down to an acceptable level that would not result in mechanical damage.

Although this may be seen as an effective and easy solution, it at the same time represents a waste of potential electrical energy, due to pitching, or in other terms, waste of investment.

C. Parts a/the system

1) Variable Speed Drive: With an energy source as intermittent as wind energy, use of variable speed pumps is a necessity. This variable speed drive is controlled by the wind turbine generator and includes: rectifier, DC link capacitor and variable frequency inverter (Fig.9). This later component will adjust the speed and torque of the pump depending on the available power. This should allow the pump to continue operating smoothly without sudden changes under variable wind conditions.

Power Grid Rectifier In veTter f------j DC Unk

Fig. 9 Variable Speed Drive

2) Pressure Stabilizer: Since, pressure seems to be the most critical issue when it comes to wind powered desalination with its irreversible effects on the membranes, it is extremely vital for it to be dealt with. The simplest way that comes to mind, in order to stabilize the pressure, would be storage, but since, a minimum of pressure is required to be

able to produce fresh water through the reverse osmosis system, a pressurized water tank would be needed [10].

From that point, a choice can be made on whether to have a continuous or non-continuous operating system. In the non­continuous case feed water is stored and it is taped when needed. On the other hand, the continuous system uses solenoid valves (figure 10) controlled by pressure sensors. The number of valves that open will depend on the pressure within the tank [19]. The higher the pressure, the more valves will open, to keep it within the acceptable operating range for the membranes.

Pressure Sensors Feed Water

Valves

Fig.lO Pressure-Controlled Valves

3) Energy Recovery Device: One positive aspect of the reverse osmosis process is that there is a very small drop pressure across the RO vessels; in other words, the concentrate stream keeps most of the initial pressure of the feed water. Therefore, energy recovery devices have been designed to make use of this significant amount of pressurized brine that would, otherwise, be wasted through direct discharge [20], [21]. That pressure will be transferred to some of the feed water (Fig. I I), hence increasing the pressure within the pressurized tanl<- faster than in typical conditions, allowing for more water production.

High PresSlJre Feedwoter � ••••••• � High Pressure Brine

Low Pressure Feedworer ---- '--_____ L..----" � Low Pressure Bn·ne

Fig. 11 Energy Recovery Device

Most ERDs incorporate positive displacement technology and can be highly efficient going up to 96% of efficiency.

D. Cost Analysis

For the last part, the cost per unit of water produced will be estimated.

Starting with the capital cost, the purchase of a 5.1 kW wind turbine used for this analysis, a variable speed pump, a small RO system, will cost around $13 700. According to Energy Recovery Inc (ERI), ERDs cost around 4% of the capital cost, which amounts to $548 [22].

Concerning, the operational costs, besides maintenance, an annual change of RO membranes is assumed; a yearly amount of $300 each was assumed. Thus, for a lifetime of about 20 years and an interest rate of 5%, the net present value of the operational costs would be $4522.

Summing both the capital and operational costs would give us, as a final total cost, a value of $18770.

With an average wind speed of 4m.s·1 leading to a daily average of around 3m3 of water produced, during the 20 years lifetime of the project, the total water production would be 21 900 m3.

Therefore, this wind powered desalination system will be producing fresh water at a cost of 7.35 MAD/m3•

IV. CONCLUSIONS

By using a low wind speed, and a low water production, the cost of water produced does not go beyond 7.35 MAD which is a highly satisfying cost, especially when compared with other countries, where the cost can go up to 30 MAD/m3, in some of the Greek Islands, for example, where powered desalinations systems with high wind integration have been implemented [2].

Water Cost ($) per m3 Vs. Wind Speed (m.s·l)

1 0,8

§ 0,6 t; 8 0,4

0,2 o

4 6 8 Wind Speed (m.s·l)

Fig. 12 Water production cost per wind speed

10 12

Eventually, production cost could be much lower in windy regions, and on higher implementation scale (figure 12). Therefore, it is safe to say that independent wind powered desalination systems will have a bright future in the coming years. For the Moroccan context, we believe that our proposed system will be suitable for implementation in regions where there's urgent need for water (either industrial or sanitary); and thus all the coastal southern Saharan region. In these regions all required conditions are present: important wind resource (the Sahara trade winds), raw material (sea and brackish water), in addition to a permanent need for fresh water.

However, it is important to mention that this system stays economically attractive only for low saline water desalination. Thus, we propose for future work conducting additional experimentation and research to effectively optimize the system and make it economically attractive for all desalination cases.

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