Desalination by solar powered membrane distillation systems

Desalination 308 (2013) 186–197

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Desalination by solar powered membrane distillation systems Mohammed Rasool Qtaishat a,⁎, Fawzi Banat b a b

Department of Chemical Engineering, University of Jordan, Amman, Jordan Department of Chemical Engineering, The Petroleum Institute, PO Box 2533 Abu Dhabi, UAE

a r t i c l e

i n f o

Article history: Received 18 October 2011 Received in revised form 14 January 2012 Accepted 22 January 2012 Available online 25 February 2012 Keywords: Membrane distillation Desalination Solar energy Solar collectors

a b s t r a c t Membrane distillation (MD) is a hybrid membrane-evaporative process which has been of interest for desalination. MD requires two types of energy, namely, low temperature heat and electricity. Solar collectors and PV panels are mature technologies which could be coupled to MD process. The interest of using solar powered membrane distillation (SPMD) systems for desalination is growing worldwide due to the MD attractive features. Small scale SPMD units suitable to provide water for human needs in remote areas where water and electricity infrastructures are currently lacking have been developed and tested by a number of researchers. The combination of solar energy with MD has proven technically feasible; however, the cost of produced water is relatively high compared with that produced from the commercial PV–RO process. The production of commercial, reliable, low cost and long lasting MD modules will put this process on the front edge of desalination technologies. The aim of this article is to present the main features of MD along with its basic principles. Efforts of researchers in coupling MD with solar energy and their cost estimates are reviewed as well. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The demand on fresh water is growing steadily and is becoming one of the worldwide challenges. The World Health Organization (WHO) estimates that 20% of the world's population has inadequate access to drinking water. Although over two-thirds of the planet is covered with water, 99.3% of the total water is either too salty (seawater) or inaccessible (ice caps). Since water is potable only when it contains less than 500 ppm of salt, much research has gone into finding efficient methods of removing salt from seawater and brackish water. These are called desalination processes. Desalination of seawater is a promising alternative to compensate for the shortage of drinking water. Generally, desalination can be accomplished using a number of techniques. These may be classified under the following categories:Thermal processes that involve phase change such as Multi-Effect Distillation (MED) and Multi Stage Flash (MSF). Membrane processes that do not involve phase change such as Reverse Osmosis (RO) and electro dialysis (ED).Hybrid process that involve both membrane and phase change such as membrane distillation (MD). The thermal desalination processes depend on the evaporation of water by the addition of heat provided by the sun or by combustion processes, this was one of mankind's earliest forms of water treatment and is still a popular treatment solution. On the other hand, the development of modern polymeric materials in recent years has led to the production of membranes which allow the selective ⁎ Corresponding author. E-mail addresses: [email protected] (M.R. Qtaishat), [email protected] (F. Banat). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.01.021

passage of water in liquid or vapor state or ions and thus providing the basis for membrane desalination processes. Among those membrane processes, RO is the leading commercial membrane desalination process which requires applying high pressure to overcome the osmotic pressure. It is worth mentioning that both, thermal and RO are the leading desalination processes in the water market [1]. However, those processes suffer from drawbacks and some technical difficulties which are: i) They are considered energy intensive either by the heat demand (i.e. thermal processes) or by the high pressure demand as in reverse osmosis process, this high energy consumption generates more pollutants and undesired emissions. ii) The scaling and fouling problem is one of the major challenges that adds to the complexity and cost of those processes. iii) The membrane cost and its durability in the membrane processes are still immature subjects that require more research and development. These drawbacks affected the economic feasibility of those processes, which necessitates the search for alternative, environment friendly and sustainable desalination. Membrane distillation (MD) is a promising new comer to the desalination processes which can be coupled to low-grade and renewable energy source such as wind and solar energy. The developments in the use of renewable energy sources (RES) have demonstrated that it is ideally suited for desalination, when the demand of fresh water is not too large. The rapid escalation in the costs of fuels has made the RES alternative more attractive. In certain remote arid regions, this may be the only alternative. The interdependence of water and energy is increasingly evident due to their territorial, environmental and economic implications.

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Innovations in the area of energy supply can improve the economic viability of prospective desalination plants considerably. Recently, considerable attention has been given to the use of renewable energy including solar, wind and geothermal as sources for desalination, especially in remote areas and islands, because of the high costs of fossil fuels. Solar energy can be used for seawater desalination either by producing the thermal energy required to drive the phase-change processes or by producing the electricity required to drive the membrane processes. It should be clarified that membrane distillation (MD) has not been yet commercialized for large-scale desalination plant in spite of its attractive features especially the possibility of coupling to lowgrade source of energy, this is due to the lower flux of MD and some technical problems such as the membrane wetting. However, much research has gone into developing new membranes for MD that overcomes those membrane design drawbacks [2–6]. MD applications are not limited only to desalination, since lower operating temperatures have also made membrane distillation attractive in the food industry where concentrated fruit juices and sugar solutions can be prepared with better flavor and color [7], in medical field where high temperatures can sterilize biological fluids [8], and in the environmental applications such as removal of benzene and heavy metals from water [3–6]. The purpose of this research paper is to provide a state-of-the-art review on membrane distillation systems associated with solar energy for seawater and brackish water desalination. This article presents the membrane distillation principle, configurations, mathematical models and economic feasibility.

2. Membrane distillation process Membrane distillation (MD) is a hybrid of thermal distillation and membrane processes. MD is a relatively new process that is being investigated worldwide as a low cost and energy saving alternative to conventional separation processes such as distillation and reverse osmosis [2–6]. Membrane distillation (MD) process is not commercialized yet for large scale industry. The reason behind this is that MD process flux is lower than the commercialized separation processes. The principle of membrane distillation is illustrated in Fig. 1. Conventionally, membrane distillation (MD) is a thermally driven process in which a microporous membrane acts as a physical support separating a warm solution from a cooler chamber, which contains either a liquid or a gas. As the process is non-isothermal, vapor molecules (water vapor in the case of concentrating non-volatile solutes) migrate through the membrane pores from the high to the low vapor pressure side; that is, from the warmer to the cooler compartment.

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Generally, the transport mechanism of MD can be summarized in the following steps: • Evaporation of water at the warm feed side of the membrane. • Migration of water vapor through the non-wetted pores. • Condensation of water vapor transported at the permeate side of the membrane. 2.1. Membrane distillation configurations Among membrane distillation processes, variation exists as to the method by which the vapor is recovered once it has migrated through the membrane. These alternatives are as follows: 2.1.1. Direct contact membrane distillation (DCMD) DCMD is the oldest and most widely used process, having liquid phases in direct contact with both sides of the membrane. The vapor diffusion path is limited to the thickness of the membrane, thereby reducing mass and heat transfer resistances. Condensation within the pores is avoided by selecting appropriate temperature differences across the membrane. It is worth mentioning that in DCMD configuration the heat losses by conduction through the membrane matrix is higher than other configuration due to the existence a continuous contact between the membrane surfaces and the feed (hot) and permeate (cold) solutions. 2.1.2. Air gap membrane distillation (AGMD) AGMD has an additional air gap interposed between the membrane and the condensation surface. This gives rise to higher heat and mass transfer resistances. Although heat loss by conduction is reduced, the penalty is flux reduction. The use of an air gap configuration allows larger temperature differences to be applied across the membrane, which can compensate in part for the greater transfer resistances. 2.1.3. Vacuum membrane distillation (VMD) The vapor is withdrawn by applying a vacuum on the permeate side. The permeate-side pressure is lower than the saturation pressure of the evaporating species and the condensation of the permeate takes place outside the module. 2.1.4. Sweeping gas membrane distillation The permeating vapor is removed by using an inert gas stream which passes on the permeate side of the membrane. Condensation is done externally and involves large volumes of the sweep gas and vapor stream. Fig. 2 shows the different configurations of MD. 2.2. Membrane distillation advantages

Membrane

Feed side

Permeate side (Cold)

(Hot)

Membrane pores Fig. 1. Principle of membrane distillation.

The benefits of membrane distillation compared to other more popular separation processes stem from: • 100% (theoretical) rejection of ions, macromolecules, colloids, cells and other non-volatiles; • lower operating temperatures than conventional distillation; • lower operating pressures than conventional pressure-driven membrane separation processes; • reduced chemical interaction between membrane and process solution; • less demanding membrane mechanical property requirements; • reduced vapor spaces compared to conventional distillation processes. The last benefit is considered one of the amazing advantages of MD process, since the large vapor space required in conventional distillation column is replaced in MD by the pore volume of a microporous membrane, which is generally of 100 μm thick.

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Feed in

Liquid permeate out

Feed in Condenser membrane

membrane

Sweep gas out Product

Feed out

Liquid permeate in

Feed out

Sweep gas in

SGMD DCMD Feed in Feed in Condenser Permeate

membrane

Coolant out Air gap

membrane

Condensing plate Feed out

Coolant in

Vacuum pump

Feed out

Product AGMD

VMD

Fig. 2. Membrane distillation configurations.

Conventional distillation relies on high vapor velocities to provide intimate vapor-liquid contact while MD employs a hydrophobic microporous membrane to support a vapor–liquid interface. As a result, MD process equipment can be much smaller, which translates to saving in terms of footprint, and the required operating temperatures are much lower, because it is not necessary to heat the process liquids above their boiling points. Feed temperature in membrane distillation typically ranged from 60 to 90 °C, although temperature as low as 30 °C has been used [1–6]. Therefore, lowgrade, waste and/or alternative energy sources such as solar and geothermal energy can be coupled with MD systems for a cost efficient, energy efficient liquid separation system. 2.3. Membrane distillation disadvantages The main disadvantage of MD process is the drawback of membrane wetting. The wettability of the microporous membranes is a function of three main factors: the surface tension of the process solution, membrane material and the membrane structure. To overcome the membrane wetting: the process solution must be aqueous and sufficiently dilute. This limits MD for certain applications such as desalination, removal of trace volatile organic compounds from wastewater and concentration of ionic, colloids or other nonvolatile aqueous solutions [9]. 2.4. Membrane distillation membrane

2.4.1. High liquid entry pressure (LEP) This is the minimum hydrostatic pressure that must be applied onto the liquid feed solution before it overcomes the hydrophobic forces of the membrane and penetrates into the membrane pores. LEP is characteristic of each membrane and permits to prevent wetting of the membrane pores. High LEP may be achieved using a membrane material with high hydrophobicity (i.e. large water contact angle) and a small maximum pore size. However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane becomes low. 2.4.2. High permeability The MD flux will “increase” with an increase in the membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In other words, to obtain a high MD permeability, the surface layer that governs the membrane transport must be as thin as possible and its surface porosity as well as pore size must be as large as possible. In fact, the relationship between the membrane pore size and the mean free path of migrating molecules determines the dominant diffusion mechanism. In MD, air is trapped within the membrane pores with pressure values close to the atmospheric pressure if no vacuum

Table 1 Some commercial membranes commonly used in membrane distillation. Membrane

As a matter of fact, commercial microporous hydrophobic membranes, made of polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE, Teflon), available in capillary or flat-sheet forms, have been used in MD experiments although these membranes were prepared for microfiltration purposes [9]. Table 1 summarizes some of the commercial membranes commonly used in MD processes together with some of their characteristics [9]. Recently, the desired characteristics for MD membranes have been specified, [10]. As it is well known, a MD membrane must be porous and hydrophobic, with good thermal stability and excellent chemical resistance to feed solutions. The characteristics needed for MD membranes are the following:

Manufacturer

Material

Thickness (μm)

Gelman

PTFE/PPa

178

Trade name TF200 TF450 TF1000 GVHP HVHP S6/2 MD020CP2N a

Millipore

PVDF

AkzoNobel Microdyn

PPc

b

110 140 450

Average pore size (μm) 0.20 0.45 1.00 0.22 0.45 0.2

Porosity (%)

80 75 70

Flat-sheet polytetrafluoroethylene membranes supported by polypopylene net. Flat-sheet polyvinylidene fluoride membranes. Polypropylene capillary membrane: number of capillaries in a membrane module: 40; effective filtration area: 0.1 m2, inner capillary diameter: 1.8 mm; length of capillaries: 470 mm. b c

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is applied vapor permeates through the porous membrane, as a result of molecular diffusion, Knudsen flow and/or the transition between them [2–6]. The calculated MD flux considering Knudsen mechanism is higher than that considering the combined Knudsen/molecular diffusion mechanism. 2.4.3. Low thermal conductivity In MD heat loss by conduction occurs through both the pores and the matrix of the membrane. The conductive heat loss is greater for thinner membranes. Various possibilities may be applied to diminish the conductive heat loss by using: i) Membrane materials with low thermal conductivities. This does not necessarily guarantee the improvement of the MD process because most hydrophobic polymers have similar heat conductivities; at least the materials have thermal conductivities with the same order of magnitude. ii) Membranes with high porosity, since the conductive heat transfer coefficient of the gas entrapped within the membrane pores is an order of magnitude smaller than that of the membrane matrix. This possibility is parallel to the need of high DCMD permeability as the available surface area of evaporation is enhanced. iii) Thicker membranes. However, there is a conflict between the requirements of high mass transfer associated with thinner membranes and low conductive heat transfer through the membrane obtained by using thicker membranes. MD can be commercialized for large scale industry if the above listed membrane requirements are satisfied, as a result, in recent years, the MD research attention has gone into preparing membranes specifically for the MD applications. For example, Fang et al. 2004 [11], prepared asymmetric flat-sheet membranes from poly (vinylidene fluoride-co-tetrafluoroethylene) by the phase inversion method. Those membranes were tested by DCMD configuration and the results were compared to PVDF flat-sheet membranes prepared by the same procedure. Their new membranes exhibited higher flux than those of the PVDF membranes. They also prepared membranes from poly(vinylidene fluoride-co-hexafluoro propylene) [12] and found that the DCMD performance of these membranes was better than that of the PVDF membrane. Li and Sirkar 2005 [13] and Song et al. 2007 [14], designed novel hollow fiber membrane and device for desalination by VMD and DCMD. The membranes were commercial polypropylene (PP) membranes coated with plasma polymerized silicone fluoropolymer. Permeate fluxes as high as 71 kg/m2.h were achieved. Bonyadi and Chung 2007 [15], used the co-extrusion method to prepare dual layer hydrophilic/hydrophobic hollow fiber membranes for MD. PVDF was used as a host polymer in the dope solution, where hydrophobic and hydrophilic surfactants were added. A flux as high as 55 kg/m 2.h was achieved using DCMD configuration. In a series of publications, Qtaishat et al. 2009 and 2010 [16–21], presented the concept of hydrophobic/hydrophilic composite membranes for MD. It was shown that this type of membranes satisfies all the requirements of higher flux MD membranes as mentioned earlier. Since the very thin hydrophobic layer is responsible for the mass transfer, on the other hand the thick hydrophilic layer, the pores of which are filled with water, will contribute to preventing the heat loss through the overall membrane. The hydrophobic/hydrophilic membrane was prepared by phase inversion method in a single casting step. A hydrophobic surface modifying macromolecules (SMMs) was blended with a hydrophilic base polymer. During the casting step, the SMMs migrated to the air/polymer interface since they have lower surface energy. Consequently, the membrane top-layer becomes hydrophobic while the bottom layer becomes hydrophilic. These membrane were proved to be workable membranes in MD, furthermore, their flux data were much higher than the commercial PTFE membranes.

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3. Heat and mass transfer membrane distillation In MD, the driving force for water vapor migration through the membrane pores is the temperature difference between the feed/ membrane interface temperature (Tmf) and the permeate/membrane interface temperature (Tmp). Due to the heat losses in MD process, the membrane/interface temperatures are different from the bulk temperatures. This could be considered as one of the MD process drawbacks. This temperature difference leads to a decrease from the theoretical driving force, which is defined as the difference between the bulk feed temperature (Tbf) and the bulk permeate temperature (Tbp). This phenomenon is known as temperature polarization. The temperature polarization coefficient (TPC) is defined as the ratio between the actual driving force and the theoretical driving force [22]; as a result the temperature polarization coefficient is expressed mathematically as the following:

TPC ¼

T mf −T mp : T bf −T bp

ð1Þ

It is impossible to measure the membrane/interface temperatures experimentally; usually these temperatures are evaluated by performing a heat balance that relates them to the bulk temperatures [22]. In order to solve this heat balance for membrane interface temperatures, the heat transfer coefficients in the adjoining liquid boundary layers to the membrane should be evaluated. Generally, the boundary layer heat transfer coefficients are evaluated using empirical correlations for the determination of Nusselt number, and a wide variety of these correlations is shown in Table 2 [22]. It is worth mentioning that each shown empirical correlation is valid for certain flow regime and module geometry. In a recent article, Qtaishat et al. [23] solved the heat balance and evaluated experimentally the membrane surface temperatures via applying different empirical correlation that takes into account the temperature variation effect on the physical properties of both feed and permeate solutions. 3.1. Heat transfer The following heat transfer analysis considers the DCMD configuration; however the same analysis could be applied to other MD configuration with some modifications. In DCMD, the heat transfer can be divided into three regions as shown in Fig. 3; that are: (i) heat transfer in the feed boundary layer, Qf; (ii) Combination of both conductive heat transfer through the membrane and heat transferred because of water vapor migration through the membrane pores, Qf; (iii) heat transfer in the thermal permeate boundary layer, Qp.

Table 2 Empirical correlations for evaluating Nusselt number in MD. Empirical correlation [22]

Flow regime

Nu ¼ 1:86ðRePrÞ =3 Nu = 3.66 Nu = 4.36 Nu = 0.097Re0.73Pr0.13 1 Nu ¼ 1:95ðRePrÞ =3 0:64 1 =3 Nu ¼ 0:13Re Pr 1 Nu ¼ 0:023Re0:8 Pr =3 1 Nu ¼ 0:036Re0:8 Pr =
3 0:14 μ Nu ¼ 0:027Re0:8 Prc μ bf

Laminar Laminar Laminar Laminar Laminar Laminar Turbulent Turbulent Turbulent Turbulent

1

mf

Nua ¼

ðf =8ÞRePr  2  1 1:07þ12:7ðf =8Þ =2 Pr =3 −1

Nua ¼

ðf =8ÞðRe−1000  2ÞPr  1 1þ12:7ðf =8Þ =2 Pr =3 −1

Turbulent

a The friction factor, f, in these correlation was estimated by: f = (0.79 ln(Re) − 1.64)− 2.

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Feed out Mf, out Tbf, out

Heat and mass fluxes

Combining Eqs. (2)–(4), the heat flux can be written as follows:

Permeate in Mp, in Tbp, in

0

1−1  1 1 1A
@ Q¼ þ þ T bf −T bp : hf hm þ J w ΔHv hp T −T

Tb,f

mf

Tm,f Permeate boundary layer

Feed boundary Jw layer

As a result, the overall heat transfer coefficient (U) for the DCMD process may be written as: 0

Tm,p

T b,p

Dry pore Feed in Mf, in Tbf, in

Hydrophobic membrane

Permeate out Mp, out Tbp, out

These heat transfer mechanisms can be expressed mathematically as follows: Through the feed solution thermal boundary layer:
 Q f ¼ hf T bf −T mf :

ð2Þ

ð3Þ

Through the permeate solution thermal boundary layer:
 Q p ¼ hp T mp −T bp :

ð4Þ

In the above equations, hf is the feed boundary layer heat transfer coefficient, hp is the permeate boundary layer heat transfer coefficient. Jw is the permeate flux, Tmf and Tmp are the membrane/feed interface temperature and membrane/permeate interface temperature, respectively. ΔHv is the latent heat of vaporization, hm is the heat transfer coefficient of the hydrophobic membrane, which can be calculated from the thermal conductivities of the hydrophobic membrane polymer (km) and air trapped inside the membrane pores (kg).

hm ¼

kg ε þ km ð1−εÞ δ

ð5Þ

where δ and ε are the thickness and porosity of the hydrophobic membrane, respectively. The evaporation efficiency, EE, is defined as the ratio between the heat transferred because of water vapor migration through the membrane pores and the total heat transferred through the membrane [22]. Mathematically, the evaporation efficiency is expressed by Q m;M:T J w Hv
: EE ¼ ¼ Q m;M:T þ Q m;cond J H þ h T −T w v m mf mp

ð9Þ

mp

3.2. Mass transfer


 J w ¼ Bm pmf −pmp

ð6Þ

ð7Þ

ð10Þ

where pmf and pmp are the partial pressures of water at the feed and permeate sides evaluated by using Antoine equation at the temperatures Tmf and Tmp, respectively; such as the following ð11Þ

where P v is the water vapor pressure in Pascal and T is the corresponding temperature in Kelvin. However, the water vapor pressure decreases with increasing the salt concentration in the feed water according to Raoult's law as follows [9]: v

P i ¼ ð1−xi ÞP

v

ð12Þ

where xi is the weight fraction of salt in water. Various types of mechanisms have been proposed for transport of gasses or vapors through porous membranes: Knudsen model, viscous model, ordinary-diffusion model, and/or the combination thereof. The governing quantity that provides a guideline in determining which mechanism is operative under a given experimental condition is the Knudsen number, Kn, defined as the ratio of the mean free path (λ) of the transported molecules to the pore size (diameter, d) of the membrane; i.e. Kn = λ/d. In MD, mass transport across the membrane occurs in three regions depending on the pore size and the mean free path of the transferring species [22]: Knudsen region, continuum region (or ordinary-diffusion region) and transition region (or combined Knudsen/ordinary-diffusion region). If the mean free path of transporting water molecules is large in relation with the membrane pore size (i.e. Kn > 1 or r b 0.5λ, where r is pore radius), the molecule-pore wall collisions are dominant over the molecule–molecule collisions and Knudsen type of flow will be the prevailing mechanism that describes the water vapor migration through the membrane pores. In this case, the net MD membrane permeability can be expressed as follows. K

At steady state, the overall heat transfer flux through the whole DCMD system, Q, is given by Qf ¼ Qm ¼ Qp ¼ Q:

mf

  3841 v : P ¼ exp 23:328− T−45

Through the membrane:
 ¼ hm T mf −T mp þ J w ΔH v :

1−1 1 1 1A @ U¼ þ þ : hf hm þ J w ΔHv hp T −T

In MD process, the mass transport is usually described by assuming a linear relationship between the mass flux (Jw) and the water vapor pressure difference through the membrane distillation coefficient (Bm) [22]:

Fig. 3. Heat and mass transfer in DCMD.

Qm

ð8Þ

mp

Bm ¼

  2 εr 8M 1=2 3 τδ πRT

ð13Þ

Where ε, τ, r, δ are the porosity, pore tortuosity, pore radius and thickness of the hydrophobic membrane, respectively; M is the molecular weight of water, R is the gas constant and T is the absolute temperature. The pore tortuosity is usually in the range of 1–2. However, it cannot be measured experimentally directly. It is possible to evaluate the effective porosity per effective unit length of the

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membrane (ε/δτ) by performing the gas permeation test that is detailed elsewhere [9]. In MD process, air is always entrapped within the membrane pores with pressure values close to the atmospheric pressure. Therefore, if Kn b 0.01 (i.e. r > 50 λ), molecular diffusion is used to describe the mass transport in continuum region caused by the virtually stagnant air trapped within each membrane pore due to the low solubility of air in water. In this case the following relationship can be used for the net DCMD membrane permeability. D

Bm ¼

ε PD M τδ P a RT

ð14Þ

Where Pa is the air pressure (assumed to be 1 atm), P is the total pressure inside the pore assumed constant and equal to the sum of the partial pressures of air and water liquid, and D is the water diffusion coefficient. The value of PD (Pa m 2/s) for water–air can be calculated from the following expression [9,22]. −5 2:072

PD ¼ 1:89510

T

ð15Þ

Finally, in the transition region, 0.01 b Kn b 1 (i.e. 0.5λ b r b 50λ), the molecules of water liquid collide with each other and diffuse among the air molecules. In this case, the mass transport takes place via the combined Knudsen/ordinary-diffusion mechanism and the following equation is used to determine the water liquid permeability [22]. C

Bm ¼

   −1 3 τδ πRT 1=2 τδ P a RT þ 2 εr 8M ε PD M

ð16Þ

4. Solar collecting technologies coupled with membrane distillation

191

determined by their material properties. All cell materials lose efficiency as the operating temperature rises. The high temperature has negative effect on the electrical output of the PV module, especially the dominant crystalline Si based cells, where their conversion efficiency degrades by about 0.4–0.5% per degree rise in temperature [25]. Tan et al. [26] performed high temperature–humidity tests on performance degradation of PV cells. It was found that the degradation is directly related to the passivation integrity, and the inception of moisture causes a significant degradation in the short circuit current and maximum power output. The tracking flat PV system is one of the methods to increase the PV power generation. The increase of solar energy capture due to sun tracking is region by region depending on the local meteorological conditions. Abu-Khader et al. [27] performed an experimental investigation on the effect of using two-axis sun-tracking systems on the electrical generation of a flat photovoltaic system to evaluate its performance under Jordanian climate. It was experimentally found that there was an increase of about 30–45% in the output power for the North–South axes-tracking system compared to the fixed one. PV electricity generation costs currently lies between 0.24 and $0.72/kWh, according to the system type and the solar irradiation. Such costs are expected to descend to the $0.13–0.31/kWh range [29]. Power conditioning equipment (e.g. charge controller, inverters) and energy storage batteries may be required to supply energy to a desalination plant. Charge controllers are used for the protection of the battery from overcharging. Inverters are used to convert the direct current from the photovoltaic module system to alternating current. The electricity produced can be used to power pumps for desalination, mostly for membrane technologies. The photovoltaic technology connected to a reverse osmosis (RO) system is commercial nowadays. However, the high cost of PV cells is still one of the major challenges facing the widespread use of this technology. 4.2. Solar thermal

Solar collectors can be used to provide the heat (Solar Thermal) or electrical energy (Solar Photovoltaic) requirements to operate a membrane distillation system. The main solar technologies that could be coupled with membrane distillation are briefly reviewed below. 4.1. Solar photovoltaic Photovoltaic (PV) cells are key components of PV applications that convert solar energy into electricity through the transfer of electrons. PV can be thought as a direct current (DC) generator powered by the sun. At present, there are three generations of PV cells: crystalline silicon (c-Si) technologies (1st generation), amorphous silicon thin-film (TF) technologies (2nd generation) and Nano-PV technologies (3rd generation). Crystalline silicon are mature and reliable technologies currently dominating the PV market (about 82% of global cell production in 2009) [23]. The conversion efficiency of c-Si lies between 15% and 18% [24]. The TF technologies are currently the main alternative to c-Si (17% market share in 2009) [23]. In addition, thin film (TF) PV technologies are presently the lowest-cost to manufacture. The production cost of cadmium telluride (CdTe) thin film module is currently the least; $0.76/Wp [23]. However, scarcity of key component materials has been highlighted as a potential barrier to both large scale deployment and reductions in TF technology cost. In particular, major concerns have been raised for indium and tellurium availability and potential risks for the TF PV technologies that utilize them, i.e. cadmium telluride (CdTe) and copper indium gallium (di) selenide (CIGS) [28]. The photovoltaic cell photo current is directly proportional to the solar intensity. The performance of the solar cell depends on the cell temperature. Solar cells work best at low temperatures, as

Solar collectors are well-known devices which are usually used to absorb and transfer solar energy into a collection fluid. The thermal energy can be achieved in solar stills, collectors, or solar ponds. Solar collectors are usually classified according to the temperature level reached by the thermal fluid in the collectors (Table 3) [29]. Low temperature collectors are those operating in the range below 80 °C while medium temperature collectors are those operating in the range from 80 to 250 °C. Low temperature collectors provide low-grade heat that is not useful to serve as a heat source for conventional desalination distillation processes but is of interest for membrane distillation process. Medium temperature collectors can be used to provide heat for thermal desalination processes by indirect heating with a heat exchanger. Evacuated tube collectors produce temperatures of up to 200 °C and thus can be used as an energy source for thermal desalination processes [30]. High temperature collectors such as parabolic troughs or dishes or central receiver systems can concentrate the incoming solar radiation onto a focal point, from which a receiver collects the energy using a heat transfer fluid. The high thermal energy content can be used directly in thermal desalination processes or can be used to generate electricity using a steam turbine. Sun tracking can improve the collector efficiency. Large-scale desalination applications require large collector areas. A solar pond is a body of liquid which collects solar energy by absorbing direct and diffuse sunlight and stores it as a heat. Salt gradient solar ponds (SGSP) rely on a salt solution (the salts most commonly used are NaCl and MgCl2) of increasing concentration with depth to suppress natural convection. Warm concentrated brine at the bottom of the pond is prevented from rising to the surface and losing its heat because the upper portion of the pond contains less salt and is,

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Table 3 Solar energy collectors [29]. Collector type

Concentration ratio

Solar pond Flat plate (FPC) Improved flat plate (IFPC) Evacuated tube (ETC) Compound Parabolic Collectors (CPC) Parabolic trough (PTC) Linear Fresnel (LFC) Parabolic dish reflector (PDR) Central receiver

1 1 1 1 1–5 15–40 15–40 100–1000 100–1500

Typical temperature range (°C) 50–100 30–80 80–120 50–190 70–240 70–400 70–290 70–930 130–2700

Tracking No No No No No Single axis Single axis Two axes Two axes

Palenzuela et al. [32] considered the combination of desalination technology into concentrating solar power (CSP) plants for the planned installation of CSP plants in arid regions. The authors presented a thermodynamic evaluation of different configurations for coupling parabolic-trough (PT) solar power plants and desalination facilities in Abu Dhabi as a case for dry locations in the Middle East and North Africa (MENA) region. Since solar insolation is intermittent, a thermal energy storage system should be incorporated to run the desalination process round the clock. One of the solutions to utilize fluctuating solar energy on a continuous basis is to incorporate thermal energy storage (TES) system. Three types of TES systems are in commercial use; (1) sensible heat storage, (2) latent heat storage, and (3) thermo chemical storage systems. The most widely used TES is the sensible heat storage system [33]. 4.3. Performance parameters in SPMD

therefore, less dense than the lower portion. Whereas the top temperature is close to ambient, a temperature of 90 °C can be reached at the bottom of the pond where the salt concentration is highest. A typical profile of density and temperature within a solar pond is shown in Fig. 4. Heat is extracted by passing the brine from the storage zone through an external heat exchanger. This heat can be used in a special organic-fluid turbine to generate electricity, provide energy for desalination, and to supply energy for space heating in buildings. Solar ponds have large storage capacity allowing seasonal as well as diurnal thermal energy storage. The annual collection efficiency of useful heat for desalination is around10–15%. Larger ponds tend to be more efficient than smaller ones due to losses at the pond edge. Solar ponds are particularly suitable for desalination plants as waste brine from desalination can be used as the salt source for the solar pond density gradient. Using desalination brine for solar ponds not only provides a preferable alternative to environmental disposal, but also a convenient and inexpensive source of solar pond salinity. Gracia-Roderiquez (2002) [21] reported that solar pond-powered desalination is one of the most cost-effective methods. Many projects are currently under preparation to make possible large concentrating solar power (CSP) plant developments in arid regions, such as the Shams 1 solar power station initiative. The Shams 1 CSP will feature 768 parabolic trough collectors over 6,300,000 ft 2 of land. Shams 1's parabolic trough collectors collect sunlight and convert it into thermal energy. The Shams solar power station is being built in the city of Madinat Zayed, located 120 km south west of Abu Dhabi, in the United Arab Emirates (UAE). Construction of phase 1 of the solar project, Shams 1, commenced in July 2010 and is expected to be completed by 2012. Upon completion, Shams 1 will be the first solar farm in the Middle East and the largest concentrated solar power (CSP) plant in the world. The project is estimated to cost $600 m [31].

Fig. 4. Typical salt gradient solar pond.

The gained output ratio (GOR) and the thermal recovery ratio (TRR) of the system are the most important performance parameters used in thermal desalination processes as well as in solar powered membrane distillation processes. The GOR is the ratio of thermal energy required to produce distillate water to the actual thermal energy consumed in the feed side. Mathematically, the GOR is calculated from:

GOR ¼

md ΔH v mh CpðT hi −T ho Þ

ð17Þ

where md is the distillate flow rate (kg/h), λ the latent heat of vaporization (J/kg), mh the feed flow rate (kg/h), Cp the feed specific heat (J/kg K), Thi, Tho the feed temperatures (in K) at the module inlet and outlet. The TRR is the theoretical energy needed for distillate produced divided by the total thermal energy input. In the SPMD, the total thermal energy input is the solar energy incident on the solar collector. As such, the TRR can be defined as:

TRR ¼

md ΔHv AI

ð18Þ

where A is the solar collector area (m 2), and I is the global irradiation (W/m 2). The TRR of a SPMD plant is measure of its efficiency to produce distillate. 5. Coupling membrane distillation with solar energy collectors Coupling membrane distillation modules with solar energy collectors has been of interest for many researchers over the world because MD can tolerate fluctuating and intermittent operating conditions as well as it requires low grade thermal energy. Two alternative configurations of coupling solar energy with MD are illustrated in Fig. 5. The solar-assisted MD desalination system (Fig. 5a) comprises solar thermal collectors which feeds hot water to the MD module. The heat is supplied to the MD module either directly or through a heat exchanger. Electricity needed is either supplied from the electric grid or from an auxiliary diesel generator to drive all pumps and other electrically powered devices. The solar stand-alone MD desalination system (Fig. 5b) is similar to the solar-assisted MD desalination system in all aspects except that solar powered PV collectors integrated with direct current (DC) battery cells and electric current inverters are used instead of the diesel generator to supply the necessary electricity. Membrane distillation modules were coupled with flat plate collectors, vacuum collectors, solar ponds, solar stills, and parabolic troughs as detailed below.

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5.1. Coupling with vacuum or flat plate collectors The first publication in this field came from Australia where Hogan et al. [34] from the University of New South Wales described a 0.05 m 3/day system using a 3 m 2 flat plate solar collectors. Hollow fiber membrane distillation module with heat recovery was used in their study. The authors reported that the thermal and electrical energy consumption was 55.6 kWh/m3. The calculated flux of 17 liters per day per square meter of collector area was comparable to that reported for solar MSF and ME plants. As reported by Thomas [35] a solar-powered membrane distillation system was installed by the Water Re-use Promotion Center in Tokyo, Japan, in 1994. Flat plate module and a 12 m 2 field of vacuum tube collectors were used. Automatic controls start up the desalination system whenever sufficient sunlight is present to provide hot water and electricity for pumping from the solar collectors and PV panels. The plant had a maximum productivity of 40 liters per hour. Four autonomous solar-powered membrane distillation plants were developed through SMADES EU-funded project [36]. First a so-called “compact” system was designed and tested to generate process parameters for the design of the so-called “large” system. Three compact systems were installed in Jordan, Morocco, and Egypt. The compact system is simple one loop desalination designed to produce about 100 l of distilled water per day. As such, no thermal heat storage tanks, no electrical storage (battery), no complex but a simple and reliable control was needed. The main components of the system are: two flat-plate solar collectors with a corrosion-free absorber that can directly be used to heat up the salty water, one spiral wound membrane distillation module with heat recovery, feed pumps, PV module with a DC/AC converter, and feed and distillate storage tanks (Fig. 6). One of the “compact” units was installed in the city of Irbid in northern Jordan in August 2005 and fed with brackish water [37,38]. The key design data of the compact system

193

are listed in Table 4. The distillate flow rate was about 120 liters per day during the summer months, and about 50 liters per day during the cloudy winter days. The distillate conductivity was less than 600 μS/cm. The large system was installed in the city of Aqaba on the Red Sea coast and fed with untreated seawater in February 2006 [37,38]. The system consists of two loops. The desalination loop is operated with seawater and is separated from the collector loop (operated with tap water) by a titanium corrosion resistant heat exchanger. This arrangement allows for the use of economic standard components in the solar collector without the need of cost-intensive corrosion resistant materials. Four spiral wound membrane distillation modules exactly the same as those used in the compact systems were operated in parallel. A schematic of the setup is shown in Fig. 7. The design capacity of the Aqaba system was 1 m 3/day. The key design data of this system are listed in Table 5. A DC/AC converter was used to convert 24 VDC delivered from the batteries into 230 V AC. The capacity of the battery storage was 300 Ah. A thermal heat storage vessel was used to store the surplus energy in order to be used whenever sufficient solar radiation is not available. Due to natural fluctuations of solar radiation and temperature, the water production rate and energy requirements fluctuated between 600 and 800 liter per day and 200 and 250 kWh/m 3, respectively. During the first month of operation (February 2006), the quality of produced distillate was very good with a conductivity of less than 10 μS/cm. In March 2006, an increase in the distillate conductivity was noticed. After a thorough evaluation, it was decided to remove the deteriorated module and to operate the system with three membrane modules instead of four. The flux obtained varied between 2 and 11 liters per day per meter squared of collector area. Experimental results from the large system showed a gradual decline of the permeate flux and quality during the first five months of operation. Heating of seawater to temperatures up to 80 °C caused scale deposit on the membrane surface. Cleaning the membrane with dilute formic acid resulted in the dissolution of the deposit on the membrane surface, and the initial membrane permeability was restored [39]. Nevertheless, the information related to the membrane durability in membrane distillation (MD) is still immature. It is documented that the membrane wetting and the scale deposition on the membrane surface are the most serious problems that make the membrane unworkable in MD [4]. However, there are many membrane designers considered designing the membranes to avoid or minimize those drawback effects. Their results were very promising [13–21]. Wang et al. [40] has recently described the performance of a solar-heated hollow fiber vacuum membrane distillation (VMD) system for potable water production from underground water. The

Solar irradiation

Solar collector

Feed tank

MD module

Feed pump PV PV module Fig. 5. Solar-assisted (a) and stand-alone (b) desalination systems.

Over flow

Background container

Distillate Refilling pump

Fig. 6. Schematic drawing of the compact system (one loop desalination system).

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Table 5 Specifications of the large unit.

Plant capacity (Average) Membrane area Solar collectors area PV-module

Compact system

Design capacity (m3/day)

1

100 l/day 10 m2 5.73 m2 106 Wp

Collector area (m2) Collector type Heat storage capacity (m3) Number of membrane modules PV (kWp) PV area (m2)

72 Flat plate 3 4 1.44 14

system has four major components, a solar energy collector, a hollow fiber membrane module, a permeation condenser and two mechanical pumps (Fig. 8). The area of the solar energy collector is 8 m 2 and the membrane total area is about 0.09 m 2. The membrane is 0.1 μm hollow fiber membrane made from polypropylene, with the inner diameter 371 mm, wall thickness 35 mm, and fiber operative length 0.14 m. The experiment results showed that the pure water flux of the system could reach 32.2 kg per hour per square meter membrane area. The performance of a desalination plant based on coupling an airgap membrane distillation module with a solar pond was tested by Walton et al. [41]. Low grade thermal energy (between 13 and 75 °C) was extracted from the pond and supplied via a heat exchanger to the membrane module. The membrane area was 2.94 m 2. The Swedish firm SCARAB (http://www.hvr.se) has built and supplied the membrane distillation module in addition to the controlling pumps and heaters. As shown in Fig. 9, hot brine was pumped from the bottom of the solar pond and circulated through a heat exchanger to supply heat to the saline solution. Cold water from the solar pond surface was passed through another heat exchanger to provide cooling. High and low temperatures for system operation were obtained by changing the flow rates for solar pond hot and cold water. The research included measuring the flux per unit area of membrane surface and conductivity of permeate over a range of feed water salinities and temperature as well as an assessment of membrane fouling. The permeate flux was fluctuating and reached a maximum of 6 L/m 2.h. Theoretical calculations, based upon measured results, indicate that membrane distillation with latent heat recovery is necessary to make the process being competitive with other thermal technologies in terms of energy use. Walton et al. (2004) [41] reported that membrane distillation is only competitive relative to reverse osmosis when low cost heat energy is available and/or when the water

chemistry of the source water is too difficult for treatment with reverse osmosis. Suareza et al. [42] developed a heat and mass transport model to evaluate the feasibility of coupling a DCMD module with an SGSP for sustainable freshwater production in an environment such as that at Walker Lake. They reported that the coupled DCMD/SGSP system is capable of providing freshwater for terminal lakes reclamation. The coupled system shown in Fig. 10 was found to produce water flows on the order of 1.6 × 10 − 3 m 3 per day per m 2 of SGSP with membrane areas ranging from 1.0 to 1.3 × 10 − 3 m 2 per m 2 of SGSP. Mericqa et al. [43] has studied the simulation of coupling VMD with solar energy to produce distillate from seawater. For this purpose solar collectors (SC) as well as salt gradient solar ponds (SGSP) were considered. Simulation results showed that VMD/SGSP could induce marked concentration and temperature polarization phenomena that reduced fluxes because of the difficulty to create turbulence in the feed seawater when SGSP are used. Using the combination of VMD/SC was more practical, as they concluded. 5.2. Coupling with parabolic trough collectors Within the frame of MEDESOL (Seawater Desalination by Innovative Solar-Powered Membrane Distillation System) project the technical feasibility of producing fresh water from seawater by integrating several MD modules (a multi-stage MD system) for a capacity range 0.5–50 m 3/d will be evaluated. The heat source of the process will be from an advanced compound parabolic solar concentrator, especially developed to achieve the specific needed range of temperatures. The seawater heater will include the development of an advanced non-fouling surface coating, as reported by Gálvez et al. (2009) [44].

Solar irradiation

Brine

Collector feild Feed pump

Storage tank Heat exchanger Distillate

MD modules

Battery

Expansion vessel Control unit

DC AC

PV

PV PV array

Fig. 7. Schematic drawing of the large system (two loop desalination system).

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Fig. 8. Flow sheet of the solar-heated MD system for producing potable water.

5.3. Coupling with solar stills Banat et al. [45] described a solar still-membrane distillation integrated system operated with artificial seawater. Hot water from the still was circulated into a tubular membrane distillation module before being returned back to the still. As such, distilled water was produced from both the solar still and the membrane distillation module. The flux of the MD module was four times higher than the flux obtained from the solar still. 6. Availability and cost Solar energy can be harnessed for MD desalination by producing the thermal energy required to drive the evaporation and by producing the electricity required to drive the pumps. The main energy requirement for membrane distillation is thermal energy. Electricity demand is low and is used for auxiliary services such as pumps, sensors, controllers etc… However, the high cost of PV modules and to less extent the high cost of solar collectors hinders the use of solar energy on wide scale. Capital costs of MD modules and corrosion resistant heat exchangers are important also. At present, no commercial MD modules are available and researchers either use modules

designed for other membrane separations or design and build their specific modules. Therefore, it is difficult to conclude if the SPMD process is really competitive with other solar driven conventional desalination processes. Very few studies on the cost of solar powered MD desalination plants have been reported in literature. Kullab and Martin [46] have presented the cost for a scaled-up solar powered air gap membrane distillation. Evacuated tube solar collectors were used to supply the thermal energy. For a yearly production of 24,000 m 3 of pure water, the cost of water production was estimated at 8.9 $/m 3. Around 70% of this cost was associated with the solar collectors. Banat and Jwaied [47] estimated the cost of potable water produced by the stand-alone compact unit to be 15 $/m 3 and 18 $/m 3 for water produced by the large unit. The authors pointed out that membrane lifetime and plant lifetime are key factors in determining the water production cost. The cost decreases with increasing the membrane and/or plant lifetime. Integrating solar power and membrane distillation desalination plants is not yet a straightforward issue and many technological aspects remain to be discussed. Large seawater SPMD desalination plants need, obviously, facilities to be located near the sea, where land cost and availability could be a significant problem. Furthermore,

Fig. 9. Flow schematic of SPMD [41].

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Fig. 10. Coupling of the DCMD module to the SGSP [42].

the solar direct normal irradiance (DNI) is normally lower on areas close to the sea, which makes concentrating solar power (CSP) plants most optimal locations to be separated from the coast. Other thermal desalination technologies such as MED or MSF could also be coupled with membrane distillation to minimize the production cost. To answer all of these issues, techno-economic analysis is needed to define the best schemes of the integration of a membrane distillation with solar energy. 7. Summary Several small and lab scale plants for MD desalination using solar energy have recently been tested. The process is deemed suitable to operate in conjunction with solar energy for small capacities. The main cost is in the initial investment. However, once the system is operational, it is extremely inexpensive to maintain and the energy has minimal or even no cost. The availability and cost of MD modules is still a serious and important issue. People not only in remote regions but also in urban areas will benefit if low cost stand-alone MD systems are developed commercially. Nomenclature Symbols A Solar collector area (m 2) Bm net DCMD permeability (s/m) d mean pore size (nm) D water diffusion coefficient (m 2 s − 1) EE evaporation efficiency f the friction factor GOR The gained output ratio h heat transfer coefficient (W m − 2 K − 1) H Enthalpy (J/kg) I Global irradiation (W/m 2) Jw DCMD flux (m/s) k thermal conductivity (w m − 1 K − 1) Kn Knudsen number Nu Nusselt number M molecular weight of water (kg mol − 1) md Distillate flow rate (kg/h) mh Feed flow rate (kg/h) p liquid pressure (Pa) Pv vapor pressure of water (Pa) P total pressure (Pa) Pa air pressure (Pa) Pr Prandtl number Q heat flux (W m − 2) T absolute temperature (K) TPC Temperature polarization coefficient TRR The thermal recovery ratio r mean pore radius (nm) R gas constant (J mol − 1 K − 1)

Re xi

Reynolds number solute mole fraction

Greek letters δ total membrane thickness (μm) ε porosity (%) ρ Density (kg/m 3) λ mean free path (nm) μ water dynamic viscosity (kg m − 1 s − 1) τ tortousity ΔHv latent heat of vaporization (kJ/mol)

Superscripts K Knudsen D molecular-diffusion C combined Knudsen/ordinary-diffusion s aqueous NaCl solution

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