Solar vacuum membrane distillation for seawater desalination

Slimane GABSI Sfax University, URECAP

Higher Institute of biotechnology Sfax, Tunisia Route Soukra BP 1175, 3038 Sfax,

slimane.gabsi@isetsfrnu.tn

Abstract-The main objective is to improve the overall

performance of membrane-based water desalination processes by

applying innovative technologies to reduce energy consumption

by using renewable energy and save energy of condensation.

The coupled collector to this installation is a flat plate collector.

To simulate solar radiation we have choose a EUFRAT model.

The model allows to obtain the out water temperature along the

day.

Therefore, we developed a mathematical model describing the

functioning of hollow fiber module using the VMD principle.

This model enables us to determine the instantaneous variation of

the distillate flow as well as the daily productivity.

We design and develop an autonomous solar desalination system

(PV - thermal collector) for a capacity about 0.5 m3/day.

The pilot plant was installed in the village of orphaned children

(8.0.8 Mahares TUNI8IE).

Experimental tests were carried to determine the permeate water

production.

Keywords- Solar energy, desalination, vacuum membrane distillation, pilot plant, performance.

I. INTRODUCTION

Tunisia is located on the southern rim of the Mediterranean basin. Like its neighbour countries, it is confronted by a problem of fresh water shortage. In fact, it has very limited water resources and has a high degree of salinity, aggravated by a large spatial and temporal disparity between southern and northern parts, and fluctuations from year to another. Tunisia has mobilized a large proportion of its water resources (surface and underground waters) using dams, transport aqueducts, small lakes, deep wells and desalination plants. For big cities and large-scale agglomeration, the problem of fresh water is currently resolved. However, seawater desalination could supply future water deficiencies. Since the 80s, Tunisia has resorted to brackish water desalination via membrane techniques such as reverse osmosis (RO). The contribution of membrane processes to the production of potable water has become a reality, e.g. the supply of Gabes and Zarzis towns], Kerkennah and Djerba Isles, as well as some tourist unities and oil companies. The quantity of drinking water produced by membrane techniques represents 55 000 m3/d. It represents l.5% of the total volume distributed by the SONEDE (National Society of Water Exploitation and Distribution). The region is characterized by an immense wealth of solar energy which is a very significant means of technologies development such as membrane distillation.

Ahmed CHEHBOUNI Cadi Ayyad University, Faculty of Sciences

Semlalia Marrakech, Morocco

Bd. Prince My Abdellah, B.P. 2390, 40000 Marrakech chehbouni@ucam.ac.ma

The desalination of brackish water or the seawater by the solar energy coupled with the membrane technique is regarded as an alternative very interesting and effective for the production of drinking water in particular in the rural and arid areas being given the availability of this source of inexhaustible and free energy in the majority of the regions. Among the techniques of membrane distillation we are interested in this study to vacuum membrane distillation (VMD). The VMD operated with a low temperature and pressure which gives the possibility to use solar energy.

II. SOLAR ENERGY IN TUNISIA

Tunisia, as well as most of the other north Africans countries, enjoys an abundance of solar radiation, the average of solar radiations exceeding 6 kWhlm3/day for the months May, June, July, August and September with a total insulation period of 3500 hlyear and 350 sunny days per year. When the clean skies, the direct solar radiation component is 80 - 95% of the total global solar radiation .

A. Modeling and validation of the meteorological model

1) Modeling of solar radiation Weather data are a function of site and weather data reconstitution models. The data weather are measured directly or provided by weather stations. For dimensioning, calculation and the estimate we use the weather data. A big number of models are provided in the literature. We cited for example the model of LIU and JORDAN which established a relation between the index of nebulosity and the index of clearness. The EUFRA T model is based on the synthesis of various work, in particular those of BRICHAMBAUT.

2) Computer code for solar energy We have designed a computer code for solar energy use and

developed an interface which represents a tool of assistance to choice the type of adequate collector. This interface is formed by two parts:

• The first relates to the simulation of solar radiation (direct, diffuse and total) and the ambient temperature and • The second the exit instantaneous temperature of each solar collector (flat plate, cylindro-parabolic, vacuum).

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

The program will calculate the sunrise and the sunset in true solar time, the various solar radiations: total, diffuse and direct receipts by a horizontal flat plate and a tilted plan of slope and orientation. for three models (Eufrat, Perrin de Brichambaut and Liu and Jordan), and the ambient temperature at any time of day. The comparison of the results of simulations obtained by the EUFRA T Model and the experimental values (taken by means of a weather station installed in ENIG) shows a satisfactory agreement (figure 1).

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6 10 16 12 14

time (h) 18

Figure 1. Example of comparison with the experimental values December 21 slope = 45°

The selected model is that of EUFRA T because it takes account of the orientation of the solar collector.

III. MODELING OF THE OUTLET TEMPERATURE OF SOLAR

THERMAL

The modeling of temperature is based on energy balances on the fluid flowing through the tube of the absorber, the absorber and a heat exchange between the absorber and the fluid between the absorber and glass, and between glazing and the environment.

It leads to differential equations of three temperatures: T F

(fluid), TA (absorber) and Tv (glass). These equations variant versus time t and the length z of the absorber. &; " ,,,t�

= ----=:.L " d, (T __ T.J .J.. 1."], (f( t) - T.J

, ;I t 4';; ::tI;; • .:;. r:!i -, � :., ;,- ,:) :' I .:.: . r :l,i - ::! ; :!� J;, . :) �

A. Choose of solar collector

Before calculation, the user must choose the collector type (flat, parabolic, vacuum).

In the case of a collector, you must choose the glazing type (single, double, etc . ... ), selectivity (selective or non selective wall) and type of material of the absorber plate (aluminium, copper, zinc, steel, silver, ... ).

From many types of solar collectors developed, three types merit further consideration: the parabolic-trough collector (pTC), the compound parabolic collector (CPC) and the flat­plate collector (FPC). The fust one is a tracking collector, whereas the last two are stationary. PTCs are generally of medium concentration ratio (15-40) whereas CPCs are generally of low concentration ratios (1.5-5). The low concentration-ratios of the CPC allow them to work without a need for tracking of the sun.

For calculations of the temperature output of any collector, it must bring the following:

• Wind velocity (m / s). • The collector characteristics: length of tube (m), the number of tubes, the outer and inner diameter (m). • The collecting area (m2). • The coolant flow (kg.m2/s)

The program will display the temperature of the coolant, absorber and glass. For a best performance and a temperature of 75°C the choice is made on the collector anti reflex ion and double glass. The collector efficiency can be written as:

17=0.76 - 2. 66 L1T

-0.009 L1T2

G G L1 T is the difference temperature between the temperature of the coolant and the ambient temperature, G is solar irradiation.

IV - VACUUM MEMBRANE DTSTTLLA TTON

Membrane distillation (MD) is a relatively new process being investigated worldwide as a conventional separation process [1-7], such as distillation and reverse osmosis. Vacuum membrane distillation consists in applying a vacuum on the permeate side of a hydrophobic microporous membrane. The water will be vaporized close to the pores and will then pass as a vapor through the membrane pores. It will then be condensed outside the module. The driving force for the process is linked to both the partial pressure gradient and the thermal gradient between the two membrane sides. VMD can be characterized by the following steps: vaporization of the more volatile compounds at the liquid/vapor interface and diffusion of the vapor through the membrane pores according to a Knudsen mechanism [1]. Permeate condensation takes place outside the module, inside a condenser or a trap containing liquid nitrogen [7].

The MD technique holds important advantages with regard to the implementation of solar driven stand-alone operating desalination systems. The most important advantages are:

• The operating temperature of the MD process is in the range of 60 to 80°C. This is a temperature level at which thermal solar collectors perform well.

• Chemical feed water pre-treatment is not necessary. • Intermittent operation of the module is possible.

Contrary to RO, there is no danger of membrane damage if the membrane falls dry.

• System efficiency and high product water quality are almost independent from the salinity of the feed water.

The choice of the membrane type system depends on a parameters nwnber, such as the costs, the fouling of the membranes, the cleaning frequency, ...

The permeate flow of the membrane is expressed by:

With: km = 4.37 10-7 slm the permeability of the membrane and P v the vacuum pressure. The saturated vapor pressure is:

3816.44 P =exp(23.1964- ) s T -46.13

V- COUPLING OF SOLAR ENERGY WITH MEMBRANE

The use of solar energy can be: * Provided by the heating of sea water by a coolant from the solar collector. The distillation module is separated from the solar system (system not integrated), *The module is immersed in the solar collector, *The module is integrated in the solar collector (integrated system).

In the configuration of the collector membrane separate several possibilities are conceivable:

• Sea water flows through the collector and the membrane. This requires a noble material for the collector (e.g titaniwn). • Either a heat exchanger between the two.

For the type of solar collector can use a flat plate collectors, a CPC collectors, a solar pond, .... The choice is based on the level of desired temperature, collector efficiency, utilizing, cost, ...

VI -EXPERIMENTAL STUDY

Figure 2 shows the Photo of the pilot plant Sioar VMD. The principal components of the vacuwn solar membrane desalination plant are:

• a membrane module (UMP 3247 R) with 806 fibers in PVDF with an internal diameter of 1,4 m, the length of the module is 1,129 m and offering a total membrane surface of 4 m2.

• a field of thermal solar collectors comprising 7 lines of 5 collectors in series.

• a field of photo voltaic solar cells. This field is !composed with 16 modules of cells LC 120 WC and 2 regulators voltage continuous 48 VDC 40A. An assembly of 8 solar tubular accwnulators 12V/230 AH (4 lines of 2 accwnulators) ensures the accumulation of the electric power produced. This installation is carried out of a power inverter 220/2 kW.

Figure 2. Photo of the pilot plant solar VMD

• Pumps for feed water and circulating heat water, • a peristaltic pwnp, which can ensure a vacuum of 5000

Pa with 100 IIh flow, • a heat exchanger with titanium plates of 26 kW power,

exchanger area 1,08 m2 and 27 plates, • a tubular condenser in titanium 60 kW power with 41

tubes, 7 mm internal diameter and 1 mm thickness, • instrumentations of control and regulation, • a tank of fresh water (1 m3) • a mixing tank which makes it possible to mix it retentat

outgoing membrane module and the supplement out of seawater.

• an electric resistance in the mixing tank thus allowing to heat its when the electric charge accwnulated in the batteries exceeds the electric needs necessary for the pumps.

A. Experiences

The experimental study involves measuring the following parameters:

• The global solar radiation using a pyranometer. • The coolant and seawater flow using flow-meters. • The different temperatures at the condenser and heat

exchanger using a temperature sensor Pt100.

B. Determination of production of desalinated water

We present the results collected March 17. This day is characterized by clear skies throughout the day with a maximwn ambient temperature of 28.5 0 C.

Figure 3 shows the variations of a solar radiation. The evolution of the distillate flow versus time is given in figure 4. The curve increases gradually at the beginning of the day and reached a maximum between 12 and 13h, and then it decreases gradually. This result is due to the incident solar energy is the most parameter affecting the production of a solar desalination unit. The daily production is about 210 kg.

1200 .... ...

... 8 ... ... ...

- J.ooo -

=:: ... ... ... ...

= 800

.:= ... - ... ..: 600

... ;e ...

..: "00 '"'" ... ... '"'" ..: 200 '0

IZl In.me 0

8 10 12 14 16 18

Figure 3: Vanatlon of solar flux functIon of tIme

25 ��-----------------------------.-----.----c

o i st i II ate, fl ow (If h) X-.,., __ -+-' __ .,-i, --c------c----c-____+_ 20 I I x l I X �I I X't'l x ! X 15 +-�----����w,�--��--�����w,��

X X X x l 10 +--r�-x-ir��'-I-r-.--r-+--r-+--r-+--r-.--�

5 i-�--���--�-+--+-�--+-�--���--���� Time

Figure 4. Permeate water production versus time on a shiny day- 1 7 March -Pressure = 7000 Pa; Feed flow rate = 21 30 kg/h, Re <670

VII. CONCLUSION

Design calculations of system were performed with Matlab, taking into consideration mass and heat transfer equations in the membrane module, and considering heat power due to the solar collector, along the year. The main objective is to improve the overall performance of membrane-based water desalination processes by applying innovative technologies to reduce energy consumption by using renewable energy and save energy of condensation. We design and develop an autonomous solar desalination system (PV - thermal collector). The pilot plant was installed in the village of orphaned children (S.O.S MAHARES). Experimental tests were carried to determine the permeate water production. While the current productivity is lower than desired, but several improvements can be conducted (increasing seawater flow rate, thermal insulation of the membrane and the tank ... ). The objective is essentially to determine the optimal operating conditions and to carry an energetic study of the pilot plant. The prospects are essentially looking for the optimal operating conditions to study the technical energy unit. While the current productivity is lower than desired, but several improvements can be conducted (increasing seawater flow rate, thermal insulation of the membrane and the tank ... ). The objective is essentially to determine the optimal operating conditions and to carry an energetic study of the pilot plant.

ACKNOWLEDGMENT

The authors address the most sincere thanks to the European Commission for its financial support within the framework for the 6th PCRD for the project Membrane-Based Desalination: An Integrated Approach MEDINA Project No: 036997.

REFERENCES

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[4] D. Wirth, C. Cabassud, Water desalination using membrane distillation: comparison between inside/out and outside/ in permeation, Desalination, 1 47, 1 39-1 45 (2002).

[5] A. M Alklaibi, Lior N., Membrane-distillation desalination: status and potential, Desalination, 1 71 , 1 1 1 -1 31 (2004).

[6] H. M. Qiblawey, Fawzi Banat, Solar thermal desalination technologies, Desalination 220, 633-644 (2008)

[7] 1. P. Mericq, S. Laborie, C. Cabassud, Vacuum membrane distillation of seawater reverse osmosis, Water Research, Volume 44, Issue 1 8, 5260-5273 October (2010).