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Small scale desalination pilots powered by renewable energy sources: case studies
Desalination 183 (2005) 151–165
Small scale desalination pilots powered by renewable energy sources: case studies S. Bouguechaa, B. Hamrounib, M. Dhahbia* a
b
LETM/INRST, BP 95 Hammam-Lif 2050 Tunisia LETM/FST, BP 1020 Le belve´de`re, Tunis, Tunisia emails: [email protected], [email protected], [email protected] Received 21 February 2005; accepted 21 March 2005
Abstract In Tunisia, water resources are very limited and have high salinities. Domestic water in rural communities has TDS reaching 2750 ppm. Inhabitants of these areas are scattered and lack proper infrastructure for the implementation of conventional desalination plants. However, these regions have an important potential of renewable energy (RE), especially solar energy where sunshine in southern regions can reach 2500 h per year. RE can be harnessed to power small-scale desalination plants in order to produce the fresh water necessary to cover the basic requirements (drinking and cooking). This paper aims to identify the potential of renewable energies, mainly solar and geothermal. It illustrates the quality of distributed water in rural communities. It presents several studies that were carried out, including solar multiple effects distillation, reverse osmosis driven by photovoltaic panels, and lately membrane distillation using a geothermal resource. The experimental results show technical promise. However, more investigations will need to optimise the operating parameters and improve economic feasibility. Keywords: Desalination; Water resources; Renewable energy; Reverse osmosis; Solar still; Membrane distillation; Tunisia
1. Introduction Tunisia is located on the southern rim of the Mediterranean basin. Like its neighbour countries, it is confronted by a problem of *Corresponding author.
fresh water shortage. In fact, it has very limited water resources, aggravated by a large spatial and temporal disparity between southern and northern parts, and fluctuations from year to year. In the central and Cap Bon areas, large extractions led to a sharp decline in water levels, causing seawater intrusion.
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.03.032
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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. In remote rural communities, the problem of potable water remains unsolved. There are about 1.5 106 inhabitants and the distributed water volume reaches 19 106 m3 per year, with an average specific consumption estimated at 45 l per capita per day, which is considerably lower than the national average (130 l per inhabitant per day). In addition, 8 106 m3 of the distributed water has a high salinity, making it unfit for human consumption. Analysis of the RE map shows the important potential sites where solar energy and geothermal energies can provide a solution to cover the energy needed for desalination. Thus, desalination powered by renewable energy presents a solution to the problem of fresh water shortage in rural dispersed communities, improving inhabitants’ living conditions and having a positive impact on the environment. The first test imitating the natural cycle of evaporation-condensation was attempted by N. Chezi in 1742. In 1872, Charles Wilson, a Swedish engineer built the first solar distillation still in Las Salinas, Chile. For more than a century, numerous investigations have been carried out to improve the overall performance of solar stills. The performance of solar stills remains lower than expected. These performances depend mainly on design and technical imperfections [1–5]. Therefore, fresh water production by solar stills did not exceed 4 kg/m2d. With the aim of overcoming these disadvantages, various authors [6–8] have proposed a wick multiple effect solar still; Cooper and Apple yard suggested a multiple solar distillation still, where the latent heat of steam would
be used to evaporate another quantity of salt water. Nevertheless, the thickness of texture and the formation of an air gap between the plate and texture substantially limited the performance. The Le Goff team [9–11] has developed the DIFICAP distillatory. This device, using a thin texture resolves the problems due to the thickness of the texture. In addition, the contact between the plate and the texture is ensured by interfacial tension, which is much greater than gravitational force Rodriguez [12] presented a systematic approach to desalination powered by RE in order to consider all the alternatives. Among all the combinations investigated, they concluded that RO powered with PV is interesting in very specific cases, such remote sunny sites. Hanafi [13] investigated the association of wind, tidal, geothermal and solar energy with different desalination techniques. He presented some guiding limits (monograms) for usage of energy sources and recommended wind energy over PV used with RO. Many RO desalination plants powered with PV were manufactured and placed in operation in several places worldwide during the last three decades. Numerous attempts and experiments were carried out in attempt to find suitable coupling procedures between RO desalination process and PV or windmills like RE resources. Keefer [14], with support from the Canadian Government, built two small systems in Vancouver, British Colombia, that were designed to demonstrate the use and optimise solar energy input (panels have a peak rating of 480 W), with battery storage to produce from 0.5 to 1.0 m3/d. They also involved comparison of different operating modes for PV/RO system. In particular, they considered the differences between direct connection of the PV/RO system, maximum power tracking, and included battery storage. By employing a variable speed, positive displacement pump with energy recovery
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from the reject brine, they claimed to be able to reduce life cycle costs by 50% relative to conventional PV/RO systems. A small PV driven RO pilot was tested in Gran Canaria Island [15]. The pilot plant, having an average production capacity of 3.2 m3/d of fresh water, is coupled to stand-alone PV system and storage batteries. The nominal power consumption is 2.35 kW. A. Hanafi [13] concludes that geothermal groundwater resources, at low enthalpy, are not expected to play a role in conventional desalination technologies (MSF or MED). However, thermal membrane distillation (MD) using a sensible heat, is driven by a solar heated source such as plane captors. Hogan [16] studied the feasibility of a solar powered desalination plant for domestic drinking water in arid rural regions of Australia. For the domestic sized plant of 50 kg/d the optimum configuration appears to be a solar collector area of around 3 m2, a membrane area of 1.8 m2 and a total heat exchange are of 0.7 m2. This configuration is cost-competitive with RO. The integration of different membrane operations is becoming quite attractive as a way for increasing the performance of the processes. Andre´s et al. [17] have coupled a MD module to a multiple effect distiller for pure water production. That study found that the best operating parameters are 85 C for a feed brine temperature at the evaporator inlet and a circulation flow of about 170 kg/h. Under these conditions, a GOR value of 3.7 and a water production of 16 kg/h may be reached. The integration of one membrane module distiller as a second step at the MED outlet permits an increase of distilled water production by about 7.5% and improvement of the energetic efficiency by practically 10%. Energetic analysis shows that MD can be driven by a low enthalpy sources as geothermal groundwater. In this paper, we present laboratory scale apparatus for a multiple effect solar still
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(MESS), RO powered PV panels (RO-PV), and membrane distillation using sensible heat of geothermal water (MD-GW), and we discuss obtained technical and economical results. 2. Quality of distributed water in rural areas In rural areas there is no drinking water network. The farming population is supplied through public fountains and animal or mechanically drawn travelling cisterns. Water is no treated and cisterns are often non-hygienic (corrosion, development of bacteria). In addition, brackish water can cause irritation of digestive tracts and skin diseases, especially for young children. Figure 1 shows the distribution of water salinity in rural regions. The country divided into four regions: Areas of very high salinities: in the south eastern regions, the salinities of distributed water are often above 2500 ppm and the percentage of water falling within this category is nearly 100%. Areas of high salinities: in the extreme south, and south western, and the CapBon regions, the salinities of distributed water are between 2000 and 2250 ppm. The percentage of water having salinity about 2000 ppm is higher in the southwest than in the extreme southern parts. Their respective percentages are 100% and 80%. Areas of moderate salinities: in the central parts of the country, the quality of water distributed is acceptable overall. Nevertheless, the percentage of high salinity in the distributed water (over 2000 ppm) reached 40–60%. Then localities were supplied with a bad water quality. Areas of low salinities: only north-western regions benefit from very high water quality and percentage of water with salinity exceeding 2000 ppm, is practically zero.
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3. Renewable energy potential
3.2. Geothermal energy
3.1. Solar energy
In Tunisia, the main disadvantage of low enthalpy geothermal energy is, of course, its low energy content and the need to use it locally. At present, most geothermal resources are recognized as geothermal water springs and not as geothermal energy sources. In most cases, geothermal energy is dissipated in towers prior to geothermal water use. The geothermal water is transferred in aqueducts to consumers or to conventional desalination plants such as the RO plant in Gabe`s [19]. In some other cases, geothermal springs are used for bathing and therapeutic treatments; they
Tunisia is fortunate to have a ‘sun belt’ due to its geographic position. The global solar radiation intensity varies from 4.5 kWh/m2d1 in the north to more than 6 kWh/m2d1 in the south with a total insolation period of 3500 h/ year and 350 sunny days per year. When the sun is well above the horizon and shines through clean skies, the direct component of solar radiation is 80–95 % of the total global solar radiation [18]. Figure 2 shows isopleths for the mean yearly solar intensity in Tunisia.
Fig. 1. Salinity of water in rural localities. , 100% , <80% TDS>2000; , 40 and 60% TDS>2000; , 0% TDS>2000. TDS>2000;
Fig. 2. Map of renewable energy resources (Solar , Contour of constant annual and Geothermal). , Regions with geosolar radiation (kWh/m3); thermal hot springs.
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are also used for heating greenhouses. Figure 2 shows 5 major geothermal districts [20]. These resources can be also found in southern regions where a lack of fresh water is acute.
4. Experimental set-ups 4.1. Multiple effect solar still (MESS) The experimental distillery, designed and patented by P. Le Goff, has dimensions of 1 m length and 0.5 m width. It is composed of three stages; each stage consisting of one plate, a very thin fabric comprising a single finely woven layer, and a steam chamber. This fabric is held in contact with the overhanging plate through the interfacial tension which is mush greater than the force due to gravity. The aluminium front plate is black painted and oriented to the sun (absorber face). The other plates are stainless steel, 0.3 mm thick. Each plate has a condensation and an evaporation face. A cover glass and a mirror are used to improve performance. Temperature sensors; radiation, wind and velocity are measured (Fig. 3). 4.2. Reverse osmosis driven photovoltaic (ROPV) The technological design of the prototype has three compartments: pre-treatment, desalination and post treatment (Fig. 4). The different compartments were described previously [21].
4.3. Membrane distillation powered geothermal resources (MD-GW)
Fig. 3. Experimental set-up of MESS.
Figure 5 shows a schematic diagram of the experimental set-up. Pre-treatment is carried out in a conical fluidised bed crystalliser (FBC), as described earlier [22]. Three MD modules are arranged in serial design. Each
one is made up of an AGMD//Max4, which is composed of two cells arranged in parallel configuration. Each cell has a hydrophobic membrane, an air gap, and a condenser exchanger.
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S. Bouguecha et al. / Desalination 183(2005) 151–165 PS Reject valve
RS
PV Panel
RO Module
CS
CS Vessel Tank
V PS Cartrige filter PS
Feed Tank
Carbon filter
Permeate Tank
Pump
CS PS
Fig. 4. Experimental set-up RO-PV.
Fig. 5. Experimental set-up MD-GW.
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5. Results and discussion 5.1. Multiple effect solar still
Solar radiation (Wm-2)
In order to undertake experimental studies in real sites, several prototypes were installed in Tunisia, Algeria and Libya. Experiments have shown high performance compared to passive distillation systems [23]. The cover glass increases the black plate temperature about 5 C and the production by over 17%. With mirror and cover glass, temperature increase reaches 11 C and the production increase is 39% (Fig. 6). Figure 7 shows the effects of variation of ambient temperature, condensate plate (n 4), evaporator condenser plates temperature (n 3 and 2) and black plate (evaporator plate n 1) temperature as a function of time. The temperature difference between successive plates is about 2 C. The difference between the black plate and the later condensate plate is
seven degrees. On cloudy days, flow rate decreased up to 50% and the black plate lost about 6 C. Figure 8 represents the variation of flux vs. time. The distillate flow rate increases when intensity of solar flux increases in a Gaussian distribution. Production is respectively 56%, 29% and 15% for first, second and third stage. This result illustrates that the number of stages must be less than or equal to three. The average production of MESS is estimated to 7–8 kg.m2.d1; this result agree with those found by B. Boukchima et al. [24, 25] in Algeria. The experimental results show the significant superiority of this type of distiller over the conventional basin type solar still. It was also found that gauze is very suitable for practical application. Irregular wetting gauze surfaces, existence of an air gap between the plaques and gauze and loss of vapour from
1000
1000
750
750
500
500
250
250 Cover glass
Cover glass + Miror
0 06:30
0 08:25
09:55
11:25
12:55
14:25
15:55
17:25
Time (h.min.s) Fig. 6. Variation of solar radiation vs. time with cover glass and mirror. glassþMirror.
, Cover glass;
, Cover
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S. Bouguecha et al. / Desalination 183(2005) 151–165 325 Ambient T Cond. plate n˚4 T of Evap.-cond. n˚3" T of Evap.-Cond n˚2 T of Evap.n˚1 (black plate)
Temperature (T in K)
318
311
304
297
290 06:30:00
09:30:00
12:30:00
15:30:00
18:30:00
Time (t in h.min.s)
Fig. 7. Variation of temperature vs. time (with mirror and cover glass). *, Ambient T; , Cond. plate n 4, –, T of Evap.–cond n 3; & T of Evap.–cond n 2; , T of Evap. n 1 (black plate). 1200 Cumulative
Product volum (Q 10
– 3Lm– 2)
1000
Stage n˚1 Stage n˚2
800 Stage n˚3 600
400
200
0 08:00
10:00
12:00
14:00
16:00
18:00
ee
Fig. 8. Variation of flux vs. time for three stages and accumulated product. ~, Cumulative; *, Stage n 2; , Stage n 3.
&,
Stage n 1;
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gaskets are responsible for the limited performance of MEES.
integrated membrane vessel. During a cycle, the product flow oscillates between 2.38 and 2.03 kg/h, a variation of 14%.
5.2. Reverse osmosis
5.3. Membrane distillation
Table 1 summarises the optimum operating conditions of the RO prototype for feed water salinity 2800 ppm The curves in Fig. 9 represent respectively variations of solar radiation (9a), available power (9b) and absorbent power (9c) vs. time. We remark that radiation solar has a Gaussian curve function, just as time and available power also have the same form. Absorbent power increases when solar radiation increase, at 640 Wm2 solar radiation, nominal power is reached, and the storage dissipation system began its automatic intermittent mode. We observed the motor pump group running for 50 s and the stopping for 250 s. (represented on Zoom zone). This intermittent mode operated for 4 h 25 min (10 h 30 min–14 h 55 min). After 14 h 55 min, the motor pump group operated continuously until 17 h 30 min. At this time, the current was 0.98 A, while the start-up current was 1.26 A. Figure 10 represents product flow and pressure vessel as function of motor pump group cycle (e.g. adds times of start-up and stop of motor pump group). Pressure and product flow were strongly correlated, in accordance with the solution diffusion model prediction. At the end of the cycle, pressure continues to decrease, while product flow remains constant (2.04 kg/h). This phenomenon is attributed to the elasticity of
The desalination of brackish or seawater can be accomplished by essentially two procedures: thermal distillation processes and membranes processes. The thermally driven membrane desalination is considered a kind of hybrid process, in which a micro porous hydrophobic membrane separates a warm solution from a cooler chamber, which contains either a liquid or gas. Vapour molecules migrate through the membrane pores from the high to the low vapour pressure side, that is, from the warmer to the cooler compartment. The separation mechanism of membrane distillation is based on VapourLiquid Equilibrium (VLE). Problems in the geothermal systems arise mainly from the loss of carbon dioxide as the pressure is reduced. When the water in the well moves close to the earth surface, conversion to calcium carbonate takes place with the loss of CO2. A fluidised bed crystalliser is used first as a pre-treatment compartment [19]. Figure 11 represents the variation of volume vs. the time at different temperature of feed solution. We note that the volume given by the Max 4 module increases linearly with time Figure 12 represents the permeate flux as a function of the concentration of sodium chloride. The permeate flux decreases insensitively for a variation of feed concentration from 3 to 35 g/L, the permeate quality stills
Table 1 Optimum operating conditions of the RO prototype for feed water salinity Aperture reject brine valve
/4
/2
3/4
5/4
3/2
Conversion rate Product flow Specific energy consumption
28 1.8 300
30 2.15 255.6
37 2.45 80
31 1.80 612
20 1.35 1080
3 0.55 3132
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1000
Solar radiation (E in wm-2)
800
600
400
200
0
a)
e
Available power (Pav inW)
600
450
300
150
0 b)
300 300 Zoom (Absorber power (Pab in W)
150
c)
0 0:00
200
Box
0:02
0:05
100
0 06:30
08:30
10:30
12:30
14:30
16:30
Time (t in h and min)
Fig. 9. (a, b, c): Variation of solar radiation, available and absorber power vs. time.
18:30
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2.3 Product flow (Q in kg/h)
3.7
2.2
3.1 2.1
Vessel Pressure (P 105 Pa)
4.3
2.4
2.5
2 0
150
290
440
580
Time (t in s) Vessel pressure
Product flow
Fig. 10. Product flow and vessel pressure vs. time (aperture reject valve 3/4). , Vessel pressure.
, Product flow;
0.9 T=318 K T=323 K T=328 0.6 Volum (V in L)
T=333 K T=338 K
0.3
0 0
30
60
90
120
Time (t in mn)
Fig. 11. Volume vs. time. , T = 338 K.
, T = 318 K;
, T = 323 K; —X—, T = 328;
, T = 333 K;
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S. Bouguecha et al. / Desalination 183(2005) 151–165 2.4
Permeate flux (Q in Kg/2m.h)
1.8
1.2
0.6 T=313
0
1
3
T=343
T=333
T=323
9
12
36
Concentration (C in g/L)
Fig. 12. Permeate flux vs. feed concentration.
, T = 313; , T = 323;
constant at 6 s/cm. The flux reduction can be attributed to several causes, such as vapour reduction due to the salt effect, temperature and concentration polarization at membrane surface. Schneider et al. [18] found similar results of marginal feed concentration effect on permeate flux and quality. Figure 13 represents mass flux vs. temperature for three MD stages arranged in serial design. The global conversion rate reaches 25% (10% due to the first stage, 9% and 6% are respectively realised by the second and third stage). The MD configured in three stages improves the energy conversion significantly.
, T = 333;
, T = 343.
minimum Es (80 kJ/kg) was obtained for 16% reject rate or 3/4 aperture reject brine valve value. Therefore, this position offers the best operating condition for a direct renewable
5.4. Specific energy consumption Specific energy consumption corresponding to MESS, RO-PV and MD-GW set-ups is summarised in Table 2. We note that RO-PV for optimum reject brine aperture has the lowest value and MESS has the highest one. The
Fig. 13. Mass flux vs. temperature for three MD stages.
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S. Bouguecha et al. / Desalination 183(2005) 151–165 Table 2 Specific energy consumption
Experimental Results (kJ/kg) Theoretical Results (kJ/kg)
MESS
RO-PV
MD-GW
1500
82
111
180–240
14, 4–18
40–50
energy coupled during a clear day. The singleeffect solar still produces 4–5 L m2, with specific energy consumption around 7000 kJ kg1, the multiple solar stills and evaporation type solar stills, respectively with consumption about 1800 and 800 kJ kg1. A. Joyco et al. have obtained a minimum specific energy consumption of 92 kJ kg.1 [26]. The large difference between predicted and experimental values can be attributed to: MESS can not produce 15 kg/d (theoretical value advanced by Le Goff); the maximum is 7–8 kg.d1, for a sunny day. RO driven by PV panels has higher No think than RO using conventional energy, because a storage dissipation system has low efficiency. In MD, the flux has an exponential form vs. temperature. Second and third stage of MD coupled to geothermal resources operate at low temperature; consequently, the flux and efficiency have a low values. 5.5. Techno-economic analysis The cost of water produced by desalination processes depends on: (1) the total capital investment in the plant, (2) the cost of operating, maintenance and repair of the facility. It is assumed in this analysis that a portion of the investment set-up is amortized each year, so there will be a steadily decreasing
principal on which interest is charged annually. If the amortization schedule is one which provides for equal annual payments of principal plus interest (convention mortgage retirement plan), the average de-amortization balance over the life of the installation will be on the order of half the original investment, so the approximation annual interest cost will be on the order of half the prevailing interest rate times the first cost. A simple techno-economic analysis can be carried out when we consider the capital cost of the components of the system and we determine the annual cost of each one, the sum divided by the annual production. More exact treatment involves combining the amortisation (n) and interest (r) charges into a single mortgage retirement charge or capital recovery factor (AP) by use the following equation: "
1
#
AP ¼ r 1 þ r n 1 1 þ 100
5.6. Basic data The quantity representing a basic requirement of potable water (cooking and drinking) for a household (5 members by family) is 20 kg/day at TDS equal to 0.5 g/L. In these conditions each process will be designed as follows: Two units of MESS are needed: each unit has 1 m2 surface and 3 stages, and produces 7.5 kg/d of distillate water. This quantity can be blended with available brackish water at 3 g/L; we obtained 10 kg/d at 0.5 g/L per unit. RO-PV: must be operating during 8 h. Produced water has high quality and it can be consumed without secondary treatment.
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Table 3 Basic design and cost data used for the studied cases Item
Life time
MESS Basis Cost
Modules of (RO/MD) Stainless Steel wood FBC PV panels Pumps Control devices Piping þ Tanks Mirror þ Glass Carbon/cartridge filters Sum Cap. invest. Cost ($/L) O& M Cost ($/L) Total ($/L)
5 10 5 15 20 7 7 10 5 10
– 504 350 – – 300 500 100 280 2034
MD-GW must be operating during 10 h, no modification of the experimental set-up used is necessary. The prototype produces 17 kg/d distillate water; an additional volume of brackish water at 3 g/L is evaluated to 3.6 kg/d, so we obtained 20.6 kg/d at 0.5 g/L. The cost of water produced by these various processes have not been reported as such, but it is estimated that an annual fixed cost of about 17.3% would cover depreciation, interest, and insurance. The load factor is estimated at 330 days/year. Operating and maintenance cost is estimated as 1/6 of annual cost. Table 3 shows that the cost of the produced water is estimated at $0.05/L for MESS, $0.08/L for RO-PV and $0.13/L for MD-GW. We note that despite its low thermodynamic efficiency and excessive specific energy consumption, MESS presents the lowest product water cost compared to other desalting processes RO and MD driven by PV and GW energy respectively. In fact,
RO-PV
Annual Cost
Basis Cost
Annual Cost
700
164.22
MD-GW Basis Cost
Annual Cost
2100
492.66
700 – 300 500 50 – 25 37250
57.74
59.12 82.11
50.28 83.79 11.73
286.60 0.04 0.01 0.05
– 1500 500 500 50 – 75 3325
97.98 83.79 83.79 05.89 08.80 444.47 0.07 0.01 0.08
50.28 83.79 05.89 02.93 693.29 0.11 0.02 0.13
where water demands are modest and where the climate is favourable, solar distillation appears to have a substantial advantage over the RO-PV and MD-GW processes. 6. Conclusions This work was carried out to evaluate the performances of three small scale desalination prototypes of type MESS, RO and MD driven by RE with the aim of optimising operation as a function of energy availability. Small-scale desalination systems powered by RE are technically viable. MESS can be applied to desalt either brackish or seawater. In the both cases MESS gives high level purity of product water. Hence, MESS remains an attractive process for sunny climates such as Tunisia, and it can found successful application in remote areas. MD is adapted to use waste energy, but geothermal water has several inconveniences such as hardness, and MD coupled to FBC is not adapted to small scale capacity. However, MD
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is a promising process for desalination (high purity distillate). MD is not significantly limited by osmotic pressure, so MD can desalt very saline water, and specific energy consumption is very low. MD coupled to solar heat collectors presents an adequate design for small scale desalination. Experimental study of RO-PV has shown that the storage dissipation system removes the perturbation due to the fluctuation of solar radiation and is caused by short cloud passages. Economic feasibility has been demonstrated for MESS, but RO-PV and MD-GW remain expensive solutions.
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Small scale desalination pilots powered by renewable energy sources: case studies S. Bouguechaa, B. Hamrounib, M. Dhahbia* a
b
LETM/INRST, BP 95 Hammam-Lif 2050 Tunisia LETM/FST, BP 1020 Le belve´de`re, Tunis, Tunisia emails: [email protected], [email protected], [email protected] Received 21 February 2005; accepted 21 March 2005
Abstract In Tunisia, water resources are very limited and have high salinities. Domestic water in rural communities has TDS reaching 2750 ppm. Inhabitants of these areas are scattered and lack proper infrastructure for the implementation of conventional desalination plants. However, these regions have an important potential of renewable energy (RE), especially solar energy where sunshine in southern regions can reach 2500 h per year. RE can be harnessed to power small-scale desalination plants in order to produce the fresh water necessary to cover the basic requirements (drinking and cooking). This paper aims to identify the potential of renewable energies, mainly solar and geothermal. It illustrates the quality of distributed water in rural communities. It presents several studies that were carried out, including solar multiple effects distillation, reverse osmosis driven by photovoltaic panels, and lately membrane distillation using a geothermal resource. The experimental results show technical promise. However, more investigations will need to optimise the operating parameters and improve economic feasibility. Keywords: Desalination; Water resources; Renewable energy; Reverse osmosis; Solar still; Membrane distillation; Tunisia
1. Introduction Tunisia is located on the southern rim of the Mediterranean basin. Like its neighbour countries, it is confronted by a problem of *Corresponding author.
fresh water shortage. In fact, it has very limited water resources, aggravated by a large spatial and temporal disparity between southern and northern parts, and fluctuations from year to year. In the central and Cap Bon areas, large extractions led to a sharp decline in water levels, causing seawater intrusion.
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.03.032
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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. In remote rural communities, the problem of potable water remains unsolved. There are about 1.5 106 inhabitants and the distributed water volume reaches 19 106 m3 per year, with an average specific consumption estimated at 45 l per capita per day, which is considerably lower than the national average (130 l per inhabitant per day). In addition, 8 106 m3 of the distributed water has a high salinity, making it unfit for human consumption. Analysis of the RE map shows the important potential sites where solar energy and geothermal energies can provide a solution to cover the energy needed for desalination. Thus, desalination powered by renewable energy presents a solution to the problem of fresh water shortage in rural dispersed communities, improving inhabitants’ living conditions and having a positive impact on the environment. The first test imitating the natural cycle of evaporation-condensation was attempted by N. Chezi in 1742. In 1872, Charles Wilson, a Swedish engineer built the first solar distillation still in Las Salinas, Chile. For more than a century, numerous investigations have been carried out to improve the overall performance of solar stills. The performance of solar stills remains lower than expected. These performances depend mainly on design and technical imperfections [1–5]. Therefore, fresh water production by solar stills did not exceed 4 kg/m2d. With the aim of overcoming these disadvantages, various authors [6–8] have proposed a wick multiple effect solar still; Cooper and Apple yard suggested a multiple solar distillation still, where the latent heat of steam would
be used to evaporate another quantity of salt water. Nevertheless, the thickness of texture and the formation of an air gap between the plate and texture substantially limited the performance. The Le Goff team [9–11] has developed the DIFICAP distillatory. This device, using a thin texture resolves the problems due to the thickness of the texture. In addition, the contact between the plate and the texture is ensured by interfacial tension, which is much greater than gravitational force Rodriguez [12] presented a systematic approach to desalination powered by RE in order to consider all the alternatives. Among all the combinations investigated, they concluded that RO powered with PV is interesting in very specific cases, such remote sunny sites. Hanafi [13] investigated the association of wind, tidal, geothermal and solar energy with different desalination techniques. He presented some guiding limits (monograms) for usage of energy sources and recommended wind energy over PV used with RO. Many RO desalination plants powered with PV were manufactured and placed in operation in several places worldwide during the last three decades. Numerous attempts and experiments were carried out in attempt to find suitable coupling procedures between RO desalination process and PV or windmills like RE resources. Keefer [14], with support from the Canadian Government, built two small systems in Vancouver, British Colombia, that were designed to demonstrate the use and optimise solar energy input (panels have a peak rating of 480 W), with battery storage to produce from 0.5 to 1.0 m3/d. They also involved comparison of different operating modes for PV/RO system. In particular, they considered the differences between direct connection of the PV/RO system, maximum power tracking, and included battery storage. By employing a variable speed, positive displacement pump with energy recovery
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from the reject brine, they claimed to be able to reduce life cycle costs by 50% relative to conventional PV/RO systems. A small PV driven RO pilot was tested in Gran Canaria Island [15]. The pilot plant, having an average production capacity of 3.2 m3/d of fresh water, is coupled to stand-alone PV system and storage batteries. The nominal power consumption is 2.35 kW. A. Hanafi [13] concludes that geothermal groundwater resources, at low enthalpy, are not expected to play a role in conventional desalination technologies (MSF or MED). However, thermal membrane distillation (MD) using a sensible heat, is driven by a solar heated source such as plane captors. Hogan [16] studied the feasibility of a solar powered desalination plant for domestic drinking water in arid rural regions of Australia. For the domestic sized plant of 50 kg/d the optimum configuration appears to be a solar collector area of around 3 m2, a membrane area of 1.8 m2 and a total heat exchange are of 0.7 m2. This configuration is cost-competitive with RO. The integration of different membrane operations is becoming quite attractive as a way for increasing the performance of the processes. Andre´s et al. [17] have coupled a MD module to a multiple effect distiller for pure water production. That study found that the best operating parameters are 85 C for a feed brine temperature at the evaporator inlet and a circulation flow of about 170 kg/h. Under these conditions, a GOR value of 3.7 and a water production of 16 kg/h may be reached. The integration of one membrane module distiller as a second step at the MED outlet permits an increase of distilled water production by about 7.5% and improvement of the energetic efficiency by practically 10%. Energetic analysis shows that MD can be driven by a low enthalpy sources as geothermal groundwater. In this paper, we present laboratory scale apparatus for a multiple effect solar still
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(MESS), RO powered PV panels (RO-PV), and membrane distillation using sensible heat of geothermal water (MD-GW), and we discuss obtained technical and economical results. 2. Quality of distributed water in rural areas In rural areas there is no drinking water network. The farming population is supplied through public fountains and animal or mechanically drawn travelling cisterns. Water is no treated and cisterns are often non-hygienic (corrosion, development of bacteria). In addition, brackish water can cause irritation of digestive tracts and skin diseases, especially for young children. Figure 1 shows the distribution of water salinity in rural regions. The country divided into four regions: Areas of very high salinities: in the south eastern regions, the salinities of distributed water are often above 2500 ppm and the percentage of water falling within this category is nearly 100%. Areas of high salinities: in the extreme south, and south western, and the CapBon regions, the salinities of distributed water are between 2000 and 2250 ppm. The percentage of water having salinity about 2000 ppm is higher in the southwest than in the extreme southern parts. Their respective percentages are 100% and 80%. Areas of moderate salinities: in the central parts of the country, the quality of water distributed is acceptable overall. Nevertheless, the percentage of high salinity in the distributed water (over 2000 ppm) reached 40–60%. Then localities were supplied with a bad water quality. Areas of low salinities: only north-western regions benefit from very high water quality and percentage of water with salinity exceeding 2000 ppm, is practically zero.
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3. Renewable energy potential
3.2. Geothermal energy
3.1. Solar energy
In Tunisia, the main disadvantage of low enthalpy geothermal energy is, of course, its low energy content and the need to use it locally. At present, most geothermal resources are recognized as geothermal water springs and not as geothermal energy sources. In most cases, geothermal energy is dissipated in towers prior to geothermal water use. The geothermal water is transferred in aqueducts to consumers or to conventional desalination plants such as the RO plant in Gabe`s [19]. In some other cases, geothermal springs are used for bathing and therapeutic treatments; they
Tunisia is fortunate to have a ‘sun belt’ due to its geographic position. The global solar radiation intensity varies from 4.5 kWh/m2d1 in the north to more than 6 kWh/m2d1 in the south with a total insolation period of 3500 h/ year and 350 sunny days per year. When the sun is well above the horizon and shines through clean skies, the direct component of solar radiation is 80–95 % of the total global solar radiation [18]. Figure 2 shows isopleths for the mean yearly solar intensity in Tunisia.
Fig. 1. Salinity of water in rural localities. , 100% , <80% TDS>2000; , 40 and 60% TDS>2000; , 0% TDS>2000. TDS>2000;
Fig. 2. Map of renewable energy resources (Solar , Contour of constant annual and Geothermal). , Regions with geosolar radiation (kWh/m3); thermal hot springs.
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are also used for heating greenhouses. Figure 2 shows 5 major geothermal districts [20]. These resources can be also found in southern regions where a lack of fresh water is acute.
4. Experimental set-ups 4.1. Multiple effect solar still (MESS) The experimental distillery, designed and patented by P. Le Goff, has dimensions of 1 m length and 0.5 m width. It is composed of three stages; each stage consisting of one plate, a very thin fabric comprising a single finely woven layer, and a steam chamber. This fabric is held in contact with the overhanging plate through the interfacial tension which is mush greater than the force due to gravity. The aluminium front plate is black painted and oriented to the sun (absorber face). The other plates are stainless steel, 0.3 mm thick. Each plate has a condensation and an evaporation face. A cover glass and a mirror are used to improve performance. Temperature sensors; radiation, wind and velocity are measured (Fig. 3). 4.2. Reverse osmosis driven photovoltaic (ROPV) The technological design of the prototype has three compartments: pre-treatment, desalination and post treatment (Fig. 4). The different compartments were described previously [21].
4.3. Membrane distillation powered geothermal resources (MD-GW)
Fig. 3. Experimental set-up of MESS.
Figure 5 shows a schematic diagram of the experimental set-up. Pre-treatment is carried out in a conical fluidised bed crystalliser (FBC), as described earlier [22]. Three MD modules are arranged in serial design. Each
one is made up of an AGMD//Max4, which is composed of two cells arranged in parallel configuration. Each cell has a hydrophobic membrane, an air gap, and a condenser exchanger.
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S. Bouguecha et al. / Desalination 183(2005) 151–165 PS Reject valve
RS
PV Panel
RO Module
CS
CS Vessel Tank
V PS Cartrige filter PS
Feed Tank
Carbon filter
Permeate Tank
Pump
CS PS
Fig. 4. Experimental set-up RO-PV.
Fig. 5. Experimental set-up MD-GW.
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5. Results and discussion 5.1. Multiple effect solar still
Solar radiation (Wm-2)
In order to undertake experimental studies in real sites, several prototypes were installed in Tunisia, Algeria and Libya. Experiments have shown high performance compared to passive distillation systems [23]. The cover glass increases the black plate temperature about 5 C and the production by over 17%. With mirror and cover glass, temperature increase reaches 11 C and the production increase is 39% (Fig. 6). Figure 7 shows the effects of variation of ambient temperature, condensate plate (n 4), evaporator condenser plates temperature (n 3 and 2) and black plate (evaporator plate n 1) temperature as a function of time. The temperature difference between successive plates is about 2 C. The difference between the black plate and the later condensate plate is
seven degrees. On cloudy days, flow rate decreased up to 50% and the black plate lost about 6 C. Figure 8 represents the variation of flux vs. time. The distillate flow rate increases when intensity of solar flux increases in a Gaussian distribution. Production is respectively 56%, 29% and 15% for first, second and third stage. This result illustrates that the number of stages must be less than or equal to three. The average production of MESS is estimated to 7–8 kg.m2.d1; this result agree with those found by B. Boukchima et al. [24, 25] in Algeria. The experimental results show the significant superiority of this type of distiller over the conventional basin type solar still. It was also found that gauze is very suitable for practical application. Irregular wetting gauze surfaces, existence of an air gap between the plaques and gauze and loss of vapour from
1000
1000
750
750
500
500
250
250 Cover glass
Cover glass + Miror
0 06:30
0 08:25
09:55
11:25
12:55
14:25
15:55
17:25
Time (h.min.s) Fig. 6. Variation of solar radiation vs. time with cover glass and mirror. glassþMirror.
, Cover glass;
, Cover
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S. Bouguecha et al. / Desalination 183(2005) 151–165 325 Ambient T Cond. plate n˚4 T of Evap.-cond. n˚3" T of Evap.-Cond n˚2 T of Evap.n˚1 (black plate)
Temperature (T in K)
318
311
304
297
290 06:30:00
09:30:00
12:30:00
15:30:00
18:30:00
Time (t in h.min.s)
Fig. 7. Variation of temperature vs. time (with mirror and cover glass). *, Ambient T; , Cond. plate n 4, –, T of Evap.–cond n 3; & T of Evap.–cond n 2; , T of Evap. n 1 (black plate). 1200 Cumulative
Product volum (Q 10
– 3Lm– 2)
1000
Stage n˚1 Stage n˚2
800 Stage n˚3 600
400
200
0 08:00
10:00
12:00
14:00
16:00
18:00
ee
Fig. 8. Variation of flux vs. time for three stages and accumulated product. ~, Cumulative; *, Stage n 2; , Stage n 3.
&,
Stage n 1;
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gaskets are responsible for the limited performance of MEES.
integrated membrane vessel. During a cycle, the product flow oscillates between 2.38 and 2.03 kg/h, a variation of 14%.
5.2. Reverse osmosis
5.3. Membrane distillation
Table 1 summarises the optimum operating conditions of the RO prototype for feed water salinity 2800 ppm The curves in Fig. 9 represent respectively variations of solar radiation (9a), available power (9b) and absorbent power (9c) vs. time. We remark that radiation solar has a Gaussian curve function, just as time and available power also have the same form. Absorbent power increases when solar radiation increase, at 640 Wm2 solar radiation, nominal power is reached, and the storage dissipation system began its automatic intermittent mode. We observed the motor pump group running for 50 s and the stopping for 250 s. (represented on Zoom zone). This intermittent mode operated for 4 h 25 min (10 h 30 min–14 h 55 min). After 14 h 55 min, the motor pump group operated continuously until 17 h 30 min. At this time, the current was 0.98 A, while the start-up current was 1.26 A. Figure 10 represents product flow and pressure vessel as function of motor pump group cycle (e.g. adds times of start-up and stop of motor pump group). Pressure and product flow were strongly correlated, in accordance with the solution diffusion model prediction. At the end of the cycle, pressure continues to decrease, while product flow remains constant (2.04 kg/h). This phenomenon is attributed to the elasticity of
The desalination of brackish or seawater can be accomplished by essentially two procedures: thermal distillation processes and membranes processes. The thermally driven membrane desalination is considered a kind of hybrid process, in which a micro porous hydrophobic membrane separates a warm solution from a cooler chamber, which contains either a liquid or gas. Vapour molecules migrate through the membrane pores from the high to the low vapour pressure side, that is, from the warmer to the cooler compartment. The separation mechanism of membrane distillation is based on VapourLiquid Equilibrium (VLE). Problems in the geothermal systems arise mainly from the loss of carbon dioxide as the pressure is reduced. When the water in the well moves close to the earth surface, conversion to calcium carbonate takes place with the loss of CO2. A fluidised bed crystalliser is used first as a pre-treatment compartment [19]. Figure 11 represents the variation of volume vs. the time at different temperature of feed solution. We note that the volume given by the Max 4 module increases linearly with time Figure 12 represents the permeate flux as a function of the concentration of sodium chloride. The permeate flux decreases insensitively for a variation of feed concentration from 3 to 35 g/L, the permeate quality stills
Table 1 Optimum operating conditions of the RO prototype for feed water salinity Aperture reject brine valve
/4
/2
3/4
5/4
3/2
Conversion rate Product flow Specific energy consumption
28 1.8 300
30 2.15 255.6
37 2.45 80
31 1.80 612
20 1.35 1080
3 0.55 3132
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1000
Solar radiation (E in wm-2)
800
600
400
200
0
a)
e
Available power (Pav inW)
600
450
300
150
0 b)
300 300 Zoom (Absorber power (Pab in W)
150
c)
0 0:00
200
Box
0:02
0:05
100
0 06:30
08:30
10:30
12:30
14:30
16:30
Time (t in h and min)
Fig. 9. (a, b, c): Variation of solar radiation, available and absorber power vs. time.
18:30
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2.3 Product flow (Q in kg/h)
3.7
2.2
3.1 2.1
Vessel Pressure (P 105 Pa)
4.3
2.4
2.5
2 0
150
290
440
580
Time (t in s) Vessel pressure
Product flow
Fig. 10. Product flow and vessel pressure vs. time (aperture reject valve 3/4). , Vessel pressure.
, Product flow;
0.9 T=318 K T=323 K T=328 0.6 Volum (V in L)
T=333 K T=338 K
0.3
0 0
30
60
90
120
Time (t in mn)
Fig. 11. Volume vs. time. , T = 338 K.
, T = 318 K;
, T = 323 K; —X—, T = 328;
, T = 333 K;
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Permeate flux (Q in Kg/2m.h)
1.8
1.2
0.6 T=313
0
1
3
T=343
T=333
T=323
9
12
36
Concentration (C in g/L)
Fig. 12. Permeate flux vs. feed concentration.
, T = 313; , T = 323;
constant at 6 s/cm. The flux reduction can be attributed to several causes, such as vapour reduction due to the salt effect, temperature and concentration polarization at membrane surface. Schneider et al. [18] found similar results of marginal feed concentration effect on permeate flux and quality. Figure 13 represents mass flux vs. temperature for three MD stages arranged in serial design. The global conversion rate reaches 25% (10% due to the first stage, 9% and 6% are respectively realised by the second and third stage). The MD configured in three stages improves the energy conversion significantly.
, T = 333;
, T = 343.
minimum Es (80 kJ/kg) was obtained for 16% reject rate or 3/4 aperture reject brine valve value. Therefore, this position offers the best operating condition for a direct renewable
5.4. Specific energy consumption Specific energy consumption corresponding to MESS, RO-PV and MD-GW set-ups is summarised in Table 2. We note that RO-PV for optimum reject brine aperture has the lowest value and MESS has the highest one. The
Fig. 13. Mass flux vs. temperature for three MD stages.
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S. Bouguecha et al. / Desalination 183(2005) 151–165 Table 2 Specific energy consumption
Experimental Results (kJ/kg) Theoretical Results (kJ/kg)
MESS
RO-PV
MD-GW
1500
82
111
180–240
14, 4–18
40–50
energy coupled during a clear day. The singleeffect solar still produces 4–5 L m2, with specific energy consumption around 7000 kJ kg1, the multiple solar stills and evaporation type solar stills, respectively with consumption about 1800 and 800 kJ kg1. A. Joyco et al. have obtained a minimum specific energy consumption of 92 kJ kg.1 [26]. The large difference between predicted and experimental values can be attributed to: MESS can not produce 15 kg/d (theoretical value advanced by Le Goff); the maximum is 7–8 kg.d1, for a sunny day. RO driven by PV panels has higher No think than RO using conventional energy, because a storage dissipation system has low efficiency. In MD, the flux has an exponential form vs. temperature. Second and third stage of MD coupled to geothermal resources operate at low temperature; consequently, the flux and efficiency have a low values. 5.5. Techno-economic analysis The cost of water produced by desalination processes depends on: (1) the total capital investment in the plant, (2) the cost of operating, maintenance and repair of the facility. It is assumed in this analysis that a portion of the investment set-up is amortized each year, so there will be a steadily decreasing
principal on which interest is charged annually. If the amortization schedule is one which provides for equal annual payments of principal plus interest (convention mortgage retirement plan), the average de-amortization balance over the life of the installation will be on the order of half the original investment, so the approximation annual interest cost will be on the order of half the prevailing interest rate times the first cost. A simple techno-economic analysis can be carried out when we consider the capital cost of the components of the system and we determine the annual cost of each one, the sum divided by the annual production. More exact treatment involves combining the amortisation (n) and interest (r) charges into a single mortgage retirement charge or capital recovery factor (AP) by use the following equation: "
1
#
AP ¼ r 1 þ r n 1 1 þ 100
5.6. Basic data The quantity representing a basic requirement of potable water (cooking and drinking) for a household (5 members by family) is 20 kg/day at TDS equal to 0.5 g/L. In these conditions each process will be designed as follows: Two units of MESS are needed: each unit has 1 m2 surface and 3 stages, and produces 7.5 kg/d of distillate water. This quantity can be blended with available brackish water at 3 g/L; we obtained 10 kg/d at 0.5 g/L per unit. RO-PV: must be operating during 8 h. Produced water has high quality and it can be consumed without secondary treatment.
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Table 3 Basic design and cost data used for the studied cases Item
Life time
MESS Basis Cost
Modules of (RO/MD) Stainless Steel wood FBC PV panels Pumps Control devices Piping þ Tanks Mirror þ Glass Carbon/cartridge filters Sum Cap. invest. Cost ($/L) O& M Cost ($/L) Total ($/L)
5 10 5 15 20 7 7 10 5 10
– 504 350 – – 300 500 100 280 2034
MD-GW must be operating during 10 h, no modification of the experimental set-up used is necessary. The prototype produces 17 kg/d distillate water; an additional volume of brackish water at 3 g/L is evaluated to 3.6 kg/d, so we obtained 20.6 kg/d at 0.5 g/L. The cost of water produced by these various processes have not been reported as such, but it is estimated that an annual fixed cost of about 17.3% would cover depreciation, interest, and insurance. The load factor is estimated at 330 days/year. Operating and maintenance cost is estimated as 1/6 of annual cost. Table 3 shows that the cost of the produced water is estimated at $0.05/L for MESS, $0.08/L for RO-PV and $0.13/L for MD-GW. We note that despite its low thermodynamic efficiency and excessive specific energy consumption, MESS presents the lowest product water cost compared to other desalting processes RO and MD driven by PV and GW energy respectively. In fact,
RO-PV
Annual Cost
Basis Cost
Annual Cost
700
164.22
MD-GW Basis Cost
Annual Cost
2100
492.66
700 – 300 500 50 – 25 37250
57.74
59.12 82.11
50.28 83.79 11.73
286.60 0.04 0.01 0.05
– 1500 500 500 50 – 75 3325
97.98 83.79 83.79 05.89 08.80 444.47 0.07 0.01 0.08
50.28 83.79 05.89 02.93 693.29 0.11 0.02 0.13
where water demands are modest and where the climate is favourable, solar distillation appears to have a substantial advantage over the RO-PV and MD-GW processes. 6. Conclusions This work was carried out to evaluate the performances of three small scale desalination prototypes of type MESS, RO and MD driven by RE with the aim of optimising operation as a function of energy availability. Small-scale desalination systems powered by RE are technically viable. MESS can be applied to desalt either brackish or seawater. In the both cases MESS gives high level purity of product water. Hence, MESS remains an attractive process for sunny climates such as Tunisia, and it can found successful application in remote areas. MD is adapted to use waste energy, but geothermal water has several inconveniences such as hardness, and MD coupled to FBC is not adapted to small scale capacity. However, MD
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is a promising process for desalination (high purity distillate). MD is not significantly limited by osmotic pressure, so MD can desalt very saline water, and specific energy consumption is very low. MD coupled to solar heat collectors presents an adequate design for small scale desalination. Experimental study of RO-PV has shown that the storage dissipation system removes the perturbation due to the fluctuation of solar radiation and is caused by short cloud passages. Economic feasibility has been demonstrated for MESS, but RO-PV and MD-GW remain expensive solutions.
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