pv and thermally driven small-scale, stand-alone solar desalination

Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt

249

PV AND THERMALLY DRIVEN SMALL-SCALE, STAND-ALONE

SOLAR DESALINATION SYSTEM W ITH VERY LOW MAINTENANCE NEEDS

Hassan E. S. Fath*, Samy M. Elsherbiny and Alaa A. Hassan

Mech. Eng. Dept., Alexandria Univ., Alexandria, Egypt E-mail [email protected]

Matthias Rommel, Marcel Wieghaus and Joachim Koschikowski Fraunhofer Institute for Solar Energy Systems ISE, Germany

Mostafa Vatansever Fentec, Turkey

ABSTRACT This paper presents the thermal performance of a membrane distillation (MD) solar desalination unit located in the Mechanical Engineering Department site, Alexandria University, Alexandria, Egypt. The unit is designed and installed as a part of a partially funded European Commission (EC) project named; “PV and thermally driven small-scale, stand-alone desalination system with very low maintenance needs (SMADES)”. The basic innovative MD principle and module is highlighted. The Alexandria MD unit is described and the unit performance is presented for the 6-months of its operation period. The unit performance covers; the transient changes in the unit productivity, unit feed water, brine and product water temperatures and conductivities, unit salt rejection, solar collector and MD process efficiencies. The unit performance, in clear and cloudy days, for typical summer and winter months, is presented. For a sunny day, of 7.25 kWh/day say, the results indicate that the unit produces about 11.2 liter/day for every square meter of collector area. The relatively high productivity, above that of the conventional solar still, is due to the partial recovery of the condensation energy. The overall unit productivity has been correlated against the solar irradiation as:

1.67-.day)(kWh/m radiation Daily 1.666 .day)(lit/m productionDaily 22 ×= Keywords: Desalination, Membrane Distillation, Solar Energy INTRODUCTION The international rapid developments, the industrial growth, and population explosion all over the world have resulted in a large escalation of demand for fresh water. On the other hand, the surface water (rivers and lakes) pollution caused by industrial and agricultural wastes and the large amount of sewage, limit the suitability of many fresh

Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt 250

water availability resources. By the beginning of this century, fresh water shortages and quality became an international problem confronting human groups and countries. The problem is more apparent in the Arabian and Middle Eastern/North African (MENA) countries due to the very limited natural resources of fresh water. Desalination (desalting) has been presented for decades as a suitable alternative for the partial solution of the world fresh water crisis. Desalination of brackish and sea water can provide the need of drinking water without any serious impact on the environment. As a result, there has been a dramatic world wide increase in the number and capacity of desalination plants, Wangnick [1]. Although it seems more expensive in areas with surface or ground water availability, it is not so in areas 300 – 500 km far from fresh water resources. For large water production, conventional desalination technologies as MSF, MEE, RO, VC,…etc. have been proven to be technically and economically suitable. However, for (i) small communities of limited fresh water demand (up to 10 m3/day), (ii) areas far from the sources of water and energy (fuel & electricity), and (iii) communities of small technical capabilities, solar desalination is more applicable. Solar desalination in many respects might be an ideal solution to small communities in many MENA countries, as shown by Malik et al. [2] and Fath [3], due to the following facts: i- Many of these countries enjoy an abundant solar intensity (Annual daily average

is between 200 – 300 W/m2) and large annual sun hours (3000 – 5000 hrs/year), and therefore incident energy of about (5 – 8 kWhr/m2 day)

ii- The diurnal and seasonal fluctuations in solar desalination productivity are intrinsically linked to the fluctuating water demand,

iii- Solar energy is almost available in every location and, in addition, is an environment friendly energy resource (with no CO2 emission)

Solar desalination can be divided into direct and indirect technologies. In the direct methods, the solar energy collector and desalination component are an integral unit, e.g. solar stills, see references [4 to 10]. A general rule of thumb for simple solar stills is that a solar collection area of about 1 m2 is needed to produce 3-5 liters of water per day. Thus, for a 1 and 10 m3/day facility, a land area of about 250 and 2500 m2 would be required, respectively. These areas may even be doubled to allow for spacing. Thus, a large solar collection area is required with the resultant high capital costs. To improve the performance of solar stills, multi-effect solar stills were used. In the indirect method, the solar energy is first collected, converted to usable heat or electricity then it is used as an energy source for the different desalination technologies. Many researchers have investigated and developed different indirect solar desalination systems, PV-RO, and Solar thermal-MEE units, etc., [11 to 15]. Each of these indirect solar desalination systems has its positive and negative features and none of them has proved to be the best either technically or economically.


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SMADES PROJECT SMADES is an EC partially funded project in solar desalination. The overall objective of the SMADES project is the development of stand-alone desalination systems for arid and semi-arid remote regions with a lack of drinkable water but a high solar irradiation. The modular system set-up is based on the highly innovative membrane distillation (MD) technology. The system integrates both solar thermal and solar PV energies. The desalination energy is supplied entirely by solar thermal collectors and the electrical auxiliary energy is supplied by PV. To gather a detailed present state of technology and problems on small scale stand-alone desalination plants, a technical study was carried out in the beginning of the project. A second study, after reasonable developments have been carried out to discover the need and the social impact for potential users and to focus on the legal frame works and economic influences. The technical focus of the project is on the improvement of desalination modules based on Membrane Distillation (MD) for stand-alone systems. The MD technique is particularly suitable for maintenance-free and solar supplied systems. The used membranes have a high resistance against different water composites and operate in a temperature range of 60 to 90°C in which collectors achieve a high efficiency and good performance. The developed technique was modular and covered a wide range of capacities (150 Lit/day up to 10 m3/day) so that systems can be adapted to any particular demand. Two different types of MD modules as well as one type of heat exchanger were developed based on the same spiral-winding technology used for the MD modules. For very small and compact applications, the MD module was equipped with internal heat recovery. The aim is to achieve Gained Output Ratios (GOR’s) of 6 to 8 and low module pressure losses (0.2 to 0.6 bar) to reduce pump energy. For larger applications (larger than 1 m3/day) MD modules without internal heat recovery but with an external heat exchanger were used. The aim was to increase the module output (the output of modules without internal heat recovery can be two to three times higher), to decrease production costs and to increase the energy efficiency of the whole system (GOR’s > 10) by connecting several MD modules to a single high efficiency heat exchanger. MEMBRANE DISTILLATION Membrane Distillation (MD) is an innovative membrane technology, [16 to 21]. Contrary to membranes for RO, MD membranes are hydrophobic. This means that, up to a certain limiting pressure, the membrane can not be wetted by liquid water. In Figure (1), the principal set-up for MD module with internal heat recovery is sketched. For MD system, on one side of the membrane there is a lower temperature, for example 60 °C, then there exists a water partial pressure difference between the two sides of the membrane and thus water evaporates through the membrane. The water


vapor condenses on the other side of the membrane and the distillate is formed. Some experience was gathered in recent experimental investigations with spiral wound MD modules with a basic set-up as sketched in Figure (2). The experiments show promising results.

Figure (1) Membrane Distillation Principles

Figure (2) Spiral Wound MD Module

The advantages of the MD systems for stand-alone units were found to be: • The process works at low temperatures (60 to 90°C) which is important for a

high efficiency of solar thermal collectors. (It is also possible to utilize waste heat from diesel engines or heat from co-generation plants.)

• Contrary to RO-systems, no chemical pre-treatment of the feed water is necessary. Simple pre-filtration is sufficient.

Heat Source

Heat Exchanger

Condenser

Evaporator

Membrane

Brine

Feedwater

Distillate


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• There are no problems in the intermittent operation mode. Also long times of dry-out situations are not important because the membrane is hydrophobic.

• The membranes are non-sensitive against algae, etc. • The modules work at atmospheric pressure. It is not a vacuum technology as in

MSF/MEE or pressurized technology as in RO. • The salinity of the feed water has almost no influence on the efficiency of the

process. • Membrane distillation produces very pure distillate. The electrical conductivity

is in the range of 2-10 µS/cm. • Due to the modular set-up systems, a high range of water capacity units can be

built. In the present SMADES project, MD technology was further developed to be applied and adopted to the conditions for solar thermally driven desalination systems. A screening of suitable materials was carried out to achieve long time resistant systems. Control strategies for long term maintenance-free operation was developed. Lessons learned from earlier operating experience of solar thermal driven pilot plants with different distillation devices (for example Multi Effect Humidification) showed that more emphasis has to be given to a well developed total system design. To the present knowledge of the proposed project, the MD technology holds more development potential than other thermally driven desalination techniques for small stand-alone systems. LAYOUT OF ALEXANDRIA COMPACT SYSTEM Figures (3) and (4) show the photo and the flow diagram of Alexandria compact system, respectively. This system was installed and mounted on the roof of the thermal laboratory, mechanical engineering department of Alexandria University, Alexandria, Egypt, in June 2005 and has been working properly since this time till now. The objective of installing this system is to carry-out experimental investigations with long term maintenance free operation under real weather conditions. The system is supplied by water from a 500 litre feed storage tank mounted at the end of the top section of the solar collector. From that storage tank the cold feed water is pumped to the condenser section of the MD module. The feed water is preheated using the latent heat of condensation of the distillate. The preheated feed water leaves the condenser and then enters the bottom section of the solar collector. The heat absorbed by the solar collector during the day time is transferred to the feed water (which may be sea water or brackish water). The heated feed water leaves the collector area on the top, passes a degasser bottle to free it from gases and then enters the evaporator side of the MD module. Part of the heated feed water is evaporated through the membrane while the concentrated brine is recirculated back to the feed storage tank. The distillate after being condensated is either collected in a distillate tank for real use or fed back to the raw water tank to form a closed loop of experiments.


Due to the separation of distillate, the water level in the storage tank decreases and the salt concentration in this tank increases as the brine comes back to this tank from the membrane module. To overcome the increase in salt concentration, the storage tank is refilled with fresh feed water from a well or another source of raw water. When the liquid level in the feed water storage tank drops to half the tank height, a switch activates the refilling pump and the storage tank is refilled with fresh feed water. The refilling process continues until the water level reaches about 90% of the storage tank height at which level another actuator switches the refilling pump off. The system is self-running since the pumps and all other electrical devices are supplied by the PV. The operation is only possible during day time because there is no other source of electricity used. The solar collector consists of three rectangular similar sections connected in series. The dimensions of each section are 2020 mm × 1020 mm × 80 mm with a total area of 2.06 m2 and an aperture area of 1.91 m2. Each collector section consists of 10 absorber finned tubes made from aluminium and connected in parallel to the header tubes. Each absorber fin has a width of 100 mm with a thickness of 2.0 mm. The aluminium finned tubing is equipped with Cu-Ni10 tubes to resist the corrosion if sea water is used. This material withstands hot salt water and is often used in large scale desalination plants. The outer diameter of the riser tubes is 10 mm and the diameter of the header tubes is 20 mm.

Figure (3) A photo of MD Alexandria Compact System


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Figure (4) Flow diagram of MD Alexandria Compact System The riser tubes are joined to the header tubes by brazing. In order to minimize the thermal contact resistance between the aluminium absorber profile and the Cu-Ni10 riser tube, the riser tube is pushed into the open cylindrical back site of the aluminium profile, and then the tubular aluminium profile and the riser tube are grouted together. The sketch in Figure 5 shows a cross section of the absorber profile. The four ends of the header tubes are passed through the collector casing to achieve a high flexibility with respect to collector field design and collector field interconnection.

Figure (5) Absorber fin profile with grouted Cu-Ni10 tube In order to achieve the operation temperatures of the membrane distillation (MD) system of 80 to 90°C, the solar thermal flat plate collector has to have a selectively coated absorber to reduce the heat loss by radiation. The absorber plate is made from aluminium. Its selective surface is applied by an electro-chemical process. The achieved characteristic values for the selective surface are 0.93 absorpitivity and 0.18 emissivity.

Aluminum fin profile Cu-Ni10 tube


The glass cover has a low iron content to reduce losses due to the absorption of radiation by iron particles. A low-iron glass cover is used to increase transmission to the absorber. The transmission of the solar radiation for glass with low iron is about 10% higher than for standard window glass with high iron content. The transmissivity of the glass used is 0.89. The back side of the collectors as well as the walls are insulated with mineral wool with thickness 60 mm on the back side and 30 mm on the walls. The collector casing is made of anodised aluminium. This material is resistant against salty air and strong weather conditions as sandstorm. The collector efficiency, η, curve shown in Figure (6) was determined according to the code EN12975-2 at the test centre for solar thermal collectors at Fraunhofer ISE in Germany.

Figure (6) Collector Efficiency Curve of the FENTEK Cu-Ni10 collector The measuring instrumentation and the data acquisition system were adapted to the different requirements of the compact system. So, the system which focuses on experimental investigations is highly equipped with sensors and with an electronically data acquisition system. Figure (7) provides a list of all sensors and their locations in the hydraulic loop. The sketch represents the highly equipped system for experimental investigations.


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Figure (7) Sensors list and Layout Plan

UNIT PERFORMANCE RESULTS & DISCUSSION The unit is continuously running since its complete installation and collection of full measurements started in July 2005. The data are taken every 10 seconds for all temperatures, flow rates, electric conductivities and global solar radiation as indicated in Figure (7). To show the performance, data for a clear day (17 Aug, 2005) and for a cloudy day (30 Oct., 2005) are presented. Figure (8) shows the collected data for the clear day from sunrise to sunset. The maximum solar intensity during this day, Iglob

was 995 W/m2 and the working evaporator inlet temperature was above 60°C. The electric conductivities for feed and distillate water are 526 and 3 µS/cm respectively. The daily production is 64 Lit/day (11.2 Lit/m2day) while the accumulated solar energy is 41.6 kWh/day (7.25 kWh/m2day). For the cloudy day, Figure (9) shows a maximum solar intensity of 1231 W/m2. However, the total solar energy is 29.5 kWh/day (5.15 kWh/m2day) and the daily production is 23.6 Lit/day (4.11 Lit/m2day). The electric conductivities for feed and distillate water are 672 and 2 µS/cm respectively. The relation between the daily radiation and daily production is shown in Figure 10. It is clear that the distillate daily production is directly proportional to the daily total radiation. A linear correlation over a 6-months operation period is given as:

1.67-.day)(kWh/m radiation Daily 1.666 .day)(lit/m productionDaily 22 ×= (1)


The scatter about the correlation is mainly due to solar radiation at early morning or close to sunset where there is no production since evaporator inlet temperature is too low to operate the MD. The maximum and minimum condenser and evaporator temperatures are shown in Figure (11). The values are almost constant over the operating period with an average temperature difference in evaporator of 43 °C and 18 °C in the condenser. The solar collector and MD process efficiencies are defined as:

∑

∑×

−×=

AI

TTCpm

glob

cicofeedc

)(η (2)

∑

∑×

×=

AI

HeatLatentm

glob

distprocessη (3)

where: A = collector area (m2) mdist = distillate flow rate (kg/s) mfeed = feed water flow rate (kg/s) Iglob = Global solar irradiation (W/m2) Tci = collector inlet temperature Tco = collector outlet temperature Figure (12) shows an average collector efficiency of 50% and a process efficiency of 90% which indicates only 10% heat losses from the MD. The percentage of salt removal from the feed water is defined as:

100)tyconductiviFeed

tyconductividistillate1(alSalt Remov% ×−= (4)

The percentage of salt removal is found to be about 99.5%.


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Compact System Alex. 17th Aug.

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6:007:00

8:009:0010:0011:0012:0013:0014:0015:0016:0017:0018:0019:0020:00

C

l/

h

mik

roS

/cm

0

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600

800

1000

l/h

m

ikro

S/c

m

Tcond_in Tcond_out Tevap_in Tevap_out Cond_dist

V_dist Cond_feed V_feed Iglob

Figure (8) Data for a Clear Day

Compact System Alex. 30th Oct.

0

20

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7:00

8:00

9:00

10:0011:0012:0013:0014:0015:0016:0017:0018:0019:00

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ikro

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800

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l

/h

mik

roS

/cm

Tcond_in Tcond_out Tevap_in Tevap_out

V_dist Cond_feed V_feed Iglob

Figure (9) Data for a Cloudy Day


y = 1.6656x - 1.6697

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8

Daily Radiation (kwh/m^2.day)

Dai

ly P

rod

uct

ion

(l/m

^2.d

ay)

Figure (10) Effect of Daily Radiation on Daily Production

0.0

10.0

20.0

30.0

40.0

50.0

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1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 101

Day No.

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p (

C)

Min Tcon

Max Tcon

Min Tev

Max Tev

Figure (11) Temperature Distributions


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0

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0 10 20 30 40 50 60 70 80 90 100Day No.

Eff

icie

ncy

%

Collector Eff.

Process Eff.

Figure (12) Solar Collector and MD Process Efficiencies CONCLUDING REMARKS

1- A full description of a desalination unit based on membrane distillation is

given. The unit is self operating using a PV panel to run the feed pump and solar collectors to heat up the feed water. Condensation energy is recovered in the condenser channel of the membrane to preheat the feed water.

2- A sample of the measurements for a clear day and a cloudy day is presented and indicates a high productivity of 11.2 Lit/m2.day for a total solar energy of 7.25 kWh/m2.day.

3- A correlation between the unit productivity and total solar radiation is given.

4- The unit shows a high salt rejection performance as it reduced the electric conductivity of the feed water from 670 to about 3 µS/cm, for the product. This gives a salt rejection percentage of about 99.5%.

5- The MD process efficiency is about 90% and the solar collector efficiency is about 50%.

6- The system is very suitable and promising for arid areas in the Arabian and north African regions.


ACKNOWLEDGEMENT

The authors would like to acknowledge the EC for partially funding the project “PV and Thermally Driven Small-Scale, Stand-Alone Solar Desalination System with very Low Maintenance Needs – SMADES”, contract No: ICA3-CT-2002-10025. REFERENCES [1] Wangnick, K., “2004 - IDA Worldwide Desalting Plants Inventory; Report No.

18” IDA report (2004).

[2] Malik M.A.S., Tiwari G.N., Kumar A. and Sodha M.S. “Solar Distillation”, Pergamon Press (1982).

[3] Fath, H. E. S., “Solar Desalination: A promising Alternative for Water Provision with Free Energy, Simple Technology and a Clean Environment”, Desalination, Vol. 116, pp. 45-56 (1998).

[4] Fath, H. E. S. and Elsherbiny, S. M., “Effect of Adding a Passive Condenser on Solar Still Performance”, Int. J. of Solar Energy, Vol. 11, pp. 73-89 (1991).

[5] Fath, H. E. S. and Hosny, H. M., “Thermal Performance of a Single Slopped Basin Still With Inherent Built-in Additional Condenser”, Desalination, Vol. 142, pp. 19-27 (2002).

[6] Fath, H. E. S. and Ghazy, A., “Solar Desalination Using Humidification De-humidification Technology”, Desalination, Vol. 142, pp. 119-133 (2002).

[7] Fath, H. E. S., Elsherbiny, S. M., and Ghazy, A., “Transient Analysis of a New Humidification De-humidification Solar Still”, Desalination, Vol. 155, pp. 187-203 (2003).

[8] Fath, H. E. S., Elsherbiny, S. M., and Ghazy, A., “A Naturally Circulated Humidifying/Dehumidifying Solar Still with a Built-in Passive Condenser”, Desalination, Vol. 169, pp. 129-149 (2004).

[9] Nafey, A. S., Fath, H. E. S., El-Hlaby, S.O., and Soliman, A. M., “Solar Distillation Using a Single Stage Humidification-Dehumidification Processes (I) Numerical Investigation”, Energy Conversion & Management, Vol. 45, pp. 1241-1261 (2004).

[10] Nafey, A. S., Fath, H. E. S., El-Hlaby, S.O., and Soliman, A. M., “Solar Distillation Using a Single Stage Humidification-Dehumidification Processes; (II) Experimental Study”, Energy Conversion & Management, Vol. 45, pp. 1263-1277 (2004).

[11] Gocht, W., Sommerfeld, A., Rautenbach, R., Melin, T., Eilers, L., Neskakis, A., Herold, D., Horstmann, V., and Muhaidat, M.; “Decentralized Desalination of Brackish Water by A Directly Coupled Reverse-Osmosis Photovoltaic-Systems – A Pilot Plant Study in Jordan”. Renewable Energy, 14, pp. 287-292 (1998).


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[12] Muller-Holst, H., Engelhardt, M., and Scholkopk, H.; “Solar Thermal Seawater Desalination Systems for Decentralized Use”, Proceedings of the Sixth Arab International Solar Energy Conference, AISEC-6, 29 March-1 April, Muscat, Sultanate of Oman, ISBN 99921-66-43-6, pp. 317-324, (1998).

[13] Herold, D., Horstmann, V., Neskakis, A., and Plettner-Marliani, J., “Small Scale Photovoltaic Desalination for Rural Water Supply-Demonstration Plant in Gran Canaria”. Renewable Energy 14, pp. 293-298 (1998).

[14] Gracia-Rodriguez, L., “Seawater Desalination Driven by Renewable Energies: A Review”, Desalination 143, pp. 103-113 (2002).

[15] Kersman, S., Reheinlander, J., and Gabler, H., “Seawater Reverse Osmosis Powered from Renewable Energy Sources-Hybrid Wind/Photovoltaic/Grid Power Supply for Small-Scale Desalination in Libya”, Desalination 153, pp. 17-23 (2002).

[16] Findley, M.E., “Vaporization through Porous Membranes”, Ind. Eng. Chem., Process Des. Dev., Vol. 6, p. 226, (1967)

[17] Gore-Tex, D.W., “Membrane Distillation”, Proc. 10th Ann. Conv. Water Supply Improvement Assoc., Honolulu, July 25-29, (1982).

[18] Carlson, L., “The New Generation in Sea Water Desalination- SU Membrane Distillation System”, Proc. First World Congress on Desalination and Water Reuse, Desalination, 45, p. 221, (1983).

[19] Schofield, R.W., Fane, A.G., Fell, C.J.D., “Heat and Mass Transfer in Membrane Distillation”; Journal of Membrane Science, 33, pp. 299-313 (1987).

[20] Andres, M., Doria, J., Khayer, M., Pena, L., Mengual, J. “Coupling of a Membrane Distillation Module to a Multi-Effect Distiller for Pure Water Production”. Desalination 115, pp. 71-81 (1998).

[21] Banat, F., Jumah, R., Garaibeh, M.; “Exploitation of Solar Energy Collected by Solar Stills for Desalination by Membrane Distillation”, Renewable Energy 25, pp. 293-305 (2002).

[22] Rommel M., Koschikowski J, and Wieghaus; “Thermally Driven Desalination Plants Based on Membrane Distillation”, Int. Conference, RES for Island–Tourism & Water, Crete-Greece, May 26-28 (2003).

pv and thermally driven small-scale, stand-alone solar desalination

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