design of a solar powered desalination system for use in south africa

7/28/2019 Design of a Solar Powered Desalination System for Use in South Africa

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DESIGN OF A SOLAR POWERED DESALINATION SYSTEM FOR USE IN

SOUTH AFRICA

GR Hartwig and AB Sebitosi University of Stellenbosch, Matieland, South Africa

ABSTRACT

This paper looks at the feasibility and design of a solar

powered desalination system for use in South Africa.

Furthermore, a simulation is constructed in order to

predict the fresh water output of such a system for a

specific location from a meteorological database. The

chosen system uses a Multi Effect Distillation (MED)

unit coupled with a flat plate solar collector to deliver

200 litres of fresh water per day from a collector area

of 10.42 m2. Experimental verification of the solar

collector is undertaken and compared to theMeteonorm data used in the simulation.

1. INTRODUCTION

Water is the most valuable resource for the survival of

human beings. Without water, a human being will notsurvive for more than a couple of days. Globally water is becoming a scarce resource and our constant developmentof the world is increasing the pressure on our water resources. Eventually over development will startdestroying the environments which sustain these water resources.

The goal of the project is thus to ascertain whether it isfeasible to desalinate sea and brackish water for use in theWestern and Northern Cape areas and if by usingavailable meteorological data for a given location, onecan predict the fresh water output of such a system for thespecific location. Included in this report is a literature

study on desalination and solar collections system andanalytical calculations in conjunction with simulations to

design a system from a given specific fresh water output.Furthermore the report contains a simulation usingmeteorological data to attain the feasible amount of freshwater produced by a system for the given location andfinally an experimental setup to evaluate the solar collector output. Not included in this report are detaileddesigns for a solar collector and evaporation unit

discussed in the report and also no experimentalevaluation of the distillation unit.

2. SOLAR ENERGY COLLECTORS

Collecting the energy of the sun is a simple process, yetthere are many methods to accomplish it. There is thedirect method, which includes among other salinity

gradient ponds, flat plate collectors and evacuated tubecollectors. Indirect methods include parabolic trough

collectors, dish collectors and tower collectors. All of themethods are evaluated and compared against therequirements of the current project

The chosen concept for the project is the flat platecollector. The main reasons for this decision are the

following. No tracking device is necessary for theoperation. Also the design is simple resulting in areduction in costs, operation requirements andmaintenance. Finally, an industry exists in South Africa tosupport and aid flat plate solar collectors.

3. DESALINATION METHOD

Desalination of saline water can be achieved throughvariety of methods and although most show great potential, only the most appropriate methods areconsidered and analysed. Methods include multi-stageflash distillation (MSF), multiple effect evaporation(MED), vapour compression (VC), freeze separation,electro dialysis reversal, reverse osmosis (RO), nano

filtration and solar still [1].

It is found that the MED process is the most suited for thegiven project. The forward feed configuration together with horizontal falling film evaporators is decided on bycomparing various attributes described by Ettouney andEl-Dessouky [2].

Other reasons include simple design, construction,operation and maintenance that all contribute to creating aremote system capable of operating with minor attentionand care. This cannot be said for systems such as RO andMSF and is very much needed for areas where technicalknowledge and monitoring is limited.

4. COMBINED SOLAR AND DESALINATION

METHODS

With both the solar collector and the evaporation processes chosen, combining the two systems does notnecessarily constitute the best combination. RO combinedwith photo-voltaic collectors is a highly attractive systemthat is currently widely available as can be seen by the

various suppliers available on the internet. Research onthe subject has been done by Eduardo Zarza et al. [3],

who compared a RO coupled with photovoltaic systemand a MED coupled with a parabolic trough system. Theresults showed that the solar powered MED system proved to be the better option.

Furthermore, boiling of the water through the MED

system ensures no harmful bacteria and organisms are present in the fresh water produced. This is a highly



valuable quality of the system for developing countriessuch as South Africa and the rest of Africa.

5. MULITPLE EFFECT DISTILLATION

MODELLING AND SIMULATION

The platform used is MATLAB. Other platforms such asMicrosoft Excel and FLUENT are encountered in

simulations found in literature [4] [5], but are notfavourable since the author is proficient with theMATLAB platform.

The simulation model chosen is a model used by Ettouneyand El-Dessouky [2]. Two models are described, acomplex and a simplified one. The simplified model isdecided on since it is not only a simplified mathematical

model but also does not incorporate components such as pre-heaters and flash boxes. These components areassociated with large complex plants and do not fit into a

robust simple plant designed for ease of maintenance andoperation.

Certain assumptions are made to develop the model for

the forward feed MED system. Firstly, constant specificheat for seawater at different temperatures and

concentration. Secondly, constant thermodynamic lossesand constant heat transfer area for all of the effects in thesystem. There is no flashing of vapour inside effects andformed vapours are free of salt particles. The feedseawater entering the first effect is at saturationtemperature of that effect. Energy dissipation to the

environment is negligible and lastly the driving force

behind the heat transfer in the effects is the difference between the condensation and evaporation temperatures.

A flow diagram of the model is provided in Figure 1. Theflow diagram shows the main steps involved in the modeland how the iterations occur.

6. SOLAR COLLECTOR MODELLING AND

SIMULATION

The simulation of the solar collector is conducted in asimilar manner to the MED simulation. Again MATLABis used as the preferred platform to work with for the

same reasons as before. For the task at hand, a simplemodel is chosen, as with the evaporation model, to assistwith the interconnectivity between the two simulations

and input data.

Figure 1: MED simulation flow diagram



Figure 2: System layout

A highly comprehensive handbook for the study of solar collection is provided by Duffie and Beckman [6]. In

depth calculations and evaluations are provided as well assimulation techniques and examples. Al-Ajlan et al [7] provides a simple simulation model for a flat plate solar collector incorporating various equations provided byDuffie and Beckman [6] with the main equation being

As always since a simple model is used, manyassumptions are made. For the given model the followingassumptions are made by Duffie and Beckman [6] for useof the equations. Steady state performance is assumed. A

sheet and tube type collector is simulated. The header pipes are small enough in area to be neglected and

provide inform fluid flow for the riser pipes. The glasscover does not absorb solar energy that can affect lossesfrom the collector, is assumed to be opaque to infraredradiation and the temperature drop through the cover isnegligible.

Furthermore, heat flow through the glass cover and back

insulation is one dimensional. The sky is assumed to be a black body at a given temperature. No temperaturegradients are assumed to exist around pipes. Temperaturegradients in the flow direction and between pipes are

considered independent from each other. Collector properties are not influenced by temperature. Ambient

temperature is constant. The influence of dust on the glasscover and shadows created by the collector’s sides arenegligible.

Figure 2 provides a layout diagram of the entire proposedsystem. Various subsystems are shown on the layout.

Detailed designs of these systems are needed but havethus far not been undertaken. As is the case with the MED

model, a flow diagram is presented in Figure 3 which

depicts the major steps in the model.

7. RESULTS

Data from the experiment and the simulation areformulated in the following section. The two sets of datatake various steps in order to produce a format from

which results can be visually attained.



Input the parameters of the

solar collector and the solar radiation

Calculate the overall heat

loss, efficiency factor andheat removal factor

Assume mean absorber

plate temperature

Calculate useful energy gain

Calculate mean absorber

plate temperature

Difference greater than

tolerated error?

Determine difference

between initial and endmean absorber plate

temperature

Output usefulenergy gain

Calculate new

mean absorber

plate temperature

Yes

No

Figure 3: Solar collector simulation flow diagram

7.1 SIMULATED RESULTS

The results from the simulation start with the dataset provided by Meteonorm. The dataset is provided in aMicrosoft Office Access format which is then convertedto the Microsoft Office Excel format. From here the

dataset is converted to a text file format for easy transfer to MATLAB. MATLAB reads the text file and arrangesthe data in a manageable matrix. The simulation is then

able to read the matrix and use the data to produce a newset of data which reflects the simulated solar collector output in Watt hours per square meter.

The daily energy dataset is then used to provide agraphical representation of the daily energy acquired by

the simulated solar collector. Figure 4 provides the dailyenergy output of the simulation as well as the daily energydata input provided by Meteonorm.

Figure 4: Solar simulation output versus Meteonorm

input

7.2 SOLAR COLLECTOR EXPERIMENTAL

RESULTS

The experimental results start at the Steca controller. Thecollector logs the data from three temperature probes andone flow meter and stores it on a SD memory card. The

data is presented in a comma separated value formatwhich Microsoft Office Excel can read. By viewing thedata with the Steca TS Analyser [8], provided free of

charge by Steca on their website, a visual representationof the data is provided as well as the function to view the

data in Excel where it is organized in cells. From thisformat the data can be saved as an Excel workbook and processing can occur to render custom presentations of the data.

The Steca TS Analyser provides a selection of the variousinput and output values that needs to be presented. This

includes temperature readings, flow rate and pump output

percentage. Another useful tool incorporated into theanalyser is the calculation of the power output in Watts.This calculated value is also presented in the Excel outputdocument. Figure 5 provides a photo of the experimentalsetup.

Figure 5: Experimental system for solar collector

7.3 SIMULATED MULTIPLE EFFECT

DISTILLATION SYSTEM

The distillate production rates are found through the useof various formulas. Together with the solar collector energy output the distillate output for the system iscalculated. Figure 6 provides a yearly output of theoretical fresh water output per square meter of collector per day. From this graph an estimated collector

area can easily be attained for a required distillate water production.



Figure 6: Simulated fresh water output

7.4 SIMULATED VERSUS EXPERIMENTAL

RESULTS

In Figure 7 the results from the experiment on the solar

collector is presented in comparison with the simulated

solar collector data for the same time period.

Figure 7: Solar Collector Experimental Results versusSimulated Results

8. DISCUSSION

With a performance ratio of nearly 4, the system producesa maximum fresh water output per day of around 45 L/m2

and an average of 29 L/m2. This is comparably with a

simulated system by Kalogirou [9] which produces a

maximum of 130 L/m2

per day with a performance ratioof 8 and using parabolic solar collectors. Although morethan double the output, the performance ratio is also morethan double and the excess can be attributed to using

parabolic collector instead of flat plate collectors.

The solar basin stills near Ladysmith [10] in South Africa produces a maximum of 6 L/m

2per day which is well

below 45 L/m2

per day from the simulated system. Aconsiderable increase in fresh water production is thus possible for these communities. Another experimentallyverified simulation described by Qiblawey and Banat [11] produces on average 25 L/m2 per day with a performance

ratio of 3.5 which is very similar to the given simulatedresults. Similarly a three effect multiple effect evaporator

described by Ahmed et al. [4]} produces up to 14 L/m2

per day of fresh water. Unfortunately the solar radiation

received by all these systems is different and thus thesystems cannot be compared directly in terms of output per square meter of collector.

Unfortunately various problems still exist that hamper thefeasibility of the project. This includes the gathering of

sea water for use in the system. Ideally the system wouldneed to be located close to the ocean. However, using brackish water as an alternative source is also possibleand would increase the possible usable sites.

The total cost of the experimental evaluation of the solar

collector is R10 500. The total cost for a system using asingle flat plate solar collector is estimated to be aroundR20 000. This cost would increase five times if a systemis needed that must support an entire household.

9. CONCLUSION

The project undertaken here examined a solar thermal

operated desalination system. The motivation for the project sprung from the need to look at solar energy as anenergy source to power desalination. Fresh water is an

ever increasing requirement for the growing world population and the sources are diminishing. By usingsolar energy to create fresh water from the ocean, a possible limitless source of fresh water is unlocked.

The area that was investigated is South Africa with the

main focus being on the western coast. The West Coast of South Africa holds great potential for a solar powereddesalination system due to large amounts of solar radiation in close proximity with the ocean.

The goal for the project was to design a solar desalination

system for a given target fresh water output. The target

was the free of charge six kilo litres of fresh water per month per household that the Free Basic Water Scheme of the South African government provide. Furthermore, itwas needed to be able to predict the fresh water output of a specific system for a given location. This is done byusing solar radiation from a meteorological database,Meteonorm, to serve as an input to a simulation that predicts the fresh water output for that specific location

and system.

The project looked at various solar thermal collectionmethods and desalination processes in literature. Previouswork on the subject was investigated to better understand

the current environment that exists for solar desalination.From this certain methods and processes were further examined and compared. The final chosen concept for

solar collection was the flat plate solar collector and for the distillation process a multi effect evaporator systemwas chosen. For each of the systems a mathematicalmodel was also decided on to form part of the requiredsimulation.

Designing of the solar desalination system for a specificoutput made use of the mathematical models of the

processes to provide parameters for the system. Theresulting system uses a solar collection area of 10.42 m

2

to provide the prescribed six kilo litres of fresh water withthe location being Cape Town. The system is designed to be accompanied by a rain water harvesting system. This isdue to the solar collector doubling in size if the required



amount of fresh water needs to be produced with winter solar radiation values resulting in an overly expensive

system.

Simulation of the system was carried out together with theMeteonorm data. Hourly solar radiation values are

provided by Meteonorm which are fed into the solar collector model. The output from the model is summed up

into daily usable energy values. This daily value isconverted into a feed sea water flow rate at 70 ˚Celsiuswhich can then be fed into the multi effect evaporator.The evaporator model then provides a fresh water outputaccording to various parameters. The simulation is run for each day of the year to produce a yearly fresh water yieldfor a given system.

Experimental verification of the simulation was the nextstep. Unfortunately only the solar collector part could beverified due to time constraints. An experimental setup

was done for a flat plate solar collector. Temperaturesensors, a system controller, circulation pump and holdingtank filled with sea water was used in the setup.Collection of the sea water was an intensive process and

close proximity of the system to the ocean is preferred.Data was captured by the controller and stored on a

memory card that was regularly downloaded onto acomputer. Data was processed into a more manageableform and subsequently visually presented as graphs.

Results from the simulation showed that a given systemcan produce an average of 29 litres of fresh water per day

per square meter of solar collector. The performance ratio

of the system is 3.76 which is in line with the assumptionfor multi effect evaporator systems that state that the performance ratio of the system is nearly equal to theamount of effects used in the system. The results alsocompare favourably with other similar systems inliterature.

The experimental results were not as ideal as would have

been hoped for. This is mainly due to a large number of cold weather fronts that passed over the Western Cape aswell as temperature sensors malfunctioning on certaindays. The experimental data acquired did however, notmatch the simulation data but a similar upward trend can

be seen. Furthermore, the simulation results alwaysseemed to be more conservative compared to theexperimental results. Valuable knowledge in terms of sea

water acquisition and practicality of the system was also provided through the experiment.

The project has thus been able to design a framework for a system capable of providing a targeted fresh water output and also to predict the fresh water output from a

specific system for a given location. Experimentalverification of the multi effect evaporators and the

evaluation of a rainwater harvesting system arerecommended for future development of the project. Also

the ability of the system to be totally powered by solar energy should be investigated as this will further enhancethe value of the system. Finally the construction of a pilot

plant is envisaged to provide fresh water for a ruralhousehold.

The potential for this technology to help with alleviatingwater problems in South Africa is enormous. The systemwould also not put pressure on an already lacking power

system as is the case with reverse osmosis systems that iscurrently being implemented in coastal towns. Water

would be free of harmful bacteria which is a very large problem in not only South Africa but also the whole of Africa. This technology can thus be the answer to manyof our countries problems and it has the benefit of being powered by renewable energies, a must in the currentglobal energy environment.

Currently the project is shifting focus from being

exclusively a sea water desalination system to a morewidely applied system. This includes using the system torecycle grey water from households. By implementing the

system in such a way, various current problems areovercome or simplified. This includes the collection of feed water and the upper temperature limit of the feedwater.

10. REFERENCES

[1] Al-Karaghouli, A., Renne, D. and Kazmerski,L.L.: "Solar and wind opportunities for water desalination in the Arab regions," Renewable and Sustainable Energy Reviews, Vol. 13, 2009, pp.2397-2407

[2] Ettouney, H. M. and El-Dessouky, H. T.: Fundamentals of Salt Water Desalination: Elsevier Science Ltd, 2002.

[3] Zarza, E., Ajona, J.I., León, J., Genthner, K. andGregorzewski, A.: "Solar thermal desalinationresearch project at the Plataforma Solar deAlmeria," International Solar Energy Society, Vol.

1, Part II, 1991, pp. 608-622.

[4] Ahmed, M., Shayya, W. H., Hoey, D., Mahendran,A., Morris, R., Al-Handaly, J., "Use of evaporation ponds for brine disposal in desalination plants,"

Desalination, Vol. 130, 2000, pp. 155-168.

[5] Hatzikioseyian, A., Vidali, R. and Kousi, P.

National Technical University of Athens. [Online].

May 2009. HYPERLINK

"http://www.metal.ntua.gr/uploads/3024/179/

Modelling_and_thermodynamic_analysis_of_

a_MED_plant_for_seawater_desalination.pdf"http://www.metal.ntua.gr/uploads/3024/179/Modelling_and_thermodynamic_analysis_of_a_MED_plant_for_seawater_desalination.pdf

[6] Duffie, J.A. and Beckman, W.A.: Solar Engineering of Thermal Processes Third Edition.Hoboken: John Wiley & Sons, Inc., 2006.



[7] Al-Ajlan, S.A., Al Faris, H. and Khonkar, H.: "ASimulation Modeling for Optimization of a Flat

Plate Collector Design in Riyadh, Saudi Arabia," Renewable Energy, Vol. 28, 2003, pp. 1325-1339.

[8] Steca. [Online]. Aug 2009. HYPERLINK

"http://www.steca.com/index.php?main|48be3fbf00c70|2"http://www.steca.com/index.php?main|48be3fbf00c70|2

[9] Kalogirou, S.: "Use of parabolic trough solar energy collectors for sea-water desalination,"

Applied Energy, Vol. 60, 1998, pp. 65-88.

[10] Water Research Commission. [Online]. March

2009. HYPERLINK

"http://academic.sun.ac.za/polymer/WRC/Sol

ar%20Dist.pdf"http://academic.sun.ac.za/polymer/WRC/Solar%20

Dist.pdf

[11] H.M. Qiblawey and F Banat, "Solar ThermalDesalination Technologies," Desalination, Vol.220, 2008, pp. 633-644.

11. AUTHORS

Principal Author: Gerhard

Hartwig holds a BEng degree inMechatronics from the University of Stellenbosch. He is currently workingon his masters’ degree on the topic of this paper.

Co Author: Ben Sebitosi holds a PhD degree in

Engineering from the University of Cape Town and iscurrently a senior lecturer and researcher at the Centre for Renewable and Sustainable Studies at the University of Stellenbosch.

Presenter: The paper is presented Gerhard Hartwig.

design of a solar powered desalination system for use in south africa

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