high-energy efficiency desalination project using a full titanium desalination unit and a solar pond...

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ELSEVIER Desalination 136 (2001) 199-212 DESALINATION www.elsevier.com/locate/desal High-energy efficiency desalination project using a full titanium desalination unit and a solar pond as the heat supply G. Caruso a, A. Naviglio a, P. Principi b*, E. Ruffmi b °University of Rome "La Sapienza", DINCE, C so V. Emanuele II, 244, 00186 Rome, Italy b Energy Department, University of Ancona, Via Brecce Bianche, 601 O0Ancona, Italy Fax +39 (071) 280-4239; email: [email protected] Received 26 July 2000; accepted 9 August 2000 Abstract The project concerns the design, optimization, construction, assembling, start-up and extensive monitoring of an experimental plant consisting of a full-titanium desalinator coupled with a small solar pond. The operational tests took place at the site of the solar pond of the University of Ancona (Italy). Data collected during the start-up and operation of the plant under various conditions are being utilized for improving expertise on heat recovery with highly corrosive fluids, on co-generation plants aimed at producing electricity and fresh water, and on desalination fed by solar energy. Some preliminary economic evaluations are discussed. Keywords: Solar-powered desalination; Multiple effect; Solar ponds; Solar desalination economics 1. Salt-gradient solar pond applications A salt-gradient solar pond is a particular type of solar pond which relies on a salt solution of increasing concentration with depth. A salinity- gradient solar pond is an integral collection and storage device of solar energy, and it can be used irrespective of season. Usually the gradient solar pond is filled with a solution of sodium chloride *Corresponding author. having a concentration varying from a very low value at the surface at near saturation at the bottom. Fig. 1 shows the cross section of a salinity- gradient solar pond with representative tempera- ture and salinity profiles on the three typical regions. It is possible to see the lower warm and upper thin convective zones and the central gradient zone which has the function to insulate the storage zone. Presented at the conference on Desalination Strategies in South Mediterranean Countries, Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean, sponsored by the European Desalination Society and Ecole Nationale d'Ingenieurs de Tunis, September 11-13, 2000, Jerba, Tunisia. 0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S001 l-9164(01)00182-5

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ELSEVIER Desalination 136 (2001) 199-212

DESALINATION

www.elsevier.com/locate/desal

High-energy efficiency desalination project using a full titanium desalination unit and a solar pond as the heat supply

G. C a r u s o a, A . N a v i g l i o a, P. P r i n c i p i b*, E . R u f f m i b °University of Rome "La Sapienza", DINCE, C so V. Emanuele II, 244, 00186 Rome, Italy

b Energy Department, University of Ancona, Via Brecce Bianche, 601 O0 Ancona, Italy Fax +39 (071) 280-4239; email: [email protected]

Received 26 July 2000; accepted 9 August 2000

Abstract

The project concerns the design, optimization, construction, assembling, start-up and extensive monitoring of an experimental plant consisting of a full-titanium desalinator coupled with a small solar pond. The operational tests took place at the site of the solar pond of the University of Ancona (Italy). Data collected during the start-up and operation of the plant under various conditions are being utilized for improving expertise on heat recovery with highly corrosive fluids, on co-generation plants aimed at producing electricity and fresh water, and on desalination fed by solar energy. Some preliminary economic evaluations are discussed.

Keywords: Solar-powered desalination; Multiple effect; Solar ponds; Solar desalination economics

1. Salt-gradient solar pond applications

A salt-gradient solar pond is a particular type o f solar pond which relies on a salt solution of increasing concentration with depth. A salinity- gradient solar pond is an integral collection and storage device o f solar energy, and it can be used irrespective of season. Usually the gradient solar pond is filled with a solution of sodium chloride

*Corresponding author.

having a concentration varying from a very low value at the surface at near saturation at the bottom.

Fig. 1 shows the cross section of a salinity- gradient solar pond with representative tempera- ture and salinity profiles on the three typical regions. It is possible to see the lower warm and upper thin convective zones and the central gradient zone which has the function to insulate the storage zone.

Presented at the conference on Desalination Strategies in South Mediterranean Countries, Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean, sponsored by the European Desalination Society and Ecole Nationale d'Ingenieurs de Tunis, September 11-13, 2000, Jerba, Tunisia.

0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved

PII: S001 l-9164(01)00182-5

200 G. Caruso et al. / Desalination 136 (2001) 199-212

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Gr o~ len± z o n e

~ o r o g e z o n e

L _ _ _ Spr~n 9 Foil S

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Sol~niCv, V. Tempero tu re , °E

Fig. 1. Typical cross section and thermal and salinity profiles of a salt-gradient solar pond.

The salt-gradient zone insulates the storage zone and inhibits the phenomena with which the heat water rises from the bottom to the surface and loses its heat to the external air. Heat stored in the bottom can be extracted to be used for several thermal applications for which the main advantages are as follows: • low investment costs per installed collection

area • thermal storage is incorporated into the

system at the same cost • very large surfaces to collect solar energy can

be built

Only if the abundance of inexpensive salt and level land are verified is it possible to say that the use of solar energy is economic; therefore, it makes economic sense to locate the pond near an inexpensive salt supply such as salt works or an industry that produces brine as a by-product.

When these crucial factors are matched with the deliverables of a pond, this technology can be applied successfully in: • water heating • agricultural processes as crop drying, shelter

heating, greenhouse heating • desalination • electrical power generation • production of marine chemicals.

From these particular areas it seems that the most suitable application is desalination.

Solar ponds located in arid regions where drinking water is scarce and abundant brackish water is available may be the best sources of energy for water desalination. There are two reasons for this. First, the solar pond can provide the thermal energy required for the process, and second, the concentrated waste brine from the desalting process can be used for salt-gradient maintenance because salt is continuously diffus- ing from the bottom at the highest salinity to the surface at very low salinity. In this manner the often difficult problem of pollution due to water production can be solved.

The solar pond desalination system can be employed to solve drinking water problems in areas where fresh water is scarce and to growing agriculture communities around the solar pond in arid areas. If the solar pond is also used for electric power generation, the plant will be autonomous in a area water pipelines and electric lines are absent.

2. Solar pond construction considerations

After the solar pond site selection and prepa- ration, the most important elements of a solar pond are basically a containment volume obtained by earthwork, a filling of a salinity- stratified solution when the native soil does not provide low permeability a membrane liner and

G. Caruso et al. / Desalination 136 (2001) 199-212 201

at the end a system for heat removal from the heat stored zone and associated equipment for pumping.

The usual technology used for building agriculture water retention basins can be employed to construct earth berms, installing membrane liners, insulation and mechanical systems, and this results in low construction costs. The solar pond site selection is very important, the ideal solar pond site should have several essential characteristics: • easy access to water to fill the pond and for

operation and maintenance • access to salt and free salt available to reduce

costs • dry soil to minimize thermal losses • free from moving water table to minimize heat

losses • easily compacted soil and with good cohesion

for wall and structural stability

Often some of these items are contradictory; in fact, for example, if the soil is good for draining, it has low cohesive properties and then it is difficult to compact for berm construction, but in any case it is possible to reach a compromise.

Fig. 2. Characteristics of the basin.

Fig. 3. View of Ancona solar pond.

3. Ancona solar pond research facility

The construction of the solar pond in Ancona was a continuation of the solar pond research begun in 1982 by the Department of Energy, University of Ancona. The pond is located on an area of the renewable energy laboratory that was donated to the University of Ancona by the municipality. The choice of the site with suitable soil and ground characteristics was not available. In fact, soil testing showed water at a depth of 1.5 m below ground level.

The solar pond has an average collecting area of 625 m 2 and is 3.5 m deep. Fig. 2 gives the dimensions of the pond, and a view of the laboratory area with the lake are given in Fig. 3. To minimize construction costs, the earth

excavated was used to build a berm. The 2 and 1 slope was selected to prevent soil movement. Experience shows that this method of building the bund has problems of compacting the earth, especially at the bottom.

The original design consisted of a 2-m deep excavation and the construction of a 1.5-m high berm, giving a total pond depth of 3.5m. This design was changed when water was detected at a depth of 1.5m below the ground with the possibility of great heat losses. In the new design this unexpected situation was, however, exploited to detect potential losses of brine into the soil through possible minute damages to the liner. In this design the bottom of the pond was sloped slightly towards the south and covered with 0.5 m

202 G. Caruso et al. / Desalination 136 (2001) 199-212

of gravel and with 5 cm of sand to maintain a smooth soil surface.

A 20-em diameter cement pipe was buried across the south slope up to the bottom of a well. The top of the well outside the pond is about 5 m from the bund. The water flowing into the well is periodically tested for possible salt content. If the water level under the liner rises due to rainfall, automatic pumping in the well can be initiated to reduce the water level. The water from the well will run into an adjacent waterway.

3.1. Liner

The 1-mm thick EPDM sheeting for solar pond lining was selected because of its ability to retain its physical properties even at very adverse conditions. The lining required was 900 m s. This was made from smaller sheets mounted in the factory. Since the joining technique using hot binding by inserting a strip of rubber between sheet edges by thermomeehanieal presses required particular attention, it was done in the factory. The liners cover the entire internal surface of the pond and extend over the top of the berm.

Initially the sheet was anchored in a trench around the perimeter on top of the berm. After the installation the liner was exposed to heavy rains, and this resulted in mud from the top of the berm accumulating at the bottom of the pond. It was very difficult to drain the mud out of the pond and clean the liner surface. To resolve this problem, a ditch 0.3 m wide and to 0.3 m deep was excavated along the centreline of the top of the berm, and the liner edges were buried under- neath drain tiles 0.3 m in diameter placed in the ditch. This system was connected to the sewage system of the solar pond area.

3. 2. Heat extraction system

The actual heat extraction system permits heat removal by pumping out brine and passing it

through the titanium shell and tube heat- exchangers of the desalination unit. After the heat exchange the cooled brine is returned to the pond at a lower level than sucking. Pumping brine with the external heat exchanger was preferred to an internal plastic pipe heat exchanger previously used to prevent the interface between the gradient zone and lower convective zone erosion.

3.3. Operation

After an inactivity period the operating of the Ancona solar pond was compromised. In fact, in March of 1998 some maintenance was necessary; for example: • the area of the solar pond was cleaned • the basin was emptied and the solid mass

removed; • the laying of the liner was controlled • the bridge was consolidated

In April of the same year, the solar pond was again filled by the Zangrando method with 110t of salt, and the salinity gradient was established. The temperature and salinity profiles were measured in the central zone of the solar pond. • For the temperature measurements a copper-

constantane thermocouple was used, fixed on a graduated pole. The signals were transmitted to a digital microprocessor thermometer, Delta Ohm, Model HD 9016.

• For the salinity measurements, weighing samples to a different density of extracted solution were used.

The measurements were made at 10-cm intervals in depth.

In Figs. 4 and 5 the temperature and salinity profiles at the beginning of loading and operating are shown. In fact, the profiles are not normally shaped because only after the absorption of solar energy do they reach the typical slope.

Figs. 6 and 7 show the pond salinity and temperature profiles during the first months of

G. Caruso et aI. / Desalination 136 (2001) 199-212 203

350 350 [

300 300 1

25C _250 ]

15( 150 1

I0( 1001

50 i0

15 17 19 21 23 25 27 29 31 33 35 0 2 4 6 8 10 12 14 16 18 T [°el S [%1

Fig. 4. Temperature profile after a loading week. Fig. 5. Salinity profile after a loading week.

320 30O 280 280 240 22O

| ,oo

- e - ql-l.g

02-ago

4O 20

v r ' "*' ' i " ' | i v w i

20283088404880888088 Tenmmmm T rq~'q

320 300 280 280

i 240 220 200 180 160

~14~ 120 1

40 20 0

-'4'- 16-lug -e-26-1ug

0 i ! i ' i u

4 8 12 16 20 Salinity 8 [%]

i i

24 28

Fig. 6. Temperature profile in steady state. Fig. 7. Salinity profile in steady state.

204 G. Caruso et ai. / Desalination 136 (2001) 199-212

operating in the period July-August. It can notice the formation of a linear gradient zone with the thickness o f 80°C and a lower convection zone where the temperature and salinity have reached a value of about 65°C and 24%, respectively.

4. Economic and technical considerations for a solar pond coupled with a desalinator

For economic evaluation purposes for solar pond generating heat, Tables 1 and 2 give the construction and operating costs of a laboratory solar pond. The main construction costs of solar ponds are the excavation and lining of the internal surface and salt.

Table 1 Capital costs per unit area of the Ancona solar pond, G_.,/m 2

Land Free Design and performance analysis 2.3 Excavation-diking 15 Water 0.5 Salt 9.2 Liner 9 Wide strips of plastic net for wave control 1 Instrumentation monitor and control 4 Heat extraction system 2 Diffuser 0.3 Piping and other pumping systems 1 Total 44.3

Table 2 Operating cost of the Ancona solar pond, E/y

Maintenance 1200 Capital recovery 1700 Operator time 800 Electricity 500 Salt 200 Total 4400

The development of thermal processes of desalination as multi-effect distillation (MEF) and multi-stage flash desalination (MSF) permit operation with a temperature range around 60- 70°C. In this case it is possible to use solar pond technology as a heat source where seawater or brackish water is available to produce fresh water (Fig. 8).

It has been demonstrated that it is possible to separate water and salt economically from seawater by use of solar pond thermal energy. A control aim of the work is that in these technologies the brine produced by desalting may be used for practical pond operation to maintain the appropriate salinity gradient.

During the life of the pond, natural phenomena of salt diffusion up the surface to the air are actuated. In this case the interface between the upper convective zone and the gradient zone goes in deep to reduce the gradient zone thickness. To modify the salinity profile and to assume the solar pond stability, it is necessary to use localized injection with a salt solution to different level by the diffuser.

To control several physical phenomena the injection technique can be used in many cases. In this case the salt to produce brine to inject inside the pond can be taken from the seawater desalting process. This reduces the pollution due to the production of salt from the desalination plant.

5. M ~ T C desalination system

The main part o f a ME-TC desalination plant (Fig. 9) is a multi-effect evaporator with its auxiliaries and ancillaries. An ejector vapor compressor enables the plant to operate also according to the thermocompression process.

The advantages of this plant solution are the following: • simplicity and compact construction • plant can be operated with no recirculation

G. Caruso et al. / Desalination 136 (2001) 199-212 205

i Ir p-

FEED WATER ~r DRAIN U C Z M A K E - U P

/ Villi

D.AUNAT.ON ~ H..,.~.~on ~

c/" .....c. !::: ....... x__L_ ....

EVAPORATING POND T O T H E ~ ~ l ~

B R I N E T A N K BLOW DOWN

INJECTION TANK

Fig. 8. Schematic of solar-pond-heated desalination system.

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EJ lo2 @ ~-~=~,, ¢>

DES#L/NAT/ON PI.,,4NT PROCESS DIAGRAM I F 7 b F 1 L ~ : ~ L~ Yl.4 ] ~ : . , ,

Fig. 9. Desalination plant flow diagram.

206 G. Caruso eta]./Desalination 136 (2001) 199-212

• low pumping power • high performance ratio/unit of installed heat

transfer surface area • stable operation • low operating labor cost

In this project, a full-titanium desalination unit was specifically designed. It was conceived after the experience gained with a preceding plant (three stages, 10m3/d of distillate production). It was designed and built in the framework of a project partially financed by the EC (contract Thermie No. SE 303/94 IT), con- cerning the design, optimization, construction, assembling, start-up and extensive monitoring of a full titanium desalination plant, with a production capacity of 30 m3/d.

Data collected during the manufacturing tests and most of all during the start-up and the operation of the plant under various conditions and alignments are being used for the improve- ment of the know-how on (1) heat recovery with highly corrosive media, (2) co-generation plants aimed at producing electricity and fresh water, and (3) desalination fed by solar energy. One of main targets of the project is the assessment of data which will allow a comparison of the additional cost of the full titanium desalination plant with respect to traditional technology, with the benefits of (1) better heat transmission through tube bundles, which means better perfor- mance; (2) reduced requirement of chemicals and of maintenance activities; and (3) improved plant reliability and duration.

The multi-effect process is used when low- temperature heat is available (60-90°C) from a solar pond or from a steam turbine. Thermo- compression configuration needs medium- pressure steam (9-16 bar) to operate; this steam is produced by a steam boiler fed by conventional fuel or by a solar system. In Table 3 the main process parameters of the desalination plant are reported.

Table 3 Process parameters of a 30 m3/d desalination plant

Multi- Vapor effect compression

Production, t/h 1.25 1.25 No. of effects 4 4 Seawater flow, t0a 22 11 make-up flow rate, t/h 4 4 Heat input, kW 240 at 65°C 154.7 Heat performance, 690 444

kJ/kg water Efficiency, kg NA

Inlet vapor temp., °C 55 Distilled temp., °C 41.4 Seawater inlet temp., °C 25 Drain outlet temp., °C 33 Solar pond exchange 43.2 make-up temp., °C

Thermo-ejector motor NA vapor

Ejector outlet temp., °C NA First effect temp., °C 49.6 Fourth effect temp., °C 34.6 Blowdown temp., °C 35

5.73 dist,/kg steam

NA 41.4 25 33 NA

218 kg/h; p -- 10 bar; T = 180

60 49.6 34.6 35

The seawater desalination plant (Fig. 9) consists of a multi-effect evaporator with its auxiliaries and ancillaries. An ejector vapor compressor allows the operation of the plant according to the thermo-compression process. The entire plant has been designed for simplicity of control and stability in operation. The plant is capable of operation at reduced capacity from 100% to 50%.

The feed water to each effeet is sprayed over the tube bundles by spray nozzles. Their locations on the distribution headers ensure a uniform flow distribution over the tube bundles to avoid areas of low flow or drying out and the following formation of scales on the tube surface. The tube bundle in each effect is horizontal.

G. Caruso et al. /Desalination 136 (2001) 199-212 207

6. Tests performed

Several tests have been performed during the experimental phase aimed at verifying the performance of components and equipment.

Aiter the commissioning tests, performed in April and May 1997, the start-up of the plant occurred in early June, 1997. The first tests were performed in thermocompression mode until August 1997. From September to May 1998 the plant was operated in multieffect mode, simulating the heat from the solar pond using steam from a boiler, due the unavailability of the solar pond. From June to August 1998, tests using the heat provided by the solar pond were carried out. Generally, the desalination plant was operated a week each month from June 1997 to August 1998. The installation of solar collectors, foreseen by the University of Ancona to integrate the thermal power of the solar pond, was not possible before the end of the project. Therefore, multi-effect tests with heat from the solar pond have been carried out with a heat input lower than the design value (about 50%).

The operating hours in stable conditions (production tests) of the plant may be sum- marized as follows: • Commissioning tests: (ME with boiler) 120h • Thermocompression mode (TC) 80 h • Multi-effect mode (ME), heat from a boiler

160 h • Multi-effect mode (ME), heat from solar pond

6O h Some problems connected to vacuum loss

through some leaks in the titanium welding were revealed and solved. The vacuum system was extensively tested and its operation is now satisfactory. Further problems related to hydraulics in the distillate line and in the feed water line were also faced. Some modifications in the piping were studied and performed. Other problems related to the vapor thermocompressor arose in some tests and some modifications were needed.

Only a limited number of tests has been performed so far with the solar pond due to the problems encountered in reaching high tempera- tures inside the pond, either for unfavorable weather conditions or technical problems due to pond maintenance. In the tests performed, the solar pond/desalinator heat exchanger seemed to operate successfully. Some minor problems occurred in the liquid pipeline (downcomer), where the concentrated solution produced in the exchanger is driven only through gravity into the evaporator shell of the first stage. This was due to an insufficient difference of height between the two nozzles imposed by lay-out requirements.

In Table 4 some averaged values obtained in the three operating modes are reported. In Figs. 10 and 11, the specific electric consumption and heat performance (thermal heat for unit of distillate produced) are shown. The values corresponding to 100% are design values, not tested during the experimental campaign due to the limited thermal power available. As for the solar pond tests, only 30% and 50% of the design power has been tested due to the discussed limitations of the available solar pond. The specific electric consumption values reported in Fig. 10 do not include the contri- bution of the vacuum system. This additional consumption, due to the additional water flow rate through the high pressure main pump for

Table 4 Production and thermal energy consumption averaged values

Operation Heat Production, Heat mode input, kg/h performance,

kJ/h kJ/kg

ME (heat from 360,000 540 667 a boile0

TC 360,000 480 750 ME (heat from

solar pond) 420,000 600 700

208 G. Caruso et al. / Desalination 136 (2001) 199-212

Heat performance

ME (boiler) ME (S.P.) TC

D30%

1150%

ii66%

0(100%)

Fig, 10. Specific electric consumption during the experimental campaign compared to the design value (100%).

g-,

i

Oz t , __ - - - - | I r a - - __1__ __Jk .__ | . . . . . . . . . . . . L I __ _ _

ME (boiler) ME (S.P.) TO

Fig. 11. Heat performance during the experimental campaign compared to the design value (100%).

pump for the vacuum hydro-ejector, may be assumed about 3 kW for the production of 1 m3/h of distillate.

7. Operating costs

The operating costs of the specific project have been strongly affected by the particular site selected to install the plant. Furthermore, the mode of operation for only a few hours each day,

due to the availability of a limited source of salt water, suggests evaluating the average cost only during stable operation of the plant. The start-up and shut-down phases that during the experi- mental campaign have very much affected the economics of the production, due to their incidence on the global time of operation, are not taken into account.

The average consumption of natural gas has been about 15Nm3/h and the electricity

G. Caruso et al. / Desalination 136 (2001) 199-212 209

,

2.5'

2

1.5

1

0.5

0 ME (boiler)

Fig. 12. Water cost (operating cost, CeJm 3 ).

ME (S.P.) TC

consumption has been 8 kW, including the feed pump, the vacuum system and the extraction pumps, during the TC and ME operation modes (the last one with expanded steam). In the ME mode, when the operation was carried out in conjunction with the solar pond, no natural gas consumption was needed, but an increment of 3 kW for the solar pond pump was recorded for the electrical consumption.

With an average production of about 1 m3/h of distillate, the specific consumption, including the vacuum system contribution, is: • natural gas: 0.015 Nm3/kg of distillate (TC

and ME mode) • electricity: 8 kWh/m 3 of distillate (ME mode);

5 kWh/m 3 (TC mode); 11 kWh/m 3 (solar pond)

If a cost of 0.13 ~ / N m 3 for natural gas and of 0.1 6/kWh for electricity is considered (industrial use), a total cost of about 2.68 6/m 3 of distillate is obtained (ME-steam mode without solar pond) and 2.4 {~]m 3 (TC mode). In ease of main energy supply provided by the solar pond, an operation cost due to energy of about 1 6/m 3 of distillate is obtained (Fig. 12).

Personnel and maintenance costs are not meaningful because the operation of the plant was carried out as a research activity, so several interventions of maintenance personnel were

requested during the many checks. As for operation personnel requirements, during steady- state operation, a technician was required, part- time.

Chemical consumption was null during the year of operation-monitoring, without any decre- ment of the plant's performance. It may be noted that the plant operated for 9 months with town- supply drinkable water, artificially salted with a suitable content of NaCI. The water remained stagnant inside the storage tank with formation of biological fouling detected on the main filter of the feed line.

8. Economic viability

Some preliminary considerations about the economic viability to be expected depend on the possibility of a production in series of the plant. Our forecast for a series-production is an extra cost with respect to a conventional realization of about ~50,000.

With reference to a target production of 30m3/d, the thermal power requirement (PN) is 240 kW and the thermal power saving (PS) with a full titanium desalination plant and a conven- tional has been estimated about 72kW.

In order to evaluate the economic viability of the project, an economic value is here associated

210 G. Caruso et al. / Desalination 136 (2001) 199-212

to the PS. Should the saved thermal power be provided by a conventional thermal plant, we could assume: • cost o f fuel (methane): 0.13 ~/Nm 3 • average efficiency of the boiler during the

year: 0.8 • heat content of fuel: 8250 kcal/Nm ~ =

34,540 kJ/Nm 3

We obtain a cost of the thermal energy to the user of Y = 0.015 E/kWh, including the depreciation (D) of the thermal plant. Therefore, the energy saved yearly would be:

Q = P S ' N h / y = 72" 8760 = 630,720 kWh

The economic saving would be:

E = Q • Y = 630,720" 0.015 = 94606/y

With reference to the economics of the solar pond, if we refer to a new solar pond with the same characteristics as the Agip Petroli solar pond, but incorporate the experience of construc- tion and operation of the Margherita di Savoia solar pond, the cost of energy "produced" by the pond is:

Y' = 0.062 ~/kWh.

In this case the energy supply cost for the inno- vative full titanium desalinator would be:

E ' = P N " N" Y' = 240" 8760" 0.062

= 130,400 ~/y

With reference to the economics of an extrapolated commercial solar pond, the cost of energy produced would be one-tenth the preceding value. In this case the economic saving due to the innovative full titanium desalinator would be:

E" = 13,040 ~/y

A traditional desalinator would have produced the same amount of distillate with an energy cost (if fed through commercial fuel):

E* = 39,100 E/y

In order to evaluate the operating and main- tenance costs (M) for a full titanium desalinator, we have to take into account, among others, the cost of the following items: • operation personnel • chemicals • spare parts • maintenance of tube bundles and box cleaning

For a comparison with a desalinator manu- factured with traditional technology, the difference in costs for operational and mainten- ance must be recognized mainly with reference to (1) chemicals and (2) maintenance for tube bundles and box cleaning. A full titanium desalinator requires significantly fewer chemicals than a traditional one. The requirements for maintenance tube bundles and cleaning boxes are also expected to be significantly lower.

Globally, with reference to a full operation during the year, the saving of cost (M) = -40006 may be assumed for the full titanium desalinator of 30 m3/h.

Thus, the following value of pay-back times would result: • Pay-back time referred to a comparison

between a full titanium desalinator and a traditional desalinator both fed through conventional fuel:

t _ _ _ C _ 50,000 = 3.71 y E - M 9 4 6 0 + 4000

Pay-back time referred to the full titanium desalinator fed by a 1 km 2 solar pond and compared with a traditional desalinator fed through commercial energy:

G. Caruso et al. / Desalination 136 (2001) 199-212 211

I I I

[] commercial - 1 km2 S.P. • commercial - fossil • present - 1 km2 S.P. • present - fossil

0 2 4 6

Y e a r s

Fig. 13. Pay-back time (with respect to traditional technology).

8 10 12

C 50,000 t ! ~

E * - E " - M 39,100-13,040+4000

= 1.67 y

Considering an extra cost of 150,0006 for the present realization (prototypical) with respect to a conventional one, the pay-back times would be 11.1 y and 5.13 y, respectively. In Fig. 13, the pay back times obtained for the present realization and a commercial realization using the present technology with respect to traditional technology are shown.

At the moment, only a few projects of solar ponds with a meaningful size have been imple- mented worldwide. The main two causes negatively affecting dissemination of solar pond technology are: (1) a rather high cost of the solar pond and (2) a rather low cost of commercial fuels for some years.

The experience gained has shown that with the present market prices of fossil fuel, a solar pond is competitive only for a very high extension. Nevertheless, in southern Europe and in North Africa, especially in remote areas or islands, conditions for attractive economics of solar pond may exist also for relatively limited

dimensions of the basin (ranging between 20,000 and 100,000 m2). Several countries have shown a real interest in solar ponds. In several conferences the technical and economic validity of this technology for the production of pure water has been recognized.

We believe that there will be in the future an expansion of solar pond construction. This will occur mainly in hot climate regions, and new opportunities may be envisaged for European technology. Obviously, in case of an increase of cost of oil products, the economics of desali- nation through solar ponds would improve accordingly. For a short-medium term, the construction of some units in Mediterranean countries and the in Middle East may be envisaged.

The commercial exploitation of the tech- nology of the project may be recognized over a short term more in the field of electric energy production. In fact, there is an increasing requirement for pure water at the sites of thermal electric plants, and some main utilities are analyzing the possibility of satisfying the internal needs and, eventually, additional local needs through desalination units fed by a co-generation scheme.

212 G. Caruso et ai. / Desalination 136 (2001) 199-212

9. Environmental impact

The impact of the project on the protection of the environment is very beneficial for two reasons, in addition to that of the improvement in heat performance (lower primary fuel consump- tion if traditional fuels are used).

The first one is the lower amount of chemicals which is expected for the operation of the full titanium desalinator: a traditional desalinator requires chemical additives in the amount of 42~g of polyphosphates/m 3 of raw seawater and of 17,300g of acid (HC1, H2SO3)/m 3 of raw seawater. The innovative desalinator is expected to require a lower amount of chemicals. There- fore, the environmental impact is potentially very limited. During the experimental campaign, no chemicals were used, and a negligible effect was revealed in plant performance. To better assess the need of chemical dosing, further tests will be performed in the future and a detailed analysis of the surface conditions of the evaporator tubes will be carried out accordingly. A very prelimi- nary estimate indicates that the chemical additive requirement is about 25% of the usual amount needed in desalinators manufactured with traditional materials.

The second one is the material of the desalinator, which is much less susceptible to corrosion and which does not release to the environment corrosion salts, oxides and ions.

10. Conclusions

From the technical point of view, the project involving the design, manufacture, and prelimi- nary testing of an innovative full titanium desalinator, coupled with a solar pond, may be judged as satisfactory. The performance of the plant has been extensively confirmed during a one-year experiment. Further studies and the continuing development in the design should lead to quite interesting commercial perspectives for this technology, with the possibility of economic

profitability in remote areas, also coupled with renewable energy sources conversion systems.

Most of the initial targets of the activity have been successfully reached. Further tests will be carried out in order to obtain full comprehension of fouling performance and the resistance of titanium.

Unfortunately, problems connected with the unavailability of a large-size, fullly operating solar pond have not yet permitted extensive experiments on the performance of a combined solar pond/desalinator system. Nevertheless, the information collected seems sufficient to provide useful guidelines to improve the design of the plant and to allow a more suitable selection of some critical components.

References

[1] World Resources 1998-1999--Aguidetotheglobal environment, Report by the World Resources Insti- tute, UN Environment programme, UN Development Programme and the World Bank, 1999.

[2] J.R. Hill, C.E. Nielsen and P. Golding, Salinity- Gradient Solar Ponds, CRC Press, Boca Raton, FL, 1989.

[3] A.H. Khan, Desalination Processes and Multistage Flash Distillation Practice, Elsevier, Amsterdam, 1986.

[4] G. Cesini and P. Principi, Solar pond design and construction in central Italy, Int. Conf., Progress in Solar Ponds, Cuemavaca, Mexico, 1987.

[5] M. Pacetti, P. Principi and F. Sabetta, Solar Energy, 34(4/5) (1985) 297.

[6] M. Calm, G. Caruso, L. Grarniccia and A. Naviglio, Proc., 45th Congresso Nazionale ATI, Cagliari, 1990, pp. 27-35.

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[8] P. Principi, R. Ricco and E. Ruffini, Proc., 51st Congresso nazionale ATI, Udine, 1996, pp. 1249-1259.

[9] G. Caruso, A. Naviglio and P. Principi, Use of solar energy for seawater desalination: a solar pond assisted ME-TC desalting plant, ITEEC, Marrakesh, Morocco, 1997.